FMEA for wastewater plants: how to prioritise critical assets

FMEA ranks wastewater asset failures by severity, occurrence, and detection. The output is a prioritised maintenance plan grounded in risk, not in habit. A plant with 4,000 distinct asset items cannot maintain everything at the same intensity. FMEA is how you decide what gets the attention.

This guide walks through Failure Mode and Effects Analysis specifically for wastewater treatment plants, with worked examples on aeration blowers, secondary clarifiers, and disinfection systems. If you are building a PM programme from scratch, or reviewing an existing one, FMEA is the mechanism that turns a spreadsheet of assets into a defensible reliability strategy.

What FMEA actually is

FMEA is a structured method for identifying how an asset can fail, what the consequences are, and how likely and detectable those failures are. Each identified failure mode gets three scores: severity (how bad is the consequence), occurrence (how often does it happen), and detection (how likely are we to catch it before it turns into a real event). The product of the three scores is a Risk Priority Number (RPN), and RPN sorts the population.

The method originated in aerospace and automotive engineering in the 1960s and is now standardised across many industries by the IEC 60812 international standard on failure modes and effects analysis. Its water utility application is documented in Water Research Foundation reliability research over the last two decades.

Scoring severity, occurrence, and detection

Each dimension is scored 1 to 10, calibrated to the wastewater context.

Calibration matters. Different teams working on the same assets can produce very different scores if the scale is not anchored. Spend the first FMEA session calibrating on a handful of well understood failure modes so the scoring is comparable across the rest of the plant.

The Risk Priority Number

RPN is Severity multiplied by Occurrence multiplied by Detection. The theoretical maximum is 1000 (10 x 10 x 10) and the practical maximum in a well maintained wastewater plant is around 500 to 700. Any RPN above 200 warrants immediate mitigation planning.

Worked example: aeration blower FMEA

Consider a centrifugal aeration blower at a secondary treatment plant. The FMEA session might produce the following failure modes.

The two highest RPN items (discharge valve seat wear at 192, bearing failure at 168) get the most attention. Discharge valve wear was not previously on the PM programme; adding an annual leak test closes the detection gap. Bearing failure was on the programme; the vibration threshold gets tightened and the PM interval reduced.

Worked example: secondary clarifier FMEA

Weir short circuiting shows up as the highest RPN, driven by the low detection score. This is a common finding: hydraulic short circuiting is invisible to routine inspection unless the operator is specifically looking for it. A short annual weir level survey closes the detection gap and reduces RPN by a factor of 3.

Worked example: UV disinfection FMEA

Sensor drift stands out at RPN 252, with severity 9 (pathogen breakthrough risk) and detection 7 (only visible when the reference calibration catches it). This is the highest priority mitigation and drives the calibration schedule.

The FMEA session in practice

Assemble the right team

An FMEA session needs an operations lead, a maintenance lead, a reliability engineer, and someone with process expertise. Sessions can run half a day for a single asset or up to two full days for a complete process area. Facilitator neutrality matters; RPN scores can be swayed by whoever speaks loudest if the facilitator does not enforce discipline.

Identify failure modes systematically

Work through each subsystem of the asset. Do not skip modes that seem unlikely. The value of FMEA is in surfacing the low probability high consequence modes that never appear in the routine inspection.

Score each dimension separately

Score severity first (does not depend on the asset condition), then occurrence (historical failure rate), then detection (how would we catch this before impact). Discussing severity and occurrence together conflates them and produces inconsistent scores.

Compute RPN and sort

Once scored, RPN sorts the failure modes. Anything over 200 gets a mitigation plan. Anything between 100 and 200 gets a periodic review. Below 100 is monitored but not actively worked on.

Assign mitigations

Each mitigation should have an owner, a due date, and a mechanism to verify it is in place. Mitigations that never get implemented are worse than no FMEA at all, because they create a false sense of coverage.

Re score after mitigation

Once the mitigation is in place, re score detection or occurrence and recalculate RPN. This tells you whether the mitigation actually reduced risk or just added work.

Integrating FMEA output with the CMMS

FMEA is not a one time exercise. The value comes from feeding the mitigation plan into the CMMS PM library and revisiting the RPN scores when new failure data accumulates.

• Each mitigation becomes one or more PM templates in the CMMS.
• The FMEA RPN becomes an attribute on the asset in the CMMS.
• Failure events reported in the CMMS feed back to the FMEA occurrence score.
• New failure modes discovered in operation get added to the FMEA library.
• FMEA is re run every 3 to 5 years for critical assets, on major upgrades, and after any significant failure event.

FMEA vs HAZOP

HAZOP (Hazard and Operability Study) is a related methodology often used in process plants for safety focused analysis. HAZOP is more structured around process deviations from design intent (high flow, low temperature, no flow), while FMEA is more equipment focused. Wastewater plants that carry significant chemical handling (chlorine, sodium hypochlorite, sulphuric acid) often run HAZOP alongside FMEA to cover both equipment failure and process safety. Choose FMEA for equipment reliability, HAZOP for process safety, and both when both dimensions matter.

FMEA vs Reliability Centered Maintenance

FMEA is one tool; Reliability Centered Maintenance (RCM) is the broader methodology it fits inside. RCM adds functional analysis (what does each asset actually do), consequence classification (safety, environmental, operational, hidden), and maintenance strategy selection (predictive, preventive, run to failure, redesign).

For most wastewater utilities, a well disciplined FMEA is sufficient to build a defensible PM programme. Full RCM is worth the additional investment at large utilities with complex process trains and stringent regulatory profiles.

Workshop facilitation techniques

An FMEA session runs on the quality of facilitation. Effective facilitators enforce three disciplines: separate identification from scoring, enforce the calibration table, and log dissent explicitly. Separating identification from scoring means the team first lists every failure mode without discussing likelihood or consequence, then goes back and scores each. This prevents anchoring on the first failure discussed. Enforcing the calibration table means every score has to reference the anchor conditions, not the facilitator gut feel. Logging dissent means preserving alternative viewpoints in the record even when the group settles on a majority position. The IEC 60812 standard includes recommended facilitation practices in its normative annex.

Updating the FMEA over time

An FMEA is not a one time exercise. Every incident that occurs should trigger a review: was this failure mode in the FMEA? If not, why not, and what should be added? Was the scoring accurate given the actual failure occurrence? What mitigation might have caught this earlier? Utilities that treat FMEA as a living document typically show measurable RPN reduction over 3 to 5 years, whereas utilities that treat it as a one time deliverable see FMEA quality drift as personnel change.

FMEA programme metrics

• Coverage: percent of critical assets with a current FMEA.
• Mitigation closure rate: percent of high RPN mitigations implemented within committed date.
• RPN trend: are RPNs decreasing over time as mitigations take effect.
• Failure event correlation: are unplanned failures happening on high RPN or unexpected modes.
• Re score cadence: FMEAs updated on schedule.

Frequently asked questions

Summary

FMEA turns an asset register into a prioritised risk map. Severity, occurrence, and detection produce a Risk Priority Number that sorts the population. High RPN failure modes get mitigation. The output feeds directly into the CMMS PM library, becoming the basis of a defensible reliability programme grounded in evidence rather than habit. For wastewater utilities under regulatory pressure and constrained by staffing, FMEA is the single highest leverage exercise in the reliability toolkit.

Next reading

• What is a CMMS for water utilities?
• Preventive vs predictive maintenance
• How a CMMS reduces unplanned downtime in pumping stations
• RCA for wastewater plants
• Browse the wastewater plants directory