The control parameters for the MBR process are divided into four categories: sludge system parameters, membrane operation parameters, biochemical environment parameters, and return & sludge discharge parameters. Each parameter directly affects the process operation performance and effluent quality. Parameter adjustments must follow the principle of "stability and gradual change"; drastic adjustments are strictly prohibited. The specific parameters are as follows:
I. Sludge System Parameters
1. MLSS (Mixed Liquor Suspended Solids)
| Parameter | Control Range | Significance | Adjustment Principles & Precautions |
|---|---|---|---|
| MLSS | 8,000 – 12,000 mg/L; For denitrification/low temperature: 10,000 – 12,000 mg/L; For industrial wastewater: can be adjusted to 12,000 – 15,000 mg/L |
Represents the total microbial mass in the bioreactor; a core indicator of biochemical reaction efficiency. The high MLSS capability of MBR is a key advantage, enhancing degradation efficiency and shock load resistance. | Precautions: 1. Too high: accelerates membrane fouling, rapid TMP increase, increased energy consumption. 2. Too low: insufficient degradation capacity, effluent COD and ammonia nitrogen exceedance. 3. Adjustment: controlled by waste sludge discharge. If MLSS is too high, increase sludge discharge; if too low, reduce sludge discharge and increase the sludge return ratio. |
2. SRT (Sludge Retention Time)
| Parameter | Control Range | Significance | Adjustment Principles & Precautions |
|---|---|---|---|
| SRT | 15 – 30 days; For high denitrification demand: 20 – 30 days; For low temperature (<15°C): 25 – 30 days |
Determines the microbial community structure. Nitrifying bacteria have a long generation cycle (10-20 days) and require a long SRT to be retained, ensuring ammonia nitrogen removal. Also affects sludge activity and membrane fouling. | 1. Too short: loss of nitrifying bacteria, ammonia nitrogen exceedance; sludge is highly active but concentration is difficult to maintain. 2. Too long: sludge aging, reduced activity, accelerated membrane fouling, increased effluent COD. 3. Adjustment: controlled by the volume of waste sludge discharged. SRT = Total sludge mass in bioreactor ÷ Daily waste sludge discharge volume. |
| SV30 (Sludge Volume after 30 min settling) | 80% – 95% (significantly higher than conventional processes due to high MLSS) | A quick indicator for judging sludge settling performance and concentration, assisting in adjusting MLSS and sludge discharge volume. | 1. >95%: indicates excessively high sludge concentration or poor sludge settleability; increase sludge discharge. 2. <80%: indicates insufficient sludge concentration; reduce sludge discharge and increase the return ratio. 3. Note: SV30 in MBR should not be judged by conventional process standards; 80%-95% is generally normal. |
| SVI (Sludge Volume Index) | 80 – 150 mL/g (consistent with conventional processes) | Precisely characterizes sludge settleability and compactness; helps determine sludge bulking, which indirectly affects membrane fouling rate. | 1. >150 mL/g: sludge bulking, loose flocs, prone to blocking membrane pores; increase DO, control F/M, increase sludge discharge. 2. <80 mL/g: sludge mineralization or aging, poor activity; reduce sludge discharge, supplement nutrients. 3. Monitor regularly, at least once a week. |
| F/M (Food to Microorganism Ratio) | 0.05 – 0.2 kg BOD₅/(kg MLVSS·d) (Lower than conventional processes) |
Reflects the "food supply vs. demand" ratio for microorganisms. A low F/M favors nitrification and sludge stabilization, and reduces membrane fouling. | 1. Too high: microorganisms metabolize vigorously, sludge flocs become loose, accelerating membrane fouling. 2. Too low: sludge aging, reduced activity, decreased degradation efficiency. 3. Adjustment: adjusted through influent flow rate and MLSS. If F/M is too high, increase MLSS or reduce influent flow. |
II. Membrane Operation Parameters
| Parameter | Control Range | Significance | Adjustment Principles & Precautions |
|---|---|---|---|
| Membrane Flux | 15 – 25 L/(m²·h); Submerged MBR: 15 – 20 L/(m²·h); External (sidestream) MBR: 20 – 25 L/(m²·h) |
The permeate flow rate per unit membrane area per unit time; a core indicator of membrane operation efficiency, directly determining treatment capacity. | 1. Too high: sharply accelerates membrane fouling, rapid TMP increase, risk of membrane damage. 2. Too low: low treatment efficiency, wasted energy. 3. Adjustment: adjusted by controlling the suction pump frequency and on/off cycle based on TMP changes. Avoid sudden flux increases. |
