Wastewater wetlands effective in nature?

Over the last decade or so there has been a substantial growth in the number of man-made wetlands being commissioned to treat wastewater. It's easy to see why; cheap to build and operate, usually effective and good for nature. Is this one of those rare commodities, a win-win situation where both man and environment benefit?

Constructed wetlands are being used to aid in the treatment of wastewater across the world. One type of wastewater that this technique is being used for is to reduce the impact of mine drainage. Containing heavy metals and highly acid water, this type of water pollution can have major effects on aquatic ecosystems. The breach of the Nangiles Audit at Wheal Jane tin mine in Cornwall during January 1992 sent highly toxic concentrations of cadmium, zinc, arsenic and iron into the Carnon River and into Falmouth Bay. Of more concern to water quality is that of chronic pollution incidents such as can be found affecting the River Pelenna in Wales where acid mine drainage (AMD) from disused coalmines cause metal pollution, particularly iron discolouration and precipitation. Britain as a whole has had a long industrial history built on its abundant coalfields. As these workings close, a way of controlling pollution efficiently and effectively will need to be put in place.

In relation to mine drainage, the processes at work within the wetlands is still not fully understood, however three dominant processes seems to be at work:
1. Redox reactions; the change in metal ion characteristics in reducing or anaerobic environments.
2. Microbial actions; changes to metal ions as a result of respiration of sulphate reducing bacteria.
3. Substrate reactions; ion/cation exchange with the substrate, adsorption and complexation.

Although these processes are not fully understood, well-designed wetland systems have achieved some very good results. An ideal example of the effectiveness of a constructed wetland for acid mine drainage is:
Tennessee Valley Authority 950 Coal mine.
A wetland system was installed to reduce acid mine drainage from a closed mine near Flat Rock, Alabama. The mine authority had been spending approximately $28,500 annually on chemical treatment of the drainage when in 1986 it opted to build a wetland at a cost of $41,000 and annual monitoring costs of $3,700. Outflow from the wetland was within statutory limits most of the time. Despite being planted with only two reedbed species; Typha latifolia and Scirpus cyperinus, the system performed well and also quickly established itself. Within 6 months of completion 32 taxa of macroinvertibrates had become established (only 2 taxa present before completion) and within 12 months 20 different plant taxa were found within the wetland. This healthy ecosystem resulted in the main problem of the wetland when muskrats were attracted and burrowed into dykes causing a breach of the channels and therefore occasional reduced outflow quality.

With valuable metals being retained within a small area, the option of recovering this resource becomes a possibility. Experiments have shown that in some instances, metals can accumulate in plant and algal tissues. Metal accumulation occurs naturally over a period of time and concentrations can reach levels where recovery is viable. Natural examples are bog iron deposits; manganese nuggets and gold nuggets are also known to have been formed through bacteria or algae precipitating ions out of water. Studies have shown that apical leaves of Pistia stratiotes can concentrate a number of metals including copper, cadmium, lead and zinc. Eichhornia crassipes has also been shown to be an effective plant at concentrating metals. Studies are looking at the effectiveness of other plants in concentrating metals including Lemna lemna. Green algae species seems to be effective at concentrating metals. The most effective species seems to be Oedogonium, Mougeotia and Microspora. Studies seem to demonstrate that most metals are susceptible to concentration.

So, are there any negative aspects of using wetlands? Early signs seem to show that even when wetland systems fail to work as expected the consequences are usually still better than not doing any treatment at all. The main problems appear to be related to design criteria and particularly insufficient residence time for the wastewater. Although other problems have arisen. There have been cases where wetlands have failed to work because of insufficient low weight carbon compounds being available for bacteria. This is a problem with mine water that may not have sufficient available organic compounds until the wetland achieves maturity, which can take upto 5 years. In some instances this has been solved by the addition of sewage into the wetland. Another problem that has occurred in some areas is protection of endangered species. Wetlands quickly become home to a wide diversity of life and if protected species move into the wetland, that can cause management problems.

Wetland systems are still a fairly recent technique and one possible problem, particularly when dealing with mine discharge is biomethylation of metals. Methylation of metals can produce extremely toxic bioaccumulating pollutants. Methylmercury for instance is a powerful and persistent neurotoxin and methyltin is also poisonous to the central nervous system. Many other metals also have toxic methyl compounds. Biomethylation of metals tend to happen in reducing environments in the presence of bacteria or vitamin B12 derivatives; the very conditions found within wetland systems. As the years progress and these systems become more established, we should monitor the top predators of these systems for signs of accumulation of these pollutants.

Overall, the use of constructed wetlands to deal with the persistent pollution associated with disused mines of all types can lead to major improvements in the environment. It can lead to the establishment of one of natures most productive and threatened habitats. Provided that we are aware of the potential problems and are prepared to make changes to protect the integrity of the ecosystem then this technology really can be considered 'green' and sustainable.