Sewage Treatment Plant

medium IMO Required 96 models total

A sewage treatment plant breaks down and disinfects blackwater from toilets and greywater from galleys before discharge, using biological digestion or electrochemical treatment, and is inspected under MARPOL Annex IV rather than the oil record book regime that governs bilge water.

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What defines a sewage treatment plant

A marine sewage treatment plant (STP) treats blackwater from toilets, and often greywater from galleys and laundries, to a standard that permits discharge closer to shore than untreated sewage would allow. It sits under a different regulatory regime from the oily water separator: MARPOL Annex IV governs sewage, with its own discharge distances, effluent standards and survey cycle, separate from the Annex I rules that cover oil in bilge water. Two treatment principles dominate the market -- biological plants that use aerobic bacteria to digest waste over a multi-stage retention period, and electrochemical plants that break down and disinfect waste using an electrolytic cell, typically generating disinfectant in situ from seawater. Biological plants need a stable feed and enough retention time to keep the bacterial culture alive; electrochemical plants respond faster to intermittent loads but consume more power and electrode life.

Sewage treatment plant flow scheme — Flow scheme of a three-chamber biological sewage treatment plant: inlet sewage is aerated in the first chamber, settles in the second chamber with sludge returned to the first, and is disinfected in the third chamber before discharge overboard.

Main components

Aeration or treatment chamber

On biological plants, a series of chambers -- aeration, settling, and often a final disinfection stage -- where an air blower keeps the bacterial culture aerobic and mixed with incoming waste.

Electrolytic cell

On electrochemical plants, a cell with plate or tube electrodes through which seawater or the effluent itself passes, generating disinfectant in situ rather than dosing a stored chemical.

Comminutor / macerator

A grinding stage ahead of treatment that reduces solids to a size the biological or electrochemical process can handle, and prevents rag and debris from blocking downstream pumps.

Effluent monitoring

A sample point, and on many installations a continuous monitor, for the parameters the discharge standard sets -- typically suspended solids, faecal coliform count and residual chlorine -- checked before the effluent is allowed overboard rather than to a holding tank.

Selection and sizing

Design population -- crew complement plus passengers where applicable -- against the plant's rated daily flow.
Peak load pattern: passenger vessels see sharp morning and evening peaks that a plant sized only on average daily flow cannot absorb.
Trading pattern: a ship spending long periods in port or in special areas needs enough holding capacity or treated-discharge capability to cover restricted zones.
Power consumption, particularly for electrochemical plants, against the electrical load budget of the vessel.
Space for retention time in biological designs -- undersizing the tankage is the most common design shortfall.

Regulations and class

MARPOL Annex IV requires an International Sewage Pollution Prevention (ISPP) certificate for ships above the size threshold it sets, backed by an initial and periodic survey of the plant and its discharge arrangements. The effluent standard the plant must meet, and the distance from land at which even treated sewage may be discharged, both come from Annex IV; special areas and some port state requirements are stricter than the baseline. Class surveys check the plant's operating log, the treatment performance against its type approval, and that the discharge piping correctly separates treated discharge from any direct overboard bypass -- a bypass valve left open or unsealed is one of the most common port state control deficiencies found on this equipment.

Typical faults

Fault	Cause	Consequence
Biological process fails	Bacterial culture killed by disinfectant or strong cleaning chemicals, or an extended low-load period with no feed	Effluent quality out of spec, weeks needed to re-establish a healthy culture
Electrode wear on electrochemical cells	Normal consumption over operating hours, accelerated by low-salinity feed water	Falling disinfection performance, eventually failing effluent monitoring
Macerator blockage	Foreign objects such as wet wipes or rags entering the system despite do-not-flush notices	Loss of flow through the plant, backup into the sanitary system
Odour and foaming	Overloading beyond design population, or aeration blower undersized or fouled	Crew complaints, and often a sign the treatment is also failing to meet discharge standard

What to look for in a supplier

Type approval certificate under MARPOL Annex IV covering the effluent standard actually required for the vessel's trading area.
Rated capacity matched with margin to the vessel's actual population and peak load pattern, not just average daily flow.
Spare parts availability for electrodes, blower diaphragms or membranes specific to the model.
Clear operating and troubleshooting documentation the crew can follow, since this equipment is usually maintained by ship's staff rather than a specialist.

A biological plant that has sat unused during a long yard period needs to be reseeded and given time to re-establish its bacterial culture before sailing -- opening the discharge valve on day one, before the culture has recovered, is how an otherwise sound plant fails its first effluent check after a refit.

4000h

Service Interval

5 yr

Class Survey

15 yr

Typical Lifetime

Typical Manufacturers

Scanship DVZ Jets Hamworthy

Inspector Mode Show All 96 total

16 manufacturers · 96 models

Evac

24 ✓ 20 verified

EcoTreat 2

Application

Compact marine sewage treatment plant

Common Failures & Inspection Points

Area: International Sewage Pollution Prevention Certificate (ISPPC) validity and authenticity
Check: Verify ISPPC is valid, properly issued for vessel size (≥400GT or ≥15 persons), signed and endorsed by flag state authority, not expired
Area: Sewage treatment plant IMO type approval certificate missing or invalid
Check: Confirm STP holds valid IMO MEPC 227(64) certificate (or MEPC 159(55) for older units), check certification class (STP type approval letter), verify not lapsed
Area: Direct sewage discharge valve not sealed or incorrectly secured
Check: Inspect discharge valve seals, verify valve is sealed shut when in port or within 3 nautical miles from shore, confirm mechanical lock/weld, check pump discharge isolation
Area: Sewage treatment plant not operational or inadequate capacity for crew
Check: Test plant start-up sequence, verify blower/pump operation, check aeration and treatment tank water levels, confirm adequate treatment time (typically 8-15 hours retention), measure actual vs. design capacity, review operational log
Area: Monthly effluent quality testing records missing or deficient (BOD, COD, TSS, E.coli)
Check: Request last 12 months of effluent test reports, verify parameter compliance with MEPC 227(64) limits (BOD ≤25mg/L, COD ≤125mg/L, TSS ≤35mg/L, fecal coliform <250 CFU/100mL), check testing lab certification, confirm disposal location recorded

Typ-universelle Inspektionspunkte fuer Sewage Treatment Plant (verifiziert, 2026-06).

Sewage Treatment Plant

What defines a sewage treatment plant

Main components

Aeration or treatment chamber

Electrolytic cell

Comminutor / macerator

Effluent monitoring

Selection and sizing

Regulations and class

Typical faults

What to look for in a supplier

Typical Manufacturers

16 manufacturers · 96 models

Evac

Hamworthy/Wartsila

ACO Marine

Jowa

Detegasa

RWO Water Technologies

Victor Marine

Wärtsilä Hamworthy

Hamann

Selmar

Deyuan Marine

DVZ

Headhunter Inc.

Jets

Scanship

Wärtsilä