Integrated treatment of shrimp effluent by sedimentation, oyster filtration and macroalgal absorption: a laboratory scale study

Effluent water from shrimp ponds typically contains elevated concentrations of dissolved nutrients and suspended particulates compared to influent wat er. Attempts to improve effluent water quality using filter feeding bivalve s and macroalgae to reduce nutrients have previously been hampered by the h igh concentration of clay particles typically found in untreated pond efflu ent. These particles inhibit feeding in bivalves and reduce photosynthesis in macroalgae by increasing effluent turbidity. In a small-scale laboratory study, the effectiveness of a three-stage effluent treatment system was in vestigated. In the first stage, reduction in particle concentration occurre d through natural sedimentation. In the second stage, filtration by the Syd ney rock oyster, Saccostrea commercialis (Iredale and Roughley), further re duced the concentration of suspended particulates, including inorganic part icles, phytoplankton, bacteria, and their associated nutrients. In the fina l stage, the macroalga, Gracilaria edulis (Gmelin) Silva, absorbed dissolve d nutrients. Pond effluent was collected from a commercial shrimp farm, tak en to an indoor culture facility and was left to settle for 24 h. Subsample s of water were then transferred into laboratory tanks stocked with oysters and maintained for 24 h, and then transferred to tanks containing macroalg ae for another 24 h. Total suspended solid (TSS), chlorophyll a, total nitr ogen (N), total phosphorus (P), NH4+, NO3-, and PO43-, and bacterial number s were compared before and after each treatment at: 0 h (initial); 24 h (af ter sedimentation); 48 h (after oyster filtration); 72 h (after macroalgal absorption). The combined effect of the sequential treatments resulted in s ignificant reductions in the concentrations of all parameters measured. Hig h rates of nutrient regeneration were observed in the control tanks, which did not contain oysters or macroalgae. Conversely, significant reductions i n nutrients and suspended particulates after sedimentation and biological t reatment were observed. Overall, improvements in water quality (final perce ntage of the initial concentration) were as follows: TSS (12%); total N (28 %); total P (14%); NH4+ (76%); NO3- (30%); PO43-(35%); bacteria (30%); and chlorophyll a (0.7%). Despite the probability of considerable differences i n sedimentation, filtration and nutrient uptake rates when scaled to farm s ize, these results demonstrate that integrated treatment has the potential to significantly improve water quality of shrimp farm effluent. (C) 2001 El sevier Science B.V. All rights reserved.