Water quality changes following deep well injection of BWRO concentrate. Results from the BWRO pilots Noardburgum and Zevenbergen
Brackish water reverse osmosis (BWRO) is an interesting source treatment option for drinking water, and under serious consideration in the Netherlands. The drivers consist of environmental problems like salinization of fresh water wells, drawdown of water tables in phreatic aquifers and increasing costs to produce drinking water from polluted, fresh groundwater. The major obstacle for large scale application of BWRO is the disposal of the membrane concentrate. Deep well injection into a (more) saline, confined aquifer is considered to be the best solution to this waste water problem, though permitting of deep well injection is still problematic. Bottleneck is the potentially negative effects of BWRO concentrate injection on the water quality of the disposal aquifer.
The effects of BWRO concentrate injection on the hydrochemistry of the disposal aquifer were studied at two pilot locations in The Netherlands: Noardburgum (water supply company Vitens) and Zevenbergen (Brabant Water). At both locations, brackish water was abstracted, desalinated using reverse osmosis (50% recovery) and the membrane concentrate was injected into a deeper, confined aquifer. Both BWRO concentrates fitted neatly into the natural chemical environment of the disposal aquifer. This was demonstrated for the major constituents and trace elements. At the Noardburgum site, injected BWRO concentrate passed through the aquifer almost unaltered, despite supersaturation towards several carbonate and phosphate minerals. At Zevenbergen, there were many hydrogeochemical interactions between the injectate and the aquifer, which resulted in a complex pattern of water quality changes, including precipitation of calcite and siderite, dissolution of minerals rich in magnesium and strontium, and sorption of phosphate, silica, arsenic and nickel from the injected solution. In comparison with the native groundwater, injection had no negative effects on the water quality of the disposal aquifer at Zevenbergen. At Noardburgum, iron was the only water quality parameter to increase (substantially) upon injection, resulting from the high iron levels in the BWRO concentrate. From an environmental viewpoint this increase in iron concentrations was irrelevant.