[caption id="attachment_38598" align="alignright" width="178"] Pic 1: Bulrush (Typha latifolia) growing in a constructed wetland for urban stormwater treatment in Co Galway. (Photo: FH Wetland Systems)[/caption] For your chance to win a copy of Féidhlim Harty’s new book, 'Permaculture Guide to Reed Beds – Designing, Building and Planting your Treatment Wetland', email your answer to the following question to mcarrigan@engineersireland.ie on or before Friday, 17 November: what are the three main system types within the general category of treatment wetlands? It has been a busy few weeks in the news for Irish waterways. First, storm Ophelia brought widespread power shortages, which shut down wastewater treatment systems in many parts of the country. The consequence of this was a direct pollution impact in any areas reliant on electricity for good sewage treatment. Then just a week later, the Environmental Protection Agency's (EPA's) new 'Urban Waste Water Treatment in 2016' report highlighted the serious gaps in Ireland's sewage treatment infrastructure. In a nutshell, we are well over a decade past our deadline to provide secondary treatment for all urban sewage discharges. This means that sewage is still being discharged directly into our coastal waters, either raw or with only primary settlement, and causing ongoing water pollution (whether we have electricity or not!). The embarrassment (and opportunity) of the whole situation is that there is a very straightforward solution to both problems. Constructed wetlands and reed beds are a cost-effective solution already employed by many small urban areas around the country. They can provide high quality effluent treatment with zero ongoing electricity requirements and are also ideally suited to the small- to medium-sized urban areas that are so seriously in need of attention. Thus, while many areas have been identified in the national news as having inadequate treatment (see Fig 1 below, and map to right), other more positive stories have escaped notice. The constructed wetlands at Lemybrien in Co Waterford, Kiltimagh in Co Mayo and Glaslough in Co Monaghan, for example, have worked away quietly with no need for electricity inputs during the recent storm. They continue to provide treatment, tertiary polishing and, in some cases, urban stormwater treatment into the bargain.

Constructed wetlands and sewage treatment

Constructed wetlands and reed beds have been used for municipal sewage treatment in many counties (see Fig 2) and have been used for domestic sewage treatment in probably every county in the country.

Fig 1: EPA lists of urban areas discharging untreated sewage (from Urban Waste Water Treatment in 2016)

[caption id="attachment_38591" align="alignright" width="264"] CLICK TO ENLARGE Pic 2: Map of raw sewage discharges in Ireland (Source: EPA Urban Waste Water Treatment in 2016)[/caption]

EPA list of large urban areas discharging untreated sewage:

  • Cobh, Co Cork
  • Passage West/Monkstown, Co Cork
  • Ringaskiddy/Crosshaven/Carrigaline, Co Cork*
  • Youghal, Co Cork
  • Killybegs, Co Donegal
  • Arklow, Co Wicklow

*The footnote in the EPA report states that most of this area is now connected to a new treatment plant, but some raw sewage is still discharged.

EPA list of small urban areas discharging untreated sewage:

  • Clare: Ballyvaughan, Clarecastle, Kilkee, Kilrush and Liscannor
  • Cork: Ballycotton, Castletownbere, Castletownsend, Inchegeelagh, Ringaskiddy village, Timoleague, Whitegate/Aghada
  • Donegal: Bundoran, Burtonport, Coolatee Housing Scheme, Falcarragh, Kerrykeel, Kilcar, Moville, Ramelton, Rathmullan, Howth (Doldrum Bay)**, Rush
  • Galway: Ahascragh, Carraroe, Roundstone, Spiddal
  • Limerick: Foynes, Glin
  • Louth: Omeath
  • Mayo: Belmullet, Killala, Newport, Arthurstown, Ballyhack
  • Wexford: ArthurstownBallyhack, Duncannon, Kilmore Quay, Avoca

** The footnote in the report states that this is a secondary discharge within the area covered by the Ringsend wastewater discharge licence. It caters for a population of approximately 120
 

Fig 2: List of municipal reed beds and wetlands

  • Carlow: Hacketstown4
  • Cavan: Ballyconell1
  • Clare: Kilmurry1
  • Cork: Boherbue2, Cloyne3
  • Dublin: Colecot1
  • Galway: Williamstown1, Moycullen3
  • Kildare: Ardclough2
  • Mayo: Hollymount3, Kiltimagh2
  • Meath: Kildalky1
  • Monaghan: Tydavenett1, Clones1, 4, Drum1, Threemilehouse1, Glaslough5
  • Offaly: Geashill3
  • Roscommon: Ballyfarnon2, Cloonfad2, Keadue2.
  • Waterford: Lemybrien1, Tooraneena1
  • Wexford: Ballygarret1, Ferns1, Piercestown3

1 Free Water Surface constructed wetland receiving primary settled effluent.

2 Free Water Surface constructed wetland receiving secondary treated effluent.

3 Subsurface flow gravel reed beed receiving secondary treated effluent.

4 Gravel media sludge treatment/drying reed beds.

5 Integrated constructed wetland receiving primary settled effluent.

Sources: Wetlands of Ireland and personal experience. The above systems represent design input from many Irish wetland and reed bed designers. Note that the above list generally excludes smaller housing estate systems, many of which are now also managed as municipal treatment systems.

