Correlation between biological agents and levels of heavy metals in municipal sewage sludge.

During the period 2003-2004, 110 samples of sewage sludge were examined from 18 treatment plants located in the territory of the whole of Poland, the majority in the Lublin Province. Samples of fermented and dehydrated sewage sludge were taken from sludge drying beds. The bacteriological and parasitological determinations covered bacteria of the Salmonella genus and eggs of the helminths Ascaris spp., Trichuris spp., and Toxocara spp. Chemical examinations for the presence of the heavy metals: chromium, zinc, cadmium, copper, nickel and lead were carried out by the method of atomic absorption flame spectrophotometry. The results were subjected to statistical analysis with the use of ANOVA and Student's t-tests. In the sewage sludge samples examined, Salmonella bacteria were isolated from 9 samples (8.2 % ), and the eggs of helminths from 31 samples (28.1 % ). The eggs of Ascaris spp. were most frequently detected--in 24 samples of sewage sludge (21.8 % ), followed by Trichuris spp. eggs--in 11 samples (10.0 % ), and Toxocara spp. eggs--in 6 samples (5.4 % ). Several samples of sewage sludge were infected by eggs of all the parasites examined (ATT). A very high variation was observed with respect to Salmonella bacteria and ATT eggs according to the origin, type of sanitation and duration of deposition of sludge. Levels of chromium, zinc, copper and lead in the sewage sludge samples, in the majority of samples examined, remained below allowable values. Very high values exceeding the allowable values for the levels of heavy metals were observed only in the samples of sewage sludge from one treatment plant. This concerned a cadmium level (201.1 mg/kg d.m.) which exceeded by 20 times the most rigorous allowable contents of this metal for sewage sludge designed for agricultural use, and nickel (288.7 mg/kg d.m.)--the standard being exceeded 3 times. A significant variation of the levels of heavy metals was observed depending on treatment type. A longer period of deposition of sludge on plots (over 6 months) causes a significant decrease of cadmium level, a decrease in the amount of Salmonella bacteria, and total elimination of Toxocara eggs. In the majority of cases no significant correlation was observed between the level of heavy metals and biological contaminants. A weak correlation was found between the concentrations of lead and chromium and the presence of the eggs of helminths (ATT), as well as between the concentration of zinc and the presence of Salmonella bacteria.