Environmental, health and handling impacts of using chemical assisted sedimentation on land application of sludge from Cronulla Sewage Treatment Plant

Quality and handling costs are considerations for agricultural usage of sewage sludge. These considerations were studied in three parts when CAS was trialled at Cronulla STP: (1) for its effect on the raw sludge in (a) the % total and volatile solids, (b) production volume variants, (c) the variants in the levels of Al, As, Bo, Ca, Cd, Cr, Co, Cu, Fe, Hg, K, Mg, Mo, Mn, Na, Ni, P, Pb, Se and Zn. Groups (a), (b) and (c) were measured by dried and ashed weights, sludge hydraulic volumes, ICP—AES and ICP—MS assays, respectively. The quality control of the measurements for groups (a) and (c) was emphasised, ensuring data patterns were not sampling/analytical/dosing‘ solution relayed, (2) for the potential effects of the raw sludge on operational steps of stabilisation, dewatering, stockpiling, (3) the potential effects on plant uptake of nutrients, contaminants and hydrologic effects of sludge/soil mix. CAS was dosed using coagulant aids of FeCl3 and polyacrylate and polyacrylamide. The dosing was at a rate of 0, 22.5, 33.8 and 45 mg of FeCl3 to one litre of influent, with combinations of copolymers ranging from 0.2 to 0.4 mg in two treatment trains; the main focus was at the 0.2 mg level. The increase in sludge Fe was found to be approximately 3000 ppm (dry weight) for background, to the range of 30000 to 50000+ ppm (dry weight) with dosing. Comparison of Fe data against dose rate indicated significant differences in the log slopes for the two treatment trains. Fe mass balance between calculated and assayed Fe values suggested a lack of dose rate accuracy, at the lower range of dosing for one treatment train and at the higher range for the other train. To overcome the inaccuracies in dosing, the analyte responses reported were based on analyte/Fe comparisons, transposed back to analyte/true dose rates. Analysis of variance indicated phase factors were significant and diurnal variability factors not significant (P = 0.01 level)