Coparrison of Treatment Methods

On-site Wastewater Treatment Systems – Technical issues UNESCO-IHE, MUI Department, Elisabeth v. Münch, PhD

On-site wastewater treatment systems - a brief overview of technical issues Elisabeth v. Münch, PhD

Department of Municipal Infrastructure, UNESCO-IHE, Westvest 7, 2611 AX Delft, The Netherlands February 2005

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TABLE OF CONTENTS 1.

Definition of Terms _____________________________________________________ 4

2.

Evaluation and Classification of Disposal Sites _______________________________ 5

3.

Processes for Decentralised Wastewater Treatment Systems _____________________ 6 3.1. Anaerobic Treatment Processes_______________________________________ 6 3.1.1. Septic Tanks ___________________________________________________ 6 3.1.2. Upflow Anaerobic Sludge Blanket Reactor (UASB) ____________________ 7 3.2.

Ponds ____________________________________________________________ 7

3.3. Upgrading Pre-Treated Wastewater ___________________________________ 8 3.3.1. Intermittent Sand Filter ___________________________________________ 8 3.3.2. Rock Filters ____________________________________________________ 8 3.4. Aerobic Suspended Growth Systems___________________________________ 9 3.4.1. Sequencing Batch Reactor (SBR) ___________________________________ 9 3.4.2. Extended Aeration_______________________________________________ 9 3.4.3. Membrane Bioreactor (MBR) _____________________________________ 10 3.5. Aerobic Fixed Film Systems _________________________________________ 11 3.5.1. Rotating Biological Contractor (RBC) ______________________________ 11 3.5.2. Submerged Aerated Filter (SAF) __________________________________ 11 3.5.3. Moving Bed Bioreactor (MBBR) __________________________________ 11 3.6.

Constructed Wetlands______________________________________________ 12

3.7. Other Issues ______________________________________________________ 12 3.7.1. Nutrient Removal ______________________________________________ 12 3.7.2. Sludge Management ____________________________________________ 13 3.7.3. Odour________________________________________________________ 13 3.8. 4.

5.

Comparison of Processes ___________________________________________ 13

Final Disposal Methods _________________________________________________ 16 4.1.

Final Disposal Methods for Normal Site Conditions _____________________ 16

4.2.

Final Disposal Methods for Difficult Site Conditions ____________________ 16

4.3.

Final Disposal Methods for Adverse Site Conditions ____________________ 16

Wastewater Collection, Treatment and Disposal for Sewered Areas ______________ 17 5.1.

Collection Systems_________________________________________________ 17

5.2. Treatment Systems ________________________________________________ 17 5.2.1. Pre-engineered Package Plants ____________________________________ 17 5.2.2. Individually Designed and Constructed Systems ______________________ 18 5.3. 6.

Disposal and Reuse Systems _________________________________________ 18

References____________________________________________________________ 18

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1. DEFINITION OF TERMS On-site Treatment The term on-site treatment is usually used to refer to “small” wastewater treatment plants with local effluent disposal (most often subsurface disposal). It usually involves a very basic sewerage system to convey wastewater to the treatment process. The treated effluent pipes are usually very short, because the aim is to locate the treatment process and the disposal site in close proximity to each other. Small Treatment Plants The International Water Association Specialist Group on Small WWTP has defined a small wastewater treatment plant (WWTP) as being one for less than 2000 person equivalents (pe) or a daily flow of 200 m3/d. In comparison, the Norwegian classification system calls a treatment plant “small” if it treats the effluent of 35-500 persons. Indeed, there is no clear-cut definition of what size exactly is “small” for WWTPs. WWTPs that have the capacity to treat effluent from < 35 persons (i.e. scattered dwellings of < 7 houses) are called mini-treatment plants (or on-site plants) in Norway. These plants are only used in cases where soil infiltration cannot be used due to impermeable soils. Mini-treatment plants normally treat all the wastewater from the house (grey as well as black water), and they normally use pre-treatment in a septic tank followed by a unit based on biological or chemical processes or combinations of the two. Decentralised Wastewater Management There is no generally accepted definition of what “decentralised” wastewater management means. It is usually used to imply something “small” and “compact”. Decentralised wastewater management employs collection, treatment, and disposal/reuse of wastewater from individual homes, clusters of homes, isolated communities, industries or institutional facilities, as well as from portions of existing communities at or near the point of waste generation. Decentralised systems maintain both the solid and liquid fractions of the wastewater near their point of origin, although the liquid portion and any residual solids can be transported to a centralised point for further treatment and reuse (Crites and Tchobanoglous, 1998). Decentralised WWTPs will normally (but not always) receive a more concentrated wastewater than centralised plants, and flow and variations in composition are greater. Generally speaking, the smaller the system, the larger the variations in flow and composition will be. Other Wastewater Treatment Terms The conditions present in a biological treatment system are commonly classed into the following three categories: Page 4


