1
1 INTRODUCTION In present scenario the cost of conventional energy is increasing day by day and demand for such energy sources is also rising so it has become necessary to utilizing bio-gas as a fuel for domestic and industrial purpose is the most economical reliable and time tested method for conserving energy. Bio gas is also known as swamp gas sewer gas fuel gas, marsh gas, wet gas and in India more commonly as gobar gas. Bio-gas consists of 60-65% methane (CH4), 35-40%. Carbon dioxide (CO2) and traces of hydrogen sulfide (H2S) ammonia (NH3). 1.1 HISTORICAL BACKGROUND OF BIOMETHANATION The first person to observe the phenomenon of Biomethanation was ALESSONDRO VOLTA of Italy was back to 1776 he wrote to a friend to that consumables air was being produced continuously in lakes and ponds in the variety of Como in northern Italy. Volta observed that when he distributed the bottom sediment of the lake, bubbles of gas would rise to surface. He also noticed that more bubbles come of when sediment contains more plant material. In 1806 William Henry showed that Volta’s gas was identical with methane gas was identical with methane gas. Humphrey Davy in the early 1800’s observed that methane was present in farmyard manure pieces. In 1808 Davy conducted the first laboratory experiment to produce methane by anaerobic fermentation was wasted. In the initial periods anaerobic fermentation was carried out mainly as a municipal waste treatment process and energy recovery has not of primary concern. In 1895 biogas from a waste treatment plant in extra. In England was collected and use to light nearly streets. 2
The interest in biogas reserve further fill up during second world war. French scientists to particular interest in advocating gas technology in French colonies in Africa. During this period fuel Starred French and Germans used biogas as a fuel for vehicles and farm tractors. Followed the war several nation such as UK, USA, CANADA, RUSSIA, China, Kenya, Uganda, S.A. and India showed interest in Biomethanation. Broker 1956 postulated that anaerobic digestion is two step of multistep biological process. Kirsch and Sakes 1971 elaborated Barkers postulate and suggested involvement of two stage conversion of organic matter to methane. The concept of two stage anaerobic digestions was modified by meinorney and Bryant (1981) suggested three steps anaerobic digestion is now considered as balanced three stage process in which three group of micro organisms work. However series of energy crisis which rocked the world from 1973 onwards coupled with concern for environment. Protection interest in Biomethanation. 1.2 BIOENERGY India predominantly has on agricultural population with about 70% people living in village the development of village depends to a great extent on the availability of the energy. The potentiality of use of biogas as a suitable energy alternative requires vital importance. The importance and utility of biogas in our country can be extracted from the fact that the cattle population in India is nearly 287 million assuming the avg. wet drug obtained per day to be 10 Kg. and the collection rate of 66% the total availability of cow drug would be 575 million tones per annum. This it self would enable production of 22, 425 million m2 of biogas which can replace oil to an extent of 13,900 million liters apart from which manure which is also obtained and as per rough estimate 200 million tones of organic manure per annum will be obtained.
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1.2.1 Bio energy from Solid Wastes If all solid wastes are collected and used as a fuel it can supply about more than 10% of the total requirements. Urban wastes include household, sewage, and commercial, institutional. Manufacturing plants and demolition wastes. Agricultural wastes include animal manure, crop waste and forest and hogging residues. The only available wastes in this sector are those animal waste generated on large feed lots and diaries and the portion of those crop wastes i.e. biogases and fruit tree pruaings not readily required back to the soil The various sources to obtain biomass for production of bio energy are numerous. However, they can broadly be categorized in following main heads. 1. Agricultural Wastes 2. Community Wastes 3. Animal Wastes 4. Industrial Wastes
1.3 FEW STORIES FROM WASTE TO ENERGY
1. Bio gas from wastewater at starch and Glucose mfg. unit Versa Biotek ltd, samalokot in A.P. produces starch and liquid glucose from about 40,000 metric tones of maize and about 25,000 metric tones of tapioca tuber, per annum. The process also generates about 1600 cum per day liquid waste. A Biomethanation plat based on up flow Anaerobic sludge Blanket secondary treatment has been installed for treating the waste water. The Biomethanation plant has been generating about 8000 cum. Biogas everyday for the last one year. The biogas is not only leading to substantial saving in cost of fuel in the boiler but also results in large saving on cost of chemicals, which were required for wastewater treatment prior to installation of this project the payback period of this project works out to about four years. 4
2. Power from Biogas at Distillery About 12000cum biogas per day being produced from Bio methanation plants installed for treating distillery Wastewater (Spent wash) at K.M. sugar mills (Distillery) Faziabad, U.P. is being utilized for generating power through the steam turbine route for meeting the total electricity requirement of their distillery as well as that of their residential colony. The project has been performing satisfactorily for last four years and has been generating an avg. of about 4 lakh units of electricity every month. The payback period for such a project works out to be about 3 - 4 years.
3. Power from Biogas About 21000 cum. Biogas per day being produced from Biomethanation plants treating distillery wastewater (spent wash) at kanoria chemicals and industries Ltd. Ankelshwar, Gujarat is being utilized from generation of power required for their captive use. The project is based on two internal combustion energies fuelled of own biogas, each of 1.003 cum capacity. The waste heat of the flue gases of the engines, which is at a temp of more the 5000C, is also being utilized for generation of about 1.5 tones per hour steam at about 1300C. The steam is used for meeting process heat requirement. A H2S removal plant based on a bio-chemical terminology has also been installed to avoid the corrosion of biogas engines. The project has been performing satisfactory for the last three years and generating about 10 lakh units of electricity every month. The payback period of this project works out is about 3 years.
