Treated water from the denitrification filters flows to the chlorine contact tanks where disinfection takes place. Once disinfected, residual chlorine is removed and the water is discharged to the South Platte River.
In 1992, the Littleton/Englewood WWTP converted existing gas disinfection system to an alternative liquid chemical method. The liquid chemical disinfection system consists of low pressure storage tanks, peristaltic pumps, and an oxidation reduction potential (ORP) control system. The process uses a 10 to 12 percent solution of sodium hypochlorite for disinfection and a 34 percent solution of sodium bisulfite for effluent dechlorination.
The chemical and pumping systems are housed in separate buildings. Four 4,500 gallon hypochlorite storage tanks are located within a secondary containment area sized to hold the volume from at least one full storage tank. Two tanks are always kept empty. In the event of a spill, provisions allow the chemical to be recycled from the secondary containment area back into the empty or a partially filled tank or allow wasting to the plant headworks. A peristaltic pump is plumbed from each tank. Pump revolution totalizers indicate when pump hoses should be changed.
The two 6,000 gallon sodium bisulfite tanks are equipped with bottom heaters which maintain tank temperatures at 20 degrees C to prevent crystallization in the metering and distribution equipment. Tanks vent outdoors and overflow piping has water traps to eliminate fumes from entering the storage room. There is also a peristaltic pump provided for each bisulfite tank with revolution totalizers to indicate when pump hoses need changed.
The Strantrol 900 ORP (oxidation reduction potential) system monitors the ORP value of the chlorine contact tank influent. The system adjusts pump speed in response to plant flow changes and variance from the ORP setpoint. The dechlorination ORP monitors the chlorine contact tank effluent and adjusts the bisulfite pumps to maintain the setpoint.
Five identical chlorine contact tanks provide adequate detention time for disinfection. Each tank is drained and cleaned four times a year.
An automatic sampler is programmed to collect a sample for every 500,000 gallons of final effluent.
Four plant water pumps pull water from the chlorine contact tanks and supply water for pump seals, the headworks sluice system and irrigation water. The water flows through strainers to remove particles.
For more information on the liquid chemical system and the ORP control system please visit the library and check out the K-I-S-S presentation; available as text and a slide show.