SYSTEM AND METHOD FOR THE TREATMENT OF WASTEWATER USING PLANTS
Field of the invention
The present invention relates to a method for the treatment of wastewater. More particularly, the present invention relates to a method and device for the treatment of wastewater. Background
The prior art is replete with inventions relating to the treatment of wastewater effluents. The following United States patents are not directly related to the use of wetlands but also deals with a variety of problems related to wastewater treatment. Horsley et. al. (U.S. Pat. No. 5,549,817) describes a storm water treatment system/apparatus designed to collect the first flush of storm water runoff, which consists of majority of pollutants, and to treat this effluent using sedimentation, filtration and constructed wetlands. The apparatus is design as a self contain integrated component amenable to mass production. It includes the sedimentation tank surrounded by ring shaped tank which houses the subsurface flow type constructed wetland, multiple component can be used for larger capacity installation. Raw storm water enters the central sedimentation chamber of the apparatus, which is divided into multiple segments designed to provide a series of sedimentation/ filtration chambers. A removable filter bulkhead sequenced from coarse to fine sediment sizes divides each chamber. Effluent from the final sedimentation chambers is discharged to the constructed wetland system, which surrounds the central sedimentation basin. Water flows through the perimeter wetland system in subsurface / saturated conditions and contacts with the root zone of the system where biochemical interaction attenuate pollutants.
Dufay, (U.S. Patent No. 6,159,371) described a constructed wetlands remediation system for the remediation of wastewater effluents. The system comprises utilizing one or more wetlands cell. Each cell comprises a wastewater effluent inlet, a remediated wastewater outlet, a bed (e.g. gravel with a depth of at least 12 inches, a top located below the horizon, an effluent inlet end and a remediated wastewater outlet end), a liner for preventing leakage of the wastewater from the cell to native soils, and remediation means for reducing waste in the wastewater effluent. Bed comprises at least two substantially distinct layers of gravel, with the layers having a greater gravel size (e.g. thickness of at least 6 inches) near the bottom of the gravel bed and a progressively smaller gravel size (e.g. thickness of at least 2 inches) near the top of the gravel bed.
DeBusk, (U.S. Patent No. 5,733,453) has described about wastewater treatment system and method, which reveals a vertically stacked system contains at least two different
treatment process zones, one atop the other and separated by a water-impermeable barrier, which reduces land area requirements. Water to be treated is directed from one treatment process to the other sequentially and exits the system in a purified condition. A vertically stacked water treatment system comprises a water-impervious treatment bed and water treatment system consists of layer of soil, rocks, gravel, and shredded recycled tires. Plantation grown in treatment system will help to remove contaminants from wastewater.
Wolverton (U.S. Patent No. 4,415,450) describes a method for treating wastewater using microorganisms and vascular aquatic plants disclose a method for treating wastewater comprising the steps of subjecting the wastewater to an anaerobic settling step for at least 6 hours and passing the Hquid effluent from the anaerobic settling step through a filter cell in an upflow manner. Effluent is passed first to the action of anaerobic and facultative microorganisms and then to the action of aerobic microorganisms and the roots of at least one vascular aquatic plant.
Gatiff US Patent No. 6,189,262 describes (Method of Treating Industries Wastewater, 2001) about modular cubes with numerous cavities include a rooting medium such as soil, organic, inorganic or semi organic material and tree in each cavity, whereby a long and arrow root system is established for each such tree. The cubes with the vegetation were all used to remediate a contaminate site, including discharged industrial wastewater.
Eifert US Patent No. 5,0093,975 (Enhanced subsurface flow constructed wetland, 1999) described about a subsurface flow constructed wetland that treats a variety of flowing wastewater effluents, provide pre-treatment clog reducing wastewater sludge disintegration and adds pretreated nutrients to wastewater so as to enhance microbial growth therein for improvement for improving the effectiveness and efficiency of wastewater. Treatment of constructed wetland includes one or more treatment cells having a soil or fine stone substrate cap covering the wastewater to be treated within a soil, gravel, organic or synthetic material media-matrix. The substrate cap is populated by natural plants, such as Phragmties sp., Typha sp. and /or scirpus sp. having root systems extending within the substrate cap. The root systems extend from the substrate cap downward into the media matrix accommodating the wastewater being treated be the root serve to biologically mediate, filter and/ or absorb undesirable components from the wastewater so as to cleanse and thus treat the wastewater.
