Activated Sludge:

Activated Sludge:

Definition

Is defined as a ‘Suspension’ of microorganisms, both living and dead’ in a wastewater. The microorganisms are active by an input of air (oxygen) thus known as activated-sludge. Activate-sludge is that sludge which settle down in a secondary sedimentation tank  after the sewage has been freely aerated and agitated for a certain time in an Aeration tank.

Working Mechanism

The activated-sludge contains numerous bacteria and other microorganisms, when it is mixed with raw sewage saturated with oxygen, the bacteria perform the following function.

1. Oxidize the organic solids.
2. Promote coagulation and flocculation and convert dissolved, colloid and suspended solids into settle able solids. In practice the following operations are carried out in an activated - sludge process.
3. The sewage is given treatment in the primary sedimentation tank. The detention time is kept as short as 1-1/2 hours.
4. The settled sewage form the Primary Sedimentation Tank is the mixed with the required quantity of activated-sludge in the aeration tank. The mixture of activated-sludge and wastewater in the aeration tank is called ‘mixed liquor or mixed liquor suspended solids MLSS or MLVSS mixed liquor volatile suspended solids’.
5. The Mixed Liquor Suspended Solids is aerated for 6-8 hours in the aeration tank, called the hydraulic detention time according to the degree of purification. About 8 m³ of air is provided from each m³ of wastewater treated. The volumes of sludge returned to the aeration basin is typically 20 to 30% of wastewater flow air supply 8-10 m³ of sewage
6. The aerated Mixed Liquor Suspended Solids resulting in the formation of floc particles, ranging in size from 50 to 200pm.which is then removed in the secondary sedimentation tank by gravity settling, leeching a relatively clear liquid as the treated effluent. Typically greater than 99% of suspend solids can be removed in the clarification step.
7. Most of the settled sludge is returned to the aeration tank (and is called return sludge) to maintain the high population of microbes that permits rapid breakdown of the organic compounds. Because more activated-sludge is produced tan is desirable in the process, some of the return sludge is diverted or wasted to the sludge handling system for treatment and disposal.

Derivation of F/M Ratio:

Q = Flow of Sewage ( m³/day)
BOD =  organic matter (mg/l)
FOOD = Q ( m³/day) x BOD (mg/l)
FOOD = Q x BOD / 1000 (Kg of BOD/ day)
V = Volume of Aeration ( m³)
MLSS = Mixed liquor suspended solids (mg/l)
Micro-organisms = V ( m³) x MLSS (log/l) / 1000 = V x MLSS / 1000 (kg of MLSS in aeration tank)

F/M ratio:

A parameter of organic loading rate in the design aerated sludge parameter in the design of Trickling Filter in organic loading   rate = kg of BOD / m³-d

F/M ratio = 

F/M ratio = BOD / MLSS x t kg of BOD / Kg of MLSS/day
FM ratio varies between 0.2 -0.5 day^-1

• F/M ratio -0.5 day-1 has a good settleabilty of a sludge. ( even in some cases it can go to 1)
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• F/M ratio 70.5 day^-1 Food is more so the bacteria will move the effluent (failure of the system)
• If high F/M ratio, filamentous bacteria will also grow. They not settle easily because of long tails, get entangled with each other. Food to micro organism ratio(F/M) is a common used parameter in the activated-sludge process which is defined as

Activated Sludge Process Design

Design of Activated Sludge Systems:

Design of activated-sludge process involves details of sizing and operation of the following main elements.

1. Aeration tank (reactor)-capacity and dimensions.
2. Aeration system-oxygen requirements and oxygen transfer system.
3. Final sedimentation tank – (deifier)
4. Return activated sludge system.SV1
5. Excess activated sludge withdrawal system and subsequent treatment and disposal of waste sludge. Since the whole process takes place in al liquid medium the hydraulic regime essentially in the aeration tank and final sedimentation tank.
6. MLSS – a mixture of settled sewage + activated sludge dissolved oxygen < 2mg/l

Design Criteria

1. F/M ratio = 0.2 – 0.5 day -1 or 0.2 – 0.5 kg BOD's / kg MLSS – d
2. Detention time (aeration time) of sewage = 6 to 6 hours
3. MLVSS or MLSS = 1500 -3000 mg/l
4. Air supply = 10 m³/ m³ sewage treated
5. return sludge = 0.25 -10 of Q (influent sewage flow) Qr / Q = 0.20-0.30 = Vs/100Vs (Volume of sludge)
6. Depth = 3-5m
7. L=W ratio =5:1
8. SVI 50-150 ml/gm

Sludge Volume Index (SVI-TEST)

It is the measure of the settleability and compatibility of sludge and is made from a laboratory column setting test.

