The use of constructed wetlands for water pollution control has a long standing tradition in urban, peri-urban, rural, agricultural and mining environments. The capacity of wetland plants to take up nutrients and to filter organic matter has been widely discussed and presented in diverse fora and published in hundreds of articles. In an ever increasingly complex global world, constructed wetlands not only play a role in providing safe sanitation in decentralized settings, shelter for biodiversity, and cleansing of polluted sites, in addition, they produce biomass that can be harvested and used for the production of fodder and fuel. The United Nations University Institute for Integrated Management of Material Fluxes and of Resources (UNU-FLORES) was established in December 2012 in Dresden, Germany, to assess the trade-offs between and among resources when making sustainable decisions. Against the backdrop of the Water-Energy-Food Nexus, which was introduced as a critical element for the discussions on sustainability at Rio +20, the UNU was mandated to pay critical attention to the interconnections of the underlying resources, namely, water, soil and waste. Biomass for human consumption comes in the form of food for direct use, as fodder for livestock, and as semi-woody biomass for fuelling purposes, be it directly for heating and cooking or for the production of biogas and/or biofuel. Given the universal applicability of constructed wetlands in virtually all settings, from arid to tropical, from relatively high to low nutrient loads, and from a vast variety of pollutants, we postulate that the biomass produced in constructed wetlands can be used more extensively in order to enhance the multi-purpose use of these sites.
Water supply, pollution control, agricultural resource management, and energy production have been treated traditionally as independent problems to be solved separately. It is becoming more apparent that interconnections among cycles of water, nutrients, and energy can now be exploited in systems that perform multiple functions optimized to provide services for human communities, while improving ecosystem health. Properly designed and operated, constructed wetlands for the treatment of wastewater can provide functions that include safe sanitation in decentralized settings, shelter for biodiversity, and cleansing of polluted sites. In addition, constructed wetlands produce biomass that can be harvested for the production of fodder and fuel.
The concept of considering interconnected systems is exemplified by the Water-Energy-Food Nexus, which was introduced as a critical element for the discussions on sustainability at Rio +20. UNU was mandated to pay critical attention to the interconnections of the underlying resources, namely, water, soil and waste. The United Nations University Institute for Integrated Management of Material Fluxes and of Resources (UNU-FLORES 2017) was established in December 2012 in Dresden, Germany, to assess the trade-offs between and among resources when making sustainable decisions.
Using wetlands to treat wastewater addresses at least three major elements in sustainability: water and sanitation, energy, and greenhouse gas (GHG) emissions. Sustainable use of resources is a key element in achieving goals on ending poverty established initially by the Millennium Development Goals. Established by the United Nations Conference on Sustainable Development, Rio + 20, in 2012, the further evolved Sustainable Development Goals, SDGs (UNDP 2017), articulated the three pillars of sustainable development: environmental, social, and economic. Of the 17 SDGs, at least two are relevant to the integrated use of constructed wetlands: SDG Goal 6: Ensure access to water and sanitation for all and SDG Goal 7: Affordable and clean energy. Climate change is addressed in SDG Goal 13 and has been more fully considered in the 2015 Paris Agreement (Climate Action 2015) of the Conference of Parties of the UN Framework for Climate Change. Here, nations committed to sufficiently reduce their GHG emissions to limit global warming to less than two degrees Celsius by the end of this century.
Wastewater treatment embodies aspects that include human health, ecosystem stability, energy, and GHG emissions. Worldwide, nearly 60 million kg/year of biochemical oxygen demand (BOD) are removed by wastewater treatment plants, along with over 4 million kg of nitrogen and nearly a million kg of phosphorus (Corcoran 2010). In municipal-scale wastewater treatment plants, the carbon associated with BOD is typically converted to microbial biomass, which in turn is removed as a dense organic sludge, which, when stabilized, dried or composted, can be used as an energy or nutrient source. These types of treatment systems are highly efficient in terms of the physical space they occupy and the amount of BOD that can be removed per unit cost. But they are also expensive, complex, centralized, and require vast amounts of energy (three percent of all electricity use in the United States, for example) and trained personnel to operate and maintain them (McCarty et al. 2011).
Wastewater treatment technology is unevenly distributed worldwide. In North America and Western Europe, for example, more than three quarters of wastewater produced is treated, compared with West and Central Africa, Southern Asia, and East Asia, where more than three quarters of wastewater discharged is not treated (Corcoran 2010). Not surprisingly, investments in sanitation infrastructure reflect economic prosperity. Ashley & Chapman (2006) estimated that investments in infrastructure necessary to meet minimum health and environmental standards by the year 2030 range up to 1.2 percent of gross domestic product (GDP) for high-income countries, while low-income countries need to invest up to 6.3 percent of their GDP to bring treatment facilities up to standard.
