Effects of heavy metals on the survival of heterotrophic bacteria in waste stabilization ponds.

Waste stabilization ponds are potentially the most important method of wastewater treatment in hot climates. They are the cheapest and simplest of all wastewater treatment technologies and are capable of providing very high quality effluent. This work is mainly based on the field and laboratory investigations done at both University of Dar es Salaam waste stabilization ponds and the Vingunguti waste stabilization ponds in Dar es Salaam, Tanzania. The ponds are treating domestic wastewater and municipal wastewater, respectively. Samples for laboratory examination of the effect of heavy metals were collected from the University of Dar es Salaam waste stabilization ponds. These ponds were chosen because their wastewater do not contain large concentration of heavy metals. Heavy metals were artificially added to produce concentrations of up to 20.0 mg/1 and their effects on the survival of heterotrophic bacteria were examined. Field examination of the effects of heavy metals were carried out at the Vingunguti waste stabilization ponds which treats wastewater containing industrial wastes. Copper, Lead and hexavalent Chromium metal ions were tested for the toxicity to the heterotrophic bacteria populations in the University of Dar es Salaam waste stabilization ponds. Manganese, Copper, Iron and Chromium are some of the heavy metals which were detected at the Vingunguti waste stabilization ponds. The laboratory experiments showed that the mortality rate of heterotrophic bacteria was significantly influenced by the concentration of heavy metals. The bacteria mortality rates at concentration of 5 mg/1 were 0.015 h-1, 0.019 h-1, and 0.014 h-1 for Lead, Chromium and Copper, respectively. When higher heavy metal ions concentration of 10 mg/1 was applied, there was a very sharp drop increase in the mortality rates of heterotrophic bacteria to 0.039 h-1 0.037 h-1 and 0.029 h-1 for Lead, Chromium and Copper, respectively. The mortality rate further increased to 0.065 h-1 for Lead, 0.048 h-1 for Chromium and 0.047 h-1 for Copper, when the concentration of metal ions was increased to 20 mg/l. Field studies at Vingunguti ponds have shown that the mean concentration of heavy metal was 1.7 mg/1 (0.6,- 10.1) for Copper, 0.2 mg/1 (0.04 - 1.2) for Chromium, 1.3 mg/1 (0.36 - 6.4) for Iran, and 4.8 mg/1 (1.0 - 14.5) for Manganese. The pH values followed a customary trend and increased from the influent towards the final effluent. The higher pH values seem to cause the metal ions to precipitate, this was observed at the Vingunguti ponds where metal ions concentration was the highest at anaerobic ponds outlet whose pH values were minimum.