Decentralised treatment of concentrated sewage at low temperature in a two-step anaerobic system: two upflow-hybrid septic tanks T. A. Elmitwalli*, S. Sayed**, L. Groendijk**, J. van Lier***, G. Zeeman*** and G. Lettinga*** * Department of Civil Engineering, Benha High Institute of Technology, P.O. Box 13512, Benha El-Gedida, Benha, Egypt (E-mail: t_elmitwalli@hotmail.com) ** Van Hall Institute, P.O. Box 1528 BV Leeuwarden, The Netherlands *** Sub-Department of Environmental Technology, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands Abstract The decentralised treatment of concentrated sewage (about 3600 mgCOD/l) at low temperature was investigated in a two-step anaerobic system: two-anaerobic hybrid (AH) septic tanks (each 0.575 m 3 ). The two reactors were placed in a temperature controlled-room and the hydraulic retention time (HRT) was 2.5 days for each reactor. The system was fed with concentrated domestic sewage, mainly black water from about 40 toilets flushed with only 4 litre of water and a limited amount of grey water. The system showed high removal efficiency for the different COD fractions. Mean removal efficiencies in the two-step AH-septic tank at 5 days HRT and 13 o C were 94, 98, 74 and 78% for total COD, suspended COD, colloidal COD and dissolved COD respectively. The results of short run experiments indicated that the presence of RPF media in the AH-septic tank improved the removal of suspended COD by 22%. The first AH-septic tank was full of sludge after 4 months of operation due to the high removal of particulate COD and the limited hydrolysis at low temperature conditions. Therefore, a simple mathematical model was developed based on ADM1 (IWA, 2002). Based on the experimental results and the mathematical model, only a one-step AH septic tank is required. An HRT of 5.5-7.5 days is needed for that one-step AH septic tank to treat concentrated domestic sewage at low temperature of 13 o C. Such system can provide a COD removal as high as 87% and will be full of sludge after a period of more than a year. Keywords ADM1, Anaerobic digestion; decentralised treatment; domestic sewage; septic tank; wastewater INTRODUCTION Developing countries suffer from the lack of improper wastewater collection and treatment facilities, especially in rural areas. In Egypt, more than 95% of the Egyptian rural area are not provided with wastewater collection and treatment facilities. There are about 4000 Egyptian rural areas with a population ranged from 1000 to 20000 capita. The wastewater produced from houses in these Egyptian rural areas are mainly collected and treated in septic tanks. The high construction, operation and maintenance costs for centralised wastewater collection and treatment represent an obstacle for the Egyptian government to install such systems in Egyptian rural areas. On the other hand, most of the wastewater produced in developing countries is collected and treated up to tertiary treatment. In The Netherlands about 95% of the produced wastewater is treated in central facilities (Bogte et al., 1993). The remaining 5% is produced from different sources, like remote houses, farms and recreation facilities for which connection to municipal wastewater is too costly. From 2005, it is forbidden in The Netherlands to discharge municipal wastewater into surface water and soil without adequate treatment. Also, in USA, on-site treatment (mainly septic tank) for domestic sewage serves about 20% of the US population, > 20 million homes (Scandura and Sobsey, 1997). Therefore, decentralised treatment can represent a suitable option for the treatment of sewage not only for rural areas in developing countries, but also for unserved areas with wastewater collection and treatment facilities in developed countries. 373