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4. Biogas from Slaughterhouse Wastes: Alkabeer exports Ltd. (AKEL) hare an integrated meat processing unit at rudraram village in medak dist. Of A.P. solid and liquid and liquid waste being generated during slaughtering and processing of meat is being treated through Biomethanation plants. While about 2000cubm. Biogas is being generated from liquid wastes thus reducing the COD content by 75-80% and BOD content and 85-90%; about 2500 cum. Biogas is being generated from solid wastes. Adoption of biomethanation technology has resulted in saving of furnace oil as well as chemicals used for treatment of wastewater. The sludge from the anaerobic digester is dried and is being marketed as a nutrient rich soil conditioner.
5. Biogas from Tannery Wastes In India have about 1600 tanneries with total processing capacity of 0.7 million tones of raw hide and skies. Fleshing and sludge are the two wastes emanating from tannery and treatment of tannery wastewater. A project for demonstration of application of Biomethanation technology for treatment of tannery fishing and sludge from tannery effluent treatment plant has been set up at Visharam tanner’s environmental system, Melvisharam, Tamilnadu. The plant has been designed to handle about 3 tones of tannery fleshing and 2 tones of primary tannery fleshing and 2 tones of primary sludge from. The generated biogas is then used for generation of electricity in a dual fuel engine.
1.4 Pollution from Distilleries Distillery wastewater posses a serious threat to water quality in several regions of the country lowering of PH value of the stream, increase in organic load, depletion of oxygen content, destruction of aquatic life and bad smell are some of the major pollution problems due to distillery wastewater. The high BOD causes depletion of dissolved oxygen and proves very harmful to aquatic life. In some 6
cases, particularly in Maharashtra the colour problem in groundwater is so acute that distilleries have to provide separately potable water to surrounding villages. Indian standards for disposal of industrial effluent (4) Sr.No
Characteristics
1. 2. 3. 4.
BOD (mg/lit.) COD (mg/lit.) PH Suspended solids/L. Total dissolved solid (mg/lit.) Oil and grease (mg/lit.) Sulphide (mg/lit.) Chloride (mg/lit.) Sulphate (mg/lit.)
5. 6. 7. 8. 9.
Tolerance limit for effluent discharge In to land In to public On land for surface sewers irrigation IS: 2490 1974 IS: 3306 1974 IS: 3306 1965 30 500 500 250 5.0-9.0 5.5-9.0 5.5-9.0 5.5-9.0 100 600 -
2100
2100
10 2 -
100 600 1000
30 600 1000
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2 TREATMENT OF DISTILLERY EFFLUENT 2.1 COMPOSITION OF SPENT WASH (6) The distillery wastewater known as spent wash is characterized in its color, high temp, low PH, high ash content and contains high percentage of dissolved organic and inorganic matter of which 50% may be present as reducing sugars. It contains about 90-93% water and 7-10% solids, sugar being 2-20% and protein 10-11% in the dry spent wash. The metals present in spent wash are Fe – 348mg/lit. Mn – 12.7 mg/lit, Zn – 4.61 mg/lit. With Cu – 3.65mg/lit., Cr – 0.64 mg/lit. , Cd – 0.48mg/lit., Co – 0.08 mg/lit. With electrical conductivity in the range of 15-23ds m-1 . Indian spent wash contains very high amounts of potassium calcium, chloride, sulphate compared to spent wash in other countries. Organic compounds extracted from spent wash using alkaline reagents are of humic in nature. Similar to those found in the soil excepting that fulvic acid predominates over humic acid. Indicative spent wash quality of typical sugarcane molecules based distillery in India (6) Sr. No.
Parameter
Units
Concentration Range
1.
Color
-
Dark brown
2.
Odor
-
Sugar smell
3.
Temperature
C
80-90
4.
P.H.
5.
Total solid
0
4–5 mg/lit
8
52000 - 86000
6. 7.
Total suspended solid Total volatile solid
mg/lit
3000-5000
mg/lit
40000-60000
8.
BOD
mg/lit
30000-70000
9.
COD
mg/lit
65000-130000
10.
Chlorides (Cl)
mg/lit
1000-1500
11.
Sodium (Na)
mg/lit
40000-60000
12.
Calcium (Ca)
mg/lit
2000-3500
13.
Potassium (K)
mg/lit
8000-11000
2.2 TREATMENT OF DISTILLERY EFFLUENT The characteristics of spent wash do not allow in discharge into a water body, hence it requires treatment physiochemical treatment such as sedimentation with the addition of coagulant and other additives such as alum, lime, ferric chloride, activated charcoal etc. have been found to be unsatisfactory, only the biological treatments are most often found to be effective which are amply demonstrated by adoption of these methods by all the distilleries. Despite installing huge anaerobic lagoons, aeration tanks and solar drying pits, the problems of pollution control in cane molasses distilleries in India have not been solved yet therefore, severe water pollution problems in the nearly rivers and lakes are frequently encountered as the partially treated effluents find access to water bodies. Land disposal of effluent could also be thought as on alternate for reducing pollution, as its application in agricultural fields improves almost all factors involved in soil fertility and provides condition for nitrogen assimilatory into the soil. This is the most important effect leading to increases in yield and quality of crops. These observations bear significance from the point of view of status of 9
distilleries in India and their impact on the Agri-environment, many distilleries in India are following their effluent for application on land as direct irrigation water, spent wash cake and spent-wash press mud compost. 2.3 ALTERNATIVE TREATMENT METHODS FOR WASTE WATER (3)
Distillery waste treatment can be Classified into four groups
1. Biogas generation of about 25 lit of biogas can be produced per lit of Spent wash. 2. Potash recovery – About 44 tones of K2SO4 per day can be produced from 2,30,000 Gallons of spent wash. 3. Production of Yeast as an animal feed from spent wash. 4. Treatment for removal of organics for water pollution control.
