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Constructed Wetlands For Wastewater Reuse In Agriculture

Executive Summary

Deteriorating quality and increasing scarcity of water in Iraq demands a rethink in the way wastewater is managed. We show here that a constructed wetland can assist an existing sewage treatment plan not only to be lower than actual Iraqi effluent standards, but to also meet water quality criteria for reuse in agricultural production.

Scarcity is one of several challenges to the Iraqi waters. The increasing demand from both drinking water and agriculture is overshadowed by uncertainties related to further dam construction in upstream countries. The overall aim of this study is to establish if a constructed wetland built on the available land would be sufficient to reduce the effect of wastewater effluents on the Euphrates, and meet the more stringent standards of water reuse for agriculture.

Results of a simple simulation using the actual data from operation records indicate that a 40 ha surface-water wetland would be required to bring the ca. 17,000 m3/day effluent in reach of the BOD5 and TSS water quality requirements for vegetables and fruits likely to be eaten raw.

Simulation of the effect of the constructed wetland, on daily effluent to the Euphrates River, shows a significant improvement in the capacity to meet more stringent standards. The 40 ha wetland was constructed and optimized for BOD5 and TSS removal, it would significantly help reduce eutrophication risk and bring effluent quality closer to the standards for wastewater reuse for growing vegetables and fruits likely to be eaten raw. The need to strictly adhere to existing standards and to further reduce risk of eutrophication should therefore be approached with an objective of wastewater reuse in mind. Constructed-wetlands are low-cost and effective tools already widely used for this purpose. However, they require more land then engineered solutions.

Introduction

11ArtificialwetlandCanberraAustralia.jpgScarcity is one of several challenges to the Iraqi waters. The increasing demand from both drinking water and agriculture is overshadowed by uncertainties related to further dam construction in upstream countries. The need to strictly adhere to existing standards and to further reduce risk of eutrophication should therefore be approached with an objective of wastewater reuse in mind. Constructed-wetlands are low-cost and effective tools already widely used for this purpose. However, they require more land then engineered solutions.

Image is courtesy of the Global Water Forum - http://www.globalwaterforum.org/resources/images/

1.1 A Water Stress Iraq

Iraqi waters face serious challenges in availability and in quality. By 2025, the combination of intensive dam construction in upstream countries, forecasted climate change impacts, the pervasive unsustainable wastewater treatment and a lack of a strategy for reuse, could lead Iraq to be classified as a water-stressed country. Water resources planners are always looking for additional sources to meet freshwater demand when the sources are limited in their regions. Wastewater reuse may become a suitable alternative to substitute water shortage in agricultural purposes for a scenario of using large quantities of treated wastewater in irrigation. The way was staged already in 1958 when the United Nations Economic and Social Council provided an approach to sustain this concept through the following statement:
"No higher quality water, unless there is a surplus of it, should be used for a purpose that can tolerate a lower grade" (in Hespanhol, 2007).

With a further reduction in water availability expected with climate changes, the search for appropriate technologies and alternative sources to solve this problem has become a pressing issue, especially when global trends in population growth acceleration and rapid economic development will most likely lead to acceleration in freshwater withdrawals (UN Water, 2009). In Arab region particularly, (Iraq is a part of this region) there is now calls for an improvement in municipal waters treatment to create an affordable supply to mitigate the high dependence on uncertain trans-boundary rivers flow (the Euphrates and Tigris). The total consumption of water in the Arab region is distributed among three different sectors: agricultural, industrial and domestic. Water use for agricultural purpose is 88 %, 7% for domestic and 5% for industrial sector (Mohamed F., 2004). These mentioned values exhibit an important role of improvement to wastewater collection and treatment to meet water shortages which are expected to be more complicated in the coming decades.

1.2 Wastewater as a Valuable Resource

The huge burden on scarce fresh water resources calls for the incorporation of the reuse of treated wastewater in water conservation and demand management strategies. Reuse planning is distributed among several applications such as irrigation in agriculture, reuse in aquaculture, groundwater recharge and industrial recycling or reuse.

In this context, reuse application in irrigation can reduce the cost in wastewater treatment technologies and fertilizer applications. For instance, nutrients removal (such as N and P), which has high costs in wastewater treatment, is not always necessary when the treated water is reused in agriculture or aquaculture. In contrast, reuse of treated wastewater for agricultural purposes has benefits for crops through the recycling of nitrogen and other nutrients. A study was done by the World Bank which estimated that farmers can save about $130/ha/year in fertilizer costs through using treated municipal effluents (Mohamed F., 2004). Studies in several countries have further shown that crop yields can be increased if wastewater irrigation is properly provided. For example, field experiments in India recorded an increase in crop yields using wastewater-reuse in comparison with freshwater (Table 1).

Table 1 Increases in crop yield production from wastewater reuse in irrigation projects in Nagpur, India.

Table1.jpg

Source: Shende, 1985 cited in Hespanhol, 2007

a. yield in ton /ha annually
b. years of harvest used to calculate average

1.3 Wastewater Reuse and Public Health Risks

Public health is the most critical issue in reuse application. Policy-makers are always facing the trade-off between public health protection and the ethical questions of whether to prevent the use of wastewater by farmers as the limited source of water that is available for them (Drechsel et al., 2010). According to this position, WHO supported the policy-makers in developing countries by establishing guidelines for microbiological standards where wastewater reuse is permitted (Drechsel et al., 2010).

