Mechanical filtration is often the first step in the recirculation loop. The task is to remove faeces and other particles from the water before they break down in smaller particles. Smaller particles are harder to capture and result in adverse water quality conditions. Organic loading increases bacterial growth, reduces the biological filter performance and decreases water quality.

In the farm designs drum filters are usually the tool of choice for solid removal. This is basically a halve submerged rotating cylinder, covered with a fine screen. Waste water is led into the drum and passes through the screen, leaving solids trapped against the screen. The drum then rotates dirty cloth out of the water, where water jets are used to spray the captured solids into the waste water collection drain. A normal screen size has openings of 36-40 micron, which allows for capture of parasites. The drawback of drum filters is the cost of investment, the complexity, operating cost and the fact that very fine solids are not captured.

Static bed biofilters (SBBF) can be used as a relatively cheap and low-tech alternative for capturing of solids. A big advantage is the fact that they can capture all solid sizes. The downside is that there is bacterial degradation of the organic solids, that can affect water quality and the fact the parasites are not captured. A combination of SBBF and drum filters can be beneficial.

Belt filters can be the tool of choice for rapid removal of large solids from the water. This can include wasted feed but for example also dead fish. The advantage of the belt filter is the fact that solids are lifted gently from the water. In drum filters large solids are sometimes rolling back into the water time and again, breaking the solids and eventually worsening the situation.