Low footprint effluent treatment

The design of Singapore's Kranji STW provides the solution to many of the future challenges facing urban planners - as the demand for land for residential and commercial development increases, consultants and contractors can no longer assume that there will be large open areas to provide buffer zones for odours, and space for conventional treatment facilities. David Fullerton and James Currie of Binnie Black & Veatch report on the approach taken at Kranji where new primary settlement tanks, aeration tanks and final clarifiers occupy a footprint approximately a third of that of the existing activated sludge plant.

In 1992 Binnie Black & Veatch was commissioned by Singapore’s Ministry of Environment to design and construct improvement and extension works to the Kranji STW. Kranji is situated in the north-west of Singapore and is a conventional activated sludge plant of 75Ml/d DWF capacity.

Egg-shaped sludge digesters at Kranji

This $260M project consists of containing and treating the odours from the existing process units; providing new compact and covered works to extend the existing works; and supplying additional covering works.

Odour control

Containing and treating odours was the first task. It comprised of covering the existing inlet works, detritors, primary sedimentation tanks, aeration and final clarifiers with purpose-designed, pre-fabricated sectional Fibre Reinforced Plastic (FRP) geodesic aluminium dome covers.

In addition, associated FRP foul air extraction ductwork was provided to convey odorous gases to a total of six odour treatment plants. This was completed in 1997.

Odour control was also an important part of the extension works while at the same time helping in the reduction in plant size.

Saving space

In Singapore the existing major STWs, including Kranji, are the conventional ‘open tank’ design with treatment units laid out over relatively large areas with a buffer zone of up to 1km from the plant, in which only industrial development is allowed. These buffer zones have been provided to ensure that any odours generated from the treatment plant have minimum impact on the community.

However, there is limited space available on the island for residential and other development and these buffer zones represent a valuable land resource, which could be released for development.

Where possible the covered and compact works have been designed on a modular basis to enable the Kranji STW to treat an ultimate inflow of 300Ml/d in stages. The initial new compact and covered works provide additional treatment for a DWF of 75Ml/d but with provision for the treatment of peak flows up to 220Ml/d. The design of the liquid treatment stream and the anaerobic digestion plant was modularised.

The new main treatment module contains the primary settlement tanks, aeration tanks and final clarifiers on a footprint which is approximately a third of that occupied by the existing activated sludge plant, serving the same hydraulic capacity. Three 7800m3 egg-shaped digesters have been constructed and interconnecting pipe galleries will facilitate the addition of nine egg-shaped digesters in the future.

To cater for the ultimate design inflow of 300Ml/d, a total of four main treatment modules and 12 egg-shaped digesters will be necessary.

Mechanical plant of sufficient capacity has been installed to treat the initial flows of 75Ml/d DWF and 220Ml/d wet weather flow.

The unusual aspects of the design are in the main treatment module where, in order to reduce the plant footprint, many of the components are substantially deeper than usual.

The 12 rectangular double-tray primary sedimentation tanks are each approximately 35m long, 5m wide and 9m deep; the six aeration tanks are approximately 56m long by 11m wide and 10m deep and equipped with more than 3000 ceramic dome fine bubble diffusers.

The 12 final clarifiers are substantially deeper than usual at 9m. Because the groundwater is only a few metres below the surface, the base slabs are more than a metre thick to provide sufficient mass to prevent uplift when empty.

The primary sedimentation tanks and final clarifiers are rectangular double-tray tanks scraping sludge to a common hopper at one end. Scum is also conveyed by the chain scrapers to the opposite end of the tanks where it is removed on a regular basis by electrically actuated scum-removal devices. Overall the module will be approximately 200m long and 80m wide, extending from some 8m above ground to – in some areas – 12m below.

The various structures that make up the new works are connected by a series of underground service galleries. The main gallery runs north/south and connects the main treatment module with the inlet works and power house and connects to the sludge gallery serving the new digesters. The galleries have interconnecting process pipework and cabling serving the new facilities.

The new plant will be controlled by a state-of-the-art integrated monitoring and distributed control system. Monitoring and control will be based on a distributed control system in which an individual plant area responds automatically to predetermined input signals, e.g. level or flow.

The new inlet works will screen and degrit flows from the southern part of the Kranji STW catchment prior to mixing with flows from the northern part. The latter will continue to be screened and degritted in the existing inlet works. Combined flows will then be distributed to the two process streams for treatment.

Sludge from the new plant, once it has been digested in the egg-shaped digesters, will be dewatered to 28% DS content and stored in three 5000m3 steel silos prior to removal from the works.

Biogas generated during sludge digestion will be stored in two 21m diameter floating bell gas-holders prior to use in the power house. The new power house will contain two dual fuel generators each rated at 3000 kVA with space for another four units.

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