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HomeMy WebLinkAboutNC0061492_Maury Engineering Design Basis_20180308BACKGROUND Maury Sanitary Land District operates an existing 0.225 MGD WWTP in Green County off Sylivant Farm Road. Originally, all drinking water in the service area was provided from groundwater. Now the source of the water varies. It can be groundwater or surface water. When the source is groundwater, alkalinity is adequate in the influent wastewater to accomplish nitrification. When the drinking water source changes to surface water, the alkalinity in the influent wastewater is not adequate for nitrification. For the last few years the operator has been supplementing the alkalinity by adding two 50 pound bags of powdered lime per day, seven days per week. The District requested we evaluate a method to automate alkalinity feed in order to reduce labor requirements. : : � : _ _� `ZK��►f.'I 17 �1� X117 Lime slaking was eliminated because of the capital cost, and because of the complexity of operation. Magnesium hydroxide was considered because it is considered the safest of the options. Magnesium hydroxide will not raise the pH in the WWTP above 9.0 SU. Overdosing the chemical will not hurt the WWTP. The operator is also not damaged because of contact with the chemical. It is essentially Milk of Magnesia. However, it is a suspension and must remain continuously mixed. It will easily set up and clog a chemical feed line. MgOH would also have to be stored in a tank in a heated building. 135 LBS of MgOH = 100 LBS of the lime this requires 12 to 15 gallons per day or 0.5 to 0.625 gallons per hour N ontinuously fed. The MgOH costs about $5.00 per gallon based on a 500 gallon shipment. This would cost approximately $75.00 per day. Caustic Soda (NaOH) was also considered. Care must be used to make only small adjustments to the rate A chemical feed because unlike MgOH it is easy to over feed the NaOH. Overfeeding can result in violating the pH limit in the NPDES Permit, and could affect other permit parameters as well. 430 LBS of Caustic = 100 LBS of Lime. At a 50% solution 22.5 gallons of Caustic would be required. At a 25% solution 45 gallons of Caustic would be required. A 50 % solution begins to solidify at temperatures in the mid 50's OF. At a 25% solution the temperature can go down to 0 OF. The 25% solution costs approximately $1.25 per gallon based on 500 gallon shipments. The owner will have less work and less cost using a 25% solution of Caustic. DESIGN AROUND CAUSTIC A single double walled tank is being proposed. The double wailed tank provides spill containment. There is no degradation of the Caustic with age. The chemical supplier suggested a minimum tank should be able to hold 10 days to two weeks of storage. The tank should be polyurethane which is impact resistant and is compatible with Caustic. We are proposing approximately 60 days of storage. A 3000 gallon tank provides 66.6 days of storage. Purchasing Caustic in bulk can result in significant savings. The tank must be able to handle liquids with a 1.9 Specific Gravity. One such tank has dimensions of 104" diameter and ill be fitted with quick disconnect fittings to eliminate the potential for spilling 142" height. The tank w during the filling process. A simple 5 gallon bucket can be used as a drip pan following filling of the tank with the quick disconnect. The tank will be located adjacent to a driveway to allow the chemical supplier to see both his truck and our storage tank while he is filling the tank. This reduces the opportunity for overflow during filling. Tanks of this nature cost between $9,000 and $15,000 depending on accessories. Plastic -Mart, Snyder, Chkw; mtainer, Plastic all Super Store, and Miller Plastics all can provide a 3,000 gallon double walled polyethylene spill containment tank. The double walled tank provides spill prevention. A 12" slab with two layers of #5 rebar at 9" on center each way will provide a suitable foundation for the chemical storage tank. The chemical feed pumps need to be compatible with Caustic, and be able to deliver 45 gallons per day continuously. This is 1.875 gallons per hour. When feeding Caustic, a peristaltic pump is strongly preferred over a diaphragm pump. One pump that meets the feed rate requirement is the Blue White A2 Pump with a pump range of 0.045 to 4.5 gallons per hour. The chemical feed rate can be adjusted manually, in small increments based on the pH the daily pH readings that are taken by the operator. A single pump would be used, because the operator can revert to bagged lime as a backup if the pump goes down. The Flex-A-Prene tubing, which is undulated by rollers typically to cause the pumping action, has 12000 to 3000 hour life. Pumps like this cost approximately $3,000 each. They are positive displacement pumps. TDH is not really an issue. Blue White, Knight, Flex Master, Longer Pumps, and Verderflex all make peristaltic pumps. The pump needs a 20 amp 115 volt outlet. The outlet is available in the existing chemical feed room. The chemical feed room was initially used to house the ferrous chloride pump. For many years, in lieu of operating a chemical feed pump for the ferrous chloride, the District has been simply drip feeding the ferrous chloride into the wetwell of the return sludge pump station. The room has been abandoned for its ts original function. Therefore, the room is available for chemical feed of NaOH. A process schematic is attached. SAFETY Access is restricted to the site by a locked gate. The chemical is stored in a double walled spill containment tank. The tank is filled using a quick disconnect to reduce the potential for spills. The tank is positioned so the driver can see both his truck and the tank he is filling. There will be an emergency eyewash in close proximity to the tank and chemical feed pump. The proposed tank's more than 50 feet from the property line, approximately 500 feet from the closest stream, 900 feet from the closest residence, and 900 feet from the closest well. A simple 5 gallon bucket can be used as a drip pan Following filling of the tank with the quick disconnect. FLOODPLAIN MAPPIN G The Elevation in the vicinity of the proposed chemical feed tank is approximately 65. The MSLD WWTP is clearly out of the floodplain. Two flood maps are att ached. BLUE LINED STREAM MAPPING Both the USGS Map and the NRCS Maps are provided that show the relationship of the existing WWTP to the Streams. 1 I _ :s �. •CaRGnaw Eat[ cap cc TMAURY SANITARY ' Wei Nk . 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