Supply Chain - Wastewater Treatment - Sludge Handling & Treatment

Under the new consents Macclesfield WwTW is required to serve a 2035 population equivalent of 82,679 with a final effluent quality of 0.3 mg/l Total Phosphorus (TP), tighter ammonia consents of 2mg/l ammonia on a 95th %-ile basis, and 12 mg/l ammonia as an upper tier limit. The works must also provide UMON_3 spill and UMON_4 flow to full treatment monitoring.

The works has a discharge permit with a flow to full treatment limit of 61,430 m3/day and a dry weather flow limit of 28,500 m3/day.

The area for the inlet works had limited working area space. To minimise the interface with existing live plant at the inlet, the team chose to use an off-site manufactured precast construction approach for a majority of the new inlet works. The precast design incorporated 218 precast panels and two precast raker beams. Utilising this construction technique, with design for manufacture and assembly (DfMA), had several advantages including:

The delivery strategy involved 79 shipments direct from Shay Murtagh Precast in Ireland, as opposed to a minimum of at least 250 concrete wagons. This resulted in substantial carbon savings in transportation of rebar and concrete; approximately 50% reduction in embodied carbon, along with minimised local nuisance and disturbance and reduced material wastage.

At the time of writing (July 2024) the precast inlet has passed its water test and mechanical and electrical installation is in progress.

In addition, a new interception chamber has been constructed to collect flows from Macclesfield and Bollington Sewers alongside three new coarse screens and screen management within the precast inlet. Four new fine screens, a new grit removal stage situated within the precast inlet, storm flow to full treatment (FtFT) separation, and MON_3 and MON_4 monitoring are also included.

A new wet well submersible primary treatment feed pumping station has been constructed which comprises four pumps to transfer FtFT plus returns to a new distribution chamber, which splits flows to the north and south primary settlement tank groups.

Three new launder pumps have been installed to transfer water to enable cleaning of the launder trough. There is a new below-ground culvert to connect storm water overflow to the existing storm tank feed channel. Drainage from Pump Station 2 and the admin building has been diverted into the new inlet works. Furthermore, there is a new MCC kiosk at the new inlet works. The site solution will reuse the existing storm tanks.

The inlet works precast concrete approach resulted in substantial carbon savings in transportation of rebar and concrete. There was a reduction of approximately 50% in embodied carbon, along with minimised local nuisance and disturbance. There was a 42% reduction in time to construct against the original in situ plan and reduced material wastage.

The secondary treatment area solution includes a new final settlement tank distribution chamber, replacing the half bridge scrapers and reusing the existing final settlement tanks (FST). Each FST features new dedicated return activated sludge (RAS) and surplus activated sludge (SAS) pumps and new scum pipework.

Mobile Organic Biofilm (MOB)TM technology has been constructed in the secondary treatment area as an innovative and efficient solution. The media is a first for the wastewater industry’s (being a green, renewable plant-based media with a high surface area) and this is now on site, with the test plant running with parameters in place to measure settleability.

The MOBTM plant has been configured to allow future adaptation to an integrated fixed film activated sludge (IFAS) system including overall sizing, zoning and aspect ratio of the cells as future-proofing.

The MOBTM technology will be able to achieve nitrification and enhanced biological phosphorus removal (EBPR). The solution intends to improve settleability, help treatment capacity, provide nutrient removal, and enhance process stability.

There will be new pipework connecting the existing primary settlement tanks (PSTs) to the new MOBTM plant and a new wet well submersible MOBTM feed pumping station. There will be five blowers from Aerzen Machines to provide air demand from the MOBTM plant, two drum screens on the SAS line to recover MOBTM media, and six mixed liquor recycle pumping stations.

Whilst the civils work is complete, the mechanical and electrical installation is ongoing. The blowers have arrived on site and the secondary treatment area MCC kiosk has been delivered.

The tertiary treatment solution utilises Mecana tertiary pile cloth filters (TPCFs) for solids removal and these have arrived on site. The sludge handling system features:

There are new thickened sludge transfer pumps, a new roof on an existing sludge tank and reuse for thickened sludge storage, and a new odour control system for the surplus activated sludge.