An aerobic state is characterised by the presence of oxygen (may or may not contain nitrate). An anaerobic state is characterised by the absence of oxygen or nitrate. An anoxic state is characterised by the absence of oxygen, but presence of nitrate. This latter distinction between anaerobic and anoxic is only important for discussing biological nutrient removal concepts. Person equivalent or p.e. (the amount of wastewater discharged per person; usually taken to mean 200 L/d/person and 60 g BOD/d/person). BOD: BOD stands for “biological oxygen demand” and is a measure for the strength of the wastewater with regard to organic matter. It is expressed as mg/l or kg/d. Effluent: The water flow leaving a wastewater treatment system. 2. EVALUATION AND CLASSIFICATION OF DISPOSAL SITES Table 1 below summarises the characteristics of so-called “normal”, “difficult” and “adverse” site conditions. The type of site condition is an important factor in choosing the effluent disposal method. Table 1. Classification of site conditions (after Qasim, 1999) Normal site conditions

Difficult site conditions

Adverse site conditions

Subsoil type

Suitable for percolation

Low permeability

Impervious

Water table depth

Deep

High

Very high

Bedrock

Unfractured

High and fractured

High and fractured

Ground surface slope

Favourable

Unfavourable

Steep

Flooding

None

Occasional

Frequent

Distance from water supply wells, buildings, escarpments

Far away

Quite close

Close

Lot size

Large

Small

Small

Disposal methods for septic tank effluent (see also Section 3)

Gravity flow over a conventional percolation trench or bed.

Further treatment needed (aerobic processes or intermittent sand filters). Periodic dosing of a disposal field by pump or dosing syphon.

Drastic change in water conservation, treatment and recycling.

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Bouma (1979) makes the following recommendations regarding soil classification: The separation between “suitable” and "unsuitable” soils has a somewhat arbitrary character, if only because of the wide variety of soil and site factors considered and their different relative significance. The factors of high ground water or bedrock, for example, create different problems and call for a different analysis than the factor of slow permeability or steep slope. The following broad definition for acceptable performance of any on-site soil system is proposed: “Acceptable on-site disposal and treatment of domestic liquid waste implies complete infiltration in the seepage system at all times, followed by transformations during soil percolation to the effect that neither the ground water aquifer nor surface waters are contaminated at any time to a degree that is unacceptable in terms of human health or environmental quality”. 3. PROCESSES SYSTEMS

FOR

DECENTRALISED

WASTEWATER

TREATMENT

The wastewater treatment processes used for decentralised wastewater systems are in principle the same as used for larger centralised wastewater systems, i.e. physical, chemical and biological processes as well as combinations of these. Certain processes are better suited for the small-scale application than others, and these are described in detail in this chapter. An overview and comparison of the processes described below is provided at the end of this chapter. 3.1. Anaerobic Treatment Processes 3.1.1. Septic Tanks A septic tank has the following functions: Settling chamber for solids; Storage for sludge and scum; and Anaerobic digestion and breaking down of the waste solids Anaerobic bacteria (organisms that live without oxygen) feed on the sludge, reducing its volume. Soluble organic matter is released from the sludge into the effluent. Methane and carbon dioxide are also produced and vented from the tank through the house vent. Only about 40% of the sludge volume is reduced in this manner, however, and the accumulated solids must be pumped from the tank once very 2 to 3 years. Septic sludge (so-called “septage”) has to be treated and disposed by one of several available methods (see Section 3.7.2). If solids are not pumped out, the tank will fill, re-suspend the accumulated solids, and wash them into the absorption fields where they quickly clog the soil pores. The liquid effluent from the septic tank is discharged to a distribution box and then to a drainfield. Effluent coming from the septic tank is not of a high quality nor is it consistent, but this is not necessary if a suitable soil type and area is used for final subsurface disposal. The tank does remove up to 60% of the BOD and 70% of the suspended solids. Indicator microorganisms, which are microorganisms that indicate the possible presence of disease-causing bacteria, are not reduced to low levels. Page 6


