CHARACTERIZATION OF SEWAGE-SLUDGE AND TOXICITY EVALUATION WITH MICROALGAE

Heavy metal contents (Cd, Cr, Co, Ph, Ni and Zn) of digested sludge, c ollected from sewage treatment works (Shatin (ST), Shek Wu Hui (SWH), Tai Po (TP) and Yuen Long (YL)) in 1992, 1993 and 1994, were analysed by atomic absorption spectrophotometry after sequential extraction by 1 M KNO3, 0.5 M KF, 0.1 M Na4P2O7, 0.1 M EDTA and 6 M HNO3. It was fou nd that the major forms of Cu, Ph, Ni and Zn were in the sulphide phas e, organically-bound phase, adsorbed phase and carbonate phase, respec tively. A dinoflagellate (Dunaliella tertiolecta) and a diatom (Thalas siosira sp.) were used as bioindicators to test the toxicity of digest ed sludge samples collected in 1994. Each of the algae were inoculated to elutriates of digested sludge at different concentrations. The tox icities of the four types of sludge, according to 96 h-IC(50)s, were r anked as follows: TP > ST > SWH > YL for D. tertiolecta and ST > SWH > TP > YL for Thalassiosira sp. For D. tertiolecta, a significant corre lation was found between algal inhibition and ammonium-N, both in slud ge (p < 0.05) and elutriates (p < 0.01).