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Water Pollution Control Division /  Description of Wastewater Treatment Facilities

Preliminary Treatment Facilities | Primary Treatment Facilities |
Secondary Treatment Facilities | Tertiary Treatment Facilities |
Effluent Disinfection


Preliminary Treatment Facilities

All of the sewage flowing into the plant first enters the Pumping Station (Structure No. 1). The Pumping Station has four mechanically cleaned screens, which remove large debris from the entering flow. The screenings are then transported by a belt conveyor to a container to be loaded onto trucks for disposal. Pre-chlorination facilities have been provided within the influent chamber for odor control.

Four pumps are available to pump the sewage to three aerated Grit Chambers (Structure No. 2). The grit is removed from the chambers by bucket scrapers and elevators. Three blowers provide the air for the Grit Chambers. The grit removed from the bucket scraper is discharged onto a reversible belt. The grit is discharged to a container to be loaded into a truck for disposal.


Primary Treatment Facilities

Four rectangular equalization tanks accept flow from the grit chambers to provide primary treatment. A traveling bridge skims and scrapes sludge to the sludge trough at the influent end of the tank. Cross collectors in each equalization tank more sludge deposited in the trough by the bridge collectors to the center pump suction line.


Secondary Treatment Facilities

The secondary treatment system accepts effluent from the equalization tanks. This system is a two-stage oxygen activated sludge process. The purpose of the Stage 1 process is to satisfy the carbonaceous oxygen demand of the organic pollutants. Two parallel trains in the Oxygen Transfer Basins (Structure No. 16) provide contact with the influent and activated sludge. The effluent is then settled within four circular Clarifiers.

A portion of the sludge from the Stage 1 Clarifiers is withdrawn and returned to the Stage 1 Oxygen Transfer Basins. A portion of this sludge is wasted from the process and disposed of in the Sludge Disposal Structure.

The purpose of the Stage 2 process is to achieve nitrification, i.e. the conversion of nitrogen in the form of the ammonia ion to the nitrate form and to satisfy the nitrogenous oxygen demand. Together with the Stage 1 process they satisfy the total BOD. Since Stage 2 is more susceptible to hydraulic or other upsets, provisions have been made to bypass flow from the Stage 1 processes to the plant's chlorination facilities during high flows. This should safeguard the stability of the Stage 2 operation. Four trains in the Oxygen Transfer Basins (Structure No. 19) will provide contact with the activated sludge. The effluent of these basins will be settled in four circular Clarifiers (Structure No. 20), which are similar to the Clarifiers in Stage 1.

A portion of the sludge from the Stage 2 Clarifiers is withdrawn and returned to the Stage 2 Oxygen Transfer Basins, while the waste portion from this stage is combined with the Stage 1 sludge and processed in the Sludge Disposal Structure (Structure No. 4).

The purpose of the returned activated sludge pumping system is to maintain a sufficient concentration of biomass in both the Stage 1 and 2 Oxygen Transfer Basins so that the required degree of treatment can be obtained within the desired time interval. There are two distinct structures for RAS 1 (Return Activated Sludge Stage 1) and RAS 2 pumping equipment (Structure Nos. 5 & 6 respectively).


Tertiary Treatment Facilities

The Intermediate Pump Station (Structure No. 7) raises the flow from the Stage 2 process so it can flow by gravity through the tertiary process. The final polishing is accomplished by six automatic backwash sand filters located in the Filter Structure (Structure No. 9). The filter effluent is used for backwash, which is then discharged to the plant influent. The effluent from the filters flows to the Chlorine Contact Tank (Structure No. 26) where it is disinfected.


Effluent Disinfection

Effluent disinfection at the plant is accomplished utilizing sodium hypochlorite. Sodium hypochlorite is delivered in bulk to storage tanks located in Structure 8. The sodium hypochlorite distribution system provides for the prechlorination of the plant influent, post-chlorination of the plant effluent, chlorination of process waters and chlorination of the sand filter influent. Provisions within the system have been made for the future chlorination of the Stage 1 return activated sludge flow.

Post-chlorination is accomplished within two Chlorine Contact Tanks (Structure No. 26). Each of these tanks has been designed to provide a 15 minute detention time for chlorine contact at a maximum design flow of 60 MGD. Dechlorination is accomplished with sodium bisulfite addition at the end of the chlorine contact tanks just prior to discharge.

An automatic hypochlorination disinfection system will utilize an effluent flow sampling system and residual analyzer to measure for final residual.

About Amherst

The Town of Amherst was established in 1818 and celebrates its 200th Anniversary this year. The town has a geographical area of 53.6 square miles and a population of greater than 122,000.

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