Modern conventionally designed septic systems are composed of four basic components: Building sewer Septic tank Distribution box Drainfield (or leach field) Typically, septic tanks are made of concrete or fiberglass, although other materials such as steel, redwood and polyethylene have been used. Regardless of the material of construction, a septic tank must be watertight and structurally sound. The size of the septic tank system is critical in terms of its performance. Several tanks, often arranged in series, are applied for schools, summer camps, parks and motels. 3.1.2. Upflow Anaerobic Sludge Blanket Reactor (UASB) The UASB is a tank filled with anaerobic granular sludge with good settling properties. Influent wastewater is distributed at the bottom of the UASB reactor and travels in an upflow mode through the sludge blanket. The anaerobic degradation of organic substrates occurs in this sludge blanket, where biogas is produced. The gases produced under anaerobic conditions (methane and carbon dioxide) serve to mix the contents of the reactor as they rise to the surface. Critical elements of the UASB reactor design are: Influent distribution system Gas-solid separator Effluent withdrawal design Modifications to the basic UASB design include adding a settling tank or the use of packing material at the top of the reactor. The presence of a settler on the top of the digestion zone enables the system to maintain a large sludge mass in the UASB reactor, while an effluent essentially free of suspended solids is discharged. The UASB reactor has the potential to produce higher quality effluent than septic tanks, and can do so in a smaller reactor volume. Whilst it is a well-established process for large-scale industrial effluent treatment processes, its application to on-site domestic sewage is still relatively new. 3.2. Ponds Wastewater can be treated in ponds, which are also referred to as lagoons or waste stabilisation ponds. These are shallow, man-made (earthen and/or lined) basins, which can be provided with or without mechanical aerators or covers (for anaerobic ponds). They provide BOD and pathogen removal, and can also provide nutrient removal. The stabilised sludge has to be removed periodically. The approach to pond design has been largely empirical; it is nowadays commonly based on volumetric and organic loading rates, hydraulic retention times and temperature. Ponds require a relatively large land area, which is a disadvantage if space is limited or expensive. Their main advantage is that they are easy to maintain and operate. Ponds are usually classified as follows: Anaerobic ponds – usually 3-5 m deep; solids settling and anaerobic digestion occurs Page 7