As wastewater reuse in irrigation reduces the pressure on the conventional freshwater resource, there are possibilities and constraints which should be considered. In contrast with benefits that have apparently been gained, there are serious risks. In this respect, the most affected groups are the farm workers due to the duration and intensity of direct contact with wastewater and contaminated soil (Drechsel et al., 2010). Recent epidemiological studies that surveyed groups of rice farmers in Vietnam using wastewater gave evidence of increased diarrhoea and skin irritations and infections (Drechsel et al., 2010). The financial gains from agricultural production using these wastewater utilities can lead farmers having to pay for medication to treat themselves. This situation enhances the need for farmers to be educated about the risks that can arise when using wastewater for irrigation. Furthermore, consumption of irrigated production also set up an exposure route to health risks. Several studies demonstrated higher Ascaris infections in both adults and children consuming uncooked vegetables irrigated with wastewater. Several records for diarrhoeal outbreaks have been associated with the consumption of vegetables irrigated with wastewater (WHO, 2006).

Table 2 shows the Iraqi standard for effluent discharged to watercourses in comparison to the more stringent standard for reuse in some of the neighbouring countries (Kuwait, Saudi Arabia and Oman) adopted from WHO guidelines (2006). So far, there is no specific standard for reuse in Iraq (WHO, 2006).

Table 2 Iraqi Effluent standards for discharge to watercourses and the stringent standard for wastewater reuse in agriculture (WHO, 2006)

Table2.jpg

Water quality requirements for vegetables and fruits likely to be eaten raw

Study Site

The An Najaf wastewater plant was built in the mid 1970s to service 140 000 P.E.. The plant’s treated effluent reaches the adjacent Euphrates River via a channel draining the surrounding agricultural lands.

2.1 Plant Description

An Najaf sewage treatment plant is a big-scale of centralized treatment system. It was constructed in the mid 1970s by a Danish company and was renovated in 2003 by an American company, Bechtel. It consists of four units of trickling filter system. The expected maximum wastewater flow is 28000 m3/day. The plant is located nearby the Euphrates and surrounded by agricultural and some rural residential areas. Besides that, an available area is existed for any expected upgrading. See Figure 1.

Figure1.jpg

Figure 1 Image for some components of An Najaf sewage treatment plant (Photographed by Amar A. Ismail, 2008)

2.2 Constructed wetlands as a Solution

Constructed wetlands use same processes that occur in natural wetlands (when they are designed properly). The components of the wetland system; vegetation, selected substrate, water column, and communities of microorganisms that developed naturally, all contribute to control treatment mechanisms into the wetland. Wetlands’ processes are influenced by the change in environmental conditions; whether these conditions are of diurnal changes, including variation in temperature and dissolved oxygen content, or seasonal changes including variation in daylight hours from season to season and temperature. Vegetation growth, chemical reactions, microbiological activities increase during warmer seasons (Malcolm B. et al., 1997).

Wetlands provide a diversity of micro-environmental species which play an important role in pollutant removal processes. Various processes occur within plants strands, the water column and on the substrates. The wetland substrates typically contain a high proportion of organic matter, coming as a result of annual vegetation production, whereas sediment and litter that accumulate in the wetlands provide an ideal condition for chemical and microbial processes.

2.3 Wetlands performance

Typically, removal efficiency is based on the inflow-outflow comparisons of the pollutant concentration (i.e., % reduction) or the assessment of the pollutant mass being removed by unit of wetland surface and time (i.e. kg/m2/yr). In evaluation of performance process, it is important to select criteria that accurately reflect the actual performance of the wetland in relation with understanding of the wetland objectives established (Kadlec and Knight, 1996). In this context, the characteristics of wastewater that are most treated in constructed wetlands depend on pre-treatment processes. They play a role in the alleviation of wastewater concentration loading.

In a similar manner, DeBusk et al. (2001) reported that hydraulic retention within the wetland treatment system (HRT)affects the pathogen removal efficiency. Studies with different wetland configurations have shown 1-2 log in viral and bacterial reduction. A study recently done on constructed wetland by Redder et al.(2010) reported that infections with the protozoan parasites (Cryptospridium and Giardia ssp.) are mostly occurring with crops irrigated by treated wastewater and they caused diarrhoea. In this study, Redder et al.(2010) concluded that constructed wetlands can easily achieve a reduction to a rate of ≈ 2 log for the protozoan pathogens. Redder et al.(2010) also confirmed that, although natural wastewater treatment systems (constructed wetlands) are more effective in protozoan parasites reduction than conventional treatment (mechanical), but no correlation between protozoan parasites reduction and other organisms reduction.

On the other hand, combination of minimal wastewater treatment system (constructed wetlands in this case) with drip irrigation and washing vegetables, after harvesting, can easily achieve a 6 log unit of pathogen reduction (Drechsel et al., 2010).

