Drogheda Wastewater Treatment Plant

The Drogheda Wastewater Treatment Plant (WWTP) was designed for a capacity of 101,600 PE (Population Equivalent) to serve the greater Drogheda area and parts of East Meath.

An extensive programme of works at the Drogheda Wastewater Treatment Plant and the local wastewater network is underway. A significant amount of work has already been completed including major works to the odour control units, the three primary settlement tanks and two aeration tanks at the treatment plant.

We are continuing to assess additional capital maintenance works over and above the current project works and, if deemed appropriate, will seek the allocation of appropriate investment and resources for this work.

Current programme of works underway

Uisce Éireann is delivering an extensive programme of upgrade works at the plant and on the network.. Works already completed include - Operational works to odour control units; operational maintenance works to three primary settlement tanks and two aeration tanks.

Covering of Primary Settlement Tanks and On-going Technical Assessments (further investment needs and plans)

We are actively working on a capital project (with investment allocated) to cover the Primary Settlement Tanks and to provide new odour control units to treat extracted odours from this location. Design and procurement for these works is underway, and we are working with our suppliers to seek to ensure timely manufacture and installation. The timeframe for installation of the primary settlement tank covers and associated ancillary works will be subject to on-going review as the project progresses but is currently estimated to be within the following time period: the end of Q2 2026 to the end of Q4 2027*.

*Note - end of Q2 2026 is currently being best case estimation.  As is always the case with any significant capital works there is a possibility that complexities arise that could impact that estimate.

We will continue to actively review operational data from Drogheda Wastewater Treatment Plant and other similar sites in Ireland (and internationally) in order to seek to define and evaluate, appropriate, proportionate and sustainable additional mitigation measures for the site, which might be considered by Uisce Éireann.

Upgrades to the Sludge Farm and Anaerobic digesters

Uisce Éireann is progressing with plans to replace the anaerobic digesters on a phased basis over the next number of years while the plant remains operational. To safely facilitate these works the digestion process has been switched off, and the digestion of sludge on-site has therefore ceased pending replacement of the digesters.

The replacement of the anaerobic digesters at the site represents a significant investment by Uisce Éireann in the continued upgrade of the plant. The capital upgrade works on these digesters are advancing on site presently and include replacement of 1 no. anaerobic digester under the current programme. While the digester construction is now completed, it won’t be brought back into operation until the rest of the sludge farm upgrades have been completed.

Anaerobic Digester no. 1 project update

Assessments of the existing equipment – that interact with the digester - is ongoing, as part of the process to seek to bring the now completed Digester No.1 back into service. Concerns have been raised particularly in relation to existing pipework and with the control systems. Preliminary findings indicated that we could not safely bring Digester 1 back into service at this time.

Following further assessments of the existing equipment at the sludge farm, we now understand that the sludge farm in its entirety needs to be upgraded prior to Digester No.1 being brought back into service. This was unanticipated at the outset of the project.  We have undertaken a workshop 2 to ascertain the available options. We are now in the process of scheduling the detail design, planning (if required) and construction durations. The outcome thus far is that we have an increased volume of Construction works now thought to be required, and the associated assessments, expanded design, detail design, planning and pre-construction work required has resulted in the indicative dates now showing the recently replaced digester and upgraded sludge farm being back into operation in Q1 2029. 

In addition to the progress on the above, we have brought forward the Centrifuge procurement and are currently procuring new centrifuges which include integrated Odour extraction. However, the installation of these centrifuges is now part of the overall project construction works which will commence in 2026. We have also carried out an assessment of all the Odour control units onsite and a workshop to review options for their replacement is currently scheduled for May / June 2024.

Uisce Éireann have also completed a review of the rest of sludge farm for the site, in order to identify any further works that may be deemed necessary and we are progressing with procurement and installation of new and advanced (technology) centrifuges for dewatering of sludge which have integrated odour extraction. We are also advancing with sludge farm odour abatement unit works on the site.  It is anticipated that all works to the sludge farm will be completed by Q1 2029.

On-going construction works are not anticipated to be disruptive as they will take place at the plant between 9am and 5pm. There is a possibility that some works may cause unavoidable and inevitable temporary odours, but would be very limited and of short duration.  We will seek to notify impacted customers in advance of such works happening via our web page (news updates).

We will continue to provide on-going progress updates via our website on works underway and those which might be planned for the future.

Works on the Network

Given the extent and distance of the sewer network in Drogheda, wastewater can potentially be held in the sewer for a long period of time before arriving at the WWTP for treatment (particularly during the summer season). To reduce potential odours from the sewer network the following measures are being implemented:

Uisce Éireann have in recent years commenced with temporary/ seasonal application of an odour neutralising treatment at the East Meath pumping station and on incoming flow at the WWTP itself. Seasonal odour neutralising treatment will continue to be applied at the above pumping stations over the 2024 summer season.

Network jetting continues to be  undertaken  in Drogheda to clear general blockages in the network that could give rise to odours and Uisce Éireann have also completed works to seal manhole covers at a number of specific locations in the network, to seek to prevent the release of any odours that might arise.

Other potential causes of Odour

It is important to note that odours can arise for a variety of reasons, such as lack of rainfall, which impacts the dilution effect in a combined sewer network; temperature, which can naturally give rise to odour issues; and the extent/condition of industrial loading into the sewer network together with increased agricultural activity in the area.

For more information on the importance of not flushing inappropriate items down the toilet please see our Think Before You Flush page.

Independent Odour assessments

In Q2 of 2021 Uisce Éireann commissioned an independent odour report for the Drogheda WWTP, the results of which are available below.