Facultative ponds – 1 to 2.5 m deep; aerobic treatment in the top layer and anaerobic degradation in the deeper layers (if mechanical aeration is provided, this type of pond could be turned into an entirely aerobic pond) Maturation ponds – 1 to 1.5 m deep, entirely aerobic; active algal biomass is maintained throughout entire depth; pathogen removal; can be filled with floating macrophytes (e.g. duck weed, water hyacinth) A typical pond treatment system consists of three ponds in series, where the first pond is anaerobic, the second facultative and the third a maturation pond. 3.3. Upgrading Pre-Treated Wastewater Anaerobic treatment or pond treatment may not achieve the required effluent quality. In this case, add-on technologies may be used. Examples of these include: Intermittent sand filters (see below) Rock filters (see below) Microstrainers Dissolved air flotation Floating aquatic plants (as mentioned above in relation to maturation ponds) Constructed wetlands (see Section 3.6) 3.3.1. Intermittent Sand Filter Intermittent sand filters can upgrade the effluent quality from septic tanks, UASBs or ponds to advanced secondary or even tertiary levels. The media grains of the sand filter provide a large surface area for many different organisms to live on, which results in a stable process. Clogging of the filter bed occurs gradually, allowing an operator time to respond to upcoming process failure. When properly designed and operated, a filter bed can operate for several years before maintenance is required. When that time comes, the filter bed can be restored within a few hours. The typical sand filter is a concrete or PVC-lined box filled with a specific sand material. A network of small-diameter pipes is placed in a gravel-filled bed on top of the sand. Septic tank effluent is pumped under low pressure through the pipes in controlled doses to insure uniform distribution (Qasim, 1999). The effluent leaves the pipes, trickles downward through the gravel, and is treated as it filters through the sand. A gravel under-drain collects and moves the treated wastewater to discharge (pumped or gravity). The accumulation of solids occurs in a 50 to 80 mm layer that must be removed periodically. The total filter area required for an intermittent sand filter is determined by dividing the average flowrate by the design hydraulic loading rate. One spare filter should be added to ensure continuous operation because it may take several days for a cleaning event to be completed. 3.3.2. Rock Filters Rock filters remove suspended solids by sedimentation as pond effluent flows through the void spaces in the rocks. The accumulated algae are then degraded biologically. The advantages of the rock filter are its simplicity of operation and its relatively low construction Page 8


cost. Odour problems can occur, especially in wastewater containing significant concentrations of sulfate (greater than 50 mg/L). 3.4. Aerobic Suspended Growth Systems The conventional activated sludge process is the best-known suspended growth aerobic system, and is the process most commonly used in large, centralised WWTPs. It can also be used in small package plants. Four process elements are common for all activated sludge systems (for a detailed description of this system the reader is referred to Tchobanoglous et al. (2003)): A flocculent, aerated slurry of microorganisms (which is called “mixed liquor suspended solids” or MLSS) is utilised in a bioreactor to remove soluble and particulate organic matter from the influent wastewater; Quiescent settling is used to remove the MLSS from the process stream, producing an effluent that is low in organic matter and suspended solids; Settled solids are recycled as a concentrated slurry from the clarifier back to the bioreactor; Excess MLSS (sludge or biosolids) is wasted from the bioreactor to control the solids retention time to a desired value. Air is provided to the bioreactor by mechanical surface aerators or diffused air aeration (using blower and diffusers). In general, the inherent instability of the process means that it must be controlled carefully – a much easier proposition in one central plant. Because the process provides no physical barrier, poorly treated effluent is released immediately if problems, accidents or equipment failures lead to a process breakdown. Once the process is disturbed it can take hours and often days for it to re-establish itself. There are many process variants to the basic activated sludge process, the main ones being described briefly below. 3.4.1. Sequencing Batch Reactor (SBR) The SBR process is a fill-and-draw-type reactor that acts as aeration basin and final clarifier. Wastewater and biomass are mixed and allowed to react over several hours in the presence of air. At a certain point in time, the aeration is turned off and the mixed liquor in the reactor is allowed to settle, thereby removing the need for a separate settling tank. After a short settling period, the clarified treated effluent is discharged via a specially designed decanter. One design variant is that the decanter follows the liquid level down enabling only the clear, treated effluent to be discharged, while the biomass continues to settle. Once the treated effluent is discharged the reactor is available to treat a further batch of wastewater. In this way, the process operates on a batch treatment principle, with the operations being sequenced. Two or more SBRs are usually operated in parallel unless a sewage storage tank is used. 3.4.2. Extended Aeration The extended aeration process utilises a large aeration basin where a high population of microorganisms is maintained (resulting in a long hydraulic and solids residence time compared to the conventional activated sludge process). Many types of prefabricated package plants utilise this process. Page 9


