Below Render: Courtesy Clancy Moore Architects: winner of the Davey Prize in the AR Emerging Architects 2019: creating a civic presence in keeping with the historic site
Since 2014, the Environmental Protection Agency had identified several locations in Ireland where raw sewage is being discharged into our seas and rivers. Irish water is investing in vital new infrastructure to enable these unacceptable practices to stop by 2025.. One of the biggest projects undertaken so far is located in Arklow, a growing commuter town, just 1.5 hours from the capital of Dublin. Working in partnership, Wicklow County Council and Irish Water are developing a new wastewater treatment plant for the catchment area. This vital infrastructure will provide a state-of-the-art wastewater treatment plant that will stop the discharge of untreated wastewater into the Avoca River and the Irish Sea, ensuring that Arklow meets the relevant EU legislative requirements and will provide residents with a new resilient wastewater network that can grow from 18,000 PE (population equivalent) to 36,000 PE. Irish Water is procuring this scheme under a Design-Build-Operate contract with a programme of 4 years and a delivery date of 2025.
Ayesa was appointed by Irish Water to provide consultancy services across all phases of this critical WwTP project. This project continues our historical involvement in delivering water infrastructure in the area, having previously been appointed as Engineer for the Arklow Water Supply Scheme and on the Arklow (FRS) Flood Relief Scheme. Our understanding of the whole water cycle in the catchment has enabled us to take a holistic, strategic approach. There were some scheme overlaps between flooding and wastewater schemes, and we have used this to bring value and resilience to shared interfaces. The effects of climate change, seen in more severe weather, were addressed as part of the upgrade works. Two overflows (one at the WWTP) and another at the South Quay will provide emergency relief for excess storm flows due to an extreme weather event or an extended power outage at the Wastewater Treatment Plant. Without these overflows, high tides and weather events could trigger flooding risks.
GATES 1 and 2: DESIGN OF PREFERRED ROUTES: Our initial appointment was to work with Arup on Gates 1 and 2 to design the preferred routes for the new interceptor sewers to transport wastewater to the treatment plant and create a new outfall pipe to discharge the treated effluent safely to the Irish Sea. This was challenging due to the location of the existing sewers, river flows, and the aggressive marine environment. We also provided a support function to Irish water by undertaking CPOs, and landowner agreements and assisted in the oral hearing on the Environmental Impact Statement.
From the start, modelling, analysis and surveys were key to developing the design, location and routes of the interceptor sewers and the outfall. We used a detailed verified hydraulic model of the network, using InfoWorks ICM, an advanced integrated catchment software for modelling complex hydraulic and hydrologic network elements quickly and accurately. The model determined future drainage requirements and planning by pinpointing hydraulic, operational, and structural deficiencies. Our team conducted rigorous analysis and surveys: these included: flow & rainfall flow & load, topographical, marine, noise and appropriate assessments. Studies showed that wastewater flows into low-lying areas twice per year during flooding events, particularly at the South Green. The topography of Arklow and the termination of all outfalls at the river provided only two viable locations for the proposed new sewer effluent interceptors. These were located at the North and South Quays. The scheme also needed a tunnel shaft that crosses both sides of the Avoca River to transfer waste to the wastewater treatment plant.
In terms of the outfall, we analysed a potential river or marine outfall. However, it became clear that the river discharge would be too onerous and that a marine outfall was the most efficient solution. Our team investigated several options, including a final cost-efficient solution - which reduced the length of the deep tunnel to the plant. We used hydrodynamic and water quality modelling to inform the design and the route for the outfall - which revealed that the long-marine outfall route with a 1000m submarine pipe was the most suitable taking into account the flow currents into Arklow Bay, proximity to beaches, inlet works, stormwater storage tanks, an existing G.E. (General Electric) sub-sea electricity cable and a pump sump needed for stormwater overflows.
GATES 3 AND 4:
The project has secured the required planning, land and licences to proceed, and Irish Water has appointed Ward & Burke as the contractor. Following a tender competition, Irish Water appointed Ayesa as the Employer’s Representative, and we are responsible for contract administration services across Gates 3-4. In August 2021, construction began on the new €140m wastewater plant. On the town network sites, the underground shaft tunnel running along the North Quay for the new interceptor sewer has been completed. The river crossing tunnel and the installation of the south Quay interceptor sewer will continue into 2023. In the summer of 2022, the 1000m outfall which will safely discharge treated wastewater was sunk off the coast of Arklow. The pipe was shipped in three sections from Norway, each measuring almost 310m in length, to its final location.
The scheme includes
- A pumping station with an ultimate design capacity of 304 litres/sec to pump flows to a new Wastewater Treatment Facility, which in the first phase will serve a population of 18,000; (future of 36,000 PE)
- Twin siphons 100m in length under the Avoca River;
- New sewers to provide for future development
- Stormwater separation and rationalisation of combined sewer overflows
- Interceptor sewers ranging from 525 to 1800mm were constructed along the north and south banks and a section of the river in difficult ground conditions.
Benefits & Innovation
Our expertise in tunnelling and geotechnics combined with our expertise in Water and network design enabled us to devise the optimum solution to find the optimal alignment for the interceptor sewers given the constraints arising from the existing sewer locations and the challenging ground conditions. As the lead engineer on the Arklow Flood Relief Scheme (FRS,) we identified physical overlapping between the schemes to integrate our approach. This WWTP brings multiple benefits to health and environmental integrity and will help facilitate economic and social development for Arklow town, waterfront, beach area, and surroundings. It will also mitigate flood risks which have been a recurring problem over the past century.
Stakeholder & Community Engagement
The project team minimises dust during construction by spraying exposed earthworks during dry spells, sweeping the hard surface roads, erecting hoarding around the site, speed and vehicle access restrictions, and wheel washes at the site exit points. A Full-Time Community Liaison Officer was appointed to liaise with stakeholder concerns throughout each work phase, and regular communications were posted on the website page. During the works, a traffic management plan was agreed upon with the local country council which limited construction vehicle movements to outside of morning and evening traffic times.