ABSTRACT
Nowadays, performance studies on the amperometric total residual oxidant (TRO) sensor are only in the bench test stage and have not been conducted under specific maritime conditions with Ballast Water Management System (BWMS). In this study, the application of the amperometric TRO sensor in land-based biological efficacy (BE) testing, operation and maintenance (O&M) testing, as well as shipboard (SB) testing, was explored by comparing with the existing di-phenylene-diamine (DPD) TRO sensor. The results showed that the average TRO measurement deviation between the amperometric sensor and the DPD sensor was within ±10% in valid BE test cycles and the O&M testing exceeding 47 operating hours. The TRO value measured by amperometric sensor exhibited significant fluctuations, but the improved control logic could achieve smoothing out the fluctuation, with stability comparable to or even higher than that of the DPD sensor. The practicality and reliability of the amperometric sensor in electrochlorination-based BWMS were further verified through SB testing.
HIGHLIGHTS
TRO measurement deviation with the amperometric sensor was within an acceptable range.
TRO signal output with the amperometric sensor was affected by salinity.
Smoothing control logic could mitigate the measurement fluctuation caused by the TRO sensor.
The reliability of the amperometric sensor was further verified through shipboard testing.
INTRODUCTION
Ballast water is used to maintain the ship's stability and maneuverability and control the trim and heel of the hull (Thach & Phan 2023). However, ballast water contains various exclusive organisms from both ballast and de-ballast regions, recognized as the major vector for the invasion of alien organisms potentially (Duan et al. 2023). It is estimated that 7,000–10,000 different species globally every day, such as marine microorganisms, plants and animals, are transferred through the ballast water pathway (Lakshmi et al. 2021). The arbitrary discharge of untreated ballast water can inevitably cause ecosystem disorder and lead to significant economic losses (Marbuah et al. 2014; Kurniawan et al. 2022).
In order to reduce the risk of alien species introductions, the International Maritime Organization (IMO) adopted the International Convention for the Control and Management of Ships' Ballast Water and Sediments (referred to as the Convention) in 2004. The Convention entered into force on September 8, 2017, with clear limits on the discharge concentration of living organisms in ballast water (Regulation D-2) (IMO 2004). In addition to the IMO, the United States also promulgated a Ballast Water Discharge Standard (BWDS) for its territorial waters in 2012 and the final rule of the United States Coast Guard (USCG) entered into force on the same year (USCG 2012). Therefore, Ballast Water Management System (BWMS) become necessary for international ships' navigation (Sayinli et al. 2022).
BWMS, based on electrochlorination, are widely adopted in the shipping field to meet regulation D-2 of its higher inactivation efficiency (Cha et al. 2015). According to statistics, approximately 40% of BWMS evaluated by IMO use electrochlorination technology and electrochlorination BWMS account for approximately one-third of BWMS-type approved by USCG (Zimmer-Faust et al. 2014; USCG 2024). Given that ballast water treatment can generate ecotoxic total residual oxidants (TROs) potentially, both the Group of Experts on Scientific Aspects of Marine Pollution (GESAMP-BWWG) of IMO and the United States Environmental Protection Agency (USEPA) propose requirements for monitoring TRO in BWMS (USEPA 2013; GESAMP-BWWG 2015–2022; Kim et al. 2016). Therefore, accurate TRO measurement is crucial for BWMS process control.
Currently, TRO measurement mainly includes the di-phenylene-diamine (DPD) method and the amperometric method. TRO sensors based on the DPD method employ colorimetric assays utilizing chemical reagents. Total oxidants in the sample water react with these reagents and the higher the oxidant concentration, the darker the color (Buchan et al. 2005; ISO 2017). Although TRO sensors based on the DPD method were already type-approved due to their high accuracy and stability and widely used in electrochlorination-based BWMS, there are also many issues in actual operation (Zimmer-Faust et al. 2014; Zhou et al. 2020). For example, the DPD TRO sensor is usually installed online and an elaborate sampling line is arranged for TRO measurement. Additionally, reagents are indispensable for the measurement and should be replaced regularly within months. The DPD reagent is active after preparation and should be kept at a low temperature, which makes it more challenging on board ships with higher ambient temperatures. Sample drain after analysis needs to be led back to the main ballast line or neutralized before discharge, due to residual oxidant in the sampling water.
Amperometric TRO sensors are essentially electrochemical, producing a current proportional to chlorine concentration (Zimmer-Faust et al. 2014). Compared with DPD TRO sensors, amperometric TRO sensors are reagent-free and can be installed in line with direct insertion into the main ballast line, which greatly reduces economic costs and installation difficulties. Nowadays, the amperometric TRO sensor with three electrodes, including a platinum working electrode (WE), a platinum auxiliary electrode/counter electrode (CE) and a silver/silver chloride reference electrode (RE), has been successfully developed. This solved the problems of frequent calibration, low measurement accuracy and unstable output signal caused by double-electrode TRO sensors (Li et al. 2022). However, the performance characteristics of amperometric TRO sensors are only at the research stage and cannot be directly applied to BWMS. Differences in factors, such as seawater quality, ship environment and durability, can significantly affect the TRO sensor's performance (Badalyan et al. 2009; Zimmer-Faust et al. 2014). Therefore, it is vital to conduct a comprehensive performance evaluation of the amperometric TRO sensor with BWMS.
