Waste mussel shells could become a low-tech, natural tool to filter pollutants from New Zealand waterways if field testing by University of Canterbury engineers proves successful.

Associate Professor Aisling O'Sullivan and Professor Tom Cochrane, from the University of Canterbury Department of Civil and Natural Resources Engineering, are exploring the use of waste seashells as a solution to reduce contaminants, such as nitrate and phosphorus, from our streams.

Associate Professor O'Sullivan says green-lipped mussels are a huge industry in New Zealand, producing over 90,000 tonnes of shells every year. 'When the meat is taken out, the shell becomes waste material. Approximately half of these shells go to landfill. We started looking at using those shells as an active substrate that we could put into something like a filter and treat the water as it passes through.”

Professor Cochrane says shells are made of calcium carbonate, which helps to remove pollutants, but they also have other properties that enhance the removal of contaminants, and they are more effective than an alternative such as mined limestone.

Associate Professor Aisling O'Sullivan has used shells to treat mine drainage and remove metals in Aotearoa New Zealand since 2006, and the pair thought they could now apply them to target nutrients in waterways and, specifically, nitrate leaching from the agricultural sector.

Farming is a major source of nitrate contamination in groundwater. Nitrate leaching is caused when there is more nitrate in the soil than crops can use, and it becomes an environmental pollutant when it reaches ground water or other waterways.

Using mussel shells as a solution would help to combat nitrate leaching and meet climate change targets and would also help to reduce waste going to landfill by converting it into a high value, reusable product.

The technology would help enable farmers to meet new regulations by reducing the number of nitrates leaching from their land, while providing a natural lime fertiliser and soil enhancer from the waste seashells. They will also work with shellfish and fertiliser industries, councils, and iwi to meet regulation requirements.

'We believe this solution will have a reduced carbon footprint because we're diverting waste from landfill and at the same time reducing the need for imported and synthetic fertiliser currently used on farms. Our solution is a regenerative approach that supports a circular economy,” says Associate Professor O'Sullivan.

'It's low-tech, green technology that is reusing waste that would otherwise go to landfill, it is economically viable, and it could be reused after it's captured the nutrients. All these aspects make it highly attractive to develop and implement.”

Professor Cochrane says the solution could work in several ways, one being through the use of drainage pipes.

'A lot of the agricultural area has drains, and we could implement a treatment system at the end of those drains before it goes into a stream.”

The pair previously worked together on an award-winning and patented invention called ‘Storminator' that removes heavy metals, such as zinc, from roof stormwater in urban areas.

'With the Storminator we were trying to remove contaminants that end up in urban streams, focusing on metals and zinc and copper, and so one of the solutions we were exploring was using shells,” says Professor Cochrane.

Associate Professor O'Sullivan says they have demonstrated using waste shells to remove acidity and metals very successfully, but the team hasn't yet explored using them to remove nutrients, which is quite different technology.

'The next step is to see how we can optimise waste seashells to remove nitrate and phosphate and then design and develop modular systems we could deploy in the field specifically for nutrients. We know nitrate is a problem on farms in Canterbury and Waikato - so that's a good first application to validate the testing.”

New Zealand has set ambitious targets for nitrate reduction to be met by 2025 and achieving them will require innovative and regenerative technologies as well as land-use improvements.

The University of Canterbury engineers are developing technology that will enable them to meet the recent freshwater reforms. Associate Professor O'Sullivan says the current alternative systems used to remove nutrients have limitations and sometimes unintended consequences.

'Using waste seashells as a filter would help address these issues. There are constructed wetlands or permeable reactor barriers that target nitrate in agricultural settings, but they're quite expensive to construct, they're limited in terms of how much nitrate they can remove, and their performance is lower in cooler months. We're looking at overcoming those limitations with our design.”

Associate Professor O'Sullivan plans for the filters to be reused at the end of their lifespan – either as a ‘regenerated' filter or being crushed to make an organic fertiliser that can be applied back on the farm