In addition, there is a pump mixer on the thickened sludge tank, and a new wet well submersible liquor returns pumping station comprising fixed speed pumps to return filtrate from the rotary drum thickeners and the Mecana tertiary pile cloth filters.

Finally, there is a new final effluent wash water pumping station comprising a packaged booster set inside a new kiosk and a new externally located buffer storage tank and automatic backwashing filter will be installed at the pumping station.

Whilst defining the solution for Macclesfield WwTW, it was identified through value engineering that the final settlement tanks, the existing sludge tank, and the storm tanks could be reused, and the requirement for additional primary thickened sludge storage could be removed.

The MOBTM technology eliminates the need for internal sieves and additional main flow screening. There is no micro-plastic generation and therefore less environmental risk, and improved settlement of sludge.

Further value engineering opportunities included:

For such a large scale, complex project with three working areas on site, there have been health and safety challenges and without collaboration from the outset, the project would have not been a success. Despite there being multiple sub-contractors on site at the same time, limited working areas, working on a live operational site, operations interface, and stakeholder interface, the project team have worked seamlessly together to meet the project’s requirements.

The next stages of the project include connecting the power upgrade to all three sections of site, seeding flows through the MOBTM plant, and diverting the flows through the new inlet and integrated treatment works.

Site-wide software installation, commissioning end to end testing, and optimisation of the solution follow, in order to meet the December 2024 regulatory commitment date.

The existing works comprised an inlet works (single screen and grit vortex), a primary settlement tank, four biofilters, one final settlement tank, final effluent monitoring, a storm tank and returns, and two sludge holding tanks.

The driver for the new works is a tightening in the Total Phosphorus (TP) and ammonia consent that came into force December 2024.

The existing works is a fully pumped works with two catchment pumping stations (from Akeferry and Westwoodside) pumping to the inlet. The process of the new works starts off with primary treatment, where two screens from Huber Technology and a grit vortex from SPIRAC Ltd capture and remove inorganic materials (e.g. wet wipes and rocks) to be taken to landfill. After preliminary treatment, the flow enters a balancing tank to maintain flow to full treatment (FFT) during higher flows, preventing premature storm spills.

The secondary treatment consists of a pocket UCT ASP (University of Cape Town activated sludge process), 37.5m (length) x 11m (width) x 6m (height), two 8.95m internal diameter final settlement tanks (286.6m3 total volume) and a RAS return pump station, with integrated SAS thickening facility that leads to the sludge storage tank.

The ASP consists of two anaerobic pockets (total volume 115m3) to allow the site to utilise biological phosphorus removal, where the phosphorus removing organisms have a competitive advantage, due to there being no free oxygen for other organisms to use.

These pockets then flow into two anoxic zones (total volume 115m3) in which free oxygen is available for denitrification purposes. The flow then enters three aerated pockets (total volume 1,513m3) with tapered airflow, reducing in the percentage of supplied oxygen in each subsequent pocket - this is to reduce the amount of dissolved oxygen and nitrates recycled back to the anoxic zone in the form of RAS.

As this is a UCT configurated ASP, there is also a fixed speed denitrified mixed liquor pump, recycling anoxic effluent back to the anaerobic feed, this is to maintain anaerobic conditions within the anaerobic pockets.

Ferric sulphate is dosed into the final settlement tank distribution chamber (to act as a top-up dose in case EBPR is not consistently meeting the 1 mg/l target of total phosphorus off the ASP). The flow then splits equally to the two final settlement tanks which have half-bridge scrapers. The sludge is drawn off by the return activated sludge (RAS) return pump station that feeds into the start of the ASP to maintain the cyclical process.

A secondary dose of ferric is supplied to the Mecana pile cloth filter (pre-TSR dose) from Eliquo Hydrok Ltd. This allows precipitation of soluble phosphorus, to further polish both the suspended solids and total phosphorus to meet the low 0.2 mg/l consent. The flow is sampled and monitored at the final effluent kiosk, before discharging to the outfall.

The surplus activated sludge (SAS) is directed towards sludge handling facilities which include a Huber S-DISC sludge thickener and polymer dosing equipment. The thickened sludge will then be held in a new sludge holding tank to be taken from site for use at other Severn Trent treatment works.

