1

A Theoretical Environmental Impact

Assessment of the Use of a Seawater

Scrubber to Reduce SOx and NOx Emissions

from Ships

Dr. Brigitte Behrends and Prof. Dr. Gerd Liebezeit

Research Centre Terramare

Wilhelmshaven, Germany
2

CONTENTS

1 Introduction......................................................................................................5

1.1 Air pollution...................................................................................... 5

1.2 Contribution of shipping to air pollution ........................................... 7

1.2.1.1 Legislation:..................................................................................................... 8

1.3 Techniques to reduce atmospheric emissions in power plants.......... 9

1.3.1Techniques for the removal of sulphur from petroleum 9

1.3.2Flue Gas Desulphurisation 10

1.4 Technologies to reduce atmospheric emissions from ships............. 12

- the seawater scrubber......................................................................... 12

The following reactions should take place: ..................................................................13

2 NO2 + H2O  HNO2 + HNO3 ....................................................................................13

3 HNO2  HNO3 + 2 NO + H2O...................................................................................13

2 HNO2  H2O + N2O3 ................................................................................................13

N2O3  NO + NO2.......................................................................................................13

2 NO2 + H2O  HNO3 + HNO2 ....................................................................................13

overall: 3 HNO2  HNO3 + 2 NO + H2O ..................................................................13

1.4.1Composition of the effluent 14

1.5 Questions of concern ...................................................................... 14

2 Element cycles...............................................................................................15

2.1 Sulphur........................................................................................... 15

2.1.1Sulphur: an important component of the world economy Production 15

2.1.2Sulphur emissions to the atmosphere 17

2.1.2.1 Natural emissions of sulphur.................................................................... 17

2.1.2.2 Global anthropogenic sulphur emissions ............................................... 17

2.1.2.3 Particulate sulphate.................................................................................... 18

2.1.3Deposition of sulphate and sulphuric acid 18

2.2 Nitrogen cycle................................................................................. 19

2.2.1Emissions of nitrogen compounds to the atmosphere 19

2.2.1.1 Natural emissions of nitrogen .................................................................. 20

2.2.1.2 Anthropogenic emissions of nitrogen ..................................................... 20

3 Composition of seawater...............................................................................22

3.1 Marine sulphur cycle....................................................................... 23

3.2 Marine nitrogen cycle ..................................................................... 25

3.3 The pH of seawater: the carbonate system..................................... 26

4 Laboratory measurements ............................................................................30

4.1 Impact of diluted sulphuric acid (pH 4) on seawater ...................... 30

4.2 Impact of diluted sulphuric acid (pH4) on brackish waters ............ 32

Deleted: 13
3

4.3 Summary of laboratory experiments .............................................. 34

4.4 Impact of the acidic effluent of the Krystallon Scrubber on the pH of

the receiving water................................................................................ 34

4.5 Estuaries......................................................................................... 35

4.5.1Harbour areas like the Port of Rotterdam 37

4.6 The Baltic Sea ................................................................................. 38

4.6.1Vital inflow from the North Sea 38

4.7 Acidification of surface waters ....................................................... 39

4.8 Experiences with acid disposals at sea. .......................................... 40

4.9 Eutrophication effects..................................................................... 40

5 Environmental impact of scrubbing waters - Synthesis .............................42

Figures

Fig. 1: Mean composition of atmospheric nitrogen compounds (nmol m-3) in the German Bight _______21

Fig. 2: Scheme of the microbiological cycle of sulphur and its possible influence on the atmosphere ___23

Fig. 3: Schematic seasonal variation of nutrients and phytoplankton biomass in temperate areas. _____25

Fig. 4: Major reactions and pathways in the marine nitrogen cycle _____________________________26

Fig. 5: Impact of sulphuric acid (pH 4) on the pH of seawater (S = 29 PSU)______________________31

Fig. 6: Percentage of sulphuric acid (pH 4) in brackish water with resulting pH (t = 0 and t = 1 h). ___33

Fig. 7: pH ranges that support aquatic life ________________________________________________36

List of Tables

Tab. 1: Comparison of estimates for global exhaust gas emissions from ships. ____________________8

Tab. 2: World production and consumption of sulphuric acid ________________________________16

Tab. 3 Important Atmospheric nitrogen oxides ___________________________________________20

Tab. 4 Average sea- and freshwater compositions___________________________________________23

Tab. 5: Percentage of sulphuric acid (pH 4) in brackish water with resulting pH (t = 0 and t = 1 h) __32

Tab. 6: Contribution of nitric acid to the acidity of the effluent _______________________________35

Tab. 7: Ports in the EU ranked by estimated annual emissions of NOxin 2000 ___________________37

Tab. 8: discharge of sulphate and nitrate per hour ____________________________________________43
4

List of used abbreviations

amu atomic mass units

CCN cloud condensation nuclei

DIN dissolved inorganic nitrogen (NH4

+

, NO2

-

, NO3

-

)

DMS Dimethyl sulphide

DON dissolved organic nitrogen

FGD Flue gas desulphurisation

IMO International Maritime Organization

Mam-Pec Computer model to generate predicted environmental concentrations

MCR Maximum continuous rating

NOx Oxides of Nitrogen

PAH polycyclic aromatic hydrocarbons

PON particulate organic nitrogen

PSU practical salinity units

SOx Oxides of Sulphur

SWS seawater scrubber

VOC volatile organic compounds

Formatted: Italian (Italy)
5

1 Introduction

The burning of fossil fuels is one of the most significant contributor’s to atmospheric

pollution. For several decades concern has been expressed about the major pollutants,

carbon dioxide, sulphur dioxide and nitrous oxides. Carbon dioxide is considered to be

largely responsible for global warming while the latter two gases contribute amongst

others to acid rain. Additionally, NOx enhances eutrophication of terrestrial and aquatic

ecosystems. A large number of emission reducing

technologies have been developed for power stations

(see chapter 1.3), but the shipping industry has made

less progress in limiting emissions. This is despite the

fact that recent estimates suggest that shipping is a

major factor in global S and N cycles (chapter 2.1). The

main objective of this report is therefore the reduction

of atmospheric emissions from ships, by use of a seawater scrubber technology (flue

gas desulphurisation process).

1.1 Air pollution

Air pollutants are substances that are introduced into the atmosphere via

anthropogenic activities. Air pollution occurs both in gaseous and particulate forms

which, when present in excess, are harmful to human health, buildings and

ecosystems. Five major impacts determine the classification of pollutants under the

traditional policy field, Air Pollution:

 acidification of soil and water by pollutants such as sulphur oxides and nitrogen

oxides;

 eutrophication of ecosystems, especially by nitrogen compounds;

 damage to buildings sensitive to the same substances;

 formation of tropospheric ozone from so-called ozone precursors, e.g. volatile

organic compounds, nitrogen oxides and carbon monoxide, thus indirectly

affecting human and animal health and vegetation;

 direct effects on human health and ecosystems, e.g. through high atmospheric

concentrations of particles and volatile organic carbon compounds (VOCs).

Although some of these compounds are also produced by natural processes, the main

environmental problems result from human activities, such as the burning of fossil

fuels (coal, oil and natural gas). Air pollutants may be transported over considerable

distances, affecting air quality, ecosystems, lakes and other surface waters,

Eutrophication is the process

whereby a body of water

becomes over-enriched with

nutrients which results in

overgrowth of algae and

depleted oxygen levels in the

water
6

groundwater, soils and buildings also in areas remote from the pollution source. Public

concern is focussed mainly on damage to forests and smog, although in Scandinavian

countries emphasis is also placed on the acidification of lakes. The atmospheric

pollution by sulphur dioxide and nitrous oxides is described in detail in chapters

2.1.2.2 and 2.2.1.2.

Particulates

Particulate air pollution originates from diverse sources. One major source of primary

particle emissions is fuel combustion, including that by mobile sources such as diesel

and gasoline-powered vehicles. The relative contributions of these different sources to

the particle concentration at a given site may vary significantly. Atmospheric particles

consist of organic and inorganic substances and are present in liquid or solid forms.

They are subdivided according to diameter: coarse particles >2.5 μm, fine particles

<2.5 μm. A further distinction is to classify particles as either primary or secondary,

according to their origin. Particles that are emitted directly into the atmosphere are

termed primary particles, whereas secondary particles form during atmospheric

reactions. The primary particles contain carbon, hydrocarbons, metal oxides and other

solid materials, which are primarily formed during combustion processes. Fine

particles also include secondarily formed aerosols, smaller combustion particles and

re-condensed organic and metallic compounds with low volatility. Most secondary

particulate matter occurs as ammonium sulphates and nitrates formed from reactions

involving sulphur dioxide, nitrogen oxides and ammonia. Shipping contributes to both,

primary and secondary particles. Natural atmospheric emissions of particulates also

derive from volcanic activities.

1. Primary particulates:

Flue gases contain soot particles, which are produced by incomplete combustion of

fossil fuels. Due to the ability of these particles to serve as condensation nuclei, soot

contains polycyclic aromatic hydrocarbons (PAHs), a family of semi-volatile organic

pollutants encompassing compounds such as extremely small amounts of anthracene,

and some pyrene, or benzo[a]pyrene. Some PAHs from combustion products have

been identified as carcinogenic. Combustion-derived PAHs are rapidly transferred from

the atmosphere to aqueous systems and accumulate in the sediment.

2. Secondary particulates

Secondary particles (including sulphate and nitrate aerosols) are formed from gaseous

pollutants (SO2, NOx, NH3). Ships emit sulphur dioxide and nitrous oxides through the

Formatted: French (France)
7

combustion of fossil fuels. The primary air pollutants sulphur dioxide and nitrogen

oxides are oxidised as they are dispersed in the atmosphere, forming sulphuric acid

and nitric acid respectively, which may be deposited downwind as acid rain. Secondary

particles can be transported over long distances.

