One major challenge in this study was to investigate the performance of an osmotically driven membrane process, such as forward osmosis (FO) in a case of using raw wastewater that was obtained from a municipal wastewater treatment plant, with a focus on the flux decline and solute diffusion. First, to determine the effect of suspended solids (SS) in wastewater, wastewater was used containing 20 SS mg/L and filtered by a 0.45 mm filter to remove the SS. The results showed that a noticeable flux decline was observed in the case of the existing SS, but flux was slightly decreased without the SS. Furthermore, a larger decline in reverse salt flux (Js) was also obtained with the SS, thus it can be implied that cake enhanced osmotic pressure (CEOP) phenomenon occurred. In other words, the SS could accelerate membrane fouling, resulting in a flux decline and hindered reverse salt diffusion. There was also a comparison of the FO performance using wastewater and a MBR permeate. As hypothesized, it has been found that wastewater resulted in a higher flux and reverse salt flux (Js) decline through the consecutive fouling experiments (four times), but a MBR permeate also brought about substantial flux decline, which was contrary to what was conjecture. These findings indicate that an effective method to control fouling, such as pretreatment and cleaning, may be required even if the treated water resulting from wastewater treatment is used as feed water. As the fouling was getting severe, UV254 rejection was gradually decreased. That phenomenon might possibly be attributed to the increased humic substances on the membrane surface, changes in the membrane characteristics, and Js decline.