Wengrzvenk US Patent No. 5, 174,897 (Constructed Wetland to control nonpoint source pollution, 1992) describe a unique design of a surface flow constructed wetland to control non-point source pollution but all applications relates to surface flow wetlands without mention or subsurface flow modifications.
Simmering et. al., US Patent No. 5,690,827 (Sewage treatment system using peat and a constructed wetland, 1997) disclosed a system and method of sewage treatment which consists of primary settling chamber e.g. septic tanks, peat filter and subsurface constructed wetlands system. Constructed wetland cell is generally planted with common wetland plants such as reeds (Phragmites), cattails (Typha) and Bullrushes (Scirpus).
Todd et. al., US Patent No. 5,486,291 (Ecological fluidized method for the treatment or polluted water, 1996) described a system which comprises of a fluidized bed container containing a layer of coarse media with a layer of fine media above it which has a buoyancy similar to the polluted water and particularly pumice rock having a high surface to volume ratio.
Hondulas US Patent No. 5,337,516 (Treatment of polluted water using wetland plants in a floating habitat, 1994) detailed the invention consisting of a wastewater treatment basin and a number of emergent wetland plants in floating containers adopted to float in the wastewater basin such that root system treat the wastewater. Morrision et. al US Patent No. 5,156,741 (Wastewater treatment system and method,
1992) describes a system of wastewater treatment and method incorporating a subsurface flow constructed wetland using turf grass. A recirculatory system is used for the subsurface wastewater along with aeration of subsurface water throughout the bed to enhance turf grass root growth in the treatment media. All the previous work related to this area of invention use certain types of plants in treatment of wastewater. However, the existing knowledge does not indicate the use of elephant grasses (Pennisetum purpurem) in combination with other plants such as cattails (Typha sps), canny lily (Canna sps), dwarf palm (Cyperus alternifolius), arrow arum (Peltandra Virginia), etc with the three types of media in one place with their varying configurations in the treatment zone. Objects of the invention
The main object of the present invention is to provide a system for the remediation of wastewater effluents, both the design and operation of which invokes special and temporal aerobic and anaerobic environments within the treatment system to facilitate a wide range of biotic and abiotic processes for removal of pollutants and in certain instances nuisance and pathogenic organisms - although the principal mode of removal is related to a wide range of microbially mediated redox conditions, which provide both aerobic and anaerobic conditions.
Another object of the present invention to provide a simple and effective method for treating wastewater mainly from domestic origin without requiring the addition of chemical
agents, nil energy requirement, low cost and least manpower intensive operation and maintenance.
Still another object of the present invention is a method for treating wastewater that does not require the installation of larger facilities conventionally used such as activated sludge treatment plant, trickling filter, aerators etc.
Still another object, of this invention is to provide a method for removing pollutants from wastewater by means of selected vegetation, widely available in certain areas of India and Indian sub-continent in temperate climate where the temperature ranges between 5-50° C.
Still another object, of the invention is use of elephant grasses (Pennisetum purpurem) individually or in combination with other plants from aquatic or semi-aquatic origin in cells of three types of media in the treatment zone.
Yet another object of this invention is to provide a method for removing pollutants by using different rocks and gravel types with a view to utilize their properties of adsorption and microbial activities. Yet another object of invention is to provide root system for secondary treated wastewater either in a media filled cell or without the media with only root system. Summary of the invention
The present invention is useful for treatment of wastewater. Here, wastewater means domestic wastewater, commercial establishment wastewater, animal wastewater, and agricultural runoff as well as storm water runoff. The system of the invention is a natural wastewater treatment system relying on ecosystem processes, such as nitrification and mineralization for contaminant removal. The system design also effects physical removal of fine suspended solids. This system consists of channel or cell or bed, filled with crushed stones, sand and crushed bricks or similar materials and planted with aquatic or semi aquatic plants species such as cattails (Typha spsj, elephant grasses (Pennisetum purpurem) canny lily (Canna sps.), dwarf palm (Cyperus alternifolius), arrow arum (Peltandra Virginia), elephant ear (Colocasia esculentd), sweet flag (Acorus calamus), woolgrass (Scirpus sps.), yellow flag iris (Iris pseudocorus), soft rush (Juncus effuses), and horsetail (Equisetum hyemale). System consists of three zones, namely the infed or inlet zone, treatment zone and outfed or outlet zone. In treatment zone, aquatic or semi aquatic plants are grown through which the treatment of wastewater is carried out using bacteria attached to the roots profile of the plants. Treated wastewater may be used for gardening, irrigation purpose or recreational uses such as bathing, golf parks, etc or discharged to the lakes, creeks, estuaries or sea. Treated water can also be used for residential purposes after conventional water treatment.