Definition

The sludge volume index is defined as ‘the volume in mm occupied by 1 gm of sludge after it has settled for a specified period of time’ generally ranging from 20 min to 1 or 2 hr in a 1 – or 2-l cylinder. One-half hour is most common setting time allowed to the mixed liquor to settle for 30 min. ( larger cylinder is desirable to minimize bridging of sludge floe and war effects). Take the reading let Vs is the settled volume of sludge (ml/l) in 30 min.

* If SVI is 50 - 150 ml/mg, the sludge settle ability is Good.

Return Activated Sludge System:

1. The activated sludge form the underflow of the final setting tanks should be returned to the inlet of the aeration tanks at a rote sufficient to maintain the MLSS concentration at the design value.
2. The flow are needed for return-sludge is determined form the incoming sewage flow rate and the concentration at which the sludge is with drawn form the final setting tanks.

Hence a simple measure of the underflow concentration form the setting tanks is required. The parameter conventionally employed for this purpose the sludge volume index, SVI which is defined as 4 the volume occupied by sludge containing 1.0g of sludge soiled (dry weight) after 30 min setting and thus it has ht units ml/g. Some time represented as SDI i.e sludge density index. Once the SVI and operating MLSS concentration (x) is known, the required rate of activated sludge return can be determined

R = 100 / [ 106/ (x) (SVI) -1] where r = return sludge flow rate as a % age of incoming sewage flow.

SEDIMENTATION:

It is the removal of solid particles form a suspension by settling under gravity.

CLARIFICATION:

It is a similar termn which refers specifically to the function of a sedimentation removal.

THICKENING:

It means the separation of water from Suspended Solids

where R = return sludge flow rate (ML/D) for Q in ML/D)

SURFACE GEOMETRY OF FINAL SEDIMENTATION TANKS:

VARIATION OF THE ACTIVATED SLUDGE PROCESS:

1. Activated sludge was introduced in 1941 and has undergone many variations and adaptations.
2. The main objective of many modifications has been to increase the loading capacity of the basic plug flow activated sludge plant by provision of optimum condition design parameters for different variations are summarized in table. It is worthy of note that 5 modifications tapered aeration step aeration the CMAS process, the pure oxygen system and the deep shaft process all aim at either the improvement of oxygen transfer efficiency t the efficient distribution of available oxygen to match demand. A flow sheet of most of the commonly used variations is similar to that of CAS (Conventional Activated Sludge).

CONVENTIONAL ACTIVATED SLUDGE:

Volumetric loading = kg of BOD
                                      m³-d
Aerial loading rate = gm of BOD
                                      m³-d
Td = V/Q in days and grater than 5 days.

ALGAL-BACTERIAL SYMBOPSTS:

The combined and mutually- been facial action of algae and bacteria in this process is called algal-bacterial symbioses.

• Shock loading (CSTR)
• BODu

Aerated Lagoons:

Aerate lagoons are activated sludge units operated without sludge return. Historically they were developed from waste stabilization ponds in temperate climate where mechanical aeration was used to supplement the algal oxygen supply in winter. It was found, however that soon after the aerations were put into operation the algal disappeared and the microbial flora resembled that of activated sludge. Aerated lagoons were now usually design as completely mixed not-return activated sludge units. Floating aerates are most commonly used to supply the necessary oxygen and mixing power.

Sludge Treatment:

Anaerobic sludge treatment cell Primary Sedimentation Tank and Secondary Sedimentation Tank are basically organic these can treated to aerobic.

• Anaerobic ponds and septic tank are for waste water treatment .
• Sludge treatment = Anaerobic sludge treatment.

COLD DIGESTION:

• Two stage digestion up
• High rate digestion up
• Fixed film processes. A swm zone

SLUDGE DIGESTION:

SLUDGE: the concentrated impurities settled at the bottom of the flower bed of sedimentation tanks.

Digestion:

To decompose or breakdown by heat and moisture or chemical action. (to invent food equable forms)

Sludge treatment:

Aerobic digestion it is defined as ‘it is the use of microbial organisms in the absence of oxygen I for the stabilization of oxygen materials by conversion to mean and inure produce including CO₂.

