Ringsend wastewater treatment plant: a case study from Dublin, Ireland

Ringsend wastewater treatment plant, Dublin, has outstanding achievements in technical design and environmental sustainability to its credit

Executive summary

The project consists of the upgrade of one of the largest wastewater treatment plants (WWTPs) in Europe, treating the effluents of 1.6 Million people and related industries (paper, brewery). The concepts developed integrate a complete and advanced series of treatment steps that can therefore serve as models for WWTPs worldwide. They include innovative treatment and reduced energy consumption and solids production, as well as low environmental impact and emissions. 

Environmental excellence was demonstrated by locating the plant, with a peak capacity of 11.1 m3/s, on a very restricted site with 15 ha of surface area, already partially occupied by existing facilities. In addition, historic ruins and one of the main breeding grounds for Brent Geese were preserved. The facility was integrated into the urban landscape, on a sensitive Oceanside used for daily leisure activities by the residents, with odours contained and treated, and noise and pollution emissions minimised. 

Advanced treatment standards had to be incorporated such as the European Bathing Water Directive and high effluent quality for partial nitrogen removal and disinfection.  The solids production is reduced by thermal hydrolysis and high rate digestion, leading to pathogen free final residuals are that are reused as fertiliser. Energy from solids digestion is converted to electricity, coming covering more than half of the plants needs and enhancing self-sufficiency and environmental sustainability.

To meet such a large array of challenging objectives, both technical and environmental, a series of new technologies were integrated into a new, very innovative treatment line. All those novel process units are the largest of their kind, and their use at this scale and their combination has never been demonstrated previously: 

- for the effluent treatment (of 11 m3/s) - lamella primary settlers, stacked sequencing batch reactors, UV disinfection

- for the solids treatment (of 105 t DS/d) - mechanical high solids thickening, thermal hydrolysis, high rate digesters, energy conversion, thermal drying.      

The Dublin Bay project showed how technical challenges can be solved with simple and reliable technologies. New concepts to enhance energetic self-sufficiency and produce high quality fertiliser were demonstrated, which will enable solids treatment to be more cost effective and environmentally compatible in the future.   

The Dublin bay project

Project background

For centuries the River Liffey and its estuary, Dublin Bay, have been at the heart of the business and recreational life of the city of Dublin. Miles of sandy beaches and swimming areas in the heart of the city distinguish Dublin from other great cities worldwide, and Dubliners and tourists alike enjoy a special relationship with the Bay. They gaze upon it, walk along the shore, play on the beaches, swim, sail, fish, and windsurf. Dublin Bay is considered the city’s finest amenity. 

By the 1990s, however, the Bay’s amenity value was increasingly threatened by deteriorating water quality. Wastewater from homes and businesses in most of Dublin was pumped to the original Ringsend Wastewater Treatment Works (RWWTW) for primary treatment before release directly into Dublin Bay. Wastewater residuals were dumped at sea three times a week, and wastewater from the northern part of the city was discharged untreated into the sea.

However, in the 1990’s, the implementation of the Urban Wastewater Treatment, the Dumping of Sludge at Sea and the Bathing Water Directives in combination with the Dublin Bay Water Quality Management Plan has transformed completely the water quality in the Bay and restored it once again to one of the countries most popular amenities. In so doing Dublin could soon  become one of the few European capitals with a ‘Blue Flag’ beach.

A major component of the transformation of Dublin Bay  has been the development of a new treatment plant to provide biological and tertiary wastewater treatment to approximately 1.7 million people. The new Ringsend Waste Water Treatment Works (RWWTW)  is the largest wastewater treatment project in the history of the Irish State and the first to be awarded on a design-build-operate basis. It is also considered to be one of the most advanced wastewater treatment systems in the world. 

The project objectives

Planning for the Dublin Bay Project began in 1994, and the Dublin Corporation. as the Dublin City Council was then known, had the foresight to plan for the most advanced secondary and tertiary wastewater treatment technologies. With implementation of the Dublin Bay Water Quality Management Plan, the Bay would meet EU standards—and be clean for the first time in more than three centuries. Dublin would also become one of the few European capital cities with “Blue Flag” beaches, suitable for swimming within the greater city limits.

The comprehensive Dublin Bay Project consisted of four distinct elements delivered under separate contracts. Further to a new wastewater treatment plant at Ringsend to replace the original treatment plant, the project also included:

• a temporary solids drying facility to allow solids from the existing works to be disposed of on land rather than sea; 

• a pumping station at Sutton to intercept wastewater from north Dublin and pump it to Ringsend via a submarine pipeline; and 

• a 10.5-km (6.5-mi) submarine pipeline to transfer sewage from the Sutton Pumping Station to the new works at Ringsend.

