CP-1994-15
Assessment of four rainwater catchment designs on cistern water quality
Faisst, Eric W., and Roger S. Fujioka
This paper reports on an experiment in which rainwater was collected from the same corrugated metal roof of a given building using for different collection designs, each with an identical 55-gallon plastic cistern tank as the catchment device. The four variations in design were a covered tank with screen device (Gutter Snipe), a covered tank with screen and foul-flush diverter, a covered tank with screen and sand/gravel/charcoal filter, and an uncovered tank as a control. The drinking water quality of the four cisterns was evaluated by testing for concentrations of bacteria (total bacteria, total coliform, fecal coliform, and hydrogen sulfide producing bacteria), conductivity, turbidity, PH, and phosphates. The results indicated that tank 4 (samd/gravel/charcoal filter) demonstrated the lowest levels of indicator bacteria, ranging from 0 to 6 fecal coliforms per 100 ml (average of 1.3 CRU/100 ml); followed by tank 3 (foul flush) with a range of 0 to 168 CFU/100 ml (average of 17.8 CFU/100 ml); tank 2 (screen only) with a range of 0 to 388 CFU/100 ml (average of 51 CFU/100 ml); and tank 1 (uncovered) with a range of 0 to 2.59 x 104 CFU/100 ml (average of 2340 CFU/100 ml). However, concentrations of at least one of the three fecal indicator bacteria (total coliform, fecal coliform, and hydrogen sulfide producing bacteria) were often recovered from these waters, indicating that the systems designs were not able to meet U.S. microbiological drinking-water standards. All four systems were below the maximum turbidity level of 5 NTU established for individual sources. These cistern waters were also determined to have very high concentrations of total bacteria, always exceeding 500/ml, the minimum level at which interference with the recovery of total coliform bacteria has been reported. Fecal (44.5°C) and total (33.5°C) coliform results were compared with the hydrogen sulfide MPN method (room temperature). The hydrogen sulfide test correlated better with fecal coliform results as compared to talk coliform results after a 24-hour incubation period.