Engineering perspective of electrochlorination system for ballast water treatment using carbon dioxide

1. Introduction

Ballast water is transferred by vessels to ensure stability during voyages. Ballasting and deballasting operations are performed to offset changes in vessel displacements associated with unloading and loading, respectively. However, the various biological substances contained in ballast water can affect native species and cause ecological changes in the ocean [1].

To prevent invasive species from translocating via ballast water, ballast water should be treated and disinfected in accordance with the United States Coast Guard (USCG) and International Maritime Organization (IMO) regulations. In 2019, the IMO announced a list of approved ballast water treatment systems (BWTS) in accordance with the procedure. The 45 BWTS consists mainly of filtration, electrolysis, UV, ozone treatment, and chemical injection systems [2]-[3]. Among the BWTS, electrochlorination is preferred because it provides relatively low power consumption, cost, footprint area, and higher disinfection efficacy [3].

To enhance the removal of marine organisms, Cha et al. [4] examined the synergistic effects of carbon dioxide (CO₂) injection on electrochlorination disinfection of Artemia franciscana. It is well known that injected CO₂ can be converted into carbonic acid (H₂CO₃^*) in water, leading to a decrease in the pH of water. At low pH, hypochlorous acid (HOCl) is formed more than the less powerful oxidative hypochlorite (OCl^-). HOCl is 80-200 times stronger than OCl^- in terms of pathogen disinfection [4]. In general, exhaust gas is a mixture of gases and contains mainly CO₂, NO_x, and SO₂. Under these circumstances, CO₂ generated from combustion engines can be a valuable source for the elimination of microorganisms (e.g., zooplankton and phytoplankton) because the exhaust gas can minimize energy consumption instead of the CO₂ gas generation system. However, there are insufficient documents on the composition and treatment efficacy of exhaust gas in ballast water treatment to draw clear conclusions [5].

In this study, we focused on the influence of treatment efficacy by utilizing exhaust gas as a CO₂ source and proposed a promising conceptual design for a new type of BWTS.

2. Experimental

3. Results and Discussion

Figure 1 shows the inactivation of Artemia sp. at different pH values in the presence of 4ppm TRO concentration. CO₂ was used to adjust the pH of test water. When the pH was lowered from 8.2 to 5.5, the inactivation of Artemia sp. increased by approximately 3, 7, and 29 times on days 1, 2, and 3 of holding time, respectively. This agrees well with previous results that CO₂ injection strengthens disinfection efficacy against microorganisms compared to the electrochlorination system alone [4][6]. According to earlier studies, the efficacy of electrochlorination relies heavily on HOCl concentration in water. Previous studies also explained that reduced pH increases the HOCl concentration in the solution, and the disinfection efficacy of HOCl is much higher than that of ClO^- (considering that the pKa of HOCl is 7.53) [4][6]. Meanwhile, estimates showed that the disinfection efficacy greatly increased as the decomposition reaction of foreign substances accelerated and biological toxins increased on day 3.

Figure 1:

Inactivation of Artemia sp. with respect to different pH. The initial TRO concentration was 4 ppm and pH was controlled by CO2 injection

Figure 2 shows dissolved oxygen (DO) concentration as a function of pH. Overall, the DO concentration steadily decreased with increasing holding time and constantly decreased with decreasing pH, indicating that disinfection efficacy is related to DO concentration. There was no significant change in the concentration between 1 and 3 days of holding time at pH 5.5, implying that the enhanced inactivation of Artemia sp. on day 3 was not directly correlated with changes in DO concentration. Although TRO concentration decay was more noticeable in only electrochlorinated water than in electrochlorinated water with CO₂ addition [4]. Our results did not show a drastic difference in the TRO concentration decay profiles (data not shown here). Further studies are needed to elucidate the cause of the enhanced disinfection efficacy.

Figure 2:

Dissolved oxygen (DO) concentration according to pH change

It is worthwhile to review the technique that was most effective for disinfection. As shown in Figure 3, the inactivation efficacy of Artemia sp. was 98.8%, 99.4%, and 99.9% in CO₂ alone, 4 ppm TRO, and 4 ppm TRO + CO₂ on day 3 of the holding time, respectively. From an engineering perspective, CO₂ can be constantly injected to maintain a certain level of disinfection in ballast water tanks, and electrochlorination systems with CO₂ addition can be used when entering the United States, where USCG ballast water treatment regulations are applied. Accordingly, it would be helpful to reduce the power consumption of BWTS.

