Characterization and stabilization of chemically assisted primary sedimentation process (CAPS) sludge

Abstract

Chemically assisted primary sedimentation process (CAPS) is adopted as Stage I of the Strategic Sewage Disposal Scheme (SSDS) implemented by the Hong Kong government. CAPS, using chemical coagulants to treat wastewater, is employed in a new sewage treatment plant (STP) on Stonecutters Island. The production of these large quantities of CAPS sludge creates disposal and utilization problems and stabilization of sludge before land disposal is one of the development trends for the disposal of sludge. In the present study, the effect of different coagulants on the performance of CAPS was investigated by jar tests and a large scale test. The various physical and chemical properties of chemically-treated sludge and the biologically-treated sludges collected from different STPs were compared. Then, the effect of pulverized fuel ash (PFA) and lime (CaO) stabilisation on the CAPS sludge was studied. The suitability of the alkaline stabilized sludge for land application purposes was also evaluated by (i) short -term cup trials on growing two edible crops, Chinese radish and barley, in stabilized-sludge amended soil mixtures; (ii) long-term pot trials on planting tall wheat grass in amended soil. Lastly, the impact of disposing stabilized sludge in landfill was studied by lysimeter tests. The jar test and pilot-scale results showed that CAPS could achieve better removal efficiencies of suspended solids, total nitrogen and phosphorous. It was found that there were significantly differences in both physical and chemical properties between the chemically modified sludge and the biological treated sludges. Also, the use of PFA and lime was able to render the stabilized sewage sludge meeting the USEPA's criteria for Process to Further Reduce Pathogens (PFRP) in addition to producing a final product with improved handling characteristics. The results from plant bioassay indicated that it was feasible to plant on a mixture of natural soil and stabilized sewage sludge provided the type of soil used is carefully selected. Lastly, liming in sludge delayed the gas production by maintaining high pH levels. All in all, alkaline stabilization can be viewed as an alternative to treat sewage sludge. Besides, the stabilized product has potential to be used as a soil conditioner in landscaping purposes. But more work needs to be done on the detailed microbiological and geotechnical properties of the CAPS sludge, the optimum application dosage of applying stabilized sludge to soil, and the long-term behavior of the stabilized sludge when disposed of in the landfill environment.