The temporary solids drying facility commenced operation in 1999; the pumping station began operation in the summer of 2003, and the pipeline was laid under Dublin Bay in the summer of 2001. The new RWWTW began operation in the summer of 2003, just over three years after construction began.

The Ringsend wastewater treatment works

The new works

In October 1999 Dublin Corporation awarded Contract No. 2 of the Dublin Bay Project to the ABA Consortium comprising Ascon Ltd, Black & Veatch (UK) Ltd and Anglian Water.

This contract comprised the design and construction of a new works on the 15-hectare site of the existing works at Ringsend followed by a 20-year operation period. The contract was awarded with a whole of life cost of Euros 250m, and comprises two phases: a design and build stage and a 20-year operation and maintenance period.

As a subsidiary of Black & Veatch (UK) Ltd, Black & Veatch Contracting Ltd undertook the significant role of providing process, mechanical, electrical /ICA engineering design, project management, site installation, commissioning and training services to the project.

Process summary

The new works will be located on the 15-hectare site of the existing works at Ringsend, on the banks of the River Liffey where it flows into Dublin Bay. 

The plant has been designed to accommodate flows and loads relating to the year 2020 design horizon, with a full flow to treatment of 11.1m3/sec and an average biological load of 98,400 kg/d BOD. This approximates to 1.64 million Population Equivalents. The effluent being discharged has to satisfy quality requirements for the Full Flow to Treatment summarised in the Table below.

Sludge produced from the works for disposal to have a minimum dry solids content of 92% and to have pathogen levels based on US EPA Regulation 40 CFR Part 503, Class A Sludge, which allows unrestricted reuse in agriculture.

Parameter95 % -ile Compliance StandardNot to be exceeded Standard

BOD mg/l2550

COD mg/l125250

TSS mg/l3587.5

Amm.N mg/l18.7547

Faecal Coliforms (FC)/100ml100,000(80 % -ile compliance between May and August only)

In summary the new works comprises the following processes:

• Inlet screening of entire flow. 

• Flow splitting and storm bypass. 

• Aerated grit channels for storm flows

• Screw pumps to lift Full Flow to Treatment (FFT) to the aerated Fats, Oil and Grease and Grit removal channels (FOGG)

• Removal of FOGG from FFT 

• Primary Settlement Units for FFT using Lamella Plate Settlers.

• Sequencing Batch Reactors (SBR) for achieving the secondary treatment.

• Disinfection by Ultra - Violet Radiation. 

• Hydrolysis and digestion of sludge. 

• Power generation in CHP plant using enhanced gas production.

• De - watering of sludge 

• Drying of sludge 

• Odour Control. 

• Holding of storm flows and recycle.

The challenges

Constructing a plant of this magnitude and complexity presented the project delivery team with some extraordinary challenges. One such challenge was the location, which was hemmed in by the historical ruins of a fort and an easement which had to be protected and retained in its historical state. 

The existing 25 year old treatment facility, was located in the centre of the site and provided primary treatment for the wastewater flows from Dublin, this facility  had to remain fully operational during the construction of the new works. 

One of the major construction issues with the new works related to the ground conditions. As the site sat on an old landfill, the reclaimed land had very low bearing pressures and required extensive piling to support the new structures. 

At 15 hectares the site is extremely small for a plant of this capacity. A novel and innovative combination of treatment technologies were chosen in order to ‘shoe-horn’ the treatment process into the available area. Compact, simple processes such as lamella plate settling tanks and stacked Sequence Batch Reactors were chosen to achieve the performance goals within the space constraints. 

The design also includes some novel environmental features. To dispose of processed solids on land without incurring high landfill costs, the plant was designed to minimize solids production and achieve Class A biosolids that can be applied to land for agricultural benefit. The treatment scheme includes a thermal hydrolysis plant to increase both solids destruction and the yield of biogas, which is used to generate much of the electricity needed to operate the facility. 

Another special challenge involved relocating a grassland feeding area for the annual visit of the Brent Geese. About 15% of the world population of these birds who breed in the Arctic Canada come to Dublin for the winter. The large areas of grassland around the existing works at Ringsend was a favourite feeding ground for them. A new 2 hectare area adjacent to the existing works was provided to facilitate the winter arrival of these Geese.

Innovative technology and environmental excellence

Because of the challenging constraints, such as the restricted site, and the requirement to keep the existing facilities operational, prohibiting any deterioration in the effluent standard from the works during construction, it was necessary to propose innovative technology processes into the liquid and sludge treatment streams. For the liquid stream lamella plate settlers were chosen for primary treatment followed by Stacked Sequence Batch Reactors (SBRs) for secondary treatment followed by tertiary treatment in the form of Ultra-violet (UV) disinfection.

