Sewerage Treatment plant- An Operational Overview
Let us take an example and understand it better. Explained below is the sewerage treatment plant in sharjah having a treatment capacity of 75000 cub m /day. The block diagram showing the operation of the plant as well as the pictre of settlement tank can be seen above.
Now let us move in detail to each section shown in the block diagram
The typical treatment works is divided into
Inlet area-Flow distribution chamber & Balancing tanks
Mixed Liquor Distribution Chamber
Secondary Treatment -Aeration Tank & Blower House
Final Settlement Tank
Tertiary Treatment-Backwash Wash water Pumping station & Rapid Gravity sand Filter
UV Treatment & Chlorination Plant
Effluent pumping station
RAS SAS
Return Liquor Pumping station
Sludge Treatment- Sludge Thickeners ,Sludge Dewatering
Filtrate Pumping Station
Inlet Area: The flow from different parts of Sharjah Municipality converges in inlet area. STP Phase 7 when commissioned will act as the gate way for sewerage for treatment purpose. The flow distribution chamber in the inlet area distributes the sewerage to different phases by controlled valves for treatment purpose. The operation of the valves is controlled by a PLC which monitors the inward flow and level of sewerage under treatment in different phases. If the load on the plant is surplus i.e. if the inward flow exceeds the treatment capacity the sewerage is stored temporarily in the balancing tank and thereby utilizing the plant capacity during night time. Balancing tank12 capacity is nom. 12,600m³ and split into 8 no. sections. All eight tank sections shall be connected by penstocks at high level, such that if required any one tank section can be isolated. The screens10 (3 nos) in the inlet area are capable of removing the raw sewerage and these grits are flushed out with the help of a grit elevator11. The detritor in the screen area consists of power operated grit collecting, cleaning and discharging mechanism.
Mixed liquor Distribution Chamber(MLDC) : The mixers here ensure homogenous mixing of the liquor with the sewage and the distribution chamber(DC) assists in dispensing the flow to different aeration tanks.
Aeration Tank(AET): The aeration tanks5 (4 nos) receives the sewerage distributed by the flow distribution chamber and it works in conjunction with the blowers6 (7 nos). Inside the aeration tank the sewage gets mixed up with air/oxygen to enable the growth of bacteria for decomposition. Submersible mixer units in aeration tank shall comprise of a motor driven blade type propeller. A single propeller type recirculation pump shall be installed within each aeration lane, to re-circulate a proportion of the mixer liquors back to the anoxic zone. Scum from the aeration tanks will discharge to the scum pumping station wet well, located on each pair of tanks.
Final Settlement Tank: The final settlement tanks7 (8 nos) are designed to remove settle-able biological floc (sludge) from mixed liquor fed to the tanks. The mixed liquor from fine bubble diffused air treatment process bioreactors reaches the centre column and then through inlet ports it moves into a centre energy-dissipating compartment. Ports in this compartment direct mixed liquor to the outer flocculating well. Settled sludge is collected hydraulically through a rotating conduit. Return activated Sludge (RAS) handling from each final settlement tank is via dedicated suction pipelines to the RAS/SAS pumping station. Wasting of surplus activated sludge (SAS) is from the final settlement tanks and utilizes the same suction pipelines as the RAS withdrawal.
Final effluent from the tanks discharges via peripheral launder channels fitted with V-notch weir plates. Settled effluent from FST reaches the tertiary lift pumping station which discharges the flow into the rapid gravity sand filters (RGSF) channels. Overflow from tertiary lift pump flows to the effluent pumping station
Backwash Washwater Pumping station: The washwater system delivers a maximum flow rate sufficient to provide 6 l/s at the hydrants and the Phase 7 process water requirements. Dirty back wash water shall be discharged to the dry well sump. The dry well sump pump shall discharge to the return liquors pumping station.
Rapid Gravity Sand Filter(RGSF): It is a part of the tertiary treatment process. The filters are designed to operate and backwash automatically. Backwashing shall be initiated automatically by time; level in the filter cell, or manually via the local MCC.
Chlorination and UV Treatment: The influent to each UV disinfection system is tertiary treated effluent from existing or new rapid gravity sand filters. After exiting the UV reactor(s) the disinfected effluent shall pass an overflow weir-penstock that maintains the minimum water level in the reactor. Downstream of the weir effluent will gravitate to the discharge pipe leading to the relevant Effluent Pumping Station sump. A chlorination system provided on the Phase 7 stream, dose chlorine at the Tertiary Pumping Station discharge (immediately upstream of the flow split to the RGSF), and again downstream of the new UV disinfection plant. A chlorine residual chart recorder is provided for final treated water at the pumping main leaving the site.
Effluent Pumping Station(EPS): A new effluent pumping station with split case centrifugal pumps connect into the existing effluent disposal system, with the potential for future connection to new effluent disposal pipelines inland. Sharjah Municipality is using the effluent for irrigation and excess of effluent is pumped into the sea.
Return Liquor Pumping station(RLPS): The return liquor pumping station receives liquor from Dirty backwash water from the RGSF, drain down water from the final settlement tanks and Site roads drainage. The close coupled centrifugal pumps in RLPS pumps the liquor to MLDC.
Return/Surplus Activated Sludge Pumping Station(RAS SAS)3: Activated sludge from the final settlement tanks will be delivered to the return/surplus activated sludge pumping station wet well. The RAS shall be pumped to the mixed liquor distribution chamber and the SAS shall be pumped to the sludge thickener feed pump wet well. Both RAS and SAS pumps are screw centrifugal type.
Sludge Treatment: The sludge treatment process takes surplus activated sludge (SAS) from Phase 7. This also includes additional flow from the scum removal system This sludge goes into a dewatering belt of sludge thickener PS where further thickening of sludge takes place. Sludge belt thickeners8 shall consist of an endless dewatering belt passing around a series of rollers. . Polyelectrolyte Mixing System in the sludge thickeners ensures intimate dispersion of conditioning polyelectrolyte agent with the incoming sludge. This sludge is then pumped by thickened sludge pumping station having belt pressers to thickened sludge storage tanks2(2 nos)(SST). Each tank is provided with fluidic mixers such that the tank contents remain fully homogenized when full. In the Sludge Dewatering section final dewatering is done by the sludge dewatering machines13 and sludge is taken to storage area and effluent moves to the filtrate pumping station.
Filtrate Pumping station(EPS): The Filtrate PS receives liquor from gravity belt thickener of sludge thickening PS and belt presses of thickened sludge PS
