The wonders of waste 05 February 2019

United Utilities’ £200 million investment at its Davyhulme wastewater treatment works in greater Manchester has put the site right at the forefront when it comes to sewage treatment

Davyhulme wastewater treatment works (WwTW) in greater Manchester, serving a combined domestic and industrial load that is equivalent to 1.2 million people, is a beacon for United Utilities’ highly sophisticated approach to sewage treatment and sludge management and disposal. It is the outcome of a recently-completed four-year upgrade to modernise the North West England water utility’s largest such treatment plant. The works deals with a massive throughput of 750 million litres per day (Mld), even as Manchester’s continued economic growth brings ever more people and businesses to the area. The capability of the works has been massively enhanced to cope with future demand and tighter environmental standards.

Here, a total of 91,000 tonnes per annum of biosolids (the ‘sludge’ produced both on site and from seven other feeder sites) can be processed via thermal hydrolysis and anaerobic digestion to produce a high-quality soil conditioner for farmers, plus biogas. The works has the flexibility to either clean the gas into biomethane for injection into the local gas supply grid (gas holder pictured inset p10), or use it as fuel to power CHP (combined heat and power) engines, generating electrical energy for the site – and even powering a fleet of electric vehicles for the site operations team.

Today, Davyhulme is one of the biggest UK wastewater treatment works, draining the entire western side of Manchester. The site operates 24 hours a day, every day. In heavy rain, flows of more than 30,000 litres per second are treated at the works; equivalent to emptying 90 road tankers every minute. And the on-site Manchester Bioresources Centre is now capable of treating sludge arising from 60% of the North West’s seven million population.

It wasn’t always this way, of course. Davyhulme started life as a ‘sewage farm in the countryside’, treating the city’s wastewater before releasing it into the Manchester Ship Canal. The Davyhulme Treatment Works opened in 1894 and has been at the forefront of innovation ever since, not least with a breakthrough in the drive to improve the treatment process when, in 1914, two chemists, Edward Ardern and William Lockett, discovered the activated sludge process. This in itself was revolutionary at the time, changing forever the way that wastewater was treated – right across the world.

In this process, wastewater containing organic matter is aerated in an aeration basin, in which microorganisms (bacteria and protozoans) metabolise the suspended and soluble organic matter. This allows part of the organic matter to be synthesised into new cells, with other matter oxidised to CO2 and water to derive energy. Before the modernisation project, Davyhulme WwTW employed two activated sludge plants running in parallel (ASP1 and ASP2, each dealing with half of the flow). ASP1 was older and it has now been replaced with a new 10-lane plant (ASP3; pictured in part p10). ASP3 now handles 60% of the flow, and ASP2 40%.

Although the activated sludge process used in the modernisation process is still the same basic one devised by Arden and Lockett, the way in which it is controlled has been massively enhanced. “These improvements in ASP3 include feedback and feed-forward control loops to ensure that we have the exact amount of air injected into the process, resulting in a much better-controlled and energy-efficient treatment of the sewage,” explains Dave Frain, Davyhulme WwTW production manager for United Utilities. “We are also able to ensure the FM ratio [food/mass (microorganisms)] is perfect at all times with this extra control function. The dissolved oxygen is now introduced from the bottom of the basin, instead of at the surface, ensuring it is uniformly maximised and efficient throughout the entire process, eliminating dead spots and unwanted settlement. The bugs receive the right amount of food and air at all times. We have also installed efficient blowers and automatic detection of sludge blankets to help us control the primary and final tanks efficiently.”

The modernisation project also included replacing the old inlet works and building a new odour-controlled inlet consisting of a two-stage screening facility. Previously, this stage of the process required manual intervention, due to the size of the works. This is now all controlled automatically via a DCS (distributed control system), and that also monitors the amount of rag being taken out of the process. “For the first time on site, we have a facility to capture fats, oils and grease that enter the works, and we now store this and send it to a green energy recovery company, which converts it into a biodiesel.”

The civils were constructed using design for manufacture and assembly (DfMA). It meant most of the concrete fabrication happened in the factory in precisely controlled conditions, then the sections were assembled on site. It’s faster, cheaper and gives much better quality control.

Laing O’Rourke’s project leader David Marsh comments: “The use of DfMA and digital engineering was identified as a key efficiency strategy from the tender design stage to final construction. By using 4D visualisation though BIM (building information modelling), we could review each construction activity to ensure constraints were mitigated. It provided a representation of changes and highlighted problems before they physically occurred on site. Compared to traditional on-site construction, the use of precast elements and off-site manufacture has saved 6,800 on-site working days, generated significant savings and reduced health and safety risks.”

The new treatment works also performs an additional treatment process, compared to the one before. “The new activated sludge process treats both ammonia and BOD (biochemical oxygen demand). The original Arden and Lockett design utilised a carbonaceous treatment for the removal of BOD only. However, we now have a nitrifying activated sludge process that also removes ammonia, which is achieved in the same way as before, but far more readily through careful control, using different parameters and design. At Davyhulme WwTW, the previous decommissioned process – ASP1 – was only designed to treat BOD, and we had a tertiary treatment plant to remove ammonia.”