The alternatives for treatment of distillery spent wash may be identified as solar drying, Ammonification and nitrification process, incineration, potash recovery, anaerobic lagoon, anaerobic filter, up flow anaerobic sludge blanket reactor, up flow blanket filter, anaerobic fluidized bed, acid-methane segregation process.
2.3.1 Solar Drying Spent wash is dried in open shallow pits and needs large surface area sediment can be recovered and used for fertilizer, these is no scope for energy recovery and causes water pollution problem.
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2.3.2 Ammonification and Nitrification Process This process is dependent on Ammonification and nitrifying bacteria which are sensitive, slow growing, Temperature and PH dependent. 2.3.3 Incineration This is direct wet catalytic combustion process and causes air pollution problem.
2.3.4 Potash Recovery It uses multiple effect evaporators with incinerator. But it is cost intensive and consume lot of energy.
2.3.5 Anaerobic Lagoon The treatment from the anaerobic lagoon is not very effective and requires large area of land as the residence time is very high the reduction in BOD, COD level of waste water is not more than 80% even with residence time of more than 3 months.
2.3.6 Anaerobic Filter Anaerobic filter is the first innovative reactor developed in 1969 Lat Young and Mc carty. It contains insert packing material to support bio-film development. The process found potential application for treatment of dilute soluble waste water. Waste water in an anaerobic filter was carried out at 4-8 days HRT and 516Kg. COD per m3 organic loading the COD removal of more than 80% was found at an avg. loading 12.5Kg. COD per m3 at 5 days HRT. Few full scale anaerobic filter plants installed in sugar industries in India for the treatment at combined sugar mill waste water.
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2.3.7 Up flow Anaerobic Sludge Blanket Reactor (USAB) (5) Up flow anaerobic sludge blanket reader was developed in the Netherlands with unique features of biomass Immobilization, without supporting media. The reactor was extensively evaluated in laboratory, pilot and full scale plant mainly with sugar industry waste water in these studies both single and two phase mode of digestion. Bio methanation of cane-sugar waste water has been examined in laboratory scale USAB reactor. The process was how to satisfactorily handle organic loading up to 13 Kg. corresponding to 4 HRT. The COD removal of above 90% was achieved. The biogas production as high as 0.5m3/kg. COD applied was achieved with methane content of 70-75% . The reactor was maintained at 30oC temp. 2.3.8 Up-flow Blanket Filter(5) The up-flow blanket filter which is a hybrid reactor mode by combining on up-flow anaerobic sludge blanket and anaerobic filter has recently been evolved. Plastic ring floating in the top third of the reactor 2/3rd volume occupied and USAB system. The reactor was operated as 27oC for treatment of sugar waste water at loading rates varying from 5-51kg COD/m3d were obtained the filter at the top of reactor was found very efficient in retaining biomass. 2.3.9 Acid – Methane Segregation Process This is modified version of anaerobic activated sludge process in which acid and methane formed are separated. It is more efficient than anaerobic activated sludge process because of low HRT. If is more efficient for energy recovery provided, the symbolic relationship between two organisms (acid and methane forms) is clearly understood.
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2.3.10 Anaerobic fluidized Bed In this treatment waste water is mixed with an approximate amount of recycled effluent and introduced at the bottom of a column with sand particle (0.30.4mm) at a rate sufficient to fluidize the medium. It is more suitable treatment of soluble waste. Sufficient power input may be require maintaining fluidization. 2.4 COMPARISON OF AEROBIC AND ANAEROBIC AND PROCESS (1) Aerobic Process
Anaerobic Process
• Short detention time
• Very long detention time
• Compact Plant
• Very large area required
• Costly
• Usually cheap
• High power requirement
• Low power requirement
• Nearly complete treatment
• Particle treatment
• CO2 and biomass produced
• Usually
• Large
amount
of
sludge
CH4
produced
addition to CO2 • Less sludge production
produced
• Less nutrient required
• More nutrient required
13
in
3 PROCESS SELECTION 3.1 METHODS FOR OBTAINING ENERGY FROM INDSUTRIAL WASTE
1)
Anaerobic digestion / Biomethanation
2)
Landfill Gas Recovery
3)
Incineration
4)
Densification / Pelletization
5)
Other Techniques
3.1.1 ANAEROBIC DIGESTION / BIOMETHANATION (9) In this process, organic fraction of the wastes is segregated and fed to a closed container (Biogas digester) where, in the presence of Methanogenic bacteria and under anaerobic conditions, it undergoes bio-degradation producing methane rich biogas and effluent biogas consists of methane, CO2, small quantities of NH3 and H2S and has a C.V. of above 5000 Kcal/m3. Depending upon waste composition, the biogas production ranges from 50-150 m3/tonne of wastes. The sludge from anaerobic digestion, after stabilization can be used as a soil conditioner or as manure depending upon its composition.
a) Biomethanation Methanogenous is a microbiological phenomenon associated in the breakdown of the complex organic matters to methane, CO2 and water in the absence of oxygen. This microbiological phenomenon how lead to an industrial process called Biomethanation by which a large part of energy from waste organic 14
materials are converted to energy rich biogas. The formation of methane has key role in Methanogenesis because it is related directly to the COD reduction of the waste. The Biomethanation of industrial waste water is more attractive because it provides a compact and economical treatment process requiring no creation producing only minimum quantities of excess sludge and generating a gaseous fuel of significant commercial value. b) Biological Process of Methanogenesis (7) The biological process of Methanogenesis is currently recognized as 3 stage degradation of complex organic materials. The organic compound present in sugar mill waste water is sucrose which is readily amenable to Methanogenesis. The sucrose is first hydrolyzed in to their monomers, glucose and fructose by extra cellular enzymes produced by fermentative bacteria. The monomers molecules are the basic substrate of fermentative organisms for energy assimilation required for cell growth and maintenance. The major route of fermentative or sugar is via pyruvic acid formation in the reaction, hydrogen is also produced from dehydrogenation of pyruvate. A number of bacterial species, called obligate proton reducing on acetogenic bacterial oxidize the higher fatty acids to acetic acid and hydrogen. Methane is formed from acetic acid by Acetoclastic Mathenogenis and from CO2, H2 , by Hydrogenoclastic Methanogenation in Methanogensis 70% of the methane come from Decarboxylation of acetate which 30% derived from hydrogen and Carbondioxide. Methane can also be formed from methanol and formic acid in small quantity which have little practical significance. The formation of methane from acetate and CO2/H2 proceeds according to the following reaction.
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CH3COO + H2O CH4 + HCO3 CO2 + 4H2
CH4 + 2H2O
The Methanogensis under normal conditions, proceeds mainly via the acetate and hydrogen route to methane. But whenever unstable condition arises due to accumulation of Hydrogen, the bacteria will adopt an alternative strategy by the formation of higher fatty acids. The accumulation of these higher Fatty acids in mixed liquor leads to the final Methanogensis more and more inhabited. Also when the PH of mixed liquor drops below the acid forming bacterial may supplant the Methanogenic bacteria and acetate buildup will occur.
c) Features of Process The process developed is based on concepts of phase separation having the methane reactor configuration as hybrid i.e. up-flow blanket filter (UBF) the phase separation was chosen because of the obvious advantages in controlling the operational parameters by providing a selective environment for Acidogens and Methanogens separately. The USAB design of reactor normally employed for anaerobic digestion of other effluents is not suitable for distillery effluents, owing to the poor granulation of biomass and subsequently its wash out from the bed reactor particularly having the matrix of high voltage and above becomes extremely expansion, this is particularly not feasible for very high effluent generations unit like distillery.
d) Advantages of Anaerobic Digestion / Biomethanation 1. Generation of gaseous fuel. 2. Can be done on a small scale. 3. No external power requirement like aerobic treatment. 4. Enclosed system enables all the gas produced to be collected for use. Green house gases emission to the atmosphere is avoided. 16
5. Free from bad order, rodent and fly menace, visible pollution and social resistance. 6. Modular construction of plant and closed treatment needs less land area. 7. Production of biogas and high grade soil conditioner.
e) Disadvantages 1. In case of digesters operated under Mesophilic temperatures, Destruction of pathogenic organisms may be less than that in Aerobic composing. However several digester systems operated. At high Thermophlic temperatures are also available. 2. It is more capital intensive compared to compositions and 3. Land fills. 4. Not suitable for wastes containing less biodegradable matter.
3.1.2 LANDFILL GAS RECOVERY (9) The waste deposited in a landfill gets subjected, over a period of time, to anaerobic conditions and its organic fraction gets slowly volatilized and decomposed, leading to production of landfill gas which contains a high percentage of methane (50%). Typically, production of landfill gas starts within a few months after disposal of wastes and generally last for 10 years of even more depending upon mainly the composition of wastes and availability of moisture. As the gas has a calorific value of around 4500 K cal/m3 , it can be used as source of energy either for direct heating/cooking applications or to generate power through IC engines of turbines.
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Advantages of landfill Gas Recovery 1. Low cost means for waste disposal. 2. The gas can be utilized for power generation or as domestic fuel.
Disadvantages 1. Insufficient gas recovery process yielding only 30-40% of the total amount of gas actually generated. Balance gas escapes to the atmosphere. (Significant source of two major green house gases, carbon dioxide and methane). 2. Utilization of methane may not be feasible for remote sites. 3. Cost of pre-treatment to upgrade the gas may be high. 4. Explosion may occur due to possible buildup of methane concentrations in atmosphere. 3.1.3 INCINERATION (9)
It is the process of direct burning of wastes in the presence of excess air (oxygen) at high temperature (about 8000C) liberating heat energy, inert gases and ash. Net energy yield depends upon the density and composition of waste, percentage of moisture and inset materials which add to the heat loss, ignition temperature, size and shape for the constituents, etc. combustion results in transfer of 65-80% of the heat content of the organic matter into hot air, steam and hot water.
