Use of Microorganisms as Important Household / Industrial Products!
Microbes or microorganisms are small organisms which are not visible to naked eye because they have a size of 0.1 mm or less. They can, therefore, be seen only under the microscope. Microbes are present everywhere inside soil, in all types of waters, in air, on dust particles, inside and outside our bodies as well as other animals and plants. They even occur in most inhospitable places where no other life forms can exist — in snow, inside thermal vents or inside geysers (with temperature of 100° C), deep inside soil, highly acidic habitats, Microbes belong to diverse groups of organisms — bacteria, fungi, protozoa, microscopic plants. Viruses, viroids and prions are also included amongst microbes. They are infectious agents. Viruses are nucleoprotein entities, Viroids are made up of only nucleic acids. Prions are proteinaceous infectious agents. The three cannot be cultured in cell free extracts. Most of the other microbes can be grown on nutritive media where they form colonies, e.g., bacteria, fungi. The colonies can be
seen with naked eyes. They are useful in the study of various aspects of microorganisms. While microbes are causal agents of most of the infectious diseases, they have also been in use by humans and nature in many important processes in homes, industries, agricultures and sewage treatment. Rather, microbes become part of many useful articles used by early humans like fermented honey (alcoholic drink mead), wines, bread, curd, cheese, separation of plant fibres, etc.
Household Products 1. Dairy Products: Lactic acid bacteria (LAB) like lactobacillus are added to milk. It converts lactose sugar of milk into lactic acid. Lactic acid causes coagulation and partial digestion of milk protein casein. Milk is changed into curd, yoghurt and cheese. The starter or inoculum used in preparation of milk products actually contains millions of LAB. (i) Curd:
Indian curd is prepared by inoculating skimmed and cream milk with Lactobacillus acidophilus at a temperature of about 40°C or less. Curd is more nutritious than milk as it contains a number of organic acids and vitamins including
seen with naked eyes. They are useful in the study of various aspects of microorganisms. While microbes are causal agents of most of the infectious diseases, they have also been in use by humans and nature in many important processes in homes, industries, agricultures and sewage treatment. Rather, microbes become part of many useful articles used by early humans like fermented honey (alcoholic drink mead), wines, bread, curd, cheese, separation of plant fibres, etc.
Household Products 1. Dairy Products: Lactic acid bacteria (LAB) like lactobacillus are added to milk. It converts lactose sugar of milk into lactic acid. Lactic acid causes coagulation and partial digestion of milk protein casein. Milk is changed into curd, yoghurt and cheese. The starter or inoculum used in preparation of milk products actually contains millions of LAB. (i) Curd:
Indian curd is prepared by inoculating skimmed and cream milk with Lactobacillus acidophilus at a temperature of about 40°C or less. Curd is more nutritious than milk as it contains a number of organic acids and vitamins including
B12. LAB present in curd also checks growth of disease causing microbes in stomach and other parts of digestive tract. Curd is eaten as such, salted or sweetened. Curd is churned to prepare lassi. It is also used to obtain butter and butter milk. (ii) Yoghurt (= yogurt):
It is produced by curdling milk with the help of Streptococcus thermophiles and Lactobacillus bulgaricus. The temperature is maintained at about 45°C (40°^6°C) for four hours. It has a flavour of lactic acid and acetaldehyde. Yoghurt is often sweetened and mixed with fruit. (iii) Butter Milk:
It is acidulated product which is formed by inoculating skimmed milk with starter culture of Streptococcus cremoris, S. lactis, Lactobacillus acidophilus, Leuconostoc species at 22°C for 18 hours. Acidulated liquid left after churning of butter from curd is also called butter milk. (iv) Sour Cream:
Cream obtained by churning of milk is inoculated with Sterptococcus lactis for producing lactic acid and Leuconostoc cremoris for imparting the characteristic flavour.
(v) Cheese:
It is one of the oldest milk products prepared with the help of microbes. The curd is separated from liquid part or whey to form cheese. Depending upon water content, cheese is of three types -soft (50-80% water), semihard (about 45% water) and hard (less than 40% water). The method of preparing cheese with the help of microbes was known in Asia and Europe long before Christ. There are several varieties of cheese with different texture, flavour and taste. Curdling is done with the help of lactic acid bacteria and enzyme rennin (= Casein coagulase, chymosin), rennet (from Calf’s stomach) or fruit extract of
Withania coagulans. In preparation of raw cheese milk is curdled with the help of lactic acid bacteria. The curd is heated gently to separate cheese from whey. Any liquid left in cheese is allowed allowed to drain by hanging it in cloth. Unripened or Cottage cheese is prepared by single step fermentation which involves inoculation of skimmed milk with cheese culture (e.g., Lac tobacillus, Acetobacter, Saccharomyces,, Rhizopus, Amylomyces) and addition of Saccharomyces rennin or rennet after 1-2 hours. Curd is placed in cloth lined porous containers for draining out whey.
Ripened cheese is prepared from unripened cheese by first dipping in brine, wiping and then maturation with different strain of bacteria and fungi. It takes 1-16 months for ripening. Large holed swiss cheese is ripened with the help of CO2 producing (causing holes) bacterium called Propionibacterium sharmanii. Roquefort cheese uses Penicillium roqueforti while Camembert cheese employs Penicillium camemberti for ripening.
2. Bread: Selected strains of Baker’s Yeast, Saccharomyces
cerevisiae, are grown on molasses. When sufficient growth has occurred, Baker’s Yeast is harvested and converted into either powder or cakes. A small quantity of Baker’s
Yeast is added to wheat flour. The same is needed. The kneaded flour is kept at a warm temperature for a few hours. It swells up. The phenomenon is called leavening. Leavening is caused by secretion of three types of enzymes by yeast. They are amylase, maltase and zymase. Amylase causes breakdown of a small quantity of starch into maltose sugar. Maltase converts maltose into glucose. Glucose is acted upon by zymase. Zymase is a complex of several enzymes of anaerobic respiration which brings about fermentation.
Fermentation of glucose mainly forms ethyl alcohol and carbon dioxide. The two cause swelling or leavening of the dough. The leavened dough is baked. Both carbon dioxide and ethyl alcohol evaporate making the bread porous and soft.
3. Dosa, Uppma and Idli: They are fermented preparation of rice and Black Gram (vem. Urad). The two are allowed to ferment for 3-12 hours with air borne Leuconostoc and Streptococcus species of bacteria. CO2 produced during fermentation causes puffing up of the dough.
4. Jalebi: The semi-liquid dough of fine flour of Wheat is fermented with yeast, fried in the form of coils and dipped in sugar syrup to obtain Jalebi. Imriti is similarly prepared from Black Gram flour.
5. Other Foods: Tempeh (Indonesia), Tofu (Japanese) and Sufu (Chinese) are fermented foods obtained from soyabean. Soy sauce is brown flavoured salty sauce fermented from soyabean and wheat. Tender bamboo shoots are used as vegetable directly as well as after fermentation. Several types of sausages are prepared by fermentation and curing of fish
and meat. Sauerkraut is finely chopped fermented and pickled cabbage.
6. SCP (Single cell protein): It is the production of microbial biomass as supplementary food for humans and animals. The common SCP are Spirulina, Yeast and Fusarium graminearum. Processing is required. SCP is rich in high quality protein, vitamins and minerals but poor in fat. Besides proving much needed proteins, SCP is useful in reducing environmental pollution as it is often grown over medium having organic wastes from agriculture and industries.
7. Toddy: It is a traditional drink of some parts of South India which is made by fermentation of sap of palms. A common source is tapping of unopened spadices of coconut. It is a refreshing drink which can be heated to produce jaggery or palm sugar. Toddy left for a few hours undergoes fermentation with the help of naturally occurring yeast to form beverage containing about 6% alcohol. After 24 hours toddy becomes unpalatable. It can be now used for producing vinegar.
Industrial Products:
Fermentative activity of microbes is used industrially to obtain a number of products. The two common ones are alcoholic fermentation and antibiotics. Methodology:
For any new industrial utilisation of a microbial activity, the technology passes through three stages —laboratory scale, pilot plant scale and manufacturing unit. The development from laboratory scale to manufacturing unit is called scaling up.
