Macrophyte Treatment Systems (MaTS)

Macrophyte Treatment Systems (MaTS)

The discharge of wastewaters into natural watercourses, ponds and wetlands is an ancient and long-established practice, though rising urbanisation led to the development of more engineered solutions, initially for domestic sewage and then later, industrial effluents, which in turn for a time lessened the importance of the earlier approach. However, there has been a resurgence of interest in simpler, more natural methods for wastewater treatment and MaTS systems, in particular, have received much attention as a result. While there has, undoubtedly, been a strong upsurge in public understanding of the potential for environmentally har-monious water cleaning per se, a large part of the driving force behind the newly found interest in these constructed habitats comes from biodiversity concerns. With widespread awareness of the dwindling number of natural wetlands, often a legacy of deliberate land drainage for development and agricultural purposes, the value of such manufactured replacements has become increasingly apparent. In many ways it is fitting that this should be the case, since for the majority of aquatic macrophyte systems, even those expressly intended as ‘monocultures’ at the gross scale, it is very largely as a result of their biodiversity that they function as they do.

 These treatment systems, shown diagrammatically in Figure 7.2, are charac-terised by the input of effluent into a reservoir of comparatively much larger vol-ume, either in the form of an artificial pond or an expanse of highly saturated soil

held within a containment layer, within which the macrophytes have been estab-lished. Less commonly, pre-existing natural features have been used. Although wetlands have an innate ability to accumulate various unwanted chemicals, the concept of deliberately polluting a habitat by using it as a treatment system is one with which few feel comfortable today. A gentle hydraulic flow is estab-lished, which encourages the incoming wastewater to travel slowly through the system. The relatively long retention period that results allows adequate time for processes of settlement, contaminant uptake, biodegradation and phytotransformation to take place.

 The mechanisms of pollutant removal are essentially the same, irrespective of whether the particular treatment system is a natural wetland, a constructed monoculture or polyculture and independent of whether the macrophytes in question are submerged, floating or emergent species. Both biotic and abiotic methods are involved. The main biological mechanisms are direct uptake and accumulation, performed in much the same manner as terrestrial plants. The remainder of the effect is brought about by chemical and physical reactions, principally at the interfaces of the water and sediment, the sediment and the root or the plant body and the water. In general, it is possible to characterise the primary processes within the MaTS as the uptake and transformation of contaminants by micro-organisms and plants and their subsequent biodegra-dation and biotransformation; the absorption, adsorption and ion exchange on the surfaces of plants and the sediment; the filtration and chemical precipita-tion of pollutants via sediment contact; the settlement of suspended solids; the chemical transformation of contaminants. It has been suggested that although settlement inevitably causes the accrual of metals, in particular, within the sed-iment, the plants themselves do not tend to accumulate them within their tis-sues. While this appears to be borne out, particularly by original studies of natural wetlands used for the discharge of mine washings (Hutchinson 1975), this does not form any basis on which to disregard the contribution the plants make to water treatment. For one thing, planting densities in engineered sys-tems are typically high and the species involved tend to be included solely for their desired phytoremediation properties, both circumstances seldom repeated in nature. Moreover, much of the biological pollutant abatement potential of the system exists through the synergistic activity of the entire community and, in purely direct terms, this largely means the indigenous microbes. Functionally, there are strong parallels between this and the processes of enhanced rhizo-spheric biodegradation described for terrestrial applications. While exactly the same mechanisms are available within the root zone in an aquatic setting, in addition, and particularly in the case of submerged vegetation, the surface of the plants themselves becomes a large extra substrate for the attached growth of closely associated bacteria and other microbial species. The combined rhizo- and circum-phyllo- spheres support a large total microbial biomass, with a distinctly different compositional character, which exhibits a high level of bioactivity, rel-ative to other microbial communities. As with rhizodegradation on dry land, part of the reason is the increased localised oxygenation in their vicinity and the corresponding presence of significant quantities of plant metabolic exudates, which, as was mentioned in the relevant earlier section, represents a major pro-portion of the yearly photosynthetic output. In this way, the main role of the macrophytes themselves clearly is more of an indirect one, bringing about local environmental enhancement and optimisation for remediative microbes, rather than being directly implicated in activities of primary biodegradation. In addi-tion, physico-chemical mechanisms are also at work. The iron plaques which form on the plant roots trap certain metals, notably arsenic (Otte, Kearns and Doyle 1995), while direct adsorption and chemical/biochemical reactions play a role in the removal of metals from the wastewater and their subsequent retention in sediments.

