THE EFFECT OF LOW CONCENTRATIONS OF ASSIMILABLE ORGANIC-CARBON (AOC) IN WATER ON BIOLOGICAL CLOGGING OF SAND BEDS

Infiltration of pretreated surface water with recharge wells is hampered by biological clogging processes in the aquifer, The influence of the concentration of easily assimilable organic carbon (AOC) in water on clogging was investigated in filter beds operated under well-defined laboratory conditions using acetate as a model substrate. Acetate concentrations in the water as low as 0.01 mg C/1 promoted clogging with the main head loss, caused by bacterial growth, in the first centimeter of the sand bed. An empirical model was developed describing a linear relationship between the operation period to reach a certain increase in head loss (T(Pi)) and the reciprocal value of the acetate concentration or acetate load [L(ac) g C/(m2 . d)] at a constant filtration rate. The rate of clogging, designated as the n-microbiological clogging factor (C(f)), is defined by the slope of the linear relationship between the square root of the increase of the head loss (P(i)1/2) and the operation time. This linear relationship demonstrated that the increase in clogging rate was constant with time. Observations at several locations with experimental recharge wells revealed that the AOC concentration is an important parameter for the biological clogging potential of water. The AOC concentration of water used for infiltration in a recharge well should be less than 0.01 mg acetate-C equiv/1 to prevent biological clogging for a period of more than a year.