Fouling experiments of nanofiltration (NF) and reverse osmosis (RO) are reported for treatment of the effluent of chemical-biological treatment plant and the original effluent of dairy industry respectively. In the experiments, a thin film composite type of spiral wound was used and fitted with flowmeters and pressure sensors, The feed water was stored into a feed tank and passed a fine filter and was pumped to membrane. Brine and permeate were recirculated back to the feed tank. Membrane fouling was investigated with 16 and 30% water recovery of a single membrane at different pressures and flowrates for RO and NF membranes respectively, Fouling is evaluated with a relationship between relative flux (J/J(o)) which is the ratio of the flux at any time during the fouling test to the initial flux and relative resistance (R-f/R-m) which is the ratio of fouling (cake) layer resistance to clean membrane resistance. Turbidity, conductivity, chemical oxygen demand (COD), total suspended solids (TSS) and total hardness were measured in the feed and permeate side of each membrane. The effluent total hardness concentrations of chemical-biological treatment plant were found greater than the influents. The results are presented in terms of the relative flux as a function of time related to hydrodynamic conditions and pollution characteristics of wastewater. The permeate water flux of RO membrane decreases more rapidly than NF membrane. the relative flux decreases with increasing the fouling layer resistance, F onto membrane surface. 50% the drop of permeate flux was observed for RO and NF membranes after 50 h and 80 h of operation, respectively. The fouling rate increases with an increase in the concentration of the wastewater constituents in the dairy industry. The relative flux decreased 10 and 20% with increasing chemical oxygen demand (COD) from 5,000 mgl(-1) to 10,000 mgl(-1) and from 45 mgl(-1) to 450 mgl(-1) for RO and NF membranes, respectively after 45 h of time. Fouling of membranes resulted in 100% increase of specific energy consumption as the relative permeate fluxes of NF and RO membranes decreased 30 and 40% respectively. The average of specific energy consumption was obtained at 6 and 10 kWhm(-3); consequently, operational costs were estimated at U.S. $0.45 m(-3) and U.S.$ 0.75 m(-3) for NF and RO units respectively. Also, operational cost for chemical-biological treatment was found at U.S. $0.30 m(-3).