 The ability of emergent macrophytes to transfer oxygen to their submerged portions is a well-appreciated phenomenon, which in nature enables them to cope with effective waterlogging and functional anoxia. As much as 60% of the oxygen transported to these parts of the plant can pass out into the rhizosphere, creating aerobic conditions for the thriving microbial community associated with the root zone, the leaf surfaces and the surrounding substrate. This accounts for a significant increase in the dissolved oxygen levels within the water generally and, most particularly, immediately adjacent to the macrophytes themselves.

 The aerobic breakdown of carbon sources is facilitated by this oxygen transfer, for obvious reasons, and consequently it can be seen to have a major bearing on the rate of organic carbon biodegradation within the treatment system, since its adequate removal requires a minimum oxygen flux of one and a half times the input BOD loading. Importantly, this also makes possible the direct oxida-tion of hydrogen sulphide (H2S) within the root zone and, in some cases, iron and manganese.

 While from the earlier investigations mentioned on plant/metal interactions (Hutchinson 1975) their direct contribution to metal removal is small, fast-growing macrophytes have a high potential uptake rate of some commonly encountered effluent components. Some kinds of water hyacinth, Eichhornia spp., for example, can increase their biomass by 10 g/m2/day under optimum conditions, which represents an enormous demand for nitrogen and carbon from their environment. The direct uptake of nitrogen from water by these floating plants gives them an effective removal potential which approaches 6000 kg per hectare per year and this, coupled with their effectiveness in degrading phenols and in reducing copper, lead, mercury, nickel and zinc levels in effluents, explains their use in bioengineered treatment systems in warm climates.

 Emergent macrophytes are also particularly efficient at removing and storing nitrogen in their roots, and some can do the same for phosphorus. However, the position of this latter contaminant in respect of phytotreatment in general is less straightforward. In a number of constructed wetland systems, though the overall efficacy in the reduction of BOD, and the removal of nitrogenous com-pounds and suspended solids has been high, the allied phosphorus components have been dealt with much less effectively. This may be of particular concern if phosphorus-rich effluents are to be routinely treated and there is a consequent risk of eutrophication resulting. It has been suggested that, while the reasons for this poor performance are not entirely understood, nor is it a universal finding for all applications of phytotreatment, it may be linked to low root zone oxygena-tion in slow-moving waters (Heathcote 2000). If this is indeed the case, then the preceding discussion on the oxygen pump effect of many emergent macrophytes has clear implications for biosystem design.

 As has been established earlier, associated bacteria play a major part in aquatic plant treatment systems and microbial nitrification and denitrification processes are the major nitrogen-affecting mechanisms, with anaerobic denitrification, which typically takes place in the sediment, causing loss to atmosphere, while aerobic nitrification promotes and facilitates nitrogenous incorporation within the vegeta-tion. For the effective final removal of assimilated effluent components, accessibly harvestable material is essential, and above water, standing biomass is ideal.

 The link between the general desire for biodiversity conservation and the acceptability of created wetlands was mentioned earlier. One of the most impor-tant advantages of these systems is their potential to create habitats not just for ‘popular’ species, like waterfowl, but also for many less well-known organ-isms, which can be instrumental in bolstering the ecological integrity of the area. This may be of particular relevance in industrial or urban districts. At the same time, they can be ascetically pleasing, enhancing the landscape while per-forming their function. These systems can have relatively low capital costs, but inevitably every one must be heavily site specific, which means many aspects of the establishment financing are variable. However, the running costs are gener-ally significantly lower than for comparable conventional treatment operations of similar capacity and efficacy. In part the reason for this is that once properly set up, a well-designed and constructed facility is almost entirely self-maintaining. However, the major contribution to low operational overheads comes from the system’s low energy requirements, since gravity drives the water flow and all the remediating organisms are ultimately solar powered, either directly or indirectly, via the photosynthetic action of the resident autotrophe community.

