COST EFFICIENCY OF BALLAST WATER TREATMENT SYSTEMS BASED ON ULTRAVIOLET IRRADIATION AND ELECTROCHLORINATION

Having come into effect, the International Convention for the Control and Management of Ships’ Ballast Water and Sediments of 2004 requires ships to process their ballast water in accordance with specific standards. Different processing methods require different use of ship power, thus affecting fuel oil consumption, ships’ energy efficiency, and the ship economics in general. This paper presents the analysis and comparison of the economic viability of systems using two dominant ballast water treatment methods on merchant ships. The cost effectiveness of these methods, UV irradiation and electrochlorination, is compared to the standard efficiency of ballast water exchange using sequential flow method as a reference. The process efficiency is measured through fuel oil consumption on board. Taking into account possible variations in efficiency due to different designs and environmental constraints and assumptions, the findings are in favor of the electrochlorination method.


INTRODUCTION
Invasive aquatic organisms have been recognized as a potential threat to marine ecosystems for more than 20 years, and the ever-continuous increase in maritime traffic is raising the potential danger of spreading of hazardous aquatic organisms and pathogens (HAOP) as well as the devastation of coastal waters.
At the International Conference held in London in February 2004, the International Maritime Organization adopted the International Convention for the Control and Management of Ships' Ballast Water and Sediments [1]. By meeting the agreed conditions, the Convention finally came into force in September 2017.
The BWM Convention defines two main ballast water management standards: -D1 -Ballast Water Exchange Standard, which requires the exchange of 95% of the volume of ballast water using either the sequential flow method or the flow or dilution method, and -D2 -Ballast Water Performance Standard, requiring ballast water treatment resulting with less than 10 living organisms per m 3 , sized ≥50 µm, as well as less than 10 living organisms per m 3 of water, sized <50 µm and ≥10 µm, with a limited number of indicator microorganisms (bacteria). Modern ballast water treatment (BWT) systems usually employ a two-step treatment approach [2].
The aim of the primary treatment process is to reduce the number of large aquatic organisms. Consequently, it considerably increases the efficiency of secondary treatment, which then reduces the amount of organic substances below the required level [3]. The primary treatment methods in use are filtration and hydrocyclone.
During filtration, ballast water is filtered by passing through a semi-permeable barrier (filter), removing particles, sediments, zooplankton, and phytoplankton larger than 50 µm.
The hydrocyclone separates solids (aquatic organisms) from liquids (in this case, ballast water), using centrifugal forces. Since the efficiency of the hydrocyclone greatly depends on the mass and density of the particles, this method is somewhat deficient in the removal of small organisms from ballast water.
Other physical methods of primary treatment of ballast water are coagulation with flocculation (sometimes prior to filtration to increase its efficiency) and filtration with crumb rubber. However, these two methods are not widely used in currently available ballast water treatment systems.
Secondary separation methods include a variety of physical and chemical methods, used alone or in combination [4,5]. The most commonly used physical methods include ultraviolet (UV) irradiation, heat, require that there should be no surviving organisms in the discharged ballast water after treatment. This is incompatible with the UV treatment method, where some organisms may survive, but with no or minimal reproduction or multiplication capabilities.
Electrolysis is an electrochemical process where direct current passing through the electrolyte (ballast water) is used to create various compounds with significant oxidation capabilities, and therefore disinfection potential. Some of those compounds are hypochlorite acid (HOCl), hypochlorite ion (OCl -), hypobromate acid (HBrO), hypobromate ion (BrO -), and hydroxyl radicals (OH -). [3] Electrochlorination is a process of creating reactive chlorine compounds by electric current passing through ballast water with high salt content. The efficiency of those systems is extremely high. Research and testing of such systems recorded a destruction of 99% bacteria, phytoplankton, and zooplankton [12]. Beside the concentration of chlorine in the water, the efficiency of the process also depends on the temperature, reaction time, and amount of any residual chlorine and its compounds. In addition, this type of ballast water treatment brings about the problem of creating the so-called 'disinfection by-products', the most dangerous of which are products resulting from the reaction of chlorine with organic material, such as chloroform, trichloronitromethane, and dichloroacetic and trichloroacetic acid. It is therefore necessary, before discharging ballast water treated by electrochlorination, to neutralize 'disinfection by-products' by adding sodium bisulfite or sodium sulfite [13,14].

