Treatment of a hard borehole water containing law levels of pesticide by nanofiltration

Intermittent, low levels of pesticide in the borehole water at Debden Road water works, Saffron Walden, England, required an appropriate treatment for its removal. A second treatment goal was a reduction of the calcium hardness by about 50%. Nanofiltration utilising DOW NF 200 was the chosen process allowing to achieve both requirements. During the first year of operation, no significant decrease in permeability was observed and the plant was cleaned only once after an incident that caused heavy scaling on the membranes of the third stage. This scaling event was also clearly visible in the monitoring graph of the differential pressure normalized for the tangential flow rate. The pesticide concentrations in the permeate always remained far below the maximum values of drinking water standards. Due to a severe drought in the area, the plant had to be run at a feed flow rate that was 35% below its design capacity. The consequence of the low flux rate was an increased calcium passage that resulted in a calcium hardness of the permeate above the desired maximum. Several solutions were proposed to increase the calcium rejection and the chosen measure was to take 7 vessels out of service until the water availability will allow running the plant at its design capacity.