[ AQUACULTURE ] Making specific use of the economic potential of water discharge
Settling tanks are an effective method for smaller aquaculture enterprises to reduce the presence of particulate substances in the water discharge.
belt filters. Microscreens require a fair amount of maintenance and energy but are characterized by reliably good and consistent cleaning results for particlebound nutrients. These “coarse contaminants� are also removed from the water discharge very quickly, which counteracts redissolution of nutrients, a process which is strongly time-dependent. The mechanical cleaning effect mainly depends on the mesh size of the filter screen, which is in practice mostly between 60 and 100 microns. This enables up to 90 percent of particulate matter and 80 percent of biochemical oxygen demand (BOD) to be removed from the water. By using extremely tight mesh sizes of 40 microns it is even possible to reduce the risk of infection due to some parasites such as Ichthyophthirius multifilis (which can measure up to 1 millimetre) and a certain proportion of Saprolegnia infections, for example (in combination with UV light or ozone). Although microscreens have no direct influence on dissolved nutrients they do, however, reduce these loads indirectly through the rapid mechanical removal of fish excrement and uneaten feed, thereby relieving the strain on biological filters, www.eurofishmagazine.com
especially in recirculating systems (RAS) with internal water recycling. To prevent clogging of the narrow meshes by the separated solids, the fine mesh material which is made of plastic or stainless steel is at intervals automatically cleaned by a backwash rinse system. Depending on the fish population and amount of feed the water required for this is on average between about 1 and 2 percent of the amount of outflowing water that passes through the microscreen filter. During this process the separated particulate dirt, i.e. the solid filtrate, is partly resuspended. It thus seems reasonable to thicken the relatively liquid slurry to increase the dry matter content and reduce the volume of sludge. This secondary sedimentation can take place, for example, in a second downstream filter screen or in a funnel-shaped settling tank, which is called a “Dortmund tank� after the location of its first use in Dortmund in Germany 1887. In a Dortmund tank the slurry water is fed from below into the tank, whereby the flow rate decreases on the way up because of the widening of the funnel. This calms the water, the solids settle out and sink down the inclined side walls to the bottom where they can be removed.
Like unprocessed water discharge, sludge from aquaculture facilities still contains significant amounts of nutrients which lead to eutrophication of natural waters if their release is not controlled. It can also, however, be used profitably for the production of secondary products. If thickened, the sludge can for example be used directly as a substrate in biogas plants (“fish manure�). This also applies to plant biomass that grows in tertiary treatment ponds or plant settling ponds thanks to the nutrients dissolved in the water discharge. The production of crops that can be marketed directly can be even more lucrative. A nice example
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of this is the culture of aquatic and marsh plants on coco mats that grow in the water discharge from a zander RAS in southern Germany. Once the plants have grown and taken root the mats can be picked up and rolled out like lawns and used for initial plant growth on re-naturalised river and lake banks. Combining fish farms with mussel and algae cultures in IMTA (Integrated Multi-Trophic Aquaculture) is also an attractive option. The Canadian salmon producer Cooke Aquaculture operates such a facility off the coast of New Brunswick. At the centre of this pilot project, which is one of the world’s first IMTA salmon farms, is a salmon farm around which several mussel farms are located. These filter the organic excrement
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Eurofish Magazine 1 / 2015
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