MBR Plus RO Dual-Membrane Water Reuse: Process Configuration, Flux Selection and Microbiological Interface

1. Why MBR Is the Ideal RO Pretreatment

Reverse osmosis succeeds or fails on pretreatment rather than on the membranes themselves. Conventional secondary effluent feeding RO through media filtration supports an average design flux of only 11 (9-13) litres per square metre per hour, while MF/UF pretreatment lifts that to 17 (13-20). An MBR separates solids across a 0.01 to 0.4 micrometre barrier, delivering permeate with virtually no suspended solids and very low turbidity, effectively giving RO feed quality equal to or better than MF/UF. The higher flux band can therefore be used, and RO cleaning intervals lengthen substantially.

2. Key MBR Values (HJ 2010-2011)

• Floor removal efficiencies: COD 90%, BOD5 95%, suspended solids 99%, ammonia nitrogen 90%.
• Submerged design values: sludge loading 0.05-0.15 kg BOD5 per kg MLSS per day; MLSS 6000-12000 mg/L for hollow fibre and 6000-20000 mg/L for flat sheet; transmembrane pressure 0-60 kPa for hollow fibre and 0-20 kPa for flat sheet.
• Design operating flux: 12-30 litres per square metre per hour for hollow fibre; 16-50 for flat sheet.
• With nutrient removal: anaerobic zone sludge concentration 20-25 g/L with dissolved oxygen not above 0.2 mg/L; membrane bioreactor concentration 6-12 g/L, dissolved oxygen 2.0 mg/L inside the cassette and 0.5 mg/L outside it; sludge return ratio 100%-500%.
• The permeate system must carry online pressure gauge, flow meter and turbidity meter; online turbidity is also the first warning of a sudden change in RO feed quality.

3. RO Flux by Feed Type

• Secondary effluent through media filtration: average flux 11 (9-13) litres per square metre per hour; lead element flux below 19; concentrate flow above 3.6 cubic metres per hour; feed flow below 12; pressure drop below 2.0 bar per vessel; element recovery below 12%; flux decline above 15% per year; salt passage increase above 15% per year.
• Secondary effluent through MF/UF, the band applied to MBR permeate: average flux 17 (13-20); lead element flux below 25; concentrate flow above 3.6; feed flow below 13; pressure drop below 2.0 bar; element recovery below 13%.
• Feed gates: SDI15 below 5, preferably below 3; free chlorine below 0.1 mg/L, practically zero; temperature 5-45 degrees Celsius; cleaning pH window 2-11; Langelier Saturation Index negative or near zero.

4. Recovery and Concentrate Disposal

• Reuse RO systems on brackish or low-salinity feed are typically designed at 65%-85% recovery, with multi-stage designs reaching 85%-95%.
• Concentration factor approximates 1/(1 - recovery): at 75% recovery the concentrate carries roughly four times the feed salinity, and at 90% roughly ten times.
• The concentrate destination must be settled before the process route is fixed. Options include recycle to the front of the plant, subject to a salt accumulation check, evaporation and crystallisation, or dedicated discharge. Overlooking concentrate disposal is the most common cause of mid-project redesign in dual-membrane schemes.

5. Energy and Operation

• Brackish and reuse RO typically runs at 0.5-2 kWh per cubic metre; MBR energy is dominated by aeration, covering both process oxygen demand and membrane scouring.
• MBR suction pumps run intermittently (the code cites 9 minutes producing, 1 minute idle) with the on/off ratio set by testing; capacity equals design flow divided by actual daily operating hours times a safety factor of 1.2-1.5.
• RO cleaning intervals are usually assumed at one to six months at the design stage; MBR ahead of the RO lengthens that interval considerably.

6. Microbiological and Disinfection Interface

The reuse pathway carries stricter microbiological limits than the non-reuse pathway. Table 4 of the GB 18918-2002 amendment sets the instantaneous faecal coliform limit for Class 1A at 10 to the power 3 MPN per litre where the effluent is reused, against 10 to the power 4 MPN per litre where it is not, a full order of magnitude difference. Consequently:

• Disinfection, whether chlorination, ultraviolet or ozone, must be sized for the reuse limit and not selected against the non-reuse figure.
• HJ 2010-2011 likewise provides that chlorination, ultraviolet or ozone may be applied where effluent microbiology is strictly controlled, and activated carbon adsorption or chemical oxidation where odour and colour control are critical.
• RO permeate is itself very low in microorganisms, but reclaimed water can be recontaminated during storage and distribution, so disinfection contact time must be checked against the worst case.

References

• HJ 2010-2011, Technical Specifications for Membrane Biological Processes
• Design Guidelines for the Design of Reverse Osmosis Membrane Systems (flux limits for secondary effluent feed)
• WHO, Guidelines for Drinking-water Quality
• Amendment to GB 18918-2002 (in Chinese)