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Linwoods, a milk processing and bakery operation near Armagh, Ireland, produces wastewater that arises from the cleaning of its equipment. The processing plant operates six days a week, and wastewater is produced seven days a week. Wastewater production is currently 385 to 400 m3 weekly, or 20,000 to 21,000 m3 annually, rising to approximately 26,000 m3 annually.
The raw wastewater is treated in a surface-aerated tank. Aeration operates continuously for 15 hours daily. It is then switched off to allow particulate matter to settle, with the effluent being drawn off the top and stored while the sludge is removed to a sludge tank. Currently, the final effluent from the treatment facility is taken by road tanker to the Water Service sewage treatment works at Armagh on a daily basis, as there is no facility for discharging the effluent direct from Linwoods’ premises.
Objective & Challenge
The client required an alternative method of managing its wastewater—one that would be more sustainable, carry a lower cost and support the local farming economy.
Partially treated wastewater was transported 7 miles by road tanker to the Water Service sewage treatment works at Armagh. This was expensive and emitted about 7.5 tons of carbon to the atmosphere annually. In addition, the Water Service was unable to accept any additional wastewater from Linwoods, which limited any potential expansion of the bakery and milk processing facility.
A plantation of short-rotation willow coppice (SRC) had been established on a neighboring farm, which is irrigated with the partially treated effluent. The high-density planting—typically 15,000 plants per hectare—and high growth rate mean that the willow readily takes up the water and nutrients applied in the wastewater. A well-established plantation of SRC willow will produce 10 to 12 dry tons of wood annually.
Plantations are harvested every two to three years, which keeps the plants vigorous and ensures rapid growth. SRC is a method of producing large quantities of wood fiber, principally for use as a fuel.
Irrigation pipe work has been installed on 7.2 hectares of the willow plantation—ultimately, a total of 12.9 hectares will allow for nonirrigated margins and buffer zones at the edges of the plantation. The irrigation system consists of the following:
The whole system is automatically controlled and run with minimum input from the farmer. The willow coppice has been divided into zones approximately 0.5 hectares in size to facilitate careful control over effluent application.
Phase I comprises 13 zones, and Phase II has 10 zones. Each zone is atomically irrigated depending on soil temperature, soil moisture, rainfall, zone irrigation volume, history, etc. Data regarding the volumes irrigated and monitoring data ultimately will be uploaded via general packet radio service to an online data-handling application.
Monitoring & Nutrient Loading
The regulator enforces compliance with the quality conditions of the waterway, the conditions of discharge, conditions for application and the other general conditions with its own and the consent holder’s individual monitoring regimes. Soil moisture and volume of effluent supplied to the plantation (by zone) is monitored daily, and the data is supplied to the regulator on a yearly basis.
A small stream crosses the site. It is sampled every two weeks to monitor its quality upstream and downstream, though there is no discharge to this waterway. Upstream and downstream water samples are analyzed every two weeks for biochemical oxygen demand, suspended solids, ToN, ToP, pH and dissolved oxygen. These findings are also reported to the regulator as soon as they are available. The soils are analyzed for phosphorus in accordance with the ‘Phosphorus (Use in Agriculture) Regulations (Northern Ireland) 2006,’ and monitoring of the effluent continues at the Linwoods factory prior to discharge.
The nutrient loading on the system at present tends to be approximately 2,800 m3/hectare/year of effluent. This supplies about 150 kg of nitrogen/hectare/year and 6 kg of phosphorus/hectare/year—both of which are well within the sustainable level of nutrient assimilation by SRC.
Financial. The present costs of transportation of the effluent seven miles to the treatment works will be replaced by the pipeline running from factory to irrigation site. The cost charged by the treatment works is now replaced by the gate fee charged by the farmer per cubic meter of effluent. This alone is providing significant savings to the factory and earnings for the willow farmer. The irrigation system is robust, needs little maintenance and requires minimal operational and running costs.
Environmental. With all the dirty water being recycled, there is no outfall to the stream. There are carbon savings in taking a vehicle off the road and diverting a waste stream from the treatment works’ activities and subsequent energy requirements. There is also the production of carbon-neutral wood fuel for biomass boilers diverting energy use derived from fossil fuels.
Community. This project is stimulating and supporting the diversification of farming as well as ensuring the supply of local non-fossil-fuel wood biomass energy for the community.