1.2 Contribution of shipping to air pollution

Shipping contributes to air pollution by the burning of fossil fuel. The emission of NOx

and SOx have different consequences for the atmosphere: NOx is involved in

production of ground level ozone but it also reacts with tropospheric ozone thus aiding

in the depletion of the ozone layer. SO2 is oxidised to SO3 and eventually forms

sulphuric acid, which contributes to acid rain and provides cloud condensation nuclei

by formation of secondary particles, thereby increasing the albedo. Owing to the

reactivity of sulphur gases in the atmosphere, most anthropogenic SO2 emissions are

deposited locally; therefore shipping contributes mainly to coastal air pollution.

Corbett and Fischbek (1997) give annual emissions of 3.08 * 106ton N and 4.24 * 106

ton S for the global fleet accounting for 14% (N) and 16% (S) of emissions from all

fuel combustion sources (Tab 1). On the other hand, only 2% of the 6,000 * 106ton

carbon dioxide emitted annually from fossil fuel combustion is related to the operation

of ships. This implies that ship engines although highly efficient at converting fuel into

power, emit very high levels of nitrogen and sulphur gases. Various estimates for the

contributions of shipping to global air pollution are presented in (Tab 1).
8

Tab. 1: Comparison of estimates for global exhaust gas emissions from ships.

Source Calculation

(1) Year

C

(2)

[106t]

SO2

[106t]

NOx

[106t]

Isensee, 2001 1997 101 6.4 6.0

IMO, 2000 M 1996 112 5.5 9.8

IMO, 2000 F 1996 117 5.8 10.3

Det Norske Veritas, 1999 M 1996 109 5.5 9.9

UNFCCC, 1997 ? 1994 109 7.7-11.5 9.3

Corbett and Fischbeck,

1997

F

1996 - 8.5 10.1

Corbett, 1999 ? 1992 123.6 8.5 10.1

Mean 1994 -

1997

114.2 6.7 9.3

(1) M = model, F = fuel based calculations

(2) Calculated from CO2 emissions

Emissions from shipping operations are not equally distributed over the seas. Most

vessels navigate relatively near shore, following the main shipping lanes. Corbett

(1999) concludes that almost 70 % of these emissions occur in a coastal zone of 400

km (216 nautical miles) width. Similarly, Oftedal (1996) estimates that 74 - 83 % of

all vessels are within 200 nautical miles (370km) of land at any time. Consequently,

the impact of shipping on NOX and SO2 levels is highest in the northern hemisphere,

particularly along the west and east coasts of the United States, in northern Europe

and the North Pacific.

1.2.1.1 Legislation:

As soon as the IMO regulation for the prevention of air pollution from ships (Marpol

73/78 Annex VI) came into force, emissions from ship exhausts had to be reduced.

Annex VI contained provisions allowing for special "SOx Emission Control Areas" to be

established with a more stringent control on sulphur emissions. In these areas, the

sulphur content of fuel oil used on board ships must not exceed 1.5 % m/m.

Alternatively, ships must fit an exhaust gas cleaning system or use any other technical
9

method to limit SOx emissions. Currently the Baltic Sea, the North Sea and the

Channel are designated as SOx Emission Control Areas in the Protocol. Marpol Annex

VI entered into force on the 19th May 2005..

1.3 Techniques to reduce atmospheric emissions in power plants

The technology for the reduction of atmospheric pollutants should be environmentally

benign. Otherwise, the problem is merely being shifted to another area. There are

three possibilities to reduce SO2 emissions from combustion processes: removal of

sulphur or sulphur dioxide before, during or after combustion.

1.3.1 Techniques for the removal of sulphur from petroleum

After World War II, production of high sulphur crude oils increased considerably. As a

result of increased use of high sulphur crudes and increasing awareness of

environmental considerations, product specifications became more stringent. The oil

industry had to develop techniques for the removal of sulphur and the platinum

reforming process was developed. However it should be noted that platinum reforming

is only suitable for white oils containing relatively low ppm levels of sulphur.

The major process used today is Hydrotreating or Hydrodesulphurisation, i.e.

treatment of the oil with hydrogen gas obtained e.g. during catalytic reforming.

In this process sulphur compounds are reduced by conversion to hydrogen sulphide

(H2S) in the presence of a catalyst. As only sulphur is removed, high yields of liquid

end products result. In addition, H2S can be washed from the product gas stream by

an amine wash. H2S is recovered in highly concentrated form and can then be

converted to elemental sulphur via the Claus-Process.

For Hydrotreating two processes are used, the liquid phase or trickle flow and the

vapour phase processes for light straight-run and cracked fractions. In both cases the

feedstock is mixed with hydrogen-rich make-up and recycled gas and reacted at

temperatures of 300 - 380 °C. Pressures of 10 - 20 bar for naphtha and kerosine and

30 - 50 bar for diesel are applied. Removal of sulphur from heavier oils such as marine

fuel oil often requires pressures of up to 200 bar. Under these conditions, S and N

compounds are converted to hydrogen sulphide and ammonia, while a part of the

aromatic compounds present will by hydrogenated.

Catalysts employed are cobalt, molybdenum or nickel finely distributed on alumina

extrudates.

More recent developments include biological conversion of sulphur-containing

petroleum components to elemental sulphur. Biological removal of hydrogen sulphide
10

and light mercaptans from refinery streams has been commercially implemented. A

biological process for removal of sulphur from diesel oils was under development for

several years, but the firm has subsequently ceased operation. Sonocatalytic

treatment (optional) with hydrogen peroxide oxidation in the presence of

iron(III)complexes at low temperatures has removed sulphur from diesel oils. Several

developers are engaged in advanced pilot plant studies seeking to commercialise such

a process. This approach will remove dibenzothiophenes, compounds which are of

particular concern to the oil industry. The effectiveness of this method for removing

sulphur from heavy fuel oils has not yet been quantified, and disposal of product

sulphones must be factored into any process design.

1.3.2 Flue Gas Desulphurisation

The removal of SO2 after the combustion process is termed Flue Gas Desulphurisation

(FGD). The different FGD processes can be classified by their technologies:

Limestone/Gypsum System

The limestone FGD is to date the most widely used process. In principle, a suspension

of crushed limestone in water is sprayed into the flue gases. The SO2 reacts with

calcium ions to form a calcium sulphite slurry. Aeration of the slurry with compressed

air oxidises calcium sulphite to calcium sulphate. After removal of the water, the

calcium sulphate can be disposed of or used by the building trade. Although the

reduction of SO2 is around 90 %, the process has the disadvantage that limestone has

to be stored and large quantities of gypsum waste is produced.

Spray Dry System

A slurry of slaked lime is used as an alkaline sorbent. The slurry is injected into the

flue gases in a fine spray. The flue gases are simultaneously cooled by the evaporation

of water. The SO2 present reacts with the drying sorbent to form a solid reaction

product, with no wastewater.

Wellman-Lord Process

In the Wellman-Lord process, hot flue gases are passed through a pre-scrubber where

ash, hydrogen chloride, hydrogen fluoride and SO3 are removed. After pre-scrubbing,

the gases are then cooled and fed into an absorption tower, where the SO2 reacts with

a saturated sodium sulphite solution to form sodium bisulphite. The sodium bisulphite
11

is regenerated after a drying step to sodium sulphite again. The remaining product -

the released and clean SO2 - may then be liquefied or converted to elemental sulphur

or sulphuric acid. The sorbent is regenerated during the combustion process and is

continuously recycled, but the products (sulphur compounds) have to be stored.

Seawater Scrubbing Process (SWS)

Experience with the FGD by seawater scrubbing process in power plants

Probably the first large sized FGD was installed at the Battersea power plant (London

Power Company) in the UK in 1930. During the power station planning approval local

authorities expressed concern about the impact on the local community of the

emissions of large quantities of SOx from the combustion of coal. The water of the

Thames was used as the major sorbent although for a short period limestone was

added. This practise was soon discontinued as it was determined that it provided no

additional benefit to the scrubbing system. The Battersea power station operated in

this mode for more than 50 years.

In 1972, ABB Environmental in Norway and Norsk Hydro started the development of a

flue gas desulphurisation process (Flakt Hydro process) using seawater for the

absorption of SO2 from flue gases. The first seawater scrubber was introduced in 1988

at the Statoil refinery in Mongstad Norway (Tokerud, 1989). The seawater scrubbing

process exploits the natural buffering capacity of seawater to absorb acidic gases

instead of producing huge amounts of gypsum. After passing through a dust collector,

the hot flue gas enters the scrubber. The scrubber is a packed absorption tower,

where the gases flow counter-current to seawater. During this process SO2 is

efficiently absorbed by the seawater. Air is supplied to oxidise the absorbed SO2 to

sulphuric acid and to saturate the seawater with oxygen. Before discharging the

effluent into the sea, further seawater is added to bring the pH back to normal

seawater values. As the Norwegian Sea is deep and well mixed, further water

treatment was regarded unnecessary (Glenna and Tokerud, 1991). Several other sea

water scrubbing flue gas desulphurisation systems have been installed in power plants

around the world.

The sulphate ions themselves are harmless, as they are major constituents of ordinary

seawater. However, the pH of the effluent may be of concern. To reduce the sulphuric

acid problem, e.g. in sensitive areas, precipitation of sulphate with limestone is

possible. This system is simple and inherently reliable with low capital and operational

costs. It can remove up to 99 % of SO2, with no disposal of waste to land. However,

heavy metals and chlorides that are also present in the scrubbing water must be
12

captured by use of a water treatment system, otherwise they will be released to the

sea with the wash water.

Conclusion of Marine FGD technologies

Each method has both advantages and limitations related to cost, removal efficiency,

operational experience and waste products produced. For ships, the lack of space is an

important criterion. Space is not only needed for the FGD itself, but also for chemicals

(depending on the technology) and waste products. Some emission control

technologies affect more than one pollutant and have different impacts on

environmental effects. FGD simultaneously removes SO2, NOx and particulate matter.