An oxidation ditch is a variation of the extended aeration process. It has a channel in the shape of a race track. Mechanical aeration with rotors is usually used to supply oxygen and maintain circulation. 3.4.3. Membrane Bioreactor (MBR) The key component of the MBR technology is a membrane that separates the activated sludge (or MLSS) from the effluent (instead of a final clarification stage). There are many different suppliers of MBR systems on the market. As a representative example, I am describing here a process using flat sheet membranes manufactured by the Japanese company Kubota. The process was developed as a result of a Japanese Government initiative to produce high quality effluent treatment plants. The discharge permeate can be reused for toilet cisterns, washdown, irrigation and more. The permeate from the MBR process is typically: < 5:5:5 mg/l BOD:SS:ammonia-N; and free of pathogens, viruses and bacteria. The process employs flat sheet membrane panels housed in stainless steel (304 or 316) units and aerated by a coarse bubble system below each unit. A series of these membranes are submerged within an activated sludge treatment tank. The aeration necessary for treatment of the liquors also generates an upward cross-flow over the membranes; essential to keep fouling of the filtration surface to a minimum. An advantage of this design is that the membrane panels are securely retained and do not touch or abrade each other whilst the units also act as a flume to ensure effective tank mixing and even distribution of the biomass. The membrane panels are manufactured with a pore size in the range of 0.1 to 0.4 μm which in operation becomes covered by a dynamic layer of protein and cellular material. This further enhances the effectiveness of this filtration performance by providing an effective pore size of less than 0.01 μm, which is in the ultra filtration range. The incoming wastewater requires screening (< 3 mm) and de-gritting prior to entering the membrane bioreactor tank. The process requires no primary or secondary settlement stages and no additional tertiary treatment or UV stages to achieve very high disinfection. The MBR system does not require flocs to be formed to remove the solids by settlement and therefore the biomass can operate at very high levels of MLSS, generally in the order of 12,000-18,000 mg/l, and as high as 22,000 mg/l. This high concentration enables a small tank volume and a long sludge age to be utilised, which substantially reduces sludge production. The hydraulic flow determines the required number of membrane units. Each membrane unit may contain up to 400 flat sheet membrane panels housed within a rectangular box, together with an integral aeration system in the bottom section of the unit. Treated effluent is removed from the membrane units using gravity head (typically 1 – 1.2 m), or a pumped suction operation can be utilised.

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3.5. Aerobic Fixed Film Systems Fixed film system can employ a process of purely fixed film, or a combination of fixed film and suspended growth. Many commercially available package plants use aerobic fixed film systems. Variants of this process are described below. 3.5.1. Rotating Biological Contractor (RBC) The Rotating Biological Contactor (RBC) supports a biologically active film, or biomass, of aerobic microorganisms. An RBC treatment system typically comprises of three units: Primary Zone: A settlement tank where wastewater enters and solids settle and are stored for subsequent removal. Anaerobic digestion may also take place. RBC: This is where the biological treatment takes place. Numerous discs attached to a shaft form the RBC assembly, which is partially submerged in a trough to create an environment for an active biomass to develop on the media. The RBC is slowly rotated to bring the biomass into alternate contact with the wastewater and atmospheric oxygen. Final Clarification Zone: Here settlement of the mixed liquor and excess biomass takes place. 3.5.2. Submerged Aerated Filter (SAF) The SAF process can be described as follows: Settled wastewater is fed from a primary tank into the first stage of a reactor at a controlled rate, where it is mixed with the aerated bulk liquid already present. Air is introduced into the reactor through a fine bubble diffuser system at the base of each chamber. A uniquely structured media is suspended over the fine bubble membrane diffuser to provide optimised contact between the oxygen-rich wastewater and the biomass. With a high surface area to volume ratio, the media supports a biologically active film of microorganisms, to treat the wastewater by using oxygen from the air provided. Manufactured from lightweight vacuum-formed PVC sheets (for example), bonded together to form packs, the media can easily be removed for maintenance. When the oxygen-rich wastewater comes into contact with the biomass attached to the surface of the media, organic pollutants are broken down by the biomass. The flow of air can be controlled to optimise the levels of dissolved oxygen within the reactor, ensuring that the process is energy efficient. 3.5.3. Moving Bed Bioreactor (MBBR) A number of suppliers offer processes that are based on free-floating submerged plastic media on which a biofilm grows. The market leader at present is arguably the Kaldnes process (Purac Ltd.), and therefore this process is described further below. The process is based on the biofilm principle, and the core of the process are the biofilm carrier elements made from polyethylene with a density slightly below that of water. These are designed to provide a large protected surface for the bacteria culture. The plastic "wheel" in the Kaldnes Moving Bed™ Process is the result of extensive research. It has been described as "an apartment with three rooms and a kitchen, where bacteria can live comfortably and tuck into hearty meals of water pollutants". (Kaldnes web site, http://www.kmt.no/process.html) Page 11


