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TECHNICAL REPORTS

Waste Management

Bacterial Removal and Protozoan Grazing in Biological Sand Filters

^a Department of Mathematical Sciences and Technology, Agricultural University of Norway, P.O. Box 5003, 1432 Ås, Norway
^b Technical Department of Ski County Council, P.O. Box 3010, 1402 Ski, Norway
^c Department of Chemistry, Biotechnology and Food Science, Agricultural University of Norway, P.O. Box 5040, 1432 Ås, Norway

^* Corresponding author (anne-marie.bomo{at}imt.nlh.no).

Received for publication September 24, 2002. The objective of the study was to investigate the importance of protozoan predation as a biological removal mechanism in sand filters used for purification of bacteria from wastewater. Eleven sand filter columns were seeded with a high dose of wastewater (70 mm d^–1) and a high concentration (10⁸ colony forming units [CFU] mL^–1) of Aeromonas hydrophila (American Type Culture Collection [ATCC] 14715) for a period of 30 d. Water samples from three filter outlets were analyzed for the concentration of A. hydrophila. In addition, one filter column was sacrificed each sampling day for the quantification of A. hydrophila, culturable bacteria (heterotrophic plate counts, HPC), total bacterial counts, and protozoa in the sand. The mean removal efficiency of A. hydrophila in the sand filter columns was 4.7 log units. The concentration of A. hydrophila in the sand filter effluent, however, had a clearly time-dependent pattern from high (log 6) and unstable concentrations to low and more stable levels (log 2). The removal efficiency of A. hydrophila correlated significantly (P = 0.0005, r² = 0.6) with numbers of protozoa in the sand filters. Significantly higher (P < 0.05) concentrations of A. hydrophila were observed in sand filter effluents from columns treated with the protozoan inhibitor cycloheximide, compared with nontreated columns. Results from the present study show that protozoan grazing plays an important role as a bacterial removal mechanism in sand infiltration systems.

Abbreviations: CFU, colony forming units • HPC, heterotrophic plate counts

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