Organic carbon dynamics in a constructed mangrove wastewater wetland populated with benthic fauna: A modelling approach

The use of wetlands for processing nutrient- and organic-rich wastewater has proved effective to prevent coastal pollution in developing countries, mainly due to low running costs (low energy consumption and training requirements) and relatively high system filtration efficiency (difference of is flushed in the system minus what is flushed out, multiplied by 100). In this study we present a management model that simulate the fate of area specific organic carbon loadings in constructed mangrove wetlands (CMW). The model was calibrated and validated by measured data obtained from a newly established CMW system in Tanzania. Sewage from a nearby hotel was pumped to a primary stabilization pond with a residence time of 12h. Subsequently, sewage was discharged into 9 unvegetated cells and 9 cells planted with mangrove trees (Avicennia marina). Each group of 9 cells was split into 3 subgroups of 3 cells receiving seawater mixed with 0, 20 and 60% sewage for six months. Raw sewage was assumed to contain 0.5 (150mgBODL−1)person equivalent. The model provided a good simulation of oxygen, carbon and nutrient dynamics in the systems; primary production (by benthic microalgae and phytoplankton); and biomass change of important faunal groups (crabs and gastropods). High sewage loadings and reduced residence time decreased the system filtration efficiency. Thus a discharge of 100L of 1PE (300mgBODL−1)m−2 into the system requires a water residence time of about 60h within the CMW to achieve the Tanzanian emission standards (30mgBODL−1). Higher mangrove fauna biomass and abundance of biogenic structures (i.e. pneumatophores and crab burrows) increased carbon mineralization in the CMW sediment by up to a factor of 2.