January 2006

Submersible membranes for dairy waste

ACWA MBR – the Middle Eastern arm of ACWA Services - has supplied Submersible Membrane Systems for a sewage and effluent treatment plant extension/upgrade project at the Almarai Trading Company’s dairy products processing facility in Al Kharj, Saudi Arabia. The Kubota flat sheet submersible membranes systems were supplied by ACWA to the main contractor for the project, WETICO-Saudi Berkefeld.

Due to increased dairy production and the introduction of new product lines at Almarai’s processing facility, higher levels of sewage and process wastewater were being discharged to the existing effluent treatment plant – originally installed by ACWA Services in 1995.

To cope with these higher discharge levels, the company appointed specialist environmental consultants McDonald Stevens Associates (MSA) - who designed the original plant in 1994 - to design the upgrade system. The MSA design philosophy was to incorporate the very latest in membrane bioreactor technology.

The Almarai Company Limited owns and operates a number of large-scale dairy farms in the Al Kharj region, with more than 50,000 head of cattle and as a horizontally integrated enterprise, processes and supplies fresh milk, juice, laban and yogurt products for wide-scale distribution and sale throughout the Kingdom of Saudi Arabia and the Gulf States.

In an area lacking natural water resources, the sewage and effluent treatment plant plays a pivotal role in the Almarai operation – providing not only an economic solution to effluent and sewage disposal, but a continuous supply of recycled water for irrigation.

For the revised project design, Kubota membranes were selected because of their reliability and cost-effective performance - particularly when producing irrigation standard effluent. At the cutting-edge of membrane technology, Kubota membranes are well-proven internationally and have the approval of the Californian Health Department for achieving its tough Title22 consent standards for irrigation reuse.

The Membrane Process

Within the bioreactor, the Kubota submersible membranes provide cross-flow solids-liquid separation and produce a high quality disinfected effluent. The process requires no primary or secondary settlement stages and no additional tertiary treatment or UV stages to achieve quality typically better than 5:5:5 mg/L BOD: TTS. NH3. The membranes provide a fixed barrier which removes all suspended solids, bacteria and viruses and providing there is adequate aeration, the system completely eliminates all traces of Ammonia.

Within the bioreactor, a series of these flat sheet membrane panels are housed in stainless steel cases, aerated from below by a coarse bubble oxygen diffuser system. An advantage of this design is that the membrane panels are securely retained and cannot touch or abrade each other. The cases also act as flumes, which ensure effective mixing of effluent and even distribution of the biomass. The membrane panels have an average pore size of 0.4 microns, which, in operation, becomes covered by a layer of protein and cellular material. This enhances the effectiveness of the filtration process by providing an effective pore size of less than 0.01 microns - a feature which puts it within the ultrafiltration range.

The membranes are arranged in series within the bioreactor tank, according to the hydraulic flow required to be treated and are designed for ease of installation and maintenance. Plants are designed so that all sludge removed is in a stable condition and suitable for direct feeding to belt presses or sludge drying beds. Sludge is removed at approximately 1.8% dry solids.

The Kubota membranes utilise a gravity withdrawal method which produces a robust solution with less wear on the membranes. The Kubota membrane has a lifespan which is by far the longest of any of its competitive equivalents, with a life expectancy up to twelve years and is covered by warranties of between 5 and 10 years.