Toxicity evaluation of sewage sludges in Hong Kong

Anaerobically digested sewage sludges collected from four wastewater treatm ent plants located in Sha Tin, Tai Po, Yuen Long, and Shek Wu Hui in Hong K ong were subjected to chemical characterization and toxicity testing to pro vide preliminary assessment of their suitability for land application. All sewage sludges were slightly alkaline with pH range of 8.3 - 8.7. Electrica l conductivity (EC; 0.69 dS m(-1)) and soluble NH4-N content (996 mg kg(-1) ) of sewage sludge from Yuen Long were lower than that of other plants. Con centrations of heavy metals were determined as total, extractable, and wate r-soluble fraction using mixed acid digestion, DTPA (pH 7.3), and distilled water, respectively. Yuen Long sludge was most polluted with Zn and Cr hig her than the pollutant concentration limits listed in Part 503 of USEPA, ow ing to the effluent coming from the tannery industry. High concentration of Ni was found in sludge from Sha Tin that originated mainly from the electr oplating industry. DTPA-extractable Zn contents were high in sludges from Y uen Long (1247 mg kg(-1)) and Shek Wu Hui (892 mg kg(-1)), while 3.7 mg kg( -1) of DTPA-extractable Cr was found in Yuen Long sludge. Metal speciation of sludges showed that Pb was major in residual phase while Cu, Cr, and Ni in organic and residual phases, and Zn did not show any dominant chemical p hase. The sludge extracts did not exert significant adverse effect on seed germination of Brassica chinensis (Chinese cabbage), but Yuen Long sludge c aused a reduction in root growth. In view of its lower EC and soluble ammon ia contents, the high concentration of Zn and Cr in Yuen Long sludge would likely be responsible for this adverse effect on root growth. Therefore, Yu en Long sludge would likely have a more serious impact on soil quality and plant growth as compared to other sludges. This would require further verif ication from greenhouse and field experiments. (C) 2001 Elsevier Science Lt d. All rights reserved.