311

Treatment Process

Pumping, Screening, Grit Removal
Sewage enters the treatment plant through multiple force mains from the three pump stations. All flows come into the plant at a central head-works facility that houses mechanically cleaned screens. These screens separate and remove large objects that could harm the plant's operating equipment. The sewage then flows by gravity to a five-channel aerated grit removal facility where sand, gravel and other heavy materials are settled out, cleaned by air agitation, and fed into storage hoppers for periodic removal and disposal in a landfill.

Primary Clarification
Grit removal is followed by primary sedimentation in circular clarifiers or sedimentation basins, where the heavier inert or organic solids settle to the bottom and are collected. Sludges from these basins are pumped to the anaerobic digesters while the partially clarified effluent overflows the weirs and moves to the aeration basins.

Aeration Basins
In the aeration basins, the incoming wastewater is completely mixed with activated sludge that is returned from the bottom of the final clarifiers. The bacteria present in the mixture remove the organic impurities from the water and in the process, grow and reproduce. After several hours in the aeration tanks, the liquid flows into the final clarifiers where the solids are settled out. A portion of the solids is returned to the aeration tanks while the excess is pumped to the anaerobic digesters.

Final Clarifiers and Disinfection
As the solids settle to the bottom of the clarifiers and are removed, a clear liquid emerges and gently flows over weirs into the chlorine contact basins. Here, the clarified effluent is mixed with chlorine to kill bacteria and other harmful organisms. After approximately 20 minutes in the presence of the chlorine, the water flows over exit weirs into a structure before entering pipes that carry the liquid out of the plant for discharge into either Bird Creek (Northside) or the Arkansas River (Southside). As the water flows over the final weirs, it is injected with sulfur dioxide, a chemical that neutralizes any remaining chlorine.

Sludge Treatment
As wastewater flows through the treatment process and solids are gradually removed, those solids are pumped through gravity thickeners on their way to the anaerobic digesters. This process allows the biosolids to settle once again with additional liquid being removed to produce a thicker mass of solids. The liquid effluent from the thickeners flows back into the plant head works where it mixes with the incoming wastewater and goes through the treatment process again, getting additional treatment before it is discharged from the plant. The settled solids in the thickeners are pumped to the anaerobic digesters. The biosolids remain in the digesters for a minimum of 20 days while biological activity continues to occur within the biosolids. Periodically, the digested solids are withdrawn from the bottom of the digesters and pumped to lagoons for storage. The biosolids remain in the lagoons for about one year during which time additional stabilization occurs. The resulting solids are then pumped into trucks and transported to area farms and ranches for field application. Each year, approximately 6,100 dry tons of biosolids are land-applied.

Excess Flow Treatment
During wet-weather periods, the ground becomes saturated and rainwater enters the joints of sewer pipes. This can cause above capacity flows into the treatment plants. The Northside Plant operates four flow-diversion facilities that are used to store excess flows during these periods. Collectively they have a storage capacity of 83.2 million gallons. Following periods of significant rainfall, when plant flows subside, this stored wastewater is released back into the sewer system and treated at a managed rate.