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When Nature Serves Engineering: Plants that Clean Water
An alternative title for this article could be: “what else will nature do for us?” No matter the title, today there is a tremendous increase in the environmental awareness. Environmental protection and all related aspects is part of our life. Probably every day we hear about a new environmental problem and its consequences. We know that the way of economic activity of the industrialized countries, mainly, along with the rapid increase of human population, resulted in the present and the expected environmental problems.
The need for ecosystem protection is pressing. Under this frame, water, this natural resource, is becoming scarce globally. Many speak of a future water crisis since, in the long term, the available potable water amount is more or less steady, while most of the water sources, which are technically easy and economically attractive to exploit, are almost exhausted. Thus, new sources, like wastewater, are needed.
Large wastewater volumes are produced every day, since practically every human activity is connected to the production of waste. The effects of the uncontrolled discharge of wastewater to surface and underground receivers are more or less known: ecosystem degradation, toxicity, eutrophication, risks for human health etc. Thus, in order to protect the environment and the aquatic ecosystems (sea, rivers, lakes, underground water), the construction of wastewater treatment plants is necessary. Wastewater treatment and management is an important issue for our modern societies. Probably most of us assume that this problem is solved, we have found the best solutions and it is not of top priority. But, in reality, there are still many parameters and challenges to be taken into account.
The first, and perhaps the most important, is – what else – the costs. Building of an end-of-the-pipe large, extensive facility of a conventional treatment plant demands a big initial investment and possesses high costs for operation and maintenance. Generally, this kind of facility is the only feasible solution for our capitals, with the largest cities having populations up to many millions of people. Then, the cost is considered as reasonable. But, what happens with our smallest cities, small/medium communities, rural or mountainous areas etc, for which the costs cannot be usually covered by the local authorities? Is it feasible to build such a heavy and big facility for each settlement or to expand the sewer network system for hundreds of miles to transfer all the wastewater volume to a big treatment plant? The answer is today leaning to be negative. Thus, somewhere here the problem begins.
Another parameter to take into consideration is, of course, the environmental impact of these facilities. In conventional biological wastewater treatment plants, an extensive use of concrete and steel is made. They require very large amounts of energy input for their operation, with all its consequences (CO2 production). They produce noise, odours, mosquitoes and a by-product (sludge) in big volumes which needs to be handled on a daily basis. In fact, although sludge represents only a few percent of the treated volume, sludge management and handling could reach up to 50% of the total facility costs. So, while we want to solve one problem (wastewater treatment and pollution reduction before the final discharge), it is possible that we create another one.
Over the last years, environmental legislation goes to increasingly stringent (at least in Europe and the USA) limits, aiming at both protecting the environment from human activities and also improving the life quality of the people. International experience has shown that it is financially viable but also environmental friendly to deal with the problem of wastewater management through the construction of small, independent and decentralized treatment units in small/medium settlements or remote communities.
It is clear that if we want to speak about sustainable development, other alternative wastewater treatment techniques should be available. And it is really amazing that nature itself provides us the means to do so. How? Some observant scientists noticed some decades ago that in natural wetlands, water purification takes place at some level. So they thought to examine the use of wetlands for wastewater treatment under controlled conditions. The outcome was the new, promising and effective alternative technology of Constructed Wetlands (CWs). In a simple way, ecological engineering became true.
CWs belong to the wider category known as natural treatment systems. Natural treatment systems simulate systems that exist in nature (e.g., natural wetlands) and are built so to exploit naturally occurring processes under controlled conditions. CWs are isolated systems, meaning that all water/wastewater pollutants do not escape to the environment. They are man-made facilities, but consist only of natural materials. They are simple systems and easy to construct. Their main characteristic is the presence of plants (usually common reeds/cattails). Within them, all components (soil/gravel, microorganisms, plants and water) contribute to the creation of the appropriate conditions, so that the wastewater pollutant load is eliminated through various natural (physical, biological and chemical) processes. They can be classified in horizontal flow (surface or subsurface) and vertical flow CWs.