The full scope of the works is detailed below:

Following a cost benefit analysis carried out between Severn Trent and MMB in 2022, the preferred solution was to construct a crude activated sludge plant (ASP) with enhanced biological phosphorus removal (EBPR).

There were several challenges that the MMB design team and site team were able to overcome by working closely together.

Sub-artesian water pressure

During ground investigation it was identified that the site had high groundwater level with sub-artesian water pressure. Since the process is gravitated after the inlet works, the depths of the structures in the outline design would have been significantly impacted by sub-artesian water, requiring significant temporary works including groundwater abstraction. The design team was able to overcome this issue by interrogating the hydraulic calculations and highlighting the opportunity with Severn Trent to raise the inlet works and the ASP structure up, with the bases of the two FSTs rising to ground level.

Because of this, the team were able to avoid the sub-artesian water pressure and reduce the groundwater impact during excavation and construction, significantly reducing construction health and safety risks, and realising environmental and commercial benefits.

Simplifying the TSR design

The original TSR design had a large bunded area that required a deep excavation and the use of more materials. Early on, the design team was able to discuss options with the supplier and other sites that had installed the same technology.

The option to part-bury the tanks enabled the removal of the large, below-ground bunded design, simplifying it to a single rectangular slab that the tanks would sit on and be backfilled around. This did not affect the hydraulics or the process and reduced the total excavation required and concrete usage which in turn, reduced the embodied carbon of the solution.

Inlet works location

A review of the construction programme and site layout highlighted an opportunity to construct the inlet works earlier whilst keeping access to the ASP and MCC area. The outline design had the new inlet works to the north of the existing inlet works. This would have prevented the MCC from being constructed as the crane would be located on top of it to service the original position.

However, during a design sprint with Severn Trent, the new inlet works was moved to the south of the existing inlet works. Whilst this meant it was located over an area with several services - since the level had been raised - the slab now sat above all the services in the area and would not clash. The site team was able to locate and prove the existing services, then excavate and construct the new slab and pipework.

Site-drainage pumping station

A review by the MMB design team of the requirements for the new site-drainage pumping station found that the existing pumping station had sufficient space, volume and was in suitable condition to take the new flows and pumps that were required for the new works. This saved programme, time, cost and risk in having to construct a new site returns pumping station in an area heavily congested with live services including the main incomer to site, as well as remove any interface with the sub-artesian water.

Design efficiencies

There were several design efficiencies that were realised during the detail design phase including:

BIM360

Several benefits were gained during the project with the close communication between the site and design teams through regular site visits, weekly meetings and the use of Issues on BIM360, which helped to streamline the production of drawings.

Reviews by the relevant design disciplines and the site team could be tracked, and buildability checks could be raised in a timely manner. This also helped to improve design decisions having the early site involvement and helped to reduce redesign. The great construction and design collaboration helped to identify opportunities to enable services to either have a reduced dig depth or be constructed out of the ground, further reducing the ground water interface. One example being the FST outlet pipework being kept above ground to the collection chamber, which removed all excavation requirements.

ASP maturation

Significant programme gain came from challenging the ASP maturation period. The initial programme had 28-weeks for maturation, however, regular meetings between process engineers from both MMB and Severn Trent were held. This led to the development of a joint commissioning plan whereby hold points and process reviews could be used to trigger the next stage of the commissioning works.

Instead of waiting for the Severn Trent standard commissioning duration, if the target was achieved sooner, a joint review would be carried out and the next stage could start. This led to the maturation period of the ASP to be reduced to 12-weeks.

Water voles

Water voles impacted works on the installation of a new culvert for crossing the drainage ditch to which the site discharged.

MMB ecologists were able to carry out a survey of the occupational drainage ditch and highlight the areas where water vole habitats were located. A review carried out by both the design team and site team, found that by moving the culvert approximately 20m further north, it would still provide a suitable crossing point to site, whilst having no impact on the water voles or habitats.

An innovation that was used at Haxey STW during the construction of the ASP were converge concrete sensors. When constructing the concrete activated sludge plant, the team identified an opportunity to trial sensors to measure the concrete strength. The structure consisted of 6m tall in situ concrete walls; the perfect set up to trial the sensors.