Accordingly, the present invention provides method for the removal of pollutants wastewater, said method comprising of (a) removal of suspended solids present in the wastewater, through a bed made up of bricks pieces, sand and or gravel after inlet, (b) subjecting the wastewater free from suspended solids to a treatment bed which has media with appropriate plant roots, housing bacterial population, from where the wastewater flows in overall horizontal direction, with flows divider having baffle arrangements for improved mixing of the effluent with the bacterial population to enable contact with the root profiles of the plant, (c) and collecting the treated water from the outlet for either use after treatment or without further treatment, or disposal on land, lakes, river or marine water body. In one embodiment of the invention, the untreated or primary treated wastewater is passed through an inlet to plant based media existing through the outlet, providing aerobic and anaerobic zones within the media for the different types of bacterial population to consume organic mater, and other pollutants in their respective zones.
In another embodiment of the invention, the system used comprises of wetland plants of one or more type contained in one or more beds dependant on the initial load of pollutants, in order to achieve the desired effluent quality at the outlet.
In yet another embodiment of the invention, a flow regime is established for the domestic wastewater through the plant roots embedded in the media comprising of crushed bricks, sand and gravel. In yet another embodiment of the invention, the horizontal level is maintained on an overall basis for the bed, the wastewater flowing through the bed in an up - down movement due to the presence of baffles in said bed.
In another embodiment of the invention, the plant root profiles are made to high contact times for better bacterial action. In yet another embodiment of the invention, the number of beds and design thereof depend on pollution load of wastewater flow through inlet with or without primary treatment.
In another embodiment of the invention, the requirement of a physical filtration unit is avoided by treating the wastewater to primary level before the inlet to the system.
In yet another embodiment of the invention, the bed topography is adjusted for the flow of wastewater without any pumping or energy input.
In another embodiment of the invention, additional number of beds are provided for further reduction of pollutants dependant on high inlet values.
In yet another embodiment of the invention, pollutants such as BOD, COD, TSS, N and P are removed.
In a further embodiment of the invention, N and P are removed in the form- of nitrogenous and phosphorous compounds.
In yet another embodiment of the invention, the plant varieties and thereby their root system differ in different beds. In another embodiment of the invention, the process comprises removal of pollutants from domestic wastewater, comprising physical removal of suspended solids present in the wastewater, through a bed made up of bricks pieces, sand and or gravel after inlet, a treatment bed which has media with appropriate plant roots, housing bacterial population, from where the wastewater flows in overall horizontal direction, flows divider with baffle arrangements for improved mixing of the effluent with the bacterial population and contacts with the root profiles of the plant, treated water from the outlet being collected for disposal.
In another embodiment of the invention, the untreated or primary treated wastewater flows through the inlet to plant based media thereafter exiting through the outlet, the media being provided with aerobic and anaerobic zones to consume organic mater and other pollutants in their respective zones.
In another embodiment of the invention, the treatment media functions to physically treat the wastewater by filtration and sedimentation of suspended solids and pathogens and sorption of phosphorous.
In another embodiment of the invention, dissolved organics in the wastewater are sorped by smaller media contained in the treatment zone, said smaller media consisting of sand due to the smaller pore size thereof which enables attachment of microbial bio-films which perform biological zone of the system.
The present invention also provides a system for removal of pollutants from wastewater, said system comprising three zones comprising a first inlet zone, a second treatment zone and a third outlet zone, said inlet zone being provided with a media comprising crushed bricks in a mixture with one of sand, stones and gravel, said inlet zone being connected to said treatment zone, said treatment zone also being with a media comprising crushed bricks in a mixture with one of sand, stones and gravel, a plurality of barriers being provided in said treatment zone to ensure the up and down flow of wastewater flowing therethrough, said treatment zone also being provided with a media comprising a rhizosphere being composed of said media, plant roots, plant rhizomes and associated microbial communities, said treatment zone being connected to a outlet zone provided with an outlet aperture for disposal of the treated wastewater.
