Characterization of raw sludge, chemically modified sludge and anaerobcially digested sludge in Hong Kong

Chemically Enhanced Primary Treatment (CEPT) or Chemically Assisted Primary Sedimentation (CAPS) is being employed at the new sewage work on Stonecutters Island as part of the Strategic Sewage Disposal Scheme (SSDS) in Hong Kong. CAPS involves the use of chemical coagulants (such as lime or ferric chloride) to induce coagulation or flocculation and let these finely-divided particles form large aggregates (floc) so that they can settle out within a reasonable period of time. In this study, five sludge samples collected from different sewage treatment plants in Hong Kong were physically and chemically characterized. They were chemically modified sludge from Stonecutters Island (CAPS) raw sludge from Tai Po and Yuen Long Sewage Treatment Plant (STP) (rTP and rYL) and anaerobically digested sludge from Tai Po and Yuen Long STP (dTP and dYL). It was found that CAPS sludge was better than other 4 sludge samples in terms of settleability and dewaterability. CAPS sludge contained significant higher amounts (p <0.01) of extractable compounds than other sludges (except NO₃ ^- for dTP, NH₄ ⁺ and PO₄ ^3- for dYL). The concentration of total N and P in CAPS sludge were significantly higher (p <0.01) than other sludges (except dYL). The concentrations of total Cu, Pb, Ni, Cd, Cr and K in the CAPS sludge were also significantly higher (p <0.01) than other sludge samples. Most of the metals (Cr, Ph, Cr and Zn) in CAPS sludge were associated with the organically-bounded phase. It is concluded that there are significant differences in both physical and chemical properties between the chemically modified sludge and biological treated sludges.