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ENGINEERING: Largest Desalination Plant Complete

 



Recent Western Cape Business News

CURRENTLY undergoing commissioning at Mossel Bay is the country’s largest seawater desalination plant, a 15Ml/day unit serving the Municipality of Mossel Bay and the adjacent PetroSA refinery.

Due to the recent worst drought in 150 years, the municipality was compelled to turn to ‘new’, innovative water sources to augment the rapidly diminishing existing resources. Work commenced on the 15 Ml/day seawater desalination project – the largest in South Africa – in May 2010, with a target completion date of November 2010. This very condensed timeline necessitated execution of the design, tender and construction stages largely in parallel to ensure the earliest possible completion, in order to avert an impending catastrophe.

SSI headed a project team that had to deliver a R200 million project in the space of eight months, from appointment, through procurement, design and construction. The design and construction stages had to be executed largely in parallel to meet the extremely tight deadline. The project involved four complex sub-projects, each under separate contracts, by independent consultants and contractors. The project was further complicated by the limited space available for the plant and the sensitive environment of the works.

The project was commissioned by the Mossel Bay municipality, but jointly funded by PetroSA, a large industrial water consumer. This public/private initiative was supported by central government through the allocation of further funding on account of the project’s emergency status and strategic importance. The project entailed hands-on interaction between the municipality and PetroSA to ensure the objectives of the project were met in accordance with the requirements of both parties. PetroSA provided a dedicated team of specialists to work in conjunction with municipality’s technical team and consultants.

The overall project comprised four ‘sub-projects’, namely seawater intake and brine discharge works, reverse osmosis desalination plant, bulk electricity augmentation and permeate water supply pump station and rising main. The sub-projects were commissioned and executed in parallel as separate contracts, by different consultants and contractors. This required detailed planning, close cooperation and intense, regular coordination to ensure timelines and interfaces were achieved.

The marine works contract comprised large diameter welded HDPE intake and brine discharge pipelines, fitted with bespoke precast concrete weight collars. The pipelines were towed into position by a tug and sunk by filling with water. A coffer dam had to be constructed through the surf zone to facilitate towing of the pipelines out to sea.

The RO plant itself was sited behind the primary dune above the beach to minimize its visual impact. The plant had to be constructed within a very confined site, which required careful planning and design to ensure optimal use of available space. The vast majority of components had to be manufactured from scratch (e.g. filters) due to their size or imported from abroad, which made the delivery to programme particularly challenging. The following components had to be constructed within an area of 1 150sq m.

Medium to large diameter HDPE pipelines aboveground (500mm dia): 1 500m -
2 000m. Possibly the only large scale HDPE pipe plant of its kind in Africa.

Six multi-media filters: 4.5m dia x 12m long, weighing 300t per filter. Possibly the largest water treatment filters in Africa.

Six RO skids (each 2.5Ml/d capacity).

The permeate water supply pump station and rising main entailed approximately 2.3km of large diameter pipeline (500mm dia), complicated by construction within a heavily industrialized area, crossing of numerous main roads, including national road, utilizing pipe ramming techniques.

Due to the fast-track nature of the project, the normal EIA process could not be followed. Therefore to ensure compliance with environmental legislation and regulations, environmental specialists and the regulatory body were consulted early on to provide input into the design.

The beach and dune areas disturbed during construction will be re-profiled and planted with indigenous species as part of the rehabilitation phase of the project.

SSI is a multi-disciplinary consultancy that tailors solutions for clients involved in infrastructure development. The company’s services cover the complete spectrum of lifecycle needs within the sectors of transport, water, environmental, energy and resources, buildings and structures and project and construction management.

 


 
 
 
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