Advantages of Incineration 1. Suitable for high calorific value waste (paper), plastics, hospital wastes etc. 2. Units with continuous feed and high throughput can set up. 3. Thermal energy recovery for direct heating / power generation. 4. Relatively noiseless and odorless. 18
5. Low land area requirement. 6. Can be located within city limits, reducing cost of waste transportation. 7. Hygienic. Disadvantages 1. Least suitable for high moisture contently low C.V. wastes and chlorinated wastes. 2. Excessive moisture and inert content in waste affects net energy recovery. Auxiliary fuel support may be necessary to sustain combustion. 3. Toxic metals may concentrate in ash. 4. In addition to particulates, chlorinated compounds ranging from HCL to Organic compounds such as dioxins, and heavy metals are a cause of concern, which requires elaborate pollution control equipment. 5. High capital and organization and management cost. 3.1.4 DENSIFICATION (9) Densification involves the process of segregating, crushing, and mixing high and low heat value organic waste material and solidifying the same to produce fuel pallets or briquettes, also referred to as Refuse Derived Fuel (RDF). This can be conveniently stored and transported and used as s Supplementary fuel for combustion process and utility boilers. The calorific value of RDF is about 4000 Kcal/kg. And it depends upon the content of combustion organic materials in the waste, additive and binder materialist acid, used in process. Densification is a waste processing method, which densifies the waste or changes its physical form and enriches its organic content through removal of organic materials and moisture.
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Advantages of Densification 1. High calorific value of pellets 3500-4000 Kcal/kg. against that of unprocessed garbage i.e. 800-900 Kcal/kg. 2. Pellets can be conveniently stored and transported and used as supplementary fuel for combustion process and utility boilers.
Disadvantages of Densification 1. The processing unit can’t be operated during the rainy season, as the garbage will be too wet. 2. High moisture content increases the cost of drying. 3. Consumes more energy than biological process. 4. Uncontrolled burning of the pellets derived from msw may lead to harmful emissions.
3.1.5 OTHER TECHNIQUES In addition to the above methods there are some other conversion techniques such as • Pyrolysis • Gasification • Alcohol Fermentation • Slurry carb Process • Plasma Arc Technology Which could be used for energy recovery from waste. From study of various methods given above and with all their advantages and disadvantages it has been concluded that Anaerobic digestion / Biomethanation process is economical and environmental friendly than any other process discussed above. Hence, Anaerobic Digestion process has been discussed in detail, in further chapters. 20
4 PROCESS DESCRIPTION 4.1 ANAEROBIC TREATMENT OF SPENT WASH – SMAT PROCESS (10) Indian company offers “SMAT process” for anaerobic treatment of spent wash. Spent wash which is often termed as ‘distillers’ distress’ happens to be a potential source of renewable energy when treated Anaerobically, this ‘liquid gold’ releases millions of kilo calories in the form of methane rich biogas that can be fed into boiler or biogas engines to generate electricity.
SMAT process: The raw spent wash has very high BOD and COD concentration of 45,000 mg/l and 100,000 mg/l. respectively. This waste is digested Anaerobically in three stages i.e. 1. Hydrolysis 2. Acid formation / Acidognenesis 3. Methane formation / Methanaogensis.
1. Enzymatic Hydrolysis Where the fats, starches and proteins contained in cellulose biomass are broken down into simple compounds.
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2. Acid Formation / Acideoenesis Where the micro-organisms of facultative and anaerobic group collectively called as acid formers, hydrolyze and ferment, are broken to simple compound into acid and volatile solids. As a result complex organic compound are broken down to short chemical simple organic acid. In some cases these acids may be produced in such large quantities that the PH may be lowered to a level where all biological activity is arrested. This initial acid phase of digestion may last about two weeks and during this period a large amount of carbon dioxide is given off.
3. Methane Formation / Methanogenesis Where organic acids are formed above then converted into methane (CH4) and CO2 by the bacteria anaerobes. These bacteria are called methane fomenters. A PH value between 6.5 to 8 is the best for fermentation and normal gas production. In controlled waste digestion the environment must be maintained suitable for the continued growth of both acid forming and methane forming bacteria. These three stages i.e. hydrolysis, Acidogenesis and Methanaogenesis, which are carried out in a single SMAT digester. The SMAT digester is filled with specially designed rigid PVC media for bacterial to get immobilized on its surface and hence very large population of bacteria is available inside the digester. The vertical straight flute media is geometrically structured and so designed that is has optimum contact with raw spent wash which in turn ensures very high biogas generation, round the year.
Biogas generation depends chiefly on two factors 1. Population of bacteria inside the digester. 2. Maximum assured contact between bacteria and food i.e. raw spent wash. 22
Both these factors are taken care of by the structured media as explained above. The reactor content is kept completely mixed re-circulating the treated spent wash 15-20 times which also helps in maintaining the reactor PH at around 7.2 without adding chemicals. SMAT is a down flow reactor with distribution network provided at the top through which the effluent is distributed uniformly over the entire area inside the digester. The treated effluent is collected from bottom so as to avoid any short circulating and is discharged at a suitable head. The biogas is collected at the top gas dome which is fitted with state of the art safety devices. It is then transported through a biogas blower for burning in boiler or biogas engine.