Reed beds and constructed wetlands - terminology

Reed beds and constructed wetlands have been functioning reliably and effectively for the past ten-to-twenty years without fuss and without fanfare, protecting the local rivers and streams as they do. They have been listed in the EPA guidance since 1999. The terms 'reed beds' and 'constructed wetlands' are often used interchangeably, but there are a few distinct system types within the general category of treatment wetlands, as follows (see also Pic 3)):
  • Soil-based constructed wetlands (or free-water surface wetlands, or integrated constructed wetlands) most closely resemble a natural marsh. These systems consist of a lined shallow basin, backfilled to c.150mm with loam soil and planted with a selection of tall wetland plants. The effluent is treated as it moves slowly through the plant stems and leaf litter that accumulates in the shallow water of the marsh.
  • Horizontal-flow gravel reed beds (or horizontal subsurface flow systems) are generally smaller in footprint area. These consist of a 600-700mm deep bed of gravel into which common reed (Phragmites australis) and other species are planted. The effluent is treated as it flows through the gravel and plant roots.
  • Vertical flow (VF) reed beds are more like raised sand-polishing filters or stone-trickling filters. These systems are smaller than horizontal-flow gravel reed beds. The VF reed bed consists of c.1m depth of gravel, of progressively smaller particle sizes towards the bed surface. Effluent is dosed over the bed surface (by pumped feed or gravity dosing mechanism) and treatment occurs as it trickles down over the gravel media and plant roots. Note that sand filled or topped VF reed beds can be prone to blocking unless the exact aggregate grades are used and the correct degree of pretreatment is achieved on a consistent basis for the lifetime of the system.
[caption id="attachment_38620" align="aligncenter" width="589"]Terminology for reed beds and constructed wetlands Pic 3 LEFT: Soil-based constructed wetlands. CENTRE: Horizontal-flow gravel reed beds. RIGHT: Vertical flow reed beds[/caption] Ponds may also be used to extend the residence time within the overall system while minimising the footprint area of the project. These have the advantage of offering an additional habitat dimension towards the end of a system, and greater penetration of UV light for extra die-off of pathogens. However, they pose a potential safety hazard and are thus often excluded from domestic systems. The three system types described may be used alone or in series on a given site to achieve the desired effluent standard, but the design guidelines for each system type are different and should not be used interchangeably. The EPA Wastewater Treatment Manuals and EPA Code of Practice provide clear guidance on sizing and on system layout for each of the treatment wetland types.

Physical, chemical and biological treatment mechanisms

[caption id="attachment_38600" align="alignright" width="178"] Pic 4: Gravel reed bed system for effluent polishing. (Photo: FH Wetland Systems)[/caption] So, how do treatment wetlands achieve the treatment standards needed without electricity for blowers or pumps? Physical, chemical and biological treatment mechanisms all come into play. Physical separation by sedimentation occurs in the slow-moving water within the system, and in the still water of the pond, if used. The gravel and/or plant stems and leaf litter further slow the flow of water and allow greater settlement of fines onto the bed of the system. Biologically, microbial activity is an important mechanism, involving essentially the same aerobic bacteria that thrive in standard mechanical systems. Rather than using air blowers, however, the plants introduce the oxygen by drawing it down to the roots where it can be utilised by microorganisms for treatment. Chemical mechanisms include redox reactions and volatilisation of nitrogen and sulphur from the system. When setting out a system on site, the layout and size need to be carefully designed to ensure that all the treatment mechanisms will properly treat the effluent prior to release back into the receiving environment. It is also important to adhere to all of the standard EPA minimum separation distance requirements and allow for the fact that constructed wetlands are open systems and must be fenced to prevent unauthorised access. Maintenance requirements and energy inputs are low compared with mechanical aeration units. That said, bear in mind that treatment wetlands are sewage treatment systems that need care and maintenance like any other. The lower the suspended solids inputs, the longer will be the lifetime of the system. Beyond standard desludging of the preceding settlement tank, however, maintenance requirements are relatively modest. Check that pipes are flowing freely and that the water levels are set to the correct level and the system will work away happily for 15-50 years, depending on the design. Naturally, not all sites are suitable for constructed wetlands or reed beds. They take up more space than conventional mechanical aeration systems and gravity throughput will only be possible where the falls are favourable. That said, where space is limited they can work well in tandem with conventional aeration units, providing tertiary polishing of secondary treated effluent along with a backup against serious water pollution during a power shortage or system malfunction. So, whether you want to design for zero carbon footprint, resilience from power shortages or to provide ongoing protection of Ireland's rivers and coastal waters, treatment wetlands can be part of the overall solution you can offer to your clients and adopt in your local area for municipal use. References: Author: Féidhlim Harty is director of FH Wetland Systems environmental consultancy and author of Septic Tank Options and Alternatives - Your Guide to Conventional, Natural and Eco-friendly Methods and Technologies. His new book Permaculture Guide to Reed Beds – Designing, Building and Planting your Treatment Wetland (right) has just been released and is available to buy here. See Harty's previous article for additional options for challenging sites for domestic systems. For your chance to win a copy of Harty's new book, Permaculture Guide to Reed Beds – Designing, Building and Planting your Treatment Wetland, email your answer to the following question to mcarrigan@engineersireland.ie on or before Friday, 17 November: what are the three main system types within the general category of treatment wetlands?