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Public Works - Wastewater Division

Treatment Plant 2 Photos:

Raw wastewater (called influent) from the sewer system enters the treatment plant at the headworks. Here, the wastewater passes through two mechanical bar screens that remove large debris, rags, sticks or other objects that may damage equipment used in the treatment process. Wastewater is then pumped to the Aerated Grit Chambers which removes sand, gravel and metallic objects. The removal of these solids improve treatment efficiency and help protect the equipment against excessive wear. The Influent then flows through a Parshall flume which measures the flow rate and the total amount of wastewater treated by the plant.


The clarifiers remove floatable and settleable materials from the wastewater. The water is detained in the clarifier for 3 hours which allows the majority of solids (sludge) to separate from the liquid portion and collect at the bottom of the tank. The solids or sludge is then mechanically collected and pumped to the digesters. Meanwhile, the floatable waste (or scum) is skimmed off the top of the water and is also pumped to the digesters. The remaining liquid portion is pumped to the trickling filter.


Wastewater is distributed over the honeycomb-like plastic filter media which is covered with a thin bacterial growth. Microorganisms grow on the media and absorb much of the organic content found in the wastewater. When the growth becomes too large, it breaks off and is passed to the next phase of treatment.


This is the final treatment step. The secondary clarifiers provide a quiescent condition which promotes settling of the bacterial growth from the trickling filters and also any solids or scum not removed during primary sedimentation. The collected sludge and scum are pumped back to the headworks to repeat the treatment process.


Sludge collected from the primary clarifiers is pumped to the digesters for treatment. Sludge from the secondary clarifiers is routed backed to the headworks and will repeat the treatment process. No sludge is collected in the trickling filters


Collected sludge is pumped into heated tanks called Digesters. Anaerobic (absence of oxygen) bacteria thrive in the digesters, and convert raw sewage sludge to an inert material (digested sludge), methane gas and carbon dioxide. This process takes about two to three weeks.


Digested sludge from the digesters is distributed to the sludge drying beds. The liquid portion of the sludge is evaporated or decanted and returned back to the headworks to repeat the treatment process. When the sludge has dried, it is tested for pollutants. When approved, the sludge will be spread on the City’s farm land where only restricted crops are grown (non-human consumption). The dried sludge is categorized as Grade B quality and is safe and relatively odor free.


Digester gas (approximately 60% methane, 40% carbon dioxide) is produced by anaerobic bacteria in the digesters. The gas is collected and fuels two engine generators each of which produces 400 kilowatts of continuous electrical power for use in the treatment plant. Waste heat recovered from the engine is used to heat the digesters.


The operations building provides a central location for plant personnel. It contains the main control room which is used to operate and monitor the treatment process. In addition, it contains offices, a laboratory, and the lunch/training room.


The City’s treatment Plant 2 laboratory maintains a Department of Health Services, Environmental laboratory Accreditation Program (ELAP) Certificate No. 2001 in the field of testing # 16 for conventional pollutants. Influent, Pri-Effluent, and Final Effluent are analyzed frequently to confirm the treatment plant is operating as designed.


Final effluent is used to irrigate restricted agricultural crops (non-human consumption). Crops consist mostly of alfalfa hay, and cotton. The reservoirs store the water until it is needed for irrigation and provides a habitat for water loving fowl.


The City owns 5,000 acres located just south of the Plant 2 treatment plant. The farm land is leased to Progressive Associate Group which uses Plant 2 effluent to irrigate restricted crops (non-human consumption) such as cotton and alfalfa. Biosolids generated from both Plant 2 and Plant 3 are spread on this farmland.


The City’s Wastewater Treatment Plant 2 also provides service to indirect discharges by accepting septage waste from residential septic tanks, seepage pits, interceptor, cesspools, restaurant grease traps, and chemical toilets. Only pre-authorized industrial waste is accepted at the City’s treatment Plant 2.

Service Area: 63.4 Square miles
Staff: 5 operators, 1 electrician, 1 mechanic, 1 Facility Worker
1 Lab technician, 1 Supervisor
Plant Capacity: 25 million gallons per day
Average Daily Flow: 14.5 million gallons per day
Bio-solids Processed: 234 tons per month
Methane Gas Produced: 166,000 cubic feet per day
Average Influent: BOD 297 mg/L, SS 359 mg/L
Average Effluent: BOD 34 mg/L, SS 21 mg/L