Compared to conventional activated sludge processes there is no sludge recycle in the MBBR process, and the final clarification stage is considerably smaller. Alternative processes also exist that use activated sludge together with freely moving media, and these are sometimes referred to as “suspended carrier” processes. 3.6. Constructed Wetlands Wetlands are comparatively shallow (typically < 0.6 m) bodies of slow-moving water in which dense stands of water-tolerant plants such as reeds are grown. In man-made, constructed, wetlands, these artificially created bodies are typically long, narrow trenches or channels to promote the occurrence of plug flow conditions. A septic tank, a primary settling basin or an anaerobic reactor to remove the bulk of the suspended solids and organic matter commonly precedes constructed wetlands for sewage treatment. Two systems are distinguished: Free water surface systems with shallow water depths; or Subsurface flow systems with water flowing laterally through the sand or gravel. Constructed wetland systems can significantly remove BOD, total suspended solids, nitrogen and phosphorus, as well as metals, trace organics and pathogens. Mosquito control and plant harvesting are the two main operational considerations associated with constructed wetlands for wastewater treatment (Qasim, 1999). 3.7. Other Issues 3.7.1. Nutrient Removal Increasingly, the removal of nutrients (nitrogen and phosphorus) is required before treated effluent can be discharged to the environment (particularly if the disposal site is a sensitive water course). Nitrogen removal is usually performed using the biological processes of nitrification and denitrification. In large, centralised activated sludge plants, this process is realised using biological nutrient removal (BNR). All BNR process configurations for nitrogen removal have in common the use of an aerobic stage (for nitrification) and an anoxic stage (for denitrification). These two stages can be incorporated into suspended growth or attached growth process designs. Phosphorus removal can also be achieved using a BNR process, and it requires the right sequence of anaerobic, anoxic and aerobic conditions. Due to the process complexity and inherent instability, it is not commonly used in small scale or on-site WWTPs. Instead, chemical P removal using iron salt dosing is commonly used (e.g. in Norway). Anaerobic treatment systems such as septic tanks or UASBs are, on their own, unable to provide nutrient removal.

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3.7.2. Sludge Management All wastewater treatment processes described above produce sludge; some produce more, others produce less. It is important to take into account how the sludge should be treated and disposed when devising a wastewater management system. Sludge from septic tanks is also called septage or faecal sludge. Treatment options for faecal sludge, or other biological sludges resulting from decentralised wastewater treatment, include: Co-composting with organic solid waste Planted drying bed Unplanted drying bed Settling/thickening tank (or only a storage tank) Settling pond Anaerobic digestion Co-treatment with sewage sludge Co-treatment with wastewater In each case, a liquid fraction and a solids fraction will be the result of treatment. The liquid fraction has to be treated further (e.g ponds, constructed wetlands). The solid fraction could be used for beneficial reuse in agriculture. Where this is not possible, the treated sludge could be disposed in landfills or burnt in incinerators (but this is unlikely to be an option for decentralised systems). For more details on faecal sludge management the reader is referred to the web site of the Swiss organisation SANDEC (www.sandec.ch), who is very active in this field. 3.7.3. Odour The potential for odour generation is an important consideration for decentralised wastewater treatment because potential odour receptors (peoples’ noses!) might be much closer than in the case of centralised WWTPs. Odour can be minimised by proper wastewater treatment process design (e.g. ensuring that the biological process is not overloaded) or with specialised odour treatment technology (such as biofilters or activated carbon adsorption). 3.8. Comparison of Processes Table 2 below provides a comparison of the processes most commonly used for on-site or small-scale wastewater treatment.

