Effect of treatment at the sewage works on the numbers and types of bacteria in sewage

Abstract

It is well known that successive stages of treatment at a sewage works reduce the quantity of suspended solids, the content of organic matter, and the biochemical oxygen demand of sewage, so that the polluting strength of the final effluent is a small fraction of that of the sewage received at the works.Much less is known, however, about the effect of treatment on the bacterial flora of sewage. An investigation has therefore been made at two sewage works at each of which treatment in percolating filters and humus tanks is followed by treatment in a sand filter or an anthracite filter. Samples taken at various stages of treatment from both works over a period of several months were submitted to bacteriological examination to find the plate count on nutrient agar and the counts of coli-aerogenes bacteria, of faecal Bacterium coli and of Streptococcus faecali8. A large number of pure cultiires of coliform bacteria and of streptococci were isolated and their characters were determined. Chemical analyses were made on the same samples as were used for bacteriological examination.The data as a whole gave a general picture of the extent to which treatment at the works reduced the numbers of bacteria in different groups, of the relative parts played by percolating filters and by sand filters in this process, and of the numbers of bacteria discharged to a river with a final treated effluent. PREVIOUS WORKOne of the earliest processes of sewage treatment consisted in applying settled sewage intermittently to slow sand filters. Early work at the Lawrence Experiment Station (Massachusetts State Board of Health, 1887Health, -1915 established the fact that if the interval between doses was sufficiently long to allow the bed to drain and dry on each occasion, intermittent filtration through fine sand yielded an effluent in which the bacterial count was a small fraction of that of the settled sewage applied to the bed. Thus a filter containing coarse clean mortar sand (effective size 0-48 mm.), 5 ft. deep, which consistently received sewage at a rate of 74,000 gallons*/acre/day removed on an average 94 % of the organic matter and 98 % of the bacteria.A similar filter with finer sand (effective size 0'06 mm.) treated 43,000 gallons/acre/day and removed 97.5 % of the organic matter and 99-99 % of the bacteria from sewage.As with slow sand filters used in the treatment of water, the efficacy of these filters depends largely on the presence of a biological film which reduces the volume of the interstices between the grains of sand and at the same time removes organic matter in the water by using it as a source of food for growth and energy for the micro-organisms in the film.

Effect of treatment in percolating filters on the numbers of bacteria in sewage in relation to the composition and size of filtering medium

“…The numbers decreased during summer and autumn falling to a minimum of 400,000 in December. The increase in numbers of bacteria in settled sewage during the warmer months has been noted on previous occasions and Allen et al (1949) suggested that growth may take place in the sedimentation tank during the warmer months of the year. Table 2 gives the results of storing fresh sewage in completely filled bottles for different periods at 6 and 200 C. Temperature had no significant effect on the total count after storage for 24 hr.…”

“…On the second occasion when 374 Effect of sewage treatment on bacterial numbers 375 100 cultures were obtained from sewage and 100 from effluent, 59 % of those from sewage and 63 % of those from effluent were E. coli Type I. These proportions are similar to those found in sewage and filter effluent by Allen et al (1949) and bear out their conclusion that the filter does not have a significant selective action on the different types of coliform organisms. Table 6 of haemocytometer counts made from March to May 1960 show that the sewage contained 4*85 x 108 particles per ml., whereas the concentration in the effluents ranged from about 2 to 8 x 107 per ml.…”

Improved methods for determining the most probable number ofBacterium coliand ofStreptococcus faecalis

“…From 67 tubes, positive at 450 C. in the 'resuscitation' method, pure cultures were isolated by streaking on glucose-yeast extract agar, incubating at 370 C. and subculturing from the resulting colonies. These cultures were tested as described by Allen et al (1949). Of 67 strains isolated from 34 samples of polluted waters all were Gram-positive cocci, in pairs and short chains, which grew at pH 9'6 and in broth containing 6'5 % sodium chloride, which survived heating at 600 C. for 30 min., and which acidified litmus milk.…”

Reduction of Net Sulfide Production Rate by Nitrate in Wastewater Bioreactors. Kinetics and Changes in the Microbial Community

“…Among the biological methods, addition of nitrate is considered one of the most suitable to control sulfide production (Jiang et al 2009;Mohanakrishnan et al 2009). However, different mechanisms have been suggested via which nitrate might reduce sulfide concentration: (1) by increasing the redox potential (Allen et al 1949;Poduska and Anderson 1981), (2) by increasing pH (Aelion et al 2000), (3) by inhibiting SRB (Yuan et al 2010;Eckford and Fedorak 2004;Jenneman et al 1986), (4) by causing a competition of SRB with heterotrophic nitrate-reducing bacteria for organic electron donors (Hubert and Voordouw 2007), (5) by the preferential use of nitrate as electron acceptor instead of sulfate by some species of SRB (Tang et al 2009), or (6) by stimulating sulfide oxidation by nitrate-reducing sulfide-oxidizing bacteria (NR-SOB) (Mohanakrishnan et al 2009;Garcia de Lomas et al 2007;Vaiopoulou et al 2005). Some of these mechanisms are not mutually exclusive, and their relative importance might change depending on environmental conditions.…”