Figure 3:

Comparison of Artemia sp. individuals: CO2, 4 ppm TRO, and CO2 + 4ppm TRO injection

Some BWTS manufacturers have attempted to use CO₂ gas generation systems as the CO₂ source. To our knowledge, there are no cases of direct application to BWTS using vessel exhaust gas. Therefore, it is important to verify if the exhaust gas emitted from a combustion engine can be applied to a BWTS. Table 1 shows the summary of auxiliary engine exhaust monitored. Maximum continuous rating (MCR) was 1,540 kW at a 75% engine load, and the exhaust gas flow rate was 7,066 kg/h. In addition, the CO₂ concentration was approximately 5.62%.

Table 1:

Summary of auxiliary engine exhaust monitored

When injected CO₂ is dissolved in water, the concentrations of [H₂CO₃^*], [HCO₃^-], and [CO₃^2-] can be predicted using Equations (1)–(3). The total inorganic carbon (C_T) is expressed by Equation (4), which is the sum of [H₂CO₃^*], [HCO₃^-], and [CO₃^2-].

The equilibrium constants for KHCO₃^- and KCO₃^2- were analytically obtained using Equation (5) [7].

where T is the absolute temperature, and a1 to a5 are empirical constants.

constants. To calculate the pH inside the ballast water tank, the concentration of H⁺ was determined using Excel software. The dissolved inorganic carbon (DIC) concentration in the ballast water tank as a function of flow capacity is shown in Figure S2.

Table 2 shows the predicted pH inside the ballast water tank as a function of ballast water flow capacity. The ballast water flow capacity will be approximately 500 m³/h when the pH is 5.5, and it is indicated as the highest disinfection efficacy. A pH was predicted to be 5.9 when the ballast water flow capacity reached 1000 m³/h, that is, under conditions where vessels are frequently operated. From these results, a suitable pH for microorganism disinfection was achieved using only the vessel auxiliary engine without any acid additives.

Table 2:

Predicted pH in ballast water tank as a function of bal-last water flow capacity. Exhaust gas ratio was fixed to 50%, and the ballast water temperature was 22°C

Finally, the IMO calls for the maritime industry to restrict its CO₂ emissions by -40% by 2030 and -70% by 2050, compared to the base level in 2008 [8]. In view of the upcoming CO₂ emission regulations, many shipbuilding manufacturers have begun researching the possibility of onboard carbon capture vessels. They have mainly focused on CO₂ capture and storage using chemical absorbents. However, it will be difficult to apply to vessels owing to corrosion caused by chemical adsorbents and the expensive operation cost required to regenerate the chemical absorbent. However, if the injected CO₂ in the ballast water tank is periodically neutralized during the voyage and discharged after attaining a pH similar to that of seawater, it is expected that CO₂ can be partially reduced [9]. The proposed concept of an electrochlorination system combined with CO₂ injection is illustrated in Figure 4.

Figure 4:

Schematic of electrochlorination system combined with CO2 injection

4. Conclusions

We investigated the inactivation of Artemia sp. using an electrochlorination system combined with CO₂ injection. At pH 5.5, the inactivation of Artemia sp. significantly increased by approximately 29 times compared to pH 8.4 on day 3 of holding time. Results showed that the main factors enhancing the disinfection efficacy were an increase in HOCl concentration with decreasing pH and a decrease in DO concentration due to the dissolution of CO₂. Results of comparing disinfection efficacies revealed that the inactivation of Artemia sp. was 98.8%, 99.4%, and 99.9% in CO₂ alone, 4 ppm TRO, and 4 ppm TRO + CO₂ on day 3 of the holding time, respectively.

From an engineering perspective, it can be concluded that the CO₂ emitted from a combustion engine can be directly used to reduce pH. Furthermore, if the injected CO₂ in ballast water tanks is periodically neutralized during the voyage and discharged after attaining a pH similar to that of seawater, it is expected that CO₂ can be partially eliminated. We hope that this study will provide insight into how to retrofit and design BWTS in the maritime industry.

Acknowledgments

This research was supported by the Korea Shipbuilding and Offshore Engineering Research Fund.

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