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Table 2. Overview of important processes for on-site / small-scale wastewater treatment Characteristic or parameter ↓

Technology

→

Septic tank

UASB

Type of process

Anaerobic

Effluent quality (overall)

Low

BOD removal N removal

Ponds

Constructed wetlands, land application

Anaerobic treatment + Sand filters

Conventional ASP, SBR, Ext. aeration

MBR

Anaerobic and aerobic

Anaerobic and aerobic

Anaerobic + aerobic/physical

Aerobic suspended growth

Medium

Low

Medium

High

High

Aerobic suspended growth + physical Very high

RBC, SAF, MBBR Aerobic attached growth, or suspended + attached growth High

Medium

Medium

Medium

Medium

Medium

High

Very high

High

None

None

Low

Low

Medium

High

High

High

Possible

Possible

1

None

None

None

Low

None

Possible

Medium

High

Medium

High

Medium

Low

Low

Medium

Energy use

Very low

Low

Low

Very low

Medium

High

Very high

Medium to High

Biogas production

Yes, but not usually captured No

Yes

No

No

Yes

No

No

No

No

No

No

No

No, except for P removal

No, except for P removal

Sludge production

Low

Low

Low

None 2

Low

High

Yes (for membrane cleaning) Low

Space requirements Ease of O&M

Medium to high 3

Medium to high

High

High

Medium

Medium to low

Very low

Medium to low

Very simple

Simple

Very simple

Simple

Medium to

Reasonably

High complexity

Reasonably

P removal Sustainability (overall)

Chemical use

1 2 3

Usually with chemical P removal. Harvesting of plants required instead of sludge production Includes space requirement for drainfield, depending on site conditions

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Medium to low

On-site Wastewater Treatment Systems – Technical issues UNESCO-IHE, MUI Department, Elisabeth v. Münch, PhD Characteristic or parameter ↓

Technology

→

Septic tank

UASB

Ponds

Constructed wetlands, land application

Anaerobic treatment + Sand filters

Process stability

Quite robust

Robust

Very robust

Robust

simple Robust

Capital cost O&M cost

Low Low

Low Low

Medium Low

Medium Low

High Low to medium

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Conventional ASP, SBR, Ext. aeration complex Can be unstable High High

MBR

Reasonably robust Very high Very high

RBC, SAF, MBBR complex Reasonably robust High Medium


4. FINAL DISPOSAL METHODS 4.1. Final Disposal Methods for Normal Site Conditions The on-site final disposal options for normal site condition include (Qasim, 1999): Seepage pits Gravel-filled absorption trenches or bed, with one of the following dosing and resting alternatives: o Septic tank effluent discharge by gravity o Septic tank with serial distribution o Septic tank with alternating trenches o Septic tank with pressure dosed distribution o Septic tank with shallow trench low-pressure pipe distribution Leaching infiltrator or chambers 4.2. Final Disposal Methods for Difficult Site Conditions The following disposal methods are suitable for difficult site conditions (Qasim, 1999): Trench and bed or leaching chambers with improved treatment Shallow sand-filled, pressure-dosed disposal field Septic tank and mound Evaporation and absorption bed Recirculating intermittent sand filter, disinfection, and surface discharge Constructed wetlands 4.3. Final Disposal Methods for Adverse Site Conditions Briefly, the following options exist for adverse site conditions: Holding tank and hauling wastes Evaporative lagoon Slow-rate land treatment Overland flow Waterless, ultra-low-flow toilets, urine-separation toilets Dual system (black water – grey water) Complete system / total recycle The last three technology options fall into the group of “ecosan” (for ecological sanitation) technologies. Ecosan describes a new holistic paradigm in sanitation based on the systematic closure of local material flow cycles, thus introducing the concept of sustainability and integrated, eco-system oriented waster and natural resource management to sanitation and water management. Ecosan systems aim to enable a near complete recovery of all nutrients, trace elements and energy contained in household wastewater and organic waste and their productive reuse in agriculture. The ecosan area is a rapidly developing field and it would go beyond the scope of this report to go into too much detail here. A very good web site on this topic is the site by the German organisation GTZ: http://www.gtz.de/ecosan/english/index.html Page 16


