WASTEWATER DEPARTMENT. Bentonville Wastewater Treatment Plant Facts:

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1 Mission: The mission of the Bentonville Wastewater Treatment Utility and staff is to protect public health and the environment through the effective treatment of wastewater. Effective wastewater treatment prevents disease, contamination of water supply, maintains clean waters for the propagation and survival of fish and aquatic life, and conserves water. Bentonville Wastewater Treatment Plant Facts: 1. There has been a wastewater treatment plant at this location, in one form or another, since about The wastewater treatment plant you see today is an Activated Sludge Extended Aeration plant and was built in Since 1985, there have been three (3) plant upgrades: 1995: Additions to Maintenance and Operations buildings. No upgrades to the wastewater treatment processes were made. 2000: Two 600,000 gallon aerobic digesters and a blower building were added to improve the solids stabilization process. Also, Anoxic basins were added at the North end of the plant for Nitrate + Nitrite Nitrogen removal. 2005: Modifications were made to the anoxic basins to accommodate biological Phosphorus removal. The Chlorine disinfection system was replaced by a U.V. disinfection system. 4. The Bentonville Wastewater Treatment Plant serves the residents, businesses and industries of Bentonville, Centerton and the Northwest Arkansas Regional Airport. 5. Design capacity: 5 MGD (million gallons per day) 6. Permit capacity: 4 MGD 7. Average daily flow for 2007: 4.6 MGD 1

2 Headworks: Where wastewater enters a wastewater treatment plant. Preliminary Treatment: Influent flow measurement and screening. Influent = Untreated wastewater flowing into the Wastewater treatment plant. 1. Influent flow measurement: Influent flow is measured by an Ultrasonic Flow Transmitter in an 18 Parshall Flume. 2. Influent screening: The influent flows through a bar screen to remove large debris such as rags, rocks, pieces of wood and just about anything else one can imagine. The screenings are transported to a landfill for disposal. Primary Treatment: A wastewater treatment process which allows substances in wastewater that readily settle or float to be separated from the water being treated. 1. The influent flows into a Grit Chamber where floatable solids and settleable solids (sand, grit & gravel) are removed. The floatable solids are transferred to digesters for further treatment and the settleable solids are transported to a landfill for disposal. 2

3 Secondary Treatment: A wastewater treatment process used to convert dissolved or suspended materials into a form more readily separated from the water being treated. This process is usually a type of biological treatment process followed by secondary clarifiers that allow the solids to settle out from the water being treated. 1. Fermentation Basin: The basin where anaerobic conditions are maintained to initiate the first step of biological Phosphorus removal. A process called Luxury Uptake is employed to remove Phosphorus from the water being treated. 2. Anoxic basin: The anoxic basin is where the denitrification process takes place. Denitrification is the process by which microorganisms reduce Nitrate (NO3) Nitrogen to Nitrogen gas that is released into the atmosphere. Dissolved Oxygen levels are maintained at a very low level, about.1 mg/l. 3. Oxidation Ditch: The oxidation ditch is where the nitrification takes place, BOD is stabilized and Phosphorus is taken back up by the biomass. Nitrification is the process by which bacteria oxidize ammonia nitrogen (NH3). BOD, Biochemical Oxygen Demand, is when the bacteria use the organic matter in the wastewater for a food source. Dissolved Oxygen is taken up during this process. All three of these processes are aerobic processes. The dissolved Oxygen level is maintained at about 2 mg/l in the oxidation ditch. 4. Final or Secondary Clarifiers: The final clarifiers are where the biomass is separated from the clean water once their work has been completed. 5. Effluent Disinfection: Effluent = The treated water flowing From the wastewater treatment plant to the receiving stream. An Ultra Violet (U.V.) disinfection system is used to inactivate most microorganisms in the effluent, including pathogenic bacteria. 3

4 Solids Stabilization: WASTEWATER DEPARTMENT 1. Gravity Thickeners: Wasted sludge is pumped from the final clarifiers to the gravity thickeners to continue the sludge thickening process. The sludge is allowed to settle for a longer period of time in the gravity thickeners, which increases the solids concentration of the sludge. The sludge enters the gravity thickeners at approximately 1.7% solids or 17,000 mg/l, and is at about 2.5% or 25,000 mg/l when it is pumped to the aerobic digesters. 2. Aerobic Digesters: Aerobic digesters are where sewage sludge is Stabilized. Sludge stabilization refers to vector attraction reduction and pathogen reduction. According to Federal Regulation 40 CFR Part 503, sludge can be stabilized to either a Class A or a Class B biosolid for surface disposal. Stabilized sewage sludge is referred to as biosolids. - Vector Attraction Reduction: 11 options available. We use S.O.U.R. & 34% volatile solids reduction. - Pathogen Reduction: 9 options available Class A : 1, 000 MPN Fecal Coliform colony forming units per one dry gram of sludge. Class B : 2,000,000 MPN Fecal Coliform colony forming units per one dry gram of sludge. 3. Belt Filter Press: Once the sludge has been stabilized, the biosolids are de-watered by means of a belt filter press. A flocculent or Polymer is mixed with the sludge to separate the water from the solids. The solids are then run through a series of rollers between two permeable belts to squeeze out as much water as possible. An average of 17% solids are produced by the belt filter press. Approximately 1,000 dry tons of biosolids are produced each year. Solids Disposal: 1. Land Application: Permitted land application sites. 2. Compost: Bentonville Compost facility. 4

5 Laboratory: The City of Bentonville Wastewater Laboratory is responsible for analyzing wastewater samples for parameters issued by the Arkansas Department of Environmental Quality (ADEQ) by means of an NPDES permit. Approximately 350 samples are logged in, analyzed and recorded by the laboratory each month. The following are the most common analyses performed: TSS : Total Suspended Solids TS : Total Solids CBOD5 : Carbonaceous Bio-chemical Oxygen Demand NH3 : Ammonia as Nitrogen Nitrates Total Phosphorous Fecal Coliform Sample Types: Composite Samples: Composite samples are taken at flow rated intervals to represent the actual flow entering and leaving the Wastewater treatment plant in a 24 hour period. Grab samples: Grab samples are taken periodically to indicate what is occurring in a specific area of the plant at a given time. Numerous analyses are performed through out the day to supply operations personnel with process control information necessary to maintain maximum plant performance. The laboratory must pass EPA Quality Assurance Tests yearly and follow strict QA/QC protocol daily to assure the accuracy of our results. The laboratory and the Pretreatment Program are subject to regular inspection by ADEQ and EPA. Laboratory personnel are also responsible for industrial sampling of three permitted industries and sampling for current domestic wastewater strength to determine allowable industrial loading parameters. 5

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