Membrane Bioreactor Configurations Compared: From EMBR to Submerged Hollow Fibre, What Changes

MBR — Membrane Bio-Reactor — combines the activated sludge method with membrane separation. Memories of classification can get elaborate: by separation mechanism there are reaction membranes, ion-exchange membranes and osmotic membranes; by nature there are natural membranes such as biofilms and synthetic organic and inorganic membranes; by configuration there are plate, tubular, spiral and hollow-fibre forms. What matters for a plant is simpler: which arrangement of membrane and bioreactor delivers the effluent you need at an energy and fouling cost you can live with.

The configuration families

The aerated-membrane bioreactor was first reported by Cote and colleagues in 1988. Using gas-permeable dense membranes such as silicone rubber, or microporous hydrophobic polymer membranes in plate or hollow-fibre modules, it achieves bubble-free aeration while holding gas partial pressure below the bubble point. Contact time and oxygen-transfer efficiency improve, aeration control gets easier, and performance stops depending on the bubble size and residence-time behaviour that conventional aeration has to fight.

The extractive membrane bioreactor, or EMBR, exists for water that microorganisms should not touch. Some industrial wastewaters are too acidic or alkaline, or carry substances toxic to biomass; where volatile toxins are present, conventional aerobic treatment strips them out with the aeration airflow, destabilising performance and polluting air. The EMBR developed by the British scholar Livingston separates wastewater from activated sludge with a membrane: the wastewater flows inside, the sludge outside, and organic pollutants selectively cross to be degraded. Because the two sides are independent circuits, microbial conditions are unaffected by wastewater quality, and operating conditions such as HRT and SRT can each be held in their optimal range.

The solid-liquid separation MBR is the most extensively studied: it simply replaces the secondary sedimentation tank of the conventional activated sludge process with membrane separation.

Why removing the clarifier changes the biology

In conventional treatment, sludge-water separation depends on how well the sludge settles, and settleability depends on aeration tank conditions — which limits the method. The clarifier caps sludge concentration around 1.5-3.5 g/L, which caps the biochemical reaction rate. HRT and SRT are interdependent, so raising volumetric load fights against lowering sludge load, the system makes large amounts of excess sludge whose disposal runs 25-40% of plant operating cost, and sludge bulking is a standing risk. MBR separates sludge retention time from hydraulic retention time. MLSS can run far higher, highly effective bacteria establish, and the biochemical rate rises. Lowering the F/M ratio cuts excess sludge, in theory to zero.

Split versus submerged

The split-type MBR puts the membrane module outside the bioreactor. Mixed liquor is pumped under pressure to the membrane, permeate passes, and solids and macromolecules return with the concentrate. It runs reliably, membrane flux is generally large, and cleaning, replacement and addition are easy. The cost is energy: reducing deposition on the membrane surface demands a high cross-flow velocity from the circulation pump, and the shear from high-speed rotation inactivates some microbial cells.

The submerged MBR places the module inside the bioreactor. Influent enters, activated sludge removes most pollutants, and water is drawn through the membrane under suction. It saves the circulation system, uses less energy and occupies less space than the split type, and has drawn particular attention in recent years. The trade-off is lower membrane flux, greater fouling propensity, and fouled membranes that are awkward to clean or replace.

Module forms and what the record shows

The plate module is the earliest form, resembling an ordinary plate-and-frame filter press: simple to manufacture and assemble, easy to operate, clean and replace, but complex to seal, with large pressure loss and low packing density. The first MBR plant was built by Dorr-Oliver in the United States in 1967, treating 14 m3/d. Japan put a water-reuse system into a high-rise building in 1977 and built two plants of 10 m3/d and 50 m3/d in 1980; by the early 1990s 39 such plants were running there with a maximum capacity of 500 m3/d. In 1997 Wessex Water built what was then the world's largest MBR at Porlock in the UK at 2,000 m3/d, followed by a 13,000 m3/d plant at Swanage in 1999. Canada's Zenon Environmental applied tubular and then submerged hollow-fibre MBRs across more than ten locations at scales from 380 m3/d to 7,600 m3/d. In the United States an MBR was built in Ohio in the early 1990s for an automobile manufacturer's wastewater at 151 m3/d. Envirogen commissioned a landfill leachate MBR in New Jersey at about 1,500 m3/d, holding bacterial concentration to 50,000 g/L and treating influent COD from several hundred to 40,000 mg/L at above 90% removal. Lyonnaise des Eaux built a 400 m3/day drinking water plant at Douchy in France in 1995 with effluent nitrogen below 0.1 mgNO2/L and pesticide below 0.02 ug/L. In China, Tsinghua University's submerged MBR pilot passed national appraisal in May 1998, and a Tianjin University team built a demonstration system in 2000 treating 25 tons per day in 10 square metres at 0.7 kWh per ton.

Special duties and the energy bill

MBR suits streams that defeat conventional biology. Nightsoil sewage carries very high organic content; Japan's NS system combines plate membranes with a high-concentration aerobic reactor, holding sludge at 15,000-18,000 mg/L, and by 1994 more than 1,200 such systems were in service. Since the 1990s the range has widened to food, aquaculture, livestock, cosmetics, dye and petrochemical wastewater. Effluent is clear enough to beat the Water Quality Standard for Non-potable Urban Reuse Water (CJ25.1-89) and can be reused directly. The cost is power: membrane driving pressure, the high MLSS that demands stronger aeration for oxygen transfer, and the cross-flow needed to scour the surface all push energy above conventional biological treatment. Membrane life and cleaning regime, not effluent quality, are what decide whether an MBR project works economically.