Decreasing phosphorus discharge in fish farm ponds by treating the sludge generated with sludge drying beds

Two sets of experiments were carried out to assess the net advantage of treating the sludge generated during fish production with sludge drying beds (SDBs). The first experiment consisted of monitoring the concentration of o-PO4-P and total phosphorus (TP) at the effluent of four 1 m2 SDB. The SDBs were set up in a fish farm facility and were fed with fresh settled sludge every week (two of them were operated at 32 kg DM/m2.year and two of them were operated at 18 kg DM/m2.year). The second experiment consisted of monitoring under lab conditions the long term aerobic and anaerobic leaching of o-PO4-P of fish farm sludge. Phosphorus leaching from SDBs was that of 3 μg o-PO4-P released/g TP and 5 μg TP released/g TP, regardless of sludge loading. Phosphorus leaching rates under lab conditions was that of 280 μg o-PO4-P/g TP and 520 μg o-PO4-P/g TP under aerobic and anaerobic conditions, respectively. Furthermore, SDBs leached phosphorus to a greater extent during high raining episodes (up to 4 mg o-PO4-P/m2 d and 7.3 mg TP/m2 d). However, the implementation of a slag filter treating the SDB effluent could reduce the phosphorus leached by the SDB, thus evidencing its potential utilization to reduce leaching variability of SDBs. The main conclusion of the present work is that of the use of a SDB may reduce the phosphorus discharged by a trout production pond (either in terms of soluble or particulate phosphorus) by 36%. More precisely, sludge treatment with a SDB will save about 0.52 kg of P/ton of fish produced and between 0.215 and 0.402 g of o-PO4-P/ton of fish produced.