Effectiveness of selected wastewater treatment plants in Yemen for reduction of faecal indicators and pathogenic bacteria in secondary effluents and sludge

The aim of this study was to evaluate the efficiency of wastewater treatment plants (WWTPs) in Yemen for reduction of faecal indicators and pathogenic bacteria in the secondary effluents and sludge. Hundred sixty bacterial isolates were obtained from 27 secondary effluents and sludge samples generated from Ibb wastewater treatment plant (IWWTP), Taiz wastewater treatment plant (TWWTP), Aden wastewater treatment plant (AWWTP₁ and ₂) and Sana'a wastewater treatment plant (SWWTP) in Republic of Yemen. Isolation of the bacteria was carried out by the direct plate method on the several selective media. The concentrations of faecal coliforms (FCs) were more than that recommended by World Health Organisation guidelines in all secondary effluents samples expect for those collected from TWWTP. FCs in the sludge from IWWTP and SWWTP were more than the standards limits recommended by United State Environmental Protection Agency (U. S. EPA) Class B, while sludge from AWWTP and TWWTP meet U. S. EPA standards limits Class A and class B, respectively. Among 160 bacterial isolates, E. coli was the most common (detected in 88.88% of the samples), followed by Streptococcus faecalis (70.37%), Klebsiella pneumonia (66.67%), Enterobacter aerogenes (59.23%), Salmonella typhi (33.33%), S. typhimurium and Shigella sonni (25.93% for each) and Yersinia pestis (22.22%). The sludge samples collected from IWWTP and TWWTP and stored for 24 weeks at room temperature (25 ± 2 °C) met the standards limits recommended by U.S. EPA, Class A.