1.4 Technologies to reduce atmospheric emissions by ships

- the seawater scrubber

For ships seawater scrubbing is a very suitable process, because

 no limestone has to be stored on board,

 no waste (gypsum) is produced, which has to be deposited on land,

 the seawater already contains substantial amounts of sulphate and nitrate

The Krystallon Scrubber seawater scrubber removes 90-95 % of SO2 and 10 - 20 % of

NOx. Additionally, the SWS removes 80 % of the particulates and 10-20% of

hydrocarbons. The use of cyclone technology ensures that the particulate material is

retained on board ship and not included with effluent overboard seawater. The

particulate sludge is then deposited ashore along with shipboard oily waste according

to local environmental and safety regulations. The system needs little extra space, as

the majority of the system replaces existing exhaust silencers of the exhaust system.

The accompanying positive side effects are the reduction of engine noise and a

reduction of the diesel smell.

As the conversion of SO2 to SO4

2-

and NOx to NO3

-

consumes oxygen, aeration of the

effluent is necessary. The system incorporates a high degree of recirculation, thus

ensuring that sulphur oxides are given adequate time and oxygen contact to be

converted to SO4. In the following calculations complete oxidation will be assumed.

The Krystallon Scrubber works by bringing water in contact with hot exhaust gas. The

exhaust gas is channelled through a concentric duct into a shallow water tank. Within
13

the water, there are a set of mixing baffles which break up large gas flow into smaller

bubbles, forcing the gas to come in contact with water, and thereby encouraging

transfer of species from gas to water. The SOx in exhaust gas is relatively soluble in

seawater, so this transfer happens to a very high degree. Larger particles (greater

than 2.5 micron) are predominantly captured in the water. Fine particles (smaller

than 2.5 micron) may pass through without capture.

Since the recirculated water is maintained at a pH of 2.0-4.0, this makes a very good

medium for scrubbing of NOx (eg Scheme 1). Measurements have shown up to 20 %

removal of NOx, and this is enhanced due to the acidity of scrubbing liquor.

The following reactions should take place:

2 NO + O2  2 NO2 + 27.28 kcal

As this is an exothermic reaction the equilibrium is shifted to the left of the equation

with increasing temperature. Accordingly, there is no reaction of NO with oxygen at

temperatures >650 °C.

2 NO2 + H2O  HNO2 + HNO3

3 HNO2  HNO3 + 2 NO + H2O

2 NO + O2  2 NO2

overall: 4 NO2 + 2 H2O + O2  4 HNO3

The following reaction also takes place at higher temperatures:

2 HNO2  H2O + N2O3

N2O3  NO + NO2

2 NO2 + H2O  HNO3 + HNO2

overall: 3 HNO2  HNO3 + 2 NO + H2O

The final product in any case will be nitric acid.

Water that is removed from the scrubber is pumped through a set of large cyclones.

These cyclones are designed to separate some of the heavy particles, as well as light

particles in a two-stage system. Underflow from the main cyclones is concentrated

again, and the underflow is fed to a settling tank for collection of soot and oil. Under

normal operating conditions this system can be run with no ongoing maintenance

(such as filter backwash or media recharge), and the resulting cleaned recirculated

water is maintained at extremely low concentrations of hydrocarbons, making it safe

for discharge to sea.
14

1.4.1 Composition of the effluent

While NOx production depends on the engine design and engine operating conditions,

SO2 production depends on the sulphur content of the fuel. During seawater scrubbing

the SO2 is finally converted to sulphuric acid and NOx to nitric acid. 95 % of the SO2 is

eliminated by this technique and ca. 20 % of NOx. The salts in the effluent are SO4

2-

,

SO3

2-

, NO3

-

and NO2

-

.

The seawater scrubber is fed with natural seawater, which is then added to the

seawater cooling system. The dilution is typically between 5:1 and 10:1. The mixture

of cooling water and buffering water is then discharged to the sea. The pH of the

effluent mixture ranges from 4.8 to 6.4 depending upon dilution rate which is a

function of engine operating conditions. The intention of this work is to analyse the

impact of the produced ions and the low pH on marine life.

1.5 Questions of concern

 Is air pollution simply converted to seawater pollution by discharging the scrubbing

water directly into the surrounding waters?

 Are there differences in the effects on fully marine and brackish waters? Is there

any breakpoint for salinity, at which the scrubbing water should not be discharged

anymore?

 Can the produced sulphuric and nitric acids be buffered in semi-enclosed systems

and brackish waters?

 Does the addition of nitrate from nitric acid lead to higher primary production of

biomass, i.e. to eutrophication?

 Is there any oxygen deficiency in the surrounding seawater by discharging

sulphuric acid?

 What are the impacts of scrubber effluent on harbour waters compared to power

stations effluents

 What is the worst-case scenario?

To answer these questions, background information on global sulphur and nitrogen

cycles and seawater chemistry is provided in the following section and then the effects

of acid discharge are discussed.
15

2 Element cycles

2.1 Sulphur

The element sulphur occurs as crystals, granular aggregates, massive layers and

powdery crusts in three different forms: amorphous, rhombic and monocline crystals.

It forms as a product of volcanic activity, around volcanic craters and hot springs, but

also by weathering of sulphide minerals and bacterial reduction of dissolved sulphate.

Sulphur melts easily and burns producing sulphur dioxide gas.

Sulphur is an essential nutrient for plants. It is also a major element of seawater and

marine sediments. In nature sulphur compounds may cycle through a series of

oxidation states (-2, 0, +2, +4, +6). Therefore, sulphur compounds may act as both

electron acceptors and electron donors. Bacteria of a wide range of genera gain

metabolic energy from either oxidising or reducing sulphur compounds. Major forms of

sulphur include sulphate and sulphide minerals, dissolved sulphate (SO4

2–

), dissolved

sulphide (HS–), and hydrogen sulphide gas (H2S). Organic sulphur is a component of

organic compounds such as humic substances, kerogen and proteins. Sulphur in fossil

fuels (mercaptans, thiophenes, mono- and polycyclic sulphur compounds) originates

from these latter sources (e.g. Guadelupe et al., 1991; Thompson, 1994; SaizJiminez, 1995; Sinninghe-Damsté et al., 1998).

2.1.1 Sulphur: an important component of the world economy Production

Elemental sulphur for industrial purposes is produced by oxidation of hydrogen

sulphide gas or reduction of sulphur dioxide and organic sulphur. These compounds

are constituents of natural gas and oil, from which they are also recovered. Of all

sulphur sources, natural sulphur from open pit mining accounts for less than 1 %,

nevertheless representing 3 * 106t/yr world-wide. Most of the sulphur is recovered

from natural gas and oil (CIEC, 1999). For the exploitation of deep sulphur layers the

Frasch process is the most important technique: Sulphur is melted with heated water

under pressure and the liquefied sulphur is then pumped to the surface with

compressed air (Hollemann and Wiberg, 1985).

Uses of sulphur

Sulphur is an essential nutrient for the growing plant. It is present in fertilisers,

usually in the sulphate form that plants can use. For example, superphosphate

contains about 11 % S, while ammonium sulphate, usually regarded as a nitrogenous

fertiliser, contains 24 % S which is more than its nitrogen content. However, about

90 % of industrial sulphur is used in the manufacture of sulphuric acid, the most
16

important sulphur compound. More sulphuric acid is produced than any other chemical

in the world. In Western Europe in 1997 over 19 * 106tonnes were produced, the

total production world-wide being estimated at around 150 * 106tonnes (Smith et al.,

2001). It is said that the economic prosperity of a country can be assessed by its

consumption of sulphuric acid. Sulphuric acid was manufactured by the lead-chamber

process until the mid-1930s. This process has now been replaced by the contact

process, involving the catalytic oxidation of sulphur dioxide.

Tab. 2: World production and consumption of sulphuric acid

1992 1993 1994 1995 1996 1997

106tonnes H2SO4

Production 145.7 132.5 137.9 148.9 151.3 155.6

Consumption 147.1 132.8 138.8 150.1 153.3 157.5

(EFMA, 2000)

Demand for sulphuric acid

The largest single sulphuric acid consumer by far is the fertiliser industry. Sulphuric

acid is used with phosphate rock in the manufacture of phosphate fertilisers while

smaller amounts are used in the production of ammonium and potassium sulphate.

Substantial quantities are also used as an acidic dehydrating agent in organic chemical

and petro-chemical processes, as well as in oil refining. In the metal processing

industry, sulphuric acid is used for pickling and descaling steel; for the extraction of

copper, uranium and vanadium from ores, in the purification of non-ferrous metals

and plating. In the inorganic chemical industry, it is used most notably in the

production of titanium dioxide.

Further applications of sulphuric acid are wood pulping processes for paper production,

some textile and fibres processes, leather tanning, the use as electrolyte in car

batteries and production of detergents and pharmaceuticals. Other end uses for

sulphuric acid include effluent/water treatment, production of plasticisers, dyestuffs,

explosives, silicate for toothpaste, adhesives, rubbers, edible oils or lubricants and the

manufacture of food acids such as citric acid and lactic acid.

Waste streams containing sulphate are generated by many industries, including

mining, metallurgical, pulp, paper and petrochemical industries. By discharging these

sulphate containing wastes to the rivers, all sulphate finally ends up in the sea.
17

2.1.2 Sulphur emissions to the atmosphere

No gaseous sulphur compound is a permanent or long-lived constituent of the

atmosphere. The short residence time for sulphurous gases is a consequence of rapid

oxidation: Sulphur and its gaseous compounds hydrogen sulphide (H2S) and sulphur

dioxide (SO2) are oxidised to sulphur trioxide (SO3), which dissolves in water droplets

of the clouds to form sulphuric acid (H2SO4). This process is responsible for acid rain.

However a significant concentration is neutralised by gaseous ammonia in the

atmosphere to form the corresponding ammonium salts e.g. ammonium sulphate (this

is a secondary particulate).

