Our society seems to believe that the more technology we throw at an obstacle the better the solution will be. In the previous eNewsletter on drum filters I mentioned that they were efficient and capable of removing small particles, but that they are very expensive and waste lots of water during cleaning, and as such do not represent the silver bullet we are looking for in solids removal.

Another method of solids removal is the stationery, submerged mechanical filter (SSMF). This is a broad category of filters that use a coarse media to allow for free water flow. As the water passes over the filter media, the solids settle against the media due to friction between the media and water causing localised low water velocity, which results in settling. The photographs below show examples bioblock (left), corrugated plastic blocks (centre) and bird netting (right), all of which are well suited to this application although the efficiency of the netting is difficult to predict due to its’ irregular volume and inconsistent shape within the water.

Coarse stone (19mm) was also used in the early days of recirculating aquaculture, but is extremely heavy and therefore requires enormous effort to clean. This makes it a less desirable option other than in Aquaponics where the size of the bed is enormous compared to the amount of fish waste it needs to deal with and blocking is less of a problem.

The ability of SSMFs to clean water is a function of various parameters, primarily the coarseness of the media as compared to the flow velocity through the media. Fine media with a slow flow rate is most efficient but blocks the quickest, whereas very coarse media with a faster flow will take longer before cleaning is required, but will occupy a massive volume within the system.

SSMF are generally installed in multiple parallel chambers so that one can be cleaned without interrupting the flow of the RAS. To clean a chamber we then isolate the chamber from the rest of the RAS, lower the water level slightly and rinse the media in that chamber. The dirty water is then dumped, the chamber rinsed, the media replaced and water again allowed to flow through the chamber.

At first glance it may seem that this cleaning wastes a lot of water, but the reality is that SSMFs actually waste less water than drum filters to perform the same function. SSMFs also need less frequent cleaning than bead filters and are not especially expensive filters to install or operate, making them a great choice for many aquaculture situations.