Units of The Plant
- Screening – To screen all the debris and floating matter which could clog the pumps and rotors. A screen basket of 0.6 x 0.6 x 0.45m with SS square tubes and SS weld mesh with 20mm openings. The basket is provided with a hook type ring for lifting the basket from the Equalization tank and emptying the debris which will be sent out along with municipal solid waste.
- Equalization tank – To provide for storage for sewage, the primary function is to provide for buffer capacity, enable pH correction. Aeration to be provided to keep the sewage in mixed condition.
- Sewage transfer pumps – Sewage transfer pumps to have grinder to shred the sewage solids into finer sizes and be able to handle upto 40mm solids.
- Sequential batch reactor (SBR) tank – RCC unit built below the ground with necessary inlet distribution arrangement, aeration system with fine pore diffusers, aerator grid. The system to be designed as completely mixed system. Two blowers for the aeration system shall be provided with one working and one standby blower. All blowers shall be provided with acoustic enclosure.
- Sludge tank / sump – Rotational moulded HDPE tank / RCC sump for collecting excess sludge for processing further in centrifuge or filter press or carting away for disposal.
- Decant / Secondary effluent collection sump – RCC unit built below the ground to provide for suitable capacity.
- Filter feed pumps – Two filter feed pumps – One working and one standby pump of suitable discharge capacity for filtering treated effluent through multi grade sand filter and activated carbon filter.
- Multi grade / Pressure Sand filter – FRP/ MS (FRP Lined) vessel multi grade sand filter with multi port valve / frontal piping for reducing turbidity in the treated effluent.
- In the secondary loop, Sodium hypo chlorite is dosed through an electronic dosing pump. Hypo chlorite is let into treated water tank and collection tank to kill and control the growth of bacteria.
- Activated carbon filter – FRP vessel activated carbon filter with multi port valve / frontal piping for adsorbing organics from the treated effluent. Granular activated carbon of Iodine number 800 grade media to provide for 10min contact time.
- Treated effluent sump – RCC unit for storage of treated water for reuse in landscaping and flushing.
- Chlorination plant – Online metered dosing pump type chlorine dosing system for chlorinating water used for flushing only.
The sewage treatment plant is based on Sequential batch reaction process. The processes of BOD removal, nitrification/de-nitrification, phosphorus removal and sludge separation are achieved continuously in a single SBR tank. The process operates on the batch inflow and batch outflow principle. The phases of aeration, settling, decantation occur sequentially and are controlled by a PLC.
The raw sewage is collected into Equalization tank, kept in missed condition by using coarse bubble diffusers / level controlled pump / submerged blower which work based on the water level in the Equalization tank. The equalized sewage is pumped from the Equalization tank to the SBR tank.
The sewage is aerated for BOD removal only or for nutrient removal also in addition in the SBR tank time of aeration and the quantity of air vary for the respective treatments. During the aeration phase the BOD removal and if desired nutrient removal such as reduction of nitrogen and phosphorus will be achieved. The process design by default shall be for the nutrient reduction, however if only BOD removal is required the number of aeration hours may be reduced, thereby the cycle time of the SBR can be reduced from the total of 8 hours to 6 hours.
A typical operation pattern of the SBR is as follows:
|Sl No||Phase||Time Required||Activity|
|1||Fill||0.5 - 1 hr||The raw sewage from the equilisation tank is pumped to the SBR tank.|
|2||React||2 – 4 hr||The sewage undergoes biological oxidation with the introduction of the diffused air for oxygen source and the micro organisms in the Sludge settled in the SBR tank use the raw sewage organics as food and transform to stable organic matter. The aeration phase is generally shorter for BOD removal and higher if nutrient removal is to be achieved.|
|3||Clarify||0.75 – 1 hr||The aeration in the SBR tank is shut off, no sewage is added leading to quiescent conditions in the SBR tank which enables the settlable matter in the mixed liquor to collase and settle down. No chemical dosing is required as the sludge in the SBR is highly oxidized and the separation of solid and liquid phase is brought about naturally.|
|4||Decant||0.5 – 2 hrs||The clear supernatant in the SBR is allowed to be decanted into a clarified water tank by the use of motorized valves which are timer controlled / relay controlled / PLC controlled. The Decant phase denotes the end of a cycle.|
|5||Filter||12 – 20 hours||The decanted supernatant is collected, filtered using a multi grade sand filter and activated carbon filter to reduce the turbidity and dissolved organics in the treated water.|
|6||Disinfect||The filtered water is disinfected using UV light to achieve disinfection and them dosed with Sodium Hypo chloride solution for achieving residual disinfection.|
The SBR process is an extension of the Activated sludge process carried out in batches. The SBR process uses the latest biomass conditioning technology. This enables the SBR system to attain nutrient control without the addition of chemicals and to out perform continuous “flow through systems”.
Choice of Food / Mass ratio:
Although the F/M ratio in the SBR system continually changes during the Fill and React phases, the average values are comparable to those in activated sludge systems. Currently most installed SBR systems are based on low load design, (in order to obtain a high degree of BOD removal and nitrification), typically with a F/M ratio of 0.05 to 0.15 kg BOD/kg MLSS. This is comparable to an extended aeration type process.
Conventionally loaded activated sludge systems operate in the range of 0.15 – 0.40 kg BOD /kg MLSS, depending on the requirements for discharge. SBR can also be designed in this range but higher the F/M ratio, the higher the excess sludge production and a decreasing nitrification and stabilization.
ADVANTAGES OF SBR PROCESS OVER CONVENTIONAL TREATMENT PROCESS
- Single unit operation and less equipment, thereby reducing capital and O&M costs.
- No Settling Tank and return sludge pumps.
- Allows storage of active sludge mass and prevents it from being lost.
- The state-of-the-art PLC control system provides a completely automatic operation of plant.
- Single integrated bioreactor reduces built-up land requirement.
- The automatic operation results in minimal supervision requirements, contributing to lower operating and maintenance costs.
- Higher treatment efficiency due to biological reduction of nitrogen and phosphorus results in better treated effluent quality.
- Low F/M ratios result in significantly less waste sludge and due to its complete stabilization, sludge can be directly disposed into environment without further treatment.
- Aerobic conditions throughout the bioreactor and absence of settling tank eliminates the potential for odor.
- Complete sludge stabilization improves its structure and mechanical dewatering characteristics.
- Cyclic operation of process allows enrichment of a variety of microbial strains which characterizes health of the aerobic reactors.
- Due to retention of excess sludge mass, bacteria maintain high metabolic rate even in low BOD regime.
- The plant is conducive for operation on low load conditions due to low occupancy rates in the initial days of a project.
The sludge generated in the system is pumped on a regular basis to a sludge holding tank for collection and further process or for carting away.
The sludge can be dewatered on site using a slow speed centrifuge or a filter press. However due to the advantages and ease of operation, it is preferred to use a centrifuge for sludge dewatering.
Control and automation:
The SBR system is basically dependent on a PLC for the control of the various functions of the SBR tank such as aerator switch on and off, opening of motorized valves etc. apart from these the following also are automated.
- The aeration for the equalization tank is controlled by a solenoid valve fitted on the inlet of the manifold for controlling the flow of air when the sewage is collected. Alternatively submerged aerator with a float switch can be installed for aeration and keeping the sewage solids in a mixed condition.