This study aimed to verify the measurement accuracy and robustness of the amperometric TRO sensor in comparison with the DPD TRO sensor. Studies included land-based (LB) testing in three salinity ranges (freshwater, brackish water and marine water), operation and maintenance testing and shipboard (SB) testing. The results could guide the application of the amperometric TRO sensor in electrochlorination-based BWMS.
MATERIALS AND METHODS
LB biological efficacy testing
Physical–chemical characteristics of challenge water for land-based testing
Test cycle . | Salinity (PSU) . | Temperature (°C) . | pH . | TSS (mg/L) . | POC (mg/L) . | DOC (mg/L) . | Duration (min) . |
---|---|---|---|---|---|---|---|
F-1 | 0.38 | 14 | 8.3 | 68 | 5.7 | 7.3 | 40 |
F-2 | 0.38 | 20 | 7.8 | 66 | 7.1 | 6.5 | 40 |
F-3 | 0.40 | 20 | 7.7 | 60 | 6.8 | 6.0 | 40 |
B-1 (not calibrated) | 15 | 9.9 | 8.1 | 60 | 8.0 | 6.6 | 40 |
B-2 | 18 | 15 | 8.4 | 67 | 7.8 | 6.9 | 40 |
B-3 | 18 | 17 | 8.2 | 71 | 7.6 | 6.9 | 40 |
M-1 (not recorded) | 29 | 17 | 8.1 | 42 | 5.9 | 7.2 | 40 |
M-2 | 28 | 19 | 8.4 | 52 | 7.6 | 7.2 | 40 |
M-3 | 28 | 19 | 8.3 | 47 | 7.0 | 7.4 | 40 |
O&M-1 (not calibrated) | – | – | – | – | – | – | 900 |
O&M-2 | 18 | 13 | 8.2 | – | – | – | 960 |
O&M-3 | 15 | 18 | 7.8 | – | – | – | 120 |
O&M-4 | 18 | 17 | 7.5 | – | – | – | 480 |
O&M-5 | 17 | 18 | 7.6 | – | – | – | 360 |
Test cycle . | Salinity (PSU) . | Temperature (°C) . | pH . | TSS (mg/L) . | POC (mg/L) . | DOC (mg/L) . | Duration (min) . |
---|---|---|---|---|---|---|---|
F-1 | 0.38 | 14 | 8.3 | 68 | 5.7 | 7.3 | 40 |
F-2 | 0.38 | 20 | 7.8 | 66 | 7.1 | 6.5 | 40 |
F-3 | 0.40 | 20 | 7.7 | 60 | 6.8 | 6.0 | 40 |
B-1 (not calibrated) | 15 | 9.9 | 8.1 | 60 | 8.0 | 6.6 | 40 |
B-2 | 18 | 15 | 8.4 | 67 | 7.8 | 6.9 | 40 |
B-3 | 18 | 17 | 8.2 | 71 | 7.6 | 6.9 | 40 |
M-1 (not recorded) | 29 | 17 | 8.1 | 42 | 5.9 | 7.2 | 40 |
M-2 | 28 | 19 | 8.4 | 52 | 7.6 | 7.2 | 40 |
M-3 | 28 | 19 | 8.3 | 47 | 7.0 | 7.4 | 40 |
O&M-1 (not calibrated) | – | – | – | – | – | – | 900 |
O&M-2 | 18 | 13 | 8.2 | – | – | – | 960 |
O&M-3 | 15 | 18 | 7.8 | – | – | – | 120 |
O&M-4 | 18 | 17 | 7.5 | – | – | – | 480 |
O&M-5 | 17 | 18 | 7.6 | – | – | – | 360 |
Operation and maintenance testing
To verify the reliability of the TRO sensors incorporated with BWMS, the operation and maintenance (O&M) test was conducted independently by a test facility that had the same system as BE testing and arranged amid the BE test cycles. The O&M test was operated the same as the BE test, but the ballast pump takes brackish water locally from the Hundested harbor in Hundested, Denmark. The treated water will be discharged directly into the harbor after the neutralization at the end of the discharging piping. Physical–chemical characteristics of challenge water for O&M testing are shown in Table 1. The sensors were operated for a total of 53 h exceeding the 50 h as specified by the ETV protocol during five O&M tests (USEPA 2010).
Shipboard testing
The SB testing was conducted on board a 300,000 DWT very large crude carrier (VLCC) which was installed with two identical sets of BWMS with treatment rated capacity (TRC) of 3,200 m3/h per system. An online DPD sensor and an in-line amperometric sensor were installed adjacently on the main ballast line of the port side to measure the TRO level during the ballasting treatment. The electrolysis unit takes up water from the sea chest in the engine room after being filtered by a 50-μm automatic back-flushing filter. Chlorine was produced by electrolysis and injected into the main ballast line in the pump room by the dosing pump. Two TRO sampling ports were arranged downstream of the injection point to measure the TRO concentration after fully mixing. DC for electrolysis was regulated in accordance with the TRO level measured by the DPD sensor to reach the dosing target of 7.5 mg/L TRO.
