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WEFTEC®.06 Available AC power at the site is important to system design to ensure the correct DC power supply. Wells and small system sites will most often have single-phase power, larger sites three- phase power. In any of these situations, the control panel will operate from 110 volts or less using a step down transformer to achieve the lower voltage. In all cases, the power service at each installation must be sufficient to assure that the AC feed line current is acceptable and meets all codes when all the equipment is in operation. System sizing is normally based upon a system’s daily demand for disinfectant at peak flow conditions. Equipment requirements must then be defined by the necessary dosing residual, the desired excess on-site generator production capacity, whether redundant generator equipment is important, the excess storage capacity to meet Class 2 reliability, and the dosing control scheme employed. Having defined the production requirements, equipment sized for 100% of necessary capacity is advisable. A more suitable size would be 50% to 70% of a generator’s rated capacity. If the sodium hypochlorite production requirement is very large, it’s advisable to put in multiple systems of smaller capacity, for example, three units of 50% generation capacity. This design allows the performance of maintenance without loss of production capacity. Redundant tanks are the accepted standard for on-site hypochlorite generation installations. These tanks provide backup storage and allow for bulk commercial hypochlorite storage in the rare event that the on-site system is inoperable for any length of time. The installation should include a mixing system to enable commercial hypochlorite dilution to a 1% solution for proper dosing control. The diluted commercial grade hypochlorite solution will have a lower rate of degradation than commercial grade hypochlorite. Dosing control schemes are available in three basic formats, manually adjusted, flow paced, and residual paced. There are instances where it is desirable to combine flow and residual pacing. This combination is accomplished using a Programmable Logic Control (PLC) system. Well sites will most often have a manually adjusted dosing system since the well pump flow is constant and the hypochlorite product has a stable concentration. Flow and residual pacing require an analog signal be sent to the control panel in turn signaling the variable speed drive on the dosing pump to adjust the flow to the proper residual. Part of the PLC function is to control the disinfection dose rate of change to prevent severe oscillations in the concentration of disinfectant applied. An average dose of one to five milligrams per liter of sodium hypochlorite is the generally required for potable water. Wastewater dosing may vary up to ten milligrams per liter. Isolated cases exist where the potable water dose required for a reasonable residual can be much higher. Two examples of this increased dosage requirement are for iron and manganese control and to treat for high ammonia levels (e.g., potable water doses up to 40 milligrams per liter of sodium hypochlorite may required to remove ammonia through breakpoint chlorination. Dosing volume and pressure will define the pump type required. Small systems will generally use a diaphragm pump for sodium hypochlorite application. Systems greater than 100 pounds per day will often use positive displacement “hose” pumps capable of higher volumes. PVDF, Copyright ©2006 Water Environment Foundation. All Rights Reserved 6349PDF Image | On-Site Sodium Hypochlorite Generation
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