Water and Effluent Treatment Magazine
Issue link: https://read.utilityweek.co.uk/i/411335
12 WET NEWS NOVEMBER 2014 ONSITE Millbrook WwTW upgrade consideration was given to the high-strength centrate and fil- trate liquors generated from the on-site sludge treatment centre (STC) that are returned into the wastewater process," he says. Analysis showed that in terms of ammonia concentra- tion, these combined liquors contributed approximately 125,000 PE in addition to the crude influent load. A detailed optioneering exercise was car- ried out to select the most effi- cient way of treating this non- crude influent. White says: "A dedicated high-rate treatment process was considered to treat the liquors at-source. However, a whole life cost assessment showed that the most cost-effective and low- est carbon approach was to combine the liquors with the crude load and treat in the Bardenpho process. "This meant that the site would comprise of a simple, robust process with an overall lower number of assets, thus representing best-value for the client. This would also allow replacement of the existing, shallow ASP tanks. However, accommodating 33,500m3 of process volume into this con- strained site presented a signifi- cant design challenge," White says. The design A detailed 3D model that included the existing site and below ground services was par- ticularly useful for planning new pipeline routes and avoid- ing clashes, says Martin Tresid- der, construction manager at 4Delivery. "Modelling of new structural assets was instrumental in eval- uating the construction sequence and phasing activi- ties. By including external con- straints such as the overhead HV cables, the model was also used to help plan safe methods of working." A significant proportion of the overall design effort was focused on the optimum civil and structural arrangement for the large Bardenpho ASP structure. "To minimise footprint, the process depth was set at 7m, which is at the upper limit for typical ASP plants. This still entailed a structure 92m long and 62m wide. Four lanes were selected to provide operational flexibility and arranged in a three-pass serpentine arrange- ment to ensure optimum plug- flow conditions." Tresidder continues: "As the structure was being designed for construction in reinforced concrete, great effort was made to ensure an efficient design. The dra˜ outline design involved a simple 700mm thick base on a precast concrete piled foundation and 500-700mm thick walls. "A co-ordinated approach was applied for development of the design, involving group dis- cussion between the teams, civil, structural and geotechni- cal engineers. Early contractor involvement was key to these discussions." Following these design work- shops it was agreed to use a layer of Vibro-stone columns as it is more cost effective than pil- ing and far less noisy. However, Vibro-stone columns required the structural design to allow for up to 50mm of potential settlement. To avoid excessively thick walls, the structural design adopted a propped cantilever system with tie-beams between opposing walls at coping level. This meant that the 7.5m high walls could taper in thickness from 500 to 350mm, as the beams help resist the hydro- static load imposed on the walls. The base slab design was also more efficient as a result of adopting this solution: 300mm thick and 500mm thick below the main walls. "Use of the innovative propped cantilever design helped save a significant vol- ume of in-situ concrete. Settle- ment was anticipated but the structure was designed to with- stand the effects, Tresidder says. "The tie beams were designed to make installation as quick and simple as possible, without need for connection into the wall reinforcement. For With two existing plants in place, space was going to be an issue. Also, the irregularly shaped footprint available to use, overhead high-voltage cables, shallow groundwater table, poor ground strength, and existing buried services all had to be considered The structural design adopted a propped cantilever system with tie-beams between opposing walls at coping level, enabling the 7.5m high walls to taper in thickness from 500 to 350mm The site comprises a simple, robust process with an overall lower number of assets