Sewage Treatment Plants

How the plants work

Annex IV defines sewage as black water from toilets and medical spaces. Grey water from showers and the galley is outside Annex IV, but many plants treat both.

• Extended aeration plants break waste down with bacteria in aerated tanks. They settle the sludge in a clarifier and disinfect the effluent with chlorine or UV.
• Membrane bioreactors use the same biology but filter the mixed liquor through membranes fine enough to hold back solids and bacteria. That gives effluent that needs little or no further disinfection.
• Electrochemical and electrocatalytic plants, such as De Nora Omnipure, treat macerated sewage by electrolysis of seawater. They are compact and need no bacteria culture, but they draw more power.

Why a rig relies on an approved plant

Annex IV applies to units of 400 GT and above, or certified for more than 15 persons. Untreated sewage may be discharged only more than 12 nautical miles from land, and comminuted and disinfected sewage only beyond 3 nautical miles. Sewage stored in holding tanks must go out at a moderate rate while the ship is en route at 4 knots or more. A drilling unit on location is never en route, so it cannot empty holding tanks at sea. Effluent from a type-approved treatment plant may be discharged on location, provided it leaves no visible floating solids and does not discolour the water.

Key specifications

Type approval is the first check

The standard depends on the installation date.

• MEPC.2(VI) for plants installed before 1 January 2010.
• MEPC.159(55) for plants installed from 1 January 2010.
• MEPC.227(64), amended by MEPC.284(70), for plants installed from 1 January 2016.

MEPC.227(64) limits thermotolerant coliforms to 100 per 100 ml and total suspended solids to 35 mg/l. It limits BOD5 to 25 mg/l and COD to 125 mg/l, with pH between 6 and 8.5. Its nitrogen and phosphorus limits apply only to passenger ships discharging in special areas, such as the Baltic Sea, so a rig does not need nutrient removal. A used plant certified only to an older standard cannot normally be installed today. The unit's International Sewage Pollution Prevention Certificate lists the approved plant.

Failure modes and inspection

• Tanks. Hydrogen sulphide attacks steel and coatings above the waterline, so check coating, thickness and internal corrosion.
• Aeration. Check blower hours and condition, and diffusers for clogging.
• Inlet. Check the macerator or screen, pumps, and level switches and controls.
• MBR membranes. Check age, permeability trend and cleaning records.
• Electrochemical plants. Check electrode wear and power supply condition.
• Disinfection. Check chlorine dosing or UV lamp hours and sleeve fouling.

Documentation

• Type approval certificate showing the IMO standard
• Operating manual with rated hydraulic and organic loads
• Maintenance records and spare parts history
• Effluent test results from service
• Membrane or electrode replacement dates
• Tank thickness and coating reports

Integration and compatibility

Check the black and grey water collection piping, including vacuum toilet systems. Vacuum systems send concentrated, low-volume sewage that some plants handle better than others. Check overboard discharge and sludge handling, tank vents and gas detection, and alarms to the vessel's monitoring system. Size the plant to the maximum persons on board, including temporary accommodation.

Choosing used or new

An unused surplus plant approved to MEPC.227(64) is the most useful used option. A plant from a recent installation can work well if its approval and capacity fit. A plant approved to an older standard may stay on the unit where it was installed, but it cannot move to another one. Evac and De Nora plants are common offshore. Solid waste equipment is under Waste management.