Disinfection of effluents from municipal sewage treatment plants with peroxy acids

Two peroxy acids, peracetic acid (PAA) and peroxy-monosulfuric acid or caro acid (CA) were tested as alternative bactericidal agents for the disinfection of effluents from municipal sewage treatment plants and their action was compared with chlorine. PAA added in a concentration of 2000 and 400 ppm, commonly used in medicin, killed in a few minutes 99.9% or more of most bacteria of the effluents (Fig. 1, A, B, C, D, E) excepted staphylococci and micrococci (Fig 1, F) and endospores (Fig. 1, G). These effects were comparable with those obtained with 5 ppm chlorine. Although these results indicate a very good killing effect, such concentrations of PAA are not applicable in practice, as they lowered the pH and increased the BOD of the effluent. When PAA was added in smaller amounts (10 and 5 ppm) no change of the pH and only a negligible increase of the BOD (Table 1) occured. These concentrations with a contact time of 15 min, gave a very good reduction of most bacterial groups, varying between 96% (total count, Fig. 1 A) and 100% (group-D streptococci, Fig. 1 E). With 1 ppm PAA the bactericidal action was slower, but after 30 minutes contact time the effects were similar to these of 5 ppm. It was also demonstrated that the effect of PAA was not affected by the number of bacteria in the effluent before treatment (Table 2), but well by the BOD. The addition of the inorganic CA in high concentrations had only a disadvantageous effect on the pH of the effluent, but no effect was noted when it was added in concentrations of 10 ppm. In this concentration, contact times of 120 min were necessary to kill about 90% of most bacteria (Fig. 2), showing that CA was only a weak disinfectant with slow action. It may be concluded that 5 ppm PAA could be a good alternative disinfectant for secondary effluents with a high degree of purification.