1. Laboratory Scale: Soon after the discovery of use of a microorganism, the maximum number of strains is searched and the most suitable strain is selected and multiplied. A laboratory scale apparatus/plant is manufactured. It has a glass fermentor (fermenter). All the parameters of the process are worked out like nutrients for the microbe, pH, aeration, disposal of C02if evolved, optimum temperature, by products, product inhibition or stimulation, time of optimum production, separation of product and its purification. Ultimately, the laboratory scale process is finalised.
2. Pilot Plant Scale:
It is intermediate stage where working of the laboratory scale process is tested cost and qualities of the product are evaluated. Glass vessels are replaced by metallic containers. The container where fermentation is carried out is called bioreactor or fermentor. Aeration system, pH corrections and temperature adjustments are perfected.
3. Manufacturing Unit: Its size is determined by the economics worked at during the pilot plant scale process. Bioreactor or fermentor is often large. Microorganisms are added in bioreactors in three ways: (i) Support growth system or on surface of nutrient medium, (ii) Suspended growth system or suspended in nutrient medium, (iii) Column or immobilised growth system where microorganisms placed in calcium alginate beads are kept in columns.
Alcoholic Fermentation: Louis Pasteur found for the first time that beer and butter milk are produced due to activity of Yeast and Yeast-like microorganisms. Yeast species used in alcoholic
fermentation are Saccharomyces cerevisiae (Brewer’s
Yeast), S. ellipsoidens (Wine Yeast), S. sake (Sake Yeast) and S. pireformis (Ginger Beer/Ale Yeast). The nutrient medium is barley malt for beer, fermented rye malt for gin, fermented rice for sake, cashew-apple for fenny, potato for vodka, fermented cereals for whisky, fermented molasses for rum and fermented juices for wines and brandy. 1. Yeast does not possess sufficient diastase/amylase. Therefore, either 1% malt or Rhizopus is used when the nutrient medium consists of complex carbohydrates as present in cereals and Potato. Hydrolysis of starch is carried out in separate tank at high temperature (55°C) for 30 minutes. The crushed food mixed with hot water for obtaining malt is called mash. The sweetened nutrient medium prior to alcoholic fermentation is called wort. 2. Bioreactor/fermentation tank is sterilised with the help of steam under pressure. The liquid nutrient medium or wort is added into the tank and sterilised similarly. It is then allowed to cool. 3. When the liquid nutrient medium is cooled down to appropriate temperature, it is inoculated with appropriate
strain of Yeast through support growth system (on the surface) or suspended growth system (inside the wort). Fermentation occurs in three ways: (i) Batch Process:
Bioreactor is very large (capacity up to 2,25,000 litres of medium). Yeast and nutrient are allowed to remain there till maximum alcohol content is achieved (6-12%). It is called wash. The same is removed and the tank sterilised for the next batch, (ii) Continuous Process:
There is a regular removal of a portion of fermented liquor/wash and addition of more nutrient, (iii) Fed Batch Process:
Nutrient is regularly fed in small quantities in the fermenter so as to optimise the working of the fermenting microbe without any inhibition, (iv) Immobilised Yeast:
Lately Yeast is being used in immobilised state in calcium alginate beads. The technique is 20 times more efficient.
4. Both Beer and Wine are filtered, pasteurised and bottled without further distillation. Beer has an alcoholic content of 3 – 6% while in wines the alcoholic content is 9-12%. Higher alcoholic content is generally achieved through direct addition of alcohol. Hops are added to wort during preparation of beer. Distillation of the fermented broth is carried out in case of other alcoholic beverages called hard liquors, e.g., gin (40%), rum (40%), brandy (60-70%). Rectified spirit is 95% alcohol. Absolute alcohol is 100% alcohol. 5. Bye-products of alcoholic fermentation are CO 2 and Yeast. A number of other chemicals can be formed with the change of nutrient medium, pH and aeration – n-propanol, butanol, amyl alcohol, phenylethanol, glycerol, acetic acid, pyruvic acid, succinic acid, lactic acid, caproic acid, caprylic acid, ethyl acetate, acetaldehyde, diacetyl, hydrogen sulphide, etc.
Antibiotics: The term was coined by Waksman (1942). Antibiotics (Gk. anti— against, bios —life) are chemical substances produced by some microbes which in small concentration can kill or retard the growth of harmful microbes without adversely affecting the host. Penicillin was the first antibiotic
to be discovered by Alexander Fleming (1928). He found that fungus Penicillium notatum or its extract could inhibit the growth of bacterium Staphylococcus aureus. The antibiotic was however, commercially extracted by efforts of Chain and Florey. The chemical was extensively used in treating wounded American soldiers in World War II. Fleming, Chain and Florey were awarded Nobel Prize in 1945. Waksman and Woodruff isolated actinomycin in 1941 and streptothricin in 1942. Waksman and Albert (1943) and Waksman (1944) discovered streptomycin. Burkholder (1947) isolated chloromycetin. Over 7000 antibiotics are known. Every year some 300 new antibiotics are discovered by means of hypersensitive microorganisms (started in 1970). Streptomyces griseus produces more than 41 antibiotics while Bacillus subtilis forms about 60 antibiotics. Antibiotics can be broad spectrum or specific. Broad Spectrum Antibiotic. It is an antibiotic which can kill or destroy a number of pathogens that belong to different groups with different structure and wall composition. Specific Antibiotic. It is an antibiotic which is effective only against one type of pathogens.
Action:
Antibiotics function either as bactericides (killing bacteria) or bacteriostatic (inhibiting growth of bacteria). This is done by (i) Disruption of wall synthesis, e.g., penicillin, cephalosporins, bacitracin, (ii) Disruption of plasmalemma repair and synthesis, e.g., polymyxin, nystatin, amphotericin, (iii) Inhibition of 50 S ribosome function, e.g., erythromycin. (iv) inhibition of 30 S ribosome function, e.g., streptomycin, neomycin, (v) Inhibition of aa-tRNA binding to ribosome, e.g., tetracycline, (vi) Inhibition of translation, e.g., chloramphenicol.
Characteristics of a Good Antibiotic: (a) Harmless to host with no side effect, (b) Harmless to normal microflora of alimentary canal, (c) Ability to destroy pathogen as well as broad spectrum, (d) Effective against all strains of pathogen, (e) Quick action. Resistance to Antibiotics:
Pathogens often develop resistance to existing antibiotics so that newer antibiotics are required to be produced. The
resistance is generally produced due to extrachromosomal genes present in plasmids. They can pass from one bacterium to another due to transformation and transduction. As a result of repeated transformation, certain strains of bacteria have become multiresistant or super bugs, e.g., NDM-1. Resistance to antibiotics comes from (i) Development of copious mucilage, (ii) Alteration of cell membrane so that antibiotic cannot recognise the pathogen, (iii) Alteration of cell membrane which prevents antibiotic entry, (iv) Change to L-form by pathogen, (y) Mutation in pathogen. (vi) Development of pathogen enzyme capable of modifying antibiotic. Production of Antibiotic:
Suitable strain of microorganism is cultivated on a sterilised nutrient medium provided with optimum pW, aeration, temperature, antifoaming agent and antibiotic precursor (if any). When sufficient antibiotic has diffused into medium, the microorganism is separated and the antibiotic is extracted from medium by precipitation, absorption or solvent treatment. It is purified, concentrated and bioassayed before packing.