Initially the team were using a PERI shutter system and part of the temporary works states that the shutters cannot be struck until the concrete has reached 5N/mm2 compressive cube strength. With the ASP on the critical path of the programme, it was imperative that the walls were poured as efficiently as possible, striking the shutters as soon as possible, then moving onto the next pour.

The concrete sensors were cast into the walls and proved extremely effective as they were able to read the temperature of the concrete, convert it to compressive strength and notify the team when the shutters could be stripped. This resulted in an efficient process between each pour, streamlining the pour schedule and reducing the reliance on concrete cubes.

Using the sensors, the team were able to save one day per section of wall poured, removing 11 days from the programme. Both time and costs were reduced due to not having to prepare 45 cubes for early strength testing, saving over £700 in engineer time and cube crushing.

The key benefits of the sensors are:

The main construction phase was completed by December 2024 with the commissioning phase subsequently starting after.

The project has been successful by the close communication between the client, site team, design team and operations. By having regular meetings throughout the week enabled both sides to raise and questions/concerns, ideas to be discussed and decisions made in a timely manner.

With flows switched to the new works, the existing works is to be decommissioned and abandoned.

Once the expansion is completed, Newthorpe flows and Heanor flows will have separate preliminary and primary treatment streams, before combining in the secondary treatment stage.

Flows from the Newthorpe catchment will arrive at the existing inlet works. This area will stay as per the current arrangement and only the mechanical equipment will be upgraded. The flows are lifted using Archimedes screw pumps to a raised inlet works structure.

This structure holds the fine screens, which are designed to capture at least 80% of all the solids above 6mm. After the screens, a detritor will settle all grit and inorganics such as sand; removing them from the process stream to avoid damage to the mechanical equipment.

The wastewater from the catchment then flows to the primary treatment area, where the first physical separation of particles using gravity to settle the fines at the bottom of the two existing primary settlement tanks is carried out.

The effluent from the primary settlement tanks, that is currently connected to a secondary treatment consisting of trickling filters and hummus tanks, will now be diverted to the new works’ expansion.

Flows from the Heanor catchment area that arrive at Heanor will be pumped 2.5km to a newly build inlet works at Newthorpe. Similar to the existing inlet works, the new inlet works will feature 2D 6mm screens provided by Longwood Engineering Company Ltd and a 4.5m detritor provided by Tuke & Bell Ltd.

After the detritor, the flows merge, without having been settled, with the Newthorpe flows. Keeping the Heanor flows unsettled is key to provide the right load for the process to work.

The main biological treatment happens in this secondary stage, where the aim is to reduce to the consented levels certain parameters such as Phosphorus, ammonia, iron and suspended solids. The process selected for Newthorpe is an enhanced bio phosphorus removal process in a modified University of Cape Town (UCT) configuration.

The first stage in the process is the anaerobic zone, where the polyphosphate-accumulating organisms (PAOs) thrive in an environment with no oxygen nor nitrogen introduced. In this stage, the PAOs accumulate volatile fat acids which will help them being biologically competitive in the aerobic phase of the treatment.

The next stage is the anoxic zone, where recirculated active sludge (RAS) from the secondary settlement process is introduced.

This RAS recirculates the bacteria through the system as well as introduces nitrogen oxides which have been created in the aerobic section by the oxidation of ammonia (NH4). In this anoxic zone, the nitrogen oxides are transformed into nitrogen gas which escapes to the atmosphere.

The third stage of the process is the aerobic zone in which air is introduced. Under these conditions, three main processes concur:

The final stage of the secondary treatment is the secondary sedimentation on the final settlement tanks, where the rich-P sludge is removed from the system and either recirculated to the anoxic zone; or thickened and wasted to produce other products such as gas and cake on sludge digestion processes.

The final stage of the process aims to further reduce the P levels in the effluent by capturing non-organic phosphorus.

Ferric sulphate is dosed upstream of the tertiary solids removal unit to cluster the particles into larger floccules that are then removed by a physical filtration system. The system selected in Newthorpe is FilterClear (mixed media filtration), designed and manufactured by Bluewater Bio Ltd. Once commissioned it will be the largest FilterClear system in wastewater treatment.

MMB and Severn Trent will be working together during the commissioning of the plant to understand the emissions of greenhouse gases in sewage treatment works and how to optimise the plan to reduce them. Instrumentation to measure NO and N2O will be installed on the ASP.

