Sewerage Treatment in an Integrated Constructed Wetland
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Proceedings of the Institution of Civil Engineers Municipal Engineer 162 December 2009 Issue ME4 Pages 199–205 doi: 10.1680/muen.2009.162 .4.199 Paper 900038 Received 07/09/2009 Accepted 25/09/2009 Dan Doody Rory Harrington Mark Johnson Oliver Hofmann Don McEntee Keywords: Senior Engineer, Senior Scientific Officer, Senior Executive PhD student, Edinburgh Chartered Engineer, environment/groundwater/ Monaghan County Department of the Engineer, Monaghan Napier University, UK Dublin City Council, sustainability excavation/sewage Council, Environment, Heritage County Council, Ireland treatment & disposal Monaghan, Ireland and Local Government, Monaghan, Ireland Dublin, Ireland Sewerage treatment in an integrated constructed wetland D. Doody CEng MIEI, R. Harrington PhD, MFS (Yale) BAgSc, M. Johnson CEng MICE CEnv. MCIWEM MCIOB, O. Hofmann Dipl. Biol. and D. McEntee BE, MEngSc, MICE, MIEI An integrated constructed wetland was constructed in humus tank. The effluent from the treatment plant discharged to 2006/2007 to treat combined sewage from the village of the nearby Glaslough Stream, a tributary of the Mountain Water Glaslough in North Monaghan, Ireland. With a design River. By the 1990s, the plant was heavily overloaded and a capacity of 1750 population equivalent, the plant became manifestly poor quality of effluent was being produced, leading operational in October 2007 and the current load is to prosecution of MCC by the Eastern Regional Fisheries Board around 800 population equivalent. Untreated influent is (ERFB). In 2001, the sewerage system was extended and a pumped directly to a receiving sludge pond; the liquid temporary package plant was installed with the treated effluent then flows by gravity through five sequential vegetated discharging directly to the Mountain Water River. By early ponds and the effluent discharges directly to the Mountain 2003, the temporary plant was in turn ‘experiencing difficulties’, Water River. The banks of the ponds feature equestrian leading to further written complaints from the ERFB. riding trails. Monitoring equipment has been installed in each pond and early results are very encouraging. In the During this period, MCC commissioned a preliminary report on first year of operation, effluent quality consistently the construction of a traditional treatment plant with a design surpassed the quality of the receiving river. The system capacity of 350 population equivalent (pe) and a final effluent can robustly cope with large variations in volumetric and standard of 20/30 (biological oxygen demand (BOD)/suspended biological oxygen demand loadings of the influent. The solids (SS)). The preliminary estimated cost (in October 1989) construction and maintenance of the wctland show was IR£205 000 (J260 000). By January 2002 the design significant cost savings compared with traditional sewage capacity had been revised upwards to 650 pe and the cost treatment plants in the county. The extensive monitoring estimate had risen to J950 000. Applying construction cost work being undertaken at Glaslough wctland will help inflation, this equates to an estimated 2008 cost of J1 530 000 promote the wctland concept and its application to a for a traditional plant of 650 pe capacity. range of water quality management issues such as leachate (from landfills), sewage sludges, drinking water 3. ICW CONCEPT source protection and flood amelioration. Initial work on the development of the ICW concept has been carried out on a number of farms in the Anne Valley, County Waterford over the last twenty years. By allowing liquid waste 1. INTRODUCTION streams (from a variety of sources) to flow sequentially through Glaslough is a historic village located some 10 km northeast of a number of vegetated ponds, ICWs (through a combination of Monaghan town in Ireland that has developed around the physical, chemical and biological processes) have the capacity to renowned Castle Leslie and the picturesque Glaslough Lake produce a high-quality effluent. In the Anne Valley during the (Figure 1). This paper describes the background, construction 1990s, a number of on-farm ICWs were constructed to treat dirty and early operation of the Glaslough–Castle Leslie pilot farmyard water. An ICW was also constructed to treat domestic integrated constructed wetland (ICW), constructed to treat effluent from the village of Dunhill. Prior to 1990, water in the domestic sewage effluent from Glaslough. This project is part of Annestown Stream was of very poor quality with little or no fish a unique initiative by the Department of Environment, Heritage life. Today the quality of the Annestown Stream is approaching and Local Government (DEHLG) in treating liquid waste streams good-quality status and sea trout have returned to the stream in shallow vegetated ponds. It is a cooperative undertaking by after an absence of many years.1 Monaghan County Council (MCC), Castle Leslie Estate, DEHLG and the University of Edinburgh. In August 2003, a