2.1.2.1 Natural emissions of sulphur

Natural emissions of sulphur into the atmosphere from biogenic sources and from

volcanoes are estimated to be around 60 * 106 S yr-1(2 * 1012 mol S yr-1). These

values range from 46 to 124 * 106 S yr-1(Whelpdale, 1992). In addition, sea spray

contributes by the release of sulphate, to a large amount of sulphur emissions. This

estimate is very imprecise with 35 to 315 * 106 S yr-1, of which around 90 % cycles

back into the oceans.

Finally, the biogenic production in the open sea contributes to almost half (46 %) of all

natural sulphur emissions to the atmosphere (Whelpdale, 1992). Dimethyl sulphide

(DMS), the main volatile sulphur species emitted by marine phytoplankton in the

oceans, plays a major role in the atmospheric sulphur cycle (Lovelock et al., 1972;

Nguyen et al., 1984; Andreae and Raemdonck, 1983, Bates and Cline, 1985)

In short, DMS released by marine phytoplankton enters the troposphere and is

oxidised there to sulphate particles, which then act as cloud condensation nuclei (CCN)

for marine clouds (Charlson et al., 1987). Both the nature of the DMS oxidation

mechanism as well as the quantification of the extent to which the oxidation product

H2SO4 contribute to new CCN is largely unknown. Further, DMS has an indirect impact

on the acidity of rainwater. The atmospheric DMS concentration varies seasonally,

dependent on phytoplankton blooms. The DMS production in the Southern Indian

Ocean has been reported to be by a factor of 20 or higher in the summer compared to

the winter (Sciare et al., 2000).

2.1.2.2 Global anthropogenic sulphur emissions

The emission of sulphur compounds into the environment is undesirable because of

their acidifying characteristics. The global anthropogenic sulphur emissions have been

estimated to be in a range from 60 to 110 * 106t S yr-1(2 – 3 x 1012 mol yr-1) for the
18

years 1976 to 1985 (Moller, 1984; Hameed and Dignon, 1988). The combustion of

fossil fuels accounts for 80 to 85% of the total.

2.1.2.3 Particulate sulphate

Sulphate compounds are emitted as primary (directly released) and secondary

particles. The sources of primary sulphate particles include open pit mines, dry

lakebeds, and desert soils. Other sources of primary sulphate particles are various fuel

combustion processes from both stationary and mobile sources.

Secondary sulphate particles are produced in the atmosphere when oxides of sulphur

(SOx) are transformed into particles via physical and chemical processes. The main

source of SOx is stationary and mobile fuel combustion. The types of sources that emit

SO2 are generally located in urban areas. Secondary sulphate particles formed from

SO2 emissions can be transported over long distances. These transported sulphate

particles can comprise a significant portion of the ambient sulphate in some rural

areas.

2.1.3 Deposition of sulphate and sulphuric acid

Gases in the atmosphere can reach the earth’s surface by wet and dry deposition

processes. For wet deposition, precipitation is the delivery mechanism to the surface.

The gases can dissolve in clouds and rain drops or adsorb to frozen precipitation

elements. Sulphate particles represent good condensation nuclei and may be

scavenged by the clouds.

Those deposition processes, which do not involve precipitation, are collectively termed

dry deposition. Sulphate can be chemically bound or adsorbed to small particles,

which then reach the surface by gravitational settling (Whelpdale, 1992). The direct

adsorption of SO2 to particles is still poorly understood.

Secondary particles formed through chemical reactions are typically much smaller

(<2.5 μm diameter) than particles derived from physical processes (>2.5 μm

diameter). The smallest particles in the atmosphere are the result of high temperature

combustion and gas to aerosol conversions.

Depending on the relative humidity and the presence of other atmospheric gases like

ammonia, the sulphuric acid formed will be removed directly in precipitation, or

neutralised and converted to ammonium sulphate. The atmospheric conversion of

sulphur dioxide via sulphuric acid to ammonium sulphate (Scheme 1) is an example of

a gas to aerosol conversion process.
19

Scheme 1

H2SO4(g)  H

+ + HSO4-

HSO4

-  H+ + SO42-

NH3(g) + H2O(g)  NH4

+ +-OH

2 NH4

+

(g) + SO4

2-

(g)  (NH4)2SO4(aq)

Thus, particles form through complex gas phase, heterogeneous and liquid phase

reactions.

2.2 Nitrogen cycle

Elemental nitrogen (N2) is gaseous and the major constituent of air (78.09 % Vol.). By

this, 99 % of all nitrogen on earth occurs in the air. Chemically bound nitrogen can be

found as nitrate, and organic bound nitrogen in e.g. proteins or nucleic acids. Pure

nitrogen can be simply produced by fractionation of liquid air (Linde Process).

By its ability to have oxidation states ranging from –3 up to +6, numerous

combinations with other elements are possible. The most important reaction partners

are hydrogen (reduced form) and oxygen (oxidised forms). Nitrogen compounds,

especially ammonia, are of great importance for the world economy for use, among

others, as fertiliser (Hollemann and Wiberg, 1985).

Nitrogen is also an essential nutrient for life, but only a few plants are able to use

elemental nitrogen as source. A variety of different bacteria are able to oxidise or

reduce nitrogen compounds to gain energy.

2.2.1 Emissions of nitrogen compounds to the atmosphere

Nitrogen dioxide (NO2) is a reddish brown, highly reactive gas that is formed in the air

through the oxidation of nitric oxide (NO). Nitrogen oxides (NOx), the term used to

describe the sum of NO, NO2 and other oxides of nitrogen, play a major role in the

formation of ozone. The major sources of man-made NOx emissions are hightemperature combustion processes, such as those occurring in automobiles and power

plants.
20

Nitrogen oxides also contribute to the formation of acid rain and to a wide range of

other environmental effects, including: potential changes in the composition and

competition of some species of vegetation in wetland and terrestrial systems, visibility

impairment, acidification of freshwater bodies, eutrophication of estuarine and coastal

waters, and increases in levels of toxins harmful to fish and other aquatic life. The

consequence of eutrophication is an explosive algae growth leading to a depletion of

oxygen in the water.

2.2.1.1 Natural emissions of nitrogen

Atmospheric reactions of nitrogen are much more complex than the atmospheric

reactions of sulphur. Ammonia (NH3) is the most reduced form of nitrogen, and is

released in small quantities from anaerobic degradation of organic matter containing

nitrogen. The major contribution to natural emissions is from agriculture. Similar to

hydrogen sulphide, ammonia reacts with the hydroxyl radical to form oxidised nitrogen

species. Nitrogen oxides are released to the atmosphere from both natural and

anthropogenic sources. The two most common nitrogen gases released to the

atmosphere from biological processes are nitrous oxide (N2O) and nitrogen dioxide

(NO2).

2.2.1.2 Anthropogenic emissions of nitrogen

Combustion processes release mostly nitrogen oxide (NO) and nitrogen dioxide (NO2).

The exact composition of nitrogen oxides emitted from combustion processes varies

with the temperature of the combustion process, therefore the nitrogen oxides from

combustion are often referred to as NOx to indicate the uncertainty in chemical

composition.

Like sulphur, the modern global nitrogen cycle is very different from the "Preindustrial" nitrogen cycle. The difference is the large amount of nitrogen added to the

atmosphere through combustion processes. The excess atmospheric nitrogen oxides

contribute to acid rain in the same way that excess sulphur oxides do. Tab. 3 lists the

important atmospheric nitrogen oxides and their oxidation states.

Tab. 3 Important Atmospheric nitrogen oxides

Nitrogen oxide Oxidation state

N2O +1

NO +2

NO2 +4

HNO3, N2O5 +5

Deleted: Tab. 3
21

Atmospheric nitrogen compounds are transported over long distances (Schulz et al.

1998). During transport they undergo specific physical-chemical changes (Fig. 1).

NOx-compounds are typical anthropogenic precursors of oxidised nutrients like nitrate.

The extremely complicated reactions within the atmosphere are described in detail by

Holland (1978). For example, a dramatic change in the composition of sea-salt

aerosols in coastal areas has been observed by Schulz et al. (1998). This change is

caused by the reaction of sodium chloride with (anthropogenic) nitric acid. The

chloride of the sea-salt is exchanged by nitrate and hydrochloric acid is formed:

NaCl + HNO3  NaNO3 + HCl

By this reaction up to 55 % of the original sodium chloride in aerosols may be

replaced by sodium nitrate. These extremely hygroscopic aerosols are easily deposited

because of their high mass.

Sodium chloride containing particles, originating from sea spray, are highly enriched in

the coastal atmosphere. Contents of up to several 10 mg Cl-/m3 have been

determined which rapidly decrease landwards. Thus, the reaction outlined above will

take place preferentially in coastal areas and hence a remarkable amount of nitrogen

deposition occurs in the coastal area. Although higher NOx emissions may occur in

regions remote from the coast the chloride exchange reaction does not play a major

role in NOx removal here due to the low level of atmospheric NaCl. Schulz et al.

(1998) regard this reaction as one of the most important sinks for oxidised nitrogen

compounds in the coastal marine atmosphere.

Deleted: Fig. 1
22

NH4

+

(177.7)

NH3(53.5)

NO3

-

>1 μm

(50.9)

NO2(187.3)

NO3

-

<1 μm

(25.7)

PAN, HNO2,

N2O5(1.8)

HNO3(28.5)

NO (13.3)

Fig. 1: Mean composition of atmospheric nitrogen compounds (nmol m-3) in the

German Bight (Schulz et al., 1998), PAN: peroxyacetylnitrate.

3 Composition of seawater

Seawater is a steady-state system. It is a solution of salts of nearly constant

composition, dissolved in variable amounts of water. There are well over 70 elements

to be found in seawater but only 11 make up >99 % of the dissolved salts; all

occurring as ions, i.e. electrically charged atoms or groups of atoms. These major ions

behave conservatively. This means that they have constant ratios (Tab. 4), both to

one another and to the salinity in almost all ocean waters, and that they do not

participate in (mostly biological) reactions to such a degree as to experience a

noticeable change in concentration. This property of seawater allows the calculation of

concentrations of conservative constituents from salinity data.

