December 2006

Water re-use in Dubai

ACWA Emirates LLC - the Middle Eastern arm of ACWA Services - has been awarded the design and supply contract for a Submerged Membrane Bioreactor (sMBR) Plant at the prestigious "The Greens" residential complex in Dubai UAE. The order was placed by Metito (Overseas) Ltd, with EMAAR as the end client.

Installed to upgrade an existing sewage treatment plant, the new MBR stage will use 'Kubota' Flat Sheet Submersible Membranes to produce a high quality, fully disinfected effluent for irrigation purposes and is capable of processing effluent flows up to 12,000 m3/day with <5mg/L BOD, 5mg/L Ammonia and 1mg/L Suspended Solids concentrations.

The decision to opt for a submersible flat sheet membrane system was based on a variety of factors, including flexibility, its ability to be installed on a very low footprint and the high quality of effluent it produces - typically >6 log removal of coliforms and > 4 log removal of viruses. The process requires no primary or secondary settlement stages and no additional tertiary treatment or UV stages to achieve unrestricted irrigation quality effluent.

Modular flat sheet membrane systems are easily expandable. They require 75% less total aeration tank volume when compared with conventional systems and are not affected by the presence of hair or fibrous material which often causes tangling in alternative hollow fibre type systems. Because of this, no primary clarification or advanced screening to less than 3mm is necessary.

Submersible flat sheet membranes have proved to be extremely cost-effective as they are low on maintenance and require little more than weekly monitoring and half yearly membrane cleaning with dilute (0.5%) Sodium Hypochlorite solution. Due to the processes of gravity filtration and continuous cleaning by coarse bubble scouring, flat sheet membranes have an extended lifespan and can be expected to provide up to 10 years effective and reliable service.

The Process

The existing conventional treatment plant installed at "The Greens" site is to be upgraded from 3,000 to 12,000m3/d by simply adding a membrane stage at the end of the existing process.

Before passing to the MBR process, raw effluent from an existing anoxic tank - which converts Nitrate to Nitrogen - will pass by gravity to two existing aeration tanks. The tanks are being upgraded to allow sufficient aeration with high solids concentrations by a fine bubble diffuser system. Originally, the sludge concentration would have been 3,000-4,000mg/L in the aeration tanks, but with the MBR design, the same tanks will utilise sludge concentrations between 10,000 and 12,000 mg/L. This allows for a much higher BOD and Ammonia load treatment in the same sized treatment tanks.

After the anoxic an aeration tanks, the flow of effluent will pass to the new MBR stage. This comprises three concrete tanks in which ACWA is to install 42 double deck membrane units (each containing 400 membrane panels) - three to a tank. Sets of units have an automatic flushing valve which provides daily cleaning of the membrane's coarse bubble air diffusers. The flushing valve will open for a short duration to allow sludge to be drawn into the diffuser and, via a venture effect, the sludge and air is discharged through the flush valve. This operation cleanses the diffuser of any blockages and completely eliminates the need to flush the system with wash-water. Sludge in the MBR stage is recycled to the anoxic tank.

MBR in operation

Within the MBR tanks, the flat sheet membrane panels, housed in stainless steel cases, will be aerated by a course bubble diffuser system. The membrane panels are securely retained and cannot touch or abrade each other, while the cases also act as flumes to ensure effective tank mixing and even distribution of the biomass. The system provides cross-flow filtration in which mixed liquor flows parallel to the membrane, whilst water permeates through it. This cross flow process prevents any fouling of the membrane surfaces.

Membrane panels have an average pore size of 0.4 microns, which in operation becomes covered by a dynamic layer of protein and cellular material. This further enhances the performance effectiveness of the filtration process by providing an effective pore size of less than 0.01 microns, which puts it in the range of ultrafiltration.