The Al Ansab sewage treatment plant (STP) serves the central area of Muscat, the capital of Oman. When commissioned in 2010, the plant was the largest flat-plate membrane bioreactor facility in the world, with an initial capacity of 55,000 m3/day. Expansion to 125,000 m3/day is under design and when operational in 2016 the plant will once again be one of the largest MBRs of its kind in the world. With limited space available, and a desire for consistent high quality effluent for full public access applications, the membrane bio-reactor (MBR) process was selected as the treatment technology for the plant. The plant equipment and tankage installed under the first phase was sized to serve a maximum month average flow rate of 55,000 m3/day, while buildings, hydraulics, interconnecting piping and electrical power supply were sized to serve the second phase total capacity of 84,000 m3/day.

Kubota EK400 flat sheet membranes of 0.4 μm pore size were selected for the MBR. Eight membrane tanks were provided in the first phase, each having 40 double stacked membrane units. Each double stacked unit has 400 membrane panels making the total membranes sheets installed in the first phase over 120,000 panels. Al Ansab MBR plant expansion to the second phase is currently under detailed design and the new Kubota membranes module RW400 will be used in this phase.

The plant was designed to accept sewage delivered by sewer networks. Due to delay in completing the sewer network the new plant was commissioned using trucked sewage diverted from the old plant. This is a non-standard condition for the MBR plant, and Haya implemented several modifications in anticipation of this situation. The high volume of trucked wastewater was addressed with flow and load equalization systems to dampen the impact of shock loading. The Al Ansab STP is performing very well and complying with all the required effluent standards. Many lessons have been learned in both the design and operation of such a large MBR plant.