The impact of scrubber washwater on inland waters

Background : The International Maritime Organization (IMO) has set limits on sulphur content in 19 fuels for marine transport. However, vessels continue to use these residual high sulphur fuels in 20 combination with exhaust gas cleaning systems (EGCS or scrubbers). Next to high sulphur, 21 combustion of these fuels also results in higher emissions of contaminants including metals and 22 PAHs. In scrubbers, exhaust gases are sprayed with water in order to remove SOx, resulting in 23 acidic washwater with elevated contaminant concentrations discharged in the aquatic ecosystem. The number of vessels with scrubbers is increasing rapidly, but knowledge on washwater quality 25 and impact are limited. Results : The scrubber washwater is found to be acidic with elevated concentrations of e.g. zinc, 27 vanadium, copper, nickel, phenanthrene, naphthalene, fluorene and fluoranthene. Model 28 calculations on the effects of scrubber (20% of vessels) discharge on aquatic systems showed a 29 decrease in pH of 0.015 units and an increase in surface water concentrations for e.g. naphthalene 30 (110% increase) and vanadium (17% increase). 31 Conclusions : The IMO established sulphur regulations to mitigate the impact of high sulphur 32 emissions of the maritime sector. However, the use of open loop scrubbers as an abatement 33 technology will not reduce their contribution to the acidification. In addition, different types of scrubbers discharge washwater that is acute toxic for aquatic organisms. However, washwater is 35 diluted and the compounds for which a large increase in surface water concentrations was 36 calculated (Naphthalene > Phenanthrene > Fluorene > Acenaphthene > Vanadium) were not the compounds that already exceed their respective Water Quality Standards (WQS). Nevertheless, existing WQS exceedances of ‘priority hazardous substances’ (Water Framework Directive) that are also identified in the washwater indicate that coastal waters and estuaries, often with large ecological value, are already under pressure. In these areas the discharge of scrubber washwater should be discouraged.

PAHs. In scrubbers, exhaust gases are sprayed with water in order to remove SOx, resulting in 23 acidic washwater with elevated contaminant concentrations discharged in the aquatic ecosystem.

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The number of vessels with scrubbers is increasing rapidly, but knowledge on washwater quality 25 and impact are limited.   To comply, ships can use compliant low S fuel oil or alternative fuels which are low in S, such as 70 liquefied natural gas (LNG) or methanol. The IMO S limits only apply to atmospheric emissions. 71 Conseqently, it is allowed to continue to use high S fuels in combination with an exhaust gas 72 cleaning system (EGCS or scrubber). In scrubbers, the exhaust gases of vessels are sprayed with 73 liquid in order to remove the SOx before it will be emitted to the air. Scrubbers are capable of 74 removing up to 95% of the SOx in the exhaust gases and meet the IMO S exhaust limits 8 . Two system, also referred to as seawater scrubbing technology, the exhaust gases are sprayed with 77 seawater at a high flow rate, and the SOx in the exhaust gas is trapped and converted to sulphurous 78 acid (SO3 2-) and sulphuric acid (SO4 2-). The washwater generated in the scrubber is discharged in 79 the surrounding surface water. Alternatively, closed loop systems use freshwater as the scrubbing 80 medium, which is pre-treated with sodium hydroxide (NaOH). This washwater recirculates in the 81 scrubbing system. The scrubbing capacity is maintained by dosing extra NaOH and periodically 82 discharging smaller volumes of washwater. Often, 'hybrid systems' are installed, whereby vessels 83 can shift the scrubber operation between open or closed loop mode.

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Given the fairly recent changes in the IMO S regulations, the amount of vessels equipped with 85 scrubbers is still limited, but changing rapidly. According to Clarksons Worldfleet Register, 86 consulted in November 2019, nearly 3000 scrubbers have already been installed, which 87 corresponds to 3% of the total number of vessels and 16% of the gross tonnage. This implies that 88 mainly large vessels invest in a scrubber. Additionally, 15% in numbers or 35% in gross tonnage 89 of all vessels ordered at this moment (November 2019) will have a scrubber installed. From an 90 economical perspective, scrubbers are an attractive option, particularly for larger vessels 9, 10 . In 91 order to comply, the choice between using the more expensive low S fuels or the installation of a 92 scrubber depends largely on the price difference between both, low S fuels and common heavy 93 fuels 11 . Under most conditions, the scrubber installation costs are recouped within the span of 94 maximum several years 12, 13 . The number of scrubbers is predicted to continue to increase after the 95 implementation of the more restrictive global sulphur cap in 2020.

