The treatment of grey water in two upflow anaerobic sludge blanket (UASB) reactors, operated at different hydraulic retention times (HRTs) and temperatures, was investigated. The first reactor (UASB-A) was operated at ambient temperature (14–25 °C) and HRT of 20, 12 and 8 h, while the second reactor (UASB-30) was operated at controlled temperature of 30 °C and HRT of 16, 10 and 6 h. The two reactors were fed with grey water from ‘Flintenbreite’ settlement in Luebeck, Germany. When the grey water was treated in the UASB reactor at 30 °C, total chemical oxygen demand (CODt) removal of 52–64% was achieved at HRT between 6 and 16 h, while at lower temperature lower removal (31–41%) was obtained at HRT between 8 and 20 h. Total nitrogen and phosphorous removal in the UASB reactors were limited (22–36 and 10–24%, respectively) at all operational conditions. The results showed that at increasing temperature or decreasing HRT of the reactors, maximum specific methanogenic activity of the sludge in the reactors improved. As the UASB reactor showed a significantly higher COD removal (31–64%) than the septic tank (11–14%) even at low temperature, it is recommended to use UASB reactor instead of septic tank (the most common system) for grey water pre-treatment. Based on the achieved results and due to high peak flow factor, a HRT between 8 and 12 h can be considered the suitable HRT for the UASB reactor treating grey water at temperature 20–30 °C, while a HRT of 12–24 h can be applied at temperature lower than 20 °C.
Keywords: anaerobic digestion, domestic wastewater treatment, ecological sanitation, grey water