4.2 PROCESS OF THE ANAEROBIC TREATMENT WASTEWATER TRANSPORT Raw effluent i.e. raw spent wash from the distillery is carried to the treatment site through suitably designed channel or a closed pipe depending upon the topography of the site. Raw spent wash is then received in a sump. The sump is constructed in RCC M 20 and lined with suitable protective lining to protect sump from corrosive nature of raw spent wash. The raw spent wash which is at the temp. 900C is passed through the heat exchanger before feeding it to the reactor for bringing down the temperature to 36-400C. Pumps are installed to transfer raw spent wash from sump to reactor through heat exchanger pump discharge is taken to a feed tank located at centre of reactor.
4.3 SMAT REACTOR The SMAT reactor is erected and fabricated at site using mild steel plates of designed thickness conforming to IS 226. The SMAT reactor is used on floating type foundation. The roof of the reactor is fixed type supported in grid of ISMB. 23
The reactor is painted from inside using chlorinated rubber paint, whereas the outside surface is painted by synthetic enamel or aluminum paint. The SMAT reactor is partially packed with structured media out of PVC. The structured media is provided in the form of modules. This specialty of the media lies on offering very large surface area at a void ratio of 90%. The surface area provided by media is around 95-105 sqm per cum. The entire media remains submerged in the reactor content. The bacteria developed on media surface take upon organic content of wastewater to metabolize it and produce biogas and biomass. The reactor content is kept under constant recirculation pumps. To achieve optimized mixing the recirculation pump suction network is placed next to the bottom of the reactor. This suction network is designed in such a way that it sucks reactor content from entire bottom cross sectional area. All recirculation pumps their discharge into roof feed tank.
4.4 TREATED EFFLUENT DISCHARGE As SMAT reactor is down flow, the treated effluent is collected from the bottom of reactor. To utilize head available, the overflow arrangement is so designed that treated effluent is discharged at suitable head.
4.5 BIOGAS RECOVERY The biogas produced by anaerobic digestion inside the reactor is collected at the Gas Dome. The gas dome is placed at Reactor Roof and it’s fitted with all essential safety equipment such as breather valve, flame arrestor etc. The biogas is then conveyed to blower for further utilization is boiler or biogas engines.
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4.6 Advantages of this Process 1. No dilution water required for treating COD up to 110,000 mg/l. 2. Higher organic loading rate in terms of COD per m3 of reactor volume and thus requires smaller tank. 3. Special Media :
SMAT reactor employs honeycomb type, specially
designed geometrically structured rigid PVC media to immobilize the bacteria inside the reactor. This immobilization of the bacteria on the specially designed performed media with the result that (I) the bacteria are neither washed away not settled at the bottom of the reactor tank. (II) There is an optimum (designed contact) between the bacteria and the organic impurities contained in raw spent wash, ensuring continuous, generation of biogas at the optimum. 4. Extremely quick restart within 48-72 hrs, even after long factory shut down of 6 to 8 weeks as bacteria are always available in active condition attached to very large surface area provided by the media. Also there is no loss of biogas. 5. No biomass/ anaerobic sludge recycling required as bacteria are always present in very large quantity in the form of bio-film or media. 6. Most Rugged System :
Can withstand variation in flow, PH, COD,
concentration. 7. Very high bacterial population as bacteria are attached to media surface in the from of bio-film. The film neither gets washed away not settles at the bottom hence SMAT digester continuously generates higher amount of Biogas. 8. Higher digestion : BOD reduction up to 90% and COD reduction up to 70% . 9. High resistance to toxicity and shock loading 10. No smell nuisance. 11. Aesthetically pleasing and extremely neat and clean system. 25
12. Payback period of two years due to higher and continuous biogas generation 13. Lower operation and maintenance cost power is required only for feed and recirculation pumps. 14. Lowest life cycle cost as no loss of biogas in startup after shutdown.
26
27
5 FACTORS AFFECTING BIO DIGESTION OR GENERATION 1. PH or the hydrogen ion concentration 2. Temperature 3. Total solid content of the Feed material 4. Loading rate 5. Seeding 6. Uniform feeding 7. Diameter to depth ratio 8. Carbon to Nitrogen ratio 9. Nutrients
10. Mixing or stirring or agitation. 11. Retention time or rate of feeding. 12. Type of Feed stocks. 13. Toxicity due to end products. 14. Pressure. 15. Acid accumulation inside the digester. 1) PH or Hydrogen ion Concentration PH of the slurry changes as various stages of the digestion in the initial acid formation stage in the fermentation process. PH is around six or less and more of carbon dioxide is given off. In the latter two three weeks time, the PH increases as the volatile acid and nitrogen dioxide components are digested and CH4 is produced. 28
The ideal values of PH for digestion of sewage solid are reported to be in the range 7 to 7.5. But slightly higher value of 8.2 has been reported to be optimum for digestion of raw material.
2) Temperature Methane bacteria work
best at a temp between 35 to 38 .The fall in
production starts at 20 and stops at 10 .The temp is very important factor since it affects the bacterial activity directly.
3) Total solid content The total solid content in raw material should be 8 to 10%. The adjustment of total solid content helps in bio-digesting the material at faster rate.
4) Loading Rate Loading is defines as the amt of raw material fed to digester per day per unit volume most plant operate loading rate at 0.5 to 1.0 kg. Of volatile solid per m3 per day 2f loading rate is so high that add will accumulate and fermentation will stop.
