Unrestricted use of reclaimed secondary effluents for irrigation is a major goal in countries suffering from water shortage. Reverse osmosis desalination is used to provide high quality waters with reduced salinity. In order to allow water production with high economic efficiency, fouling in the membrane installation needs to be minimized. Biofouling, caused by microorganisms synthesizing high-molecular biofilms, is of major concern. Biofouling reduces the water production rate and thus increases the costs of the process. Deeper knowledge on its formation and its impact on membrane performance is needed. This is relevant especially for large-scale treatment plants, where process conditions change over length and time and influencing factors on fouling formation occur in combination. Thus, in the present thesis a membrane test cell was developed which enables the investigation of biofouling under validated, representative conditions of full-scale modules. Biofouling was studied in order to determine its impact on membrane performance. Also, appropriate, cost-effective pre-treatment prior to the reverse osmosis process minimizes fouling. Therefore, biofiltration and its suitability as stand-alone pre-treatment was studied when reusing secondary effluents with reverse osmosis. The developed membrane test cell of 1 m length can be assembled with further test cells to simulate a spiral wound module alone, as well as several modules in series in a pressure pipe. The test set-up enables the systematic study of fouling formation integrative over the full length of industrial spiral wound modules. All performance parameters (feed channel pressure drop, permeability/flux, and salt passage) can be monitored over the full length and locally connected to accumulated foulants (non-destructive fouling diagnosis). Validation studies demonstrated that the hydraulic conditions (relationship between pressure drop and flow velocity, as well as the flow profile) are exactly as in real spiral wound modules. Each test cell is a representative, validated system of full-scale dimensions and hydraulics. It was further found that for fouling formation investigations, feed spacers with the same thickness as the feed channel height need to be used. In this way, accurate experimental measurements, especially of feed channel pressure drop, are ensured. With the developed test cells, the impact of biofouling on membrane performance was determined under conditions similar to practice. Biofouling resulted in a decline of all membrane performance parameters. Feed channel pressure drop was affected earliest and most severely, indicating its suitability as a sensitive biofouling monitoring parameter. Salt rejection was moderately impacted by biofouling and influenced by several process parameters, reducing its applicability as monitoring parameter. It was further found, that most biofilm accumulated in the lead parts of the membrane test cells with a declining gradient towards the tail sections. The gradient of biofouling over the length of the membrane installation was directly referred to the declining availability of easily assimilable substrate. It emphasizes the importance to reduce the concentration of biodegradable nutrients in the feed to the membrane installation as suitable strategy to restrict biofouling. The high amount of biofilm deposits in the lead parts caused feed channel pressure drop increase over the lead test cell and affected negatively the performance of the downstream test cells: The tail test cells showed a moderate decline for the permeability (flux) and salt rejection. Biofiltration improved the quality of secondary effluents as tertiary treatment. It successfully reduced the load of substances (microbes, dissolved organic matter, biopolymers, particles) reportedly contributing to fouling of subsequent membrane processes. Especially biopolymers of secondary effluents, which are major membrane foulants, were identified to be completely biodegradable. The biopolymers were estimated to be of colloidal size. Yet, the removal of these organics was suggested to be completely caused by biodegradation; neither filtration nor adsorption mechanisms played a role to retain biopolymers and dissolved organic carbon within the biofilter. However, a combined study of biofiltration and reverse osmosis revealed, that the improving effect of biofiltration as pre-treatment on membrane performance was lower than expected. Although, both biofouling and organic fouling were reduced on the reverse osmosis membrane, only marginal improvement on performance parameters was found. The adsorption of small non-biodegradable substances on the membrane as an organic fouling layer in the early stages of the process, as well as the difference in fouling layer composition were probably reasons for the findings. Thus, the successful application of biofiltration as pre-treatment is highly depending on the feed water source and the foulant layer formation. For the present case biofiltration as stand-alone pre-treatment is not recommended; a combination of biofiltration with subsequent e.g. flocculation and UF could be more beneficial.