Feasibility of using Bivalve Molluscs to Reduce the Suspended Solid Content in Shrimp Farm Effluents
G.L.S.N. LIYANAGE, W.M.T.B.WANNINAYAKE and U EDIRISINGHE
The shrimp industry is now constrained by problems linked with environment due to direct discharge of farm effluent to the water source. In treatment systems, some bivalves can be utilized to reduce the suspended solid content in farm effluents and the present study was carried out whether species of bivalves namely Meretrix casta and Perna perna can be used for this purpose in Sri Lanka.
Different densities of bivalves (20 m-3, 40 m-3, 60 m-3, 80 m-3), from each species were separately allocated in effluent water and suspended solids were measured after 6 hours. Twenty bivalves from both species were separately subjected to the effluent water (2 m3) at two different salinity levels (20 ppm and 30 ppm) and suspended solids were measured at 6 hour intervals. Two different volumes of water (l m3 and 2 m3) were allowed to be utilized by 20 bivalves from each species and suspended solids were measured at 2 hour intervals.
In both species, with the increase in the number of individuals, the suspended solid concentrations reduced. Perna perna was found to be significantly more efficient in reducing the suspended solids than M. casta (p<0 05). At 30 ppt salinity, there was significant difference (p<0.05) between the two species in their suspended solid reduction ability. The species which was capable of reducing suspended solids at both salinity levels was P. perna, M. casta showed a significantly lower (P<0.05) suspended solid reduction ability at 30 ppt salinity. Significantly higher (P<0.05) reduction in suspended solid concentrations of both species was observed when total volume used was l m3. However, after 30 hours of purification no significant difference (P >00 5) was found between the two volumes.
Results of this study revealed that bivalves such as P. perna and M. casta can be utilized in effluent treatment systems and suspended solid concentration can be reduced from 850 mg/m3 to less than half of the concentration within a period of 24 hours using 20 bivalves in 1 m3 of water.