Nitrous oxide is produced during the biological removal of ammonia in nitrifying ASPs. Monitoring nitrous oxide emissions (along with nitrite) during commissioning at Newthorpe will not only provide STW with a baseline process emissions factor (EF) for the site but also with valuable insights into the mechanisms resulting in nitrous oxide production.

At Heanor, the main structure, which is the new transfer and storm pumping station structure, is currently in its final stages.

The site will be operational in its final configuration by December 2024 to meet the regulatory requirements.

The 2.5km 450mm dual rising main connecting Heanor STW to Newthorpe STW is now completed by with only the pressure testing and its final connection to Newthorpe STW pending. The rising main will be ready to transfer flows in October 2024, to aid with the EBPR maturation process ahead of the regulatory commitment.

The route of the rising main crossed several historical mine sites. MMB’s geotechnical team conducted an investigation to understand the status and position of the mine entries and any other mine-related features along the route. Additionally, a watching brief was in place to quickly identify any non-natural ground features.

Thanks to this effort all the risks were mitigated, and the rising main was safely constructed by Johnston Trenchless Solutions.

At Newthorpe, all major civil structures are completed, and on the site, a major operation of backfilling, electrical and mechanical installation is currently ongoing.

In 2024, the Newthorpe STW expansion project has been awarded two Green Apple Awards in the categories of Building & Construction and Sustainable Water Management. The project also won the first Ground Engineering Awards for MMB for the UK Project with a Geotechnical Value of between £500k and £1m.

Newthorpe STW and Heanor Milnhay STW have been merged in one of the major AMP7 Severn Trent projects, with Mott MacDonald Bentley as principal designer and contractor. The expansion to Newthorpe STW will be one of the largest enhanced bio-phosphorus treatment sites in the region covering a population equivalent of 100,000 and will achieve a tightened permit of 0.2 mg/l, whilst minimising the use of chemicals to remove phosphorus.

The project is currently on track to achieve the biological maturation of the EBPR by its regulatory date in December 2024.

The driver behind the latest upgrades and growth to Perth STC stems from a greenfield development project called Bertha Park which will see the construction of more than 3,000 new homes and employment land within the Perth & Kinross Council area. This new development will see the works population equivalent (PE) rise from c.70,000 to c.105,000.

In parallel with the Perth (Bertha Park) WwTW project, the Perth IFAS project is being undertaken by a separate delivery team. This will see the installation of integrated fixed-film activated sludge (IFAS) media into the activated sludge lanes. This project aims to add an IFAS stage to the two combined activated sludge (CAS) lanes at Perth WwTW, converting the final two-thirds of each lane.

The work involves new civil construction, installation of pipework, valves, instrumentation and control systems. New fine bubble diffusers and additional blowers will be installed to meet air requirements, and IFAS media will be added to support growth until 2033.

Perth WwTW treats sewage from Perth and the surrounding areas. Influent sewage gravitates to the WwTW. Sewage exceeding 570 l/s is diverted to storm storage, while flows under 570 l/s are screened and go through grit removal prior to passing through four primary settlement tanks (PSTs). Flows over 1,100 l/s are spilled at an inlet overflow.

Primary settled sewage then enters a selector tank and is split between two activated sludge plants; the fine bubble diffused air (FBDA) plant, and the surface aeration plant. Mixed liquor from these plants goes to three final settlement tanks before discharge into the River Tay.

Return activated sludge (RAS) is recycled back to the selector tank, and surplus activated sludge (SAS) is co-settled in the primary settlement tanks. Co-settled primary sludge is stored in a sludge storage tank, which also receives imported sludge. The sludge is processed by Veolia who dewater it and transport the sludge cake off-site for disposal.

This screenshot below from ESD’s 3D model highlights the key areas being upgraded within the scope Bertha Park. The blue area highlights the inlet works and PST upgrades, yellow shows the aeration upgrades, red covers the new sludge dewatering area, purple details the new sludge reception No. 2 area, and the orange area is the FSTs and distribution chamber.

Inlet works (blue)

The inlet works are undergoing extensive repairs and upgrades to numerous areas. Inlet screens 1 and 2 will be replaced with new screens, compactors and launder channels, and a new vehicle turning area will be constructed for skip replacement.

