Marine Sewage Treatment Plant Capacity & Holding Tank Tool

Sewage Generation • STP Hydraulic Load • Organic Load • Holding Endurance • Treated Effluent Retention • N-1 Capacity • Voyage Legs • Shore Reception

Marine Sewage Treatment Plant Capacity & Holding Tank Tool

Screen shipboard sewage treatment capacity as a hydraulic, organic-load and storage problem. Simple mode compares onboard generation, effective STP capacity and holding endurance. Advanced mode adds source registers, multiple sewage treatment plants, untreated and treated-effluent tanks, voyage restrictions, Baltic passenger-ship nutrient-capable treatment, shore reception, N-1 resilience and sensitivity analysis.

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Capacity screen, not a discharge authorization.The tool never decides whether sewage may be discharged. Route, distance from land, ship type, approved equipment, Administration requirements, port/coastal-State restrictions and actual effluent quality remain controlling.

GenerationDaily sewage hydraulic load

TreatmentEffective STP capacity margin

RetentionHolding endurance and backup

People & Wastewater Load

Persons onboardCrew, passengers and other persons included in this planning case.

Sewage generation factorL/person/day. User-entered vessel planning factor, not a universal MARPOL value.

Additional wastewater routed to STPm³/day, for grey water or other streams actually routed to the plant.

Planning / retention perioddays.

Sewage Treatment Plant & Holding Tank

STP rated hydraulic loadingm³/day from the vessel's approved/type-approved plant basis.

STP operational derate% available capacity for fouling, maintenance or operating margin.

Usable sewage holding capacitym³.

Current holding inventorym³.

Maximum planned tank fill% of usable capacity.

Normal operating basisSelect the actual planning basis. The tool does not determine discharge legality.

Planning Limits & Vessel Basis

Vessel service

Untreated tank planning ceiling% of usable capacity.

Treated-effluent tank planning ceiling% of usable capacity.

N-1 STP coverage target% of baseline hydraulic load.

Wastewater Source Register

Enter vessel-specific source loads. The sample figures are illustrative planning inputs only. Grey water is counted only when it is actually routed to the sewage treatment plant or holding system.

Use	Source / group	Persons / units	L/unit/day	BOD5 g/unit/day	Duty %

Sewage / Effluent Holding Tanks

Use	Available?	Tank service	Capacity m³	Current fill %	Usable %

Sewage Treatment Plant Register

Hydraulic and organic ratings should come from the plant's approved/type-approved basis. For Baltic Sea Special Area passenger-ship discharge cases, mark only units whose approval includes the applicable nutrient-removal standard.

Use	Available?	Hydraulic rating m³/day	Organic rating kg BOD5/day	Derate %	Max planned h/day	Baltic passenger nutrient approval?

Voyage / Operating Leg Register

A discharge leg must be independently verified by the user. “STP + retain treated effluent” keeps the treatment plant operating but stores its effluent. “Retain influent” gives the STP no processing credit for that leg.

Use	Phase	Days	Generation factor %	Handling mode	STP h/day	Untreated shore reception m³	Treated shore reception m³	Baltic Annex IV Special Area?

Plant / Pollution-Prevention Checks

Type-approval loading basis checkedHydraulic and organic ratings reflect the installed plant's approved certificate / documentation. Plant availability and maintenance status checkedBlowers, pumps, membranes, aeration, disinfection and controls are represented by the entered derate. Effluent quality / sampling status checkedActual discharge quality remains a separate operational requirement outside this capacity model. Holding tank levels and alarms checkedStarting inventories and usable volumes are based on credible tank indications. Shore reception / standard connection checkedReception volumes entered in the voyage plan are realistically available. Grey-water routing confirmedOnly wastewater actually connected to the STP / sewage holding system is included. Baltic passenger nutrient approval checked where relevantPassenger-ship Special Area discharge uses only units marked with the applicable nutrient-removal approval. Local and coastal-State restrictions reviewedAny no-discharge or retention requirement is represented in the operating legs.

Regulatory boundary: MARPOL Annex IV allows several approved sewage-system arrangements and sets discharge conditions, but holding-tank capacity itself is to the satisfaction of the Administration, taking account of ship operation, persons onboard and other relevant factors. This tool provides engineering screening only and does not certify compliance or authorize discharge.

Technical basis: MARPOL Annex IV regulation 9 requires an approved sewage treatment plant, approved comminuting/disinfecting system with storage, or a holding tank sized to the satisfaction of the Administration. MEPC.227(64), as amended by MEPC.284(70), ties STP approval to designed hydraulic and organic loading and sets effluent performance standards. Passenger ships discharging treated sewage in the Baltic Sea Annex IV Special Area require the applicable additional nitrogen and phosphorus removal standard. Local rules can be stricter, including no-discharge zones that prohibit both treated and untreated sewage discharge. IMO PPR 13 in February 2026 agreed draft Sewage Record Book and Sewage Management Plan amendments in principle, but those draft provisions are not treated here as adopted requirements.

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