Treatment Options Available To Us

Primary treatment of municipal waste involving settling and retention removes very few viruses. Sedimentation effects some removal. Virus removal of up to 90 percent (which is a minimal removal efficiency) has been observed after the activated sludge step. Further physical-chemical treatment can result in large reductions of virus titer, coagulation being one of the most effective treatments achieving as much as 99.99 percent removal of virus suspended in water. If high pH (above 11) is maintained for long periods of time, 99.9 percent of the viruses can be removed.

Of all the halogens, chlorine at high doses (40 mg/1 for 10 min) is very effective, achieving 99.9 percent reduction. Lower doses (for example, 8 mg/ 1) result in no decrease in virus.

As a result of several studies, the following conclusions regarding viruses in sewage warrant consideration: (1) primary sewage treatment has little effect on enteric viruses; (2) secondary treatment with trickling filters removes only about 40 percent of the enteroviruses; (3) secondary treatment by activated sludge treatment effectively removes 90 percent to 98 percent of the viruses; and (4) chlorination of treated sewage effluents may reduce, but may not eliminate, the number of viruses present.

The current concept of disinfection is that the treatment must destroy or inactivate viruses as well as bacillary pathogens. Under this concept, the use of coliform counting as an indicator of the effectiveness of disinfection is open to severe criticism given that coliform organisms are easier to destroy than viruses by several orders of magnitude.

An important concept is that a single disinfectant may not be capable of purifying water to the desired degree. Also, it might not be practicable or cost effective. This has given rise to a variety of treatment combinations in series or in parallel. The analysis further indicates that the search for the perfect disinfectant for all situations is a sterile exercise. It has been estimated that in the United States only 60 percent of municipal waste effluent is disinfected prior to discharge and, in a number of cases, only on a seasonal basis. Coupling this fact with the demonstration that various sewage treatment processes achieve only partial removal of viruses leaves us with a substantial problem to resolve.

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