BTS | HARDING PRIZE COMPETITION 2025
These systems utilise specific mechanical and
chemical processes to target problem contaminants and can utilise flocculation, coagulation, hydro-cyclones, filter presses, dewatering screens, centrifuges and more. These plants look at moisture contents of spoil for reuse/disposal and the problem determinants in the water for removal. It should be noted that these slurry treatment plants
and water treatment plants are modular and temporary assets by nature, mirroring a construction project’s duration; therefore, permanent plants have not been considered in this review.
3. ROADMAP TO WATER RECYCLING The road map for water recycling at Align followed two specific innovations: a new chemical treatment at the STP; and, a bespoke plant at the WTP. In the following sections, the paper sets out the
four main phases of works that were associated with these two innovations chronologically and also my involvement in achieving these works. ● Moving from Hydrated lime to a polymer at the STP Filter press
● Dismissal of the pre-operational condition within the Wastewater Discharge Consent
● WTP trials and new WTP ● Water Recycling
The final section will then explore the benefits that have been realised by the recycling network and how this has directly impacted the sustainability for the project, and, further, how it can impact the future of tunnelling construction.
3.1. Innovation 1 – Polymer in the Filter Press At the commencement of the project, the STP had been set up to initially use the standard treatment chemicals of hydrated lime and sulphuric acid in the filter press process; there was an option to use a polymer to optimise the process. This removed high pH byproducts, extra site deliveries and reduced the amount of exposure and handling, and so improved management of the environmental and safety hazards. If continued to be used then 17,500 tonnes of lime
would be needed, producing 4.5 million tonnes of filtrate water at pH 12 that would need treatment using 8,400 tonnes of sulphuric acid. The alternative option would see 4,600 tonne of polymer used and that would generate the 4.5 million tonnes of filtrate water ready for reuse at pH 8. Previous filter press tests had been commissioned
by STP manufacturer, MS. However, those tests were carried out within different geology, used bentonite, additional flocculants, and were undertaken with different polymers. Further, this project would also be the first full industrial-scale application. As the STP Engineer for the project, I was responsible
for championing the investigation of this polymer in the filter press with the STP manufacturer, through the STP
20 | May 2025
production teams. Additional work had been done by the supplier ahead of the on-site trials to determine the best polymer to use - FLOPAM MS PRESS 3 - and the dosage rates to trial. The STP circuit had been operated in ‘closed
loop’ to the trial and contaminant build up from this phase needed removal by flushing the slurry system, undertaken by draining the filtrate water tanks to the WTP ponds and diluting the excavated slurry with fresh water during production. I worked with the authorising body, the EA, during
this time to get the required approvals to discharge a volume of water to the pond system. The trial overlapped with this phase prior to ensure no adverse effect on the data by running in ‘closed loop’ and concentrating any potential contaminants within the system. The system was returned to ‘closed loop’ for a section of production and the sample suite repeated for comparison. During these works, various samples were taken throughout with a mixture of in-situ tests, inhouse laboratory tests and external chemical suite analysis. At completion of the polymer trial, the plant was then
switched back to lime and the testing regime was re- run for a base comparison. The conclusion for the testing revealed that the
polymer is a comparable replacement for lime in a productivity sense with many added environmental and health and safety benefits. A leachate report was also undertaken to analyse the long-term effects in the chalk cake; and works were initiated to allow use of the polymer exhaustively. During this process I monitored the STP performance
through a variety of reports and observations. I orchestrated the testing regime and provided the key logistics to ensure that the samples taken were transported to their respective laboratories in the correct conditions.
3.2. Wastewater Discharge Consent Included within the pre-operational conditions of the sites Environmental Consent was a specific requirement around the suitability of the polymer. To discharge this polymer, a Compliance Assessment Report (CAR) was needed, consisting of the three trial documents. One of the reports was on the physical assessment of the trial that I authored, and collated the information from the trial and resulting test data, and others by my colleagues on the chalk leachate and filtrate water discharge quality. This process then ignited many conversations upon
which various stakeholders were invited to the site. I then embarked on touring the STP to show first- hand the potential benefits of the polymer, large scale application and the efforts to make the project as sustainable as possible. The results of the report submission and the following
engagement visits resulted in the acceptance of the CAR, thus allowing process water treated at the STP to be discharged to the Water Treatment Plant.
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