Deleted: Tab. 4
23

Tab. 4 Average sea- and freshwater compositions

Freshwater (river) Seawater

mg/L weight % mg/L %

Sodium Na+7 5.83 10800 30.72

Potassium K

+

2 1.67 400 1.14

Magnesium Mg2+ 4 3.33 1300 3.70

Calcium Ca2+ 15 12.50 410 1.17

Chloride Cl8 6.67 19400 55.19

Bicarbonate HCO3

-

58 48.33 140 0.40

Sulphate SO4

2-

12 10.00 2700 7.68

Silica SiO2 14 11.67 2 0.01

(internet site)

As well as major elements, there are many trace elements in seawater - e.g.,

manganese (Mn), lead (Pb), gold (Au), iron (Fe) and iodine (I). Most of these occur in

parts per million (ppm) or parts per billion (ppb) concentrations. In contrast to the

major elements, the relative abundance of trace elements are variable.

Although freshwaters display high variability in their composition the most abundant

ions in river water are bicarbonate and calcium (Tab. 4). Compared to seawater, the

abundance of bicarbonate is only 2.6 times less. Bicarbonate plays an important role

in pH buffering (chapter 3.3).

Non-conservative substances dissolved in seawater are, among others, some gases

(e.g. oxygen and carbon dioxide) and inorganic nutrients. These are essential for the

growth of plants, including algae. Major nutrients include nitrate, phosphate, and

silicate (the latter required only by silicate depositing organisms). Nutrients are

usually depleted in surface waters, where plants grow, and are found in higher

concentrations in deeper waters, where the plant and animal remains that sink from

surface waters decay.

3.1 Marine sulphur cycle

The large amount of sulphate in seawater (2.65 g L-1) derives from volcanic activities

and degassing at the seafloor. Further, sulphates reach the oceans via river flows, but

the concentration in seawater remains constant at around 2.65 g L-1. A small part of
24

the total sulphate load in rivers comes from the natural weathering of pyrite and

gypsum. In addition, the water cycle carries recycled sulphate of marine origin.

However, most of the riverine sulphate derives from human activities, e.g. mining,

erosion and air pollution. Current fluxes are double those in pre-industrial times.

Sulphate is conservative in oxic oceans but not in anoxic basins or within sediments.

Sulphate is used by sulphate reducing bacteria to form HS or H2S. Bacteria carry out

various transformations of sulphur (Fig. 2).

Fig. 2: Scheme of the microbiological cycle of sulphur and its possible influence on

the atmosphere

1. Sulphate reduction in anaerobic environments (CH2O represents organic matter such as

carbohydrates):

2 CH2O + 2 H+ + SO4

2-  H2S + 2 CO2 + 2 H2O

This reaction is analogous to aerobic respiration but with SO4

2-

rather than oxygen

acting as the terminal electron acceptor in the oxidation reaction. The H2S produced

may precipitate as authigenic minerals such as greigite, mackinawite and pyrite or

may be used in one of the following reactions:

2. Sulphur-based (anaerobic) photosynthesis:

2 H2S + CO2  CH2O + 2 S + H2O

Deleted: Fig. 2
25

This reaction is probably the earliest form of photosynthesis using H2S rather than H2O

as the hydrogen donor in the reduction of CO2. Today it is employed by green and

purple sulphur-bacteria.

3. Chemoautotrophy under oxic conditions:

4 H2S + CO2 + O2  CH2O + 4 S + 3 H2O

This reaction is performed by species of Thiobacilli in environments with free

elemental sulphur or with H2S, for example near deep-sea hydrothermal vents.

3.2 Marine nitrogen cycle

In contrast to sulphate, nitrate is strongly linked to primary production in the sea. The

general seasonal development of nutrient concentrations in seawater is characterised

by a decrease during springtime (Fig. 3) when phytoplankton blooms, transforming

inorganic nitrate into organic nitrogen compounds such as proteins or nucleic acids. In

summer the nutrients reach a minimum, leading sometimes to a limitation of certain

nutrients. Remineralisation processes in the sediment may support a second

phytoplankton bloom in late summer. In autumn, nutrients increase because of low

primary production to maximum concentrations in winter.

NUTRIENTS

PLANKTON

BIOMAS

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

low pH

high pH

Fig. 3: Schematic seasonal variation of nutrients and phytoplankton biomass in

temperate areas.

Deleted: Fig. 3
26

Deposition of organic material (plankton and detritus) into surface sediments supports

benthic mineralisation. Mineralisation of organic nitrogen compounds occurs in several

enzymatic steps (proteins - peptides - amino acids). Finally ammonia is liberated from

amino acids by deamination. Degradation of organic material by heterotrophs leads to

dissolved inorganic nitrogen (DIN) again (Schlegel, 1981). Ammonia production is

highest under anoxic conditions (Fig. 4). The concentration of ammonia in the

porewater is governed by production, adsorption to particles, diffusion and reaction

processes.

Fig. 4: Major reactions and pathways in the marine nitrogen cycle (Lohse, 1996)

DIN = dissolved inorganic nitrogen (NH4+, NO2-, NO3-), DON = dissolved

organic nitrogen, PON = particulate organic nitrogen.

In the presence of oxygen, ammonia is oxidised in two steps via nitrite to nitrate. This

process is called nitrification and is performed by the bacteria species Nitrosomonas

spp. and Nitrobacter spp. These are restricted to the upper oxic part of the sediment

column. Nitrate is the end product of nitrification and can diffuse into the overlying

water column or deeper into the sediment where it undergoes further reactions.

During denitrification nitrogen and dinitrogen oxide are released to the atmosphere,

the latter contributing to the greenhouse gases. During summer months when input of

organic material is high and hence high remineralisation rates prevail, oxygen

deficiency can impede nitrification. Under these conditions ammonification becomes

the dominant process.

All these processes are seasonal dependent and are in exchange with the atmosphere.

Quantification is difficult. The marine system also receives anthropogenic nitrogen via

the rivers and the atmosphere.

3.3 The pH of seawater: the carbonate system

The pH of surface seawater usually ranges from 8.1 to 8.9 and is therefore slightly

alkaline. This is largely due to the presence of carbonate (and other weak acid)

species in natural waters. These are needed to balance the excess positive charge of

the major cations. There is a balance between carbonate, and dissolved and

atmospheric carbon dioxide. When carbon dioxide dissolves in water, carbonic acid is

formed and thus the pH becomes lower due to increased acidity. Dissolved inorganic

carbon (CO2

*

, HCO3

- + CO32-

) varies by ~20 % due to vertical transport in the water

column and remineralisation of both CaCO3 and organic matter.

Natural changes of seawater pH are related to primary production, which converts

inorganic (carbon?) to organic carbon, and then degradation of the produced material

Formatted: English (U.K.)

Deleted: Fig. 4
27

(Fig. 3). Photosynthetic activity increases the pH while the opposite, organic matter

remineralisation, leads to a decrease in pH values.

The ability to neutralise acids and bases to a certain extent is termed the buffer

capacity or alkalinity of seawater. Total alkalinity is found by measuring the amount of

acid (e.g. sulphuric acid) needed to bring a sample of seawater to a pH of 4.2. At this

pH, all the alkaline compounds in the sample are "used up". The result is reported as

milligrams per litre of calcium carbonate (mg L-1 CaCO3).

The amount of bicarbonate and other weak bases in seawater buffers the system, thus

keeping the pH within a narrow range. Dissolved CO2 and carbonates belong to the

buffer system and are all related by the following four equations:

1. CO2 + H20 <------> (CO2)aq

2. (CO2)aq + H20 <------> H2CO3 (carbonic acid)

3. H2CO3 <------> H

+ + HCO3-

(bicarbonate)

4. HCO3

- <------> H+ + CO32-

(carbonate)

(CO2)aq and H2CO3 are difficult to measure and are usually given as CO2

*

. The solubility

of CO2 in water is 1.7 g L-1(0.04 mol L-1) and is governed by Henry’s Law, assuming

ideal gas behaviour:

SCO2 = [CO2] = KH* pCO2

i.e. the solubility (S) of a gas is proportional to its partial pressure (pA).

Henry's law is derived from

KD = [CO2(aq)]/[CO2(gas)]

[CO2(gas)] = pCO2/RT {Dalton's law of partial pressure}

[CO2(aq)] = (KD/RT) pCO2 = KH* pCO2 {KH [mol L-1atm-1]}

As the carbonate system encompasses polyprotic acids, it is able to buffer against

changes in pH caused by addition of either acid or base:

H

+ + HCO3- <------> H2CO3

H

+ + CO32- <------> HCO3-

OH- + HCO3

- <------> H2O + CO32-

OH- + H2CO3 <------> H2O + HCO3

-

Deleted: Fig. 3
28

As calcium carbonate (solid) reacts with water in presence of free hydrogen ions, the

solid calcium carbonate dissolves forming free Ca+2 ions and free bicarbonate ions,

hence consuming free hydrogen ions, decreasing hydrogen ion activity, and by

definition increasing pH (becoming more basic). i.e. calcium carbonate acts to

neutralise or buffer the solution by consuming hydrogen ions.

Both carbonic acid and bicarbonate are weak acids with pK values of pK1 = 6.5, pK2 =

10.4. When pure water is equilibrated with CO2 the system is defined by two

independent variables (in addition to total pressure), e.g. temperature and pCO2. If,

besides temperature, one other variable such as pCO2, [CO2*] of [H+] is known, then

all components can be calculated from the equilibrium concentrations, using Henry's

law, the acidity constants and the proton condition or charge balance.

The pH of a pure aqueous CO2 solution is defined by the charge balance:

[H+] = [HCO3

-

] + 2 [CO3

2-

] + [OH-]

If the system is kept in equilibrium with constant pCO2, the pH can be varied only by

the addition of a base, CB or an acid, CA. Electroneutrality must be observed for such

an addition.

