Behavior of DOC and AOX using advanced treated wastewater for groundwater recharge

Laboratory studies with continuous-flow columns containing aquifer material were conducted to evaluate the potential of sorption and biodegradation of halogenated organic compounds (AOX) and dissolved organic carbon (DOC) in tertiary effluents during soil–aquifer treatment (SAT). To improve water quality, powdered activated carbon (PAC) treatment and the use of ozone were also investigated. Batch experiment results indicated that sorption processes are not significant concerning DOC and AOX removal in SAT. Biodegradation of DOC during SAT can be described by a first-order macroscopic kinetic model. However, 65% of DOC was not biodegradable during SAT. These refractory compounds are similar to humic and fulvic acids and are probably formed in the activated sludge treatment or are present already in the drinking water which has a DOC level of 3–5 mg l−1. The elimination of AOX under anoxic conditions is probably based on cometabolism. Poorly degradable DOC does function as a cosubstrate in AOX-cometabolism and it is likely that further AOX-degradation will occur until local background concentration are reached. PAC (100 g m−3 W-35, Norit Co.) preferentially adsorbed refractory, high molecular weight fractions and up to 50% of the AOX concentration in tertiary effluent was reduced. Ozone treatment (0.86 and 1.89 g O3 g−1 DOC0) improved the biodegradability of refractory organic compounds and cosubstrate concentration for AOX-cometabolism in subsequent SAT. This research concludes, that refractory DOC is more important in soil–aquifer treatment systems than AOX compounds, commonly considered to be of greater concern.