5. WASTEWATER COLLECTION, TREATMENT AND DISPOSAL FOR SEWERED AREAS 5.1. Collection Systems The main types of collection systems of relevance here are 4 : Conventional gravity system Small-diameter, variable-slope sewers Pressure sewers Vacuum sewer 5.2. Treatment Systems The options for small wastewater treatment systems in sewered areas are principally the same as for the on-site systems discussed above. The only difference is in the disposal mechanism, which is typically, but not always, by discharge to a watercourse. As a result of this, the requirements for effluent quality may be higher, or lower, compared to on-site systems. 5.2.1. Pre-engineered Package Plants Most package plants are complete factory-built, prefabricated systems shipped for installation. The site preparation may require excavation and a concrete foundation slab. Package plants utilise one of the following processes: Biological o Extended aeration activated sludge o RBC o SAF o MBBR Physical/chemical o Coagulation/precipitation o Filtration o Membranes Pollution control is big business. It is driven by regulations and public perception. Many vendors are trying to sell equipment. You need carefully evaluate the many products and services offered. The following approach to working with suppliers is recommended (Edwards, 1995): Determine if their experience is relevant to your situation Investigate their reputation in the business Define your problem before you call them Establish what you expect them to provide Obtain a cost estimate for their services and equipment Ask for design, installation, and training proposals in writing Ask them to test their equipment in your business Have necessary site work done before equipment arrives for installation 4

For further information, please refer to the lecture notes of van Duijl on Sewerage and Drainage.

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Set the stage for site visits and training. Special considerations for wastewater treatment in developing countries or countries in transition include: Operator skill required Frequency of O&M work required Complexity of maintenance work required Local support from supplier Operating costs (chemicals, electricity, spare parts) The Internet is a valuable source of information about package plants. Keyword searches for “package”, “packaged”, “compact”, or “small” wastewater treatment plants will yield numerous web sites of suppliers of such technology. Some examples are listed below: http://www.keeprocess.com/index.html http://www.copa.co.uk/products/products.asp http://www.aquatechtrade.com http://www.aquamax.net/ http://www.conderproducts.com/ 5.2.2. Individually Designed and Constructed Systems Processes that are individually designed and constructed for sewered areas include (for further process details see also Section 3): Septic tank, Imhoff tank (not common for sewered areas) Ponds (anaerobic or aerated) Oxidation ditch SBR RBC Trickling filters Combined attached and suspended growth processes (e.g. SAF) Constructed wetlands 5.3. Disposal and Reuse Systems The main options for disposal and reuse systems for sewered areas include: Subsurface disposal (see also Section 4) Constructed wetlands (see also Section 3.6) Land application Surface water discharge (the most common method for centralised wastewater treatment) Indirect reuse

6. REFERENCES Bouma (1979) Subsurface Applications of Sewage Effluent. In: Planning the Uses and Management of Land; re-printed in EURO Summer School (2000) Page 18


Crites, R. and Tchobanoglous, G. (1998) Small and decentralized wastewater management systems, WCB/McGraw-Hill, Boston Edwards, J.D. (1995) Industrial Wastewater Treatment - A Guidebook. CRC Press, Inc., Boca Raton, Florida, USA. Lens, P., Zeeman, G., and Lettinga, G. (2001) Decentralised Sanitation and Reuse - Concepts, Systems and Implementation. IWA Publishing, London. (based on EURO Summer School (2000) DESAR Decentralised Sanitation and Reuse, WICC, June 18-23, Wageningen University, Sub-department of Environmental Technology, The Netherlands) Tchobanoglous, G., Burton, F.L., and Stensel, H.D. (2003) Wastewater Engineering, Treatment and Reuse. Metcalf & Eddy, Inc., McGraw-Hill (4th edition). Other Recommended Reading Material: Water Science & Technology, 48, 11-12 (2003): Small Water and Wastewater Treatment Systems V (Selected Proceedings of the 5th IWA International Specialised Conference on Small Water and Wastewater Treatment Systems, held in Istanbul, Turkey, 24-26 September 2002)

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