Constructed wetlands provide a low-cost, low-maintenance alternative to traditional wastewater treatment and have been widely used in both centralized and decentralized systems (Zhang et al. 2014). The physical space necessary for constructed wetlands is larger than that for municipal technologies (Kivaisi 2001), but comparable to other more traditional low-technology alternatives. For example, to treat domestic-strength wastewater using constructed wetlands in a temperate climate would require an area on the order of 7 m2/person (Rousseau et al. 2004), while a non-aerated facultative lagoon would require 2 to 5 m2/person and 0.2 to 0.5 m2/person for an aerated lagoon (Kivaisi 2001). Wetland systems still occupy a larger area than lagoon systems, but provide additional ecosystem services, including aesthetics, biodiversity, wildlife refugia, and nutrient capture for reuse (Yang et al. 2008). However, perhaps the most significant benefits of constructed wetlands for wastewater treatment may be their capability to offset GHG emissions and to produce energy (e.g., Liu et al. 2012).
Greenhouse gas emissions
Constructed wetlands affect GHG cycles both in the treatment of wastewater and in the production and use of their biomass. With regard to infrastructure and treatment, CO2 emissions occur during construction, harvesting, and in the decomposition of organic matter. During their operation, constructed wetlands are net emitters of methane, but sinks of CO2 through their accumulation of organic matter and are net GHG sinks (Liu et al. 2012; Mander et al. 2014). In addition, traditional wastewater treatment systems emit 591 kg CO2 for every kilogram of nitrogen removed, while wetland systems emit 0.9 kg CO2 per kg nitrogen (Liu et al. 2012).
Use of the biomass produced by wetlands impacts GHG cycles as well. Factors that need to be considered include bioenergy production and transportation and the offsetting effects of biomass as a fuel source relative to the fuel it replaces. A hypothetical example is a wetland system which produces solid fuel that replaces a wood fuel source. This can result in a decrease in deforestation, with positive impacts related to the continued sequestration of carbon in the above-ground biomass of the forest itself and preservation of organic carbon contained in soils that would not be eroded (Fargione et al. 2008).
Considering the water-energy nexus, traditional wastewater treatment is a significant consumer of energy. In the United States, wastewater treatment accounts for about 3% of the national electricity load (USEPA 2006). Municipal wastewater treatment plants consume up to 2.2 megajoules (MJ = 106 joules = 10−3 GJ or gigajoules) per cubic metre of water treated (McCarty et al. 2011). Many facilities in industrialized regions produce methane through sludge digestion that is either used as a heat source within the treatment plant, used for some other renewable energy purpose, or simply flared and wasted (McCarty et al. 2011). Constructed wetlands as wastewater treatment systems require far less energy to treat wastewater (6.8% of the energy demand of a traditional activated sludge plant, according to Shao et al. (2013) and have the potential to be net suppliers of energy and contribute to the bioenergy portfolios of countries.
The production of bioenergy to reduce reliance on fossil fuels has been controversial. Biomass energy has the potential to contribute up to 50 EJ (Exajoules = 1018 joules), or about ten percent, to the global energy supply by 2035. However the primary source of biomass for this energy would be terrestrial energy crops (Slade et al. 2014). Debate is usually centred around the use of arable land to meet energy security needs at the expense of food security (Müller et al. 2008). Additionally, the water demands to grow energy crops are significant and may be prohibitive and threaten water security in water-scarce regions (Strzepek & Boehlert 2010). Other competing uses for bioenergy resources include wood for forest products and protection of biodiversity (Smeets et al. 2007).
Constructed wetlands used to treat wastewater have the potential to provide a sustainable bioenergy source without placing burdens on water resources or displacing other food or energy crops. Liu et al. (2012) projected that if constructed wetlands treated all of the wastewater in China, a land area equal to less than 20% of China's fallow land would be required and could produce 8.2 × 107 GJ/year in bioenergy, enough to meet the energy demands of about two million households in China (Zheng et al. 2014). There are clearly practical constraints at this scale, however the use of constructed wetlands to treat wastewater instead of traditional activated sludge systems saves in energy costs to treat the wastewater (Shao et al. 2013) and produces a bioenergy resource.
Here, we consider the potential for constructed wetlands to purify wastewater while contributing to renewable water and energy cycles. We address the following questions: Which wetland plant species can be used and why? How can wetland biomass be converted to usable energy at a scale appropriate for developing countries? And, how can constructed wetlands be integrated into resource management programs in developing countries?
Our work is a synthesis of recent research and practice related to constructed wetlands for wastewater treatment and biomass as a green energy source. We consulted peer-reviewed literature from the research community, international governmental and non-governmental organizations, and such grey-literature sources as technical guidance documents and white papers. Online searches were conducted using Google Scholar (Google 2017) and Thompson Reuters Web of Science (Clarivate Analytics 2017). Our searches included the following topics: Constructed wetlands for wastewater treatment, wetland biomass for biofuel production, bio-energy crops and process, process design for bio-energy, energy and green energy portfolios for developing countries, global wastewater treatment, GHG emissions related to wastewater treatment and biofuel production, and life cycle assessment for wastewater treatment and energy production. References were organized using the open-source reference database system, Zotero (Zotero 2017). References assembled for this research will be available through the United Nations University FLORES institute website (UNU-FLORES 2017).
