Mangrove wetlands as wastewater treatment facility: a field trial

Field work has been conducted in a 300-hectare natural mangrove intertidal wetlands in Shenzhen, a newly developed city in southern China, to study the feasibility of using mangrove wetlands as a sewage treatment facility. The present paper reports the results obtained in the recent year, between December 1994 and December 1995. Two parallel elongated sites (Sites A & B, each 180 m x 10 m) extending from land to sea were chosen for study. Since September 1991, Site A has received settled municipal sewage three times a week during the low ebb tide period when sediments at landward regions were dry. The hydraulic loading was 20 m(3) per discharge and wastewater was soaked into the sediments within 50 m of the discharge points before the next incoming tide. Site B served as a control. Over the past months in 1994 and 1995, surface sediments and plant leaves were collected at identified locations in two sites at every six month intervals. The impact of sewage on mangrove plant growth was assessed by monitoring plant height, diameter and number of trees using the fixed plot technique. The plant density, stem diameter and tree height of two dominant mangrove species, Kandelia candel and Aegiceras corniculatum, found in Site A were comparable with those of Site B. No significant difference was detected between two sites in terms of plant growth and death rates. These results indicate that sewage discharge over a period of about two years did not exhibit any apparent effect on plant growth. The nutrient and organic matter concentrations of surface sediments in Site A were also not significantly different from those found in Site B, except at the very landward regions (2 to 40 m away from landwards). The nutrient concentrations of sediments collected in sampling locations near the discharge points of Site A were however significantly higher than that of the control. In both sites, the organic C, total N and P, NH4+-N and NO3--N concentrations in the surface sediments exhibited a descending trend from landwards to seaward regions, with notably higher values found in the landward locations. Seasonal variation in NH4+-N content was obvious, and more ammonium nitrogen was recorded in July than in December. Leaf samples of the two dominant plant species collected from Site A had similar total N and organic C concentrations as those from Site B. These findings suggest that mangrove intertidal wetlands are of great potential for natural wastewater treatment, and are unlikely to produce any harmful effect on the higher plant communities.