Types of waste water
Water is important for ships being used for cleaning and cooling and machinery, hold washing, exhaust scrubbing 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 is.
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 washwater from scrubbers after any hazardous sludge has been removed.
There are IMO guidelines for the washwater from scrubbers which most operators will apply but, since they are guidelines, they impose 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 needs the gas to be cleaned by scrubbing before the gas can be recirculated into the engine.
There is a great deal of misinformation 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, harbours 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
- 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 15ppm 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.
Cargo hold and tank cleaning
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.
Ships that carry bulk cargoes, whether liquid or dry, often carry different cargo types on individual voyages. There are some exceptions such as crude carriers, ore carriers and of course LNG carriers as these are ships built for specialist trades. For other ship types, when different cargoes are carried, different standards of hold or tank cleanliness will be demanded by the contract of carriage.
Even crude carriers need to clean their tanks between voyages, but this is done for a different reason. Crude oil is a very dirty and viscous cargo and during discharge significant quantities will cling to the tank sides and not be able to be pumped out unless some cleaning is carried out. If cleaning is not done, then overtime the carrying – and therefore earning capacity of the vessel – will be reduced.
Historically ships would use seawater to dislodge the remaining cargo during the ballast voyage and discharge the resulting mixture at sea. It was also normal for ballast to be carried in cargo tanks and this again was discharged direct to the sea without treatment. Today, ships are required to have separate tanks dedicated for ballast use.
The practice of seawater washing was one of the reasons why OILPOL – the precursor to MARPOL – was considered necessary. The next practice aimed at reducing the amount of oil being lost was known as ‘load on top’ and began in the 1960s. This still involved seawater washing but instead of discharging the mixture, it was allowed to settle out over the ballast voyage with the oil being pumped back into the cargo tanks and the water – which did still contain oil but in much lesser amounts than previously – being discharged at sea.
With the advent of MARPOL in the 1970s, a new technique for tank cleaning – known as crude oil washing (COW) – was developed. In this method, during discharge some of the cargo is heated making it less viscous and this cargo is sprayed onto the tank surfaces washing the cargo off and into the main tank. This method is still used today and has mainly solved the problem of residual cargo affecting capacity and earnings.
Tankers that are carrying oil cargoes considered as harmless or non-polluting can still discharge some tank washings into the sea. These cargoes are mostly vegetable oils and similar products. However, there are limitations and regular changes are made to the list of permissible cargoes.
Because of their construction, tankers are difficult to inspect for cleanliness but the same is not true of bulk carriers and for this type of ship, cleaning the holds between cargoes and to high standards is usually essential. So long as the cargo itself is not considered hazardous and no hazardous chemicals are used in the cleaning process, the wash water can be disposed of at sea subject to certain conditions – mainly the ship is at least 12 nautical miles from land, is not in a restricted area and is underway. If the cargo or any chemicals are considered as hazardous under MARPOL rules, the wash water can only be discharge ashore into reception facilities.
Again, the lists of permissible substances is regularly updated by the MEPC. 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 vegetable oils and waxes that are controlled by these amendments. Residue/water mixture generated during the prewash is
to 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.
At MEPC 74 in May 2019, the committee approved resolution MEPC.313(74) containing amendments to MARPOL Annex II affecting cargo residues and tank washings of persistent floating products with a high viscosity and/or a high melting point – persistent floaters. When operating in the areas defined as North West European waters, Baltic Sea area, Western European waters and the Norwegian Sea, the revised prewash procedure for persistent floaters shall be applied. This procedure is to be included in a revised and approved procedures and Arrangements Manual. Appendix IV of MARPOL Annex II provides a format for this manual and guidance on the subject of persistent floaters has been added.
Under the changes, 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 ship is proceeding en route at a speed of at least 7 knots;
- The discharge is made below the waterline through, and in accordance with the design of, the underwater discharge outlet;
- The discharge is made at a distance of not less than 12 nautical miles from the nearest land in a depth of water of not less than 25m.
The entry-into-force date of these amendments was adjusted to take place on 1 January 2021.Black and grey waste water treatment
Black and grey waste water treatment
MARPOL Annex IV has been in force since September 2003 and applies to ships of 400gt and above and ships below 400gt that are certified to carry more than 15 persons. It contains a set of regulations regarding the discharge of sewage into the sea, ships’ equipment and systems for the control of sewage discharge and requirements for survey and issuance of the International Sewage Pollution Prevention Certificate (ISPP) that all ships subject to the regulation must carry.