delegation from the Castle Leslie Estate and MCC visited the Anne Valley to see the ICW concept in 2. BACKGROUND operation, and this resulted in the publication of an article in The sewerage collection system serving the village of Glaslough The Sunday Times supporting the use of ICWs for treating is a combined foul sewer and, until 2001, delivered flows to a municipal sewerage.2 The trustees of Castle Leslie were small treatment works (dating back to the 1940s) consisting of convinced of the benefits of treating the sewerage from primary settlement tanks, a single-pass biological filter and a Glaslough in an environmental manner and concurred with the Municipal Engineer 162 Issue ME4 Sewerage treatment in an integrated constructed wetland Doody et al. 199 Glaslough IRELAND Monaghan N54 Belfast N2 Clones Monaghan Castleblaney Dublin London MONAGHAN Carrickmacross N2 Figure 1. Glaslough location map ecological standards being pursued by the Estate. The trustees of river water body is 0?025 mg/l for high status and 0?035 mg/l Castle Leslie agreed to lease some 6?5 ha of land within the for good status.3 The MRP of the effluent from Glaslough ICW walled estate to MCC for use as a treatment plant. during the first year of operation was 0?025 mg/l. 4. BENEFITS OF ICW 5. DESIGN, LAND LEASE AND PLANNING In Ireland over the past 10 years, the national parks and wildlife The site, enclosed within the black line in Figure 2, is located service of DEHLG has been uniquely developing a more robust within the Castle Leslie Estate walls on the banks of the and sustainable approach to the use of constructed wetlands. Mountain Water River. It is surrounded by woodland and Categorised as surface-flow type wetlands, they are similar to demanded sensitive development in terms of landscape fit, natural free surface water wetlands. The holistic approach, biodiversity, amenity and habitat enhancement. termed integrated constructed wetland (ICW), has been suc- cessfully applied to deal with a range of effluent types (e.g. The design capacity of the ICW was 1750 pe (current loading is farmyard runoff, industrial waste and sewage). <800 pe). This allowed a ‘footprint’ map to be drawn up and negotiations to commence on a 99-year lease of the 6?74 ha of The concept is based upon the free surface flow of water land required (the water surface of the constructed ponds covers through a series of sequential linked shallow ponds vegetated 3?25 ha). The lease agreement included the provision of with a range of emergent plant species. While the footprint of equestrian riding trails on the banks of the wetland ponds. the ponds is much larger than those usually used for similar Figure 3 shows the layout of ICW. hydraulic loadings in reed-bed type systems they are generally less costly to build and easier to maintain. ICWs consistently Site investigation works were carried out in September 2005. deliver better quality effluent than reed-bed wetlands due to Ground vulnerability maps prepared by the Geological Survey their long retention time (90–100 days). Where there are of Ireland (GSI) indicated that the site was located in an area of environmental constraints in the local receiving waters, an ICW low vulnerability to groundwater contamination. A planning can be designed to have zero discharge to the local waters. An application was submitted to MCC on 8 July 2005 and planning ICW comprises a diversity of plant species, facilitating permission was granted on 5 September 2005 of the same year. microbial and animal diversity, and is generally appealing for A land lease agreement was signed with the Trustees of Castle recreation and amenity. Due to the slow movement of water Leslie in September 2006 at an initial cost of J233 000 through the ponds, suspended matter is deposited and there is adequate time for both aerobic and anaerobic digestion of organic matter. ICWs also demonstrate good reduction of nitrates and phosphates (generally greater than 95%). Reduction of faecal indicators is of the order of 99% due to the long retention times and the complex ecology of the aquatic system. Furthermore, ICWs also remove endocrine disrupters such as oestrogen and testosterone, which are poorly removed in conventional wastewater treatment systems. An ICW effectively creates the important nexus between quality, quantity and amenity in a way that no other traditional sewerage treatment works can. In rivers, phosphates are the controlling pollutants. High levels of phosphates create algal blooms, which deplete oxygen levels and kill fish and invertebrates. The level of molybdate reactive phosphorus (MRP) set out in the consultation paper on Figure 2. Aerial photograph of the Glaslough ICW site Environmental Objectives (Surface Water) Regulations 2008 in 200 Municipal Engineer 162 Issue ME4 Sewerage treatment in an integrated constructed wetland Doody et al. Pond 1 Discharge Mountain Water River to river Pond 5 Sludge Sludge Pond 2 pond 1 pond 2 Stream diverted around edge of site Pumping station Incoming raw sewage Pond 3 Pond 4 Stream diverted around Stream edge of site Figure 3.