Antibiotics are obtained from lichens, fungi, eubacteria and actinomycetes. The common antibiotic from lichens is usnic acid (Usnea and Cladonia). Amongst eubacteria, two account for most antibiotics, Bacillus (70%) and Pseudomonas (30%). Fungi yield a number of antibiotics like penicillin, patulin and griseofulvin (Penicillium species), cephalosporins (from marine fungus Cephalosporium acremonium), antiamoebin (Emericellopsis), polyporin (.Polystictus sanguineus), clitocybin (Clitocybine gigantea), citrinin (Aspergillus clavatus, Penicillium citrinum), clavacin (Aspergillus clavatus), etc. Most famous drugs are got from actinomycetes, especially Streptomyces, e.g., streptomycin, chloramphenicol, tetracyclin, terramycin, erythromycin. Other antibiotic yielding actinomycetes are Streptosporangium, Streptoverticillium, Micromonospora, Nocardia and Actinoplanes, etc. Some antibiotics are modified to enhance their potential. They are semisynthetic, e.g., ampicillin, oxocillin. Uses:
Antibiotics are used:
(i) As medicines for treatment of a number of pathogenic or infectious diseases. Because of antibiotics and their newer more potent forms, a number of formidable diseases are now curable, e.g., plague, typhoid, tuberculosis, whooping cough, diphtheria, leprosy, etc. (ii) As preservatives in perishable fresh food articles (e.g., meat and fish), pasteurised and canned foods, (iii) As feed supplement for animals, especially poultry birds because they enhance growth.
Chemicals, Enzymes and Other Bioactive Molecules: Microbes are being used for commercial and industrial production of certain chemicals like organic acids, alcohols, enzymes and other bioactive molecules. Bioactive molecules are those molecules which are functional in living systems or can interact with their components. A number of them are obtained from microbes.
Organic Acids: A number of organic acids are being manufactured with the help of microbes. The important ones are as follows: 1. Acetic Acid:
It is prepared from fermented alcohols with the help of acetic acid bacteria, Acetobacter aceti. Alcoholic fermentation is anaerobic process, but the conversion of alcohol to acetic acid is aerobic one. As soon as 10-13% acetic acid is formed, the liquid is filtered. It is used after ripening as vinegar. The type and quality of vinegar depends upon substrate used for alcoholic fermentation and ripening. For other purposes, acetic acid is purified. The organic acid is employed in pharmaceuticals, colouring agents, insecticides, plastics, etc. 2. Citric Acid:
It is obtained through the fermentation carried out by Aspergillus niger and Mucor species on sugary syrups. Yeast Candida lipolytica can also be employed, provided its nutrient medium is made deficient of iron and manganese. Citric acid is employed in dyeing, engraving, medicines, inks, flavouring and preservation of food and candies. 3. Lactic Acid:
It was the first organic acid to be produced from the microbial fermentation in starchy and sugary medium.
Lactic acid fermentation is carried out by both bacteria (e.g., Streptococcus lactis, Lactobacillus species) and fungi (e.g., Rhizopus). The acid derived from fungal sources is costlier but is of high purity. Any starchy or sugary medium is used. Lactic acid is used in confectionery, fruit juices, essences, pickles, curing of meat, lemonades, canned vegetables and fish products. It is also employed as mordant in tanning, printing of wool in the preparation of plastics and pharmaceuticals. 4. Gluconic Acid:
The acid is prepared by the activity of Aspergillus niger and Penicillium species. Calcium gluconate is used widely as a source of calcium for infant, cows and lactating mothers. It is also used in preparation of pharmaceuticals. 5. Butyric Acid:
The acid is produced during fermentation activity of bacterium Clostridium acetobutylicum. Rincidity of butter is also due to its formation. 6. Alcohols:
Ethanol, methanol, propanol and butanol are alcohols that can be produced commercially by fermentation activity of
some fungi (e.g., Yeast, Mucor, Rhizopus) and bacteria (e.g., Clostridium acetobutylicum, C. saccharotobutylicum). The alcohols are important industrial solvents.
Enzymes: Enzymes are proteinaceous substances of biological origin which are capable of catalysing biochemical reactions without themselves undergoing any change. The word enzyme was coined by William Kuhne (1867) after the fact the yeast provided the most well studied bio-catalytically controlled reactions of alcoholic fermentation (Gk. en- in, zyme- yeast). Buchner (1901) found yeast extract to have enzymatic activity. The number of enzymes now runs into several thousands. All of them are macromolecules (large sized molecules) with a specific three-dimensional shape. Enzymes are substrate specific and carry out a specific catalytic action. They work best at room temperature and near-neutral pH with the exception of several digestive enzymes. Use of enzymes in biotechnology had a number of problems which have been largely overcome by the technique of immobilisation of enzymes inside artificial cells or gels.
About 300 enzymes are being used in industry and medicines. Most of them are obtained from microbes. 1. Proteases:
They are enzymes that degrade proteins and polypeptides. Proteases are obtained from Mortierella renispora, Aspergillus and Bacillus species. The enzymes are used in: (i) Clearing (Chill proofing) beer and whisky, (ii) Cleaning of hides, (iii) Softening of bread and meat, (iv) Degumming of silk, (v) Manufacture of liquid glue, (iv) Manufacture of detergents capable of removing proteinaceous stains. 2. Amylases:
They degrade starches. Amylases are obtained from Aspergillus, Rhizopus and Bacillus species. The enzymes are employed for: (i) Softening and sweetening of bread,
(ii) Production of alcoholic beverages (e.g., beer, whisky) from starchy materials, (iii) Clearing of turbidity in juices caused by starch, (iv) Separation and desizing of textile fibres. Amylase, glucoamylases and glucoisomerases are employed in conversion of com starch into fructose rich com syrup. Incidentally fructose is the sweetest of the sugars. Therefore, com syrup is sweeter than sucrose solution. It is used in sweetening and flavouring soft drinks, biscuits, cakes, etc. 3. Rennet:
It is an extract from the stomach of calf that contains enzyme rennin. Rennet or chymosin is now being obtained from Mucor and Endothio species. Withania and fig (ficin) also yield similar product. 4. Lactases:
They are obtained from Saccharomyces fragilis and Torula cremoris. The enzymes convert lactose (milk sugar) into lactic acid. Lactic, acid can coagulate milk protein, casein.
Lactases prevent crystals formation (sandiness) in dairy preparations like ice-cream and processed cheese. 5. Streptokinase (Tissue Plasminogen Activator or TPA):
It is an enzyme obtained from the cultures of some haemolytic bacterium Streptococcus and modified genetically to function as clot buster. It has fibrinolytic effect. Therefore, it helps in clearing blood clots inside the blood vessels through dissolution of intravascular fibrin. 6. Pectinases:
They are obtained commercially from Byssochlamys fulvo. Along-with proteases, they are used in clearing of fruit juices. Other uses are in retting of fibres and preparation of green coffee. 7. Lipases:
They are lipid dissolving enzymes that are obtained from Candida lipolytica and Geotrichum candidum. Lipases are added in detergents for removing oily stains from laundry. They are also used in flavouring cheese. Cyclosporin A:
It is an eleven membered cyclic oligopeptide obtained through fermentative activity of fungus Trichoderma polysporum. It has antifungal, anti-inflammatory and immunosuppressive properties. It inhibits activation of Tcells and therefore, prevents rejection reactions in organ transplantation. Statins:
They are products of fermentation activity of yeast Monasciis purpureus which resemble mevalovate and are competitive inhibitors of p-hydroxy-p-methylglutaryl or HMG CoA reductase. This inhibits cholesterol synthesis. Statins are, therefore, used in lowering blood cholesterol, e.g., lovastatin, pravastatin, simvastatin.
Introduction
Figure1: general scheme of sewage treatment which shows the flow from primary treatment to tertiary treatment, and solid sludge digestion is also shown.
Sewage treatment is a process in which the pollutants are removed. The ultimate goal of sewage treatment is to produce an effluent that will not impact the environment [1] . In the
absence of sewage treatment, the results can be devastating as sewage can disrupt the environment.
The general processes of sewage treatment are primary, secondary and tertiary treatment. Primary treatment involves physical separation of sewage into solids and liquid by using a settling basin. The liquid sewage is then transferred to secondary treatment which focuses on removing the dissolved biological compound by the use of micro-organisms. The micro-organisms usually use aerobic metabolism to degrade the biological matter in the liquid sludge. Then tertiary treatment is required to disinfect the sewage so that it can be released into the environment. The solid sewage separated from primary treatment is transferred t o a tank for sludge digestion which involves anaerobic degradation using micro-organisms [2].
physical environment
Figure 2: sewage composition in a urbanized city [17
The environment of the sewage treatment plant has to be controlled precisely because bacteria are sensitive to the oxygen level, pH level, temperature, and level of nutrient. I n order for efficient degradation of biological matter to occur, these factors are controlled manually.