RESULTS AND DISCUSSION
Our analysis of constructed wetlands as wastewater treatment systems considers selection of the plants themselves, the potential for harvested biomass as an energy source, and some of the challenges in implementing these systems in the integrated management of water, nutrient, and energy cycles.
Wetland plants for wastewater treatment
In a literature survey of 643 constructed wetlands from 43 countries, Vymazal (2013) found that of the 150 plant species used, the five most common genera were Typha, Scirpus (Schoenoplectus), Phragmites, Juncus, and Eleocharis. Most of the wetlands surveyed were in North America (56%) and Europe (22%). In a survey of 36 constructed wetlands in developing countries, Zhang et al. (2014) found that Phragmites and Typha (35% and 10%, respectively) were the most commonly used plants. In a review of constructed wetlands in African countries, Mekonnen et al. (2015) listed 12 wetlands, which used primarily Phragmites, Typha, and Cyperus papyrus.
With regard to suitability for energy production, two of the five most commonly used wetland plants worldwide, Typha and Phragmites (according to Liu et al. 2012), are also superior biomass producers. The remaining three most commonly used wetland plants, Scirpus, Juncus, and Eliocharis, are all notably slower growing, and less well suited as energy crops (Figure 2). The highest energy producer, A. donax, has been relatively under-used in constructed wetlands.
The selection of plant types for constructed wetlands has not necessarily been based on the potential of the plants to produce biomass energy, or even based on their efficiency at pollutant removal, although both of these parameters vary widely from plant to plant. Plant selection has likely been based more on local availability, suitability to local climate conditions, and tolerance to influent water quality (Wu et al. 2015). Vymazal (2011) noted that mixes of species may be more effective than single species in withstanding seasonal and other ambient variations. Protecting regions from introduction or spread of invasive, non-native species has also been an important consideration (Zedler & Kercher 2004; Piwpuan et al. 2014).
Effects of harvesting
A harvesting program for treatment wetlands needs to optimize both pollutant removal and biomass growth. With regard to pollutant removal, uptake of nitrogen and phosphorus is mediated by microbial systems in the soil and root zones of emergent wetland plants (Vymazal 2007). For aerobic processes in the root zones, adequate resupply of oxygen from the wetland plants themselves is necessary (Wang et al. 2014). An ongoing maintenance program can prevent accumulation of detritus, which can impair performance and cause re-release of nutrients (Thullen et al. 2005). Harvesting to collect biomass as a renewable energy source provides this benefit, and with proper timing of harvests, can optimize biomass growth for enhanced performance in carbon and nutrient removal (Thullen et al. 2005).
With regard to regrowth after harvesting, Jinadasa et al. (2008) studied growth and uptake rates in a tropical setting. Regrowth of Scirpus and Typha was rapid after first harvesting, but progressively slower after subsequent harvests. Frequent harvesting can slow biomass production, but frequent thinning, rather than complete removal of a plant, results in the highest biomass production (Sale & Wetzel 1983; Jinadasa et al. 2008). Sustainable long-term production of biomass requires maintaining plants in a rapid-growth phase through this thinning process (Thullen et al. 2002). This approach is consistent with strategies to mimic natural wetland systems in early successional stages (Thullen et al. 2005).
Potential for energy production
Biomass generated by constructed wetlands can be converted to energy in three broadly defined processes: direct combustion, biogas production, and bioethanol production. The relative advantages and disadvantages of these processes vary with the scale of system employed. Constructed wetlands for wastewater treatment can be applied in settings that range from small villages to mega-cities, but for the purpose of this analysis, we will focus on the range of wetland sizes already in operation in developing countries. Zhang et al. (2014) surveyed treatment wetlands ranging in size from 0.25 m2 to over 30 ha. Excluding the two largest wetlands (17.4 and 35.2 ha), the remaining 56 wetlands had a mean area of 412 m2, but a median area of only 22 m2. Considering that treatment wetlands serve roughly 7 m2/person (Rousseau et al. 2004), most constructed wetlands support communities of less than about 60 people. At this scale, direct combustion and biogas production are likely the most economical bioenergy alternatives.
