Dropping the Secondary Clarifier: What an MBR Buys You and What It Costs

The idea behind a membrane bioreactor is a single substitution: take the secondary clarifier at the end of biological treatment and replace it with membrane modules. Everything else follows from that. Activated sludge concentration in the bioreactor stays high, the organic load of the biological stage rises, the footprint of the whole facility shrinks, and because the sludge load is kept low, excess sludge drops as well. Membrane-separation equipment immersed in the aerobic basin retains activated sludge and macromolecular organics in the tank. MLSS concentration in an MBR can reach 8,000-10,000 mg/L or higher, and sludge age (SRT) can be extended beyond 30 days.

What Retention Buys

Keeping the biomass in the tank changes the ecology. Because retention is effective, an MBR preserves long-generation-cycle microorganisms, so the wastewater gets deep purification; nitrifiers multiply fully in the system, nitrification becomes obvious, and deep phosphorus and nitrogen removal comes within reach. The mechanism is the complete separation of HRT and SRT - hydraulic retention time no longer dictates how long the sludge stays, which makes operation control flexible and stable.

Effluent quality is the visible payoff. Under high MLSS and microfiltration the product water is stable and good, with low SS and turbidity. Main pollutant removals reach COD ≥ 93% and SS = 100%. Product water has near-zero SS and turbidity, and treated low-pollution municipal wastewater can be reused directly as mid-water or on-site reclaimed water. Microfiltration also retains most bacteria, so disinfectant dose falls and the reused water is safer. MLSS at 6,000-10,000 mg/L cuts aerobic basin volume, and the enclosed design keeps nuisance, noise and odour low.

MBR Is Also a Water Strategy

China is a water-short country, and wastewater treatment and reuse is an effective way to develop and utilise water resources. Reuse means treating urban and industrial sewage via MBR and similar equipment for non-potable purposes - greening, flushing, ornamental-water replenishment - while reserving clean water for drinking. Urban and industrial sewage is locally available, which avoids long-distance conveyance and prevents sewage leakage during transport from polluting groundwater. Since the 1980s, MBR has drawn increasing attention as a research hotspot. An internationally emerging water-treatment technology of the 1990s, it combines biological treatment with membrane separation: first biochemical degradation of organics and cultivation of dominant bacteria to block pathogens, then membrane filtration of suspended solids and water-soluble macromolecules to bring turbidity down to discharge standard.

China's MBR research is less than a decade old but has progressed rapidly in three directions. The first is exploring combinations of different biological processes with membrane units, extending from activated sludge to contact oxidation, biofilm, combined activated-sludge/biofilm and two-phase anaerobic processes. The second is studying the factors, mechanisms and models of treatment performance and membrane fouling, to find the conditions and parameters that minimise fouling and improve capacity and stability. The third is broadening scope from domestic sewage to high-concentration organic wastewater from food and brewery plants and refractory industrial wastewater from petrochemical and dyeing operations - though domestic sewage remains primary. MBR is now applied in over ten countries including the USA, Germany, France and Egypt, at scales from 6 to 13,000 m³/d.

Configurations

MBR membranes are of two types, organic and inorganic, with organic membranes - typically polyethylene and polypropylene - the common choice. Separated MBRs usually use UF modules with molecular-weight cutoff generally 20,000-300,000; a larger cutoff gives larger initial flux, but not necessarily larger long-run flux. The immersed, or integrated, system places membrane modules directly in the reactor, draws filtrate by suction, and generates the cross-flow needed for surface cleaning by air scouring from diffusers set just below the membrane, with mixed liquor rising along the flow and creating shear at the membrane surface.

Aerobic MBR serves both municipal and industrial treatment - reusable effluent for municipal, and specific pollutants such as greasy and oily contaminants for industrial. Anaerobic MBR uses high-efficiency membrane retention to solve the problem of anaerobic sludge washing out, which also strengthens the reactor: taking UASB plus membrane as an example, the anaerobic MBR no longer needs a three-phase separator for solid-liquid-gas separation. In a two-phase anaerobic MBR, membrane separation raises acidogen concentration in the acidogenic reactor and holds macromolecules there for hydrolysis and fermentation, maintaining high acidification. Because an MBR has no aeration, high-concentration anaerobic MBRs all use the separated type to keep the sludge suspended. A Membrane Aeration Bioreactor, MABR, uses the membrane for gas transfer instead - usually supplying oxygen to aerobic processes - which enables bubbleless aeration and greatly raises oxygen-transfer efficiency.

Fouling Is the Real Operating Problem

Once the membrane is selected its physicochemical properties are fixed, so operating mode becomes the main factor in MBR fouling. Sludge concentration and mixed-liquor viscosity affect flux, and so does mixed-liquor filterability - sludge characteristics and biology both drive flux decline. Adding PAC and coagulant improves sludge-water separation by forming larger, less viscous flocs and reducing clogging, but excessive coagulant limits sludge activity and hurts capacity and performance. Standard mobile modular design keeps installation fast and phased expansion simple, which is why MBR suits retrofits of old plants - often it is just a matter of adding MBR modules. PLC control makes operation and maintenance easy and automation feasible. One practical detail: a mid-water system collecting bath wastewater needs a hair filter, because hair and fibre will otherwise clog pumps and the MBR and can paralyse the whole system.

Where CCAS and CMF Still Compete

The CCAS process - the Continuous Cycle Aeration System - is a continuous-feed SBR aeration system improved from SBR (Sequencing Batch Reactor), and it remains a reasonable alternative where membranes are not wanted. Pretreatment requirements are low: only a 15 mm gap mechanical screen and a grit chamber. During aeration, sewage and sludge are in perfect complete mixing, so BOD and COD removal reaches 95%. The repeated aerobic-anoxic and aerobic-anaerobic cycles strengthen phosphorus uptake and nitrification-denitrification, giving nitrogen and phosphorus removal above 80%. During settling the whole tank is in ideal settling conditions, so effluent SS is very low.

Continuous microfiltration, CMF, is the other membrane option. Conventional methods need complex treatment to meet RO/NF feed requirements, while CMF needs only one filtration step to produce high-quality pretreatment water directly as RO/NF feed, with yield above 95%. The company's polypropylene hollow-fiber elements run raw water outside the membrane and permeate inside, at high recirculation ratio and high velocity in the tubes to reduce fouling, with air-water backwash scouring the surface - effluent turbidity near zero.