Greywater Reuse Systems

A constructed wetland is a man-made, engineered, marsh-like area designed and constructed to treat wastewater. Constructed wetlands are a cost-effective alternative to conventional treatment systems, simple to both install and operate. Furthermore, constructed wetlands are low-cost technologies which are able to control environmental pollution.

Constructed wetland technology is currently evolving into an acceptable, economically competitive alternative for many wastewater treatment applications, and reed beds effectively remove N and P and the quality of effluent is better than secondary treatment at a conventional wastewater treatment plant.

Reed bed systems have many applications in low-income countries, because of the low operating and maintenance costs; maintenance which local people could be trained to do. In a study with Typha beds it was found that local people in Tanzania could also use Typha for biogas production, compost, raw material for basket weaving and for roofing essentials. However, there is a general lack of knowledge of sanitation and water recycling techniques in middle and low-income countries, which often cannot meet their water supply effectively.

All three biodegradation processes, namely aerobic, anoxic and anaerobic, are expected in wetlands and thus are applicable in greywater treatment. Aquatic plants remove pollutants by directly assimilating them into their tissue and by providing a suitable environment for micro-organisms to transform pollutants and reduce their concentrations.

The role of wetland plants is to provide appropriate environments for microbial attachment and growth, deterring flow and retaining suspended solids. They are also known to stimulate the soil activity by root excretions and reduce the volume of effluent by transpiration. Furthermore, both the activity of plant enzymes and the accumulation of certain mineral elements inside the plant body have a linear correlation with the removal of pollutants from water.

Macrophyte trials at many sewage treatment plants have produced marked improvements in all aspects of water quality, and reed bed systems are consistently cheaper per head of population and have low day to day operational requirements than conventional treatment plants which mainly rely on chemical and physical processes.

All parts of a plant can be involved in the wastewater treatment process. Besides rocks and soils, roots provide the all-important surfaces on which bacteria collect and grow. Stems and leaves act as natural aerators to funnel oxygen to the roots, shelter the water from wind, and shade the aquatic environment, preventing the growth of algae. Symbiosis ensures: marsh plants absorb the metabolites produced by the bacterial degradation of the organic compounds while the microbes exploit the metabolites released from the plant roots. In essence, both use each other’s wastes.

Types of Plants in Reedbed Systems

Suitable plants for greywater trials include members of the families Cyperaceae, Juncaceae and Typhaceae. The selected species should have a high production rate and show a high standing crop throughout the year. Other criteria include: high oxygen transport capability, tolerance to adverse concentrations of pollutants, tolerance to adverse climatic conditions, resistance to pests and disease and ease of management. Aquatic plant species should also be selected based on the following criteria: rapid and relatively constant growth rate, ease of propagation, capacity for adsorption of pollutants, tolerance of hyper-eutrophic conditions, ease of harvesting, and potential usefulness of harvested material.

Emergent macrophytes selected for growth in artificial systems should be robust in habit, have a high biomass throughout the year, and be readily available in the local area. In the SW of Western Australia potentially useful species fall into the categories of "sprouters", which are mainly established by rhizome transplants, or "seeders", which are better established by seed. The best growth was obtained when mean water level coincided with the sediment surface, and reduced growth occurred at slow flow rates when sediments became anaerobic. Reduced biomass occurred at both low and high water levels.

Constructed wetlands using species of Schoenoplectus, Triglochin and Phragmites have been found to remove phosphorus and nitrogen from wastewaters, and that the efficiency of removal depends on the design of the wetland, retention time and hydraulic loading. This is supported by a study of Schoenoplectus validus in both horizontal and up-flow systems, where removal of BOD, TN and TP were positively correlated with retention time, and that the horizontal beds were more efficient at nutrient removal than the up-flow beds.

Almost without exception, reed beds in the UK and Europe have been planted with Phragmites, and species of Phragmites, Iris and Typha have been demonstrated to remove heavy metals, such as lead, copper, zinc, nickel and cadmium, but less effectively than that of soil/substrate. However, Typha and Phragmites should not be used in domestic wastewater treatment systems in Australia because of the massive seasonal release of wind-blown seeds.

Relatively few native Australian species have been studied in detail and, therefore, many would be suitable for future research. Examples include the emergent macrophytes Phragmites karka and Triglochin spp, and submergents such as Vallisneria.

Role of Plants in Nutrient Removal

Wastewater treatment by wetland plants has little to do with the plants, but is primarily accomplished by aerobic and anaerobic micro-organisms attached to the surface of the substrate in which the plants are established.

The inundated roots of aquatic plants provide a mainly anaerobic environment for bacteria to convert nitrate into nitrogen gas and metabolise solids. Root growth in aquatic environments is prolific, providing an interface where nutrients are absorbed and bacteria flourish as they metabolise organic matter. A variety of algae then grow in the water, on the roots and on the walls of tanks, ponds or aquatic systems, and this also contributes to nutrient removal and assimilation.