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The use of scrubbers result in a shift of the environmental impact of S from emissions to the 97 atmosphere towards a direct discharge into aquatic systems. Further, the high S fuels used by 98 vessels with scrubbers are generally heavy fuel oils (HFO), which are residual fuels incurred during 99 the distillation of crude oil. Together with high S emissions, these fuels are known to result in 100 higher emissions of other hazardous species including metals and polycyclic aromatic 101 hydrocarbons (PAHs) compared to low S distillates such as marine gas oil (MGO). These 102 contaminants originate from higher concentrations of e.g. metals and PAHs in the fuel and larger 103 emissions during combustion of this residual fuel 14 . In general, scrubbers are found to reduce the 104 atmospheric emissions of SOx or PM to a level that is comparable to emissions when operating on 105 MGO 15-19 . But scrubbers are an end-of-pipe solution and a substantial part of the emitted 106 compounds will be trapped in the scrubber washwater, discharged in the surrounding surface water 107 with potential consequences for aquatic ecosystems 17,[20][21][22] . Existing studies are limited, mainly 108 focus on open marine systems and conclude that the overall impact of scrubber use on pH changes 109 and contaminant concentrations is expected to be small under most conditions 21, 23-25 . Yet, the long 110 term accumulation of contaminants caused by scrubber discharges can be of concern in aquatic 111 systems where ships are numerous and discharge into a relatively restricted water bodies including 112 coasts, estuaries or harbours 26,27 .

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Data on washwater contaminant concentration are scarce, often proprietary and rarely published.

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In present study, an extensive dataset on washwater contaminant concentrations and acidity is 115 compiled, based on own measurements and received and published datasets. This data allowed us 116 to calculate the impact of scrubber use on water quality for two scenarios (10% and 20% scrubber 117 use) for the Antwerp harbour docks and the Scheldt estuary. While the IMO regulatory framework 118 primarily focuses on atmospheric emissions, also the discharge of washwater is regulated to a 119 certain extent. Washwater discharge criteria were set for pH (min. of 6.5, measured at 4 m from 120 the overboard discharge point), for PAHs (max. 50 µg L -1 PAH Phe equivalent at a flow rate of 45 121 m³ MWh -1 ) and turbidity (max. 25 NTU (Nephlometric turbidity units) above the inlet water  (Table S1). Samples were taken from a tap  (3) from literature. All data was combined in a database (Table S1). Parameters with many values 163 below limit of quantification (BLOQ) were excluded from further analysis (Table S2). To calculate the impact of discharged washwater on the water quality two scenarios were defined. 166 Scenario LOW assumed that 10% of the total ship emissions were treated by scrubbers and vessels 167 discharged at average washwater concentrations. Scenario HIGH assumed a 20% treatment by 168 scrubbers and discharge at 90 th percentile of washwater concentrations (Table 1 for scenario HIGH,   169   Table S3 for scenario LOW).

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The calculated scenarios included all fuel used by the vessels, main and auxiliary engines. For 171 manoeuvring and berthing in harbours, auxiliary engines are typically used. As these auxiliary 172 engines are not always connected to the scrubber, the calculated changes in metal and PAH 173 concentrations in the harbour docks surface water could be an overestimation.

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The contaminant input Jc (kg y -1 ) to the water bodies (harbour docks and estuary) was calculated  (Table S1).

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Note however that some vessels with closed loop systems will not discharge any washwater, as  Table 1 for concentrations).  For changes in TA, SumCO2 and H2SO4, the CO2(aq) in scrubber effluent was assumed to be in 219 equilibrium with CO2 in flue gas, assumed to have a fixed partial pressure of 0.   (Table S1). Metal and PAH concentrations are found to be elevated compared to surface 250 water concentrations or Water Quality Standards (WQS) (Fig. S1, S2). The variation in  (Table 1).

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Some of the pollutants that are present in scrubber washwater are already exceeding (Flu, Pyr) or 353 close to exceedance (Ni, Zn) of their respective WQS in the surface water of the harbour docks or 354 the Scheldt estuary (Fig 3). However, the compounds for which a large increase in concentrations  show a decrease in pH of 0.015 units caused by washwater discharge (Fig. 4). The alkalinity will 409 comparably decrease slightly with 6 µmol L -1 or 0.16% and total sulphate concentrations will 410 increase with 3 µmol L -1 or 0.  Ethics approval and consent to participate: Not applicable.

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Consent for publication: Not applicable.

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Availability of data and material: All data generated or analysed during this study are included in 531 this published article and its supplementary information files.

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Competing interests: The authors declare that they have no competing interests.   Figure S1: All scrubber washwater metal concentrations