The obtained high values for ABmax (76%) and maximum COD removal in the UASB reactor (84%) showed the potential of the UASB reactor in grey water treatment. Moreover, the results demonstrated that a high CODt removal (52–64%) was obtained in the UASB reactor at HRT between 6 and 16h and wastewater temperature of 30 C. Accordingly, the UASB reactor has a significantly higher CODt removal at a short HRT as compared to the septic tank, the common system of grey water pretreatment. The temperature of grey water ranges between 18 and 38 1C and it can be considered similar to that applied in this research, if short connection between the grey water sources and the UASB reactor, and proper isolation of this connection are applied. Consequently, installing of the UASB reactor treating grey water in the cellar of the buildings will be a suitable option. Thus, the losses in the wastewater temperatures can be minimised and the energy losses from warm water in the buildings can be utilised for extra heating of the reactor, when it is required. For keeping the temperature of the reactor at about 30 1C and minimising the heat loss, a good insulation is required. Accordingly, the thermal resistance of the insulator should be X5Km2/W. For each insulator, the thermal resistance is known and, therefore, its thickness can be determined.