Electrochemical Nitrate Destruction

It is 4 a.m. and I am cracking open a water valve on a borehole site. There is a suspicion of light on the horizon and the deer are out.

This site is operational, supplying drinking water to 100,000 people in the surrounding villages and towns. Demand is on the very edge of available supply. For that reason I can only operate my process treatment plant between 4 and 6 in the morning. At this hour demand for water is low, before most other people are up. The site is an hours drive away from home. But this is a very beautiful site, and my favourite time of day.

I have a problem to solve. See what you think to this;

1. This area has expanded rapidly and has a great demand for water
2. The area is the driest in England, experiencing the lowest rainfall
3. Potable water is, of course, of the highest quality and very safe to drink.
4. Nitrate is harmful to human health, especially infants
5. Regulations stipulate the maximum concentration of nitrate allowed in the water
6. Known nitrate removal processes produce a volume of wastewater, highly concentrated in nitrate and, usually, brine.
7. There is no water to waste
8. Nitrate is harmful to the environment, depleting oxygen and killing fish
9. There are strict regulations prohibiting discharge of nitrate to water courses
10. Water is applied to soil locally to fertilise crops. It enters water courses and groundwater.

My challenge was to develop a water treatment process that will remove nitrate to the required safe drinking level, do not discharge nitrate to the environment, yet maintain sufficient potable water volume for supply.

Fortunately I enjoy a good logic problem. I even gave up my sudoku habit to make time to complete my engineering chartership. Engineering is all about problem solving. That is what makes it such a great career. And it is why I am on an operational groundwater supply works at 4 a.m.

There were many such sites in my career as a process engineer. I'd arrive in my little car, just large enough to house a valve key in the boot. Having selected a novel technology to investigate by technical evaluation I completed negotiations with early innovators, completed the planning and the logistics. It was never straightforward and of course there were no precedents or procedures. By definition innovative work is an unknown. Everything was an adventure. One Saturday morning I found myself on site, escorting in a shipping container that arrived from Australia. It had docked earlier that week in Felixstowe, and I had sailed down on my weekend off to see it.

After months of work I was finally in a Norfolk market town making preparation for my technology's arrival on the back of a flat bed truck. How was I to have known that this town held a street market on Saturdays? My pilot plant, on the back of a lorry made its way down the high street through the narrow gap between market stalls.

Arriving on site I found that, overnight, the site access road had been dug up and now sported a huge gap where a hole had been dug . I hastily needed steel plates, and some amazing influencing skills. We were not done yet as the driver was adamant he could not offload. But I got there. And now the work would start. Months of trial operation, sampling, analysis and optimisation lay ahead.

Let me tell you about one promising technology I trialled, and how it addressed the conundrum of supplying safe clean drinking water in a nitrate vulnerable zone (NVZ).        

Electrochemical Nitrate Destruction

Having completed a PhD in electrochemical remediation of wastewater, at Cranfield University's specialist post-graduate School of Water Sciences this technology research was right up my street. To test it on ground water was even more exciting given my previous award from the Groundwater Conference at Sheffield University. Combining these skills with my water industry knowledge was immensely satisfying.

Electrochemical remediation of nitrate-contaminated water produces high quality drinking water, and negates the need for a high nitrate wastewater stream. Let me tell you how it works;

First Nitrate is removed from the water by exchanging it with chloride ions on a resin media. This addresses the need to remove nitrate from water to make it suitable for drinking water. However there results from this a high nitrate concentration waste water stream.

This cannot be discharged to the environment as it is high in nitrate and nitrate is harmful to the environment, depleting oxygen and causing eutrophication.

Instead the high nitrate effluent is treated electrochemically.

Electrochemical reduction of nitrate:

An electrical current is applied to the water. This causes the nitrate ions, NO3 to undergo reduction at the cathode, converting them into nitrogen gas, N2.

NO3- + 6H+ + 6e --> 1/2 N2 + 3H2O

As you can see from the equation, reduction here refers to chemical reduction, as opposed to a 'lessening' or reducing an amount of. Electrochemical reduction is the gain of electrons, shown as + 6e in the equation.

The final product, N2 is released to the atmosphere. Nitrogen gas in the atmosphere is harmless. Our atmosphere is already 80% Nitrogen gas. It is the main component of air.

