Abstract

 A simple biological filter incorporating a common bivalve (oyster) and locally available seaweed (Gracilaria sp.) and fish to process shrimp farm effluent in a closed re-circulating water system was conducted. Pond water recycling and underwater columns aeration system are some innovations that hold much promise for sustainable aquaculture. Twelve units of 225m2 earthen ponds (3 units each for bivalves, sedimentation or settling pond and the rest as shrimp ponds) were used. Treatments differ based on the manner how oysters and fish were stocked with control (oysters only without fish in biofilter ponds). Shrimp (Penaeus monodon) juveniles at 30 pieces/m2, 10 pieces/m2 for oyster and tilapia were stocked. The treatments were; 1) oysters in biofilter pond planted in bamboo raft directly near the discharge/effluent pipes integrated with tilapia, 2) oysters in same manner of planting + tilapia but scattered all over the pond, and 3) oysters only in biofilter pond (Control). Seaweeds were planted in nylon cages along the re-circulating water channel of all treatment ponds. The seawater for culture was pumped once into the experimental set-up using a ¾ hp submersible pump that flowed into compartments (biofilter, biomanipulator and sedimentation ponds) at a rate of 600-800 m3/cycle.   Environmental monitoring has been carried out to detect important water quality changes during the process of recycling. Pond water recycling and use of biofilters (bivalves and algae) resulted in better disease control that maintained presumptive vibrio (1.1-4.5 x 102 cfu/ml), luminous bacteria (1.0-5.0 x 102 cfu/ml) and total bacterial count (1.5-7.2 x 104 cfu/ml) compared to inlet water (2.3-9.7 x 104 cfu/ml) and service canals for recycling ( 2.0-6.6 x 103 cfu/ml) during culture owing good water quality for culture and improved effluent quality. Nitrogen and phosphorus load in shrimp ponds mainly come from feeds containing high protein, that are not fully utilized. Using SEAFDEC formulated shrimp feed given 3-4 times a day for 120 days of culture, total net discharge of nutrients apparently reduced as effluents undergo the simple process of filtration and water recycling. The system was kept running all throughout the day except when feeding. Remarkable decrease of TP from shrimp pond (0.20 ppm) to biofilter (0.19 ppm) and sedimentation pond (0.11 ppm) finally filtered by seaweeds in the canal (0.08) was observed in treatment 3 compared to treatments 2 and control that ranged from 0.47-0.19 and 0.33-0.16 ppm respectively. Similarly, TN content of the culture water was evident treatment 1 (3.8-3.2 ppm) unlike in Control (5.5-3.8 ppm) and treatment 2 (4.7-3.5 ppm). The nutrients were practically reduced to a safe level to be pumped back to shrimp pond. Integration of algae, fish and sedimentation-settling pond is an effective biological process for treatment of shrimp farm effluent. The process resulted in culture and effluent water with acceptable standard water quality criteria for re-use or discharging out without harmful effects on the growth and survival of shrimps and can yield an additional income for farmers by producing secondary products like oysters, fish and seaweed.