The study compares how centralised and decentralised sewage systems in Sydney affect the amount of pathogens and nutrients that end up in drinking water sources, showing that decentralised systems can release more nitrogen and, in worst‑case scenarios, higher levels of phosphorus, Cryptosporidium and viruses.

Data collected from centralised and decentralised sewage treatment plants throughout Sydney's drinking water catchments was used to calculate the relative catchment loads of Cryptosporidium, enteric viruses, nitrogen and phosphorus for an initial screening assessment. Loads were assessed at median and 90 percentile values for expected and worst-cases scenarios. The expected scenario in the Sydney drinking water catchments is that decentralised systems (servicing 32,800 people) provide similar total loads to centralised systems (serving 70% of the catchment population) for total phosphorus (37,090 kg x y(-1)), Cryptosporidium (10(11) oocysts x y(-1)) and enteric viruses (9.1 x 10(13) y(-1)), but higher loads of total nitrogen (237,610 vs. 136,740 kg x y(-1)). Decentralised systems, however, were predicted to have higher loads in the worst-case scenario with 620,620 kg x y(-1) TN, 82,040 kg x y(-1) TP, 7.3 x 10(13) Cryptosporidium oocysts x y(-1) and 9 x 10(15) enteric viruses per year. Greater load variability was experienced with decentralised systems, which presumably reflects less reliability in their current operation and maintenance. Overall, catchment water quality is therefore not only affected by sewage disposal methods, but also failure issues. Decentralised system disposal to land may afford a degree of mitigation that can be enhanced, if the degree of failure is reduced.