Nutrient and Heavy Metal Retention in Mangrove Sediment Receiving Wastewater

Abstract

Soil column studies were carried out to examine the capacity of mangrove sediments in retaining wastewater nutrients and heavy metals. Synthetic wastewater of three different concentrations, namely diluted sewage (DW), medium sewage (MW) and concentrated sewage (CW), were applied to the columns daily over a period of 54 days. Leachate from each column was collected and analyzed. The study revealed that the concentrations of ammonium in the leachates from all sewage treatments decreased dramatically in the first week with a pattern similar to the control. After this initial decrease, ammonium contents increased rapidly especially in the column treated with CW, then remained at a steady level. At the end of the experimental period, the concentrations of ammonium found in the leachate were in the declining order of CW > MW > DW > control. Organic nitrogen, nitrites and nitrates were not detected in the leachates from all sewage treatments. This suggested that denitrification might have occurred and some of the nitrogen from sewage might have been retained in the mangrove sediment. The changes in leachate K concentration were similar to that of NH4+-N content. On the other hand, the soluble phosphorus and heavy metal contents of leachates from sewage treated columns were similar to those of the control. Most of the heavy metals, including Cu, Zn and Cd, were not detected in the leachate. The sediment data showed that NH4+-N, ortho-P, and heavy metals were accumulated on the top layer of the soil column and their contents decreased with the depth of the soil column. The highest metal content was found in the column treated with concentrated sewage. It is clear that mangrove sediments acted as a good filter/trap for phosphorus and metals, but were less efficient for ammonium nitrogen.