Nitrate depuration of secondary sewage effluents in mangrove sediments

The sewage treatment plant (STP) at La Parguera, on the southwest coast of Puerto Rico, discharges an average of 228,000 dm³ of secondary sewage 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 of actual and potential denitrification using acetylene block and substrate disappearance techniques indicate that mangrove sediment microbial communities are capable of depurating 10 to 15 times the nitrate added in the STP effluent. Plots of porewater salinities vs nitrate concentrations show exponential decay of nitrate concentration. Our observations confirm the potential of mangrove sediment-microbial communities for nitrate depuration of secondary sewage effluents.