Assessment of the impact of excessive chemical additions to municipal wastewaters and comparison of three technologies in the removal performance of pathogens and toxicity

SummaryThe removal efficiencies of pathogens such as Salmonella (S), helminths ova (H), protozoan cysts (P), total coliforms (TC), faecal coliforms (FC) and faecal streptococci (FS) by three treatment processes: aerated lagoon (AL), activated sludge (AS) and anaerobic membrane bioreactor (MBR) were evaluated by means of standard microbiological numeration methods. The micro-toxicity and phyto-toxicity of wastewaters were monitored by LUMIStox and germination index (GI) of Lepidium sativum tests.The results of municipal wastewaters receiving industrial effluents such as Sfax and Mahres were compared with other municipal wastewaters receiving mainly domestic effluents such as Ksour-Essaf.The anaerobic MBR allowed an effective removal of 100% for all the microorganisms tested. The average removal of TC, FC, FS, S, H and P was 1.65 log10, 1.42 log10, 1.23 log10, 0.91 log10, 52.23% and 76.15% in AL system and 0.62–0.84 log10, 0.87–0.93 log10, 0.71–0.78 log10, 0.81–2.71 log10, 59–74.1% and 59.84–72.2% in AS processes, respectively. LUMIStox and GI of L. sativum tests demonstrated that Ksour-Essaf wastewater (KW) was fairly toxic, while Sfax wastewater (SW) exhibited a high toxicity. This toxicity caused the inhibition of the anaerobic consortia when the MBR was fed with the SW. Moreover, the COD and the micro-toxicity increased during the day, parallel to the industrial and domestic activities resulting in the toxic character of SW during the day. SW treated in the AL remained toxic whereas the toxicity of KW treated in the AS or in anaerobic MBR decreased considerably. However, the anaerobic MBR showed a high efficiency in removing toxicity for both SW and KW. The quality of the anaerobic MBR permeate largely conforms with the microbiological WHO guidelines for unrestricted irrigation.