As B is equivalent to the concentration of a monovalent cation (e.g. [Na+] from NaOH)

and A is equivalent to the concentration of a monovalent anion (e.g. [Cl-] from HCl).

CB + [H+] = [HCO3

-

] + 2 [CO3

2-

] + [OH-] + CA

CB - CA = CT (    [OH-] - [H+]

where, CT = total CO2 concentration;  is the Bunsen coefficient (0 = KH * 22.414

[atm1]; and (CB - CA) is the acid neutralising capacity of the solution with respect to

the pure solution of CO2. This difference is defined as the alkalinity, [Alk].

[Alk] C C (β 2β ) [OH ] [H ] B A 1 2

 

     

0

2

β

KH pCO

This model has three independent variables in addition to total pressure. It is

therefore sufficient to give T, pCO2, and alkalinity to define the system. The pH is given

by [Alk] and pCO2.

There are other bases apart from those present in the carbonate system which

contribute to the total alkalinity of seawater. These are B(OH)4

-

, HPO4

2-

and PO4

3-

and,

under specific environmental conditions, hydrogen sulphide HS-.

If this is included then

CB + [H+] = [HCO3

-

] + 2 [CO3

2-

] + [B(OH)4

-

] +[OH-] + CA
29

CB - CA = CT (    BTB



 [OH-] - [H+]

with BT = [H3BO3] + [B(OH)4

-

] and B = [B(OH)4

-

]BT

and

(β 2β ) B β [OH ] [H ]

β

K p

[Alk] C C - 1 2 T B

H CO

B A

 

      

0

2

In the system containing only carbonate species, alkalinity is commonly referred to as

titration or carbonate alkalinity (CA). Incorporating other bases alkalinity is then

referred to as total alkalinity (TA). CA and TA differ by about 100 μequivalents L-1

only.

Total alkalinity of seawater averages 116 mg L-1and is greater than fresh water,

which varies significantly, depending on the drainage area. TA can have a total

alkalinity of 30 to 90 mg L-1. Brackish waters like in estuaries, where river water is

mixed with seawater, have TA varying between these values.

The pH values after addition of acid or base can be calculated using the above

equation(s) taking into account ionic strength effects on the constants. Corrected

values are tabulated.

The neutralisation of sulphuric acid in seawater occurs in two steps:

1. H2SO4 + H2O  HSO4

- + H3O+ pKa (H2SO4) = -3

2. HSO4

- + H2O  SO42- + H3O+ pKa (HSO4-

) =1.92

Both sulphuric acid and hydrogen sulphate have low pKa values (-3 and 1.92,

respectively), and are hence strong acids which dissociate completely. Therefore all

the protons of sulphuric acid contribute to the pH-shift. The carbonate system will

buffer the additional protons by releasing CO2 in order to keep the pH constant. One

molecule of sulphuric acid will therefore produce 2 molecules of CO2. The equations 1-

3 of the carbonate system (see above page 27) will shift from the right to the left

sides, finally releasing the dissolved CO2 to the atmosphere.
30

4 Laboratory measurements

A model of Abdulsattar et al. (1977) allows the prediction of the pH of effluent from

seawater scrubbers, but leads to an overestimation of acidity (Baalina, et al., 1997).

The reason is that the oxidation of SO2 to SO4

2-

is not taken into account nor is the

impact that the temperature of the seawater has on pH (Hunter, 1998). The

calculations in the previous chapter are based on an ideal seawater composition and

do not take into account other seawater constituents (e.g. organic acids like amino or

humic acids). As the above described equations are simplified, and some data is

difficult to obtain or absent, laboratory measurements were carried out at Terramare

to quantify the ability of seawater to buffer sulphuric acid solutions.

4.1 Impact of diluted sulphuric acid (pH 4) on seawater

To mimic the conditions of the seawater scrubber effluent, concentrated sulphuric acid

(98 %) was diluted with seawater to a pH of 4 (assumed worst case for scrubber

effluent, Ref 1.4.1). The amount of sulphuric acid in 1 L was 1.25 g H2SO4. Assuming

that this effluent would be discharged directly to the sea, this solution was added in

small portions to natural seawater (salinity = 29 PSU), taken at the Nassau Bridge in

Wilhelmshaven, Germany in October 2002. The concentration of the sulphuric acid

was recalculated because of the changing end-volume. The pH shift caused by the

addition of sulphuric acid was measured directly (Fig. 5). Formatted: English (U.S.),

Check spelling and grammar

Deleted: Fig. 5
31

0

100

200

300

400

500

600

7.4 7.5 7.6 7.7 7.8 7.9 8 8.1 8.2 8.3 8.4 8.5

pH

concentration of sulphuric acid in se

a

w

ater

[mg/L]

raw data

Y = (A+X)/(B+C*Xexp2)

Fig. 5: Impact of sulphuric acid (pH 4) on the pH of seawater (S = 29 PSU)

corresponds to regression line.

The addition of 2.48 mmol (68 mg) sulphuric acid to 1 L seawater caused a pH shift of

pH 0.1 units only. This amount of sulphuric acid corresponds with a mixture of 1:10

(v:v, sulphuric acid in seawater pH 4 : natural seawater). Even a mixture of 1:5 (5.08

mmol L-1) only caused a pH shift of 0.23.

The scrubber effluent is diluted with natural seawater ten times. Thus, the

requirements of US-EPA (1976) that changes in pH shall not exceed 0.2 units for

marine waters are fulfilled.

The regression line in Fig. 5 was calculated using the Lab-Fitting software.

The equation for the regression line is:

Y = (A+X)/(B+C*X2); R2 = 0.999

A = -8.43463908

B = -0.00536647

C = 0.00006588

Deleted: Fig. 5
32

4.2 Impact of diluted sulphuric acid (pH4) on brackish waters

In a second experiment, the same test was carried out with brackish water (Ems-Jade

Kanal, Wilhelmshaven, Salinity = 14 PSU). Different ratios of the original and the

acidified brackish water (pH 4) were mixed. The pH was measured directly after

mixing, and after stirring the solutions for 1 hour again. Within this time, the

carbonate system was able to buffer the pH by release of CO2 to the atmosphere,

while the pH increased again (Tab. 5, Fig. 6).

Tab. 5: Percentage of sulphuric acid (pH 4) in brackish water with resulting pH (t = 0

and t = 1 h)

Percent sulphuric acid

in brackish water (pH

4)

[%]

Percent natural

brackish water

[%]

pH

measured

immediately

pH

measured

after 1 hour

 pH between

t = 0 and

t = 1 h

0 100 7.60 7.63

10 90 6.63 7.50 0.87

20 80 6.52 7.35 0.83

30 70 6.45 7.02 0.57

40 60 6.31 7.01 0.70

50 50 6.08 6.88 0.80

60 40 5.85 6.47 0.62

70 30 5.63 6.15 0.52

80 20 5.38 5.65 0.27

90 10 4.54 4.95 0.41

100 0 4.03 4.07

Deleted: Tab. 5

Deleted: Fig. 6
33

0

10

20

30

40

50

60

70

80

90

100

3 4 5 6 7 8

pH

percentage

acidifie

d brackish w

ater

pH immediately measured

pH after 1 h stirring

Fig. 6: Percentage of sulphuric acid (pH 4) in brackish water with resulting pH (t = 0

and t = 1 h).

The test with the brackish water (S = 14 PSU) showed that the same dilution factor of

1:10 caused an immediate shift of about 1 pH-unit. After one hour the pH returned to

almost its original value (pH = 0.1). These preliminary experiments show that the

addition of sulphuric acid in seawater to pure seawater is not only dependent on the

amount of sulphuric acid, but also on time. The carbonate system reacts in two steps:

first the production of dissolved (CO2(aq)), second the release of gaseous carbon

dioxide (CO2 (g)) from the system, to keep the pH almost constant.
34

4.3 Summary of laboratory experiments

The experiments with the hypothetical effluent of the seawater scrubber (SWS) (pH 4)

showed that even with a dilution factor of only 10, the discharge would not change the

pH of the receiving seawater more than 0.2 standard units. Brackish water reacted

with a spontaneous pH shift around 1 standard unit, but within one hour it had almost

returned to the original pH (pH shift 0.1).

These measurements were carried out in closed systems, where only gas-exchange

was allowed. However the results for any experiments conducted to demonstrate the

impact of releasing a sulphuric acid discharge from a (moving) ship into the sea, must

take into account realistic conditions. For example, the extent to which the effluent

mixes with the seawater is dependant on the speed of the water at the time of

discharge (a parameter which varies with ship speed), and the tidal currents which

vary hourly. In estuaries, the exchange rate with the open sea and the inflow of

freshwater are important factors.

4.4 Impact of the acidic effluent of the Krystallon Scrubber on the pH

of the receiving water

In chapter 1.4.1 the theoretical composition of the effluent is described. The lowest pH

would be 4.8. The experiments were carried out with acidified seawater pH 4,

representing an assumed worst case. Next to sulphuric acid, nitric acid contributes to

the acidity of the effluent, but to a lesser extent.

1. NOx reduction is around 20 %, compared to 95 % SO2 reduction

2. Nitric acid delivers only one hydrogen ion per mol, whereas sulphuric acid delivers

two hydrogen ions.

Based on data for different scenarios (ship in service and ship in harbour) and sulphur

contents of the fuel, the composition of the resulting effluent was calculated (Tab. 6).

Noticeable is the changing amount of cooling water, which is finally discharged. This

variable has a significant impact on the concentrations.

The maximum relative contribution of nitrate to the acidity is calculated to be 9.66 %,

but this is of minor importance regarding the pH of the discharged effluent and its

impact on the environment. The proportion of nitric acid is relevant, as it delivers

nitrate to the marine system.

