Technical Report No. 52
SLUDGE DIGESTION PRACTICES ON OAHU, HAWAII
Jeong-rong Chen, Reginald H.F. Young
Center for Engineering Research
ABSTRACT
Characteristics and treatment efficiency of the sludge digestion facilities at four sewage treatment plants of the City and County of Honolulu, one each at Kaneohe, Kailua (both secondary treatment-trickling filter plants), and Pearl City (primary treatment), which utilize anaerobic digestion system and the fourth at Mililani (secondary treatment-activated sludge), which utilizes an aerobic digestion system were studied over a two-week period at each plant. Included in the study were both raw and digested sludges.
Overall results indicate that the digestion systems were operating generally within accepted ranges for parameters tested. The Kaneohe plant had a volatile solids reduction of 64 percent, solids retention time of 143 days, and methane: carbon dioxide gas yield percentage off 67:33. The Kailua plant had a volatile solids reduction of 30 percent, solids retention time of 32 days, and methane: carbon dioxide gas yield percentage of 66:34. The Pearl City plant had a volatile solids reduction of 64 percent, solids retention time of 10 days and methane:carbon dioxide gas yield percentage of 64:36. The Mililani plant had a volatile solids reduction of 49 percent with a sludge age of 36 days.
It was concluded that the three anaerobic digestion systems were operating satisfactorily although not at optimum efficiency. The Mililani plant achieved a reasonable digestion efficiency although operational problems were indicated because of low pH and ORP values for the sludge and high specific resistance values for the digested sludge.
It was recommended that a program of regular monitoring of sludge characteristics and digestion treatment efficiency be initiated to assist in plant operations at all sites