Direct combustion involves minimal processing, whereby harvested biomass is either dried and used as a fuel source, or formed into briquettes by shredding the biomass, mixing it with an additive, and compressing it into briquettes (Li & Zhang 2011). Wichmann (2016) assessed the transfer of agricultural biomass technology to the processing of Phragmites for biogas production, direct combustion, and thatching. Direct combustion has several advantages for small-scale applications. The fuel is easily transported and can be adapted to traditional solid-fuel cooking stoves or for heating (Tucho & Nonhebel 2015). Direct combustion also has low infrastructure costs and a low level of expertise necessary to operate and maintain systems.
A hypothetical community of 60 people would require a wetland area of about 420 m2. If the wetland produces 15 tonnes dry mass ha−1 year−1, a moderate level of productivity according to Laurent et al. (2015), the wetland can supply the community with 630 kg year−1 of dry biomass. The dry biomass has an energy content of about 16 MJ Kg−1 (Kitzler et al. 2012), so the wetland can produce about 10 GJ year−1. An average household in Ethiopia requires about 7 GJ year−1 for cooking (Tucho & Nonhebel 2015) and there are about 5 persons per home (USAID 2001), so the annual energy requirement for cooking in this community of 12 homes is about 84 GJ. The biofuel produced by the treatment wetland can therefore supply about 12 percent of the cooking fuel needs of the village.
The other likely alternative for a community of 60 residents is to convert the harvested wetland biomass to methane in an anaerobic reactor. Biogas production requires more coordination and complexity, but allows mixtures of other available organic matter to increase biogas yield (Marchetti et al. 2016) and has the potential to employ proven, small-scale biogas technologies. For example, the Gobar gas plant was designed to be a simple, family-scale anaerobic digester designed to process livestock waste, but which can be modified to treat a combination of organic materials (Pullen 2015). Small-scale anaerobic digesters are already common in many parts of the world, with more than 45 million units in Asia and 4 million in India. With more than 200,000 biogas units in operation, Nepal has the highest number of small-scale biogas digesters per capita in the world (Gebreegziabher et al. 2014).
For our hypothetical community of 12 households, a constructed wetland could collect and treat the wastewater and a biogas unit could convert the wetland biomass to biogas at a conversion rate of about 18.5 GJ Kg−1, which is slightly higher than that for direct combustion (Liu et al. 2012), for an energy supply of about 14 percent of the community's energy needs for cooking. A biogas system requires the community to make a greater commitment to infrastructure, training, operation, and maintenance than for a direct combustion system. Distribution of the biogas can also be a challenge. But because biogas reactors can also use livestock, agricultural residue, and nearly any other form of organic waste as a fuel source, these systems have the potential to provide a community with almost complete energy independence.
Opportunities and challenges
Clearly these systems are sustainable in the environmental sense, but to apply them in developing countries, they also need to be sustainable financially and in terms of community ownership. The single most common failure in the long-term success of development projects has been the lack of commitment of local populations to operate and maintain facilities, due at least in part to the delivery of technology inappropriate to, or unacceptable by, local communities (Kumar & Corbridge 2002). Constructed wetlands for wastewater treatment and biofuel production have the potential for sustainability in all its definitions, if care is given to community acceptance from project inception. A vision for successful implementation may include a community's consideration of the system as a resource that both protects their health and provides energy security, self-determination, and independence.
Constructed wetlands for wastewater treatment can be the cornerstone in integrated management of water, nutrient, and energy cycles. To return to our original questions:
Which wetland plant species can be used and why? The selection of plants will be affected by local climate, availability of plants, and the need to prevent or control invasive species. The fastest-growing plants that can also thrive under conditions of frequent pruning will be best suited for this application. The plants most commonly used in constructed wetlands, which also have fast growth rates, are Phragmites and Typha. Less commonly used, but desirable due to its fast growth, is A. donax.
How can wetland biomass be converted to usable energy at a scale appropriate for developing countries? In order of increasing complexity, three appropriate technologies are direct combustion, biogas production, and bioethanol production. At the smaller scales now in operation for treatment wetlands in developing countries, direct combustion and biogas production are the most promising technologies. The direct combustion of dry biomass harvested from treatment wetlands can provide more than 10 percent of the cooking energy needs of a small village. If a community can make sustained investment in the more complex technology of biogas production, other sources of organic matter can be added to harvested wetland material and potentially provide complete energy independence for the community.
How can constructed wetlands be integrated into resource management programs in developing countries? Treatment wetlands can be the common thread that creates the soil-water-waste nexus in a community. The wetlands themselves can improve the health and sanitation conditions in a village. Energy provided by wetland biomass can offset scarce and environmentally damaging sources, such as collection of firewood. At the smallest and least complex level, dried or briquetted biomass can be combusted as a cooking or heating fuel source. More complex biogas and bioethanol reactors, made economically feasible with wetland biomass feedstocks, can improve a community's energy security when additional organic sources are used. These can include livestock, household, and agricultural wastes. This conversion of a waste product into an energy resource can be an effective strategy in ‘closing the loop’ in water and energy cycles and results in improved environmental conditions as well as increased food and energy security.