CASE STUDY
In the case study presented here, an analysis of the annual operational cost of the two most widely used BWT methods (UV irradiation and electrochlorination) was carried out. The analysis involved an LPG vessel similar to the recent series 26xx of the shipbuilder Hyundai Heavy Industries Co. Ltd. Ulsan, Korea, with a capacity of 54.335 DWT, length 225 m, and width 37 m. The type and size of the ship was selected based on the fact that its size was close to the average size of the ships commonly used on ocean-going voyages and that ballast water loading/unloading frequency and quantities on LPG ships were between the corresponding frequencies and quantities on dry cargo ships and tankers.
Energy consumption for each method was estimated using publicly-available data on commercially available BWM systems shown in Table 1. It is noted that differences among various producers using the same method are insignificant, thus the average value was used.
ultrasound, and magnetic and electrical water treatment. The chemical methods in use involve biocides, chlorine, ozone, hydrogen peroxide, chlorine dioxide, and other chemicals.
From the 55 available and IMO-approved BWT systems (BWT systems with basic authorization [6]), 20 (36%) use UV irradiation, and 20 (36%) use electrolysis/electrochlorination for secondary treatment, i.e., the disinfection of ballast water. Those two BWM methods are the most commonly used BWT systems on ships [7]. One of the latest systems with basic IMO approval, in addition to the standard filtration in primary treatment, combines those two methods during secondary treatment (ECS HYBRIDTM System, TECH-CROSS Inc., Busan, South Korea). Interestingly, during 2014 and 2015, most of the newly approved systems or, to be more specific, seven out of ten systems approved in this two-year period, used electrolysis as a method of treatment and only one (HYBRIDTM) used UV irradiation.
Consequently, the comparison and analysis of the economic viability of those two most frequently used BWT methods (UV irradiation and electrochlorination) based on fuel oil consumption on board are presented.

ULTRAVIOLET IRRADIATION AND ELECTROCHLORINATION
Ultraviolet irradiation (UV), as a method of ballast water treatment, is based on the impact of UV rays on living cells [8,9]. It is carried out by using low pressure or high pressure UV lamps. Low pressure UV lamps (emitting UV-C radiation light with a wavelength of about 254 nm, the so-called 'short-wave' or 'antimicrobial' light) have the strongest effect on the nucleic acid, damaging the DNA in particular. If the cells activate the so-called 'repair mechanisms', the damage may not be terminal, which significantly influences the effectiveness of the treatment. High pressure UV lamps emit UV-A (400-315 nm, the so-called 'long-wave' or 'black light'), UV-B (315-280 nm or medium-wave light), and UV-C radiation. To some extent, implementation of UV light may create ozone, a gas with a significant biocidal effect. BWT systems using UV irradiation typically contain several UV lamps placed along the flow of ballast water. Maintenance of UV systems is relatively simple since it only requires replacement of damaged lamps or occasional cleaning of lamp sleeves.
It is essential that organisms present in ballast water are exposed to UV light long enough for DNA damage to occur [10]. Namely, if the flow is too fast, there is not enough time for UV light to affect organisms. On the other hand, if the flow is too slow, the lamps can overheat and failures may occur.
Recognizing the fact, the US regulations for ballast water quality differ from the IMO standards regarding residual organisms after water treatment [11]. They where: The loading coefficient is defined as the ratio between the average load and the total load. The loading coefficient used in this case study was calculated by the shipyard in 'Analysis of the electric plant loads' of the mentioned series.
The electrical power was calculated as: where: The fuel oil consumption during one exchange of ballast water was calculated as follows: where: U -fuel oil consumption SFOC alt -alternator specific fuel oil consumption The engine specific fuel oil consumption SFOC is 187 g/kWh. SFOC was taken into account along with the following ISO parameters: -Inlet pressure to the turbocharger: 1000 mbar -The temperature of air entering the turbocharger: 25°C -The temperature of the coolant in the air cooler: 25°C -Lower calorific value of fuel oil: 42,700 kJ/kg -Tolerance: 5%, at 100% -Efficiency of self-excited synchronous generator: h=0.97 -SFOC alt alternator: 192.78 g/kWh The analysis was conducted by first calculating the cost of ballast water exchange using the sequential method in accordance with the D1 standards as a reference.
The aforementioned process includes the exchange of ballast during the ballast voyage and under favorable weather conditions, in accordance with the requirements of the D1 standard.
In these circumstances, an exchange requires 48.10 hours (the time and quantity used for the calculation are in accordance with the ship's "Ballast Water Management Plan" endorsed by the Recognized Organization on behalf of ship's flag as fully compliant with relevant articles of the Ballast Water Management Convention 2004 for 'Water ballast exchange condition' -Summary Table of Typical Sequences, p.26.) of pump operation time. The sequential method was chosen because it requires only one exchange of the total amount of ballast water as opposed to the dilution method, which requires three exchanges and, from the energy efficiency standpoint, it is less acceptable and thus was not taken into account. The proposed process assumes proper hull stability, bending moments within prescribed limits, and favorable propeller immersion during the exchange of ballast water at sea.
The total operational cost of exchanging ballast water in accordance with the D2 standards was then analyzed by comparing the two methods of secondary ballast water treatment: UV irradiation and electrochlorination. Predictive calculation was made for those systems, based on unit energy consumption of commercially available systems.
Finally, the annual operational cost of those methods implemented on the ship was estimated, presupposing 12 full trips per year.

