Agenda - Public Works Committee - 10/19/2021

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Surface Water Treatment March 12, 2020 Page 2 Treatment Processes The preliminary treatment process proposed for a Northwest Metro surface WTP assumes that the UK surface water supply will be classified as Bin 1. If %Q4 PAC C U LAW additional treatment is required, a future UV or chlorine dioxide addition process can be SOn LIMENING implemented to assist in meeting additional RAW WATER treatment requirements. PUMP STATION W swfla Process Train WER All) KW Fib I ��,� As depicted in the process diagram, a potential II �, a[T , a t .; process train to treat raw surface water from the I Mississippi River includes raw water pumping, ' chemical addition, lime softening, filtration, chlorine contact, and finished water pumping. DUAL!FILTER CLWWEU This process is very similar to other major surface water treatment plants in Minnesota including SPRWS, the City of Minneapolis, and the City of St. Cloud. The chemical addition includes potassium permanganate (KMnO4)for oxidation, powdered activated carbon (PAC)for taste and odor, and coagulant to help with floc production. SPRWS uses granular activated carbon (GAC) in their filters rather than PAC. Coagulation/Flocculation and Lime Softening In coagulation, a chemical such as alum is added to the water to encourage larger particles to form in the flocculation process. The flocculation and settling occur in the clarifier. Hardness in water is primarily caused by calcium and magnesium ions (Ca2+and Mgt+). Hardness causes scaling on water fixtures, dishes, and appliances. Lime (Ca(OH)2) is added to the raw water to raise the pH. When the pH is raised sufficiently, calcium and magnesium precipitates (solids)form and they are settled out of the water in clarifiers. Lime from a silo or other storage method is fed into slakers which add water to the lime and the slurry is then fed into the raw water. The raw water and lime enter one of the clarifiers at the WTP where the softening process occurs and the sludge is removed from the water. Recarbonation The water from the clarifiers enters a recarbonation basin where carbon dioxide is bubbled into the water to lower the pH. This process stabilizes the water and prevents additional precipitation. The pH of the water in this process is lowered from approximately 11 to 9. wash troughs filter tank Conventional Filtration (Cony) -_ _----- Conventional filtration is considered for its benefits in reduction of suspended particulates. Typical conventional filters used in water { treatment are rapid, deep bed, dual media, gravity filters that utilize layers :A. .:.... of both sand and anthracite for media. Typical depths are 12" sand and i 12"-24" anthracite. The particulates removed in conventional filtration include microbial contaminants, turbidity, trihalomethane (THM) coal - precursors, as well as those precipitates formed in pretreatment (anthracite) filtered water processes. sand to clear well garnet perforated laterals Chlorine Contact coarse gravel O 1999 Enoyolopaedia Britannioa,Ino. After filtration, chlorine is added and the water enters a chlorine contact basin. The chlorine contact basin is provided to allow time for the chlorine to inactivate Giardia and viruses. The amount of time and chlorine concentrations are dictated by the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR).