ABSTRACT Salsnes Filter delivers a technology used for primary treatment of wastewater. The technology could be described as a rotating belt sieve with continuously air cleaning of the sieve cloth. The process is referred to as ‘Salsnes filtration’ throughout the text. Salsnes filtration can point out particularly high treatment efficiency, but it is not fully understood how this is obtained. Furthermore it is not clear how the process should be operated to achieve ‘optimum performance’. In an attempt to understand this, both a theoretical and an experimental approach have been applied; hence the paper consists of two main parts. In the theoretical part, different fouling mechanisms are described, and it is explained how these affect the hydraulic resistance of the sieve cloth and the resulting filtrate flux through it. In the experimental part, sieving under different pressure conditions has been explored. It was not succeeded in obtaining data suitable for statistical analyses, but some useful suggestions were given. Most importantly, it may not be beneficial to establish a particle cake under high pres-sure conditions since this provides low permeability. It was found that upholding of high pressure was sufficient to counteract this, but that a relatively small decrease in pressure results in a significant decrease in filtrate fluxes. It is concluded that Salsnes filtration should strive for a minimum decline in the ratio between pressure difference and hydraulic resistance in order to achieve the most uni-form flux through the belt sieve, which is thought to ensure the highest treatment efficiency.