Robert Mulhall takes a look at the key issues faced at the outset of the Arklow Water Supply Scheme and gives an overview of the different aspects of work that were required to develop this new public water supply
Civil

Eleven years after its initiation and following the investment of over €10.5 million, the Arklow Water Supply Scheme was finally completed in May of this year. This article examines the key issues faced at the outset of the Arklow Water Supply Scheme and gives an overview of the different aspects of work that were required to develop this new public water supply.

In the period 1998-2005, the population of Arklow increased significantly due to the amount of housing development that took place within the town. The old water-supply system had reached maximum capacity. This capacity issue was exacerbated by the fact that the old supply was totally dependent on surface water sources:

  •  The Goldmines River, which is located in Woodenbridge (7km northwest of Arklow) and
  • The Impoundment Reservoir located at Ballyduff South (3km west of Arklow).

Surface-water sources provide good quality water during periods of settled weather, but can be susceptible to periods of poor quality after heavy rain has fallen. They are also vulnerable to the risk of contamination, which can typically be caused by fertiliser or manure runoff from agricultural land.

In Arklow, it was quite a common occurrence that the source water from the Goldmines River and Impoundment Reservoir would be unusable due to their poor quality. This occurred most often during, and after, periods of heavy rainfall. The inability to use the source water during these times put tremendous pressure on the supply, since the water-treatment plant would have to be shut down temporarily to mitigate the risk of contamination entering the water supply. The Arklow Water Supply Scheme (AWSS) was initiated to address these critical issues and ensure that the Arklow area would have a safe, high quality, water supply for the next 25-30 years.

Arklow Water Supply Scheme — objectives


Figure 1 - Scheme Overview

CLICK TO ENLARGE Fig 1: Arklow Water Supply Scheme Layout Map – Wellfield No.1 (PW12 – PW16) is located in the Avoca River Basin and yields over 300,000 litres/hour

The key objectives of the Arklow Water Supply Scheme were to:

1. Increase supply capacity and reduce reliance on surface water sources
The reliance on surface water sources was reduced through the development of multiple groundwater sources. Groundwater sources are less susceptible to variances in quality, since they are not open to the elements and are less vulnerable to the risks of contamination when compared with surface-water sources.

2. Increase the water treatment capacity

The old water-treatment plant building and gravity filter beds, which were the subject of a February 1932 journal article from the Institute of Civil Engineers of Ireland ¹, were originally constructed in 1926 and still in use up until the completion of the new treatment plant in 2015. Minor modifications and upgrades were carried out to this old treatment facility over the past 90 years, but a serious redevelopment was now imperative to ensure that Arklow would have a much higher capacity water treatment system capable of coping with future demand.

3. Reduce leakage within the treated water storage reservoirs
The treated storage water system within Arklow town consists of three reservoirs:
(i) ‘Small’ reservoir (capacity 950,000 litres, constructed in 1927)
(ii) ‘Tower’ reservoir (capacity 650,000 litres, constructed in 1973)
(iii) ‘Large’ reservoir (capacity 3,350,000 litres, constructed in 1986)

A comprehensive condition survey was carried out on the three structures in 2012, and this indicated that the ‘Large’ and ‘Tower’ reservoirs were in need of refurbishment whilst the ‘Small’ reservoir was unrepairable and would need to be replaced. Reducing the amount of treated water being lost from the storage reservoirs would significantly reduce the amount of water to be treated each year and maximise the efficiency of the newly developed supply.

Construction


Figure 2 - Well Construction

Fig 2: Production well construction – the well liner shown above prevents the well from collapsing after it has been drilled. The screened mesh section in the middle allows the water to enter the well from the surrounding ground

The first construction phase of the AWSS commenced in 2007 and involved the development of groundwater sources or production wells (PW). Sixteen potential production well locations were identified in three different townlands surrounding Arklow town. A production well was drilled at each of these locations and a yield test carried out to determine the quantity of water that could be sustainably obtained from each one.

Of the 16 that were initially identified and drilled, only 13 of these were determined to have a high enough yield to merit further development. The highest yielding wells are located in the Avoca River Basin between Arklow and Woodenbridge (PW12 – PW16) and each of these has an average yield of 65,000 Litres/hour. The water from these wells is of a very high quality and requires minimal treatment.

The fact that Production Wells 12-16 are located in the Avoca River basin, which has flooded significantly in the past, posed a potential problem. As recently as 2009, flood waters did exceed two metres above ground level in this area. In order to avoid the possibility of flood-water levels submerging all electrical control equipment in the future, which would cause catastrophic damage as well as the subsequent repair downtime, all of the electrical control elements for each well in this area were mounted on a 2.5 metre raised platform like the one shown in Figure 3.

Figure 3 - Production Well Platforms

Fig 3 – Raised control panel kiosks – Required for all of the production wells (PW12-PW16) located in the Avoca River Basin. Flood waters in this area did exceed 2.1m in height in the floods of January 2009

Following the construction of the new groundwater sources, the next phase of the scheme involved constructing new pipelines which would link the new groundwater sources with the proposed site for the new treatment plant located in Ballyduff South, Arklow. Between March 2010 and January 2012, approximately 10km of pipelines were laid. Three pipelines were constructed in total, with each one collecting the groundwater from each of the different wellfields:

  • Woodenbridge wellfield (PW12, PW13, PW14, PW15, PW16);
  • Glenart wellfield (PW9, PW10, PW11); and
  • Ballyduff South wellfield (PW1, PW2, PW3, PW4, PW5).