5) Seeding The bacteria required for acid fermentation and methane fermentation are artificially seed with digested sludge which is rich in methane formers. But seeding should be up to a certain limit because beyond a certain seed concentration, the gas production will decreases.
6) Uniform Feeding One of the factors of good digestion is the uniform feeding of the digester so that the micro organism is kept in a relatively constant organic solid concentration at all times. 29
7) Carbon Nitrogen Ratio of Input Material All living organisms require digester is a culture a bacterial nitrogen oxide to form their cell proteins from biological point of view, the element, carbon and Nitrogen are the main food of anaerobic bacteria the optimum carbon Nitrogen ratio that best suits for maximum microbiological activity is 30:1.
8) Diameter to Depth Ratio Research investigation reveals that gas production per unit volume of digester capacity was maximum when the diameter to depth ratio was in the ranges 0.66 to 1.00. But reports from the field, digester of 16ft. depth and 4 ft. to 5ft. Diameter to be working satisfactory.
9) Nutrients The major nutrient required by the bacteria in digester are C, N2,02, H2, P & S of these nutrients N2 and P are always in the short supply. 10) Mixing or Stirring Since bacteria in the digester have very limited reach to their foods, it is necessary that the slurry is properly mixed and bacteria get their food supply. It is found that slight mixing improves the fermentation, however a violet slurry agitation retards the digestion.
11) Retention Time The period of retention of the materials for biogas generation inside the digester is known as the retention period. It depends upon feed stock and temperature normal value of retention period is between 40 to 45 days and in some case 60 days.
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12) Type of Feed Stocks When feed stock is woody or contains more lignin than bio digestion of these feed stocks are combined in proportions.
13) Toxicity The digested slurry if allowed to remain in the digester beyond a certain name becomes toxic to the micro organisms and might cause fall in fermentation rate.
14) Pressure The pressure on the surface of slurry also affects the Fermentation. The rate of gas production is higher at low pressure.
15) Acid Accumulation inside the Digester Intermediate products such as acetic propionic acid, butyric acid are produced during the process of bio digestion. This cause in decrease of PH, especially when fresh feed material is added in large amount. This acid may be converted into methane by addition of neem cake. Acid accumulation is usually occurred in batch digestion Systems.
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6 AREAS OF APPLICATION 1) As a fuel for Domestic and Industrial Purpose (1) As the biogas is a non poisonous, non toxic gas which when mixed with air burns with blue flame, without soot or arch offensive small. The biogas has high octane rating and calorific value ranges between 4700-6000 Kcal. Per cum. For this above reasons biogas is used in domestic purposes like cooking etc.
2) The gas produced can be used directly for heating or for use in on engine driven generator (7) Depending upon its physical properties like; its boiling temperature is 161.50C, critical temperature is 820C and critical pressure 42 atoms it can be directly used for heating.
• Biogas Fired Water Heater One of the ways of heating water is placing it in a vessel over a gas burner. This is not on efficient practice because bath water, unlike cooking items, is needed in the order of 25 to 100 liter. The surface area of the vessel in contact with hot gases is too small (0.13m2 for a 0.4m diameter vessel) besides being open to the environment. Usually in such a system, for convenient handling , about 20 lit of water are heated at a time this is respected several times. Water heating can be performed more efficiently by the use of water jacket type heater because the heat transfer area is about 0.35m2 for a capacity of 140 liters. The gas burner is placed below at the base.
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Figure shows a typical, Biogas heater in use in India.
Design of Gas-fired Bath Water Heater of 140 liters capacity (dimensions in mm)
Typical Results are given below. • Energy output = 136 liters heated from 31 to 700C • Fuel burning rate = 1.135m3/h • Operating time = 0.75h • Overall efficiency = 73%
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3) Use of Biogas in Gas Turbines (7) The main advantages of the use of gas turbines is that the capital cost is considerably lower compared to reciprocating engines - particularly when used aircraft engines whose normal flying duty is over are utilized. There are additional advantages of easy installation, modular maintenance, quick starting and reliable operation. National Aerospace laboratory has studied the application of Rolls Royce ‘Dart’ Engines used by IAF and Indian Airlines to provide mechanical energy for 1.0 to 2.5 MW electrical generators. These can be operates on duel fuels [biogas diesel or kerosene] this approach seems to be promising. The production of electrical energy only by biogas fuel is not economical. However, cogeneration with IC engine connected with a generator and with waste heat recovery would be economical. The waste heat recovered can be cycled for digester heating.
4) Biogas Provides reasonably good fuel for both spark Ignition (Petrol) and Compression Ignition (Diesel) Engine (1) Biogas is combustion gas containing high percentage of methane, which is an excellent gaseous fuel for running internal combustion engine. It has ignition temp. Of 640 – 8400C, with acetone value of 130. The mixture containing methane, carbon dioxide and air has wide combustible range and favorable conditions for forming a mixed gas for running the engine, providing same power for equal volume.
Modification required to be done in Existing Petrol Engine to switch it on to Biogas The spark ignition engine can be switched over to biogas (100%) offer it’s starting on petrol and after initial heating. Therefore, a biogas supply pipe is provided on air manifold between air cleaner and carburetor. After the engine has
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run on petrol for 5-10 minutes the biogas supply value is opened slowly and petrol supply valve closed simultaneously.