This odour modelling report is detailed and technical, but it identifies that the most significant potential odour source at the WWTP are the Primary settlement tanks, with a number of other less significant sources. These odour sources contribute to 61% of the overall odour load to atmosphere from the existing facility.

From a follow up modelling exercise the odour plume spread associated with night-time meteorological conditions is also predicted to result in greater plume extent and impact area in the vicinity of the WWTP in comparison with daytime meteorological conditions.

Uisce Éireann now has investment in place and a capital project underway to address the covering of the primary settlement tanks.  

A link to the primary modelling report can be found by visiting the following links. Due to its size the report is divided into 3 parts:

Odour reports

Odours can arise for a variety of reasons-  environmental factors such as rainfall can impact the dilution effect in a combined sewer network, estuary, low tides or ectocarpus (a type of algae) and seasonal warm temperatures can naturally give rise to odour issues; local conditions such as non-domestic and industry sources and increased agricultural activity can also lead to odours; and actions such as illegal dumping of waste into the sewer network and flushing inappropriate items down the toilet can cause blockages in domestic and public pipes (resulting in odours).

Uisce Éireann is committed to investigating any odour complaints in relation to the Drogheda Wastewater Treatment Plant (WWTP) as and when reported.

Reporting an odour

Customers should continue to report any issues directly to Uisce Éireann’s call centre which operates 24/7 on 1800 278 278 or by email to operations@water.ie.

Customers can view our Complaint Handling Code of Practice (COP) for more information.

Description of Wastewater Treatment Plant processes at Drogheda:

Stage 1

Preliminary Treatment:
The enclosed building receives of flows from Newtown pumping station, East Meath, Bryanstown pumping station, supernatant pumping station, scum chamber pumping station and storm water tanks return flow.

This stage removes materials from the raw wastewater that could damage our equipment. This screening involves removing: paper and plastics; grit removal; flow balancing; fat and grease removal. Air is extracted from the building and treated in an odour control unit prior to release to the atmosphere.

Primary Settlement: 
There are 3 primary settlement tanks on site, only 2 tanks are required for normal service. These tanks except for the outer V-notch weirs, which are covered, and air from this element is extracted for treatment in an odour control unit.  These tanks accept screened and degritted sewage. 

In this stage wastewater flows through large primary sedimentation tanks with mechanical scrapers. These tanks use gravity to settle the solids. The solids are then transferred to sludge treatment facilities.

Stage 2 

Secondary Treatment - Aeration Stage:
There are 6 aeration tanks on-site, each with an anoxic phase for nitrogen removal. 

This stage adds large amounts of air to degrade the biological content of the wastewater. Most wastewater treatment plants treat the settled wastewater liquor using aerobic processes.

Aeration Tanks:
To meet discharge licence conditions we reduce the levels of nutrients, such as nitrate and phosphorus. We do this through biological processes in the secondary and final settlement stages.

Floc Formation:
We use bacteria and protozoa to consume soluble contaminants like sugars, fats and organic short-chain carbon molecules. A lot of the less soluble fractions are bound into clumps of fine particles that we call floc

Final Settlement: 
There are 3 final settlement tanks. Following a suitable holding time in the aeration tanks, the wastewater (termed ‘mixed liquor’ at this stage) passes forward to the final settlement tanks for separation of the biomass from the cleaner, treated water. 

The wastewater is now passed through final settlement tanks. These tanks use gravity settlement and mechanical scrapers to remove the floc. The wastewater is now passed through final settlement tanks. These tanks use gravity settlement and mechanical scrapers to remove the floc.

Control Building and Site Laboratory

Quality Control:
Our quality control measures include regular laboratory testing of the final treated wastewater. This is to ensure the cleaned water has no detrimental effects on aquatic life or other water users.

Sludge Farm:
The area known as sludge farm treats the waste removed from the treatment process. The sludge farm includes 2 anaerobic digesters, 2 gas holders, sludge thickeners, a blend tank, sludge building including centrifuges, cake storage skips and sludge storage tanks. 

The Sludge Building:
The control centre for the sludge farm.  It also houses 2 centrifuges and sludge pumps. 

Primary Picket Fence Thickener Tanks (PFTs):
Separate sludge picket fence thickener (PFT) tanks are provided for each type of sludge.  The PFT thickens the sludge. 

Sludge Tickner’s/ Blending Tanks:
Primary and final sludges are blended in this tank 

Following thickening, sludge is pumped forward to the blending tank for mixing to ensure a homogeneous mixture is pumped on to the anaerobic digestion plant.

Anaerobic Digesters:
Anaerobic digestion works by feeding the sludge into a heated enclosed reaction tank where naturally occurring anaerobic bacteria degrade the organic material. The end product is biogas and stabilised sludge. The purpose of anaerobic digestion is to convert bulky, odorous raw sludge to a relatively inert material that can be rapidly dewatered with the absence of odours and reduce the volume of sludge for processing.  

Cake Storage Skips:
Dewatered sludge (termed sludge ‘cake’) is then lime stabilised and transferred to the cake storage skips pending appropriate disposal off site.

Gas Holders:
The biogas is stored in the gas holders and can then be converted to both electricity and heat using combined heat and power (CHP) units. 

Sludge Storage Tanks:
Digested sludge is stored in these tanks in advance of being centrifuged. 

two odour control units are provided. One serves the centrifuges and the second serves the PFTs, Blending Tank and the Sludge Storage Tanks.

Storm Water Holding Tanks: 
During adverse wet weather, where the incoming flow exceeds the hydraulic capacity of the plant, dilute screened storm water will be diverted to the storm tanks for settlement prior to return to the plant for full treatment or release to receiving waters. Cleaning of these tanks occurs, via tipping buckets after each storm event.