Wastewater treatment in wetlands – an operations perspective
08 March 2016
Author: Katherine Walshe, regional operations manager, southern region, Irish Water
The use of wetlands for wastewater treatment is not a new phenomenon but is one which is gathering increased momentum within the water industry due to the environmental benefits and the relatively low operations and maintenance requirements associated with these systems.
Research in 2010 reported that aquatic plants (macrophytes) were used in wastewater treatment experiments in Germany in the early 1950s. These manufactured or constructed wetlands have been developed in various forms across the globe with the common aim of harnessing nature’s ability to clean wastewater through the physical, chemical, biological and microbial processes contained within ‘Natural Wetlands’. This article offers a perspective on wastewater treatment in wetlands from an operations point of view.
Natural wetlands are areas where land is covered by water, either saltwater, freshwater or a mixture of both. Marshes and ponds, the edge of a lake or ocean, the delta at the mouth of a river or low-lying areas that frequently flood are all wetland areas. They contain plants and vegetation that thrive in the submerged or partially submerged conditions.
The plants act as an impediment to flow and are sustained by nutrients from the soils and the water. Natural wetlands have been described as the kidneys of the landscape because of their ability to assimilate and treat water lost from the landscape before it enters into receiving waterways.
Constructed wetlands are artificial wastewater treatment systems consisting of shallow vegetated ponds planted with aquatic plants with the purpose of treating wastewater by means of natural physical, chemical, biological and microbial processes. They are designed to control the flow direction, liquid retention time and water levels. After passing through the system the treated effluent is allowed to discharge safely into a receiving surface water or ground water.
There is a wide variety of different types of constructed wetlands in use throughout the world but the systems used in Ireland can be categorised into two broad areas:
- Engineered Reedbed Systems; and
- Integrated Constructed Wetlands.
Engineered Reedbed Systems are a functional element in a treatment process, with primary and/or secondary treatment process elements provided (depending on the particular treatment process at the plant) up front of the reedbed wetland pond or ponds, which can be surface flow and/or sub-surface flow, generally with a single plant species within a gravel media and lined with polyethylene barriers. Figure 1 shows a typical layout for an Engineered Reedbed System.
Integrated Constructed Wetlands (ICWs) are a sustainable wastewater treatment system constructed with natural materials and designed to integrate into the landscape with the purpose of treating wastewater by using a series of ponds with a variety of wetland plants to clean the water before allowing it return safely to the environment.
These systems generally have a primary settlement tank or primary ponds followed by a series of vegetated wetland ponds. The Department of the Environment, Heritage & Local Government Integrated Constructed Wetlands Guidance Document for Farmyard Soiled Water & Domestic Wastewater Applications, published in 2010, explains that the ICW concept was developed to:
- integrate water flow and water quality management with landscape fit and biodiversity enhancement;
- deliver enhanced treatment with emphasis on phosphorous removal;
- provide greater system robustness and sustainability.
Figure 2 shows a typical layout of an Integrated Constructed Wetland.
Wetlands are adaptable and versatile systems that can be utilised for primary, secondary and/or polishing processes for the treatment of wastewater effluent. These systems have gained recognition for providing a sustainable treatment option for small rural settlements and provide opportunities for enhancing existing wastewater treatment plants.
Good design and correct construction is paramount for the effective and efficient operation and maintenance of wetland treatment systems, to ensure system robustness, to maximise the service provisions of the treatment plant, to extend the useful life of the assets, to facilitate operations activities and to keep ongoing operating costs to a minimum.
The operations activities for the Engineered Reedbed System will be greatly influenced by the type of process units provided at any particular site with the major operations and maintenance activities predominated by the primary and secondary elements of the treatment system. The operations activities required for the Engineered Reedbed wetlands are relatively easy due to the absence of mechanical, electrical or process equipment in that element of the treatment system.
Operations activities for the Integrated Constructed Wetland systems require regular periodic inspections with minimal follow up interventions to maintain surface flows within ponds, maintain access to and from the ponds and the removal of sediment and organic debris that builds up over a period years.
Regular operations activities for the Reedbed and ICW systems include:
- monitor and maintain water levels in the wetland;
- monitor condition of plants, minimise need for replanting of wetland plants;
- ensure even flow across the wetland area is being achieved;
- keep pipe inlets/outlets to/from wetland free from plant and vegetation overgrowth;
- monitor discharge from primary (and secondary treatment – especially for the reedbed system to prevent sludge carry over) into wetland pond;
- monitor & test discharge from wetland to discharge point of system;
- remove weeds and tree saplings;
- maintain access.
Other operations activities for the ICW systems:
- desludge primary settlement tanks, as required;
- control/remove overly dominant plants and replant wetlands plants where required;
- trim and cut grass and vegetation to maintain access along pathways and embankments around ponds, as required by with periodic maintenance this can be on monthly frequency or more during the growing season;
- remove sediment and organic debris build up in wetland pond, 10-20 years after the establishment of an ICW. This nutrient rich material is a potential future source of phosphorous and is expected to become a more valued resource in future when natural rock phosphorous reserves are depleted.
The benefits of wastewater treatment in wetlands range from protecting and enhancing the environment, increasing biodiversity, facilitating carbon capture and storage, extended service life when compared with conventional treatment systems and provide a robust, sustainable and aesthetically pleasing wastewater treatment plant.
But the key benefit of wastewater treatment in wetlands from an operations perspective is the low cost of operation of these systems, they offer real savings in terms of energy use, demand on operations staff time and resources as well as sludge production, handling and disposal – all significant considerations when faced with managing the provision of effective wastewater treatment services within challenging budgetary constraints.
Please visit www.water.ie and search for wetlands. See also
- Department of the Environment, Heritage & Local Government 2010, Integrated Constructed Wetlands Guidance Document for Farmyard Soiled Water & Domestic Wastewater Applications
- Water 2010, Constructed Wetlands for Wastewater Treatment, Jan Vymazal
- World Wildlife Org, http://www.worldwildlife.org/habitats/wetlands
- US EPA 1993, Constructed Wetlands for Wastewater Treatment and Wildlife Habitat, Case Studies
- US EPA 2000, Constructed Wetlands Treatment of Municipal Wastewaters
- Universiteit Gent, PhD Thesis 2005-2006, performance of Constructed Treatment Wetlands: Model-Based Evaluation and Impact of Operation and Maintenance, Diederik Rousseau.
- Water & Environmental Journal 2015, Phosphorus Retention and Mass Balance in an Integrated Constructed Wetland Treating Domestic Wastewater, Schultz et al.
- Proceedings of the Institution of Civil Engineers 2009, Sewerage Treatment in an Integrated Constructed Wetland, Doody, Harrington et al.