Modification done is SI Engine for Biogas fuel.
The necessary modification to be done on engine for running it on biogas is shown in frg. The biogas contain 30 – 40 percent carbon dioxide, therefore the air regulation valve is used to control air / biogas mixture. The biogas also contains moisture and sometimes traces of hydrogen sulphide which effect the engine performance and life of Cylinder / Piston. The presence of moisture will give pulsating power output or engine may close after some time. The Hydrogen sulphide will attack valves, piston cylinder, etc. thus reducing the life of engine. Therefore, it is necessary to remove moisture by passing it through a hydrous material and for removing Hydrogen sulphide, it is passed through iron chips.
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Modification to be done in Diesel Engine to operate it on Duel – Fuel The existing diesel engine can be modified without any difficulty to operate it on biogas and diesel.
Modification in Diesel Engine for Biogas Fuel.
A biogas inlet manifold is provided such that the engine can run on diesel alone when biogas is exhausted. This manifold is fitted to air inlet pipe between the inlet port and air cleaner (Fig.). The biogas is required @ 2.12 cum per hr and 20-35 percent diesel for 5hp. Duel-fuel engine.
5) Diesel Engine (CI) runs on Biogas as Dual-Fuel Comprising both oil and Gas which helps to achieve about 80% saving in Diesel: (used as Vehicle fuel) petrol engine (SI) also work on Gas having petrol Replacement of the work of 100% (1) The Biogas has been used successfully for short distance transportation of vehicle. The Gas compressed up to 120-150 Kg/cm2 , pressure in standard eight cylinders was used for operating a 95 bhp, six cylinder truck. In one go it can travel a distance of 96 Km. The pressure reduction is achieved in two stages. I (120 Kg/cm2 to 1.5 Kg/cm2) and II (1.5 Kg/cm2, to 0.02 Kg/cm2).
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Performance Characteristics of Vehicle Running on Biogas: (1) Particulars
Units
Tractor
Truck
Bus
Fuel Consumption / hr
M3/hr
12
54
54
Fuel For 100Km travel
M3
48.5
107.4
107.4
Fuel Consumption/ tone km
M3
0.32
0.15
0.26+
Vehicle speed [max]
Km/hr
20
40
45
Volume of fuel tank
M3
2x2x2
8x4x1
8x4x1
(+) 20 passengers = 1 tones km
6) Biogas can also used for Electricity Generation As per studies 1KWH of electricity can be generated from 0.7m3 of gas which can light about is electric bulbs of 60 watt. Rating for 1 hour.
Biogas Lamp:
The biogas lamps are similar to Kerosene mental lamps consuming 0.11 to 0.18 cum gas per 100 candle power and use silk mental for lighting.
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The lamp has a simple with reflector ad gives 100 candle power (60w) light. The lamp can be double mental type also. The hot gas vent holes are provided to open toward side to avoid entry of rain water. The biogas on-off lock leaves is provided to supply gas for lighting.
7) It can be used effectively for operating small engine, utilizing power for pumping water, and grinding flour by using known technology
8) In sewage treatment plants biogas is used as fuel for boiler. 9) Use of Biogas in Refrigerators (7) Biogas can be used in absorption refrigeration system without only problems as long as assured gas supply for burner is available for a Refrigerator of 230 liters capacity, the biogas consumption will be approximately 0.044 m2/l. Particular problems also arise with biogas operated refrigerators. The compositon of biogas varies substantially from day to day. The gas pressure fluctuates excessively with the amount of gas stored even in a floating drum plant, special, stable-burning jets are therefore needed – especially if the refrigerator is thermostatically controlled and the flame burns only when required. On every ignition there is a risk of the flame going cut. Gas will then discharge without burning. The gas supply must therefore automatically be cut off it the flame goes out.
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7 CONCLUSION The proper utilization of distillery wastes for production of biogas will not only reduce the load on fossil fuel ,but will also help in environmental protection due to reduction in organic dung . The importance of distillery spent wash as a source of renewable energy especially when the sources for the fossil fuel are depleting is obvious but the contribution of distillery to organic load on receiving steam is also a matter of serious concern because of limited assimilation capacity. Biological treatment of distillery effluent results in production of ‘Biogas’. Thus it is an advantage for sources of renewable non-conventional energy and waste management.
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8 BILBLIOGRAPHY 1. N. Mathur, N.S. Rathore “Biogas Production Management” and utilization”P.P. – 2-5, 89 - 143. 2. D. S. Shah, D.P. Mishra and Shrivstav “Unconventional energy Sources” (1982). 3. Soil J Acceivala, “ Wastewater treatment for pollution control”
IInd
Edition PP – 33 - 39. 4. Mahajan S.P. ,” Pollution control in Process Industry”, Total McGrowlkill PP – 103 - 105 (1988) 5. ‘ All India Seminar on Pollution control in Sugar Industry”, PP - 1- 31 , (1995) 6. Vipul Goyal, “ Chemical Engg. World”, Vol 32 No 7 PP - 43 - 47 (2002). 7. T. Nijaguna, “Biogas Technology”, Newage International Publisher PP – 224 - 259. 8. WWW. Undp.org.in (United Nation’s Development Program). 9. WWW.mnes.nic.in 10. WWW.apctt.org, (Asian and Pacific center for Transfer of Technology). 11. WWW.terlin.org
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