Objective

The overall aim of this study is to establish if a constructed wetland built on the available land would be sufficient to:

  • ensure that the Iraqi effluent standards are rarely exceeded;
  • further reduce the effect of wastewater effluents on the Euphrates; and,
  • meet the more stringent standards of water reuse for agriculture.

Simulation Approach

Step 1: Treatment area of the constructed-wetland was set using the Kadlec-Knight (1996) approach, based on the “worst case scenario” for BOD5 – defined as the highest monthly-average concentration of the two winters analyzed (Nov.-Apr.) – and a target reduction of 70%. Actual daily discharge and inflow-outflow concentrations for TSS and BOD5 were obtained from operation records for 2006 & 2007. Table 3 shows how many times (% time) the treated effluent exceeded the recommended standards. See also Figure 2.

Table 3 Percentages of readings that exceed the Iraqi standards in comparison with the more stringent standards mentioned in Table 2 with treatment system (TP only)

Table3.jpg

Figure2.jpg

Figure 2 Comparison of BOD5 and TSS effluent concentrations in both treatment systems (treatment plant, and treatment plant+wetland) with the Iraqi standard and the more stringent Standards

Step 2: Using the actual daily flow, TSS and BOD5 values of the treatment plant effluent, the theoretical outflow concentrations of the treatment wetland were simulated over the 19 months recorded using the same equation as in Step 1, but by setting the wetland size to 40 ha. These daily values were then evaluated against the effluent standards of Iraq (i.e., actual), and Oman (i.e., reuse). See Table 4 and Figure 2.

Table 4 Percentage of readings that exceed the Iraqi standards in comparison with the more stringent standards mentioned in Table 2 with treatment system (TP+CW)

ScreenShot2012-06-20at20.00.39.png

Results

The treatment efficiency of the system, as defined by a mass balance approach (i.e.,Σ inflow-mass vs. Σ outflow-mass, where the slope of a linear regression indicates the remaining fraction) shows that adding a 40ha treatment wetland would bring the BOD5 removal to 93% from 78% for a treatment plant alone, and the TSS to 87% from 78%.  Simulation of the effect of the constructed wetland on daily effluent to the Euphrates River shows a significant improvement in the capacity to meet more stringent standards. See Figure 3.

Figure3.jpg

Figure 3 Increase in removal efficiency of BOD5 and TSS in treatment system (treatment plant +wetland) in comparison with sewage treatment plant only

Conclusion

The wetland size required to have 100% compliance with Iraqi effluent standards exceed available land of the An Najaf wastewater plant. Nevertheless, if a 40 ha wetland was constructed and optimized for TSS removal, it would significantly help reduce eutrophication risk and bring effluent quality closer to the standards for wastewater reuse for growing vegetables & fruits likely to be eaten raw.

Acknowledgements

I heartily thank Dr. Jean O. Lacoursière, Professor in sustainable Water Management/Kristianstad University/Sweden, for his precious advice and assistance. Special thank also goes to Mr. Akeel Najeem, Manager of Directorate of the Environment in An Najaf Province for the provision of raw sewage data from the An Najaf sewage treatment plant.

References

DeBusk, T. et al., (2001). 'Wetlands for Water Treatment.' In: Applied Wetlands Science and Technology. Ch. 9. Edited by Donald M. Kent. Boca Raton: CRC Press LLC.

Drechsel P. et al., eds. (2010). Wastewater Irrigation and Health: Assessing and Mitigating Risk in Low-Income Countries. Chapter 2 and 3. Part 1. Published by International Water Management Institute, IWMI and International Development Research Centre, IDRC. UK & USA.

Hespanhol I. (2007). 'Wastewater as a Resource.' In: Water Pollution Control- A guide to the Use of Water Quality management Principles. Hespanhol I. And Helmer R. Published by Taylor & francis. New York.

Kadlec, R. H and Knight, R.L (1996). Treatment Wetlands. Lewis-CRC Press. Boca Raton, New York.

Mohamed F. Hamoda (2004). 'Water Strategies and Potential of Water Reuse in the South Mediterranean Countries'. Desalination 165(2004) 31-41. Elsevier.

Malcolm B. et al. (1997). 'Chemical, Biological and Physical Processes in Constructed wetlands'. In: The Constructed Wetlands Manual. Department of Land and Water Conservation, DLWC. Chapter 3. Vol.2. New South Wales, Australia.

Reddar et al. (2010). 'Constructed wetlands: Are they Safe in Reducing Protozoan Parasites?' Int. J. Hyg. Environ. Health. 123. 2010 (72-77). Elsevier.

World Health Organization, WHO (2006). WHO Guidelines for the Safe Use of Wastewater, Excreta and Greywater. Vol. 1 & 2.

World Health Organization, WHO (2006). 'A Compendium of Standards for Wastewater Reuse in the Eastern Mediterranean Region'. Document WHO-EM/CEH/142/E. Regional Centre for Environmental Health Activities, CEHA. Available at: [[org.xwiki.gwt.dom.client.Element#placeholder'>>__http:www.emro.who.int/ceha/pdf/Compendium%20wastewater%20standards.pdf__]]

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