But, why to choose these eco-tech systems? Their main benefits are both economic and environmental. First of all, the initial investment to build these systems could be significantly lower compared to conventional systems. The costs for operation and maintenance are practically negligible (even up to 90% reduced as practical experience has shown), since there are no large mechanical parts or machines (which consume energy), there are no extended concrete constructions, there is no need for complicated electronic control equipment and the energy consumption is practically zero, since the necessary energy for the treatment comes from the sun, air and wastewater (i.e., use of renewable energy sources). No specialized personnel is needed to run the facility, while the need for human presence for maintenance could be limited to only one monthly visit. And, of course, they provide multiple reuse possibilities of the treated effluent. On the other hand, they possess higher area demands. However, continuous technological improvement tends to cover this gap. Vertical flow CWs, the latest development with many modifications (e.g., upflow/downflow, tidal flow, aerated etc), attracted increasing interest over the last 10-15 years due to its additional benefits like reduced area demand and enhanced efficiency, while they have been used in some unique applications. Detailed information can be found in the newly published book title “Vertical Flow Constructed Wetlands” by Elsevier.
The high efficiency level of CWs in municipal wastewater treatment led to their application for other kinds of wastewater: acid mine drainage, agricultural wastewater, dairy – textile – olive mill – mine wastewater, industrial wastewater (e.g., paper industry, refineries), stormwater runoff, landfill leachate and airport runoff, sludge dewatering, underground water purification – aquifer enrichment and flood protection.
It is easy, therefore, to realize the environmental benefits of this technology. For example, studies have shown that the use of Vertical flow CWs for sludge dewatering (another state-of-the-art effective application) results in up to 75% lower CO2 production compared to common mechanical methods, like beltfilterpresses or centrifuges. The main characteristics of these systems, the presence of plants, provides an aesthetical and acceptable “green” appearance of the systems, attracts birds and other species of the local fauna, so that with time a new wetland is practically formed. Odours and insects are almost absent, since the water flow can be subsurface and they can be integrated into park and recreational systems. Additionally, industries that adopted the CW technology improved their “green” profile and enhanced their social image to the public (corporate social responsibility). Today, many countries have adopted this technology, like the USA, Austria, Denmark, Czech Republic, Germany, Australia and Canada, among others.
These characteristics place Constructed Wetland systems in the sustainable landscape category. The implementation of CWs provides various significant functions and advantages coupled with low cost and minimum environmental impact/footprint. These systems are the results of continuous evolution of engineering coupled with environmental/ecological concerns. They prove that the ecological way of thinking and the observation of nature can provide us the necessary means to deal with our problems. In conclusion, the message is one: back to nature! It has all the answers…
Alexandros’ new book Vertical Flow Constructed Wetlands: Eco-engineering Systems for Wastewater and Sludge Treatment is available for purchase on the Elsevier Store. Use discount code WATER14 at checkout to save up to 30% on your very own copy and an all aquatic science and books and ebooks!
About the Author
Dr Stefanakis is an Environmental Engineer, with Diploma and Doctoral degrees from the Department of Environmental Engineering, Democritus University of Thrace, Greece. He also holds a MSc degree in the field of “Hydraulic Engineering” from the Department of Civil Engineering of the same university. His Diploma His M.Sc. and Ph.D. theses focused on the ecological treatment of wastewater and sludge using natural systems, mainly on Constructed Wetlands. He has published 13 papers in international peer-reviewed scientific journals, several papers in international and national conference proceedings, technical reports, lecture notes and book chapters.
His work includes numerous experiments with Constructed Wetland systems of different types and applications. He has participated in many national and EU research projects in Greece, Portugal and Germany. He has taught at the undergraduate level on the topic of environmental protection, delivered seminars in postgraduate studies programs and has been involved in international conference organizing. He has also supervised and trained various undergraduate students. He has been awarded twice during his career for his research work. Finally, Dr Stefanakis is also a practicing environmental engineer in Greece, dealing with the design of Constructed Wetlands facilities.
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