Methods
The pump output power was calculated from the equation: Hyde Guardian Gold (UV) Hyde Marine, US [21] b) Ballast water treatment according to D2 standard -UV irradiation If it is necessary to load or unload the same quantity of ballast water, that is 17,741 m³, considering the average unit consumption of 0.11433463 kWh/m³, the total energy consumption amounts to 2,028.41 kWh.
The fuel oil consumption required for one operation: , .
. U A SFOC g 391 036 88 0 391 mT The total cost of one operation (loading or unloading) in accordance with the D2 standard using a UV system is: .

c) Ballast water treatment according to D2 standardelectrochlorination
In case of electrochlorination and assuming the same quantity of ballast, that is, 17,741 m³, as well as considering the average unit consumption of 0.0494873 kWh/m³, the total energy consumption amounts to 877.95 kWh.
The fuel oil consumption required for one operation equals: Finally, the operational cost of one operation in accordance with the D2 standard using an EC system is: .
When it comes to unloading, energy consumption is related to the energy required to drive the neutralizing agent injection pump, and it is of no importance in this case.

d) Estimated annual operational cost of ballast water treatment by using various processing methods
Assuming that a ship makes 12 trips annually, the estimated average annual cost for the above methods is shown in Table 2.
It can be noted that the sequential method in accordance with the D1 standard requires only 12 additional operations, as opposed to the UV and EC treatments in accordance with the D2 standard that require twice as many operations (24), also taking into account that energy consumption in EC treatment during unloading is of no importance.
The cost of one ballast water operation was estimated using the following expression: where: C u -total cost of one operation (loading or unloading) C j -unit price of a ton of fuel oil The unit price of a ton of fuel oil, used in accordance with the ISO 8217: 2010 RMG 380 standard, is C j =307.5 US $/mT [22].

Findings
The findings for the present three cases are as follows: In this case the operational cost of one ballast water exchange using the sequential method was calculated at: . It is quite clear from the data presented that the annual costs for different BWT systems vary significantly. However, if compared with the overall purchasing and installation costs, these costs are minimal, even if taken at the whole life-cycle level. Therefore, from the shipowner's standpoint, it seems much more appropriate to pay more attention to purchase and installation costs than to operational costs. Such an extension of operational costs indicates that maintenance activities and related costs, i.e., those costs ensuring the proper operation in day-to-day work, should be borne in a way that will bring the operations up to the highest standards.

CONCLUSION
The International Convention for the Control and Management of Ships' Ballast Water and Sediments requires the processing of ships' ballast water in accordance with predefined standards. Various IMO-approved processing methods, with different power consumption levels, are offered by different manufacturers. Since power generation onboard is closely connected with quantities of fuel oil used, it is possible to compare the effectiveness of these methods based on fuel oil consumption.
The case study presented in this paper compares two most frequently used methods with operating costs borne by shipowners using the ballast water exchange method as a method commonly used before the full implementation of the Convention on all ships.
Apart from the costs arising from acquiring ballast water treatment plants and their installation on board, one of the most influential factors for the decision makers in shipping companies is the estimation of operating and maintenance costs. The analysis presented here for the reference ship shows that systems using electrochlorination may be almost five times more cost effective than those based on UV irradiation, and almost eight times more cost effective than ballast water exchange using the sequential method.
However, fuel oil consumption efficiency is not the only relevant factor in the decision making process. In addition to fuel oil consumption, other impacts, such as environmental load, reliability, and maintenance costs, have to be taken into account.
Therefore, the final decision in selecting the most suitable system is a combination of various factors, including the installation and maintenance cost of the system, the type and age of the ship, as well as the prevailing characteristics of the sea areas where a particular ship will sail.
Further research may focus on systems reliability with associated maintenance costs and assess conclusions set in this article; however, that will be required after several years of usage.