Water. Desalination + reuse
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22 Tech Quarterly March 2018 Water. desalination + reuse C O M M E N T Nikolay Voutchkov W e are improving the e ciency of desalination and water reuse plants using a variety of factors. There are two relatively new concepts that have changed dramatically the energy e ciency of pumping feed water. One new trend, which seems to be showing bigger e ciencies, especially when we have to respond to daytime and nighttime variations in demand, is related to the three- centre design, a new concept. To adjust the ow of reverse osmosis (RO) systems, one option is to adjust the feed pump; to change the feed pressure and ow of the pump. The other option, related to the three-centre design, is to run the feed pump at a constant ow, and to increase recovery. The way we control recovery is to open the valve on the concentrate pipe. There is a produced water pipe and concentrate pipe, and when we open the valve on the concentrate pipe, that controls how much water goes through the production side of things; how much freshwater and how much concentrate will get pro- duced. To produce more drinking water, close that valve. Open the valve, more concen- trate will go out of the system and less drink- ing water is produced. At night, when demand is lower, we open the concentrate valve and produce less drinking water that way. The bene t is that, the lower the recov- ery of the RO system, the less energy is used. Therefore at night, it's more e cient to open the concentrate valve, and to run at lower recovery, than to change the feed water pump speed. This three-centre concept was intro- duced for the rst time in Ashkelon, Israel, and it's very, very e cient. The reason why it's more e cient is that concentrate is used to run an energy recovery device, and to pump more new water. In the old days, 100 per cent of the feed water into the RO was pumped by a high pressure pump, usually a centrifugal pump. This concept has changed, and the high pressure centrifugal pump feeding the RO now only pumps about 40 to 45 per cent of the feed water. The rest of it is pumped by energy recovery devices that run using the energy in the con- centrate, and are much more e cient. The high pressure pump has e ciency of 83 per cent, while the energy recovery devices have e ciency of 96 per cent. That new concept is a very neat way of recovering the energy that's in the concentrate, and reusing it to pump new water into the RO system. The lower the recovery, the more concentrate we produce, the more of the feed water that we pump at the higher e ciency rate. That makes the whole system more e cient com- pared to using the high pres- sure pump to change the feed ow into the system. This was rst introduced around 12 years ago, and ini- tially the focus was energy recovery devices in the big desalination plants. The technology has evolved and in more recent facilities, the energy recovery concept is now used for brackish water desalination and wastewater reuse. The upgraded wastewater reuse facility in Orange County, California, US, operated by the groundwater replenishment authority, is the largest in the world, using the approach of energy recovery devices for low salinity water (seawater salinity is 35,000 to 45,000 milligrams per litre, and wastewater salinity is usually 1,000 to 2,000 milligrams per litre). This energy recovery concept has evolved from seawater to brackish water and wastewater reuse. Nikolay Voutchkov is a director of the IDA, and president of Water Globe Consulting We have new concepts to reduce energy use 'The technology has evolved and energy recovery is now used for brackish desal and wastewater reuse' Nikolay Voutchkov