Chlorination for the control of biological activity during direct recharge of tertiary effluent was studied in bench-scale simulated aquifers. Both free chlorine and chloramine were studied at Darcy velocities of 360 cm/d to 720 cm/d. A free chlorine residual of 5 mg/l or above inhibited biological activity in the 1.2 m of soil that was studied. A free chlorine residual concentration of 2 mg/l prevented biological activity in the first 0.9 m of exposed soil and significant biological clogging was observed in soil greater than 0.9 m from the injection point. A 2 mg/L residual concentration of free chlorine prevented biological clogging over the 0.9 m of soil adjacent to the injection point. No chlorine addition resulted in clogging adjacent to the injection point indicating that the chloramine inhibited biological activity and allowed biological activity to occur over a greater distance from the injection point. Dissolved oxygen levels decreased to zero in aquifers where significant biological activity was observed and trihalomethane concentrations decreased in these aquifers. Free chlorine appears to effectively control biological clogging adjacent to the injection point while permitting biological activity to develop after the chlorine has decayed to a level of 0.6 mg/l or less.

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