Deleted: Tab. 6
35

The calculations are based on data obtained on the Krystallon Scrubber. The SO2

adsorbing, recirculation water is diluted with cooling water (seawater) and discharged

at a pH level between 4.5 and 5.5. The laboratory experiments showed that a dilution

of the effluent with seawater of 1:10 causes a pH-shift less than 0.2 pH units.

Tab. 6: Contribution of nitric acid to the acidity of the effluent

At Sea1In Harbour2

Sulphur content in fuel % 3.50 1.50 0.50 3.50 1.50 0.50

Specific NOx emissions3 g/kWh 20 20 20 20 20 20

SO2 removed % 95 95 95 95 95 95

NOx removed % 20 20 20 20 20 20

Discharged cooling water t/h 30 30 50 70 70 70

SO2 in cooling water mg/L 412 177 59 47 20 6.7

H2SO4 mg/L 631 270 90 72 31 10

mmol/L 6.44 2.76 0.92 0.74 0.32 0.11

Hydrogen ions delivered by

H2SO4

mmol/L 12.88 5.52 1.84 1.47 0.63 0.21

NO in cooling water4 mg/L 20.4 20.4 8.14 2.3 2.3 0.9

mmol/L 77.1 77.1 77.1 10.5 10.5 10.5

NO2 in cooling water4 mg/L 33.04 33.04 33.04 4.48 4.48 4.48

mmol/L 0.72 0.72 0.72 0.10 0.10 0.10

HNO3 equivalent to hydrogen

ions

mmol/L 3.29 3.29 3.29 0.45 0.45 0.45

Sum of hydrogen ions mmol/L 16.16 8.80 5.13 1.92 1.08 0.66

Contribution of NO2 to acidity % 20.34 37.34 64.13 23.27 41.44 67.98

1

) four main engines and two generators running 85 % MCR

2

) two Generators running 95 % MCR

3

) Marpol Annex VI limits emissions of NOx to between 10 – 17 g NOx/kWh, (worst case

assumed as 20 g NOx/kWh)

4

) based on the assumption that 70 % of total nitrogen is converted to NO and 30 % is

converted to NO2.

4.5 Estuaries

Salinity and pH in the estuarine ecosystem

In estuaries a salinity gradient is caused by the permanent freshwater input by the

rivers into marine waters. The most important estuarine characteristic is that nowhere

is variation in salinity more pronounced. As seawater mixes with river water, the

conservative seawater components will co-vary directly in concentration with salinity.

Any component that is added during mixing will increase in concentration versus a
36

conservative mixing line. A component that is lost during mixing (e.g. precipitation of

some metals as hydroxides with decreasing pH) will decrease in concentration versus

a conservative mixing line. Such non-conservative behaviour will only be evident when

the addition or loss processes are relatively rapid versus water mixing in the estuary.

Other factors causing horizontal, vertical and seasonal variability of salinity in

estuaries are:

 amount of freshwater input,

 evaporation,

 water density (vertical variability),

 tides and

 the Coriolis effect

Human activities that cause significant, short-term fluctuations in pH or long-term

acidification of a water body are exceedingly harmful. For instance, algal blooms that

are often initiated by an overload of nutrients in the rivers can cause pH to fluctuate

dramatically over a period of a few hours. Changes in pH can put great strain on local

organisms.

The pH of an estuary can vary from 7.0 to 7.5 in the fresher, low salinity sections, to

between 8.0 and 8.6 in the more marine parts. The slightly alkaline pH of seawater is

due to the natural buffering from carbonate and bicarbonate dissolved in the water

(chapter 3.3). The pH of water is critical to the survival of most aquatic plants and

animals. Many species have trouble surviving if pH levels drop below 5.0 or rise above

9.0 (Fig. 7).

Fig. 7: pH ranges that support aquatic life (source: see Internet-sites)

most acid neutral most basic

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Bacteria

Plants (algae,rooted, etc)

Carp, suckers,catfish, some insects

Bass,bluegill,crappie

Snails, Clams, mussels

Largest variety of animals

(Trout, mayflynymphs,stoneflynymphys,

caddishfly larvae)

pH ranges that support aquatic life.

Changes in pH can alter other aspects of the water's chemistry, usually to the

detriment of native species. Even small shifts in the water's pH can affect the solubility

of some metals such as iron and copper. Such changes can influence aquatic life

Formatted: English (U.S.)

Deleted: 3.3

Deleted: Fig. 7
37

indirectly. If the pH levels are lowered, toxic metals in the estuary's sediment can be

re-dissolved in the water column. This can have impacts on many aquatic species.

4.5.1 Harbour areas like the Port of Rotterdam

The port of Rotterdam is a very complex area, with vital inflow from the North Sea and

several rivers and canals. The complex geometry and hydrography of the port of

Rotterdam was approached in the Mam-Pec model (in CEPE report, 1999) with two

different scenarios. In the first scenario the geometry of the Rotterdam port area was

conceptualised as a rectangular area (2 x 20 km; 4000 ha; depth 20 m), with a 5 km

wide open front to the river in order to mimic a harbour segment with an average

water exchange of 32 % per tidal period. A second scenario, with a 10 km wide open

front to the river was chosen to represent harbour segments with a water exchange of

65 % per tidal period. The hydrodynamic exchange in the Mam-Pec model is derived

from the following parameters: density differences (between marine and freshwater),

tidal period and height, river flux and the dimensions of the rectangular port area.

Shipping:

The Port Statistics from 2001 for the Port of Rotterdam show that the number of ship

arrivals of seagoing vessels is around 30 000/annum in recent years and 133 000

arrivals of inland vessels per year. Movements of seagoing vessels are on average 82

000/a.

Within the EU the port of Rotterdam has the maximum NOx, SO2 and CO2 emissions.

Tab. 7: Ports in the EU ranked by estimated annual emissions of NOx in 2000

Rank Port Name Country NOx [kT] SO2 [kT] CO2 [kT]

1 Rotterdam NLD 3.8 3.7 219.9

2 Antwerp BEL 2.2 2.2 134.0

3 Milford Haven GBR 2.0 2.2 130.4

4 Hamburg DEU 2.0 1.9 115.5

5 Augusta ITA 1.8 2.0 121.4

6 Agioi Theodoroi GRC 1.7 1.8 107.0

7 Piraeus GRC 1.6 1.6 93.9

8 Eleusis GRC 1.5 1.6 95.1

9 Gothenburg SWE 1.5 1.5 91.1

10 Immingham GBR 1.3 1.4 82.0

reported by Entec, 2002.

A rough calculation using data from Tab. 7 indicates that on an annual basis total SO2

discharge into Rotterdam port waters will contribute about 2 x 10-4 mol H+/L assuming
38

that all SO2 will be transferred to the aqueous phase. Using data from Table 6 and

assuming the presence of 100 vessels in the port area, the H+concentrations will

increase by 5.7, 9.4 and to 16.7 x 10-6 mol H+/L h for 0.5 %, 1.5 % and 3.5 %

sulphur fuel. This corresponds to 0.5 – 2 pH units. This calculation does not take into

account the buffer capacity of seawater, the exchange of CO2 with the atmosphere,

nor the tidal exchange with the open sea. Preliminary experimental data presented

above (chapter 3.4) suggest that the re-equilibrium is rapid although effects of

continuous addition of acid need to be investigated in detail.

4.6 The Baltic Sea

The Baltic Sea is a relatively shallow inland sea surrounded by the countries of Northeastern Europe and Scandinavia. Its total area is about 377,400 km2

, and its volume

about 21,000 km3. The catchment area extends over an area about four times as large

as the sea itself. The mean depth is 55 m, while the maximum depth is 459 m in the

Landsort Deep. The Baltic also receives surface water drainage from five other

countries: Belarus, Czech Republic, Slovak Republic, Norway, and Ukraine.

The Baltic Sea is connected to the North Sea through narrow and shallow sounds

between Denmark and Sweden. The outlet consists of a series of basins separated by

shallow sills, which obstruct efficient water exchange. Consequently, it takes 25 - 35

years for all the water from the Baltic Sea to be replenished by water from the North

Sea and beyond.

Salinity in the Baltic varies from fully marine at the Skagerrak/Kattegatt boundary to

almost fresh water conditions in the Bothnian Gulf. Due to the high riverine input of

fresh water, a pronounced year-round stratification of the water column, especially in

the Baltic Proper, can be observed. This results in a reduced mixing of surface with

bottom waters. Ice coverage occurs regularly in the northern parts.

More than 500 million tonnes of cargo are transported across the Baltic Sea each year,

along many busy shipping lanes. More than 50 passenger ferries also ply routes

between Baltic ports.

4.6.1 Vital inflow from the North Sea

The inflow of water from the North Sea is the main source of oxygen for the deep

waters of the Baltic Proper, and is very significant for nutrient cycles throughout the

Baltic Sea. A stagnation period of more than sixteen years ended in January 1993

when masses of high-salinity water entered the Baltic, and for the first time since

1977, the Baltic Sea was free of hydrogen sulphide. Unfortunately, the effects of this
39

water exchange did not last, and within a couple of years the depths of the Eastern

Gotland Basin were again becoming anoxic. In 1996 hydrogen sulphide was once more

recorded in the depths of the Bornholm and Eastern Gotland Basins. Contaminants

and nutrients enter the Baltic Sea in rivers, in runoff from coastal areas, through

exchange of water with the North Sea, through atmospheric deposition, and due to

human activities at sea. A complete water renewal takes about 25-35 years, so

persistent pollutants can remain in the Baltic for a long time.

4.7 Acidification of surface waters

The deposition of atmospheric acids is not a recent phenomenon, but the extent of the

consequences is still unclear. Acidification of soils and surface waters is to some extent

a natural process but has accelerated in recent decades. The main causes of

acidification are acid rain and the use of certain fertilisers in agriculture, which reach

the oceans by surface water runoff and rivers. A drop in the pH of soil to 5.5 for

example, causes a decline in agricultural harvests as certain micronutrients become

less available and microbial turnover of nitrogen and carbon are impaired.

