Abstract
The efficiency of methods used to isolate virus from sludge was assessed by artificially inoculating sludge. Recoveries tended to be erratic and no method recovered more than 25%. A relatively cheap and simple method was finally adopted for routine monitoring of sludge samples involving elution of virus at ambient pH with 1% (w/v) skim milk, followed by concentration of the organic eluent by acid precipitation. The concentrate was then detoxicated and decontaminated by treatment with dithizone in chloroform. Infectious virus was assayed by the agar suspended plaque test. Over 400 sludge samples from 6 sewage works were assayed for enterovirus and additional wastewater treatment samples were also examined for indigenous rotavirus. The levels of enteroviruses isolated from sludge samples ranged from non-detectable to 1,429 pfu g -1 whereas over a short period up to 8 x 103 foci 1-1 of rotaviruses were detected in samples of settled sewage, activated sludge and final effluent. Generally, there was evidence of seasonal variations in the levels of all enteric viruses isolated. Over 80% of raw, consolidated raw and raw and humus mix sludge samples contained infectious virus. Anaerobic mesophilically digested primary sludges were equally infectious although virus levels were reduced by a factor between 2 and 5. Virus was less frequently isolated from consolidated digested sludges (42%), aerobic thermophilic digested sludge (50%) and surplus activated sludge (61%). However, after polyelectrolyte treatment of surplus activated sludge prior to centrifugation there was an increase in positive samples to over 80%. Chemical conditioning of raw sludge by either lime/coperas or polyelectrolyte prior to filter pressing very rarely produced an infectious cake whereas polyelectrolyte treated raw sludge samples dewatered by centrifugation were 57% positive. Enteric viruses isolated belonged to the poliovirus, Coxsackie B and echovirus groups. Other types were probably present but available methodology precluded their isolation.