Author: Grellan McGrath CEng, director RPS From the shore of Blessington reservoir on the Wicklow/Kildare border, the Ballymore Eustace Water Treatment Plant currently supplies an average of 300 million litres per day (MLD) of treated water to consumers in Dublin City, Dun-Laoghaire, south Dublin and part of Kildare. The plant was originally conceived in 1936 under the Liffey Reservoir Act, which facilitated the construction of a 166 million cubic meter impounding reservoir on the River Liffey in a catchment of 312 sq km, with the dual purposes of water supply and hydro-electric power generation for a rapidly growing city population. To date there have been a number of developments at the plant, bringing its capacity from an initial 50 MLD in the 1940s, through to 136 MLD in the mid-seventies and on to 250 MLD in 1986. The recent expansion has seen the plant reach its ultimate capacity of 318 MLD, with a peak capability of up to 400 MLD. MCOS (now RPS) were consultants to Dublin City Council for plant expansions since 1983.

Final Stage Three development

[caption id="attachment_25520" align="alignright" width="300"]PJtable Table 1 (click to enlarge)[/caption] RPS was appointed by Dublin City Council in 2004 to complete the detailed design, procurement and construction supervision of the final Stage Three development of the Ballymore Eustace water treatment plant to the maximum capacity of the River Liffey. The development included a combination of existing plant refurbishment and new plant construction to achieve a sustainable output of 318 MLD (400 MLD peak output). The critical objective of the city council was achieve the 318 MLD output as early as possible in the contract period to ensure the increasing water demands of the greater Dublin region could be met. This requirement necessitated the construction and commissioning of the new treatment elements in advance of the existing plant refurbishment, while maintaining full operational capacity at the plant throughout the contract period. To facilitate this, a detailed methodology, prescribing the sequencing of the works and the interface between the works contractors and plant operations, was developed in collaboration with the city council’s operational team for inclusion in the tender documents. As well as significant site works (pipework, ducting and landscaping) the Stage three civil works comprised 12 distinct elements outlined and numbered in the table to the right (table 1).

Design

At the core of the detailed design was the requirement to maintain a gravity flow through the plant from the twin 1,600mm intake pipes from Blessington reservoir at Poulaphuca to the outlet of the new treated water reservoir manifold at the head of the trunk mains feeding into distribution. [caption id="attachment_25525" align="alignright" width="300"]PJ 3 Treated Water Outlet Weir House[/caption] To facilitate this a detailed hydraulic model of the plant incorporating the existing and proposed treatment elements together with bypass facilities was developed and used as the blueprint for completing the general arrangement for the final plant (i.e. tank locations, sizes and depths, pipe routing, diameter and material, valve sizing and location etc). To add to the complexity, hydraulic models were also required for the numerous temporary arrangements that were required to ensure continuity of supply while refurbishing existing works and connecting in the new works. With limited head to play with there was no room for error, which was a particular challenge for the RPS design team. In addition to the use of hydraulic modelling, other innovative designs solutions included: -
  • The use of wide (6.0m) rectangular weirs at strategic points throughout the process to conserve head, provide control and give irreversible hydraulic break points;
  • The provision of a settled water manifold to equalise the head feeding the three filtration plants, leading to optimum filtration operation and minimal head loss;
  • The re-engineering of the existing contact tanks to improve disinfection contact time, which was previously not possible due to the physical configuration;
  • The ability to control hydraulics on the downstream side of the process, giving greater hydraulic head and capacity to deliver water into transmission and distribution.
The Stage Three process design was further complicated by the absolute requirement to keep the plant live at all times and at the maximum capacity. The design philosophy set out a structured approach to construction involving a detailed construction and commissioning sequence, including the phased demolition and refurbishment of elements of the existing works. The design programme, milestones and construction constraints were reviewed and optimised in a collaborative manner with Dublin City Council and incorporated in the construction documents in the form of a detailed phasing schedule, setting out the order in which the various works elements were to be constructed and commissioned, complete with schedules of construction constraints and protocols prescribing the interface between the civil and M&E contractor and the plant operations staff dealing with liaison, reporting, site access, permits to work etc. The process design included an extensive by-pass system to facilitate full uninterrupted access to plant elements in case of failure or emergency. The designs core concept sought to conserve head through the innovative use of hydraulic engineering, thereby minimising the energy required to treat the water and maximising the head available to facilitate its distribution throughout the greater Dublin region. To facilitate routine maintenance of process units the overall design philosophy allowed for process elements (sedimentation and filtration) to be taken out of operation for extended periods without compromising the overall quantity and quality performance of the plant. Prior to the Stage three development this routine maintenance provision could only be facilitated during periods of low demand and often at the risk of deterioration of plant quality performance. [caption id="attachment_25529" align="alignright" width="300"]PJ4 Reservoir Outlet Manifold[/caption] To ensure compliance with strict environmental requirements the process design included a Waste Water Management Plant (WWMP) for all waste waters (filter wash water, sludge thickener decant and sludge press filtrate), complete with full on-line discharge quality monitoring, ensuring minimal impact on the River Liffey water quality. All structures were designed to fit sympathetically into the landscape through a collaborative process involving Dublin City Council, Kildare County Council as planning authority and other stakeholders. An example of this is the 57 Ml reservoir, which is built substantially underground, with the outlet manifold the only visible element. Similarly, all mechanical plant is housed in sympathetically designed buildings, with much of the core plant rooms underground to avoid noise pollution. The design included a zero earthwork balance whereby all excavated materials were reused on site as embankments around structures and to form landscaped areas.

