Load of HeteLoad of Heterotrophicrotrophic and Nitrifying Bacteria in the Sewage Lagoon and the Receiving River Buriganga

Abstract

The present study was undertaken to investigate the prevalence of some selected heterotrophic bacteria in one of the sewage lagoons and in the river Buriganga, which receives the sewage effluents from the sewage lagoons, with a view to evaluate the effect of sewage treatment. The heterotrophic plate count (HPC) ranged from 1.1 x 10⁸ to 3.0 x 10⁹ cfu/ml in sewage lagoon from January to December 2005. The HPC values ranged from 1.1 x 10⁵ to 3.0 x 10⁹ cfu/ml in the outfall of the sewage effluents and other sampling locations in the river Buriganga. The total and faecal coliform counts of the sewage lagoon and the discharge point in the river were always very high as >2.4 x 10³ cfu/100 ml in all seasons as measured by the MPN method. Salmonellae were isolated occasionally, while vibrios were frequently isolated from all the sampling points from throughout the year. The MPN values of nitrifying bacteria were also estimated to compare with HPC values. Ammonium oxidizers ranged from 1.1 x 10² to 1.1 x 10³/ml in the sewage lagoon and 2.0 x 10¹ to 2.2 x 10³ /ml in the river water. Nitrite oxidizers showed very low MPN values. This study suggests that the water of the river Buriganga was heavily polluted. It also suggests that the oxidation or self-purification system in the oxidation lagoons and subsequent treatment of the effluents were far from satisfactory level. Even the discharged sewage in the river Buriganga remains equally contaminated, which had been demonstrated by the presence of large number of pathogenic organisms. This poses health hazards to the users of the water from the river and also poses risks for the aquatic organisms of this water body.