Sludge from fish farms makes an excellent contribution to organic plant fertilizers. If the sludge is de-watered in the right manner the market is fathomless. Its inherent quality also makes it an attractive raw material for eco fertilizer.
Fertilizer maker Grønn Gjødsel at Rakkestad, Norway, specializes in fertilizers for organic farmers. Recently a large volume of fish sludge has been used in the fabrication process, with good results.
“We have used dry sludge from two fish farms and incorporated this into one of our products. It was successful both in terms of pellet formation and with regard to nutrients. The sludge has a great potential. We now invest in a new mixing unit to ensure that our plant is fully geared for more of this raw material in the future. We will scout for fish farms that produce dried sludge of this quality”, states manager Goran Radanovic at Grønn Gjødsel.
Nitrogen is the key
The sludge comes from salmon smolt production units that filter particles from effluent, thicken and de-water the sludge. As salmon diets hold a high protein content, so does the sludge, and this is favorable for plants. The sludge is also rich in zinc and phosphorous and balances well against other natural sources of raw materials for eco fertilizers.
Ecological fertilizers are made from natural sources only. Fish farm sludge ensures a well-balanced product when mixed with chicken manure and vinasse, a by-product of the sugar industry.
«Fish farm sludge gives us the freedom to compose a balanced ecological fertilizer», states Mr. Radanovic.
Grønn Gjødsel has received acclaim from customers due to the incorporation of fish farm sludge in the product. The sludge originates from fish farms Lødingen Fisk and Sisomar.
«The sludge contains more than 90 % dry matter and is convenient to pelletize, due to a minute proportion of fish oil, which lubricates our machinery», according to Mr. Radanovic.
Grønn Gjødsel is Norway’s largest producer of eco fertilizers, in the shape of pellets, based on three main ingredients: chicken manure, bone meal, and vinasse. Grønn Gjødsel also makes a hybrid fertilizer, where fish farm sludge has been incorporated with success.
«Fish farm sludge is an exciting raw material, and could well be a new natural source of nitrogen in tomorrow’s organic farming. I cannot see any reason why the sludge would not become classified as ecological, in the same way as chicken manure and blood meal», states Mr. Radanovic.
The product range of Grønn Gjødsel includes three fully ecological fertilizers delivered in 600 kg big bags, besides hybrid products, which are organic based and enriched with key micronutrients.
” The hybrid fertilizer is a popular product. Its body is organic, so here the fish farm sludge contributes to the power of this product”, says Mr. Radanovic.
The source of sludge is smolt plants with effluent filtering, thickening, and dewatering. When planning sludge treatment it is important to keep in mind the requirements for further use.
«The key factor is the nitrogen content. The ideal temperature during gentle drying of the sludge is between 40 and 50 degrees Celcius. In this way the nitrogen is conserved. If higher temperatures are used during treatment the nitrogen will evaporate into the atmosphere and be gone. Nitrogen is in fact what the farmers pay for», explains Mr. Radanovich. Due to long-distance transport, the sludge does not give any significant return for the fish farmer, however by planning the logistics there should be a potential for profit.
«We gladly pay for the sludge, however, fish farms are placed in distant places. If the sludge is collected regionally and transported in bulk, there should be better margins in the future. Today, getting rid of the sludge in a proper manner is the primary goal for fish farmers. The value of nutrients follow the current markets», states Mr. Goran Radanovic of Grønn Gjødsel.