Leaching techniques to remove metals and potentially hazardous nutrients from trout farm sludge

A fish farm sludge high in P (2–6% w/w as dry matter), Fe (5–7%), C (40–50%) and N (0.8–4%) was subjected to a series of acid leaching treatments using HCl, organic acids, and biologically mediated acid production. Additions of biodegradable organic acid solubilized heavy metals better than HCl, while additions of 1.5% w/v glucose followed by 7 day incubation stabilized the sludge releasing 92% P, 100% Fe. The use of homo-lactic Lactobacillus plantarum starter cultures were more effective than hetero-lactic Lactobacillus buchneri, solubilizing 81.9% P, 92.2% Fe, 93.0% Zn and 96.4% Ca in the sludge. The anaerobic sludge-glucose fermentation using L. plantarum produced a leached sludge that has low heavy metal and nutrient content while affording the recovery of nutrients. The potential of these methods for practical application are briefly discussed.

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► Different types of acid leaching of aquaculture sludge were used to remove metals and nutrients.
► Addition of carbohydrate enhanced nutreint release in the presence of inoculated bacteria.
► A Lactobacillus plantarum inoclum gave good nutrient release and sludge preservation.
► This treatment was better than mineral or organic acids or the addition of carbohydrate alone.