Waste Water Treatment

All residents and commercial facilities within the City limits generate wastewater. The City's collection system collects this wastewater and transports it to the wastewater treatment plant for treatment and disposal. 

lift stationWaste water is collected from approximately 30 miles of sewer lines and 16 different lift stations to the City's Wastewater Treatment Plant for treatment and disposal. 

The Soldotna sewage collection system consists of the following:

  • Collectors: Eight inch collectors receive waste water directly from users
  • Interceptors: Interceptors ranging from eight to twenty-four inches intercept waste water from smaller areas
  • Manholes: Manholes are used as points of inspection and maintenance
  • Lift Stations: 16 different lift stations pump waste water from low points within the system to elevations where waste water can flow by gravity
  • Approximately 30 miles of collection system piping
  • Approximately 1,475 sewer service connections

waste water treatment plantThe Soldotna Wastewater Treatment Plant provides sewage treatment service for approximately 4,000 people. The design capacity is 1.2 million gallons per day with a maximum flow permitted by the NPDES permit of up to 1.08 million gallons per day. The plant removes on average 98.4% of the biochemical oxygen demand (BOD), and 97.9% of the total suspended solids (TSS). Waste from the process is transferred to an aerobic digester, then processed through a belt filter press, and ultimately transferred to the Kenai Peninsula Borough Soldotna Landfill.

The Soldotna wastewater treatment plant consists of the following:

  • Three influent screw pumps, which are each capable of pumping 1215 gallons per minute
  • Mechanical filter screen and a manual bar screen
  • Grit handling system
  • Two odor control systems
  • Parshall flume
  • Two aeration basins, each with a capacity of 316,554 gallons
  • Three air blowers each producing 1,500 standard cubic feet per minute
  • Clarifier splitter box
  • Secondary Clarifier with a capacity of 318,000 gallons
  • Secondary polishing clarifier with a capacity of 52,880 gallons
  • Two Return-Activated- Sludge pumps, each capable of 1,200 gallons per minute
  • Ultra-Violet Disinfection System
  • Waste Sludge Pump
  • Two aerobic digesters, each with a capacity of 126,225 gallons
  • Sludge feed pump
  • Polymer feed pump
  • Belt Press for sludge dewatering featuring a lime feed system and a conveyor system

influent flow grit chamberThe primary solids are removed with either a mechanical filter screen or a manual bar screen, then passed through the grit handling system located in the headworks building. The mechanical filter screen is automatically self-cleaning. It is capable of removing all materials larger than 5/8 inch in diameter, the manual bar screen serves as a backup to the filter screen. The discharge from the filter screen is discharged into the concrete screening channels leading to the grit chamber. The grit chamber is designed to remove heavy matter prior to biological treatment. After the primary solids are removed from the process, the pH of the solids is adjusted between 12 and 12.5 units and hauled to the Kenai Peninsula Borough Soldotna Landfill.

aeration basinAfter the primary solids are removed, the primary effluent enters secondary treatment. The effluent flows from the headwork's building into the aeration basin where the extended aeration activated sludge process begins. The purpose of the activated sludge process (aeration basins and secondary clarifiers) is to biologically stabilize the oxygen-demanding material present in wastewater with aerobic microorganisms. Air is supplied to the aeration basins in an effort to satisfy the needs of the microorganisms present. Extended aeration activated sludge is an operational mode in which the activated sludge process is operated at a relatively long hydraulic retention time and sludge age. The number of aeration tanks in service varies depending on the volume of wastewater being treated for the proper detention time.

Mixed liquor enters the secondary clarifiers, which slows down the flow and allows the sludge to settle and produce a clear effluent. Scum is removed from the surface of the clarifiers and transferred to the digester.

UV disinfection modulesThe clear effluent leaving the clarifier then enters the ultraviolet basins, which is designed to disinfect a flow rate of 1.2 MGD. Once disinfected, the final effluent is discharged into the receiving water (Mile 20 of the Kenai River).

belt pressA percentage of the returned activated sludge is transferred to the aerobic digester. This sludge is gathered from the bottom of the clarifiers over a twenty-four hour period. After leaving the clarifier, the sludge then becomes wasted activated sludge. The principal purpose of aerobic digestion is reduction of biodegradable solids, odor and pathogenic organisms. Aerobic digestion is a method of stabilizing sludge by aerating it in an open tank for an extended period. The solids are then pumped from the digester to the belt press.

The belt press is used to de-water digested sludge pumped out of the digester. Once de-watered, the remaining conditioned sludge is 17-20% solids. Reducing the volume of the sludge by de-watering allows  for easier handling and disposal. These solids are then hauled to the Kenai Peninsula Borough Soldotna Landfill.