This process is based on suspended growth system. Raw sewage is admitted to the inlet chamber for further processing for removal of large particles. Floating matter are trapped in the screen and removed with help of rack mechanism and put to conveyor belt for disposal through collection vehicle. Grit is removed in grit chamber and then sewage is admitted to Primary Clarifier for physical settling process. After primary settling, sewage undergoes biological process wherein biomass is activated to consume the BOD and biodegradable matter is converted to carbon dioxide and water vapor with help of aerobic bacteria. Level of dissolved oxygen and mixed liquor suspended solids are maintained with help of return sludge from secondary clarifier. Food to microbe ratio, substrate, mean cell resident time, decay rate of bacteria to enhance growth of aerobic biomass is maintained apart from aeration. After aeration, biomass attains saturation level of activity and then admitted to secondary clarifier for settling of biomass and accumulation of sufficient quantity of return sludge to help activation of biomass in aeration tank. Sludge produced is returned and wasted through raw sludge pump house or digester, wherein biomass is digested anaerobic ally and sludge concentration is increased from 1-2 % to 6-8%. Supernatant is returned back to PST for treatment. Sludge from digester is taken to centrifuge or filter press or sludge drying beds prior to final disposal and utilization as manure. Filtrate from drying beds/filter press/centrifuge would be returned to PST for further treatment. Bio-gas generated in digester could be taken to Gas Holder and utilized for power generation or flared. Description of functional requirement of each unit for CASP is discussed in brief.
1) Inlet Chamber 2) Screen Chamber 3) Detritor
4) Parshall Flume
5) Primary Clarifier (PC)/Primary Settling Tank (PST) 6) Aeration Tank
7) Secondary Clarifier (SC)/Secondary Settling Tank (PST) 8) Raw sludge pump house
9) Return sludge pump house 10) Filtrate pump house 11) Sludge digester 12) Gas holder
13) Sludge drying beds/ filter press/ centrifuge 14) Dual-Fuel gas engines/ gas flaring system
First three units are considered to be Preliminary Treatment Units. 4th, 5th and 6th are secondary treatment units and remaining are supporting units for MWwTPs. Though all the units are
designed to handle peak flows and other loadings, sludge digesters need to be designed for sufficient solids loading for effective functioning in the view of the high concentration of SS.
1. INLET CHAMBER : An Inlet Chamber is provided as receiving unit to STP with special arrangements. In case of normal operations, the sewage will be diverted to STP and in case of eventuality/ repair/ maintenance, a bypass arrangement is provided for discharge of sewage into receiving body.
2. SCREEN CHAMBER : Screening is an essential step in treatment of sewage to facilitate removal of large size floating materials like rags, plastics, tobacco pouches, wooden pieces etc. which otherwise would damage pumps and interfere with the performance of the subsequent units. This consists of bars placed parallel in vertical or inclined system across the flow to trap the floating material. Generally 20 mm spacing is considered to be adequate for STPs. However, considering the problem of pouches of tobacco passing thru’ it parallel to flow, finere screens with spacing of 12 mm followed in mesh pattern are in vogue. 50 mm x 10 mm thick flats are considered to be standard practice in India. Timer based racking mechanism cleans the screen periodically and put the screenings to belt conveyor system. Material is conveyed to collection van provided beneath the end of the belt conveyor for final disposal. CI Gates upstream and downstream to the screen are provided to control flow.
3. DETRITOR :Detritus Tank popularly known as detritor is provided after screen to remove heavy inorganic fraction of the sewage prior to physical and biological process. Difference in settling velocities between organic and organic content due to specific gravity is the basic principle followed to design the unit. Grit consists coarse sand particles, ash, clinker, egg shells and silt coated by organic matter (generally marginalized group use silt/ash for washing of utensils) etc. Grit is considered to be non-putrescible possessing higher hydraulic subsidence value than organic matter.
Grit removal, if not done, causes abnormal wear and tear to mechanical parts, scrappers/ pumps/
aerators etc. With proper hydraulic design, grit can be effectively removed from the sewage. To prevent loss of useful organic content and to prevent emanation of highly malodorous emissions from the removed grit, grit classifier based on reciprocating mechanism is provided.
4. PRIMARY CLARIFIER (PC)/PRIMARY SETTLING TANK (PST):PST removes organic matter (SS), scum and reduce BOD in quiescent condition. Brownian motion and settling characteristics of solids supported by proper overflow rate and weir loading rate enhances settlement of solids. It also settles BOD (integral part of SS) and hence reduces BOD load on subsequent units. Reduction in BOD load will have great effect on power consumption.
Plastics escaped from the screen and grit chamber would float on surface and can be removed by scum arm provided at top. Various functional parts like inlet, baffle, scrapper, outlet weir, sludge removal, telescopic valve, scum box are major components of the PST.
5. AERATION TANK : Microbes present in sewage can consume substrate (biodegradable organic matter) if favorable pH value and oxygen is available for faster consumption of organic matter. This unit is heart of the process. Various parameters are controlled to maximize efficiency.
Food to microbe ratio, power level, MLSS, means cell residence time, decay rate, volumetric loading, organic loading etc. Washout of microbes must be avoided. It is desirable to maintain DO level at 2 mg/l, which may diminish to 0.5 mg/l as an alarming level. Growth of filamentous bacteria and Nocardia must be avoided and controlled. Sufficient HRT shall be maintained to prevent washout.
Entire biomass gets activated in this system by means of suspended growth. Microbes can move to any place within reactor to get food and balance struck between microbes and substrate facilitates conversion of organic matter into active biomass.
