Survival and accumulation of microorganisms in soils irrigated with secondary treated wastewater

Secondary-treated wastewater was used to irrigate forage crops during two years (1994 and 1995). The bacteriological quality of the soil irrigated with both secondary treated wastewater (without chlorination) and with potable water was evaluated. The following three treatments were investigated in this study: i) irrigation with potable water in amount equivalent to 100% of the class A Pan evaporation reading (as taken from the nearest Meteorological Weather Station); ii) irrigation with treated wastewater in amount equivalent to 100% of the A Pan reading and iii) irrigation with treated wastewater in amount equivalent to 125% of the A Pan reading. At the end of the last growing season, soil samples were collected at two soil depths and subjected to microbiological analysis. The bacteriological analysis showed that the total aerobic bacterial counts of surface soil were similar in all irrigated plots, suggesting that the use of this wastewater did not stimulate or inhibit these microflora. Surface soil was found to have higher bacterial counts than deeper soil suggesting bacterial removal from irrigation water by the first few centimeters of soil due to natural infiltration of soil. The total coliforms ranged from 2.1 x 10(3) CFU/g to 4.2 x 10(3) CFU/g while fecal coliforms were less, ranging from 1.2 x 10(2) CFU/g to 4.2 x 10(2) CFU/g. No detectable helminth eggs were recorded in the treated wastewater used for irrigation. The results strongly suggest the necessity to treat wastewater effluents to an extent to which no or very few residual bacterial contaminants will be detected.