Same, but different – a comparison between the side-stream and full-flow system | KraftPowercon

Date
9/19/2020

Category
Marine

Posted By
Göran Stenmark

Electrochlorination (EC) systems make up nearly half of all active ballast water management systems on the global market today – and for good reason. They are efficient at treating large volumes of water, their power consumption is low, and they usually have no hold times since they treat the water directly at uptake. But did you know that there are two types of EC systems? That’s right, you can either choose a side-stream system or a full-flow system. While both systems leverage the same chemical treatment method, the practical treatment process differs between them. So, what is really the difference between them? Let’s compare the two to give you a better idea!

SAME TREATMENT METHOD – DIFFERENT PROCESSES

A full-flow system treats the ballast water at uptake by pumping it through the initial filter and the electrochlorination cell, where water molecules and salt in the seawater decompose into sodium hypochlorite. This is the active ingredient that disinfects the ballast water and kills microorganisms that have passed the filter. Once the water has passed through the cell, the ballast water tanks are filled with the treated water.

In a side-stream system, this process looks slightly different. Instead of pumping the entire water flow through the electrochlorination cell, a small percentage of water is separated from the total flow – typically after passing the initial filter – and treated in the cell. Once this smaller volume of water has been treated, it is re-injected to the total ballast water flow to disinfect the remainder of the ballast water.

Animation showcasing ballast water flowing through a side-stream system.

WATER SALINITY AND TEMPERATURE REQUIREMENTS

The smaller volume of ballast water separated in the side-stream system often requires a temperature of around 5–15°C before entering the electrochlorination cell. This is due to the chemical process taking place in the cell. Chemical reactions usually occur easier in higher temperatures, so if the water is cold, it becomes harder to decompose water and salt molecules. This reaction is needed to produce the active ingredient, sodium hypochlorite. To ensure the water temperature is high enough for the chemical reaction to take place, it usually passes through a heater before entering the cell. Apart from having the right temperature, the water also needs to have enough salinity in order to support the chemical reaction in the cell. A Practical Salinity Unit (PSU) of around 5–15 is the general range required for most systems.

In a full-flow system, the ballast water can both have a lower temperature and salinity, as the cell produces a concentration of sodium hypochlorite proportionate to the total water flow. This, however, is not the case in the side-stream system, as only a small portion of water has to produce a significantly higher concentration of the active ingredient.

Keep in mind that both side-stream and full-flow systems are dependent on the salt in the seawater to work. If your vessel operates in or enters a port with fresh water, your system might not work unless seawater is brought in a separate tank from where the active ingredient can be produced.

Animation showcasing ballast water flowing through a full-flow system.

WHEN SHOULD I CHOOSE ONE OR THE OTHER?

Unfortunately, there is no one-size-fits-all when it comes to ballast water management systems. In terms of whether or not you should go for a side-stream or a full-flow system, one of the main things you will have to consider is the ballast water flow you treat on a regular basis. If your ballast water flow is high, it will be harder to run all of it through one single electrochlorination cell in a full-flow system. Therefore, you might have to run several cells in parallel. For one, this requires a lot of free space and secondly, it isn’t very cost-effective. In this situation, you might be better off with a side-stream system, as they require less space and treat only a small portion of your total ballast water flow.

With that said, many parameters dictate whether a specific system is suitable for your needs or not. Apart from your ballast water flow, you have to take into consideration factors such as the size of your vessel, the water characteristics in which you operate, and your budget. We always recommend that you do thorough research before picking a system. Help from an expert can be a worthwhile investment as it will help you find the best possible solution for your specific needs.

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