In one embodiment of the invention, the side-walls of the treatment zone are constructed of concrete or brick work with plaster.
In another embodiment of the invention, the media in the treatment zone functions as a solid support for wetland plant growth. In yet another embodiment of the invention, a layer of small diameter material is placed at the top of the zones to enable better plant roots establishment. Brief description of the accompanying drawings
Figure 1 is a cross sectional view of the system of the invention.
Figure 2 shows the distribution of wastewater flow in the system of the invention. Figure 3 is a graph showing the performance evaluation of Elephant grass in terms of
BOD, TSS, TKN, and TP for a 2 day and a 3 day retention time.
Figure 4 is a graph showing the performance evaluation of elephant grass in terms of FC for a 2 day and a 3 day retention time.
Figure 5 is a is a graph showing the performance evaluation of cattails in terms of BOD, TSS, TKN, and TP for a 2 day and a 3 day retention time.
Figure 6 is a graph showing the performance evaluation of cattails in terms of FC for a 2 day and a 3 day retention time.
Figure 7 is a is a graph showing the performance evaluation of mix population in terms of BOD, TSS, TKN, and TP for a 2 day and a 3 day retention time. Figure 8 is a graph showing the performance evaluation of mix population in terms of
FC for a 2 day and a 3 day retention time. Detailed description of the invention
The technology of the present invention is a wastewater treatment facility. Deriving the knowledge from processes occurring in natural wetlands, the invented treatment system, though complex, attempts an integrated system in which water, plants, aquatic animals, microorganisms and the environment - sun, soil, sand, air interact to improve water and wastewater quality. The method of the invention treats wastewater by natural means i.e. without the addition of chemicals. It has been accomplished with the use of aquatic or semi aquatic plants that in association with the attendant microorganisms present with the plants root, absorbs and biodegrade organic materials and excess nutrients. These aquatic or semi aquatic plants have typically been located in natural marshlands. Treatment systems are being implemented in series on scales ranging from single family dwelling to medium size cities. Present applications include treatment of municipal wastewater, storm water, acid mine drainage, landfill leachate, and agricultural runoff.
The technology of the present invention uses filter media and natural plant root systems in combination in a form of subsurface flow of waste water for its treatment leading to removal of pollutants. Although, it can be constructed as either subsurface flow or free- water surface systems, both may be used in a treatment facility. Both are usually constructed as shallow basins or channels with a subsurface barrier to limit seepage. The free-water surface system most resembles natural wetlands with water flowing over the media surface at shallow depths. This type of system usually includes emergent or submerged plants depending upon depth of the treatment cells. In subsurface flow systems, wastewater is applied to a cell or channel filled with a porous medium such as gravel or crushed rock. The flow rate is regulated so that free wastewater does not rise to the surface. These systems may also include a variety of foliage or flowering plants growing in the porous medium to create a pleasing landscape environment. In both types, treated wastewater emerging from the system is either discharged to a surface stream or applied to land as irrigation water.
The most popularly used wetland plants to date are bulrushes, cattails, reeds and sedges. Wetland plants are morphologically and anatomically adapted to growing in a water- saturated substrate by the presence of internal gas spaces called aerenchymas throughout the plant's tissue. The most important function of the aerenchymas is to supply oxygen to the buried or submerged root and other parts of the plant species. The root and rhizomes, however, leak oxygen into the substrate creating oxidized and anoxic zones around the roots. Rhizomes, as referred to here, are roots and those portions of the root profile that give arise to new growth from the plant.
The presence of these oxidized and anoxic zones around the root profile creates a good environment for aerobic and facultative microorganisms in the rhizospheres. The aerobic bacteria consumes and reduce the suspended solids as well as other substances in wastewater that contribute to the reduction of oxygen in water referred as Biological Oxygen Demand (BOD). The conversion of ammonia compounds to nitrate compounds is function of these bacteria. A short distance away from the root or rhizome area lies in the anoxic zone, wherein oxygen is in short supply. Still at a distance away lies the anaerobic zone. In this area the facultative and anaerobic thrive, these bringing about the future reduction of suspended solids as well as the reduction of nitrate compounds, to nitrogen gas and more basic substances.