Sewage composition Sewage is composed of organic matter such as carbohydrates, fats, oil, grease and proteins mainly from domestic waste. It also contains dissolved inorganic matter such as nitrogen species and phosphorous species mainly from agricultural use [3]. It is essential to remove the nutrients before they are released to the environment
because it interferes natural habitats by altering the chemical composition such as pH or oxygen level both directly and indirectly.
Oxygen level Oxygen level is an important factor to secondary and tertiary treatment processes. Secondary treatment, oxygen is required as a terminal electron acceptor in organic matter degradation. For example, nitrification by Nitrosomonas and Nitrobacter species requires dissolved oxygen to occur [4]. Oxygen in secondary treatment is provided manually by pumping oxygen into the sewage continuously which occurs in an aeration tank [5]. In tertiary treatment, the removal of excess organic matter is enhanced by settling the sewage in a lagoon. This process is also aerobic, but it depends on the diffusion of oxygen because most organic matter has been degraded by secondary treatment [5].
pH Acidity plays a crucial role in the breakdown of organic matter because pH affects the solubility of compounds which indirectly affect the accessibility by bacteria [8]. Also, bacteria responsible for organic matter degradation are sensitive to the pH of the environment. Extremely high or low pH levels are able to kill bacteria, deposition of organic matter occurs due to lack of degradation [6]. Hence, the pH of sewage treatment is controlled to be around 7. A nitrifier in secondary treatment, Nitrosomnas requires a pH between 6~9 in order to be viable [7].
Temperature The effect of temperature is influential for secondary treatment, but it is not important in primary treatment. Bacterial growth is sensitive to temperature because high temperature can increase the fluidity of the phospholipid bilayer which leads to cell lysis. However, bacteria are known to have higher enzymatic activity at higher temperature because of increased thermal energy. For example, when thermophilicsludge treatment is compared to mesophilic treatment, the sludge biodegradability is higher with thermophilic degradation [9]. Hence the temperature has to be controlled precisely to maximize the efficiency of degradation but also allow the cell to remain viable.
Nutrients availability There are a lot of nutrients available in the sewage because of human waste and agricultural runoff [3]. Bacteria can harvest the electron from organic matter and transfer it to a terminal electron acceptor which results in the break down of organic matter and energy conservation [10].
Microbial processes There are several microbial processes, and the microbial processes can be catergorized into aerobic and anaerobic.
Aerobic After primary treatment, liquid and solid phases are physically separated. The liquid phase is treated with aeration to allow aerobic degradation of the nutrients. The two important microbial processes at this stage are nitrification and phosphorous removal. Nitrification occurs in two discrete steps. First of all, ammonium is oxidized to nitrite by Nitrosomonas.spp, and nitrite is further oxidized to nitrate by Nitrobacter.spp [4]. Phosphorous removal can occur biologically by the process of “enhanced biological phosphorous removal.” The process is demonstrated by the
cell taking up phosphorous within their cell, and the biomass is filtered [11].
Anaerobic In the liquid component of sewage, denitrifying bacteria reduce nitrate into dinitrogen gas which liberates nitrate from the sewage [13]. The solid component of the sewage separated in primary treatment is fermented by bacteria anaerobically [12].
Key microorganisms Microorganiasms can also be categorized by its metabolism.
Microorganisms with aerobic microbial process Members of the Nitrosomonas genus is a gram negative bacterium responsible for the first stage of nitrification in sewage. They oxidize ammonium into nitrite. This bacterium prefers a pH around 6-9 and nitrify optimally at 20-30°C [4]. Members of the Nitrobacter genus is a gram negative bacterium responsible for the second stage of nitrification in the sewage. It oxidizes nitrite to nitrate using oxygen as a terminal electron acceptor. The bacteria has an optimum pH of 6~8, and an optimum temperature of 0~40°C [4].
Microorganism with anaerobic microbial process Members of Pseudomonas genus is a gram negative denitrifying bacteria that use the chemical energy in organic matter to reduce nitrate into dinitrogen gas [14]. Also, members of the bacteroidetes phylum are the gram negative bacteria responsible for the anaerobic fermentation of the solid sludge [12].
Current Research
Figure 3: A general scheme of the function of microbial fuel cell
A research has shown the correlation between nutrient removal efficiency, light wavelength and light intensity. Xu et al. discovered that red and high intensity light maximizes the nutrient removal efficiency [15]. Also, the use of pre-treated sludge is found to generate electricity in a microbial fuel cell [16]. This can potentially lead to production of renewable energy.
Sewage treatment ewage treatment is the process of removing contaminants from wastewater , primarily from household sewage. It includes physical, chemical, and biological processes to remove these contaminants and produce environmentally safer treated wastewater (or treated effluent). A by-product of sewage treatment is usually a semi-solid waste or slurry, called sewage sludge, that has to undergo further treatment before being suitable for disposal or land application.
Sewage treatment may also be referred to as wastewater treatment, although the latter is a broader term which can also be applied to purely industrial wastewater. For most cities, the sewer system will also carry a proportion of industrial effluent to the sewage treatment plant which has usually received pretreatment at the factories themselves to reduce the pollutant load. If the
sewer system is a combined sewer then it will also carry urban runoff (stormwater) to the sewage treatment plant. Sewage water can travel towards treatment plants via piping and in a flow aided by gravity and pumps. The first part of filtration of sewage typically includes a bar screen to filter solids and large objects which are then collected in dumpsters and disposed of in landfills. Fat and grease is also removed before the primary treatment of sewage.
Origins of sewage[edit] Main article: Sewage
Sewage is generated by residential, institutional, c ommercial and industrial establishments. It includes household waste liquid from toilets, baths, showers, kitchens, and sinksdraining into sewers. In many areas, sewage also includes liquid waste from industry and commerce. The separation and draining of household waste into greywater and blackwater is becoming more common in the developed world, with treated greywater being permitted to be used for watering plants or recycled for flushing toilets.
Sewage mixing with rainwater [edit] Sewage may include stormwater runoff or urban runoff . Sewerage systems capable of handling storm water are known as combined sewer systems. This design was common when urban sewerage systems were first developed, in the late 19th and early 20th centuries.[2]:119 Combined sewers require much larger and more expensive treatment facilities than sanitary sewers. Heavy volumes of storm runoff may overwhelm the sewage treatment system, causing a spill or overflow. Sanitary sewers are typically much smaller than combined sewers, and they are not designed to transport stormwater. Backups of raw sewage can occur if excessive infiltration/inflow (dilution by stormwater and/or groundwater) is allowed into a sanitary sewer system. Communities that have urbanized in the mid-20th century or later generally have built separate systems for sewage (sanitary sewers) and stormwater, because precipitation causes widely varying flows, reducing sewage treatment plant efficiency.[3] As rainfall travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oiland grease. Some jurisdictions require stormwater to receive some level of treatment before being discharged directly into waterways. Examples of treatment processes used for stormwater include retention basins, wetlands, buried vaults with various kinds of media filters, and vortex separators (to remove coarse solids).[4]
Industrial effluent [edit] Main article: Industrial wastewater treatment
In highly regulated developed countries, industrial effluent usually receives at least pretreatment if not full treatment at the factories themselves to reduce the pollutant load, before discharge to the sewer. This process is called industrial wastewater treatment or pretreatment. The same does not apply to many developing countries where industrial effluent is more likely to enter the sewer if it exists, or even the receiving water body, without pretreatment. Industrial wastewater may contain pollutants which cannot be removed by conventional sewage treatment. Also, variable flow of industrial waste associated with production cycles may upset the population dynamics of biological treatment units, such as the activated sludge process.
Process steps[edit]
Overview [edit] Sewage collection and treatment is typically subject to local, state and federal regulations and standards. Treating wastewater has the aim to produce an effluent that will do as little harm as possible when discharged to the surrounding environment, thereby preventing pollution compared to releasing untreated wastewater into the environment.[5] Sewage treatment generally involves three stages, called primary, secondary and tertiary treatment.
Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment. Some sewage treatment plants that are connected to a combined sewer system have a bypass arrangement after the primary treatment unit. This means that during very heavy rainfall events, the secondary and tertiary treatment systems can be bypassed to protect them from hydraulic overloading, and the mixture of sewage and stormwater only receives primary treatment. Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment may require a separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment. Tertiary treatment is sometimes defined as anything more than primary and secondary treatment in order to allow ejection into a highly sensitive or fragile ecosystem (estuaries, low-flow rivers, coral reefs,...). Treated water is sometimes disinfected chemically or physically (for example, by lagoons and microfiltration) prior to discharge into a stream, river , bay, lagoon or wetland, or it can be used for
the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.
Simplified process flow diagram for a typical large-scale treatment plant
Process flow diagram for a typical treatment plant via subsurface flow constructed wetlands (SFCW)
Pretreatment [edit] Pretreatment removes all materials that can be easily collected from the raw sewage before they damage or clog the pumps and sewage lines of primary treatment clarifiers. Objects commonly removed during pretreatment i nclude trash, tree limbs, leaves, branches, and other large objects. The influent in sewage water passes through a bar screen to remove all large objects like cans, rags, sticks, plastic packets etc. carried in the sewage stream.[6] This is most commonly done with an automated mechanically raked bar screen in modern plants serving large populations, while i n smaller or less modern plants, a manually cleaned screen may be used. The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/or flow rate. The solids are collected and later disposed in a landfill, or incinerated. Bar screens or mesh screens of varying sizes may be used to optimize solids removal. If gross solids are not removed, they become entrained in pipes and moving parts of the treatment plant, and can cause substantial damage and inefficiency in the process.[7]:9 Grit removal[edit]
Pretreatment may include a sand or grit channel or chamber, where the velocity of the incoming sewage is adjusted to allow the settlement of sand, grit, stones, and broken glass. These particles are removed because they may damage pumps and other equipment. For small sanitary sewer systems, the grit chambers may not be necessary, but grit removal is desirable at larger plants.[7] Grit chambers come in 3 types: horizontal grit chambers, aerated grit chambers and vortex grit chambers. The process is called sedimentation. Flow equalization[edit]
Clarifiers and mechanized secondary treatment are more efficient under uniform flow conditions. Equalization basins may be used for temporary storage of diurnal or wet-weather flow peaks. Basins provide a place to temporarily hold incoming sewage during plant maintenance and a means of diluting and distributing batch discharges of toxic or high-strength waste which might otherwise inhibit biological secondary treatment (including portable toilet waste, vehicle holding tanks, and septic tank pumpers). Flow equalization basins require variable discharge control, typically include provisions for bypass and cleaning, and may also include aerators. Cleaning may be easier if the basin is downstream of screening and grit removal.[8] Fat and grease removal[edit]
In some larger plants, fat and grease are removed by passing the sewage through a small tank where skimmers collect the fat floating on the surface. Air blowers in the base of the tank may also be used to help recover the fat as a
froth. Many plants, however, use primary clarifiers with mechanical surface skimmers for fat and grease removal.
Primary Treatment [edit]
Primary treatment tanks in Oregon, USA.
In the primary sedimentation stage, sewage flows through large tanks, commonly called "pre-settling basins", "primary sedimentation tanks" or "primary clarifiers".[9] The tanks are used to settle sludge while grease and oils rise to the surface and are skimmed off. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities.[7]:9 –11 Grease and oil from the floating material can sometimes be recovered for saponification (soap making).
Secondary treatment [edit] Main article: Secondary treatment
Secondary treatment is designed to substantially degrade the biological content of the sewage which are derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. To be effective, the biota require both oxygen and food to live. The bacteria and protozoaconsume biodegradable soluble organic contaminants (e.g. sugars, fats, organic shortchain carbon molecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are classified as fixed-film or suspended-growth systems.
Fixed-film or attached growth systems include trickling filters, constructed wetlands, bio-towers, and rotating biological contactors, where the biomass grows on media and the sewage passes over its surface.[7]:11 – 13 The fixed-film principle has further developed into Moving
Bed Biofilm Reactors (MBBR)[10] and Integrated Fixed-Film Activated Sludge (IFAS) processes.[11] An MBBR system typically requires a smaller footprint than suspended-growth systems.[12] Suspended-growth systems include activated sludge, where the biomass is mixed with the sewage and can be operated in a smaller space than trickling filters that treat the same amount of water. However, fixed-film systems are more able to cope with drastic changes in the amount of biological material and can provide higher removal rates for organic material and suspended solids than suspended growth syste ms.[7]:11 –13
Secondary clarifier at a rural treatment plant.
Some secondary treatment methods include a secondary clarifier to settle out and separate biological floc or filter material grown in the secondary treatment bioreactor.
Tertiary treatment [edit] The purpose of tertiary treatment is to provide a final treatment stage to further improve the effluent quality before it is discharged to the receiving environment (sea, river, lake, wet lands, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practised, it is always the final process. It is also called "effluent polishing." Filtration[edit]
Sand filtration removes much of the residual suspended matter .[7]:22 –23 Filtration over activated carbon, also called carbon adsorption,removes residual toxins.[7]:19 Lagoons or ponds[edit]
A sewage treatment plant and lagoon in Everett, Washington, United States.
Lagoons or ponds provide settlement and further biological improvement through storage in large man-made ponds or lagoons. These lagoons are highly aerobic and colonization by native macrophytes, especially reeds, is often encouraged. Small filter-feeding invertebratessuch as Daphnia and species of Rotifera greatly assist in treatment by removing fine particulates. Biological nutrient removal[edit]
Biological nutrient removal (BNR) is regarded by some as a type of secondary treatment process,[1] and by others as a tertiary (or "advanced") treatment process. Wastewater may contain high levels of the nutrients nitrogen and phosphorus. Excessive release to the environment can lead to a buildup of nutrients, called eutrophication, which can in turn encourage the overgrowth of weeds, algae, and cyanobacteria (blue-green algae). This may cause an algal bloom, a rapid growth in the population of algae. The algae numbers are unsustainable and eventually most of them die. The decomposition of the algae by bacteria uses up so much of the oxygen in the water that most or all of the animals die, which creates more organic matter for the bacteria to decompose. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies. Different treatment processes are required to remove nitrogen and phosphorus. Nitrogen removal[edit] Nitrogen is removed through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water. Nitrification itself is a two-step aerobic process, each step facilitated by a different type of bacteria. The oxidation of ammonia (NH3) to nitrite (NO2−) is most often facilitated by Nitrosomonas spp. ("nitroso" referring to the formation of a nitroso functional group). Nitrite oxidation to nitrate (NO3−), though traditionally believed to be facilitated by Nitrobacter spp. (nitro referring the formation of a nitro functional group), is now known to be facilitated in the environment almost exclusively by Nitrospira spp. Denitrification requires anoxic conditions to encourage the appropriate biological communities to form. It is facilitated by a wide diversity of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogen, but the activated sludge process (if designed well) can do the job the most easily.[7]:17 –18 Since denitrification is the reduction of nitrate to dinitrogen (molecular nitrogen) gas, an electron donor is needed. This can be, depending on the waste water, organic matter (from feces), sulfide, or an added donor like methanol. The sludge in the anoxic tanks (denitrification tanks) must be mixed well (mixture of recirculated mixed liquor, ret urn activated sludge [RAS], and raw influent) e.g. by using submersible mixers in order to achieve the desired denitrification.
Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiary treatment. Over time, different treatment configurations have evolved as denitrification has become more sophisticated. An initial scheme, the Ludzack-Ettinger Process, placed an anoxic treatment zone before the aeration tank and clarifier, using the return activated sludge (RAS) from the clarifier as a nitrate source. Influent wastewater (either raw or as effluent from primary clarification) serves as the electron source for the facultative bacteria to metabolize carbon, using the inorganic nitrate as a source of oxygen instead of dissolved molecular oxygen. This denitrification scheme was naturally limited to the amount of soluble nitrate present in the RAS. Nitrate reduction was limited because RAS rate is limited by the performance of the clarifier. The "Modified Ludzak-Ettinger Process" (MLE) is an improvement on the original concept, for it recycles mixed liquor from the discharge end of the aeration tank to the head of the anoxic tank to provide a consistent source of soluble nitrate for the facultative bacteria. In this instance, raw wastewater continues to provide the electron source, and sub-surface mixing maintains the bacteria in contact with both electron source and soluble nitrate in t he absence of dissolved oxygen. Many sewage treatment plants use centrifugal pumps to transfer the nitrified mixed liquor from the aeration zone to the anoxic zone for denitrification. These pumps are often referred to as Internal Mixed Liquor Recycle (IMLR) pumps. IMLR may be 200% to 400% the flow rate of influent wastewater (Q.) This is in addition to Return Activated Sludge (RAS) from secondary clarifiers, which may be 100% of Q. (Therefore, the hydraulic capacity of the tanks in such a system should handle at least 400% of annual average design flow (AADF.) At times, the raw or primary effluent wastewater must be carbonsupplemented by the addition of methanol, acetate, or simple food waste (molasses, whey, plant starch) to improve the treatment efficiency. These carbon additions should be accounted for in the design of a treatment facility's organic loading.[13] Further modifications to the MLE were to come: Bardenpho and Biodenipho processes include additional anoxic and oxidative processes to further polish the conversion of nitrate ion to molecular nitrogen gas. Use of an anaerobic tank following the initial anoxic process allows for luxury uptake of phosphorus by bacteria, thereby biologically reducing orthophosphate ion in the treated wastewater. Even newer improvements, such as Anammox Process, interrupt the formation of nitrate at the nitrite stage of nitrification, shunting nitrite-rich mixed liquor activated sludge to treatment where nitrite is then converted to molecular nitrogen gas, saving energy, alkalinity, and s econdary carbon sourcing. Anammox™ (ANaerobic AMMonia OXidation) works by artificially extending detention time and preserving denitrifiying bacteria through the use of substrate added to the mixed liquor and continuously recycled from it prior to secondary clarification. Many other proprietary schemes are being deployed, including DEMON™, Sharon- ANAMMOX™, ANITA-Mox™, and
DeAmmon™.[14] The bacteria Brocadia anammoxidans can remove ammonium from waste water [15] through anaerobic oxidation of ammonium to hydrazine, a form of rocket fuel.[16][17]
Phosphorus removal[edit] Every adult human excretes between 200 and 1000 grams of phosphorus annually. Studies of United States sewage in the late 1960s estimated mean per capita contributions of 500 grams in urine and feces, 1000 grams in synthetic detergents, and lesser variable amounts used as corrosion and scale control chemicals in water supplies.[18] Source control via alternative detergent formulations has subsequently reduced the largest c ontribution, but the content of urine and feces will remain unchanged. Phosphorus removal is important as it is a limiting nutrient for algae growth in many fresh water systems. (For a description of the negative effects of algae, see Nutrient removal). It is also particularly important for water reuse systems where high phosphorus concentrations may lead to fouling of downstream equipment such as reverse osmosis. Phosphorus can be removed biologically in a process c alled enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate-accumulating organisms (PAOs), are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20 percent of their mass). When the biomass enriched in these bacteria is separated from the treated water, these biosolids have a high fertilizer value. Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron (e.g. ferric chloride), aluminum (e.g. alum), or lime.[7]:18 This may lead to excessive sludge production as hydroxides precipitates and the added chemicals can be expensive. Chemical phosphorus removal requires significantly smaller equipment footprint than biological removal, is easier to operate and is often more reliable than biological phosphorus removal.[citation needed ] Another method for phosphorus removal is to use granular laterite. Once removed, phosphorus, in the form of a phosphate-rich sewage sludge, may be dumped in a landfill or used as fertilizer. In the latter case, the treated sewage sludge is also sometimes referred to as biosolids. Disinfection[edit] Further information: Advanced oxidation process
The purpose of disinfection in the treatment of waste water is to substantially reduce the number of microorganisms in the water to be discharged back into the environment for the later use of drinking, bathing, irrigation, etc. The effectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Cloudy water will be treated less successfully, since solid matter can shield organisms, especially from ultraviolet light or if contact times are low. Generally, short contact times, low doses and high flows all militate against
effective disinfection. Common methods of disinfection include ozone, chlorine, ultraviolet light, or sodium hypochlorite.[7]:16 Chloramine, which is used for drinking water, is not used in the treatment of waste water because of its persistence. After multiple steps of disinfection, the treated water is ready to be released back into the water cycle by means of the nearest body of water or agriculture. Afterwards, the water can be transferred to reserves for everyday human uses. Chlorination remains the most common form of waste water disinfection in North America due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment. Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect microorganisms from the UV light). In the United Kingdom, UV light is becoming the most common means of disinfection because of the concerns about the impacts of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics in the receiving water. Some sewage treatment systems in Canada and the US also use UV light for their effluent water disinfection.[19][20] Ozone (O3) is generated by passing oxygen (O2) through a high voltage potential resulting in a third oxygen atom becoming attached and forming O3. Ozone is very unstable and reactive and oxidizes most organic material it comes in contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer t han chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event of an accidental release), ozone is generated on-site as needed. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for special operators.
Fourth treatment stage [edit] Further information: Environmental impact of pharmaceuticals and personal care products
Micropollutants such as pharmaceuticals, ingredients of household chemicals, chemicals used in small businesses or industries, environmental persistent pharmaceutical pollutant(EPPP) or pesticides may not be eliminated in the conventional treatment process (primary, secondary and tertiary treatment) and therefore lead to water pollution.[21] Although concentrations of those substances and their decompostion products are quite low, there is still a chance to harm aquatic organisms. For pharmaceuticals, the following substances have been identified as "toxicologically relevant": s ubstances with endocrine disrupting effects, genotoxic substances and substances that enhance the development of bacterial resistances.[22] They mainly belong to the group of environmental persistent pharmaceutical pollutants. Techniques for elimination of micropollutants via a fourth treatment stage during sewage treatment are being tested in Germany, Switzerland [citation needed ] and the Netherlands.[23] However, since those techniques are still costly, they are not yet applied on a regular basis. Such process steps mainly consist of activated carbon filters that adsorb the micropollutants. Ozone can also be applied as an oxidative method.[24] Also the use of enzymes such as the enzyme laccase is under investigation.[25] A new concept which could provide an energyefficient treatment of micropollutants could be the use of laccase secreting fungi cultivated at a wastewater treatment plant to degrade micropollutants and at the same time to provide enzymes at a cathode of a microbial biofuel cells.[26] Microbial biofuel cells are investigated for their property t o treat organic matter in wastewater .[27] To reduce pharmaceuticals in water bodies, also "source control" measures are under investigation, such as innovations in drug development or more responsible handling of drugs.[22][28]
Odor control[edit] Odors emitted by sewage treatment are typically an indication of an anaerobic or "septic" condition.[29] Early stages of processing will tend to produce foulsmelling gases, with hydrogen sulfide being most common in generating complaints. Large process plants in urban areas will often treat the odors with carbon reactors, a contact media with bio-slimes, small doses of chlorine, or circulating fluids to biologically capture and metabolize t he noxious gases.[30] Other methods of odor control exist, including addition of iron salts, hydrogen peroxide, calcium nitrate, etc. to manage hydrogen sulfide levels. High-density solids pumps are suitable for reducing odors by conveying sludge through hermetic closed pipework.