Not surprisingly, because the objective is to destroy harmful bacteria in sewage, many of the treatment methods employed are the same as those used in ballast water treatment systems.
As well as biological treatment, hypochlorination and UV are commonly employed. Some systems do not rely on a single method but combine methods to increase effectiveness. Membrane bio-reactors are also popular.
Biological treatment involves the breakdown of sewage in an organic process that can be done by aerobic or anaerobic bacteria. The latter will inevitably produce hydrogen sulphide (H2S) – which is commonly knows as ‘bad egg’ gas because of its characteristic odour – and methane. Aerobic systems, which do not have this problem, are more popular.
In a typical system the sewage is passed through a screen filter to remove toilet paper and other materials that may have been introduced to the system – a particular problem on passenger vessels. After filtering, the sewage passes to an aeration chamber (often called the bioreactor) where air is bubbled through it and the aerobic bacteria break down the sewage water.
From the aeration chamber, the effluent is moved to a hopper-shaped settling tank. Sludge collects in the bottom of the tank while relatively clean water separates out above. The water is moved to a chlorination chamber and sterilised by chlorine. Grey water is also passed into the chamber and they are treated together. After treatment, the water can be discharged overboard while the sludge returns to the aeration chamber for further treatment and organic breakdown.
A development of the biological process is the membrane bio reactor (MBR). This combines conventional biological treatment processes with membrane filtration to provide an advanced level of organic and suspended solids removal. In an MBR system, the membranes are submerged in an aerated biological reactor. They have porosities ranging from 0.035μm to 0.4μm (depending on the manufacturer), which is considered between micro- and ultrafiltration.
This level of filtration allows for high quality effluent to be drawn through the membranes and eliminates the sedimentation and filtration processes typically used for wastewater treatment. This reduces the process tankage required.
Greywater issues have remained largely absent from the maritime conscience, perhaps due to the rather more obvious and immediate effects of blackwater. But greywater, which comes from the kitchens, laundries and washing facilities for passengers and crew, can pose equally difficult challenges, particularly if a dangerous chemical finds its way into a sink, laundry or shower system.
Greywater has managed to creep under the regulatory radar, mostly because it has a tenth of the levels of nitrogen and pathogens that appear in sewage and decomposes more rapidly. But due to the high volume of untreated greywater produced by the ever-increasing number of passenger cruise ships and ferries, manufacturers have developed systems aimed at greywater treatment.
The development of greywater-oriented systems has also come about because of the need to establish ways to effect the complete degradation of organic matter found in greywater, including fat and grease. Many manufacturers are active in the sector and a variety of technologies are employed. In order to remove fat and grease, some systems make use of separation technology similar to that used for treating oily bilge water.
It is not unknown for black and grey water to become mixed due to operational reasons such as use of wrong tanks or when one tank becomes full and the only space available is in the other system’s holding tank. In 2012 at IMO MEPC 64, the delegation of the Netherlands informed the committee of some preliminary results on a survey conducted on the performance status of the sewage treatment plants installed on board ships, which indicated that a vast majority of the equipment did not meet the existing sewage treatment standards due to improper use of detergent, lack of maintenance or not following the operational instructions. Revelations such as this can mean a concerted inspection campaign will be initiated by port state control regimes.
Although MARPOL does not regulate greywater, some national and state bodies do. Regional rules vary and change frequently. In Alaska, for example, stringent limits regarding cruise ship discharges were introduced in 2000 (33CFR159 sub-part E) with greywater regulated for the first time.
In addition to Alaska’s clean-up efforts, the Great Lakes, US waters (EPA Vessel General Permit, 2013), and inland waterways in Europe (2012/49/EU) have also regulated grey water treatment in various ways, each affecting certain shipping sectors. There are already four sets of different type-approval specifications and at least five different compliancy regimes for operators and equipment makers to contend with.
Some equipment makers have developed treatment systems that can handle both black and grey water. These systems ensure compliance with regulations and also save space as there is no need to duplicate equipment.