Wastewater Treatment Process

Click here for a diagram of the wastewater treatment process.

The plant treats in excess of 3,600 million gallons/year; with a population equivalent of 250,000 with industry loadings. The Nampa Wastewater Treatment Plant removes 99% of BOD (Biological Oxygen Demand), 96% of TSS (Total Suspended Solids) and 99.6% of NH3-N (Ammonia).

Wastewater flows by gravity to the Plant Headworks where it is lifted by the Raw Sewage Pumps. Then Step screens remove sticks and rags from the raw sewage flow. Grit basins remove inorganic grit and snail shells. Rag and grit removal reduces possible plugging problems and prevents excess, while also preventing excessive pump wear.

From the grit basins, the flow is split through three flow meters which measure the raw sewage flow. The flow from each parshall flume goes to a primary clarifier.

Primary clarification removes settleable (sludge) and floating (scum) solids.

Effluent from the primary clarifiers flows to the primary effluent splitter box where the flow is split either to the trickling filters or to bypass the trickling filters. The amount of flow that goes to the trickling filters is limited by their treatment capacity.

The flow that goes to the trickling filters is diluted with returned trickling filter effluent to minimize shock loading. The mixture is then pumped to an elevated wet well where the mixture is split to the three trickling filters. The trickling filters provide biological treatment by means of a fixed microbial mass living on the rocks (media). The flow is evenly spread over the media by the trickling filter mechanism. The flow that passes through the trickling filters returns to the influent, while the rest goes to the trickling filter effluent pump station. This pump station uses screw pumps and lift pumps to lift the flow and then splits the flow to the two secondary clarifiers. These clarifiers remove the solids that grew in the trickling filters and have fallen (sloughed) off of the media.

The effluent from the secondary clarifiers goes to the secondary effluent pump station where it combines with the flow that bypassed the trickling filters. This pump station also uses screw pumps to lift the flow and then splits the flow to the Nitrification Basins. Inside the Nitrification Basins the flow is mixed with living organisms contained in the return activated sludge. The nitrification basins provide adequate detention time for the organisms to perform biological treatment which removes oxygen demanding pollutants and also converts ammonia nitrogen to the nitrate form. The Nitrification Basin has two halves with six mixed areas in each half. Air is added to each mixed area to give the organisms air to breathe.

The Nitrification Basin effluent continues to the final clarifier splitter box. Here the flow can be split between the three final clarifiers as needed. The final clarifiers capture the organisms grown in the Nitrification Basin and also any scum that gathers during treatment. Downstream of the final clarifiers, the flow goes through the chlorine mixing station where sodium hypochlorite is thoroughly mixed with the flow. The flow then goes through the chlorine contact chamber where bacteria are killed by the hypochlorite. The hypochlorite is then neutralized by sodium bisulfite that is added at the #4 Water Pump Station.

The flow then goes to the post aeration basins where air is added to raise the dissolved oxygen level in the wastewater and a defoaming agent is added to control foaming on the creek. The flow then goes to Indian Creek via the plant outfall.

Solids Handling

Sticks and rags are removed by the Step Screens and stored in the Grit Processing Building before being removed by the local sanitation service.

Grit removal at the Grit Basins are pumped to the Grit Processing Building where it is separated and stored before being removed by the local sanitation service.

The Primary Sludge is pumped to the to the Primary Digesters. The Primary clarifier Effluent flows to the bypass splitter box where flow can send either to the trickling filters or directly to the Nitrification Basins. Primary scum is pumped directly to the primary digesters.

The Secondary Clarifier sludge is returned to the plant flow upstream of the Grit Basins. It is normally sent to the Grit Basins in order to remove the snail shells that grow in the Trickling Filters. The organic matter then settles out in the Primary Clarifiers.

The Final Clarifier sludge is returned to the Nitrification Basins as return activated sludge. This sludge provides the living organisms needed to treat the sewage flow in the Nitrifications Basins.

The wasting of activated sludge comes from the return activated sludge pumps. This waste activated sludge is pumped to the flotation thickener where it is thickened. It can also be directly sent back to the headworks where it settles in the primary clarifiers.

The thickened sludge is then pumped to the primary digesters. The thickener effluent flows to the secondary effluent pump station. The scum that is collected from the final clarifiers also goes to the primary digesters.

In the primary digesters, sludge and scum are treated. This process involves heating and mixing the sludge in the absence of air (anaerobic). The volume of sludge is reduced, and pathogens are reduced or eliminated in the process. Methane gas produced during anaerobic digestion is burned in the four hot water boilers and also in the engine generator. Hot water from the boilers is used to heat and maintain the digester sludge temperature and to heat the Administration Maintenance Building. The digester gas is also used to mix the sludge in the primary digesters. Excess gas, if any, is burned in a waste gas burner.

Primary digested sludge flows by gravity to the secondary digesters where the sludge can settle. Secondary digested sludge flows either to the drying beds, the sludge holding tank, or to the blower building for belt press dewatering. Sludge in the sludge holding tank can be pumped to drying beds.