Challenging Water Quality (CWQ) & Contingency Measures
- Admin
- 54 minutes ago
- 5 min read
Practical guidance for deck officers and engineers:
When your ballast water treatment system (BWTS) suddenly starts tripping on high differential pressure, TRO alarms, or can’t keep the flow, you are very likely in Challenging Water Quality (CWQ) – and every decision you make from that moment will be checked later by PSC, flag and company.
This post gives a practical, non-brand-specific overview of CWQ, contingency measures and record-keeping, based on:
IMO Interim Guidance for CWQ (MEPC.387(81))
Liberia Marine Notice POL-014 and Marine Advisory 01/2023 (BWRB guidance)
ERMA FIRST BWTS maintenance & troubleshooting guidance as an example of maker’s instructions
⚠️ Important: This article is general guidance. The only “law” on board is your approved BWMP, Flag / RO instructions and your own BWTS OMSM/manual. Always follow those first.
1. What is “Challenging Water Quality” (CWQ)?
IMO defines CWQ as ambient uptake water whose quality (e.g. high suspended solids, turbidity, organism load) makes a properly installed, maintained and operated type-approved BWMS temporarily inoperable because:
it hits an operational limitation (shutdown or critical alarm), or
it cannot meet operational demand (minimum flow rate needed to keep cargo/ballast operations going).
Key definitions from the IMO guidance:
Operational demand – minimum BWTS flow rate defined in the BWMP that still allows safe, practical cargo/ballast operations; normally ≤ 50% of system TRC.
Operational limitation – automatic shutdown or critical alarm where OMSM tells you to stop the system, or you must shut down to protect ship/equipment/crew.
Pre-emptive bypass – bypassing before you actually hit the limit, because you expect it.
Reactive bypass – bypassing only after the system hits its limit (trip/alarm/low flow).
Typical CWQ ports: muddy rivers, dredging areas, strong plankton blooms, heavily silted ports etc. High turbidity and TSS usually show up first as filter high dP alarms and loss of flow.
2. Why CWQ is a big deal (for you)
Environment: untreated or partially treated port water in your tanks can transport invasive species and pathogens.
Compliance: IMO expects discharged ballast to meet D-2 standard whenever applicable. CWQ does not cancel the Convention – it just allows a structured way to react, decontaminate and document.
PSC focus: Incorrect BWRB entries are among the most common ballast deficiencies.
Liability: Poor records during CWQ + bypass = easy target for detentions and company investigations.
So the target is:👉 Use BWTS as much as possible,👉 Bypass only as last resort,👉 Decontaminate tanks afterwards,👉 Record & report everything clearly.
3. Pre-planning before a “known bad” port
The CWQ guidance expects ships to plan ahead for difficult ports and include CWQ procedures in the BWMP.
Before arrival to a suspected CWQ port:
Review BWMP & OMSM
Identify operational demand flow rate for your ship.
Review maker’s CWQ / high turbidity instructions (filters, TRO, UV, etc.).
Maintenance checks (example: ERMA FIRST)
Ensure filters are preserved, cleaned or serviced as per interval (manual flushing, DP checks, screen inspections).
Check electrolytic cells / UV units, cooling, TRO sensor, dosing pumps, NAT level, and flow/salinity/pressure sensors.
Confirm spares and neutralising agent are available.
Agree internal triggers
DP alarm values, minimum allowed flow before BWTS cannot support cargo operations, “when to call Master / office”, etc.
Voyage plan for decontamination option
Identify possible mid-ocean or ≥50 nm / ≥200 m areas for later BWE or BWE + BWT if bypass becomes necessary.
4. When CWQ hits: step-by-step onboard logic
4.1 Assessment & troubleshooting (before bypass)
When alarms start (high filter dP, low flow, high TRO demand etc.), follow OMSM troubleshooting first:
Verify valves/lines, check for air/strainers.
Use manual backflushing or cleaning of filter where allowed.
Reduce ballasting rate to within operational demand.
Adjust treatment intensity (e.g. TRO dosage, UV intensity) if allowed.
Confirm no fault in sensors (TRO, flow, pressure).
If after this the system still cannot meet operational demand or trips on critical alarms, you have CWQ as per IMO definition.
4.2 Alternatives to bypass
Before touching bypass, CWQ guidance asks you to consider:
Ballast less volume (only minimum safe for departure).
Spread operations over more time (slower rate).