Organic matter + H₂O amoebas CH₄+ CO₂ + NH₃+ H₂S + heat

Benefices of anaerobic digestion. Types of anabolic detectors. It’s of two types:

• Conventional (stranded) or low-rate digester or cold digester.

• High rate digesters / two stage digester are characterized by continuous miring except at time of sludge with draw.

Biological Wastewater Treatment Method

Biological Wastewater Treatment Method

It comprises of the following sub processes:

1. Aerobic biological processes
2. Anaerobic biological processes
3. Facultative biological processes

1. Aerobic Biological Processes

Aerobic Biological Processes are those where sufficed amount of dissolved oxygen is required into the wastewater to sustain aerobic action, as one of the major polluting effects of wastewater on streams results form the depletion of dissolved oxygen by the action of aerobic organisms in degrading the organic content of the waste. Practical aerobic biological treatment processes seek to to this, within the constraints of available land area and economic resources available to construct and operate treatment works.

          2. Anaerobic Biological Processes

Anaerobic Biological processes are those where micro-organisms oxidize organic matter in the completed absence of dissolved oxygen. The micro-organisms take oxygen form inorganic salts which contain bound oxygen (Nitrate NO₃, Sulphate So₄^2-, Phosphate PO₄^2-). This mode of operation is termed as anaerobic processes. Sufficiently fore dissolved oxygen is either physically difficult or economically impracticable to transfer into the wastewater to sustain aerobic action to biodegrade strong organic wastes.

Aerobic Biological Treatment Processes

There are five types of aerobic biological treatment processes used to treat municipal sewage:

1. Tricking filters
2. Rotating biological contactors (filter)
3. Activated sludge
4. Oxidization ponds
5. Aerated lagoons (used for pre treat ion industrial effluent)

Biological Treatment systems

1. Attached growth processes
2. Suspended growth processes
3. Dual (hybrid) biological treatment processes.

Trickling Filter

Introduction to trickling filter system:

The trickling filter is like a circular well having depth up to 2 meter filled with granular media like stone, plastic sheets and redwood, slag, slate.

The first tricking filter was placed in operation in England in 1893. The concept of a tricking filter grew from the contact frets which were water tight basins filled with broken stones. The limitation of the contact filters included a relatively:

1. High incidence of clogging,
2. The long retention time (a typical cycle required 12 hours, 6 hours for operation and 6 hours for resting) and relatively
3. Low loading rate. life cycle/ biological circle of bacteria: 20-30 minutes. The tricking filter itself consists of a bed of coarse material, such as stones, slates or plastic materials (media) over which wastewater is applied. Because the micro-organisms that biodegrade the waste form a film on the media this process is known as an attached growth process.

Tricking filters have been a popular biological treatment processes. The widely used design of trickling filters that can last for many years is:

Design diameter of Rock filters = 60m (2007t) and for Rock size Design diameter = 25 to 100mm

Types of Sedimentation Tanks

Typical Primary Sedimentation Tank

Rectangular Horizontal Flow Tanks

These are most commonly used for primary sedimentation, since they

• Occupy less space than circular tanks.
• They can be economically built side-by-side with common walls.
• The maximum forward velocity to avoid the risk of scouring settled sludge is 10 to 15 mm/s (06 to 09m/min or 2 to 3 ft/ min), indicating that the ratio of length to width l/w should referrals be about.
• The maximum weir loading rate, to limit the influence of draw-down currents, is preferably about 300 m³/d-m, this figure is sometime increased where the design flow is great then 3 ADWF.
• Inlets should be baffled to dissipate the momentum of the incoming flow and to assist in establishing uniform forward flow.
• Sludge is removed by scraping it into collecting hoppers at the inlet end of the tank.
• Sum removal is essential in primary sedimentation tanks because of the grease and other floating matter which is present in wastewater. The sludge serapes can return along the length of the tank a the water surface. As they move towards the outlet end of the bank, the flights then move the sum towards a skimmer located just upstream of the effluent weirs.

Circular Radial Flow Tanks

These are also used for primary sedimentation.

• Careful design of the inlet stilling well is needed to active a stable radial flow pattern without causing excessive turbulence in the vicinity of the central sludge hopper.
• The weir length aroid the perimeter of the tank is usually sufficient to give a sates factory weir loading rate at maximum flow, but at low flows, very low flow depths may result.
• To overcome the sensitivity of these tanks to slight errors in weir level and wind effects, it is common to provide v-much wares.
• Sludge removal is effected by means of a rotary sludge scrapper who moves the sludge into a central hopper, form which it is with drown.
• Scum removal is carried out by surface skimming board attached to the sludge scrapper mechanism and positioned so that scum is moved towards a collecting hopper at the surface.