| Transmembrane Pressure (TMP) | Normal: <15 kPa; Warning: 25 – 30 kPa; Chemical cleaning required: >35 – 40 kPa; Membrane scrapping (end-of-life): >50 kPa |
Characterizes the degree of membrane fouling. Higher TMP indicates more severe fouling and greater resistance to water permeation. | 1. TMP rises to 15-25 kPa: enhance online cleaning (increase frequency, extend duration). 2. TMP rises to 25-35 kPa: immediately perform online chemical cleaning, pause permeation. 3. TMP >35 kPa: perform offline chemical cleaning. 4. Monitor daily, record hourly. |
| Suction Mode | Intermittent suction: 7-9 min on, 1-3 min off; Low temperature/initial fouling: 6-8 min on, 2-3 min off |
Prevents sludge accumulation on the membrane surface caused by continuous suction, reduces membrane fouling, and extends membrane life. | 1. Strictly avoid continuous suction (no off-cycle); this rapidly aggravates fouling, causing a short-term TMP spike. 2. The off-cycle can be adjusted based on TMP; extend off-time if TMP rises quickly. 3. The on-time should be consistent, avoiding uneven cycles. |
| Membrane Scouring Aeration | 24 hours continuous; Scouring air flow rate: Submerged MBR: 10 – 15 m³/(m²·h); External MBR: no scouring aeration (pressure-driven filtration) |
Generates bubbles that agitate the membrane fibers, scouring the sludge off the membrane surface, preventing sludge attachment and mitigating fouling. Also provides a small amount of oxygen to maintain sludge activity in the membrane tank. | 1. Scouring aeration must not be interrupted (interruption >30 min can lead to rapid sludge attachment and compaction on the membrane surface, causing blockage). 2. Ensure uniform aeration to avoid localized insufficient scouring. 3. Regularly clean aeration piping and diffusers to prevent clogging and ensure stable air flow. |
| Membrane Cleaning Parameters | 1. Online Cleaning: Sodium hypochlorite concentration 500–1000 mg/L, Citric acid concentration 1%–2%; Cleaning duration: 30–60 min each; Frequency: 1–2 times/day. 2. Offline Cleaning: Sodium hypochlorite concentration 2000–5000 mg/L, Citric acid concentration 2%–3%; Soaking duration: 12–24 hours. |
Removes organic and inorganic foulants from the membrane surface and pores, restoring membrane flux and extending membrane life. | 1. Alternate use of sodium hypochlorite (for organic and biological fouling) and citric acid (for inorganic fouling) during online cleaning. 2. Cleaning chemical concentrations should not be too high to avoid damaging the membrane modules. 3. Offline cleaning is performed only when online cleaning cannot restore flux. Rinse thoroughly after offline cleaning to avoid chemical residue. |
III. Biochemical Environment Parameters
| Parameter | Control Range | Significance | Adjustment Principles & Precautions |
|---|---|---|---|
| Dissolved Oxygen (DO) | Aerobic zone: 2.0 – 3.5 mg/L; For high nitrification demand/low temperature: 3.0 – 4.0 mg/L; Anoxic zone: <0.5 mg/L; Anaerobic zone: <0.2 mg/L |
A core factor influencing microbial activity. Insufficient DO in the aerobic zone inhibits nitrification and aerobic phosphorus uptake. Excessive DO in anoxic/anaerobic zones inhibits denitrification and phosphorus release. | 1. Aerobic zone DO adjustment: control via aeration blower airflow. Increase airflow if DO is too low; decrease if too high. 2. Anoxic/Anaerobic zones: strictly avoid aeration; maintain moderate mixing intensity to prevent air entrainment causing DO rise. 3. Monitor daily, record every 2 hours. |
| pH | Overall: 6.5 – 8.0; Optimal for nitrification: 7.5 – 8.5; Optimal for denitrification: 6.5 – 7.5; Anaerobic zone: 6.5 – 8.0 |
Affects microbial enzyme activity. pH imbalance directly inhibits microbial metabolism, leading to decreased biochemical efficiency. | 1. pH <6.5: adjust by adding lime or sodium bicarbonate (prefer sodium bicarbonate for its mild, non-shocking effect). 2. pH >8.5: adjust by adding sulfuric acid. 3. Avoid drastic pH fluctuations (≤0.5 change per hour) as they can inhibit microbial activity. |
| Alkalinity | ≥100 mg/L (as CaCO₃); For high nitrification demand: ≥150 mg/L |
Nitrification consumes a significant amount of alkalinity (7.14 mg CaCO₃ alkalinity per 1 mg of ammonia nitrogen nitrified). Insufficient alkalinity causes pH to drop, inhibiting nitrification. | 1. Insufficient alkalinity: supplement with sodium bicarbonate. Avoid adding lime (can produce scale, clogging membrane modules). 2. Monitor regularly, daily. Increase monitoring frequency during periods of active nitrification. |
