June 2006

Mobile Sewage Treatment Plants

ACWA Emirates LLC – the Middle East subsidiary of UK-based ACWA Services - has supplied six Containerised, Mobile, Sewage Treatment Plants for use at desert site camps during the construction of two major gasification projects in Abu Dhabi (UAE).

Designed and built by ACWA Emirates for Consolidated Contractors Company (CCC), two of the plants will be used for treating raw sewage at the Ruwais 3rd Natural Gas Liquification site and the others at an Onshore Gas Development project in Habshan.

Treated effluent will be used as flush water for on-site ablution blocks and for spraying on desert roads to limit the amount of blown sand and improve compaction. Some of the surplus effluent will also be sold on to local cement manufacturers for use as process water.

Designed to handle sewage from an anticipated population of 7,500 site workers, the mobile treatment processes are based on Kubota Submerged Membrane Bioreactor (MBR) Technology and at each site, are capable of treating raw sewage up to a peak demand of 1,500m3/day. At the process design stage, ACWA anticipated a high peaking factor and derived a peak hourly flow rate for each plant of four times that of peak daily flow - or 166m3/hour.

The treatment process is designed to produce high quality effluent from a BOD loading of 60g/person/day, with a BOD concentration of 300mg/L and COD concentration of 600mg/L. Each plant has a peak flow capacity of 33m3/hour with the membrane tanks operating at an MLSS range of 12,000 to 18,000mg/L.

The containerised processes and associated storage tanks (each with a capacity of 300m3/hour) are fully transportable and can be easily and speedily relocated at other sites once the projects are completed. Each process is remotely operated - requiring no more than weekly monitoring and can be easily expanded to accommodate additional tanks or membrane units if required to meet any future increase in flow.

Kubota low maintenance, flat plate submerged membrane systems were selected for use on these systems because of their ability to provide a final effluent which more than meets the stringent ADNAC Water Standards for irrigation and discharge to the sea or desert terrain.

The Process

In each system, three membrane units are submerged in a bioreactor to produce a high quality, fully disinfected effluent. This is typically >6 log removal of coliforms and > 4 log removal of virus. There is no requirement for backwashing or permeate suction pumps as the membranes are cleaned automatically with course bubble air scouring.

The process does not require the use of chemicals, (only a six monthly clean with dilute sodium hypochlorite) and due to the system’s use of gravity filtration, membranes can normally be expected to function effectively for periods in excess of ten years continuous operation. Each compact process includes flow balancing, forward pumping, fine screening, membrane filtration, instrumentation and controls.

The 33m3/hour forwarding pumps, 2 duty/1 standby, operate at fixed speed according to the liquid levels in the balancing tanks. A 100mm pipeline connects a dedicated forwarding pump to the inlet level of the fine screen at each respective STP and each pumping station incorporates a level control, non-return isolation valves, a pedestal guide bar and SS chain.

In operation, the flow of sewage is directed by the dedicated forwarding pump so that each plant receives an equal amount of flow. The flow passes through a powered rotary 3mm plate screen mounted on top of each treatment tank, to remove any large particles which could impact on the downstream membrane process. The screening process is designed to protect and extend the life of the membranes, ensure cost-effective operation, reduce maintenance and eliminate down-time.

MBR Technology

In each of the six containerised processes, three simple flat sheet membrane panels, housed in stainless steel casings, are submerged in an activated sludge treatment tank and are aerated by coarse bubble diffuser systems. A major advantage of the system design is that the membrane panels are securely retained and cannot touch or abrade each other, while the cases act as flumes to ensure effective tank mixing and even distribution of the biomass.

The Kubota membrane units are manufactured with an average pore size of 0.4 microns which, in operation, becomes covered by a dynamic layer of protein and cellular material. This further improves the effectiveness of the filtration process by providing a pore size of less than 0.01 microns – well within the range of ultrafiltration.

Within the process tank of each mobile system, the membrane units are arranged in banks, each with a common automatic flushing valve, to enable daily cleaning of the ‘centipede’ air diffuser. The flushing valve opens for a short duration to allow sludge to be drawn into the diffuse via a venture effect and closes again to allow the discharge of sludge. In this way, the system does not require flushing with wash-water.

Operating as an industrial and municipal water and wastewater treatment contractor throughout the Middle East, ACWA operates from offices in Dubai, Jordan, Bahrain and Abu Dhabi. The company has only recently begun construction of a $90M contract to design and build two large RO desalination plants on Palm Jumeirah – the palm-shaped island development created off the coast of Dubai (UAE)