The Role of Macrophytes in Constructed Surface-flow Wetlands for Mine Water Treatment: A Review

Abstract

Constructed wetlands are a standard sustainable technology in waste and mine water treatment. Whereas macrophytes actively contribute to decomposition and/or removal of wastewater’s organic pollutants, removal of hydrolysable metals from mine water is not attributable to direct metabolic, but rather various indirect macrophyte-related mechanisms. These mechanisms result in higher treatment efficiency of (vegetated) wetlands relative to (unvegetated) settling ponds. Contribution of macrophytes to treatment predominantly includes: enhanced biogeochemical oxidation and precipitation of hydrolysable metals due to catalytic reactions and bacterial activity, particularly on immersed macrophyte surfaces; physical filtration of suspended hydrous ferric oxides by dense wetland vegetation down to colloids that are unlikely to gravitationally settle efficiently; scavenging and heteroaggregation of dissolved and colloidal iron, respectively, by plant-derived natural organic matter; and improved hydrodynamics and hydraulic efficiency, considerably augmenting retention and exposure time. The review shows that constructed surface-flow wetlands have considerable advantages that are often underestimated. In addition to treatment enhancement, there are socio-environmental benefits such as aesthetic appearance, biotope/habitat value, and landscape diversity that need to be considered. However, there is currently no quantitative, transferrable approach to adequately describe the effect and magnitude of macrophyte-related benefits on mine water amelioration, let alone clearly assign optimal operational deployment of either settling ponds or wetlands. A better (quantitative) understanding of underlying processes and kinetics is needed to optimise assembly and sizing of settling ponds and wetlands in composite passive mine water treatment systems.

Review of Wastewater Treatment Technologies in View of their Application in the DR Congo Mining Industry

“…At Leviathan mine (California-USA), biological treatment of mining water (Table 12) has been conducted using a bioreactor inside which a bacterial reduction of sulphates has been implemented for metal elements precipitation as sulphides [112]. This passive technique for treating polluted water is based on biogeochemical reactions that occur in a natural aquatic environment [34,44,109,113]. Wetlands are used in the treatment of industrial and domestic wastewater [114,115].…”

“…Wetlands are used in the treatment of industrial and domestic wastewater [114,115]. They act as natural filtration media wherein take place biogeochemical processes [113][114][115] that are involved in the removal of pollutants (Figure 10). Wetlands are usually swamps, shallow lakes, coastal areas, etc.…”

“…Sometimes, they consist of areas flooded naturally or artificially by water with a certain frequency. Their emergent vegetation is most often made up of plant species such as Typha latifolia, Phragmites australis, Eichhornia crassipes and other macrophytes [26,113,[115][116][117][118][119][120]. Wetlands constitute transitional ecological systems between land and the aquatic environment wherein the level of the water table is at or near the surface of the soil [121,118,119].…”

“…Wetlands are thus highly complex habitats where the interaction of biotic and abiotic factors results in a mechanical and biogeochemical filtration capable of removing pollutants from water [26,115,120]. The pollutant removal processes [26] and biogeochemical conditions that support their establishment in a wetland are described in Table 13 [34,113]. The environment that prevails inside a wetland [34,117] dictates the establishment and the extent of biogeochemical processes implicated in the depollution of water, including oxidation, reduction, chelation, adsorption, complexation, sedimentation, filtration, biosorption of pollutants by plants and their microbial digestion.…”

“…The hydrological regime, geology and sediment chemistry, the composition of the microbial community, fauna and flora, and the geomorphology and position of the wetland in a given landscape [121] influence this environment. Thus, in a wetland, oxidizing conditions prevail on the water surface up to the water-sediment interface [115] and this is not the case in the depth of the sediments where reducing (anaerobic) conditions predominate [113,115]. The bacteria (sulphate-reducing species, denitrifying species, etc.)…”