Dunnswood was deemed the flagship project where CHt and Expanded worked for the first time together to demonstrate the Laing O’Rourke group’s capabilities and expertise within the Scottish Water infrastructure sector.
It was the largest of all the SR10 projects to date.
This project featured several mixed aspects of new build off line elements and capital maintenance refurbishment work on existing plant, all coming together and controlled via a new intelligent Motorised Control Centre with associated field wiring and control through instrumentation.
The new build elements consisted of the following:
- Tertiary Treatment disc filters
- RAS pumping station
- Caustic Dosing Plant
- Disc filter and dirty backwash pumping stations
- New supernatant tank and dewatering system
The refurbishment / capital maintenance aspects consisted of the following:
- Existing ASP aerator replacement and tank modifications
- PST upgrades
- FST upgrades
- Refurbishment of inlet screens
- Refurbishment of ferric dosing system
- Upgrade of PST de-sludge system
The main quality drivers on the project were:
- EC01 Urban Wastewater Treatment Directive
- EC04 Freshwater Fish Directive
- EC10 Water Framework Directive
The applicable quality standards on the project were as follows:
- BOD spot sample - 95%ile from 10mg/l to 6 mg/l
- Ammonia spot sample 95%ile from 5 mgN/l to 1 mgN/l
- Total Phosphorus spot sample annual average - 2 mgP/l
- Iron spot sample Annual average - 2 mgFe/l
Our approach was to construct all the new major elements of plant offline and gradually bring each item of online into the process carefully, controlling the process commissioning at all times. Through this approach we could carry out several of the refurbishment works at the same time.
When the new plant was fully operational and controlling in a satisfactory manner, we were then able to start isolating items of the existing works in order to carry out the capital maintenance scope.
The majority of the civil and MEICA elements were carried out with our own in house capabilities, including the commissioning. The process commissioning was shared with our in-house delivery partners.
Throughout the project, we encouraged close relationships with the delivery team to minimise risk at all times with any potential process issues.
All lifting operations were carried out in house and this featured the use of a tower crane at Dunnswood to construct the new ASP.
The engineering behind our delivery
Although there was little opportunity to implement our Design for Manufacturing and Assembly (DfMA) strategy at Dunnswood, and the bulk of the works were carried out in house, CHt was dependant on external framework contractors to carry out specialist packaged plant items as follows:
- PST’s & FST half bridge scraper installations
- Chemical plant Installations
- Inlet Screen Refurbishments
- Tertiary treatment disc filters
These activities required close monitoring and integration, to ensure that these contractors adapted to the Laing O’Rourke and CHt way of working, and compliance with our strict Health and Safety requirements.