07 Dec 2020
It is well known that industrial aquaculture produces waste and side streams at a scale which make their reuse unsafe without further processing and often treatment. The practice of land-based aquaculture, also known as recirculating aquaculture system (RAS), creates tons of fish sludge, the leftover feed and feces that accumulates during the rearing process. When salmon, for example, are raised from egg until the smolt phase in closed ponds on land, the sludge is regarded as waste and must be treated.
Rich in nitrogen and phosphorus, sludge may also contain polluting heavy metals such as zinc, nickel and cadmium (or arsenic, mercury or lead). And while phosphorus and nitrogen are essential for all life on Earth, when released in large quantities into lakes or seas, this can cause eutrophication, the over production of algae and aquatic plants, which starves water ecosystems, including wetlands, of oxygen.
Land-based aquaculture is on the rise to meet growing consumer demand for fish. However, without a solution for processing the resulting sludge in a way that meets government regulations, fish farmers in many countries face the continual challenge of trying to do business legally and responsibly. Today, a handful of companies are providing technological and logistical support to deal with this issue, allowing farmers to focus on what they do best – raise fish.
In Norway, where salmon is intensively reared, by law, the solids must be removed from the sludge that collects in the starter culture fish basins before the water can be released into the sea; farmers there anticipate the law will eventually require as much as 90 percent solids removal. To deal with this reality, fish farmers are investing in automated, on-site sludge facilities which handle, process and treat the waste sludge, taking it up to 90 to 95 percent dry matter in a continuous process, greatly reducing transport and disposal costs. As a result, the dry solids are storage stable and the discharge water, now separated from the sludge and contaminants, can be released back to the sea or reused as part of the process.
Fish sludge processing typically involves the following key steps:
In the first step, some dewatering and sedimentation is performed. Next, the effluent water is delivered to a decanter centrifuge. In this process, a GEA sludge Decanter dewaters the remaining effluent, producing 20 to 35 percent dry solids. The percentage of water removed during this phase is heavily influenced by upstream processes. Ultimately, the higher the percentage of sludge dewatered, the less time and energy are required during evaporation and drying.
GEA sludge Decanters are specifically designed to handle sticky sludge, which in RAS is the result of the proteins and fats from accumulated and uneaten fish food. In instances where chloride levels are high, customers can choose a GEA decanter with additional scroll protection to safeguard machine performance.
A fish sludge recovery system for a leading global salmon producer in Norway. A GEA sludge Decanter pro 2500 provides efficient dewatering and features integrated remote support.
Third-party service contracts covering installation, maintenance and in some cases even remote operation, may also include hauling the sludge away for upcycling, meaning the service provider manages the transport and valorization of the dried sludge, and when local, any required reporting. This end-to-end service greatly reduces a farmer’s need to buy-in expensive water and their disposal costs. Once dried, the sludge can be:
In Norway, fish sludge processing systems that deliver food-grade fertilizer, meaning without the use of chemicals or creating hazardous by-products, are having a positive effect as far away as Vietnam. Fertilizers that return phosphorus to the soil are in short supply in Vietnam, and yet necessary for combatting plant mildew and other diseases. Norwegian fish sludge fertilizer provides both the nutrients as well as the water retention in soil for the healthy cultivation of Vietnamese coffee plants, tropical fruits, rice and rubber. In this dried form, the phosphorus from the fertilizer is much less likely to leach into the ground – a significant advantage over other types of commercial fertilizer.
"Fish waste is a resource that normally cannot be used as fertilizer in food production when heavy metals and chemical polymers are used during the sludge purification process. In these cases, it is treated as waste. However, by integrating a GEA decanter in the process, we’re able to valorize the waste into approved fertilizers, without the use of chemicals or creating hazardous by-products,” explains Jake Deighton, Product Manager Environmental Technology, GEA. “Some of this nutrient-rich fertilizer is also used closer to home, for example at Norwegian aquaponics facilities to grow salad greens and herbs."
- Jake Deighton, Product Manager Environmental Technology, GEA
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