Alpha factor improved at Shieldhall
Overloading of the treatment process at Glasgow's Shieldhall STW has been solved by the addition of a new aeration system. Monsal's lift-out design was chosen by Miller Water
The plant serves a population equivalent (PE) of 400,000 in south Glasgow and is operated by West of Scotland Water (WoSW). Miller Water was asked by WoSW to look at the problem. The company chose to develop a proposal in partnership with Monsal. A key part of providing a solution at Shieldhall was to review the existing process through conducting a preliminary audit of the secondary treatment process - in particular the aeration system.
Effective design of the retrofit aeration system required a knowledge of the key design parameters for this site. Monsal looked at the available plant data and was able to assess the current load and aeration characteristics of the current plant. The secondary treatment process consists of six large 8,500m3 oxidation ditches, 3.5m deep.
Each ditch has five large mammoth rotors rated at 45 kW each. After discussion with WoSW it was concluded that an additional 20% aeration capacity was needed on the site. The process audit concluded that the secondary treatment plant was operating with an alpha factor of close to 0.4. The alpha factor is the ratio of oxygen transfer from clean water to dirty water and is a key design parameter.
Correct selection of the alpha factor is one of the most important criteria for refurbishment design. By auditing the process and looking at plant data such as load, temperature, and dissolved oxygen (DO) Monsal was able to back calculate the operational alpha of the existing process.
Miller's remit was to achieve a fast track response to the problem and deliver a solution to a 12 week programme. A Green Book contract was agreed with WoSW and the specialist aeration package was developed via a Yellow book sub-contract. The aeration package was negotiated in parallel with the design and all key interfaces and termination points fixed by week two in order to facilitate the tight schedule.
The main aim of the project was to provide an extra 20% aeration capacity. The DO in the ditches is monitored by two probes, located in each lane. An average of their readings is used to control the outlet weirs from each lane. These weirs alter the mammoth rotors submergence and therefore their oxygenation capacity in relation to a set point of 2.0-3.0mg/l. Further control is facilitated by sequencing the rotors on/off to match the DO setpoint.
Fine bubble solution
The proposal put forward by Miller Water was for an aeration solution package that allowed for independent aeration enhancement which would provide the extra 20% via a fine bubble diffused air (FBDA) system. This was delivered by fitting two lift-out grids per ditch, each grid with a dedicated blower and control assembly - a total of 12 lift-out systems.
The design suited the fast track programme as no additional DO control or complex manifolding was required. Replication of design and use of smaller blower assemblies also facilitated fast track procurement and fabrication.
For this project Monsal modified its standard lift-out designs to be installed in situ without draining down each ditch. This was a major advantage as the existing process capability is quite stretched. The frames were installed sequentially at two frames per day, with the existing rotors turned off for only four hours during the immersion of each frame. Disruption was therefore minimised.
From contract award to aeration plant commissioning, the system was complete in just 11 weeks, one week ahead of schedule. The total refurbishment package consisted of a new power supply, civil modifications, new blower assemblies, new control panels, new pipework and the lift-out aeration systems. WoSW looked at two other project proposals before choosing Monsal's, which looked like it would be the most cost-effective. According to Mark Lewis, project engineer for Miller Water: "We were pleased with how the project was run and the final result."