A novel approach to anaerobic groundwater treatment

Abstract

Oasen has set itself the goal to “produce water of an impeccable quality and to delivery it flawlessly to its customers”. In formulating this goal, Oasen expresses its concern for unknown pollutants and the increasing concentrations of organic micro-pollutants in its source waters (primarily anaerobic groundwater). To achieve this goal, the company has devised a new treatment approach which is first to be implemented at an outdated treatment plant called ZS de Hooge Boom by 2018. The new approach centres on Reverse Osmosis (RO) as a primary barrier against pollutants, treating the anaerobic groundwater directly after abstraction. RO is followed by ion exchange (IEX) columns to remove remaining ammonium and a remineralisation step which is used to add calcium and magnesium. After these three treatment steps, the water is still anaerobic and contains methane (2700 ?g/L CH?). An additional step, capable of removing sufficient amounts of methane to produce biologically stable water, had to be designed. In contrast to conventional treatment, where part of the methane is broken down biologically in sand filters, the removal had to be achieved by a single aeration system. Additionally, experts expected large quantities of biomass to be produced on even very small concentrations of methane, which would lead to biological instability. There is however no legislation on the allowable concentration of methane and there was till now, no direct assay quantifying the effect of methane on the biological stability in drinking water. Two questions, therefore had to be answered: How much methane should be removed in order to produce biologically stable water? Which (aeration) system is most optimal to achieve this concentration? Growth potential tests were carried out on batches of samples containing various concentrations of methane. The yields of methane-oxidising bacteria (MOB) were calculated and the difference in cell volume between MOB and AOC bacteria (from control sample) was analysed. The results show that the yield of MOB on methane falls in the range of 8.6 x 10?-1.7 x 10? cells/?g CH? and that there is a factor 2 cell volume difference between MOB and AOC bacteria. Since the yield of bacteria on AOC is known (1x10? cells/?g AOC) the following bulk parameter for biological stability was derived: Bulk GP=C_AOC +C_(?CH?_4 )/Y_f =C_AOC +C_(?CH?_4 )/2.7 [?g AOC eq./L] This parameter takes into account the cell yields and the difference in cell volume so that the resulting value is a measure of the bio-volume produced on a certain amount of AOC and CH? and expresses this in AOC equivalents. With Oasen’s treatment approach, the AOC content is expected to be very low (