NITRATE DEPURATION OF SECONDARY SEWAGE EFFLUENTS IN MANGROVE SEDIMENTS

The sewage treatment plant (STP) at La Parguera, on the southwest coas t of Puerto Rico, discharges an average of 228,000 dm3 of secondary se wage effluents per day into percolation ponds located at the landward margin of the coastal mangrove fringe. Effluents flowing from the STP percolation ponds to the adjacent mangrove fringe typically exhibited nitrate levels between 0.2 mM and 1.0 mM. Experimental determination o f actual and potential denitrification using acetylene block and subst rate disappearance techniques indicate that mangrove sediment microbia l communities are capable of depurating 10 to 15 times the nitrate add ed in the STP effluent. Plots of porewater salinities vs nitrate conce ntrations show exponential decay of nitrate concentration. Our observa tions confirm the potential of mangrove sediment-microbial communities for nitrate depuration of secondary sewage effluents.