The physico-chemical quality of effluents of selected sewage treatment plants draining into river Rwizi, Mbarara Municipality, Uganda

Abstract

As population increases in urban areas, the domestic and industrial activities increase resulting in an increase in the volumes of wastewater and anthropogenic pollution, hence posing a threat to public health and environment. This study assessed the physical chemical quality of two main sewage plant effluents discharging into River Rwizi. Effluent water samples were analyzed for dissolved oxygen, colour, turbidity, total suspended solid, total iron, phosphates, alkalinity, magnesium, calcium carbonate, temperature, pH, ammonium, electrical conductivity, chloride and nitrates. Parameters were analyzed following standard methods of APHA (1985). The values obtained were compared with EPA (2001), NWSC (2015) and NEMA (1999) standards for waste water. Results showed that the mean values most of the parameters tested were higher than the recommended EPA, NWSC and NEMA standards. The mean colour was 1627.67°C and 1414.33°C in Kakoba and Taso sewage effluents respectively compared to EPA (20-150), NWSC (500) and NEMA (300) standards (p > 0.05). The mean alkalinity was 1390.17 mg/l and 1308.33 mg/l for Kakoba and Taso respectively compared with EPA (400) and NWSC (800) standards. DO had a mean concentration of 68.27 mg/l and 63.03 mg/l in Taso and Kakoba respectively compared to EPA and NEMA standard of 5 mg/l. Mg was 243.29 mg/l and 246.49 mg/l in Kakoba and Taso sewage effluents respectively compared to NEMA standard for waste water of 100 mg/l (p > 0.05). The mean pH was 8.26 and 8.16 in Taso and Kakoba sewage effluents respectively compared to NWSC and NEMA standard of 6.0 - 8.0. Phosphate mean concentration levels were 32.2 mg/l and 27.11 mg/l for Taso and Kakoba respectively compared to standards of EPA (0.5 - 0.7 mg/l) and NEMA (10 mg/l). NO3 was 10 mg/l and 5.83 mg/l in Kakoba and Taso sewage effluents respectively compared to NWSC maximum permissible limit of 5 mg/l. The mean NH4 concentration was 385.33 mg/l (Kakoba) and 50.0 mg/l (Taso) compared to the EPA guideline range (0.2 - 4 mg/l). Chloride (Cl) had a mean of 833.33 mg/l in Kakoba compared to EPA (250 mg/l), NWSC and NEMA (500 mg/l) standards. Therefore the study recommends for effective treatment of waste effluents from Kakoba and Taso sewage treatment plants before recycling in order to avoid pollution of river Rwizi.