Membrane Bioreactor Design Essentials: Interpreting HJ 2010-2011 Parameters for Engineering Checkout

1. Scope and Code Constraints

A membrane bioreactor replaces the secondary clarifier with a membrane barrier for solid-liquid separation. The bioreactor therefore runs at a biomass concentration far above that of conventional activated sludge, shrinking the biological volume while improving effluent quality.

• HJ 2010-2011 sets floor removal efficiencies for a membrane biological treatment system: COD 90%, BOD5 95%, SS 99% and ammonia nitrogen 90%.
• The specification applies to municipal wastewater and biodegradable industrial wastewater treated by membrane biological processes, and serves as the technical basis for environmental impact assessment, design, construction, acceptance and subsequent operation.
• Approved by the Ministry of Environmental Protection on 24 October 2011; effective 1 January 2012.

2. Pretreatment Requirements

• An ultra-fine screen should be installed ahead of the membrane biological treatment system.
• A hair collector should be provided where the wastewater carries significant hair or textile fibre.
• Where the BOD5/COD ratio is below 0.3, measures to improve biodegradability should be adopted.
• Sharp and hard particles must be removed before the wastewater enters the membrane bioreactor.

3. Submerged Membrane Bioreactor Design Values

Clause 6.3.3.3 of HJ 2010-2011 requires sludge loading and mixed liquor concentration to be fixed by testing; where no test data exist, Table 2 applies:

• Sludge loading: 0.05-0.15 kg BOD5 per kg MLSS per day, for both hollow fibre and flat sheet membranes.
• Mixed liquor suspended solids (MLSS): 6000-12000 mg/L for hollow fibre; 6000-20000 mg/L for flat sheet.
• Transmembrane pressure (TMP): 0-60 kPa for hollow fibre; 0-20 kPa for flat sheet.
• Freeboard 0.5-1.0 m; design water temperature 12-38 degrees Celsius, with winter insulation or heating in northern regions designed to GB 50014.

For a submerged anoxic-MBR configured for simultaneous nitrogen and phosphorus removal (clause 6.3.3.6):

• Anaerobic zone sludge concentration 20-25 g/L, dissolved oxygen not above 0.2 mg/L.
• Membrane bioreactor sludge concentration 6-12 g/L; dissolved oxygen 2.0 mg/L inside the membrane cassette and 0.5 mg/L outside it.
• Sludge return ratio 100%-500%.

4. Membrane Elements and Flux Selection

• Membrane material: PVDF or PE preferred; PP, PS, PES, PAN and PVC are also acceptable.
• Nominal pore size between 0.01 and 0.4 micrometre.
• Under design conditions, hollow fibre service life is not less than 3 years and flat sheet service life not less than 5 years.
• Element selection criteria: clean water flux 120-750 L per square metre per hour at 10 kPa; narrow, uniform pore distribution; high oxidation resistance; acid and alkali resistance; elongation below 10%.
• Design operating flux: 12-30 L per square metre per hour for hollow fibre; 16-50 L per square metre per hour for flat sheet.

5. Layout, Aeration and Permeate

• Plan layout: cassettes evenly distributed in the aeration tank, with at least 300 mm clearance between the cassette and the tank wall.
• Elevation: measured from the lowest normal operating water level, at least 400 mm from the top of the cassette to the water surface, and at least 300 mm from the air diffuser to the tank floor.
• Aeration: jet aeration combined with perforated pipe aeration, or perforated pipe combined with fine bubble aeration; the aeration grid sits below the cassettes, pipelines are sealed and free of debris; scouring air demand is fixed by testing.
• Suction pumps run intermittently (the code cites 9 minutes producing, 1 minute idle); the on/off ratio is set by testing. Pump capacity equals design flow divided by actual daily operating hours, multiplied by a safety factor of 1.2-1.5.
• The permeate system must be equipped with an online pressure gauge, flow meter and turbidity meter.

6. Membrane Cleaning

• Clean-in-place frequency: at least monthly for hollow fibre; every 2-3 months for flat sheet.
• Chemical: sodium hypochlorite unless the manufacturer specifies otherwise, dosed at 2.0 litres per square metre per event plus pipeline volume, at a concentration of 1-3 per mille.
• Sequence: stop permeate production and aeration, transfer the full chemical volume into the membrane over 30-40 minutes, soak for 20-30 minutes, then discharge spent solution to a waste liquor tank or the head of the pretreatment train.
• Off-line cleaning, with a dedicated tank and lifting gear, recovers flux lost to irreversible fouling.

7. Sidestream Configuration Envelope

• Filtration mode: cross-flow.
• Normal membrane system recovery 10%-15%.
• Cross-flow velocity 3-5 m/s; flux 40-150 L per square metre per hour.
• Operating pressure 0.2-0.4 MPa; sludge concentration 10-40 g/L.
• Freeboard 0.3-0.5 m; the recirculation concentrate tank holds at least 15 minutes of normal membrane system flow, with a 0.5-1.5 m sludge settling zone.

8. Compliance Check

MBR permeate is very low in suspended solids and turbidity, so it normally satisfies the Class 1A daily mean SS limit of 10 mg/L in Table 1 of GB 18918-2002. The amendment to GB 18918-2002, however, adds Table 4 of maximum allowable instantaneous concentrations: for Class 1A, COD 75 mg/L, total nitrogen 20 mg/L, ammonia nitrogen 10 (15) mg/L and total phosphorus 1 mg/L for plants built on or after 1 January 2006. Clause 6.3 of the amendment states that discharge concentrations must not exceed either the daily mean values in Tables 1 to 3 or the instantaneous values in Table 4; breaching either one counts as an exceedance. Design margins must therefore be verified against the instantaneous limits, not against daily means alone.

References

• HJ 2010-2011, Technical Specifications for Membrane Biological Processes for Wastewater Treatment (MEE release copy, in Chinese)
• HJ 2010-2011, English release copy (MEE)
• US EPA, Wastewater Management Fact Sheet: Membrane Bioreactors
• Oklahoma DEQ, Membrane Bioreactor (MBR) Guidance WQD-002, August 2017
• Amendment to GB 18918-2002, Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (in Chinese)