Introduction

Provision of economically and environmentally sustainable wastewater treatment for small communities remains a great challenge all over Europe, but particularly so in countries with large numbers of small and scattered settlements like Ireland. In recent reviews of Irish wastewater licensing, 515 authorisations were issued for settlements with less than 500 population equivalents (PE) and a further 359 for settlements with 500-2,000 PE (EPA 2012 and 2014). A very high failure rate with 37 per cent of tested licensed agglomerations in a 2014 survey highlighted the challenge in striving for compliance of conventional small-scale wastewater treatment plants with treatment standards for the >2,000 PE licensed category (EPA 2014). [caption id="attachment_40610" align="alignright" width="300"] Figure 1: Box plot for 95% confidence for effluent BOD concentrations across categories of wastewater treatment plants with indicative reference line for BOD ELV[/caption] Constructed wetlands (CW) may offer an economically feasible solution for this problem. For continuous optimisation of CW technology through adaption to specific climatic and other conditions it is necessary to gather data pertaining to the operation, maintenance and performance, in order to inform design and future investment decisions. The United States Environmental Protection Agency (USEPA) has already successfully implemented such an approach through compilation of the North American Wetlands for Water Quality Treatment Database (NADB) which e.g. contains data on flow, dimensions, plant species, influent and effluent analysis results. [caption id="attachment_40611" align="alignright" width="300"] Figure 2: Box plot for 95% confidence for effluent COD concentrations across categories of wastewater treatment plants with indicative reference line for COD ELV[/caption] In response to inadequate conventional wastewater treatment for small communities, NADB information has been utilised to assess constructed wetlands as a treatment alternative (USEPA, 1999). Assessment parameters for CW performance were Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Suspended Solids (SS), Phosphate (P), Nitrogen (N) and faecal coliforms from 245 locations in the US and Canada. The UK’s Constructed Wetlands Association was formed in 2000 in response to malpractice of unscrupulous contractors. This organisation maintains a frequently updated CW database containing information on 900 beds in the UK (Cooper 2007). [caption id="attachment_40613" align="alignright" width="300"] Figure 3: Box plot for 95% confidence for effluent SS concentrations across categories of wastewater treatment plants[/caption] In many other countries, such as Ireland without publicly available reference data, there remains considerable apprehension to the use of CWs. Causes are the lack of detail in information on design standards and often spurious performance claims, which reflect badly on regulation and public opinion (Babatunde, 2007). Babatunde et al. (2007) therefore recommended the compilation of a country specific database with dimension, operational and performance data on CWs, in order to avoid repetition of design errors. This study aims to fill that knowledge gap for Ireland by providing a first comparative assessment of CW performance for this part of western Europe with its temperate maritime climate. [caption id="attachment_40614" align="alignright" width="300"] Figure 4: Box plot for 95% confidence for effluent ammonia concentrations across categories of wastewater treatment plants with indicative ELVs[/caption]

Results

A review of the performance of 51 constructed wetland sites from 17 local authorities aimed to identify the best performing types of constructed wetlands and the treatment factors determining successful compliance with environmental standards. A total of 1,871 effluent results were analysed in SPSS for assessment of the parameters BOD, COD, SS, Ammonia, Orthophosphate, TP and TN. Data from secondary free surface flow wetlands (FSFW), tertiary horizontal subsurface flow (HSSF), Intergrated Constructed Wetlands (ICW) and hybrid systems CW systems were compared with those from small-scale conventional wastewater treatment plants. Correlations and analysis were also drawn with parameters such as m2/PE, seasonal variations and storm management. Data on hybrid systems was collected from research conducted by O’Hogain and McCarthy (2010) with permission from the authors. [caption id="attachment_40615" align="alignright" width="300"] Figure 5: Box plot for 95% confidence for effluent TN concentrations across categories of wastewater treatment plant[/caption] Performance assessment was based on effluent results, which were correlated with m2/PE (p<0.01). While all constructed wetlands performed well for organics removal, nutrient retention varied. Newer integrated constructed wetlands which had applied design guidelines issued by the Department of the Environment (DOE) performed best and had statistically different results (p<0.05) from all other categories. A negative correlation was proven between effluent concentrations and m2/PE. Storm management design features improved treatment performance. Mechanical wastewater treatment plants, secondary FSFW and tertiary HSSF showed a very high variance in effluent concentrations for organic and nutrient parameters. [caption id="attachment_40616" align="alignright" width="300"] Figure 6: Box plot for 95% confidence for effluent OP concentrations across categories of wastewater treatment plants with indicative ELV[/caption] E.coli concentration in effluents were excellent for integrated constructed wetlands with a mean of 89 MPN/100ml. Concerning bacteria removal, all CW categories performed well against CAS treatment plants with Ultra Violet (UV) effluent treatment. In spite of the humid climate some constructed wetlands achieved long or frequent periods of zero effluent discharge and were thus without any transfer of waterborne pollution to receptors during that time. Research conducted by by O’Hogain and McCarthy (2010) showed one system achieved zero discharge throughout a two-year period; the other only discharged after a period of very heavy rainfall, which resulted in the sites receiving a flow of 20 times DWF. The seasonal data shows that low temperature is not a factor, which affects performance in Ireland. [caption id="attachment_40617" align="alignright" width="300"] Figure 7: Box plot for 95% confidence for effluent TP concentrations across categories of wastewater treatment plants with indicative E[/caption] For all the parameters except OP, the highest variance and mean effluent concentrations, occurred during the summer months. Table 1: Effluent concentrations of E.coli bacteria MPN/100ml as means: Treatment type  No of samples Mean ICW 11 89 Secondary HSSF 3 4600 Tertiary HSSF 8 8551 Hybrid 25 256571 CAS 17 116232 CAS+ UV 59 21439 CAS + sand filter 6 13860 Extended Aeration 43 23101 SBR 7 59805 SBR + UV 22 15451 SBR + sand filter 6 78217 Trickling filter 5 4716528 Primary treatment 16 1900514

