Effects of sewage treatment on the removal of Listeria monocytogenes

Abstract

Two sewage treatment plants in Baghdad, Iraq, were investigated to assess the effects of the different treatment stages on the removal of Listeria monocytogenes. The bacteria were severely affected after the activation and digestion stages at both plants. A dramatic decrease in numbers of listerias after each of these two stages was noticed during the cold months (September-January). The organisms were able to survive these treatments and were present in the final effluent and even in low numbers in the sewage sludge cake. Sufficient dewatering of sewage sludge is recommended to obtain sewage free of listerias. Improvements in the isolation procedure of L. monocytogenes from such heavily contaminated material is also discussed.

Occurrence of Listeria spp. in Effluents of French Urban Wastewater Treatment Plants

“…Significantly lower prevalence and densities of Listeria spp. were observed in raw sludge in Summer in contradiction with previous studies, possibly due to insufficient monthly samplings and/or lack of adequate statistical tests (2,6,11,16).…”

“…Previous surveys on the occurrence of Listeria spp. in wastewater treatment plant effluents (1, 2, 4-6, 8, 11, 13, 14, 16, 20, 25), most dating before 1990 (1,2,8,13,14,20,25), have generally focused on either treated waters (4,5,8,13,14) or sludge (6,11,20), in one (1, 4-6, 8, 13, 14, 16, 20) to three (2, 11) wastewater treatment plants. The aim of the present study was to investigate, using novel methods, the current occurrence of Listeria spp.…”

“…Among the 181 nonredundant strains, the most pathogenic species, L. monocytogenes, predominated (59.1% of the strains, including seven non hemolytic and phosphatidylinositol-specific phospholipase C negative, 71.8% of the samples), followed by L. innocua (46.5 and 37.6%, respectively), both species often coexisting (20.4% of the samples). In earlier studies (1,2,8,25), only L. monocytogenes, identified by unknown methods (8,25) or conventional tests including hemolysis (1, 2), was looked for. When Listeria species were differentiated by similar tests, the API Listeria kit or a heteroduplex mobility assay which does not detect mixtures of species, L. monocytogenes was generally found to be predominant, followed by L. innocua (4,5,10,11,13,14,20).…”

Listeria monocytogenesin Multiple Habitats and Host Populations: Review of Available Data for Mathematical Modeling

“…In another study, the most probable number (MPN) of L. monocytogenes in effluents produced in the course of animal production ranged from 5000 to Ͼ18,000 cells/L, while the MPN in sewage sludge ranged from 800 to Ͼ18,000 cells/L (Watkins and Sleath, 1981). The MPN of L. monocytogenes after sewage treatment reported by Al-Ghazali and Al-Azawi (1988a) in Iraq was 3-15 cells/mL in final effluent and Ͻ3-7 cells/mL in sludge cake, although all steps in the sewage treatment process were supposed to eliminate L. monocytogenes. The same study reported that sewage treatment during the cold months (September-January) decreased the number of viable Listeria by between 85.0% and 99.7%.…”

“…Unfortunately, data required for both components of the model are currently unavailable. However, the number of L. monocytogenes in effluent and sewage sludge estimated by such a model could be validated with the existing data on L. monocytogenes in products of sewage treatment plants (Al-Ghazali and Al-Azawi, 1988a;Fenlon et al, 1996;Watkins and Sleath, 1981).…”