PHOSPHORUS REMOVAL AT WASTEWATER TREATMENT WORKS
IMPLEMENTING NEW SLUDGE TREATMENT FACILITIES AT FOUR WELSH TREATMENT PLANTS TO AID PHOSPHORUS REMOVAL
Publication Date (Web): 27 November 2015
Dr Cymru Welsh Water (DCWW) is a major UK water business servicing most of Wales. Natural Resources Wales, the Welsh environmental regulator, dentified four wastewater treatment works (WWTWs) – Gowerton, Llanelli, Parc-y-splott and Pontyberem – discharging into Carmarthen Bay, South Wales that required phosphorus removal. Effluent limits of 1 mg/L total phosphorus (TP) now apply. The paper describes how compliance was achieved and technical challenges overcome.
The key to meeting the new standard is chemical dosing to precipitate TP. However, this increases sludge production, requiring augmented sludge treatment. Ferric sulfate dosing was selected for precipitating TP; however, this drops the pH requiring sodium hydroxide dosing at some sites, while the sludge treatment augmentations were varied, but wide-ranging.
Following initial process designs, the design and build team (Morgan Sindall/Arup) with DCWW Operations confirmed the scope and cost for upgrade works at each site.
This showed that scopes and budgets were incompatible, requiring significant review and challenges to prior assumptions. Value engineering examined reuse/refurbishment of existing assets where feasible.
• Acceptance that specifications were achievable using existing assets;
• Refurbishment and reuse of available equipment;
• Rebuilding civil works utilising existing foundations;
• Demonstrating that treatment risks were insignificant, avoiding major capital upgrades.
Challenges to construction methodologies and DCWW specifications resulted in cost reductions, such as:
• DCWW specification challenge:
Resizing pipework at Gowerton WWTW, minimal velocities need not apply to final effluent pipework, as risks of encountering settleable solids were remote (pipework replacement avoided).
• DCWW specification challenge – tank level detection: DCWW had specified that chemical storage tanks required level probes to mitigate risks of spillage. Upon review DCWW accepted mitigation
using low-level pressure transducers, avoiding significant costs of fixed access to the top of tanks for maintenance of probes.
• Construction methodologies:
Use of pre-cast and pre-fabricated units Adoption of pre-cast concrete and pre-fabricated plastics components had cost, program and safety benefits.
The main challenges were accommodating plant upgrades within heavily congested works sites that had previously undergone several rounds of upgrade works. Factors involved included:
• Maintaining clearances to and protection of physical assets, both DCWW’s and that of other service providers;
• Managing complex works within restricted spaces;
• Ensuring building and plant locations allowed for vehicle turning circles;
• Interconnections between new works and existing process units; and
• Maintaining normal plant operations while implementing the upgrades.
Detailed examples of challenges faced at the sites are described in the main paper.
Implementation of chemical dosing for P Removal at these four heavily congested WWTW sites provided significant challenges for budget management and working constraints. However, with focused and collaborative stakeholders, environmental compliance was achieved by the target (31 March 2015). The P trends following chemical dosing for one of the sites are displayed in the graph below.
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