Energy efficient refurbishment of Weymouth and Portland sewage treatment works
Leonard, J., Wessex Water, UK
(free)
Abstract The project was promoted on the premise that capital maintenance work could be delivered more effectively than employing a standard ‘like-for-like’ replacement methodology. Could an alternative process system be installed on a more efficient CAPEX and OPEX basis whilst maintaining, or improving, operational stability and output? A full-floor coverage FBDA (fine-bubble-diffused-air) system was designed and installed within the existing tank structure(s). Special consideration was necessary in order to match the operating envelope of the existing air delivery system. All project cost and timescale requirements were met. An OPEX saving of 3.10 GWh, or £263,500 p.a has subsequently been realised. In addition, there has been no re-occurrence of surge events which were common with the original system. The project demonstrates that excellent energy efficiency can be achieved without detriment to operational security and performance. The plant now operates with much reduced overall power consumption whilst continuing to meet all regulatory and environmental standards.
Keywords Capital maintenance; energy efficiency; FBDA; OPEX.
Introduction Outline description of Weymouth secondary treatment process The secondary treatment stage at Weymouth STW (sewage treatment works) was constructed in 2000 to enable the works to comply with the requirements of the Urban Waste Water Treatment Directive. The design of the secondary treatment stage was based on a Sequencing Batch Reactor (SBR) process operated on a fill and draw basis. The design consisted of 4 basins each containing 2 floating down-draft mixers, 3 floating decanters and a fine bubble diffused aeration system. The SBR basins plus associated plant are contained within an underground structure measuring approximately 130m x 100m x 15m.
The treatment sequence was, and currently still is, controlled on a time basis, such that each SBR basin is filled, aerated, and emptied alternately. The treatment process for each basin is based on the following sequence:-
Mix-Fill; React-Fill; React; Settle; Decant; Waste Sludge.
The oxygen demand was originally met by a fine bubble diffused aeration system comprising of 4 fixed speed centrifugal blower units with variable output controlled via inlet and outlet guide vanes, air distribution pipework, airflow control valves, and fine bubble diffuser grids in each basin (Figure 1). The key plant design details were:
