JAN-FEB 2019

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INTECH JANUARY/FEBRUARY 2019 39 SPECIAL SECTION brain having access to all information in the system. Any necessary information from one PLC is passed to the oth- ers through the poll master. If a communication failure is detected between two PLCs, alarms are immediately gen- erated. A well site that loses communication with the poll master enters a fail-safe mode and shuts down; simulta- neously an alarm is generated and sent to SCADA to in- form operators, so that the corrective action can be taken. Action by PLCs Each individual PLC measures the water flow leaving its station. Using the flow rate (usually in gallons per min - ute [GPM]), the PLC can calculate the total number of gallons being supplied to the system. This flow total is then communicated to the poll master. The poll master uses the daily flow totals from each site, as well as the set points entered by the operator at either the LOI or SCADA, to calculate the current levels of water quality. If the water quality is outside of com - pliance parameters, alarms are generated and sent to SCADA. The appropriate wells are automatically shut down by the poll master. Each cycle of logic takes only about 60 milliseconds. Calculations in the PLC are checked and rechecked ev ery cycle, meaning operators are informed of water that is not in compliance moment by moment. Water quality objective As aforementioned, the new groundwater source has higher hardness and naturally occurring minerals, such as iron and manganese, including the regulated constitu- ents hexavalent chrome (chrome-6) and nitrate, than the surface water sources. The water quality objectives for the SFPUC Groundwater Program are to treat and blend the groundwater sources to meet drinking water qual- ity regulations, and just as importantly, to maintain the high-quality and "good tasting" water to which San Fran- cisco residents are accustomed. Groundwater treatment approach The groundwater quality from the 15 wells varies. Some wells do not require any treatment; some have minimal chemical conditioning systems; and some have more extensive treatment systems. For example, several well stations include filters to remove iron and manganese, and many wells add chlorine to match the disinfection residual in the imported surface water. In all cases, the relatively low flows of groundwater are blended into generally much higher flows of surface water, such that customers should not be able to notice a change in the aesthetic qualities of the water. In the SFGW system, the groundwater is connected and blended directly into the large Sunset Reservoir. In the RGSR system, where groundwater is blended with treated surface water in four SFPUC transmission pipelines, the blending must take place before the wa - Figure 3. SFPUC Water SCADA simplified network diagram Figure 4. Lake Merced PLC master controller. The PLC is a redundant CPU with hot backup connected to AVPN network with redundant fiber-optic cables. ter reaches the first customer on that pipeline. Figure 6 shows a schematic of the blending system configuration for a SFPUC RGSR Well Station. The RGSR Well Station transmission pipeline blending system includes the following components: l Well head compliance sample tap: for raw groundwater sam- ple collection. l Well station flowmeter: measures groundwater flow rate into the transmission pipeline. l Transmission pipeline flowmeter: measures flow rate for blending calculations and the direction of the surface water

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