Environmental Technology

Dealing with waste water


Paul Gunton
Paul Gunton
ShipInsight

09 November 2018

Dealing with waste water

Ships make use of a great deal of water some of which is taken from the sea and returned untreated and some of which needs to be treated even if it was taken onboard from clean shore sources.

Seawater for ballast use is covered in the ShipInsight Ballast Guide and other sectors of this guide. Water is important for ships being used for cleaning and cooling and machinery, hold washing and of course providing the necessities of life for the crew and passengers. The latter use of water produces two distinct streams of waste water commonly referred to as black and grey water. Black water is sewage while grey water is general cooking and cleaning waste.

On a global scale, of these two waste types only sewage is subject to regulation and Annex IV of MARPOL is where the international regulations can be found. In addition to the international standard, some jurisdictions also regulate sewage discharges. In the US, specific waters are designated as no discharge zones (NDZs) where sewage discharges are prohibited. There are no international rules applying to grey water although some contend that the chemicals used in laundry, dishwashing and cleaning can be as hazardous to the marine environment as sewage.

The waste water from cleaning and cooling machinery may be contaminated by oily wastes and its disposal is regulated under MARPOL Annex I. Not envisaged when MARPOL was first formulated, there is another type of waste water which is not yet regulated and that is the wash water from scrubbers after any hazardous sludge has been removed.

There are IMO guidelines for the wash water from scrubbers which most operators will apply but as guidelines they have no penalties if not adhered to and little if any enforcement. That may change after 2020 when scrubber use is expected to become more common.

SOx scrubbers are not the only systems that produce contaminated water from the engine exhaust stream as an Exhaust Gas Recirculation system designed to reduce NOx emissions also need the gas to be cleaned by scrubbing before the gas can be recirculated into the engine.

There is a great deal of misinformation that is spread about exhaust scrubbing not least the idea that the water used is merely fed back into the sea untreated. Exhaust gas will always contain oily residues, unburnt fuel, ash and particulates. In all cases, the oily waste must be removed to the same standards as for all water waste and this will also remove almost all of the solids as well. The sludge must be retained onboard for later disposal ashore just as with ordinary oily waste.

At MEPC 73 in October 2018, the IMO adopted resolution MEPC.307(73) containing the 2018 Guidelines on Exhaust Gas Recirculation (EGR) Bleed-off Water. The condensate of exhaust gas is generated and discharged as bleed-off water, which is handled differently depending on the fuel oil sulphur content:

• Bleed-off water, from an EGR using fuel oil not complying with the relevant sulphur limit value in MARPOL VI, should be retained onboard in a holding tank. However, it may be discharged to the sea provided the ship is en-route outside polar waters, ports, harbors or estuaries and provided the bleed-off water meets the washwater discharge criteria under the 2015 Guidelines for Exhaust Gas Cleaning Systems (MEPC.259(68)) and that samples are provided to the Administration.

• Bleed-off water, from an EGR using fuel oil complying with the relevant sulphur limit value in MARPOL VI, should either (a) meet the same requirements for EGR using non-compliant fuel oil or (b) or may be discharged to the sea provided its oil content is monitored to not exceed 15 ppm by an oil content meter that is type approved under resolution MEPC.107(49).

The Guidelines should apply to marine diesel engines, fitted with an EGR device having a bleed-off water discharge arrangement, that are initially certified under the NOx Code on or after 1 June 2019.

Finally, there is water used for hold cleaning and tank washing which may or may not be subject to special requirements. MARPOL Annex II is the international regulation for this type of waste water. The most recent changes were made at MEPC 73 with changes approved for cargo residues and tank washings of persistent floating products with a high viscosity and/or a high melting point. An approved MEPC.2/Circular contains a list of specific vegoils and waxes which are controlled by these amendments.

Residue/water mixture generated during the prewash is be discharged to a reception facility at the port of unloading. Any water subsequently introduced into the tank may be discharged in accordance with the current discharge standards in MARPOL II, regulation 13.2. The amendments are subject to adoption by MEPC 74 in May 2019.