Electrode Reactions:

The overall electrode reactions are:

At the cathode: Reduction of Nitrate

2NO3 (aq) --> N2 (g)

At the Anode: Oxidation of water to Oxygen Gas. Here oxidation is referred to as the loss of electrons. Where reduction and oxidation occur together like this it is referred to as Redox. The term OIL RIG is useful to remember this process by – Oxidation Is Loss, Reduction Is Gain.

Optimising Parameters for Nitrate Destruction Technology

It is essential to optimise operating parameters to achieve high nitrate removal efficiency and energy efficiency.

The parameters to be optimised are

• Current density
• pH
• Retention time
• Electrolyte composition
• Electrode selection

You don't have much control over the incoming nitrate concentration. But you can optimise the ion exchange preceding the electrochemical reaction to alter electrolyte composition. You can also alter the partial volume of influent from the supply works.

Electrode selection

The choice of electrode material impacts factors like nitrate removal efficiency, energy consumption, long-term stability, cost, compatibility with potable water application – and environmental sustainability considerations such as scarcity, embodied carbon, extraction and processing energy, transport location and local regulations.

Electrodes for electrochemical destruction of nitrate can be

Metal Electrodes

Metals such as titanium, or metal-coated electrodes. The electrodes can be coated with catalytic materials, or metal oxides to enhance reduction.

Carbon Based Electrodes

Carbon based electrodes can be made from graphite, carbon felt, and carbon polymer composites these offer high surface area good electrical conductivity and tunability of surface properties.

Other carbon based electrodes can be doped or modified carbon electrodes such as boron doped diamond or nitrogen doped carbon and these can improve the nitrate reduction kinetics

Metal-Oxide Electrodes

Metal oxide electrodes include TIO2 SnO2 or CeO2. These exhibit good catalytic activity for nitrate reduction The materials can be synthesised in various nano-constructed forms to enhance the active surface area.

One of the parameters I found to be successful in designing an electrochemical reactor was enhancement of surface area. I achieved this through using very fine powdered metals and by degreasing metal surfaces prior to application.

Bi-metallic Electrodes

During my PhD I researched a number of metal combinations. Most notably Fe-Cu combinations . Later in my career these would be fun to describe to the annual delegation from the Japanese water association.

Combining two or more metals can create synergistic effects and improve the nitrate reduction performance the composition of microstructure these electrodes can be tuned to optimise the activity and selectivity Other combination options include Cu-Pd and Ni-Fe.

Membrane-electrode assemblies

Integrating ion exchange membranes with the electrodes can help maintain optimal pH conditions and promote the selective transport of target ions.

This configuration can improve the overall efficiency and reduce the formation of unwanted by-products.

Addressing Disadvantages of Electrochemical Nitrate Destruction

The main consideration is energy consumption. This is especially so when considering low carbon ambitions. Optimising the process parameters to maximise efficiency is the first action to limit energy consumption.

Electrode choice and asset efficiency are important parameters to limit energy consumption.

When considering asset efficiency take action to minimise dissipation of energy by limiting friction, noise, vibration and heat generation in the system.

Also, using membrane electrode combinations for the electrode reduces formation of unwanted by-products as well as improving overall efficiency.

Renewable Energy for Nitrate Destruction

Power nitrate destruction plants with renewable energy. Nitrate destruction plants are sited on water treatment works. They are outside, in a field (hence the sunrise and the deer). This makes both solar power and wind power feasible electricity sources for this treatment process. Water supply works also operate powerful delivery pumps. There is potential to utilise hydropower from within the mains to derive electrical power.

Further Carbon Reduction Considerations

In addition to using renewable energy sources the following strategies can be considered

• Optimising maintenance and repair schedules with predictive techniques using telemetry data to target asset interventions
• Data-informed energy efficiency, utilising the methodologies from predictive maintenance for optimal pump operation
• Ensuring whole life carbon consideration at procurement throughout the supply chain

There are already a number of large scale and electrochemical nitrate destruction systems. These demonstrate the suitability of this technology. They are in operation throughout the world. These exist in The Netherlands the United States, China and Israel.

Despite its successful use at a larger scale and that the technology has existed for some time it is still considered relatively new. Further advancements in electrode materials, reactor design and system integration are needed if we are to achieve widespread commercialization and large scale implementation.

This would reduce nitrate contamination of waters and allow us to capture the advantage of lower energy consumption and lower operating costs compared to the traditional nitrate removing technology.

As a chemical engineer development of nitrate process treatments that are environmentally sustainable and provide safe clean drinking water are a worthy challenge.