Up Flow Tanks:

• up flow tanks, usually square in plan and with deep hopper bottoms, are common in small treatment plants.
• Their main advantage is that sludge removal is cared out entirely by activity and no mechanical parts are required for cleaning them.
• The steeply sloping sides usually to to horizontal concentrate the sludge at the bottom of the hopper.
• Weir loading rate is a problem only at low flows. So that v-match weirs are desirable.
• The required up flow pattern is maintained by weir troughs.
• True up flow tanks have an disadvantage on that hydraulic over loading may have more serious effects than in horizontal flow tanks.
• Any practical with a velocity lower than VP = Q/A will not removed in an up flow tank, but will escape in the effluent.

In a horizontal flow tank assuming that such particles were uniformly distributed to the flow, particle with Vp=Q/A still be removed in proportion.

Primary Treatment of Wastewater & Types of Primary Sedimentation Tanks

Primary treatment is often called clarification sedimentation or setting. This ‘unit operation’ where the wastewater is a allowed to settle for a period (≈2h) in a setting tank and so produce a somewhat clarified liquid effluent in one stream and a liquid-solid sludge (called primary sludge) in a second steam. 1. Objectives of Primary Treatment of Wastewater To produce a liquid effluent of suitably improved quality for the next treatment stage (i.e.) secondary biological treatment. To active a solids separation resulting in a primary sludge that can be conveniently treated and disposed of. 2. Benefits of Primary Treatment The benefits of primary treatment include Reduction in suspension solids Reduction in BOD Reduction in the amount of waste activated sludge (WAS) in the activated-sludge plant. Removal of floating materials (oil and geese). partial equalization of flow rates and organic load.

3. Design Criteria for Primary Treatment Plants Traditionally, the design criteria were Basic overflow rate (surface loading m3/m2-d) Depth Surface geometry Hydraulic retention time Weir rate ( m3/d-m) The above criteria are physical and while they may be adequate for design of the tank they sues nothing about the performance and operation of the sedimentation process. Therefore, additional parameters called performance criteria were established to monitor and improve the day-to-day performance and operation of the sedimentation process. Influents flow rates and their variation (daily variation) influent waste strength rates and its variation. Recycle influent streams. From activated – sludge or Septic. Supernatants form sludge de watering. Washings from tertiary filter processes.  They check efficiency of removal. Septic may have a BOD's value 30 times greater than municipal raw wastewater. Supernatants form anaerobic digestion process or filtrate back washing may also be very high in waste strength. As such the performance of a primary clarification is not solely dependent on influent flow variations.

Preliminary Treatment of Water by Screening, Grit Removal & Sedimentation

1. Screening: The first unit operation generally encountered in wastewater treatment plants is screening. A screen is a device with openings, generally of uniform size, that is used to retain solids found in the influent wastewater to the treatment pant. The principal role of screening is to remove coarse materials from the flow stream that could: Damage subsequent process equipment. Reduce overall treatment process reliability & effectiveness, or Contaminate waste way There are two types of screening processes Manually Operated Automatically Course screens (Bar Racks) Fine screens Micro screens. Coarse Screens Micro Screening Fine Screens 6 to 150mm < 0.5 k less then 6mm