TRO control logic updating for the amperometric sensor during ballasting
The control logic responses for TRO changes during the ballasting process were updated as follows.
The output reading of the TRO level was the average value of the previous output TRO level and the present actual TRO level.
The output reading would remain the same as the previous output reading if the present actual TRO level deviated from the previous output TRO level beyond the pre-set deviation ratio (>20%). The output reading would be the average value of the previous output TRO level and present the actual TRO level if the two consecutive actual TRO levels were beyond the pre-set deviation ratio.
The control loop for the regulation of current input to the electrolyser remained the same with the DPD TRO sensor.
Analytical methods
Temperature, salinity and PH were measured by in-line probes and operational flow was measured by an in-line flow meter. Chemical parameters of total suspended solids (TSS), dissolved organic carbon (DOC) and particulate organic carbon (POC) were sampled and analyzed independently by the test facility for compliance with challenge water requirements prescribed in BWMS Code and ETV. A programmable logic controller (PLC, Mitsubishi, Q series) was used for the control of the whole system and data logging. All the TRO measurements from the DPD sensor and the amperometric sensor were automatically recorded every minute during the whole testing. The data were extracted from the PLC and saved as CSV files for statistical analysis.
RESULTS AND DISCUSSION
Measurement deviation comparison during LB testing
Average TRO levels measured with the DPD sensor and amperometric sensor in LB testing.
Average TRO levels measured with the DPD sensor and amperometric sensor in LB testing.
Measurement fluctuation evaluation during LB testing
TRO levels measured under continuous operation in different test cycles.
Additionally, amperometric sensors were susceptible to interference from pH levels. As is well known, due to the sensitivity of amperometric sensors to measuring the content of hypochlorous acid, a pH of 5.0 to 7.0 is the ideal operation range for amperometric sensors. When the pH was greater than 7.0, hypochlorite ion became the main free chlorine, with hypochlorous acid significantly reduced, which would inevitably affect the accuracy of the amperometric probe (Malkov et al. 2009). Studies reported that organic chloramines could also affect the measurement of amperometric sensors (Jensen & Johnson 1990). To reduce the dependency of amperometric sensors on pH, most manufacturers typically employ internal pH compensation to improve the potential accuracy of amperometric sensors. However, due to the complexity of seawater quality, internal pH compensation also had its limitations. Temperature changes were also a major factor affecting the measurement of amperometric sensors (Zimmer-Faust et al. 2014; Li 2021). Studies indicated that three main areas affected by temperature were the membrane permeability rate, the conductivity of the electrolyte and the sample pH (Malkov et al. 2009). Although the amperometric sensor was equipped with a temperature compensation function, due to potential differences in membrane materials, electrode geometry, thermistor position, etc., no temperature compensation algorithm could accurately reflect all changes in the water matrix or the response of chlorine to these changes, inevitably causing measurement fluctuations.
Actual and processed TRO reading by amperometric sensor in O&M-2 test cycle.
SB testing verification
TRO levels in SB testing before (a) and after (b) control logic update.
The variation trend of the amperometric sensor was similar to that of the DPD sensor, but more drastic fluctuation was observed in a test without updated control logic, which could cause significant instantaneous deviations. To further evaluate this fluctuation, this study conducted an SD analysis on the SB test results before updating the logic. The results showed that the SD value (2.15 mg/L) generated by the amperometric sensor during the measurement was much higher than that by the DPD sensor (1.62 mg/L). This fluctuation was also observed in LB and O&M testing. With the update of control logic, the fluctuation of the amperometric sensor had been significantly mitigated, as shown in Figure 5(b). The SD value decreased sharply from 2.15 mg/L to 1.33 mg/L, even lower than the SD (1.58 mg/L) of the DPD sensor, which indicated that the updated logic assisted in maintaining a more stable TRO measurement with the amperometric sensor.
CONCLUSION
The following conclusions could be drawn from this study:
The acceptable average measurement deviation between the DPD sensor and amperometric sensor was within ±10% in 10 out of 11 valid LB test cycles. Salinity affected the measurement of the amperometric sensor, and measurement accuracy declined with higher salinity.
The system demonstrated satisfactory operation in natural seawater with no augmentations during the O&M testing period of more than 47 h, which confirmed that this amperometric sensor could overcome the impact of electrode passivation and met the applicability conditions.
More drastic fluctuation was inevitably observed in all the tests for the amperometric TRO sensor, although the measurement deviation was below 10%.
Smoothing the control logic of the amperometric TRO sensor could significantly reduce measurement fluctuations (about 40%), avoiding rapid changes in current input (power supply) to the electrolyser.
ACKNOWLEDGEMENT
This study was supported by the National Quality Infrastructure (2022YFF0610403).
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.