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WWTP and SRC Willow Treatment

Successful five year pilot delivered for a new sustainable, cost effective and secure waste water treatment and willow system.

The problem:

Donard is a small 250 person village in rural Co. Wicklow. Prior to 2005, wastewater from the village was treated via an old septic tank and the effluent discharged directly to a small tributary of the River Slaney, causing public health and environmental concerns. The Eastern Regional Fisheries office was particularly concerned because the stream is a designated salmonid river with sensitive wildlife.

The population of Donard was expected to rise to 600 as the village grew and further new houses were to be constructed. Wicklow County Council required a cost effective waste water treatment system that was environmentally secure to prevent any further pollution of the stream, and could cope with the predicted future growth of the town.

The solution: Screening, primary treatment and coppiced willow solution:

Wicklow County Council nominated Clearpower (prior to its joinging with Enva) in conjunction with the RPS Group to design, build and operate a new pilot waste water treatment plant (WWTP) plus willow solution for effluent treatment, effectively eliminating any discharge to the stream. Following a feasibility study and design phase in 2004, a new primary treatment works was built consisting of a new inlet works with a coarse and fine screen, followed by a new two stage septic tank and final pump sump. From the pump sump, the treated effluent is pumped via an irrigation system across an eight hectare short rotation coppice (SRC) willow plantation established on marginal land adjacent to the treatment works. The flow rate of the effluent is continually measured and the duty and stand by pumps are controlled by float switches with a GSM text alert system in the event of any issues. The willow was densely planted at 15,000 trees per hectare, and has a rapid growth rate and very high transpiration rate. The dense interwoven root mesh forms a robust absorption zone, with proven capacity to evaporate off irrigated water and recycle nutrients into wood, with no discharge to groundwater or surface water.

Wood chip harvest, replacing peat in Edenderry Power

The wood chip is harvested every three years, with the yields improved from the fertiliser value in the irrigated nutrients. The willow plantation has been harvested twice to date, producing approximately 800 tonnes of sustainable wood chip at 50% moisture content, roughly equivalent in energy terms to 190,000 litres of home heating oil. The wood chip was delivered to Edenderry Power Station, replacing the burning of peat fossil fuels and avoiding 670 tonnes of CO2 emissions to the atmosphere.

Five year environmental monitoring regime for pilot phase

To prove that the new system did not impact the groundwater under the willow plantation and to illustrate the improvements in surface water in the stream, the entire system was closely monitored for five years with water samples taken monthly from the six groundwater boreholes located through the plantation and the stream, upstream and downstream from the WWTP. Kick samples and wildlife inspections were also taken in and along the stream. For one year prior to any irrigation to the willow plantation, baseline sampling and analyses were undertaken. Samples were sent off for analysis every month and results were presented quarterly to Wicklow County Council.

The results of the 5 year monitoring programme clearly show that:

The quality of the surface water in the stream is now good and has recovered fully, and the fisheries board are reporting increased salmonid activity, with increased fauna and crustacea in kick samples.

There is no impact on groundwater from the irrigation activity across the willow plantation.

The EPA awarded the Donard WWTP and willow plantation a WWTP licence, a verification of the success of the new wastewater treatment system for Donard village.

Environmental and Economic Benefits

The SRC willow treatment system has environmental, economic and social advantages:

Environmental benefits:
  • Compliance with wastewater treatment obligations.
  • The SRC willow treatment system has no discharge – all wastewater and nutrients are taken up by the willow plants
  • Nutrients are converted into useful woodchip, which replaces the requirement to burn fossil fuels and reduces CO2 emissions to the atmosphere.
Economic benefits:
  • Lower capital costs compared with traditional secondary and tertiary treatment plants
  • Significantly lower operational costs as the willow plants do most of the work.
  • Robust, secure solution with fewer mechanical parts than a traditional system, meaning that it is less likely to break down or cause problems
Social benefits:

– Generates sustainable rural development through planting, maintaining, harvesting and delivery of willow chip fuel.
– Secures supply of cost effective, CO2 neutral, sustainable heating fuel for local communities.

The main drawback of a system like this is the land requirement for the willow system. However, with the slowdown in construction in Ireland, a number of potential sites are now available – a silver lining from the slowdown.

Plans to replicate the system at other sites in Ireland:
The replication potential of the system at other sites in Ireland has been made more likely by the construction slowdown and the increased availability of suitable affordable land. Tightening capital budgets in local authorities also favours installation of the system for smaller villages.

Replication will be most likely where the design criteria are similar to Donard – i.e.:

  • Sensitive receiving waters or low assimilative capacity
  • Available and affordable land suitable for the willow plantation
  • Topography suitable for minimal pumping
  • Good soil depth and water table sufficiently below ground level

The capital cost largely depends on the land price or long term lease agreements for the willow plantation. The operational cost is however significantly lower than the traditional primary and secondary treatment works that discharge final effluent to surface water.

“We are delighted with the performance of the prototype system in Donard village, and the comprehensive analysis over the course of the five year pilot. With increased fossil fuel prices, a need to secure local, sustainable energy supplies, and the construction slowdown freeing up land around small rural villages, this system is now a natural choice for local authorities or businesses with effluent discharge issues to manage” Simon Dick, Director