Treatment system (10) consists of three zones viz. Inlet zone (5), treatment zone (6) and outlet zone (7). Inlet zone is composed of crushed bricks or stones (3), sand (2) and crushed bricks (1). Treatment cell is the longest cell in length made up of same media as in
inlet zone. Beside these, it is also made of crushed stones (3), sand (2) and crushed bricks (1), side walls of the treatment system (10) can be constructed of concrete or brick work with plaster (11) depending upon the requirement of barriers (8) divides the treatment system (10) as wastewater (12) is distributed uniformly by measuring flow rate across the treatment system by the inlet zone (5). Wastewater then flows horizontally through the media filled channel /treatment zone (6) where it is treated by physical, biological and chemical processes. These processes take place in the rhizosphere (13), which is composed of the media, the plant roots, the plant rhizomes and the associated microbial communities. After treatment, the wastewater is collected in the outlet zone (7) and discharge through outlet aperture (14) and directed to further treatment process or to discharge into a waterway or to be used for irrigation purposes. Each component within the media filled channel or treatment zone play a role in the system.
Media in the treatment zone serve as many different purposes. The important role of the media is physical treatment of the wastewater. Filtration and sedimentation of suspended solids and pathogens occurs along with the sorption of phosphorous, and dissolved organics smaller media such as sand, are more effective in sorption and filtration than gravel or rocks because the smaller media contain smaller pore sizes area for the attachment of microbial bio- films which perform biological zone of the system. Some details of the media characteristics are presented in Table 1. The other function of the media is the solid support it gives wetland plants growth. In most cases a layer of small diameter material is placed at the top of the cells to allow for better plant roots establishment. Considering the media to use, the availability of surface for the attachment of the microbial population must be considered. Also of importance is the selection of influent wastewater to remain below the surface. If these factors are not optimized, there will be a reduction in the treatment efficiency or the system due to a lack of contact time between the wastewater and bio-film. Table 1 : Characteristics of media used in Treatment System
The plants used in the treatment system are known as emergent hydrophytes and macrophytes. The major portion of these plants (leaves and flowers) emerges above the media surface and is exposed to the air, while their roots and rhizomes are submerged
beneath the water and media. Commonly used plant species in types of system is bulrush (Scirpus), reeds (Phragmites), cattails (Typha) and elephant grass (Pennisetum). The extensive rooting structures of these species make viable options for wastewater treatment. Flowering plants such as canny lily (Canna sps.), elephant ear (Colocasia esculenta), and yellow flag iris (Iris pseudocorus) are also utilized to lesser extent. One of the important roles of these plants is the aesthetic upper that plants provide by covering the treatment bed /cell and controlling odors. These plants provide a habitat to many animals, including small mammals and birds. The plant cover also limits the amount of ponding water on the bed / cell surface that serves as breeding environments for nuisance insects such as gnats and mosquitoes. Plant roots and rhizomes provide surface for microbial growth and also aid in the filtration of solids. A major premise of the treatment method is that the plants are able to provide oxygen to the heterotrophic bacteria in the rhizosphere thereby allowing aerobic degradation of organic matter and nitrification to occur. Oxygen is transferred from the above ground parts of the plants species through airways to the roots and rhizomes. Like other aerobic organisms plants require oxygen for respiration, growth and protection from phytotoxins in the roots structure or zone. Without oxygen, the wetland plants would not be able to survive and grow. Aerated microzones are developed around the roots and rhizomes by the leaking of oxygen in the media zones. These oxidized areas in an otherwise anaerobic environment provide conditions in which the aerobic biological transformation occurs. Based on this, significant aerobic biodegradation of organic wastes and nitrification take place in these microzones.
The removal processes for each constituent of concern, occuring in treatment system of PhytoRid technology are given in Table 2. Table 2: Removal mechanisms for various pollutants
Most suspended solids are removed through sedimentation and filtration as vegetation in the treatment cells obstructs the flow and reduces velocities. In order to occur adequate filtration, the hydraulic conductivity of the cell is large enough to allow the wastewater to contact with the media. The remaining soluble organic material, left after sedimentation, is aerobically degraded by bacterial biofilm that is attached to the plants. In the treatment cells plant supply oxygen to the treatment floor through their roots, thereby promoting aerobic digestion of organic material. Some anaerobic degradation of organic material also occurs in the bottom sediments. Biofilms are formed as bacteria and microorganisms attach themselves to the plant roots, and the media filled in the treatment zone to form a biological filter from the water surfaces to the treatment floor. As wastewater passes through the thick growth of plants, it is exposed to this living biofilm, which provides a treatment process similar to that found in conventional treatment plants.