Design of screening chamber: The objective of screens is to remove large floating material and coarse solids from wastewater. It may consist of parallel bars, wires or grating placed across the flow inclined at 30o-60o. According to method of cleaning; the screens are hand cleaned screens or mechanically cleaned screens. Whereas, according to the size of clear opening, they are coarse screens (≥ 50 mm), medium screens (25-50 mm) and fine screens (10-25 mm). Normally, medium screens are used in domestic wastewater treatment. Dimensions of an approach channel Used in wastewater treatment is mostly rectangular in shape. Wastewater from the wet well of the pumping station is pumped into the approach channel from where it flows by gravity to the treatment plant. Its main function is to provide a steady and uniform flow after pumping. Select the size of bar/clear opening, say 10mm x 10 mm (medium screens) No. of bars; {(n + 1) + (n) = B}, and {Be = B – (width of bar)(n)} Head loss, hL = 0.0729 (V2 – Vh2) ------ {Vh 0.75m/sec, hL ≤ 0.5 ft} For perforated plate; amount of screening produce = (1-2) ft3/MG Length of bar; L = D/sinθ, and Lh = L * cosθ. Screen chamber. Lc = inlet zone (2-3 ft) + Lh + outlet zone {outlet zone = width of p plate + (0.5-1.0 ft)}  2. Wastewater treatment through Coarse Solids Reduction: As an alternative to coarse bar screens or fine screens, comminutors and macerators be use to intercept coarse solids and grind or shred them in the screen channel. High – speed grinders are used in conjunction with mechanically cleaned screens to grin and shred screenings that are cit up into a smaller, more uniform size for return to the flow stream for subsequent removal by downstream treatment operations and processes, comminutiors, macerators and grinders can theoretically eliminate the messy and offensive task of screening handling and disposal. Comminutors – small WWT (0.2 m3/s or 5 MGD) 6 - 20 mm (0.25 N 0.77in) 1. Comminutors: Comminutors are used commonly in small wastewater treatment plants, less than (0.2 m3/s or 5M6D). They are installed in a wastewater flow channel to screen and shred material to sizes from 6 to 20 mm (0.25 to 0.77 in) without removing the shredded solids from the flow stream. 2. Macerators: Macerators are slow speed grinders that typically consist of two sets of counter rotating assemblies with blades. The assemblies are mounted vertically in the flow channel. The blades or teeth on the rotation assembles have a close tolerance that effectively chop material as it passes through the unit. 3. Grinders: High speed grinders typically referred to as fiammermills, receive screened materials from base screen. The materials are pulverized by a high speed rotation assembly that wets the materials passing through the unit. Flow equalization is method used to overcome the operational problems and flow rate variations to improve the performance of downstream processes and to reduce the size & cost of downstream treatment facilities. To prevent flow rate, temperature, and contaminant concentrations from varying widely, flow equalization is often used. It achieves its objective by providing storage to hold water when it is arriving too rapidly, and to supply additional water when it is arriving less rapidly than desired. A smaller the screen opening, greater will be the amount of material screened. In order to improve the performance of a reactor, particularly the biological processes, it is required to equalize the strength of wastewater and to provide uniform flow, an equalization tank is design after screen and grit chamber. This may be in the line-off or off-line, as shown in the figure; 3. Grit Removal system from Wastewater: It is a Unit operation (physical). Removal of grit form waste Swater may be accomplished in grit chambers or by centrifugal separation of solids. Grit chambers are designed to remove grit, consisting of sand, gravel, sanders, or other heavy solid materials that have specific gravities or setting velocities substantially greater than those of organic particles in wastewater. Grit chambers are most commonly located after the bar screens and before the primary sedimentation. These are just like sedimentation tanks, design mainly to remove heavier particles or coarse inert and relatively dry suspended solids from the wastewater. There are two main types of grit chambers like rectangular horizontal flow types and aerated grit chambers. In the aerated grit chamber the organic solids are kept in suspension by rising aerted system provided at the bottom of the tank. a. Purpose of Grit Chamber Grit chambers are provided to: Protect moving mechanical equipment from abrasion and accompanying abnormal wear. Reduce formation of heavy deposits in pipelines, channels and conduits. Reduce the frequency of digester. b. Types of Grit Chamber Horizontal flow (Rectangular or square) (configuration type) Designing a Rectangular horizontal flow type grit chamber: Cross-sectional area, Ax = (Qdesign / Vh) for each unit (Vh ≈ 1 ft/sec), depth ≈ 3-5 ft Assuming (tD = 1-2 minutes), determine the length L = Vh * tD (Add 10% additional) Check the SLR (1200-1700 m3/m2-day) and Vs (≥ 0.01 m/sec). Grit produced is about 1.5 ft3/ML of wastewater flow. Add to depth {1ft FB + grit} Aerated Grit Chamber Designing an Aerated grit chamber: Assume a “tD” (3-4 min), determine the volume of the basin. Assume a depth (D = 08-15 ft), determine the surface area of the basin. And check the SLR (1200-1700 m3/m2-day) The amount of grit produced is about 1.5 ft3/ML of wastewater flow. Add suitable depth fro grit and free board. Calculate the amount of air required (0.2-0.5 m3/min/m length of the tank) Vortex type Grit Chamber 4. Primary Sedimentation Tank Sedimentation or setting tanks that receive raw wastewater prior to biological treatment are called primary tanks. The objective of the primary sedimentation tank is to remove readily settleable organic solids and floating material and thus reduce the suspended solid content. Efficiently designed and operated primary sedimentation tanks should remove from 50 to 70% the suspended solids and 25 to 40% of the BOD. Description: Sedimentation is carried out in variety of tank configurations including. 4.1 Circular sedimentation tank Most common-have diameters from 3 to 60m (side water depth range from 3 to 5m) 4.2 Rectangular sedimentation tank Length ranges 15 to 100m an width from 3 to 24m (length/ width ratio 3:1 to 5:1) 4.3 Square sedimentation tank They may be flat bottomed or hopper bottomed. Wastewater enters the tanks, usually at the center, through a well or diffusion box. The tank is sized so that retention time is about 24 (range 20 minutes to 3h). In the quiescent period, the suspended part ides settle to the bottom as sludge and are raked towards a central hopper from where the sludge is withdrawn. Primary sedimentation is among the oldest wastewater treatment process. Traditionally the design criteria for sizing setting tanks are Average overflow rate: 30 - 50 m3/m2/d (Typical 40 m3/m2/d) [800-1200 gal/ft2-d (Typical 1000 gal/ft2-d] Peak hourly overflow rate: 50 - 120 m3/m2/d (Typical 100 m3/m2/d) [2000-3000 gal/ft2-d (Typical 2500 gal/ft2-d] Weir loading rate: 1.5 - 2.5h (Typical 2.0 h) [1.5 - 2.5 h (Typical 2.0h)] Rectangular Sedimentation Tanks Circular Sedimentation Tanks Depth 10-16 ft (Typical 14) 3 - 3.9 m (Typical 4.3) 10-6 (Typical 14)3.39m (Typical 4.3 m) Length 50-300 ft (Typical 80-30 ft) Diameter 10-200 (Typical 40-150ft) 3-60 m (Typical 12-45m Flight speed 2-4 ft/min (Typical 3 ft/min) or (Typical 0.9 m/min) Scraper’s speed 0.02-0.05/min (Typical 0.03 Rev/min) Bottom Slope 1in/ft or Typical 0.9m/m check 1.12 ft Always provide minimum of 2 sedimentation tanks. Sludge accumulation is same for both. Sludgy accumulation 2.5kg of wet solids per m3 of flow.

