What is the problem?

Low concentrations of nitrogen and phosphorus occur naturally in waterways but too much causes the ecosystem to tip off balance. Nuisance algae and aquatic weeds can proliferate which smothers habitat for native plants, insects and fish. In extreme cases excessive plant growth can cause oxygen levels to fluctuate and cause sensitive fish and insect species to die.  

Networks of drainage ditches and sub-surface tile drains have been constructed in many lowland areas to convert wetlands into pasture suitable for farming. An unintended consequence of these drains is that they also channel excessive nutrients (such as nitrogen and phosphorus) from fertilisers and animal waste directly to waterways, rivers, lakes and estuaries, bypassing any riparian planting that might otherwise filter some of these contaminants. It’s a bit like an IV-line straight into your vein.  

In order to intercept the nutrients from tile-drains, Living Water wanted to trial some bio-filters. There are a few different types and set-ups for bio-filters and Living Water wanted to trial something that was simple and effective, so we teamed up with NIWA and Dairy NZ to trial two nutrient mitigation filters in the Waituna catchment because it’s an area where tile drains (sub-surface drains) are predominantly used. 

Who could use this information?

Landowners, Regional Councils, Land Managers, Catchment managers. 

What was done?

The filters were installed in 2016 and monitored over 3 years. 

The nitrogen filter was a 10m x 10m x 1m deep lined pit filled with wood chips. Under base flow conditions, the filters were able to reduce the concentration of nitrate-nitrogen by about 88% and by about 55% for total nitrogen. Due to anaerobic conditions the nitrogen filter created, there was an increase in the concentration of ammoniacal nitrogen. This has the potential to be toxic to aquatic organisms so further refinement of the filter construction needs to be considered.  The load reduction for nitrogen was modelled and estimated to reduce the amount of nitrate-nitrogen and total nitrogen by 94% and 49% respectively. This means that the woodchip biofilter went some way to filter out nitrogen from the tile drains but the different chemical reactions that occur at different temperatures, flows and oxygen levels mean that it might not be a reliable method for removing nitrogen all of the time.   

The phosphorus filter was a 1m3 above-ground polythene crate containing Aqual P (modified zeolite). Water from a tile drain was pumped into the filter from a catchment area of about 1 ha. It was estimated that the filter could reduce the concentration of Dissolved Reactive Phosphorus (DRP) and Total Phosphorus (TP) by about 99% and 80% respectively. However, seasonal effects combined with operational difficulties made it difficult to understand the volume of water and therefore load of phosphorus entering the filter and total load reductions were not able to be accurately determined. This was because the filter was likely to be less effective when the flow was higher. When flows are higher, there is also likely to be higher phosphorus concentrations, so while Aqual P was able to reduce the concentration of phosphorus in stable flow, setting up the filter to be effective in a real-life situation with variable flows is far more difficult.  

The laboratory assessment of the Aqual-P recovered from the filter bed suggests that after the year-long trial, approximately 32% of the sorption capacity remains, with greater capacity in the lower third of the bed than the upper material. 

Failure to ensure that the hydraulic loading rate was maintained at the required level meant that the performance of the filter material could not be assessed as required.  

What did Living Water learn from this trial? 

• Overall this study has shown that N and P filers have some potential to be part of a solution to reduce the impacts of nutrient leaching from dairy farming.  
• However, there were considerable difficulties with the set-up and monitoring of the trial that impacts the confidence we have in the results.  
• Living Water supported this trial on the basis that the potential solution would be ‘easy and practical’ for farmers to implement.  
• The lesson Living Water has learnt is the importance of keeping research real and practical.