Technical Memorandum Report No. 71
Disinfection of Bacteria and Viruses by Sludge Heat-Treatment Process at Sand Island WWTP, O’ahu, Hawai’i
Roger S. Fujioka, Wesley M. Hirano, and Philip C. Loh
The Zimpro heat-treatment process installed at the Sand Island Wastewater Treatment Plant was evaluated for its reliability in disinfecting fecal bacteria and viruses in the treated sludge. Samples of the untreated sludge and the heat-treated sludge cake were initially analyzed for fecal coliforms, fecal streptococci, and human enteric viruses. The detection of high concentrations of indicator bacteria in the sludge cake suggested that the heat-treatment process was not properly disinfecting the sludge. However, samples of sludge obtained from the plant immediately after the heat-treatment process yielded no fecal coliforms, fecal streptococci, Clostridium perfringens, or human enteric viruses, thus demonstrating that the plant heat-treatment process will disinfect the sludge of all fecal bacteria and viruses. It was subsequently determined that the heat-treated sludge becomes contaminated with fecal bacteria in the sludge storage tank, and that the use of improperly disinfected chlorinated sewage effluent to wash plant equipment or of pipes to transport untreated and heat-treated sludge contributed to the contamination of the heat-treated sludge. Additional evidence showed that the very acidic (pH 4.5-5.0), heat-treated sludge inhibited the multiplication of fecal bacteria. When the pH of the heat-treated sludge was adjusted to pH 5.5 to 6.5, fecal coliforms and fecal streptococci -but not C. perfringens -multiplied. Although fecal bacteria cannot multiply in the heat-treated sludge at pH 4.5, other bacteria will. The growth of these nonfecal bacteria in the heat-treated sludge is speculated to raise the pH to at least 5.5 and thus allow the fecal bacteria to multiply.