The root systems of plants growing in water-saturated substrates must obtain oxygen from their aerial organs via internal transport mechanisms. Oxygen leaks from the roots into the water. This, in turn, provides oxidised conditions in the otherwise anoxic substrate and stimulates both aerobic decomposition of organic matter and growth of nitrifying bacteria.

The amount of oxygen expected to be released by plants is nominal, and the limited aeration around the roots effectively ensures that anaerobic conditions will predominate unless the organic load to the wetland is low and/or aeration is possible by other means (for example, other types of plants, shallower water or mechanical devices).

Plant roots can generate a portion of this demand for oxygen, but direct oxygen transfer from the atmosphere or by artificial aeration may be required to achieve effective nitrification. Some oxygen can be expected to be supplied from the photosynthetic process of algae or submerged plant species. If photosynthesis does occur it would remove some carbon dioxide and the pH would be raised causing the volatilisation of ammonia and destruction of some bacteria.

Internal pressurisation and convective through-flow of air are common mechanisms for internal gas transport for many wetland species including Schoenoplectus validus, Baumea articulata and species of Typha, Eleocharis, Cyperus and Juncus.. Schoenoplectus validus has a relatively high resistance to air flow and thus is restricted to water less than one metre deep.

The role of plants in the treatment mechanism depends on two main parameters. These are the rate at which oxygen diffuses into the root zone (Rhizosphere) and the permeability or hydraulic conductivity of the region containing the roots.. This is why most systems use gravel rather than soil. Better filtration of solids occurs in gravel beds, although these types of beds may not significantly remove nutrients such as nitrogen and phosphorus.

The rate of oxygen release is highest near the tips of new roots and minimal in old roots and rhizomes. It is estimated that the oxygen release rate in Phragmites to be from 0.02 gm-2/day to 12 gm-2/day. It is uncertain whether wetland plants attempt to minimise their oxygen losses to the rhizosphere, as this is contrary to the widely-held belief that the design of wetland systems relies on high oxygen leakage from roots.

Furthermore, species possessing an internal convective through-flow ventilation system have higher internal oxygen concentrations in the rhizomes and roots than species relying exclusively on the diffusive transfer of oxygen. Internal transportation of oxygen in wetland plants usually occurs by passive molecular diffusion and by convective flow (bulk flow) of air through the internal gas spaces of the plants.

Plants such as pennywort (Hydrocotyle umbellata) transport oxygen 2.5 times more rapidly than water hyacinth (Eichhornia crassipes), which, in turn, transports oxygen four times more rapidly than water lettuce (Pistia stratiotes). Enough oxygen was transported in the pennywort and water hyacinth to cause 90% BOD reduction while the remaining 10% of BOD removal was due to oxygen from the air.

Wetlands have been shown to have nutrient conservation strategies involving internal cycling. Nutrients absorbed during growth are translocated to the below-ground storage organs during senescence of above-ground parts. Later, these nutrients are mobilised upwards for use by the young shoots (stems and leaves) in the next growing period.

Reallocation of biomass between compartments is essential for surviving water level changes. Species that can maintain allocation to shoots without an adverse effect on total or below- ground mass are at a distinct advantage. It seems that wetland macrophytes do possess high storage capabilities and can translocate stored nutrients from one part of a plant to another. More than 50% of nutrients were found to be stored in below-ground portions of the plants, tissues which may be difficult to harvest to achieve effective nutrient removal.

The highly productive floating plants, such as water hyacinth, have generally higher uptake capacities whereas the capacity of submerged macrophytes is lower. Even so, the area needed for wastewater treatment for phosphorus removal solely by water hyacinths would still be 30 - 50 m2 per person equivalent and that of emergents about 100 m2 per person equivalent. These figures are for whole domestic wastewater - both black water and greywater. Less area would be required for greywater treatment alone.

Regular harvesting of submerged and emergent plants removes as little as 6% of nutrients. Harvesting can upset the ecological cycles; hence, the nutrient removal process can be interfered with. If this is the case, then harvesting could be undertaken to maintain plant vigour rather than for nutrient reduction. Harvesting is only required for mosquito control, promoting new growth, and maintaining hydraulic capacity. Harvesting for nutrient removal in large systems is not practical, but for small domestic systems may be necessary.

While plants seem to only have a small role in wastewater treatment, they are an essential part of a system devised to maximise nutrient removal. For example, domestic waste treatment may involve a system where greywater is passed through a sedimentation tank, a root zone horizontal flow bed, a sand filter and an artificial pond as a final stage. In such a system, the treated water had very low BOD, total N and P and heavy metals, and was then able to be used for irrigation.

So, plants can absorb nutrients directly and provide growing areas and conditions for micro-organisms. Unfortunately, conventional reedbed systems are little more than monocultures of Phragmites, Baumea, Water Hyacinth, Typha or Schoenoplectus. Pond systems employing a wider range of species are a means to recycle more nutrients, improve treatment potential and mirror natural ecosystems. A combination of floating plants, submergents and emergents would seem to be ideal as all show a range of nutrient accumulation.