Construction

[caption id="attachment_25531" align="alignright" width="300"]PJ5 Settled Water Manifold Under Construction[/caption] Construction was carried out by a combination of BAM Civil Ltd and AECOM for civil and mechanical / electrical / process respectively. RPS was project manager supervising construction and in addition was appointed Project Supervisor Design Phase (PSDP). A dedicated site programmer tracked the contractor’s planned versus actual progress onsite and produced two-week look ahead resourced programmes. Formal systems and procedures were put in place to manage permits to work, temporary works review/design sign off and change orders. To mitigate the risks to the continual operation of the water treatment plant and provide the necessary flexibility to cater for significant seasonal and daily demand variations a buy-out was agreed with both contractors whereby the programme and sequencing of the works was modified to ensure satisfactory completion within fixed time and budgetary constraints. [caption id="attachment_25533" align="alignright" width="300"]PJ6 1,200mm Temporary Works By-pass[/caption] The collaborative environment created by the buyout agreements proved very successful, with the contractors encouraged to put forward ‘value engineering’ proposals if they felt designs could be improved, programmes shortened or costs saved without impacting the core project objectives. All milestones and targets outlined were met, which was a significant achievement considering the scale, complexity and collaboration involved. This significant programme enhancement was achieved through the employment of an innovative design solution involving the construction of a temporary 900/1,200mm by-pass pipe connecting the existing settled water channel to the filter outlet manifold, routed through the site, including an existing building that was being demolished.

Health and safety

[caption id="attachment_25536" align="alignright" width="300"]PJ7 Rapid Gravity Filter Pipe Gallery[/caption] The Contract Health and Safety file was developed electronically using Autoplay Media Studio as a front end interface to the safety file database. The safety file also included an extensive operating and maintenance record, which was included on the database as works came on stream. The result of this is that the operator has easy access to an extensive asset management electronic file, a complete asset register, a maintenance schedule, asset replacement recommendations and complete safety documentation. This combined with live data links to the extensive on site SCADA system allows the operator full visibility and transparency for life cycle asset management.

Summary

The Stage Three development of Ireland’s largest water treatment plant at Ballymore Eustace is one of the most strategically significant projects in public water supply in Ireland for 30 years. The innovative design utilised delivers quality and conserves energy on a significant scale. It provides the strength and flexibility needed in a facility that is designed to last for the next 100 years. It foresees future demand needs and higher quality and environmental standards giving the Employer confidence in the scope to expand and develop as needed. The plant utilises the latest technology in the most efficient way providing open, transparent visibility on all plant operations and performance allowing flexibility in control dealing with variations and incidents. This is almost unique in a plant of its size but the real benefits are that it is run as a professional state of the art utility, with a view to maintaining asset life and consistency. Benchmarks have been set in how water engineering should be approached in the following:
  • Ensuring consistent water quality;
  • Conserving head loss to a minimum by innovative hydraulic and process design;
  • Engineering treatment units to attain maximum capacity and efficiency;
  • Best use of and most suitable chemical systems for process treatment;
  • How energy should be conserved and managed;
  • How effective the most modern up-to-date control systems can be used as a public utility asset management tool.
The overall development sets a new standard in Irish public utility engineering, demonstrating engineering excellence and best practice both in its design and in its implementation as a multi-faceted complex project. In this way, it was possible to rebuild the largest water treatment plant in Ireland while never interrupting the water supply to its consumers in the greater Dublin region. Credits Client: Dublin City Council and Irish Water (since January 1, 2014) Consultant: RPS Contractors: BAM Civil Ltd. (Civil) and AECOM Design Build Ltd. (M&E) Images: BAM Civil Ltd, AECOM Design Build Ltd. and RPS