Municipal Wastewater Treatment Plants & Wastewater Management

Objectives of Wastewater Treatment To kill the pathogens To improve the quality of wastewater To avoid unhygienic conditions To protect the aquatic life from the toxicity wastes To make the wastewater usable for agricultural, aquaculture etc There are three constituents and interrelated aspects of waste water management.

Collection of wastewater Collection of domestic wastewater is best achieved by a full sewerage water drain age system. Unfortunately this method is most expensive and there is relatively few communities in hot climate which afford it. A modern hygienic method of night soil collection is the only realistic alternative. Treatment of wastewater Treatment is required principally to destroy pathogenic agents in sewage or night soil and to encore that it is suitable for whatever re-use process is secreted for it. Re-use of wastewater (Recycling of wastewater) The responsible re-use of night soil and sewage effluent is aqua culture and crop irrigation can make a significant contribution to a community food supply and hence it’s general social development. The best example is china where over 90% of waste after treatment is applied to land Performance criteria for Wastewater Treatment Management System The ideal system would satisfy all of the following criteria. Health criteria Water Recycling criteria Ecological criteria Nuisance criteria Cultural criteria Operational criteria Cost criteria Health Criteria: Pathogenic organisms should not be spread either by direct contact with right soil or sewage or indirectly via soil, water or food. The treatment chosen should achieve a high degree of pathogen destruction. Re-use/Recycle Criteria: The treatment process should yield a safe product for re-use, preferably in aquaculture and agriculture. Ecological criteria: In those cases land the should be considered exception when the waste cannot be re-use, the discharge of effluent into a surface water should not exceed the self-purification capacity of the recipient water. Nuisance Criteria: The degree of odor release must be below the nuisance threshold. No part of the system should become aesthetically offensive. Cultural Criteria. The methods chosen for waste collection, treatment and re-use should be compatible with local habits and social (religious) practice. Operational Criteria: The skills required for the routine operation and maintenance of the system components must be available locally or are such that they can be acquired with only minimum training. Cost criteria: Capital and running costs must not exceed the community’s ability to pay. The financial return from re-use schemes is an important factor is an important factor in this regard. However, no one system completely satisfies all these demands. The problem becomes one of minimizing disadvantages.