Inlet works

A maximum flow of 22.6m3/s is transferred to the new inlet works from the existing pump stations and sewers and the new pumping station at Sutton. Each of the inlet flows to the works is measured. The combined flow passes through 6-mm (0.25-in) fine screens. The screenings are dewatered, chopped, and compacted to a solids content of 35% before being discharged into portable waste bins for off-site disposal. 

After screening flows greater than 11.1 m3/sec pass over two fixed-length weirs for storm water treatment. The storm water is degritted before flowing to the new storm storage tanks and, ultimately, the storm sea outfall, whenever the storm flow exceeds the capacity of the storm tanks. The stored storm flow is returned from the storm tanks to the primary lamella plate settling tanks during times of low inlet flow. After screening, the main influent stream gravitates to the inlet screw pump sump in which five Archimedes screw pumps lift the influent to the channel supplying six aerated FOGG removal units. 

Degritted and degreased effluent flows from each FOGG removal unit to a common channel supplying the settling tanks. Fats, oil, and grease are removed from each aerated unit and pumped to the sludge treatment plant for blending with primary and secondary . Grit settles at the bottom of the FOGG units; it is continuously removed and then pumped to one of two grit washer/classifiers for dewatering before being discharged into skips and trucked away. 

Lamella plate settlers

Primary treatment is carried out in two banks of primary lamella settlers. Each bank contains six settling tanks, each fitted with 18 lamella packs. By inserting inclined lamella plates into the primary tanks, the surface area available for settlement can be enhanced by up to a factor of 10 while reducing the plan area of the tank. The lamella settlers have been constructed within the shell of two of the four pre-existing banks of primary settling tanks, thus maximising the use of the existing assets.

Settled sewage from the lamella packs is collected at the packs’ outlet troughs through V-notch weirs and then routed through a common outlet channel, from which it gravitates to the sump of an intermediate pumping station. Solid particles within the influent flow to the lamella primaries sink to the bottom of each tank. They are gathered by a reciprocating scraper and are transported as settled sludge to two sludge hoppers situated at the inlet end of each tank. From these hoppers, primary solids are pumped to one of two sludge holding tanks.

The primary lamella settlers are operated in co-settlement mode; the influent is mixed with surplus activated sludge from the SBRs before it is distributed between the two banks of settlers. Operational data to-date show that the resultant sludge at the bottom of the settlers is drawn off at 2% to 3% dry solids and consists of a blend of primary and surplus activated sludge under average flow conditions

The co-settlement of the primary and secondary solids has yielded various benefits, including an improvement in the amount of primary sludge removed at this stage. In addition, the higher pH of the co-settled sludge (as compared with that of primary sludge) has resulted in less separation of fats, oil, and grease from the primary settled particles and has consequently reduced the level of scum forming at the surface of the lamella settlers. The higher pH has also led to better granular formation of the primary settled particles, which enhances the drying characteristics of the sludge.

Secondary treatment using sequence batch reactors

The RWWTW features one of the largest SBR facilities in the world. Secondary treatment is achieved by 24 SBRs, which are stacked to accommodate the area constraints of the site. The plant has been configured in three SBR blocks, each with upper and lower levels with four reactors on each level. 

Each reactor measures 52 m by 39 m and has a maximum water depth of 7 m .  A two-section intermediate pumping station feeds flow to the SBRs. One section contains four low-lift pumps feeding the lower-level SBRs, and the other section contains four high-lift pumps feeding the upper-level SBRs. The whole biological treatment stage for up to 11 m3/s occupies a surface area of 2.4 hectares.

The SBRs were designed to operate in a full nitrification mode with optional alkalinity recovery or in a conventional carbonaceous (biochemical oxygen demand) removal mode. The treated effluent from both the upper and lower levels is blended to meet the effluent quality requirements of partial nitrification.

The SBRs normally follow a four-step processing cycle—fill, react, settle, and decant—carried out in 6 h, 4 h, 3 h, or 2.4 h increments as determined from the flow to the plant. Each reactor basin is configured with a pre-react zone followed by an aerobic zone, with the air supplied to both zones by a grid system of membrane air diffusers. All reactor basins are equipped to provide adequate aeration to allow nitrification to take place, but only 12 of the 24 reactor basins are required at any given time to achieve the effluent discharge standard.

Aerobic conditions are maintained in each reactor basin by providing air through a system of nine variable-volume delivery centrifugal blowers (eight duty-assist and one standby), air piping, and control valves to submerged aeration membrane diffusers in the basins. 

The blowers supply air to the basins at a rate and pressure that maintain dissolved oxygen concentrations for biological treatment—typically between 2 mg/L and 4 mg/L for the react phase. All reactor basins are equipped to provide adequate air for full nitrification. Surplus activated sludge liquor from the upper- and lower-level SBRs flows by gravity to either the inlet of the primary lamella settlers for co-settlement or to sludge holding tanks for mechanical thickening.

