De Nora – treating ballast water via “slip stream”

Jan 28 2021


De Nora Marine Technologies has pioneered a method of electrochlorination ballast water treatment where the disinfectant that eliminates aquatic invasive species is generated from a low flow, separate water supply to the actual ballast water, known as a “slip stream”.

In electrochlorination ballast water treatment, a DC current converts seawater into a sodium hypochlorite solution (NaOCl)and hydrogen gas.

 

Sodium hypochlorite is an effective disinfectant to eliminate aquatic invasive species.

 

There are two methods that electrochlorination systems can use to treat ballast water: full flow or in line and slip-stream. This is what they are.

 

In full flow systems the filter and the electrolyser are fitted directly into the main ballast line.

 

It sounds like a tidy engineering solution, but comes with a big disadvantage – all of this equipment needs to be installed somewhere between the ballast pumps and the ballast tank – which generally means the pump room.

 

The pump room is considered a ‘hazardous’ environment on tankers, so anything which can create a spark is not allowed. And installing all of this means a lot of welding – “hot work” – while there is of course a lack of available space.

 

The equipment needs to be big as the treatment capacities are large – capable of handling at least 1500m3 per hour of ballast water flow.

 

In a slip-stream electrochlorination approach, being developed by De Nora Marine Technologies, the sodium hypochlorite is generated in-situ, from a different source of saline water, requiring less than 1% of the main ballast flow for the electrochlorination process.

 

You can use any flow of saline water. The company suggests using the engine cooling water. The equipment can be housed in the engine room, which is not considered a ‘hazardous’ environment, and so work is easier. Then the sodium hypochlorite can be injected  into the main ballast water system next door via small size pipe lines.

 

The slip stream system does not eliminate hot work in the pump room – you still need to install a filter, to remove zooplankton, some phytoplankton and sediments.. But the work in the pump room is much reduced.

 

The electrolytic process also generates a dangerous by-product – hydrogen.

 

Low flow rate allows efficient degassing, says Dimitrios Tsoulos, regional sales manager EMA – marine business unit, De Nora Marine Technologies, based in Greece.

 

A further advantage of the slip stream system is that there is much more  flexibility about where and how on the ship the system is installed, he says. It is easier to break down the electrochemical system into a number of sub-assemblies, giving you more installation flexibility.

 

De Nora is not the only company offering a slip stream ballast system, but the company claims to have invented it for ballast water treatment applications and have been developing it longer than other ballast water companies.

 

Aft peak tank treatment

Another challenge with ballast water systems on tankers is what to do about ballast water in the aft peak tank. It must not mix with ballast water in the main ballast tanks, because this is considered “hazardous”, due to the risk of oil leakage into it, and the aft peak tank is in a “gas safe zone”.

 

Some companies using UV or full flow EC systems have considered fitting a completely separate ballast water system just for the aft peak tank treatment, he says. So for a VLCC you might have two large (3000m3 / hour) ballast systems in the pump room and a smaller system (300m3/hour) for the aft peak tank treatment.

 

But with a slip stream system, all which is needed is another pipe to supply sodium hypochlorite from the EC system to the aft peak tank, meaning a much simpler installation. The only equipment which needs to be installed at the aft peak tank is a small capacity filter. “With a single EC system we can treat multiple ballast lines,” Mr Tsoulos says.

 

Choice of systems

Many tanker fleet owners are still considering the best solution for their vessels, Mr Tsoulos says.

 

The market has now simplified to a choice of EC slip stream, EC full stream, UV, and chemical dosing, he believes.

 

Slip stream EC systems are most appropriate if you need to treat more than 1500 m3 an hour of water, and perhaps full flow EC and UV are more applicable to lower capacities, Mr Tsoulos believes.

 

Chemical dosing systems are most appropriate where there is limited power available on vessels, because UV and EC systems are high consumers of power. Or where the owner prefers to pay a higher cost for chemicals – perhaps because of operational limitations or the vessel’s remaining life span does not justify investing in ballast water equipment.

 

Electrolyser cells

De Nora is probably the only provider of fully approved EC ballast water systems to the maritime industry, which is also sole manufacturer of electrolyser cells, Mr Tsoulos says.

 

It has developed self-cleaning electrolyser cells, which it believes are unique. Typically, electrolysers accumulate hardness from the seawater over time. The calcium and magnesium in the seawater accumulates on the electrode plates, bridging the gap between electrodes, causing poor seawater flow, reducing cell efficiency, overheating, and potentially causing arcing.  

 

This self-cleaning feature is provided by the unique electrode coating and power reversing modules installed with a switching-mode power supply. For the electrolysers installed with the self-cleaning feature, the polarity output of the power supply is reversed after each ballasting cycle. This dissolves the small amount of hardness accumulated after each cycle and eliminates the need for acid washing for the life of the electrode.

 

Other companies have been suggesting that electrodes are cleaned with acid. But with the self-cleaning system, “the crew do not need to perform any difficult maintenance onboard carrying acids,” he says.

 

De Nora specialises in electrochemical solutions in a number of industries, doing design, manufacture and supply, making electrodes and coatings. It has been doing this for 95 years.

 

Chlorine analysers

One of the biggest complexities with EC ballast water systems is learning how to use the “Total Residual Oxidant” (TRO) analysers, which measure the chlorine concentration in treated wastewater, to ensure that it meets regulatory requirements. 

 

TRO analysers are complex for seafarers to use. It is “one of the biggest headaches,” Mr Tsoulos says.

 

De Nora offers an Oxidation Reduction Potential (ORP) analyser as an alternative, ORP (oxidation reduction potential) is a value, measured in mV that expresses the oxidizing (disinfection) power of a water.

 

It is a “simply immersed in-line instrument, easy to be installed, very easy to be calibrated during the operation,” he says. “It is an approved technology, proved to be reliable during the operations of the vessel.”

 

Dry dock postponements

Ballast water system implementations took something of a backseat during the Covid period, with shipping companies more pre-occupied with immediate issues, such as getting crew home, Mr Tsoulos says.

 

And many dry dock appointments have been postponed, due to difficulty of staff travelling to them, including company superintendents and class surveyors. For example, there are many repair yards in China, and tight travel restrictions on foreigners going to China.

 

Regulators have issued extensions to the allowable period of vessel operations between dry dock.

 

This also means that some of the ballast water system implementations De Nora had planned are now being postponed.

 

For similar reasons the number of new build vessels being launched is significantly dropped compared to previous years, which affects the market for ballast water systems, he says.

 



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