All of the pipelines constructed on the scheme were manufactured from high density polyethylene (HDPE). Aside from being corrosion resistant, polyethylene pipes have another distinct advantage over ductile iron, in that it is possible to construct a leak free joint between pipe lengths. Leak free joints result in lower operations and maintenance costs over the lifecycle of the pipeline.

Penultimate phase of AWSS


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Fig 4 – HDPE Pipeline Construction in 2010 – the pipe lengths can be flexed slightly, minimising the need for additional joints

The penultimate phase of the AWSS commenced in May 2013 and was completed in February 2015. It involved the construction of a state-of-the-art water treatment plant (WTP) located at Ballyduff South, Arklow on a site adjacent to the old WTP. The key design requirements for this new facility were that it have a treatment capacity of 6.1Ml/day, and be capable of treating both surface water (i.e. Goldmines River) and groundwater (i.e. new production wells). The resulting facility has two separate treatment process streams:

1. Groundwater treatment
The groundwater from the new production wells is already of a high quality at source, so it requires minimal treatment for suspended solids removal. However, iron and manganese removal is required. This is achieved using cascade aeration, which oxidises both the iron and manganese from their soluble states into their insoluble states, precipitating into small particles which can then be removed by subsequent filtration processes.

2. Surface water treatment
A different treatment process is required for clarifying the surface water from the Goldmines River. Surface water sources tend to contain higher levels of floating particles and suspended solids, which must be removed during the treatment process. Additional treatment steps are required in order to enable the filtration of these tiny particles.

Figure 5 - Water Treatment Plant

Fig 5 – Arklow Water Treatment Plant has a treatment capacity of over 6,000,000 litres/day, an increase of approximately 60% over that of the old WTP

The Arklow Water Treatment Plant is using a dissolved air flotation (DAF) system to promote the agglomeration of these tiny particles into larger-sized particles, which can be more easily removed. A higher percentage of these larger particles are removed by a diffused air bubble system within the DAF unit itself. Air bubbles are diffused at the bottom of the final chamber within the DAF. These bubbles rise to the water’s surface, attaching themselves to the dirt particles on the way up, causing the dirt particles to collect on the water’s surface which can then be removed. Any suspended solids remaining are then easily removed by a subsequent filtration process.

Final phase of construction


Figure 6 - Reservoir Construction

Fig 6 – Reservoir Construction in 2015– View from the top of the adjacent tower reservoir (Figure 7) of the new 1,000,000 litre reservoir under construction

The final phase of the scheme commenced in May 2015 and was completed in June of this year. The works completed during this period included the refurbishment of the ‘Large’ and ‘Tower’ reservoirs, and the replacement of the dilapidated ‘Small’ reservoir with a new 1,000,000 litre precast structure.

The refurbishment works within the ‘Large’ and ‘Tower’ reservoirs consisted of:

1. Removal of defective concrete
All defective areas identified in the reservoir structures were prepared for repair. This involved cutting and grinding all defective areas to ensure that all weak cement and contamination was removed.

2. Treatment of reinforcement and application of bonding agent
After the defective concrete had been successfully removed, the reinforcement behind was inspected for its integrity, repaired where required, and subsequently primed with reinforcement protector.

3. Repair reinstatement
After the reinforcement and surrounding concrete had been primed, a patch repair mortar was applied in the repair locations in depths required to re-establish the original concrete surface levels.

4. Application of internal protective coating
The final stage of the refurbishment process involved the application of a high strength mortar to all structural surfaces in order to protect all of the concrete surfaces and ensure a watertight seal.

Figure 7 - Tower Reservoir

Fig 7 – Tower Reservoir Refurbishment – Repair of the external cracks followed by the complete re-rendering of the outer surfaces

The completion of the AWSS marks a major milestone for Arklow town and its environs. The town now has a water supply system of which it can be very proud. The successful completion of the scheme will ensure that the area has a safe, high quality, water supply for the next 25-30 years, and one that will allow the town to develop from an economic aspect. Many elements of the old supply and treatment system were in use since 1926, so this large redevelopment & monetary investment was long overdue.

Over the past 11 years, many question marks hung over the scheme, particularly with regard to the scheme’s funding during the economic collapse. Much credit must go to Wicklow County Council, The Department of the Environment and Irish Water for recognising the critical need for this scheme and for ensuring that adequate funding was made available for its completion.

Chartered engineer Robert Mulhall is the former senior employer’s representative on the Arklow Water Supply Scheme and is now an executive engineer with Wicklow County Council.

References
1. The Institution of Civil Engineers of Ireland – Bulletin, February 1932 – The Arklow Water Supply Scheme, by PJ Mc Carthy BE.

http://www.engineersjournal.ie/wp-content/uploads/2016/07/arklow-1.jpghttp://www.engineersjournal.ie/wp-content/uploads/2016/07/arklow-1-300x300.jpgMary Anne CarriganCivilconstruction,wastewater,water,Wicklow County Council
Eleven years after its initiation and following the investment of over €10.5 million, the Arklow Water Supply Scheme was finally completed in May of this year. This article examines the key issues faced at the outset of the Arklow Water Supply Scheme and gives an overview of the different aspects...