 Aside of cost and amenity grounds, one major positive feature is that the efflu-ent treatment itself is as good or better than that from conventional systems. When correctly designed, constructed, maintained and managed, plant-based treatment is a very efficient method of ameliorating wastewaters from a wide range of sources and in addition, is very tolerant of variance in organic loadings and effluent quality, which can cause problems for some of the alternative options. In addition, phyto-systems can often be very effective at odour reduction, which is often a major concern for the producers and processors of effluents rich in biodegradable substances.

 Invariably, the better designed, the easier the treatment facility is to manage and in most cases, ‘better’ means simpler in practice, since this helps to keep the maintenance requirement to a minimum and makes maximum use of the existing topography and resources. Provision should also be made for climatic factors and most especially, for the possibility of flooding or drought. It is imperative that adequate consideration is given to the total water budget at the project planning stage. Although an obvious point, it is important to bear in mind that one of the major constraints on the use of aquatic systems is an adequate supply of water throughout the year. While ensuring this is seldom a problem for temperate lands, for some regions of the world it is a significant concern. Water budgeting is an attempt to model the total requirement, accounting for the net overall in- and out-puts, together with the average steady-state volume resident within the system in operation. Thus, effluent inflow, supplementary ‘clean’ water and rainfall need to be balanced against off-take, evaporative and transpirational losses and the demands of the intended retention time required to treat the particular contami-nant profile of a given wastewater. One apparent consideration in this process is the capacity of the facility. Determining the ‘required’ size for a treatment wet-land is often complicated by uncertainty regarding the full range of wastewater volumes and component character likely to be encountered over the lifetime of the operation. The traditional response to this is to err on the side of caution and oversize, which, of course, has inevitable cost implications, but in addition, also affects the overall water budget. If the effluent character is known, or a sample can be obtained, its BOD can be found and it is then a relatively simple pro-cedure to use this to calculate the necessary system size. However, this should only ever be taken as indicative. For one thing, bioengineered treatment systems typically have a lifespan of 15 – 20 years and the character of the effluent being treated may well change radically over this time, particularly in response to shifts in local industrial practice or profile. In addition, though BOD assessment is a useful point of reference, it is not a uniform indicator of the treatment require-ments of all effluent components. For the bioamelioration process to proceed efficiently, a fairly constant water level is necessary. Although the importance of this in a drought scenario is self-evident, an unwanted influx of water can be equally damaging, disturbing the established equilibrium of the wetland and pushing contaminants through the system before they can be adequately treated. Provision both to include sufficient supplementary supplies, and exclude surface water, is an essential part of the design process.

 One aspect of system design which is not widely appreciated is the importance of providing a substrate with the right characteristics. A number of different materials have been used with varying degrees of success, including river sands, gravels, pulverised clinker, soils and even waste-derived composts, the final choice often being driven by issues of local availability. The main factors in determining the suitability of any given medium are its hydraulic permeability and absorbance potential for nutrients and pollutants. In the final analysis, the sub-strate must be able to provide an optimum growth medium for root development while also allowing for the uniform infiltration and through-flow of wastewater. A hydraulic permeability of between 10−3 and 10−4 m/s is generally accepted as ideal, since lower infiltration tends to lead to channelling and flow reduction, both of which severely restrict the efficiency of treatment. In addition, the chemical nature of the chosen material may have an immediate bearing on system efficacy. Soils with low inherent mineral content tend to encourage direct nutrient uptake to make good the deficiency, while highly humeric soils have been shown to have the opposite effect in some studies. The difficulties sometimes encountered in relation to phosphorus removal within wetland systems have been mentioned earlier. The character of the substrate medium can have an important influence on the uptake of this mineral, since the physico-chemical mechanisms responsi-ble for its abstraction from wastewater in an aquatic treatment system relies on the presence of aluminium or iron within the rhizosphere. Obviously, soils with high relative content of these key metals will be more effective at removing the phosphate component from effluent, while clay-rich substrates tend to be better suited to lowering heavy metal content.

