By Asbjørn Bergheim*
Dealing with solid waste in aquaculture due to undigested feed and uneaten feed is a well known operational issue for fish farmers. Accumulation of solids in the fish units reduces the water quality for the fish stock and sedimentation in the recipient environment, e.g. on the sea/lake bed beneath cages and at the pipe outlet from closed systems, may represent a major ecological footprint.
In salmonid culture, the waste situation has improved over the last decade as a result of stricter guidelines, better feed quality and introduction of more efficient technology for waste removal and processing.
At a feed conversion ratio of 1.0 (kg feed/kg growth), the mechanically removable solid dry matter (DM) is about 100 g per kg of feed used or fish produced (landbased salmon/trout farms combining particle sieving and sludge dewatering). Consequently, production of 100 tons of fish means 100 cu. m of sludge containing 10 % DM.
Modern salmon smolt farms are large and produce several millions of fish every year for stocking in cage farms. The annually produced biomass in the majority of such freshwater farms is between 100 – 1,000 tons. Moreover, the ascending production of post-smolt in onshore recirculating farms (RAS) and in floating, closed cages (S-CCS) will probably represent biomasses 5 – 10 times higher, i.e. up to 10,000 tons annually per farm. These are systems fit for solids removal.
Farming rainbow trout to harvest size in inland, freshwater farms also represents substantial biomasses and sludge production in some countries. Such farms mostly load freshwater recipients, lakes and rivers, and the discharge ought to be properly treated before entering the recipient. In freshwater, removal of phosphorus is vital as regards eutrophication and well-designed and properly operated effluent treatment systems catch solid-bound phosphorus efficiently. The major part of lost phosphorus from fish farms is usually incorporated in particles.
Danish trout farmers are subject to strict regulations by the environmental authorities in order to protect freshwater bodies. Figure 1 illustrates a common way to treat produced backwash water from the effluent drum sieves (Anne Johanne T. Dalsgaard, pers. comm.). Subsequent solid settling occurs in basins with the overflow going to a lagoon for nutrient removal by plants.
Dewatered sludge from salmonid farms contains favourable levels of nitrogen and phosphorous, major nutrients for plant production, but the sludge is low in potassium. Stabilized sludge with added lime is considered an applicable organic manure spread on arable land. Analysis of heavy metals, such as chromium, chrome and lead, indicate low and harmless levels. However, the national agriculture authorities are concerned of the potential negative effects from the salt content in sludge from seawater farms. Manure application is the most common way so far to dispose/utilize the produced sludge from Norwegian smolt farms. A few reports indicate stimulated plant growth but in most cases sludge delivery to agriculture means extra costs for the fish producers.
Other, more advanced ways for sludge utilization are impending. Among these are sludge as a source for production of biogas. The research centre, Bioforsk (Norway), estimated a yield of more than 500 L methane per kg of sludge organic dry matter. Biogas is an actual fuel source, e.g. to make city buses more environmental friendly. Salmonid sludge is moreover a considerable direct energy source and contains approximately 20 MJ per kg of dry matter.
Combustion of dried sludge from aquaculture is thus a potential application and has been assessed as an alternative, more sustainable fuel source in cement factories compared to the present use of fuel of fossil origin that represents serious local air pollution.
Only a small part of the totally produced sludge in salmon and trout farming is removed and processed. A brief calculation indicates a maximum removal rate of less than 5 % of the total volume. As long as the predominant biomass production takes place in open cages inappropriate for waste handling, the coastal areas will remain the primary recipient of wastes from feed and fish.
Dr. AsbjØrn Bergheim is a senior researcher in the Dept. of Marine Environment at the International Research Institute of Stavanger. His fields of interest within aquaculture are primarily water quality vs. technology and management in tanks, cages and ponds, among others.