Coliform bacteria in an aerobic sewage treatment plant: their survival and significance as water quality indicators

Abstract

An attempt was made to determine if coliform bacteria populations in an aerobic sewage treatment plant changed in species composition and if there were any changes in their characteristics. Bacteria were isolated on media with minimal selectivity which relied on utilisation of glucose rather than lactose. Spread plate (SP) and membrane filtration (MF) methods were used and compared with coliform MF methods.

To test whether there was a change in the coliform population during sewage treatment, fifty morphological and biochemical tests were carried out on each isolate. The results were analysed by computing Euclidean distances and using complete linkage for numerical classifications to form clusters of isolates with similar characteristics. Clusters were identified by the inclusion of reference cultures of Enterobacteriaceae and by identification of a selection of isolates by API 20E methods.

Of the 857 isolates characterised and classified, 369 (43.1%) were identified as Escherichia, Citrobacter, Enterobacter or Klebsiella spp. The SP and MF methods with glucose-containing media (PRGA-SP and TGEA/PRGB-MF) detected fewer false-positive coliform bacteria than the coliform MF method using M-Endo LES agar.

Average numbers of E. coli in raw and treated sewage were 4.2 x 10⁵/ml and 1.7 x 10³/ml respectively; numbers of Citrobacter, Enterobacter arid K1ebsiella spp. Together were on average 1.2 x 10⁶/ml and 1.0 x 10⁴/ml respectively. Numbers of bacteria in clusters in which E. coli; were placed were reduced by 99.4 - 99.7% during sewage treatment, and those in clusters of Citrobacter, Enterobacter and Klebsiella spp. by 95.9-99.8%. Isolates in some clusters of these four genera were better able to survive during sewage treatment than others. E coli declined in proportion to the whole coliform population.

The numbers of bacteria represented by each cluster fluctuated markedly with time, suggesting a succession of species and biotypes of bacteria in sewage. In most cases, bacteria isolated in treated sewage were detected in raw sewage sampled at the same time, suggesting that removal of bacteria during sewage treatment was more likely than growth of any particular biotypes.

Discriminant analysis showed that there were small but significant differences between isolates from raw or treated sewage and those isolated by different methods. Biotypes of E. coli, Citrobacter, Enterobacter and Klebsiella spp. detected by the coliform and faecal coliform membrane filter methods could be distinguished most clearly from those isolated by the PRGA-SP and TGEA/PRGB-MF methods.

E. coli was the only species which could be clearly and easily separated from other coliform bacteria and species of other genera using a modified IMViC scheme. Other coliform bacteria could not be separated. Isolates identified as E. coli were placed in four separate clusters, i.e., distinct biotypes.

It was concluded that because E.coli could be most easily identified, is the most numerous coliform bacterium in faeces and the least likely to multiply in natural situations outside the gut, it should be a valid indicator of faecal contamination, even after sewage treatment.