One aspect of system design which is not widely appreciated is the importance of providing a substrate with the right characteristics. A number of different materials have been used with varying degrees of success, including river sands, gravels, pulverised clinker, soils and even waste-derived composts, the final choice often being driven by issues of local availability. The main factors in determining the suitability of any given medium are its hydraulic permeability and absorbance potential for nutrients and pollutants. In the final analysis, the sub-strate must be able to provide an optimum growth medium for root development while also allowing for the uniform infiltration and through-flow of wastewater. A hydraulic permeability of between 10−3 and 10−4 m/s is generally accepted as ideal, since lower infiltration tends to lead to channelling and flow reduction, both of which severely restrict the efficiency of treatment. In addition, the chemical nature of the chosen material may have an immediate bearing on system efficacy. Soils with low inherent mineral content tend to encourage direct nutrient uptake to make good the deficiency, while highly humeric soils have been shown to have the opposite effect in some studies. The difficulties sometimes encountered in relation to phosphorus removal within wetland systems have been mentioned earlier. The character of the substrate medium can have an important influence on the uptake of this mineral, since the physico-chemical mechanisms responsi-ble for its abstraction from wastewater in an aquatic treatment system relies on the presence of aluminium or iron within the rhizosphere. Obviously, soils with high relative content of these key metals will be more effective at removing the phosphate component from effluent, while clay-rich substrates tend to be better suited to lowering heavy metal content.

Engineered reed beds are probably the most familiar of all macrophyte treat-ment systems, with several high profile installations in various parts of the globe having made the technology very widely accessible and well appreciated. This approach has been successfully applied to a wide variety of industrial effluents, in many different climatic conditions and has currently been enjoying consider-able interest as a ‘green’ alternative to septic tanks for houses not joined up to mains sewerage. At its heart is the ability of reeds, often established as mono-cultures of individual species, or sometimes as oligocultures of a few, closely related forms, to force oxygen down into the rhizosphere, as has been previously discussed. Many examples feature Phragmites or Typha species, which appear to be particularly good exponents of the oxygen pump, while simultaneously able to support a healthy rhizospheric microfloral complement and provide a stable root zone lattice for associated bacterial growth and physico-chemical process-ing of rhizo-contiguous contaminants. Isolated from the surrounding ground by an impermeable clay or polymer layer, the reed bed is almost the archetypal emergent macrophyte treatment system.

 The mechanisms of action are shown in Figure 7.3 and may be categorised as surface entrapment of any solids or relatively large particulates on the grow-ing medium or upper root surface. The hydraulic flow draws the effluent down through the rhizosphere, where the biodegradable components come into direct contact with the root zone’s indigenous micro-organisms, which are stimulated to enhanced metabolic activity by the elevated aeration and greater nutrient availability. There is a net movement of oxygen down through the plant and a corresponding take-up by the reeds of nitrates and minerals made accessible by the action of nitrifying and other bacteria.

 These systems are very efficient at contamination removal, typically achieving 95% or better remediation of a wide variety of pollutant substances, as demon-strated in Table 7.1, which shows illustrative data on the amelioration of landfill leachates by this system.

 Nevertheless, reed beds and root zone treatment techniques in general are not immune from a range of characteristic potential operational problems, which can act to limit the efficacy of the process. Thus, excessive waterlogging, surface run-off, poor or irregular substrate penetration and the development of preferential drainage channels across the beds may all contribute to a lessening of the system’s performance, in varying degrees.