Frain continues: “The Environment Agency has set a new tighter ammonia consent of 1mg/l on the final effluent. This new plant allows us to achieve that standard with ease, while also being far more efficient, as it is using less power and can be monitored on site or remotely.”

Ahead of the switch to the new stream, the Davyhulme site underwent six months of testing. How did that go? “The new stream has ten lanes and, due to the size of the plant, we brought the lanes on in pairs to allow the biological growth of the biomass,” he reports. “This allowed the growth of the nitrifying bacteria to happen in a controlled way, without disruption to the final effluent quality.”

Commissioning a new process such as that at Davyhulme demands a vast operational undertaking, of course. Lee Donnellan, who was UU’s operations readiness manager for the duration of the modernisation project, explains: “We had to consider the start period where it doesn’t treat. In order for a treatment process to do its job, it requires a certain recipe of activated sludge to ensure the FM ratio is correct. By switching on the lanes in pairs and in a controlled way, we introduced the correct recipe from the well-established activated sludge process on site. This is what we call ‘seeding’: we seed the new process with the right type of micro-organisms that quickly establish treatment. It’s also acclimatised to the Manchester home-grown sewage, too.”

The initial phase was very much a carbonaceous process, removing BOD alone. “This then has to go through a number of sludge ages to grow the nitrifiers required to treat ammonia. Once established, we moved on to the next two lanes and so on, until all ten were completed. We were able to speed up this process by using the seed from the on-site ASP, and also the use of the on-site tertiary treatment process that was historically used to treat the ammonia. This was our fallback, if the nitrifiers needed a little more time to grow.”

Throughout this commissioning process, engineers monitored process conditions through a combination of on-site testing and samples taken to an off-site laboratory. On-site testing was a combination of the in-line instruments measuring key process parameters, such as dissolved oxygen, ammonia levels in and out of the plant, and levels of sludge in the process at different stages. Quality tests were also carried out to confirm the in-line instruments’ accuracy. These results were backed up by the results taken in United Utilities’ accredited labs in Warrington.

During the six months, UU had to adapt the parameters of the instrument control to ensure the commissioning process didn’t take a step backwards. As Donnellan points out: “You can’t just set an end-game control philosophy; each change stage requires adaption until you reach the completed state. Manual operation was widely used to get us through this, and we are now moving towards the automation stage.”

Another factor considered was the overall change to Davyhulme WwTW. “The well-established tertiary treatment was to be down-rated once we had the new nitrifying ASP, and the same can be said for the ASP2 process stream. These changes had to be balanced right through to the end, to ensure we could utilise the tertiary treatment and older ASP2 process as a fallback, while trying to achieve the end state of de-rated operation. With a live process, you can’t just turn it down and then back up again when things don’t go smoothly. It needs time to de-rate, otherwise under-loading occurs, where the microorganisms start to die and unwanted species thrive. Then, if needed, the process has to have time to uprate and treat a contingency requirement, otherwise overloading occurs whereby you don’t have enough bugs to treat. It’s a difficult balancing act, full of compromises and risk.”

Donnellan says one of the biggest challenges faced was integrating the old assets with the new ones. “We cannot stop the flow coming into the works and, as we turned the new plant on, we had to carefully manage how we turned off the old plant. As the process is a biological one, you have to allow time for it to mature. The ops readiness role helped ensure that the operational team were involved in the design phase, which includes risk management and input into the future operational design. The BIM system we used allows a CGI 3D model of the new works to be built during the design process. It allowed the engineering team and operators to use a virtual reality headset to walk through the plans and use their real-life experience – not theory – we were able to reduce issues relating to access and introduce simple, yet effective, ideas, such as moving sample points, access platforms, stairways, and pump control.” The training plan was also devised to ensure training was fit for purpose and that all the electronic operations and maintenance manuals were in a suitable format.

“A project’s main focus is on the delivery of the contract and meeting the original design specification. This assumes that operations have the required skill set, understanding or tools for the operation of a wastewater process. Projects do deliver training, but that is usually for specific assets installed, or lifting operations. What they don’t do is train operations on the inner workings of a nitrifying activated sludge process, which requires different knowledge, parameters and fault-finding to that of a carbonaceous activated sludge process.”

The ops readiness role offers a closer working relationship with operations and reports to the operations team, rather than the project team. “This ensures the real-life experience and long-term operational requirements are considered within the project world; there’s a real affinity with the ops requirements.”

Something else to factor in is the change in future operational activities required with a new process change as big as that at Davyhulme. “Skip removal, for instance, requires different contracts for different skip management activities. Also, there may be a change in resource requirements, operational task scheduling etc. Projects don’t normally consider this – the operations team are required to work this out, with some help. What the ops readiness role does is to get the operational team ready for the new way of working; and that has proven very successful here at Davyhulme.”

Brian Wall

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