This guide on scrubber washwater is intended to give an overview on the topic. Emissions from a ships exhaust stack and disposals of any kind into the sea are both emotive topics that pitch environmental groups and the industry into heated disputes and bitter recriminations.
For the last 30 years, shipowners have been contending with regulated reduction of emissions by way of MARPOL Annex VI. Beginning with NOx emissions and then shifting to SOx reduction, there are further regulations in the pipeline that might one day restrict Black Carbon, soot, Particulate Matter and even CO2 which is already being reduced by way of the EEDI.
IT is theoretically – if not as yet practically – possible to eliminate almost all of these emissions by switching to some alternative fuel although only Hydrogen eliminates all. Most analysts think that fossil fuels will remain part of the fuel mix for several decades and therefore regardless of ambitions, they must be learned to be lived with.
In reducing SOx emissions, the IMO has reached the end of the run of reduction targets with the coming into force of the 2020 sulphur cap on fuels. By the arrival of the 1 January 2020 deadline, more than 4,000 vessels had either been fitted with exhaust gas cleaning systems or were being built with or to be retrofitted with them.
Scrubber equipped ships are able to continue to burn cheaper HFO with a sulphur content above that of all other fuels. This type of fuel is normally significantly cheaper than VLSFO and ULSFO that complies with the IMO 2020 limit for global and ECAs respectively. However, throughout much of 2020, an unusually low crude price cause by lack of demand during the COVID pandemic meant that the differential between bunker grades was depressed making scrubber installation, if not uneconomic then subject to an extended payback period.
It has to be said that scrubbers have at least as many detractors as proponents. Within the shipping industry, some see them as cheating because they allow ships to continue to burn the cheapest fuel. Those owners that have opted for using a compliant low sulphur fuel are thus operating at a competitive disadvantage. For those outside the industry – and some within it – scrubbers are dirty because they merely shift pollution from air to sea.
The latter is perhaps an over simplistic view that disregards science, but which appeals emotionally to those who are hostile to ‘dirty’ shipping. Unfortunately several ports and some nation states have picked up on scrubber washwater and have banned the use of scrubbers in their waters. Ironically, the states which have banned them still permit their use on their own flag vessels operating in other countries’ waters.
Not all scrubbers do discharge chemicals to the open sea. Only open loop systems and hybrid systems operating in open loop mode do this. Closed loop systems retain and store most of the scrubber effluent although a small amount may be discharged in open waters.
It should also be understood that SOx scrubbers are not the only system that washes emissions from the exhaust stream. Exhaust gas recirculation systems designed to reduce NOx emissions also use water to cool the exhaust gases down and this results in contaminated water that needs to be disposed of.
Scrubber washwater -Removing more than SOx
When an exhaust gas is scrubbed its is passed through a stream or spray of seawater which washes out the SOx particles. Effectively, some of the water combines with the SOx to form a weak sulphuric acid solution. It also washes out most of the particulates (around 80%), soot and oil residues that would otherwise be emitted to air as they are in non-scrubber fitted ships. The washwater is not discharged directly over the side as it is first necessary to remove the oily waste that will alter be treated along with bilge water. Proponents of scrubbers point to the additional benefits of exhaust gas cleaning as another point in their favour.
Also in the washwater there may be some levels of metals that have come either from the oil being burned or from the engine. Those from the oil may be naturally occurring or have been introduced at any stage from well to bunker tank. These metals will also be present as fine articles in the exhaust of some ships not equipped with scrubbers and so are not unique to scrubber equipped ships. It would be highly unusual (but not impossible) for metals to be present in quantities considered as being harmful. IMO guidelines do not as yet set any limits for metals.
Also considered as contentious are polycyclic aromatic hydrocarbons (PAHs). These are naturally occurring substances in oils and are also produced as a result of combustion. They are also known to be carcinogenic and do accumulate in ecosystems. The IMO guidelines on washwater contain recommendations but there is some debate over the way that the levels have been arrived at. Again, while scrubbing will remove PAHs from the exhaust stream and deposit them in seawater, There are many sources of PAHs and those emitted from non-scrubber equipped vessels will also be mostly deposited naturally in the sea along with many others from non-ship sources.
The acid test for scrubber washwater
It is often the acidity of washwater that is the major criticism and here there are various factors that need to be considered. Immediately after scrubbing, the washwater is likely to be very acidic with a pH as low as 3. Washwater at this acidity should not be discharged to the sea but can be diluted with more sea water to reach the levels permitted by the IMO.
Under IMO regulations, the discharge washwater should have a pH of no less than 6.5 measured at the ship’s overboard discharge with the exception that during manoeuvring and transit, the maximum difference between inlet and outlet of 2 pH units is allowed measured at the ship’s inlet and overboard discharge.
This pH level is seen by detractors as being damaging to the environment and this is the cause of much debate. There is anecdotal evidence that some ships do discharge washwater that is more acidic than permitted but that is a matter that needs resolution.
However, some would argue that the acidity debate is overblown. In areas where there are high atmospheric levels of NOx and SOx, the pH value of rain is generally between 4.2 and 4.4. Where SOx and NOx levels are what might be considered normal, the pH value of rain is 5.6. This is well below the neutral level of 7 and much below the average for seawater which is around 8.1. The pH value of seawater varies considerably due to subsea volcanic activity and river outflows (some parts of the Amazon river have a natural pH near to that of acid rain). ‘Normal’ rainwater’s acidity comes from CO2 in the atmosphere which dissolves in rain to form a weak carbonic acid.
It would probably be in the interests of all for the acidity question to be settled and potentially this could have been done at the November 2020 MEPC 75 meeting where the item was on the agenda. However it has been deferred until MEPC 76.
So long as the argument over the rights or wrongs of scrubber use are left to emotive comments, there can never be a satisfactory outcome. Therefore scientific studies are essential to settle the matter for the future.
There have been a small number of studies on scrubber washwater although several of these are mere reworking of earlier studies with little in the way of fresh experimentation or measurement. More to the point many could only have been done using earlier generations of cleaning systems rather than the newer models now being installed.
The current IMO rules were set down over a decade ago in 2008 and many would welcome the opportunity to test whether or not the requirements are being met or bettered. Unfortunately the debate has become so polarised that neither side puts any trust in studies done by the other. Even when comparing studies the likelihood is that those doing the comparison will ignore any data that does not gel with their position.
It would be in the interest of all parties to put aside arguments and conduct a joint study or agree upon a neutral third party to do the work. Of course, if an economical and practical alternative to fossil fuels is found, then the need for scrubbers will disappear in any case.