Three new actuators are being installed for the inlet and outlet penstocks associated with screens 1 and 2, and all nine penstocks, including handwheels and frames at the inlet channel and screens, will be refurbished through shot blasting, lubrication and painting. Handrailing and toe boards for the inlet works, storm tanks and PST will be replaced, and two storm tank inlet valves will be actuated.

A new motor control centre (MCC) is also being installed to aid with the upgrades of the inlet works.

Aeration works (yellow)

The IFAS project was completed in June 2024. ESD’s scope is as follows:

In aeration lanes 1, 2 and 3, nine inlet penstocks (three per lane), three outlet penstocks (one per lane), and two cross-connection penstocks will be replaced. Adjustable outlet weirs will be replaced with fixed weir boxes and scum boards.

Six 11kW surface aerators will be upgraded to 15kW, and the MCC5 surface aeration cone feeder compartments will be updated to handle the increased motor kW. Cabling for the upgraded aerators will also be enhanced and electromagnetic current (EMC) filters will be installed within the MCC starter compartments to comply with specifications.

Final settlement tanks 1-3 (existing), distribution chamber and FST4 (new) and modifications to distribution pipework (orange)

In this area, work is also being carried out on the existing final settlement tanks (FSTs). FSTs 1, 2 and 3 will be upgraded with new v-notch weirs, replacement of existing drain-down penstocks, sludge plug valves, drain-down valves, and hydrostatic bellmouths.

Additionally, a fourth FST will be constructed, featuring a new scraper bridge, actuated hydraulic bellmouth, penstock and valves, as well as a connection to the final effluent (FE) outfall chamber.

Sludge reception No.2 (purple)

A new sludge reception facility will be constructed. This will be a replica of the existing sludge reception No.1 on site. It will include a screen with grit removal, an enclosed screenings skip, an imported sludge pumping station, a 2205m3 glass coated steel (GCS) sludge tank with mixers and decant valves, new sludge transfer pumps, an odour control system, two tanker delivery points, a road extension for drive-through access and drainage returned to the head of the works.

Sludge dewatering (red)

A new sludge dewatering system will be installed and will operate Monday to Friday, along with polymer makeup and dosing. A sludge cake transfer and trailer loading system will be housed in an enclosed building with odour control and automated doors. A liquor return pumping station with variable speed drive pumps will be installed.

Overall site

There will be a total of four new panels installed (three MCCs and one LVSB) which will support the new works areas detailed above and will be integrated with the existing works.

Due to the proximity of the River Tay, the site’s water table is tidal, posing a significant risk to the construction of the new final settlement tank. To address this, well-pointing has been implemented by SLD Pumps & Power to temporarily lower the water table in the area, allowing for the construction of FST4. This process impacts the settlement of existing structures and assets, necessitating continuous monitoring to prevent significant movement or damage. This challenge will also arise in other areas of the site during deep excavations.

Dewatering is being managed under GBR15 controls, which is valid for six months. However, since this period is likely to extend, a complex license has subsequently been applied for from SEPA to ensure ongoing compliance.

The presence of multiple principal contractors on site requires meticulous health & safety customer data management (CDM). Regular communication is crucial to ensure both contractors can work safely without interfering with each other.

Some access delays have occurred due to commissioning issues with the separate IFAS project, but ESD is working collaboratively with the client to mitigate any delays where possible.

Additionally, areas of the site have been contaminated with asbestos-containing materials and invasive species. The project team promptly notified the relevant authorities and sought guidance on appropriate remediation measures.

The ESD design was developed using Revit, a 3D modelling package, which was shared in MView, a cloud-based BIM management and collaboration product.

The 3D model has been an integral part of the project lifecycle from the early stages to developing detailed design in collaboration with stakeholders within Scottish Water. It has also helped create smoother discussions between the site team and the design team during the construction phase.

The 3D model provides much detail and clarity not only to the new works but also the existing works including underground pipes and services. It allows for a much more in-depth discussion to be had around installation and construction of new pipework which reduces the risk of errors, delays and miscommunication.

Despite its challenges, the project is progressing well. Since the start of work in mid-2023, ESD has demolished two sizeable, abandoned structures and rerouted the basement pumps to a new buffer tank and valve arrangement. The formwork, reinforcement and concrete work for FST4 are set to begin shortly.

