Development and optimization of constructed mangrove wetland systems for treatment of municipal wastewater

Abstract

The discharge of untreated or partially treated municipal wastewater deteriorates coastal and marine ecosystems. Conventional wastewater treatment methods are often too complicated and too expensive for developing countries and small communities in rural areas of developed countries. The constructed wetland treatment system with annual plants has been employed as an alternative treatment method but it needs frequent harvesting. The aim of this MPhil research is to develop and optimize a subsurface flow constructed mangrove wetland system as a secondary municipal wastewater process. A series of greenhouse studies using constructed mangrove microcosms without tidal flushing were conducted under different hydraulic retention times (HRT), mangrove plant species, salinities and the introduction of an idle period. The study also evaluates the comparability between artificial wastewater and real primary-settled municipal wastewater collected from a sewage treatment work in Hong Kong SAR in treatment performance and outcome for wastewater-borne pollutants. The results demonstrate that constructed mangrove tanks planted with Kandelia candel had significantly higher treatment efficiency than the unplanted tanks. The removal percentages of dissolved organic carbon (DOC), ammonia-nitrogen (N), inorganic-N, total Kjeldahl N and ortho-phosphate in the planted systems were 70.43-76.38%, 76.16-91.83%, 47.89-63.37%, 75.15-79.06% and 86.65-91.83%, respectively. Plant growth as well as tissue N and phosphorus (P) concentrations and uptake were enhanced by the addition of wastewater. The mass balance showed that active nitrification and denitrification processes occurred in the mangrove system, with 25-30% N lost to atmosphere, while P was mainly accumulated in sediment. The removal efficiency under 10-day HRT was better than that of 5-day but more land area is needed for longer the HRT. The introduction of an idle period significantly enhanced removal percentages of DOC and N as microbial activities in the soil were stimulated after the idle period. The denitrification potential at the end of the second treatment period was approximately 50-fold higher than that at the end of the first treatment period. Although the planted systems had better treatment performance than the unplanted ones, no significant difference in removal efficiency was found among the three mangrove species, namely Aegiceras corniculatum, Acanthus ilicifolius and Bruguiera gymnorrhiza during the four-month wastewater treatment. All planted systems effectively removed pollutants with 90% of DOC, 99% of ammonia-N, 78% of inorganic N removal, and > 97% of TKN and inorganic P removed under 5-day HRT. The total amounts of N and P accumulated in the tissues of A. ilicifolius were comparable to that of A. corniculatum and B. gymnorrhiza. However, the fate of wastewater-borne pollutants and their distribution in different components of the constructed mangrove wetland varied among the three mangrove species, indicating that the root structure and oxygen released from roots of each mangrove species might be different, which then altered the nutrient and transformation in the soil. The treatment performance of mangrove microcosms planted with A. corniculatum was affected by wastewater salinity, with a poorer rate of removal of DOC and N under high salinities (15 and 30ppt, parts per thousands). Saline wastewater reduced the denitrification potential. However, growth of A. corniculatum and tissue nutrient uptake was the highest at 15ppt. The removal percentages of DOC and P were different between artificial and real municipal wastewater under the same treatment condition, probably due to the absence of microorganisms, ions (particularly Fe3+ and Ca2+), trace elements and different forms of organic matter and P in artificial wastewater, as these are difficult to simulate. However, no significant difference in N removal was found between artificial and real wastewater. This suggested that if the formula for preparing artificial wastewater is further improved, it is possible to extrapolate data from artificial wastewater to real wastewater situations. All effluent leaving the planted mangrove systems was able to meet the effluent discharge standards of Water Control Zones set by the Environmental Protection Department of Hong Kong SAR. The present research demonstrates the feasibility of using constructed mangrove wetlands, without tidal flushing, as the secondary treatment process for municipal wastewater, even for those with high salinity.