July 2026 · BWTS Compliance · 6 min read
The filter differential pressure is climbing. The system tripped. The port water is turbid and you need to ballast. This is a challenging water quality (CWQ) event — and IMO has a specific procedure for it under Resolution MEPC.387(81). Most vessels are not documenting it correctly.
What Is Challenging Water Quality (CWQ) in Ballast Water Management?
Challenging water quality means the incoming port water — typically high in turbidity or total suspended solids (TSS) — causes a correctly installed and maintained ballast water treatment system (BWTS) to shut down or fall below its minimum required treatment flow rate.
IMO’s definition under MEPC.387(81) is precise. Two things are not CWQ:
Cold or low-salinity water. If your BWTS trips because water temperature or salinity is outside its approved operating range, that is a system operational limitation — not CWQ. The CWQ procedure does not apply.
A technical failure. If the system has stopped because a sensor has failed, a pump is faulty, or a control card has malfunctioned, that is a BWTS fault. Record it as such. Filing it as CWQ when the logs show a hardware alarm is a documentation error a PSC officer will identify.
This distinction matters. An inspector reviewing your Ballast Water Record Book (BWRB) will check the system logs against the entry. If the alarm was caused by a flow meter fault but the BWRB says CWQ, the record does not hold.
Before You Bypass: Steps the IMO Requires You to Try First
Bypass is the last resort under MEPC.387(81). Before reaching that point, the guidance requires the following steps to be attempted — and documented:
1. Reduce the ballasting rate. Slow the flow to give the filter more time to clear between automatic backflush cycles. Many BWTS can manage high-turbidity water at reduced throughput even when they cannot handle full ballast flow. Check your system’s type approval certificate for the minimum rated treatment capacity.
2. Initiate manual backflush cycles. Do not wait for the automatic cycle. Trigger it manually from the HMI. Watch the differential pressure reading. If it drops and recovers, the filter is clearing and the system can continue at a reduced rate.
3. Take only the minimum ballast required for safety. If only enough ballast is needed for safe departure, take that. Filling all tanks while bypassing the BWTS is not justified under the CWQ procedure.
4. Consider treatment at sea. If the vessel is departing within a few hours, partial ballasting at reduced rate in port — followed by decontamination and treated refill at sea — is a better outcome than a full bypass and avoids complications at the next port.
If you have taken all of these steps and the BWTS still cannot operate, you have reached the point where bypass may be necessary.
The BWTS Bypass Procedure
Under MEPC.387(81), bypass of a ballast water management system (BWMS) in CWQ conditions is a structured, documented procedure — not a shortcut.
Before initiating bypass:
- The master must conduct a vessel safety assessment
- The flag state must be notified if required by the vessel’s BWMP
- The BWMP must include a CWQ procedure — if it does not, the bypass is not covered by the IMO guidance
Only the ballast tanks strictly necessary for stability and safe departure should be filled with untreated water. Pre-emptive bypass — bypassing before the system has actually encountered a problem, based only on past experience at a port — requires prior agreement between the flag administration and the port state receiving the ballast water. It cannot be applied unilaterally.
What the BWRB Entry Must Contain After a CWQ Bypass
This is where most vessels fail PSC scrutiny. The entry exists but lacks the detail that makes it defensible. Under the revised BWRB format mandatory from 1 February 2025 (Resolution MEPC.369(80)), a CWQ bypass entry must include:
- Date, time, and port location of the event
- Tanks affected and the volume of untreated ballast water taken
- Water quality conditions observed — not just the label “CWQ” but the actual readings: filter differential pressure values, alarm codes generated, turbidity sensor readings where available
- Steps attempted before bypass — reduced ballasting rate, manual backflush attempts, minimum ballast assessment — and why each was insufficient
- Statement confirming bypass was the last available option
- Master’s safety assessment confirming the decision
- Name and signature of the responsible officer
- Decontamination plan for the affected tanks
The decontamination plan is the entry most commonly missing entirely — and its absence is immediately visible to a PSC officer reviewing the record.
Decontamination: The Step Most Crews Miss
When a ballast tank is filled with untreated water during a CWQ bypass, it is contaminated. It cannot be discharged at the next port under D-2 compliance without first completing decontamination.
The IMO decontamination procedure under MEPC.387(81):
- Discharge the tank at sea — at least 200 nautical miles from the nearest land in water at least 200 metres deep
- Flush the empty tank with treated water from the BWTS to dilute residual organisms and sediment
- Refill the tank through the BWTS under normal operating conditions
Each step must be recorded separately in the BWRB. The entry at the next port of call should show the decontamination completed, the treated refill volume, and the BWTS operational status during treatment.
If decontamination is not completed before the next port arrival, the vessel cannot discharge from the affected tank without another documented CWQ or contingency entry — and without a valid justification, the PSC officer can cite it as a D-2 violation.
What PSC Officers Look for in a CWQ BWRB Entry
Port state control officers reviewing a BWRB entry showing a bypass are looking for one thing first: evidence that treatment was attempted before the bypass was initiated.
An entry that moves directly from “system alarm” to “bypass initiated” — with no record of reduced ballasting rate, manual backflush attempts, or minimum ballast assessment — does not pass. The bypass reads as a shortcut, not a last resort.
The same event, documented in full — alarm codes, differential pressure readings, steps taken, master’s safety assessment, decontamination plan — becomes a defensible record. This is the difference between a managed CWQ event and a detainable deficiency.
Data from the 2025 CIC confirms this pattern. Of the three categories responsible for 89% of all CIC non-compliances, incorrect or incomplete BWRB entries ranked second only to BWTS operability issues. Bypass entries with missing documentation were among the most common BWRB failures identified across Paris and Tokyo MOU inspections.
What Your BWMP Must Now Include
The revised G4 Guidelines adopted at MEPC 84 in April 2026 require every Ballast Water Management Plan to contain vessel-specific CWQ procedures. A generic reference to MEPC.387(81) is no longer sufficient.
The procedure in your BWMP must describe:
- How your specific BWTS responds to high-turbidity uptake water
- The minimum treatment flow rate for your system (no greater than 50% of rated treatment capacity)
- The steps the crew must take before bypass is initiated
- The master’s safety assessment process
- The decontamination procedure for your ballast routing
If your BWMP was written before March 2024, or has not been updated since a BWTS retrofit, it almost certainly does not meet the current requirement. A PSC officer checking BWMP currency against the G4 Guidelines revision will identify this.
Common Ports Where CWQ Events Occur
CWQ events are most frequently reported at ports with high turbidity, active dredging, heavy river sediment load, or strong plankton blooms. Common locations include:
- Chinese river and delta ports (Yangtze river system, Tianjin, Nanjing)
- West African ports with high suspended sediment during seasonal river discharge
- Ports in dredging or construction activity
- Anchorages during storm events with seabed disturbance
Vessels regularly calling these ports with UV-based BWTS are at highest risk, as UV transmittance drops sharply in turbid water. If your vessel trades these routes, the CWQ procedure in the BWMP should be written for those specific conditions — not as a generic contingency.
A BWTS that shuts down in bad port water is doing its job correctly. The record you write in the next thirty minutes is what the inspector will judge.