Energy requirements[edit] For conventional sewage treatment plants, around 30 percent of the annual operating costs is usually required for energy.[1]:1703 The energy requirements vary with type of treatment process as well as wastewater load. For example, constructed wetlands have a lower energy requirement
than activated sludge plants, as less energy is required for the aeration step.[31] Sewage treatment plants that produce biogas in their sewage sludge treatment process with anaerobic digestion can produce enough energy to meet most of the energy needs of the sewage treatment plant itself .[1]:1505 In conventional secondary treatment processes, most of the electricity is used for aeration, pumping systems and equipment for the dewatering and drying of sewage sludge. Advanced wastewater treatment plants, e.g. for nutrient removal, require more energy than plants that only achieve primary or secondary treatment.[1]:1704
Sludge treatment and disposal [edit] Main article: Sewage sludge treatment
The sludges accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. The purpose of digestion is to reduce the amount of organic matter and the number of diseasecausing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting. Incineration is also used, albeit to a much lesser degree.[7]:19 – 21
Sludge treatment depends on the amount of solids generated and other sitespecific conditions. Composting is most often applied to small-scale plants with aerobic digestion for mid-sized operations, and anaerobic digestion for the larger-scale operations. The sludge is sometimes passed through a so-called pre-thickener which dewaters the sludge. Types of pre-thickeners include centrifugal sludge thickeners[32] rotary drum sludge thickeners and belt filter presses.[33][34][35] Dewatered sludge may be incinerated or transported offsite for disposal in a landfill or use as an agricultural soil amendment.
Environment aspects [edit]
The outlet of the Karlsruhe sewage treatment plant flows into the Alb.
Many processes in a wastewater treatment plant are designed to mimic the natural treatment processes that occur in the environment, whether that environment is a natural water body or the ground. If not overloaded, bacteria
in the environment will consume organic contaminants, although this will reduce the levels of oxygen in the water and may significantly change the overall ecology of the receiving water. Native bacterial populations feed on the organic contaminants, and the numbers of disease-causing microorganisms are reduced by natural environmental conditions such as predation or exposure to ultraviolet radiation. Consequently, in cases where the receiving environment provides a high level of dilution, a high degree of wastewater treatment may not be required. However, recent evidence has demonstrated that very low levels of specific contaminants in wastewater, including hormones (from animal husbandry and residue from human hormonal contraception methods) and synthetic materials such as phthalates that mimic hormones in their action, can have an unpredictable adverse impact on the natural biota and potentially on humans if the water is re-used for drinking water .[36][37][38] In the US and EU, uncontrolled discharges of wastewater to the environment are not permitted under law, and strict water quality requirements are to be met, as clean drinking water is essential. (For requirements in the US, see Clean Water Act .) A significant threat in the coming decades will be the increasing uncontrolled discharges of wastewater within rapidly developing countries.
Effects on biology [edit] Sewage treatment plants can have multiple effects on nutrient levels in the water that the treated sewage flows into. These nutrients can have large effects on the biological life in the water in contact with the effluent. Stabilization ponds (or sewage treatment ponds) can include any of the following:
Oxidation ponds, which are aerobic bodies of water usually 1 –2 meters in depth that receive effluent from sedimentation tanks or other forms of primary treatment.
Dominated by algae
Polishing ponds are similar to oxidation ponds but receive ef fluent from an oxidation pond or from a plant with an extended mechanical treatment.
Dominated by zooplankton Facultative lagoons, raw sewage lagoons, or sewage lagoons are ponds where sewage is added with no primary treatment other than coarse screening. These ponds provide effective tr eatment when the surface remains aerobic; although anaerobic conditions may develop near the layer of settled sludge on the bottom of the pond.[2]:552 –554 Anaerobic lagoons are heavily loaded ponds.
Dominated by bacteria
Sludge lagoons are aerobic ponds, usually 2 to 5 meters in depth, that receive anaerobically digested primary sludge, or activated secondary sludge under water.
Upper layers are dominated by algae
[39]
Phosphorus limitation is a possible result from sewage treatment and results in flagellate-dominated plankton, particularly in summer and fall.[40] A phytoplankton study found high nutrient concentrations linked to sewage effluents. High nutrient concentration leads to high chlorophyll a concentrations, which is a proxy for primary production in marine environments. High primary production means high phytoplankton populations and most likely high zooplankton populations, because zooplankton feed on phytoplankton. However, effluent released into marine systems also leads to greater population instability.[41] The planktonic trends of high populations close to input of treated sewage is contrasted by the bacterial trend. In a study of Aeromonas spp. in increasing distance from a wastewater source, greater change in seasonal cycles was found the furthest from the effluent. This trend is so strong that the furthest location studied actually had an inversion of the Aeromonas spp. cycle in comparison to that of fecal coliforms. Since there is a main pattern in the cycles that occurred simultaneously at all stations it indicates seasonal factors (temperature, solar radiation, phytoplankton) control of the bacterial population. The effluent dominant species changes from Aeromonas caviae in winter to Aeromonas sobria in the spring and fall while the inflow dominant species is Aeromonas caviae, which is constant throughout the seasons.[
Biogas From Wikipedia, the free encyclopedia
Pipes carrying biogas (foreground), natural gas and condensate
Biogas typically refers to a mixture of different gases produced by the breakdown of organic matter in the absence of oxygen. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste. Biogas is a renewable energy source.
Biogas can be produced by anaerobic digestion with anaerobic organisms, which digest material inside a closed system, or fermentation of biodegradable materials.[1] Biogas is primarily methane (CH 4) and carbon dioxide (CO2) and may have small amounts of hydrogen sulfide (H 2S), moisture and siloxanes. The gases methane, hydrogen, and carbon monoxide (CO) can be combusted or oxidized with oxygen. This energy release allows biogas to be used as a fuel; it can be used for any heating purpose, such as cooking. It can also be used in a gas engine to convert the energy in the gas into electricity and heat.[2] Biogas can be compressed, the same way natural gas is compressed to CNG, and used to power motor vehicles. In the UK, for example, biogas is estimated to have the potential to replace around 17% of vehicle fuel.[3] It qualifies for renewable energy subsidies in some parts of the world. Biogas can be cleaned and upgraded to natural gas standards, when it becomes biomethane. Biogas is considered to be a renewable resource because its production-and-use cycle is continuous, and it generates no net carbon dioxide. Organic material grows, is converted and used and then regrows in a continually repeating cycle. From a carbon perspective, as much carbon dioxide is absorbed from the atmosphere in the growth of the primary bio-resource as is released when the material is ultimately converted to energy.
Production[edit] Main article: Anaerobic digestion
Biogas production in rural Germany
Biogas is produced as landfill gas (LFG), which is produced by the breakdown of Biodegradable waste inside a landfill due to chemical reactions and microbes, or as digested gas, produced inside an anaerobic digester . A biogas plant is the name often given to an anaerobic digester that treats farm wastes or energy crops. It can be produced using anaerobic digesters (air-tight tanks with different configurations). These plants can be fed with energy crops such as maize silage or biodegradable wastes including sewage sludge and food waste. During the process, the microorganisms transform biomass waste into biogas (mainly methane and carbon dioxide) and digestate. The biogas is a renewable energy that can be used for heating, electricity, and many other operations that use a reciprocating internal combustion engine, such as GE Jenbacher or Caterpillar gas engines.[4] Other internal combustion engines such as gas turbines are suitable for the conversion of biogas into both electricity and heat. The digestate is the
remaining inorganic matter that was not transformed into biogas. It can be used as an agricultural fertiliser. There are two key processes: mesophilic and thermophilic digestion which is dependent on temperature. In experimental work at University of Alaska Fairbanks, a 1000-litre digester using psychrophiles harvested from "mud from a frozen lake in Alaska" has produced 200 –300 liters of methane per day, about 20% –30% of the output from digesters in warmer climates.[5]
Dangers[edit] The dangers of biogas are mostly similar to those of natural gas, but with an additional risk from the toxicity of its hydrogen sulfide fraction. Biogas can be explosive when mixed in the ratio of one part biogas to 8-20 parts air. Special safety precautions have to be taken for entering an empty biogas digester for maintenance work. It is important that a biogas system never has negative pressure as this could cause an explosion. Negative gas pressure can occur if too much gas is removed or leaked; Because of this biogas should not be used at pressures below one column inch of water, measured by a pressure gauge. Frequent smell checks must be performed on a biogas system. If biogas is smelled anywhere windows and doors should be opened im mediately. If there is a fire the gas should be shut off at the gate valve of the biogas system.[6]
Landfill gas[edit] Main article: Landfill gas
Landfill gas is produced by wet organic waste decomposing under anaerobic conditions in a biogas.[7][8] The waste is covered and mechanically compressed by the weight of the material that is deposited above. This material prevents oxygen exposure thus allowing anaerobic microbes to thrive. Biogas builds up and is slowly released into the atmosphere if the site has not been engineered to capture the gas. Landfill gas released in an uncontrolled way can be hazardous since it can become explosive when it escapes from the landfill and mixes with oxygen. The lower explosive limit is 5% methane and the upper is 15% methane.[9] The methane in biogas is 20 times more potent a greenhouse gas than carbon dioxide. Therefore, uncontained landfill gas, which escapes into the atmosphere may significantly contribute to the effects of global warming. In addition, volatile organic compounds (VOCs) in landfill gas contribute to the formation of photochemical smog.
