Denitrification beds – a creative approach

An online search of the word ‘scientist’ produces about 127 million results. Interestingly, the same search for images yields variations of a single representation – you know the one. There is no doubt that science involves test tubes and the need to occasionally wear a lab coat, but a career in science also involves imagination and creativity.

Imagination and creativity are an integral part of science

Observations are an important part of research but they require interpretation and inference. Scientists need to make the creative leap from data to possible explanations. They need creativity to come up with new ideas and to design investigations.

For Professor Louis Schipper, one such creative leap came quite early in his scientific career. As a PhD student, Louis studied a small, muddy patch of ground near Whangamatā. Twenty years later, his curiosity about this small riparian wetland has resulted in the testing and implementation of denitrification beds all over the world.

Louis’s PhD project involved the land application of wastewater from a sewage treatment plant. He was trying to figure out how much nitrogen could be sprayed onto the land while avoiding ground and surface water pollution. Overseas research indicated that less nitrogen came out of riparian wetlands than went in. At the time, people didn’t know why wetlands removed nitrogen but they thought it might be due to denitrification – the conversion of nitrate to atmospheric nitrogen gas.

Louis was intrigued by a small riparian wetland about 2 metres by 2 metres in size. Water came out of the hills near Whangamatā through this small patch of wet organic soil, and the denitrification rates were very high. Louis says, “The nitrogen was almost completely removed over 1 or 2 metres. I found that denitrification was the main mechanism for nitrate removal. Organic matter was the reason; dead leaves degrade slowly in the wet, anaerobic environment. There is no oxygen, so it slows the decomposition process. The organic matter also provides food for denitrifying bacteria. I wondered if I could work with riparian areas to improve nitrate removal. I thought the key was a unique set of circumstances: a water-saturated environment, an organic zone and nitrate. What would happen if we used a riparian area where organic matter had been removed or depleted? Could we add sawdust as an organic matter source to the path that groundwater takes and get it working?”

Using sawdust to replicate nature

Upon finishing his PhD, Louis began a new area of research in the US, but the idea of mimicking riparian zones kept bubbling away. When he returned to New Zealand, Louis secured funding to explore this idea. A local dairy factory sprayed effluent onto paddocks. Groundwater with high nitrate levels entered a nearby stream. Louis dug a trench – that he called a denitrification wall – down to the groundwater. He mixed the soil with sawdust and put it back into the trench. This wasn’t a 2 x 2 metre patch of mud but something 40 metres long. “I was nervous about whether it would work, but I thought I’ve got to take a chance and try it out,” he says.

The creative side of science looped back to the empirical aspect. Louis traded his spade for a lab coat. Groundwater samples from wells above and below the trench showed nitrate removal, but was this due to denitrification, like in the riparian wetlands? Louis made measurements of the activity of denitrifying bacteria. He was pretty certain nitrate removal was due to these microorganisms and not the nitrate being stored in the sawdust. Nearly 2 decades later, the system is still working.

Louis and other scientists around the world are researching denitrification walls and other variations such as denitrification beds. This simple technology is used in New Zealand to treat effluent from hothouses. In the US, it’s hoped they can help to reduce the amount of nitrogen going into the Gulf of Mexico and Chesapeake Bay and in California waterways.

For Louis, like most other scientists, creativity is a critical component of research. “It’s cool that I started with a little boggy part of a wetland, covered with mud from head to toe while I was doing the research. Now it’s grown into building beds over 150 metres long.”