text: Membrane bioreactors were introduced in the late 1960s, shortly after commercial-scale ultrafiltration and microfiltration membranes became available. The original designs were introduced by Dorr-Oliver Inc. and combined the use of an activated sludge bioreactor with a cross-flow membrane filtration loop. The flat sheet membranes used in this process were polymeric and featured pore sizes ranging from 0.003 to 0.01 μm. Although the idea of replacing the settling tank of the conventional activated sludge process was attractive, it was difficult to justify the use of such a process because of the high cost of membranes, the low economic value of the product (tertiary effluent) and sometimes rapid losses of performance due to membrane fouling. As a result, the initial design focus was on the attainment of high fluxes, and it was, therefore, necessary to pump the mixed liquor and its suspended solids at high cross-flow velocity at significant energy demand (of the order 10 kWh/m product) to reduce fouling. Because of the poor economics of the first-generation devices, they only found applications in niche areas with special needs such as isolated trailer parks or ski resorts.
  The next breakthrough for the membrane bioreactor came in 1989 with the introduction of submerged membrane bioreactor configurations. Until then, membrane bioreactors were designed with a separation device located external to the reactor (side stream membrane bioreactors) and relied on high trans-membrane pressure to maintain filtration. The submerged configuration takes advantage of coarse bubble aeration to produce mixing and limit fouling. The energy demand of the submerged system can be up to 2 orders of magnitude lower than that of the side stream systems and submerged systems operate at a lower flux, demanding more membrane area. In submerged configurations, aeration is considered as one of the major parameters in process performance both hydraulic and biological. Aeration maintains solids in suspension, scours the membrane surface, and provides oxygen to the biomass, leading to better biodegradability and cell synthesis. Submerged membrane bioreactor systems became preferred to side stream configurations, especially for domestic wastewater treatment.
  The next key steps in membrane bioreactor development were the acceptance of modest fluxes (25 percent or less of those in the first generation) and the idea to use two-phase (bubbly) flow to control fouling. The lower operating cost obtained with the submerged configuration along with the steady decrease in the membrane cost led to an exponential increase in membrane bioreactor plant installations from the mid-1990s. Since then, further improvements in membrane bioreactor design and operation have been introduced and incorporated into larger plants. While earlier devices were operated at solid retention times as high as 100 days with mixed liquor suspended solids up to 30 g/L, the recent trend is to apply lower solid retention times (around 10–20 days), resulting in more manageable suspended solids levels (10 to 15 g/L). Thanks to these new operating conditions, the oxygen transfer and the pumping cost in the reactors have tended to decrease and the overall maintenance has been simplified. There is now a range of membrane bioreactor systems available commercially, most of which use submerged membranes although some side stream modules are available; these side stream systems also use two-phase flow for fouling control. Typical hydraulic retention times range between 3 and 10 hours. For the most part, hollow fiber and flat sheet membrane configurations are utilized in membrane bioreactor applications.
  Despite the more favorable energy usage of submerged membranes, there continued to be a market for the side stream configuration, particularly in smaller flow industrial applications. For ease of maintenance, side stream configurations can be installed on a lower level in a plant building, and thus membrane replacement can be undertaken without specialized lifting equipment. As a result, research and development has continued to improve the side stream configurations, and this has culminated in recent years with the development of low energy systems which incorporate more sophisticated control of the operating parameters coupled with periodic backwashes, which enable sustainable operation at energy usage as low as 0.3 kWh/m3 of product.