Technical [edit] Biochemical oxygen demand (BOD) is a measure of the amount of oxygen required by aerobic micro-organisms to decompose the organic matter in a sample of aterial being used in the biodigester as well as the BOD for the
liquid discharge allows for the calculation of the daily energy output from a biodigester. Another term related to biodigesters is effluent dirtiness, which tells how much organic material there is per unit of biogas source. Typical units for this measure are in mg BOD/litre. As an example, effluent dirtiness can range between 800 –1200 mg BOD/litre in Panama. [citation needed ] From 1 kg of decommissioned kitchen bio-waste, 0.45 m³ of biogas can be obtained. The price for collecting biological waste from households is approximately €70 per ton.[10]
Composition[edit] Typical composition of biogas Compound
Methane
Carbon dioxide
Nitrogen
Hydrogen
Hydrogen sulfide
Oxygen
Formula
CH 4
CO 2
N 2
H 2
H 2S O 2
%
50 –75
25 –50
0 –10
0 –1
0 –3
0 –0.5
Source: www.kolumbus.fi, 2007 [11]
The composition of biogas varies depending upon the s ubstrate composition, as well as the conditions within the anaerobic reactor (temperature, pH, and substrate concentration).[12] Landfill gas typically has methane concentrations around 50%. Advanced waste treatment technologies can produce biogas with 55% –75% methane,[13] which for reactors with free liquids can be increased to 80%-90% methane using in-situ gas purification techniques.[14] As produced, biogas contains water vapor. The fractional volume of water vapor is a function of biogas temperature; correction of measured gas volume for
water vapor content and thermal expansion is easily done via simple mathematics[15] which yields the standardized volume of dry biogas. In some cases, biogas contains siloxanes. They are formed from the anaerobic decomposition of materials commonly found in soaps and detergents. During combustion of biogas containing siloxanes, silicon is released and can combine with free oxygen or other elements in the combustion gas. Deposits are formed containing mostly silica (SiO 2) or silicates (Si x O y ) and can contain calcium, sulfur , zinc, phosphorus. Such white mineral deposits accumulate to a surface thickness of several millimeters and must be removed by chemical or mechanical means. Practical and cost-effective technologies to remove siloxanes and other biogas contaminants are available.[16] For 1000 kg (wet weight) of input to a typical biodigester, total solids may be 30% of the wet weight while volatile suspended solids may be 90% of the total solids. Protein would be 20% of the volatile solids, carbohydrates would be 70% of the volatile solids, and finally fats would be 10% of the volatile solids.
Benefits of manure derived biogas [edit] High levels of methane are produced when manure is stored under anaerobic conditions. During storage and when manure has been applied t o the land, nitrous oxide is also produced as a byproduct of the denitrification process. Nitrous oxide (N2O) is 320 times more aggressive as a greenhouse gas than carbon dioxide[17] and methane 25 times more than carbon dioxide.[18] By converting cow manure into methane biogas via anaerobic digestion, the millions of cattle in the United States would be able to produce 100 billion kilowatt hours of electricity, enough to power millions of homes across the United States. In fact, one cow can produce enough manure in one day to generate 3 kilowatt hours of electricity; only 2.4 kilowatt hours of electricity are needed to power a single 100-watt light bulb for one day.[19] Furthermore, by converting cattle manure into methane biogas instead of letting it decompose, global warming gases could be reduced by 99 million metric tons or 4%.[20]
Applications[edit]
A biogas bus in Linköping, Sweden
Biogas can be used for electricity production on sewage works,[21] in a CHP gas engine, where the waste heat from the engine is conveniently used for heating the digester; cooking; space heating; water heating; and process heating. If compressed, it can replace compressed natural gas for use in vehicles, where it can fuel an internal combustion engine or fuel cells and is a much more effective displacer of carbon dioxide than the normal use in on-site CHP plants.[21]
Biogas upgrading [edit] Raw biogas produced from digestion is roughly 60% methane and 29% CO 2 with trace elements of H 2S; it is not of high enough quality to be used as fuel gas for machinery. The corrosive nature of H 2S alone is enough to destroy the internals of a plant.[22][23] Methane in biogas can be concentrated via a biogas upgrader to the same standards as fossil natural gas, which itself has to go through a cleaning process, and becomes biomethane. If the local gas network allows, the producer of the biogas may use their distribution networks. Gas must be very clean to reach pipeline quality and must be of the correct composition for the distribution network to accept. Carbon dioxide, water , hydrogen sulfide, and particulates must be removed if present.[22] There are four main methods of upgrading: water washing, pressure swing adsorption, selexol adsorption, and amine gas treating.[24] In addition to these, the use of membrane separation technology for biogas upgrading is increasing, and there are already several plants operating in Europe and USA.[22][25] The most prevalent method is water washing where high pressure gas flows into a column where the carbon dioxide and other trace elements are scrubbed by cascading water running counter-flow to the gas. This arrangement could deliver 98% methane with manufacturers guaranteeing maximum 2% methane loss in the system. It takes roughly between 3% and 6% of the total energy output in gas to run a biogas upgrading system.
Biogas gas-grid injection [edit] Gas-grid injection is the injection of biogas into the methane grid (natural gas grid). Injections includes biogas[26] until the breakthrough of micro combined heat and power two-thirds of all the energy produced by biogas power plants was lost (the heat), using the grid to transport the gas to customers, the electricity and the heat can be used for on-site generation[27] resulting in a reduction of losses in the transportation of energy. Typical energy losses in natural gas transmission systems range from 1% to 2%. The current energy losses on a large electrical system range from 5% to 8%.[28]
Biogas in transport [edit]
"Biogaståget Amanda" ("Amanda the Biogas Train") train near Linköping station, Sweden
If concentrated and compressed, it can be used in vehicle transportation. Compressed biogas is becoming widely used in Sweden, Switzerland, and Germany. A biogas-powered train, named Biogaståget Amanda (The Biogas Train Amanda), has been in service in Sweden since 2005.[29][30] Biogas powers automobiles. In 1974, a British documentary film titled Sweet as a Nut detailed the biogas production process from pig manure and showed how it fueled a custom-adapted combustion engine.[31][32] In 2007, an estimated 12,000 vehicles were being fueled with upgraded biogas world wide, mostly in Europe.[33]
Measuring in biogas environments [edit] Biogas is part of the wet gas and condensing gas (or air) category that includes mist or fog in the gas stream. The mist or fog is predominately water vapor that condenses on the sides of pipes or stacks throughout the gas flow. Biogas environments include wastewater digesters, landfills, and animal feeding operations (covered livestock lagoons). Ultrasonic flow meters are one of the few devices capable of measuring in a biogas atmosphere. Most of thermal flow meters are unable to provide reliable data because the moisture causes steady high flow readings and continuous flow spiking, although there are single-point insertion th ermal mass flow meters capable of accurately monitoring biogas flo ws with minimal pressure drop. They can handle moisture variations that occur in the flow stream because of daily and seasonal temperature fluctuations, and account for the moisture in the flow stream to produce a dry gas value.
Indian Subcontinent [edit] Biogas in India[55] has been traditionally based on dairy manure as feed stock and these "gobar" gas plants have been in operation for a long period of time, especially in rural India. In the last 2-3 decades, research organisations with a focus on rural energy security have enhanced the design of the systems resulting in newer efficient low cost designs such as the Deenabandhu model.