Desalination of brackish groundwater and concentrate disposal by deep well injection

In the province of Friesland (in the Northern part of The Netherlands), problems have arisen with the abstraction of fresh groundwater due to salinization of wells by upcoming of brackish water. A solution to this problem is to intercept (abstract) the upcoming brackish water, desalinate it with a brackish water reverse osmosis installation, and dispose the concentrate in a deeper, confined aquifer. The fresh-brackish interface in the source aquifer is stabilized by simultaneous abstraction of the fresh and brackish parts. After desalination, the abstracted brackish water provides an additional source for drinking water. To demonstrate the feasibility of this concept a pilot study was set up. In one year about 220.000 m3 of concentrate was produced and injected. The reverse osmosis installation was carried out under anaerobic conditions without pretreatment and antiscalant dosing. Despite the high iron concentrations (40 mg Fe/l) in the feed water, the installation performed was very stable. Although the concentrate was supersaturated toward carbonate and phosphate minerals (SI > 1), scaling or fouling of the membranes did not occur at recoveries of 50, 70, and 75%. The mass transfer coefficient or normalized flux (at 10°C) was stable at 0.85·10-8 m s-1 kPa-1. Water quality changes in the target aquifer were monitored by two observation wells, at 12 and 24 m distances from the injection well. Also the injection of the supersaturated concentrate did not lead to mineral precipitation in the target aquifer, indicating that deep well injection is technically feasible without risks of injection well or aquifer clogging. The fresh-brackish water interface in the source aquifer remained stable by the simultaneous abstraction of fresh and brackish water. This showed that the so called “fresh-keeper” concept works in practise, providing a successful remedy against salinization of fresh water abstraction wells. Overall, the pilot study showed that brackish groundwater provides an excellent, additional source for drinking water in The Netherlands or in other coastal areas worldwide, where fresh groundwater is scarce or where fresh water wells are threatened by salinization.