6. SECONDARY CLARIFIER (SC)/SECONDARY SETTLING TANK (SST) :SST permits activated biomass to settle in form of sludge and sludge settled is pumped back to aeration tank to maintain MLSS. Due to highly active biomass, lumps of biomass are formed and higher ratio of volume to surface area enhances the settlement. Now, major portion of organic content is converted into biomass, which is settled as sludge. Supernatant comprise very low BOD and SS and other parameters. In case the treated sewage is to be discharged into natural water body, the parameters comply with the norms prescribed by pollution control board.
However, in case treated sewage is to be reused for some purpose like gardening, recreational activities, tertiary treatment shall be given.
7. RAW SLUDGE PUMP HOUSE :Raw sludge accumulated in PST is generally put to SDB or sludge digester. Considering low volumes, this is very small units and used as control room for PTU.
8. RETURN SLUDGE PUMP HOUSE :Performance of aeration tank depends upon effective function of this pump house. Generally return flow varies from 25 % to 50 % depending upon concentration of sludge. Considering its’ functional importance, 100% standby pump sets are provided in this unit.
9. FILTRATE PUMP HOUSE : Filtrate from sludge drying bed/filter press/centrifuge contains high BOD and SS and hence prior to disposal into natural water body, it shall be treated. Filtrate pump house pumps back filtrate to the inlet chamber of PST and hence only treated sewage is discharged from the STP. Though quantity of filtrate is very low in terms of percentage, overall quantum is in terms of few thousand liters and hence it must be treated prior to discharge.
10. SLUDGE DRYING BEDS/ FILTER PRESS/ CENTRIFUGE : Digested or raw sludge is dewatered and converted to utilizable form of sludge cake for manure. Sludge drying bed is most common form of concentrating the sludge and it is most common practice in India.
11. SLUDGE DIGESTER : Digestion of sludge is achieved in this unit to break the long chains and convert it to simple form ready to use as manure for improved absorbance as nutrient and reduce the harmful effects on environment. Initially, design of digester with mechanical mixing of sludge posed serious problems of civil structure and blasting. This has been overcome with help of gas mixing
system, being used in India for more than 10 years with satisfactory performance. Sludge volume also reduces in this system and area required for sludge reduces to a great extent.
12. GAS HOLDER :Gas produced in digester is collected here for further utilization like power generation with uniform feeding to Dual-Fuel Engines or flaring.
13. DUAL-FUEL GAS ENGINES/ GAS FLARING SYSTEM : Gas produced in the digester is enriched with methane and high calorific value. Dual fuel engines capable of using gas (80-90%) and diesel (20-10%) can yield power sufficient to run more than half of the operations of the STP. Though it is very attractive, STPs initially set up with an objective to have captive power generation either in CASP or UASB has not been successful on large scale.
Above steps briefly describes preliminary aspects of various process units. Sufficient details are discussed about basic principles and area of attention for successful performance of the STP based on CASP. There are many more forms of the CASP like continuous flow (as described above), plug flow, Sequential Batch Reactor etc., but this is most common form easy to maintain and operable and adopted in India.
This process is having following advantages
1) Rajkot is commercial capital Saurashtra region attracting number of skilled personnel and hence availability of skilled labor/ personnel is not a problem for this process 2) Availability of spare parts is also not a problem since all the equipments like
scrapper, aerator are already installed in existing STP and RMC officials are familiar with the equipment and are easily available
3) Process is having high level of reliability, well established reaction kinetics, amenability to control, can absorb shock loads.
4) Less potential of nuisance from mosquitoes/ fly/ odour
5) Easy access to components of the system for repairs and maintenance 6) More safety of operating/ maintenance personnel
7) Since the process is proven world wide, construction techniques are known and hence easy to construct (the word conventional itself suggests its’ large scale acceptability)
8) Easy to maintain the process parameters
9) Area requirement is very low amongst all 4 processes under consideration. Hence, future expansion is possible, particularly in urban area like Rajkot –where land is scarce.
Disadvantages of the process are:
1) Sludge digesters pose operational problems due to moving mechanical parts- some of the municipal authorities had overcome this problem by using non-moving mechanical mixers. E.g. Vadodara, Delhi
2) Skilled personnel (biologist) are required to maintain process parameters like MLSS, BOD, COD, SS etc.
Unit Sizing: Conventional Activated Sludge Process
All dimensions are in m unless otherwise specified
Sr. Name of Process Unit Size of the unit Power rqnt
(W+SB) HP 1 Preliminary Treatment Units
Inlet Chamber 1 No., 6.5 x 4.5 x 3.3 -
Screen Chamber 2 Nos., 11 x 1.5 x 1.5 6+6
Grit Chamber 3 Nos., 8 x 8 x 1.1 6+3
Parshall Flume 1200 mm throat As per Int.
2 Primary Clarifier 2 Nos., 32.5 dia 3.85 ht 6+0
3 Aeration Tank 2 Nos., 36 x 72 x 4.15 400+0
4 Secondary Clarifier 2 Nos., 37 dia 3.5 ht 3+3
5 Raw Sludge Pump House 1 No., 4.5 dia, 6 m ht 15+15 6 Return Sludge Pump House 1 No., 17.5 dia, 6 m ht 60+(30+30)
7 Filtrate Pump House 1 No., 3.5 dia, 5 m ht 5+5
8 Sludge Drying Beds 28 Nos., 20 x 20 without digester, 10 Nos. with digester
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9 Sludge Digester 2 No., 24 dia, 11 ht 50+50
10 Gas Holder 2 Nos., 20 dia, 6 ht. 6+0
11 Power Generator 18 x 12 x 6 -405