A bokashi mix was added to sludge from filtering effluent at a fish farm and left for storage with parallell untreated. The treatment had no significant beneficial effect compared to untreated sludge. This supposedly due to natural fermentation during storage, initiated at the fish farm. Fish sludge was easily pelleted due to a minor fraction of fish oil, originating from fish feed in the water.

Fermentation of fish sludge with bokashi mix was tested to see if this could stabilise, enrich and reduce the loss of nitrogen due to natural denitrification. However there was no effects or benefit as compared to not treated sludge, most likely due to natural processes in fish sludge that make the pH to drop, presumably initiated at fish farm. Conclusion: Fish sludge stabilises itself “going sour”. Before drying fish sludge is today added polymers and screw pressed to reduce water content. Energy for drying and polymers are the costly inputs in processing of fish sludge to agriculture today, so any biological pretreatment that may bring the fish sludge to a more solid state is of interest in further work. New approaches where the adding of other curing agents immediately post filtering of the water could therefore be profitable. However this may include the need for changing of the bacterial flora of fish farming systems all together. As fish sludge proved successful in initial growth experiments, dried fish sludge was pelleted for further field experiments.

Figure 1: Bokashi treatment

Figure 2: Pelleted fish sludge