CHARACTERIZATION OF RAW SLUDGE, CHEMICALLY-MODIFIED SLUDGE AND ANAEROBICALLY DIGESTED-SLUDGE IN HONG-KONG

Chemically Enhanced Primary Treatment (CEPT) or Chemically Assisted Pr imary 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 th at they can settle out within a reasonable period of time. In this stu dy, five sludge samples collected from different sewage treatment plan ts in Hong Kong were physically and chemically characterized. They wer e chemically modified sludge from Stonecutters Island (CAPS) raw sludg e from Tai Po and Yuen Long Sewage Treatment Plant (STP) (rTP & rYL) a nd anaerobically digested sludge from Tai Po and Yuen Long STP (dTP & dYL). It was found that CAPS sludge was better than other 4 sludge sam ples in terms of settleability and dewaterability. CAPS sludge contain ed significant higher amounts (p<0.01) of extractable compounds than o ther sludges (except NO3- for dTP, NH4+ and PO43-. for dYL). The conce ntration 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 highe r (p<0.01) than other sludge samples. Most of the metals (Cr, Pb, Cr a nd Zn) in CAPS sludge were associated with the organically-bounded pha se; It is concluded that there are significant differences in both phy sical and chemical properties between the chemically modified sludge a nd biological treated sludges. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.