Submerged MBR Commissioning, Step by Step: Clean-Water Runs, Seeding and the 5 kPa Cleaning Trigger

A submerged membrane bioreactor (MBR) does not fail because the catalogue numbers were wrong; it fails because somebody rushed the start-up. The sequence below follows the order used on real installations: clean-water verification, sludge seeding, first filtration, steady-state operation, and the cleaning routines that keep transmembrane pressure (TMP) from drifting upward year after year.

Why commissioning decides membrane life

Everything that shortens a membrane's life — fouling, uneven aeration, grit damage, chemical shock — traces back to decisions made before the first load of raw water arrives. Treating commissioning as a checklist rather than a formality is the cheapest membrane-life extension available, and it costs nothing but discipline: verify the hardware on clean water, seed properly, then filter.

The clean-water run: verify before you seed

Before a single drop of raw water enters the tank, walk the module through a clean-water run. Confirm the air and wastewater piping is connected as drawn, that the membrane cassette is fastened firmly to the aeration box, and that the reaction tank has been cleared of soil and construction dust — grit is one of the fastest ways to damage membrane elements. Open the air release valve before filling with tap or filtered water so trapped air escapes the elements, then fill to the operating level and close the valve.

Start the aeration blower and watch two things: the aeration rate and its evenness across the module. Foam during this stage is normal — it comes from insoluble, biodegradable hydrophilic substances in the new membrane and can be ignored. Where one blower feeds several modules, measure each branch; unequal air distribution usually traces back to branch pipe diameters or a partly blocked line, and it must be equalised now, not after seeding. While the tank runs on clean water, test the control equipment, measure transmembrane pressure and water temperature at the design filtration rate and at its normal, maximum and minimum values, and record everything. Stop filtration and aeration once the performance test is complete.

Seeding: the step that cannot be skipped

Filtering raw water through an unseeded membrane is the classic way to foul it months ahead of schedule. Seed sludge should come from a plant treating similar wastewater, at a mixed liquor concentration of about 20,000 mg/L. Dose enough to lift the mixed liquor in the membrane tank above 7,000 mg/L, start raw-water feed immediately after seeding, and screen that feed through openings of 5 mm or less to keep debris out. Commercial seeding agents are not a substitute — skip them.

The operating window worth guarding

Once seeding is complete, start aeration first, then filtration, then feed. Stable performance holds only while four parameters stay in range. MLSS should sit between 7,000 mg/L and 18,000 mg/L; below the band the design performance may not be reached, above it the sludge thickens and fouls. Sludge viscosity should stay at or below 250 mPa·s — high viscosity signals a fouling sludge. Dissolved oxygen needs to read 1 mg/L or more throughout the bioreactor, and pH belongs between 6 and 8, corrected with acid or alkali. Water temperature wants 15-40°C, with cooling or insulation where climate pushes past that.

Keep a daily log of aeration rate, air outlet pressure, permeate flow, TMP, permeate and raw-water quality (BOD, COD, turbidity, total nitrogen, total phosphorus), tank temperature, dissolved oxygen, pH, MLSS, sludge viscosity and settleability (SV60 or SVI120). Trends in that log are what turn a sudden pressure jump from a crisis into a scheduled chemical clean.

Daily checks and the aeration pipe routine

Healthy sludge is tea-brown, flocculent and free of unpleasant odour; anything else calls for a look at MLSS, viscosity, dissolved oxygen, pH, temperature and BOD load. A rising TMP at constant flux means fouling — check aeration first, then sludge condition, and take corrective action including chemical cleaning where needed. Aeration pipes are cleaned once a day because scale in the aeration holes produces uneven aeration and membrane fouling together; fitting an automatic valve can do the job without an operator at the panel. The sequence is simple: stop filtration, close valve V1, open cleaning valve V2 so tank sludge flows back through the aeration holes and leaves with the air, hold V2 for one second, restore, then repeat on the second line through valves V3 and V4 before restarting filtration.

Chemical cleaning: trigger points and limits

Clean chemically when TMP at the same filtration rate climbs 5 kPa above the initial stable value, or every six months — whichever comes first. If the rise happens sooner, clean sooner; early cleaning clears surface pore blockage far better than a delayed one, and noting how long each interval lasts turns the schedule into a membrane-life forecast. Sodium hypochlorite handles organic fouling; oxalic acid and citric acid handle mineral scale and metal deposits. All three are hazardous on contact — goggles, gloves, local exhaust ventilation, and the material safety data sheet before anything else. Store them in a corrosion-resistant tank, closed against the air, out of direct sunlight, in a cool and well-ventilated place.

Feed the chemical solution slowly through the permeate header so it seeps outward through the membrane, always under gravity and never pumped: pumped injection can exceed 10 kPa and damage the elements. Keep the module submerged with at least 500 mm of water above it, aeration running throughout (lower the rate if the chemical foams), and let the solution stand one to three hours. Warm solution cleans better, but never beyond 40°C — and at low temperature cleaning may fail to restore performance at all. The first permeate after cleaning still carries chemical, so return it to the raw-water tank or treat it as waste until quality recovers.

Fault-finding in the right order

The three usual faults are abnormal aeration, climbing TMP with falling permeate flow, and deteriorating permeate quality. Work through them in sequence: aeration pipe and blower first, because blocked aeration holes are the commonest single cause of both fouling and poor permeate; then the sludge — viscosity, MLSS, settleability; only then the membrane itself, chemically cleaned before anyone assumes end of life. Permeate hoses are replaced roughly every three years, fitted firmly without straining the element or the header connection. When leak-testing pipe joints and welds under pressure, never change the pressure applied to the membrane element. For removal, drain the bioreactor, disconnect the permeate header, undo the bolts joining the element section to the aeration section and lift the section clear — releasing the anchor fixings only if the aeration section must come out too.