| Water Temperature | Optimum: 15 – 35°C; 10 – 15°C: treatment efficiency decreases; <10°C: efficiency drops sharply, nitrification nearly stops |
Affects microbial activity. Lower temperatures slow down microbial metabolism, reducing biochemical efficiency. | 1. Low temperature (<15°C): increase MLSS to 10,000–12,000 mg/L, extend SRT to 25-30 days, increase aerobic zone DO to 3.0-4.0 mg/L, consider supplementing carbon source. 2. High temperature (>35°C): increase aeration, reduce MLSS to prevent sludge aging. 3. Provide tank insulation (winter), avoid large temperature fluctuations. |
| C/N Ratio (Carbon to Nitrogen) | ≥4 (for denitrification); For high denitrification demand: ≥5 |
Denitrifying bacteria require a sufficient carbon source (organic matter) to complete denitrification. Insufficient carbon leads to total nitrogen exceedance. | 1. C/N <4: add an external carbon source (sodium acetate, glucose) upstream of the anoxic zone. Prefer sodium acetate (high utilization rate, no secondary pollution). 2. Avoid excessive carbon addition, as it can increase COD in the aerobic zone and accelerate membrane fouling. |
IV. Return & Sludge Discharge Parameters
| Parameter | Control Range | Significance | Adjustment Principles & Precautions |
|---|---|---|---|
| Sludge Return Ratio (External Return) | 100% – 200%; For denitrification/low temperature: 150% – 200%; For industrial wastewater: can be adjusted to 200% – 300% |
Returns sludge retained by the membrane tank to the bioreactor, maintaining stable MLSS in the bioreactor and supplementing the total microbial mass. | 1. Too low: insufficient MLSS in the bioreactor, decreased degradation efficiency. 2. Too high: increases energy consumption and may carry nitrates from the membrane tank into the anaerobic zone, inhibiting phosphorus release. 3. Adjustment: controlled by return pump flow rate. Adjust gradually based on MLSS changes, avoiding sudden large adjustments. |
| Internal Return Ratio (Nitrate Recycling) | 200% – 400%; For high denitrification demand: 400% – 500% |
Returns nitrified liquid (containing nitrate) from the aerobic zone to the anoxic zone, providing nitrate for denitrifying bacteria, ensuring nitrogen removal. | 1. Too low: insufficient nitrate supply, incomplete denitrification, total nitrogen exceedance. 2. Too high: increases energy consumption and may carry oxygen from the aerobic zone into the anoxic zone, inhibiting denitrification. 3. Adjustment: adjust based on effluent total nitrogen concentration. Increase internal return ratio if total nitrogen exceeds the limit. |
| Waste Sludge Discharge | Continuous, low-flow discharge; Discharge volume: controlled by SRT. Daily discharge volume = Total sludge mass in bioreactor ÷ SRT. |
Removes aged sludge and inorganic impurities from the system, maintaining stable MLSS and SRT. Also removes phosphorus-accumulating organisms from the system to achieve phosphorus removal. | 1. Strictly avoid long periods without sludge discharge: leads to sludge aging, accelerated membrane fouling, and potential exceedance of total phosphorus/total nitrogen. 2. Strictly avoid large intermittent sludge discharges: cause drastic MLSS fluctuations, affecting biochemical efficiency and membrane operation. 3. Discharge point: bottom of the membrane tank (where high-phosphorus sludge is retained). After discharge, monitor MLSS and adjust discharge volume accordingly. |
V. Core Summary of Parameter Control
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Sludge Control: Maintain high MLSS (8,000–12,000 mg/L), long SRT (15–30 days), stable SV30 and SVI, and avoid sludge bulking or aging.
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Membrane System Control: Use intermittent suction (e.g., 7 min on, 1-3 min off), 24-hour scouring aeration, strictly monitor TMP (<15 kPa target), perform regular cleaning, and prevent membrane fouling.
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Biochemical Environment Control: Maintain aerobic DO at 2.0–3.5 mg/L, strictly control DO in anoxic/anaerobic zones, pH between 6.5–8.0, alkalinity ≥100 mg/L, and C/N ≥4.
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Return & Sludge Discharge Control: Maintain external return ratio at 100%–200%, internal return ratio at 200%–400%, and discharge waste sludge continuously at a low flow rate to maintain system balance.
