The treatment plant provides primary, secondary, and tertiary treatment
for all wastewater flows up to 30 MGD as follows:
The three mechanical barscreens and one manual screen, receive raw wastewater from the inlet sewers. Rags, stones, and other course material that could cause clogging or jamming of downstream equipment are removed from the wastewater stream. The screenings are then conveyed to a compactor/washer and collected in a special waste hopper, which is hauled to a landfill. The barscreen facility is considered preliminary treatment.
GRIT REMOVAL FACILITY
Sand and grit, which has a greater settling velocity than the other material in sewage, is settled out into two 35 foot long, 11 foot wide rectangular tanks. The grit is then pumped to two cyclone seperator units where the water is seperated from the grit. The grit is then collected in two hoppers. When the hoppers are full, they are dumped into a special waste hopper. The grit facility is considered preliminary treatment.
MAIN PUMPING STATION
After the wastewater leaves the grit channels it then flows into a wet well, where five centrifugal pumps lift it 43 feet to the primary clarifiers. The pumps are equipped with variable frequency drive motors. The main sewage pump system is controlled by a programmable logic computer. Total pumping capacity of all five pumps are 44 million gallons.
Six rectangular primary clarifiers provide primary sedimentation, which removes 99% of the settable material, 75% of the suuspended solids and 35% of the biological demand from the wastewater. The sludge collected in the tanks is pumped to the digesters.
Six aeration tanks provide suspended growth activated sludge biological treatment of the wastewater. The tanks mix primary effluent, air and returned sludge from the secondary clarifiers to support a biomass that removes organic matter. Air is supplied from three 300 horsepower blowers. The suspended biomass removes 98% of the remaining BOD and 95% of the ammonia nitrogen.
After the mixed liquor leaves the aeration tanks, it then enters the clarifiers, where it settles. The clarified liquid passes over the weirs as secondary effluent. The mixed liquor sludges settle to the bottom of the clarifier and are pumped back up to the aeration tank. Some of the return sludges are pumped to pre-thickening centrifuges, where the water is centrifugally seperated from the sludge. The sludge is then pumped to an anaerobic digester.
Eight rapid disk filters treat secondary effluent or polished effluent from the dual use clarifiers. The disk filters remove 98% of the remaining suspended solids and organic material in the wastewater. The filters are backwashed to maintain solids removal efficiency. The backwashed water from the filters cell is pumped to a backwash settling tank, where the solids are collected and pumped back to the headworks of the plant or allowed to flow by gravity back to the plant. The disk filters are considered third-stage (tertiary) treatment.
After the filtered water leave the disk filter building, it is then chlorinated with a 15% sodium hypochlorite solution. The chlorine is controlled by flow, pacing off of the raw sewage pump flow signal and chlorine residuals. Plant operators have the ability to chlorinate at several locations within the plant, in order to maintain the permitted fecal coliform level as well as control algae growth in the sand filters.
When the chlorinated and filtered effluent leaves the disk filter building, it discharges into a 96 inch conduit and flows to the dechlorination facility. At this point, the effluent is dechlorinated using 38% sodium bi-sulfite. The permit for Flagg Creek WRD only requires that the District chlorinates and dechlorinates May 1st through October 31st.
PLANT FINAL EFFLUENT OUTFALL 003
After complete treatment in accord with the applicable Environmental Standards of the State and Federal Agencies, the effluent is discharged into Flagg Creek through 003 outfall. Flagg Creek is a minor tributary of the Des Plaines River.
In accordance with FCWRD’s NPDES (National Pollution Discharge Elimination System) permit, the influent and effluent are tested for certain parameters. A lab manager and lab technician perform the majority of test in-house. Their responsibilities include sampling and testing of the wastewater. Addtionally, the lab staff records data, runs test for process control and completes a DMR (Discharge Monitoring Report), which is sent to the EPA monthly.