Conclusion

Constructed wetlands and particularly ICW could be utilised more widely in Ireland as a sustainable solution for wastewater treatment of small communities. However, ICW is a relatively new treatment system and more monitoring is needed long term to assess and understand absorption capacity of phosphorous and nitrification of ammonia. In the investigated size class, CW systems could provide effective alternatives to mechanical treatment plants or complement the latter by providing a tertiary treatment step for effluents from mechanical units. Sites in catchment areas of water bodies with sensitivity towards eutrophication could utilise the hybrid systems for zero discharge. Sites with very restricted space could employ tertiary CWs with storm management. This would increase compliance with environmental standards set by the Water Framework Directive and associated legislation, for example, the Bathing Water Directive. The poor results for underperforming CW categories such as tertiary HSSF systems could be attributed to very small specific areas, poorly operated upfront mechanical plants and issues related to storm control. ICW systems appeared to perform well at nutrient retention; this was most apparent for those sites, whose construction followed Department of Environment Housing and Local Government design guidelines. The correct sizing of CW systems together with appropriate storm management are preconditions for optimal nutrient retention. This requires consideration for the optimisation of current design concepts and for future design ideas. As part of operational and maintenance procedures, dry weather flows need to be estimated for existing sites and storm control features have to be employed, in order to improve performance across all categories including mechanical sites. As an integral part of standard operating procedures, this would be a big step towards identifying and mitigating problems of small wastewater treatment systems in a timely manner.

References

Babatunde, A., Zhao, Y., O’Neill, M. and O’Sullivan, B., 2007. Constructed wetlands for environmental pollution control: a review of developments, research and practice in Ireland. Environment International, 34, 1, 116-126 Cooper, P., 2007. The Constructed Wetland Association UK database of constructed wetland systems. Water Science and Technology, 56, 3, 1-6 Department of the Environment, Heritage and Local Government, 2010. Integrated constructed wetlands. Available from: URL: http://www.environ.ie/en/Publications/Environment/Water/FileDownLoad,24931,en.pdf accessed January 2016 Environmental Protection Agency., 2014. Urban Wastewater Treatment in 2013. http://www.epa.ie/pubs/reports/water/wastewater/30086%20Urban%20Waste%20Water%20Web.pdf accessed June 2016. Environmental Protection Agency., 2012. Urban Wastewater Treatment in 2011. http://www.epa.ie/pubs/reports/water/wastewater/Focus%20on%20Urban%20Waste%20Water%20Treatment%20in%202012%20-%20web%20copy.pdf Environmental Protection Agency., 1995. Urban Wastewater Treatment Directive (91/271/EEC). Procedures and criteria in relation to storm water overflows. Available at http://www.epa.ie/pub/advice/wastewater/UrbanWasteWater2.pdf accessed June 2016 O’Hogain, S and McCarthy, L., 2010. The Operation of hybrid reed bed and willow bed combinations in Ireland- Zero discharge and the potential for no monitoring of domestic application of this combination. Proceedings 2nd International Conference on Constructed Wetlands for Wastewater treatment and environmental pollution control. UCD, Dublin. October 2010 United States Environmental Protection Agency Manual,1999. Constructed Wetlands Treatment of Municipal Wastewaters. Available from: water.epa.gov/type/wetlands/.../constructed-wetlands-design-manual.pdf This paper is co authored by Anthony Hickey1, Eadaoin Joyce1, James O’Toole1, Gerry Galvin1, Mark O’Callaghan1, Ken Conroy1 Francis Hughes1, Darran Killian1, Tommy Shrayne1, Emily Kavanagh1, Katherine Walsh1 and Joerg Arnscheidt2 1Irish Water, 2Ulster University. The paper was part of Anthony Hickey's MSc under Dr Joerg Arnscheidt at the University of Ulster. Co-authors from Irish Water reviewed the paper as part of the publishing process. The paper was published in the 'Journal of Environmental Management' on January 20, 2018. Free access to view and download the paper for the next 40 days can be attained at https://authors.elsevier.com/a/1WUj514Z6tTHYb Author: Anthony Hickey is the water and wastewater process optimisation analyst (South) with Uisce Éireann, Kilkenny. He has a diploma in civil engineering from Cork Institute of Technology; a BSc environmental science from UCC; and MSc in environmental toxicology and pollution monitoring from the University of Ulster