Evaluation of coliform tests for chlorinated secondary effluents. (1973)

The disinfection of wastewater efflu ents during the warmeather months is rapidly becoming a requirement in the U. S. In Illinois, year-round disinfection is likely. The most common method of disinfection at waste treatment plants is chlorination. Its effectiveness is usually measured by residual chlorine. The trend, however, is toward the use of indicator bac teria detection as a measure of effective ness. As requirements for effluent stan dards become focused on the limitation of certain indicator bacteria versus residual chlorine measurements, the need for de termining bacterial concentrations in chlo rinated effluents will become prevalent. Therefore, it would seem appropriate to ex plore the techniques required to assess the bacterial quality of chlorinated effluents. Two basic methods for the assay of coli form bacteria in wastewater are outlined in Standard Methods 1; they include the multiple-tube, most probable number (mpn) procedure and the membrane filter (mf) technique. However, Experience indicates that the membrane filter tech nic is applicable to the examination of saline waters but not chlorinated waste waters. x Because the mf technique is comparable to the multiple-tube procedure and is less time consuming, it seems un fortunate that the mf technique cannot be used as a control procedure by the waste plant operator who uses chlorination. McKee et al.2 reported on the lack of correlation between mpn and mf tech niques while assaying chlorinated settled wastewater for total coliforms. Because monochloramine is the predominant bac tericidal agent in chlorinated wastes, they advanced the hypothesis that partial re versibility is responsible for the discrep ancy between mpn and mf results; that is, the mf technique produces considerably fewer colonies than the number that de velop by the mpn method. Presumably, when inactivated cells are deposited on a membrane with limited nutrient availabil ity, the cells cannot rid themselves of monochloramine and therefore cannot grow. However, when inactivated cells are put in an aqueous medium rich in or ganic matter, such as lactose broth, the monochloramine may diffuse outwardly from the cells, permitting them to recover, grow, and produce gas. In the McKee et al.2 investigations, the culture media used for the mf technique was the same as that previously described.3 Dehydrated scheduled nutrient (dsn) pads were used. They contained two elements with an upper leaf impregnated with an Endo-type inhibitory nutrient. The results obtained using dsn pads with the mf tech nique were comparable to those obtained from the confirmed mpn procedures on raw settled wastewater. McCarthy et al.,4 though working initially with water, were not satisfied with the one-step, M-Endo broth mf techniques. Their work sug gested that enrichment plus an agar sub strate (e&a) was superior to the one-step technique based on a higher degree of coliform recovery. Examinations of natu ral waters and wastewater demonstrated that the e&a results were comparable to standard mpn data. From their work an agar-based medium (les M-Endo agar) was developed. Its use with the mf tech nique is basically a two-step enrichment procedure.