Apart from the inherent amenability of the organics present to anaerobic fermentation, a number of other factors are important to the technical and economic viability of the overall process. These include wastewater strength, wastewater temperature, the nature of suspended solids present and the presence of toxic materials, notably sulphates/sulphides and biocides. Analytical data is presented from various Pira research projects, describing the composition of UK mill wastewaters in terms of BOD/COD/DOC, suspended solids, the speciation of dissolved organics (eg carbohydrates, fatty acids) and inorganic sulphur compounds. In the context of anaerobic treatment, this is then discussed in terms of in-mill microbiological activity, the extent of water system closure and the type of sizing system employed.