Municipal wastewater from the secondary treatment stage was treated using various types of membranes such as regular filter paper, microfilter (MF) and UF membranes of molecular weight cut off (MWCO) of 10, 50 and 100 kDa. The membrane performance was assessed by measuring the absorbance, the BOD5 and the COD of the filtrate. The results indicated that most of the suspended solids are removed and the BOD5 and the COD were reduced by 98% and 40% respectively. The results indicate that regular filter paper is equally capable of treating the wastewater as UF and MF membranes. Membrane fouling by wastewater was investigated. Results showed that small particles cause severe membrane fouling by adsorbing within its pores. The MFI-UF index was calculated as a function of time where no stable value was reached. A new equation to calculate the fouling index was suggested. The new equation is based on measuring the relative volumes of suspension and distilled water filtered during a specific period of time. The fouling index calculated by the suggested equation was found stable with time. It has many advantages over the previously defined indices: It is easy to apply, does not involve tedious mathematical calculations, can be employed with any type of membrane and its values ranges between 0 and 1. Testing the effect of applied pressure shows that the fouling index is proportional to the applied pressure.