The acidity of a water body is dependent on the amount of acid deposited, the amount

of acid already present in the water body, and the ability of the water body to absorb

and neutralise acid (buffer capacity). Seawater has a tremendous ability to neutralise

acids, so significant acidification does not occur in coastal waters and most estuaries.

Some freshwater bodies, however, may be very sensitive to atmospheric inputs of

acidic compounds (US EPA, 1976).

The overall effects of seawater acidification on marine ecosystems are not yet clear.

For example, the potentially harmful effects of changing environmental CO2

concentrations on energy metabolism and growth of marine invertebrates and fish still

have to be studied in detail.

Acidification affects ecosystems in many ways. Aquatic organisms in acidified waters

often suffer from calcium deficiencies which can weaken bones and exoskeletons and

can cause eggs to be weak or brittle. It also affects the permeability of fish

membranes and, particularly, the ability of gills to take up oxygen from the ambient

water. Additionally, increasing the amount of acid in a water body can change the

mobility of certain trace metals like aluminium, cadmium, manganese, iron, arsenic,

and mercury. Species that are sensitive to these metals, particularly fish, can suffer as

a result. The effects of acidification on aluminium mobility has received the most

attention because this metal is toxic to fish. The effects of increasing levels of

cadmium and mercury, which are atmospheric pollutants of concern for water quality,

are also becoming known.
40

4.8 Experiences with acid disposals at sea.

The waste of titanium dioxide production contains heavy metals and sulphuric acid

(around 12 %; Rat von Sachverständigen, 1981; Carlson, 1986). Beginning in 1969

this waste was dumped into the sea about 12 nautical miles north of Heligoland in the

central German Bight with increasing amounts. This disposal was stopped in 1989,

because the heavy metal content of the acid caused dramatic changes in the

ecosystem. At that time 750 000 t of sulphuric acid (12 %) were dumped into the sea

per year. The pH of the water in the disposal area changed only slightly, due to the

buffer capacity of the receiving seawater (Dethlefsen, 1990).

Data provided by Weichart (1975) indicates that pH changes resulting from titanium

dioxide waste dumping were within the range of naturally occurring pH values. Even in

the core of the propeller wake of the dumping vessel in 5 - 10 m water depth the pH

after one hour was 7.77, increasing to 8.04 after 4.7 h. With the exception of fresh

propeller wake no indications of long term pH changes were found.

4.9 Eutrophication effects

Any inorganic nitrogen and phosphorus compound added to natural waters will

promote primary productivity. Enhanced production of biomass can lead to

eutrophication effects under certain circumstances.

In marine systems phytoplankton biomass is usually produced in -within limits- fixed

relations of carbon : nitrogen: phosphorus, the so-called Redfield ratio of 106 : 16 :1.

Thus, any mole P added will result in the fixation of 106 moles carbon and 16 moles

nitrogen.

In cases where only nitrogen is added as in scrubber effluents the extent to which

additional biomass is produced will depend not only on the actual amount added but

also on the phosphorus available for primary production. In open ocean situations P is

generally regarded as the limiting nutrient. Here, additional N will not have any effects

on planktonic biomass production.

In near-shore or harbour situations, where P is available (e.g. from riverine inputs,

runoff from agriculture or direct input of domestic sewage), addition of inorganic

nitrogen may lead to enhanced primary production.

On the other hand, addition of nitrogen may also have beneficial effects as it

counteracts the development of cyanobacteria blooms which may occur in the absence

of inorganic nitrogen when there is still sufficient phosphorus in the euphotic zone.

Certain cyanobacteria are able to use molecular nitrogen instead of nitrate or

ammonia. This is often the case in the Baltic Sea where such blooms occur regularly.
41

Here, phosphate is still present in measurable quantities even after the phytoplankton

spring bloom has reduced nitrate levels to below the detection limit (Wasmund et al.

2001).

Based on the assumption that there will be ample phosphate an input of 3.29 or 0.45

mmoles NO3

-

/L, respectively, (Tab. 4) will result in an additional biomass production of

43.6 or 6 mg/L assuming, that carbon accounts for 50 % of the produced biomass.

This figure, however, does not include further dilution of the scrubber input by

turbulent mixing along the ship's path.

Primary production, at least in temperate and polar regions, is strongly connected to

the seasonality of insolation. This results in high productivity in spring when there are

sufficient inorganic nutrients available and lower productivity in summer and early

autumn when nutrients are production limiting. Thus, relative contributions of

additional nitrogen-containing nutrients will be low in spring but higher in summer.

Furthermore, inter-annual variability will also have an effect on the relative

contribution of any additional nitrogen added.

In coastal areas and harbours neither P nor N are usually production limiting factors.

Here, it is rather the availability of light which prevents usage of inorganic nutrients by

primary producers (e.g. Colijn and Cadée 2003).
42

5 Environmental impact of scrubbing waters - Synthesis

Catalysts are used during the removal of sulphur in the production of low S fuels in

refineries. These contain transition metals (molybdenum, cobalt, nickel, zinc, copper)

which need to be replaced at more or less regular intervals. Only part of these metals

can be recycled, the remainder, especially fine material released into the gas stream,

has to be safely deposited. In addition, the amine used to remove the hydrogen

sulphide produced during Hydrotreating, can also be recycled or needs to be safely

deposited.

The costs and environmental impacts incurred from either recycling or deposition

cannot be specified on a global scale. Van Oudenhoven et al. (1993) report that in

1993, 89 refineries in Europe produced a total of 6,368 tons of spent desulphurisation

catalyst .

Thus, processes that are cheaper or more environmentally benign are attractive

alternatives. One of these is the removal of combustion gases. Of the processes listed

above Flue Gas Desulphurisation with Seawater Scrubbing (FGD-SWS) appears to be

the most attractive option for the operation of seagoing vessels.

Both laboratory experiments and field evidence indicate that acidic waste streams

from FGD-SWS introduced in full strength seawater leads to observable effects on

ambient pH only for extremely short periods of time. From the dumping of much

higher concentrated sulphuric acid in the North Sea it is known that the area affected

is restricted to a few hundred meters behind the dumping vessel.

Preliminary laboratory experiments with seawater indicate that at a 1:10 dilution the

observable pH change did not exceed 0.1 pH units. For brackish waters a time lag of

about one hour was observed. These values coincide with EPA requirements.

Considering that in this case a 12 % acid (=2.634 mol H+/L) was dumped, then it can

be expected that the maximum of about 6 mmol H+/L produced during waste

discharge from FGD-SWS, will affect neither the pH values nor the sulphate contents

of the receiving waters to a noticeable degree. This is not only due to the low

concentrations of sulphate in the waste stream. Turbulent mixing caused by ship

movements and screw rotation will provide further dilution. Natural mixing processes

will also contribute to this.

It should, however, be noted that the conclusion given above holds for fully marine

salinities only. In waters such as the Baltic with generally lower salt contents and

slightly different ionic composition, especially in the northern parts, the 1:10 dilution

may not be sufficient. Also, during harbour times when there will be no ship

movements and only limited water movement this dilution factor may be too low.
43

Detailed analysis of this case remains to be done. The other factors worth considering

are the total mass of sulphur emitted in harbour versus at sea. Vessels do not burn as

much fuel in port as they do at sea due to the lower demand for propulsion. A typical

ferry may have 27 MW of installed power available at sea, while only 3 MW of this

would be available in harbour. The amount of sulphur discharge is proportional to this

power use, thus even in brackish harbours, the sulphur dilution ratio may be

sufficient.

In the case of nitrate, open sea discharge from FGD-SWS is again due to the high

dilution rate, not likely to cause any eutrophying effects. In ports, however, the high

concentrations in the discharge stream together with restricted water exchange may

lead to nitrate values high enough to cause unwanted effects such as exceptional

phytoplankton blooms. Just as sulphur emission is linked to total engine output, so is

the nitrogen emission rates. Vessels that are moving slowly, or anchored in harbour

with only generators running will be producing much lower levels of nitrates than

under full speed at sea. The effect of 2-4 MW of engine power per ship in harbour may

be insignificant in terms of nitrate amounts. Even when main engines are started, it is

difficult to conceive of how massive amounts of fuel can be burned for propulsive

motors in a small harbour. The use of propulsion engines is to get out of harbour and

manoeuvre, so these sources are not continuous.

Tab. 8: discharge of sulphate and nitrate per hour

Sulphate Nitrate

Sulphur in fuel % 3.5 1.5 0.5 NOx emission 3,4 g/kwh 20

3MW 1,2 3MW 1,2

SO2 kg/h 37.8 16.2 5.4 NOx kg/h 57

Reduction % 95 95 95 Reduction % 20

Discharged SO4

2- kg/h 56.7 24.3 8.1 Discharged NO3- kg/h 20

27MW 2 27MW 2

SO2 kg/h 323 138 46 NOx kg/h 432

Reduction % 95 95 95 Reduction % 20

Discharged SO4

2- Kg/h 485 207 69 Discharged NO3- kg/h 153

1) In harbour situation with two generators running (3 MW)

2) Fuel consumption 180 kg/kWh

3) For 20 g NOx/kWh

4) Assuming an NOx composition of 70 % NO, 30 % NO2
44

Nevertheless, as experimental evidence is lacking, it appears necessary to conduct a

series of laboratory and field experiments to elucidate the actual changes in pH and

sulphate as well as nitrate concentrations, under a variety of conditions. Similarly, it

might be useful to adopt models developed for port sediment and water exchange, to

include pH and sulphate concentration.

Comparing these results with the harmful effects of the SO2 and NOx emissions of

ships, the use of the SWS seems to be a shortcut: most of the produced sulphate and

nitrate will reach the oceans by surface run-off via the rivers. On land however, the

acidification of soils, lakes and rivers causes considerably higher damage. The change

of atmospheric aerosol composition by NOx emissions, which has been observed in the

German Bight for example, should also be taken into account.

Other positive side-effects of the SWS are 80 % removal of particulates and noise

reduction.
45

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