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The activated sludge/contact aeration combined system (AS/CA) is created by adding biological contact filters into the rear section of the activated sludge tank. The system contains both suspended micro-organisms and fixed micro-organisms. This system combines the advantages of both single systems, and avoids disadvantages particular to the individual systems. A special feature of AS/CA is its enhanced ability to remove oil and grease, and improve the color of treated water. A pilot treatment plant was constructed and located in a slaughterhouse generating 2000 m3/day of wastewater. The test results were as follows operating with a 6-h hydraulic retention time (HRT): effluent COD = 145.6 mg/L (86% removal rate), effluent BOD = 46 mg/L (85% removal rate), effluent SS = 66 mg/L (85% removal rate), effluent oil and grease dropped from 68.6 mg/L to 21 mg/L (70% removal rate), effluent true color = 50 (83% removal rate). These values just met the minimum requirements in Taiwan's effluent standards. The pilot plant results for operating with 8-h HRT were as follows: effluent COD = 71 mg/L (94% removal rate), effluent BOD = 29 mg/L (91% removal rate), effluent SS = 43 mg/L (86% removal rate), effluent oil and grease dropped from 54.6 mg/L to 7 mg/L (87% removal rate), effluent true color = 47 (84% removal rate). These test results proved that the oil and grease removal rate increased with an increase in HRT, and reduced the likelihood of other effluent parameters exceeding effluent standards. Denatural Gradient Gel Electrophoresis (DGGE) analysis was performed on the samples. Analytical results check that compared to both a single activated sludge system and contact aeration system, the AS/CA system had relatively high level of microbe diversity and a high number of microbes. Scanning Electron Microscope (SEM) analysis also revealed degradation of oil and grease in each reaction tank. [ABSTRACT FROM AUTHOR]