Indicator organism levels in effluent from Queensland coastal STPS

Abstract

The microbiological quality of the effluent from 33 sewage treatment plants (STPs) along the Queensland coast was investigated as part of a project to develop a quantitative model to assess the health risks associated with irrigating sugar cane with treated effluent.

Initially a single sample, from each of the 33 STPs was tested to provide an indication of the microbiological quality of effluent available for irrigation use. Ten of these STPs, selected to give a range of effluent quality and treatment types, were then tested 4 to 6 times over a 12 month period to investigate in-plant variation. Microbial counts varied widely between and within STPs. Ranges for all 92 samples (with geometric means in brackets) were : thermo tolerant coliforms, 0 to 24,000,000(388) /100mL; Clostridium perfringens, 0 to 15,000 (90) /100mL; coliphages, 0 to 6,370 (4) /100mL; Cryptosporidium, 0 to 600 (<l0) /L and Giardia, 0 to 4,840 (98) /L. All of the 10 STPs which were sampled over 12 months had Giardia counts above 150/L in at least one sample, and one STP had counts above l,OOO/L for all 5 samples. No seasonal trends were evident in the counts of any of the microorganisms assayed, and no relationship was evident between microbial count and STP type, population size or STP throughput.

None of the statistically significant relationships between assays reported in this paper were strong enough for the reliable prediction of the results of one assay from the results of another.