Tertiary treatment using ultra-violet (UV) disinfection

Effluent is removed from each reactor basin at the end of the settlement phase by four weir-trough decanters connected to a common collection manifold, and then combined in a common flow channel prior to UV disinfection.  

The UV disinfection stage is designed to achieve a faecal coliform count in the final effluent of less than 100,000 CFU (Coliform Units)/100 mL on an 80 percentile basis from May through August each year. The plant is designed to disinfect an effluent flow of 11m3/sec at a minimum UV dose of 134 J/m2 with a UV transmittance of 65%. 

The flow is distributed between five channels (operated on a four-duty, one-standby basis) approximately 3.4 m wide decreasing to 2.9 m wide by 12.9m long by 1.9 m deep. A total of 990 UV lamps are installed.

Following disinfection, the final effluent flows to the existing sea outfall culvert through a flow-measuring flume. 

Sludge treatment

A complete solids treatment facility has been installed at the works capable of processing an average of 105 metric tonnes dry solids/d (115 short tons/d). Co-settled primary and surplus activated sludge, scum liquors, and digester drain down are held in two holding tanks prior to processing. From these tanks, the solids and liquor are screened and pumped to one of two solids buffer tanks for storage. Screened solids are dosed with polymer before being fed to five belt presses for thickening and dewatering. 

The dewatered solids cake, with a dry solids content of 18%, is then subjected to thermal hydrolysis treatment with the world’s largest installation of such technology for municipal solids. This process subjects residuals to pressure and temperature extremes to produce a pasteurized biosolids product and the break-up of solids for better digestion.

Two parallel solids hydrolysis streams are provided, and the solids cake is pre-heated and homogenized before being subjected to temperatures of 165 to 190˚C and pressures of 6 to 8 bar by steam injection and mixing. The hydrolyzed sludge is then depressurized and cooled to 40˚C for mesophilic digestion.

Three mesophilic digesters operate at a temperature of 40˚C with a solids-retention time of 15 days under normal sludge loading. The thermal hydrolysis increases the digestibility of the solids and, in proportion, the biogas production. The biogas produced is drawn from the top of the digesters and stored in a common gas holder with a capacity of 5,000 m3. The biogas can be used to fire the auxiliary steam boilers and to power the four combined heat and power units. Solids digestion generates over 60% of the energy (approx. 4 MW) required to run the plant under average flow and load conditions.

Digested solids flow under gravity from each of the digesters to the digested solids buffer tanks. The solids are dewatered and then dried by three dryers, each with an evaporation rate of 4 metric tonnes/h (4.4 short tons/h). The biosolids product—in a granulated form free of pathogens and vectors, with a minimum dry solids content of 92%—is applied to agricultural land.

The sludge treatment process detailed above was selected on the basis that:

• The quantity of biosolids leaving the works would be minimal

• A Class A pathogen free sludge would leave the works suitable for application to agricultural land.

• The maximum quantity of Biogas would be extracted from the Biosolids to enhance the energy efficiency of the works.

Odour control

Given the location of the works on the mouth of the Dublin Bay, the control of odour was a significant consideration in the design of the new works.  Odour treatment is provided for the inlet works and the sludge storage and treatment areas. Scrubber towers are used to treat odours from the inlet works and solids treatment area. The towers contain a fixed-bed catalyst through which diluted hypochlorite and caustic solutions are circulated. Odours from the solids hydrolysis process are treated by passage through duty/standby thermal oxidizing units.

Value and benefits

At a capital value of approx. Euros 156m (or less than 400 Euros/ (m3/d) of average flow),  the new treatment works at Ringsend together with the other projects which collectively comprise the Dublin Bay Project has already demonstrated significant environmental benefits to the Dublin Bay region and its environs. These benefits include:

• Untreated sewage is no longer discharged offshore.

• Water quality in the Bay has already improved and will meet the high standards of the Dublin Bay Water Quality Management Plan

• Dublin’s main beaches are in line to meet EU and National Bathing Water Standards. Other beaches in the Bay will also benefit, as Dublin Bay has now achieved Blue Flag status

• The dumping of sludge at sea has stopped. Sludge is now thermally dried and pasteurized, turning it into an organic-based fertilizer suitable for spreading on tillage and grass.

• Energy recovery from Bio-solids to the extent of 4MW of electricity being produced from Bio-gas combustion.

• Construction of the facility did allow to preservr historic sites and breeding grounds for geese

• By integrating the plant into a compact site, and minimising emissions, the leisure value of the Oceanside was maintained.

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