Treatment zone promotes the process of nitrification / denitrification which removes nitrogen from the wastewater. In simple terms, bacteria in the wastewater (Nitrosomonas) oxidize ammonia to nitrite in an aerobic reaction. The nitrite is then oxidized aerobically by another bacteria (nitrobacteria) forming nitrate. Denitrification occurs as nitrate is reduced to gaseous forms under anaerobic conditions in the litter layer of the treatment substrate. This reaction is catalyzed by the denitrifing bacteria such as Pseudomonas sps. and other bacteria. Plants also play an important role in nitrogen removal by providing biofilm attachment points and by supplying oxygen for nitrification in the root structures.
Phosphorous removal in the system is based mainly on the phosphorus cycle and can involve a number of processed such as adsorption, filtration, sedimentation, complexation / precipitation and assimilation / uptake. Pathogens removal in treatment system is achieved through a combination of natural die-off, temperature, sunlight, predatation and sedimentation. A proportion of pathogens are removed by sedimentation, especially those attached to particles. Biofilm filtering removes some of the pathogens by direct contact. Predatation occurs as the system provides a habitat for a variety of microorganisms, some of which are pathogen predators such as zoo plankton. The shallow water columns found in wetlands, allow the penetration of sunlight, which also destroys pathogens.
The following examples can be better understood with respect to prior art and present invention work can be utilized.
A rectangular bed or cell was made with a 2 m long, 1 m wide and 0.30 m depth filled up to 0.05 m of rocks, followed by 0.15 m of sand and 0.10m of crushed bricks. Inlet zone was filled with 0.15 m of sand and 0.15 m of crushed bricks. Plants species used for experiments were elephant grasses (Pennisetum purpurem).
The wastewater retention time was changed from 30-80 hrs. The wastewater flow rate to the cell or bed was regulated so that the water level was maintained at a height of 0.25 m from the bottom of the cell or bed. Approximately, 9 plants species were planted in the media filled bed or cell. Initially for 15 days, cell or bed were run with fresh water for stabilization of plants species. After stabilization of plants with freshwater, diluted wastewater was introduced with a schedule to stabilize the system slowly by increasing the concentration of the wastewater. The treatment cell was stabilized within 15 days. The wastewater was analyzed for 5 days biochemical oxygen demand (BOD5) at 20° C , total suspended solids (TSS), total kjeldahl nitrogen (TKN), total phosphorous (TP) and fecal coliform (FC) before and after treatment using standard methods of wastewater analysis(APHA, 20th edition, 1998).
Data in graphical form represents an average of at least 10 or more separate samples. For 2 days retention time, removal range of BOD, TSS, TNK, TP and FC was 75-80%, 68- 71%, 50-55%, 45-51% and 85-88% respectively. For 3 days retention time, of BOD, TSS, TNK, TP and FC was 78-84%, 70-75%, 52-57%, 47-55% and 89-92% respectively. Treatment efficiencies were higher in cell run for 3 days retention time as compared to 2 days retention time. Example 2
Cattails (Typha sps.) were used in place of Elephant grasses (Pennisetum purpurem ) in the set up described similar to experiment 1. The cell was run for 2 and 3 days retention time as in experiment 1. BOD reduction noticed was between 80-82 % after two days of retention compared to 81- 85 % for three days retention period. Similar trend was found for TSS, TKN, TP and FC. Removal ranges of Fecal Coliform are 90-94 % for 3 days retention time. Example 3
Elephant grasses (Pennisetum purpurem) and Cattails (Typha sps), both the plant species were used instead of only Elephant grasses as in experiment 1. The cell was run for 2 and 3 days retention time as in experiment 1. BOD, TSS, TNK, TP and FC reductions were between 82-86 %, 79-81%, 58-64%, 50-55% and 90-94% respectively after two days of
retention time where as removal ranges are for three days retention period 87-90%, 83-86%, 67-69%, 55-59% and 94-97% respectively.
The result obtained as presented above indicates that the optimum efficiencies can be achieved with a retention time between 2-3 days depending upon the level of treatment required and also the input loads.