Additionally, work has commenced on capital maintenance mechanical packages. We are preparing to intensify construction with a challenging programme focused on delivering the aforementioned areas.

ESD will continue installing large-diameter pipework to the FST distribution chamber, including any necessary tie-ins. These efforts will also include improving and replacing elements of the aeration lanes and initiating work on the sludge reception area and sludge dewatering building.

The Perth (Bertha Park) WwTW project management team is utilising Fieldview to record progress on site and to carry out daily, weekly or specified check sheets related to safety, health, environment and quality. This is a positive step forward.

The wider project team is actively engaging the supply chain through the procurement process for various workstreams and activities planned through to the anticipated project completion in August 2026.

The Containerised HUBER S-DISC re-packages proven technology for small scale sludge thickening to provide water companies with a cost-effective solution that will help address their biosolids logistics challenge.

At the heart of the Containerised S-DISC is the HUBER Disc Thickener (S-DISC); it is well proven sludge thickening technology with more than 90 references in the UK and 1071 worldwide.

The S-DISC thickens flocculated sludge using a principle similar to that of a gravity belt thickener (GBT). However, whereas a GBT uses a continuous fabric filter belt driven between two rollers, the S-DISC uses an inclined rotary filter disc with a filter media of woven stainless steel mesh. Sludge ploughs are used to assist the drainage of filtrate and a scraper blade is used to remove sludge from the disc surface, very similar to their use on a GBT. However, the stainless steel mesh is much more robust than the GBT’s fabric belt and overall the machine is very compact and easily accessible for maintenance.

The S-DISC can thicken up to 38m3/hr (Size 2) of secondary sludge to >6% dry solids content with solids capture of >97%.

The design of the Containerised HUBER S-DISC adopted a Design for Manufacture and Assembly (DfMA) approach with the focus on:

The S-DISC and ancillary equipment are mounted on a galvanised steel skid which has integral cable ducts for effective cable management and for reducing potential trip hazards associated with surface mounted cable management. The skid also includes an integral sump under the poly dosing equipment to act as a bund to capture any spillages. Level control in the bund indicates if there is liquid spilling into the bund and shuts the plant down to prevent further spillage of polymer. The bund has enough space to locate a day tank of concentrated liquid poly or polymer can be fed from an external IBC with a level control interface and IBC heating jacket power supply being provided from the container.

The skid is incorporated into a purpose built shipping container with bi-fold doors along one side and double doors at each end in order to provide unrestricted maintenance access to the plant. The plant can also be supplied as a skid mounted assembly where there is already an existing building to re-use.

An operation and maintenance risk assessment was undertaken during the design phase and then validated on the first production unit. The design brief was to ensure that this packaged solution eliminated any compromises in relation to maintenance activities.

An integrated packaged solution which includes:

Welsh Water required a sludge thickener to reduce the number of tanker movements from this site. This was motivated not simply by the need for more efficient sludge transportation but also due to the narrow access in and out of the area and a recent local hotel development. The treatment works has a population equivalent of 2,300 which increases threefold in the summer due to tourism.

The sludge thickening plan was required to thicken 16m3/hr of surplus activated sludge from 0.5% dry solids content to >5%. Having recently successfully installed an S-DISC thickener at Fishguard STW, it was deemed to be appropriate technology to use at St Davids STW. However, there was a need at St Davids to provide a building or enclosure for the sludge thickening plant as none existed on this isolated site.

A Containerised HUBER S-DISC size 1 was identified as the ideal solution. The container was based on a 20’ high cube ISO shipping container which included heating, lighting and ventilation. By offering a package solution, the need for on-site construction and M&E installation was minimised. All assembly work was undertaken at Huber’s Chippenham works and the completely assembly was fully tested prior to dispatch. The control system was design and manufactured in-house by Huber ensuring compliance with Welsh Water’s electrical specifications including the use of Mitsubishi PLC automation.

The unit was delivered in late 2022 and installed on a pre-prepared plinth. It was then commissioned in March 2023. The S-DISC thickened sludge to 5.5% dry solid content with a polymer consumption of <2 kg/tds. Filtrate quality was <200 mg/l SS demonstrating a 98% solids capture rate.