Optimize treatment settings within limits.
Coordinate with port about timing or berth (if possible) to allow lower flow.
4.3 Bypass – last resort, not first button
If nothing works and you cannot maintain safe operations, you may need to bypass the BWTS. The guidance makes two key points:
Bypass must be a last resort.
You should limit the damage:
Only the minimum safe volume;
As few tanks and lines as possible;
Treat any fraction of the flow that you still can (partial bypass if your system allows).
Every bypass event creates a “contaminated” tank which will later need decontamination before you can safely discharge.
5. Decontamination after bypass (BWE / BWE + BWT / flushing)
After you have taken unmanaged water due to CWQ, the next goal is to “clean” those tanks so future discharges meet D-2.
5.1 Ballast Water Exchange (D-1)
Liberian guidance (and BWM Convention) requires BWE to be done with:
Preferably ≥ 200 nm from land and water depth ≥ 200 m;
But at least ≥ 50 nm from land and ≥ 200 m depth when 200 nm is not possible;
≥ 95% volumetric exchange:
For flow-through: pump at least 3 × tank volume;
For sequential: empty/strip and then refill properly.
BWE can be used without BWTS (pure D-1) or as part of BWE + BWT when BWTS is again operational.
5.2 BWE + BWT and flushing (decontamination)
IMO CWQ guidance recommends, after a bypass event:
BWE + BWT – exchange the unmanaged water and refill with treated mid-ocean water, and
Flushing of tanks/piping (where possible) to remove remaining contaminated pockets and sediments.
Your BWMP should describe how BWE + BWT is done on your ship and in which sequence.
6. Contingency measures – when things still don’t go as planned
Sometimes you may not be able to do BWE/BWE+BWTS (weather, routing, ship safety). In such cases, IMO and flag allow contingency measures, which are described in the CWQ guidance and in Liberian POL-014.
Typical contingency options (to be agreed with Flag / port and included in BWMP) can include:
Change of discharge plan – keep ballast on board, don’t discharge in that port.
Alternative BWE area – use a designated area or different leg of voyage.
Use of reception facility – discharge ballast to shore reception where available.
Partial discharge – only discharge tanks that are confirmed to meet D-2, keep the rest.
Whenever contingency measures are used, you must:
Inform next port / coastal State in advance, explaining the situation.
Make clear BWRB entries describing what was done and why.
7. Record-keeping: how to protect yourself during CWQ
Liberian BWRB guidance gives detailed model entries for: ballast without BWTS, exchange without BWTS, ballasting/discharging with BWTS, BWE + BWT, exceptional conditions, etc.
Some key points that PSC and flag look for:
Every operation recorded without delay, signed by officer in charge; each page signed by Master.
Use correct items and structure (3.1 ballasting, 3.2/3.3 exchange, 3.3 discharge, 3.5 exceptional etc.).
Distances from land and water depth always recorded for BWE.
When BWTS is bypassed or inoperable, make a separate “exceptional/other” entry explaining:
Why BWTS was not used (CWQ/filter clogging etc.);
Reference to CWQ guidance / BWMP;
Notifications made to flag/port.
For combined operations (e.g. BWE + BWT), guidance stresses that a remarks entry should refer to reason and BWMP reference.
Good BWRB entries often make the difference between:
“PSC note only” vs
“Deficiency / possible non-compliance”.
8. Practical checklist for CWQ events
When you suspect CWQ, you can think in this simple sequence (adapt it into your BWMP):
Prepare & maintain – BWMP reviewed, OMSM followed, BWTS maintained (filters, TRO, dosing, sensors).
Assess & troubleshoot – alarms, low flow, DP, TRO; try all OMSM measures first.
CWQ triggers met? – operational demand not met or operational limitation reached → now formally CWQ.
Avoid bypass if possible – ballast less, slow rate, optimize settings, coordinate with port.
Bypass as last resort – minimal safe volume, fewest tanks; treat any fraction you can. Record everything.
Plan and carry out decontamination – BWE/BWE+BWTS and flushing at first safe area (≥50–200 nm, ≥200 m as applicable).
Use contingency measures if still not possible – in coordination with flag/port.
Keep BWRB and logs clean – clear entries for failure, bypass, exchange, decontamination and restoration of system, following national guidance.
“Documents above are provided for study and reference. Always follow your own ship’s approved BWMP, BWTS OMSM and Flag/RO instructions.”
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