Closed Loop Scrubber

What defines the closed-loop design

An open-loop scrubber draws seawater, uses its natural alkalinity to neutralise sulphur oxides, and discharges the wash water back to sea after treatment. A closed-loop system instead uses freshwater as the washing medium in a recirculating loop, dosing it continuously with sodium hydroxide to maintain the alkalinity that seawater would otherwise supply for free. The wash water is cleaned, cooled and sent back to the tower rather than overboard, with only a small controlled bleed-off discharged and the rest retained on board or processed into sludge for shore disposal.

Main components

Absorber tower

Same core design as an open-loop unit — spray nozzles distribute wash water countercurrent to the exhaust gas flow, stripping SOx and particulates.

NaOH dosing skid

Caustic soda is stored as a concentrated solution and metered into the recirculation line to hold pH within the operating band the system needs to keep absorbing SOx effectively.

Process water treatment unit

Continuously bleeds a fraction of the loop through separators and filters to remove PAH-bearing sludge and particulates, keeping the recirculating water clean enough for repeated use.

Holding and bleed-off tanks

Buffer capacity for wash water when the ship is in a zero-discharge zone, plus a controlled bleed-off line for waters where limited discharge is permitted, each fed from separately monitored outlets.

Selection and sizing

Sizing turns on exhaust gas volume at maximum continuous rating, the fuel sulphur content the ship is designed to burn, and the freshwater and NaOH storage the trading pattern demands — a ship spending long periods in zero-discharge coastal waters needs far more holding tank capacity than one on open ocean routes. NaOH consumption scales with fuel sulphur content and running hours.

Regulations and class

Operation sits under MARPOL Annex VI and the IMO 2020 sulphur cap, with wash water quality such as PAH, turbidity, pH and nitrate governed by the IMO Exhaust Gas Cleaning Systems Guidelines (MEPC.259(68)). A growing number of port states and coastal zones prohibit scrubber discharge outright, which is the main operational reason ships fit closed-loop rather than open-loop or hybrid units. Continuous monitoring and logging of wash water parameters is required, and class surveys check the discharge monitoring equipment as part of the annual survey.

Typical faults

What to look for in a supplier

• Wash water treatment performance data against MEPC.259(68) discharge limits, not just SOx removal efficiency
• NaOH dosing system rated for the ship's expected ambient temperature range, including cold-climate crystallisation risk
• Holding tank and bleed-off capacity matched to the actual trading pattern, not a generic reference ship
• Track record of discharge monitoring equipment accepted by the relevant class society and port state control

Check zero-discharge zone maps before every voyage plan revision, not once at newbuilding, because the list of ports and waters banning scrubber discharge has grown faster than most operators' standing orders account for.