Because large variations occur in the concentrations of viruses that enter treatment plants from season to season and from place to place, and even during a 24-hour period, field studies on the removal of viruses by treatment processes require temporal coordination of sampling. Quantitative methods for concentrating viruses must be developed to measure accurately the efficiency of virus removal by treatment processes in field situations. Extended settling, and storage of sewage and raw waters, reduce virus levels and deserve further study. Oxidation ponds must be reevaluated with regard to temporal matching of influent and effluent samples and with special care to prevent short-circuiting. Conventional and modified activated sludge plants must be reassessed with temporal matching of samples. Coagulation of viruses with metal ions requires field evaluation, and virus removal by filtration through sand and other media, under constant salt and organic loadings, needs both laboratory and field evaluation. A comparative study of water disinfectants related to specific conditions is needed. The toxicity, carcinogenicity, and teratogenicity of products resulting from disinfection must also be assessed. Other matters for investigation are: methods for quantitatively detecting viruses adsorbed on solids, the virus-removal capability of soils, better virus indicators, virus concentration in shellfish, the frequency of infection in man brought about by swallowing small numbers of viruses in water, the epidemiology of virus infection in man by the water route, the effect of viruses of nonhuman origin on man, and the occurrence of tumour-inducing agents in water.

Disinfectants↗

PubMed1973

Removal of viruses from sewage, effluents, and waters. I. A review.

All sewage and water treatment processes remove or destroy viruses. Some treatment methods are better than others, but none is likely to remove all of the viruses present in sewage or in raw water. Primary settling of solids probably removes a great many of the viruses in sewage because viruses are largely associated with the solids. Long storage of effluents or water is destructive to viruses. Activated sludge is the best biological method for removing viruses from sewage. Trickling filters and oxidation ponds are erratic, the latter probably because of short-circuiting. Coagulation with metal ions is the most effective single treatment method for removing viruses from sewage and from raw waters, according to laboratory studies at least. Lime is the best coagulant for these purposes in the rapidly virucidal high pH range. Polyelectrolytes also can sediment viruses. Rapid filtration through clean sand does not remove viruses, but filtration of coagulated effluents does, probably because the layering floc itself adsorbs viruses. Clays and carbon adsorb viruses to some extent, but the process is not efficient. Ultimately, disinfection should help to produce virus-free waters for drinking and virus-free effluents for discharge into waters with which man may come into contact. Because disinfection is not a simple matter, disinfectants must be selected according to need. Effluents and waters containing solids can probably be disinfected only by heat or by penetrating radiation, waters discharged into streams should not be disinfected with anything that will injure or kill aquatic life (unless the toxic products can be neutralized), and drinking-waters should carry a disinfecting residue.

Adsorption↗