Nitrogen and Phosphorous Filter Trials

Research & Monitoring – With Descriptor

About the project

Living Water has contributed to a DairyNZ project involving the installation of two nutrient mitigation filters. NIWA (National Institute of Water and Atmospheric Research) has installed the filters – one for Nitrogen and one for Phosphorous – to assess their ability to remove nutrients from water flowing through farm land dominated by tile drainage. The goal is to assess filter performance and provide certainty about the efficiency and cost-effectiveness of these solutions.

We invested in this project because of its potential to provide a practical solution for farmers to reduce their impact on the environment and improve the quality of the water flowing into Waituna Lagoon.


  • Reduced contaminants entering waterways


  • Nitrogen and Phosphorous filters installed 2016
  • Monitoring completed 2017
  • Final report on nitrogen removal completed April 2019
  • Final report on phosphorus removal completed July 2019
Cain Duncan

Cain Duncan

Fonterra Sustainable Dairying Advisor, Waituna
N & P filter

Project Implementation

Started in 2016, completed 2019

Progress: 100%


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. Furthermore, Living Water supported this trial on the basis that the potential solution would be ‘easy and practical’ for farmers to implement. As a third-party funder of this work, we were not able to influence or be involved in this project as much as we had initially envisioned. The lesson Living Water has learnt through this contract is the importance of keeping research real and practical.

The nitrogen filter was a 10m x 10m x 1m deep membrane lined pit filled with wood chip. 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. Because of the anaerobic conditions in the filter bed there was an increase in the concentration of ammoniacal nitrogen caused by the filter bed. 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.

  • The phosphorus filter was a 1m3 above-ground polythene crate containing Aqual P, a modified zeolite. Water from a tile drain was pumped into the filter from a catchment area of about 1 ha between May 2016 and August 2018. Although seasonal effects combined with operational difficulties made it difficult to understand the volume of water (and therefore load of phosphorus) entering the filter, 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. Load reductions were not able to be accurately determined.
  • 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.
  • NIWA and the Project Manager acknowledge that failure to ensure that the hydraulic loading rate was maintained at the required level did not meet the contract conditions, which in turn did not allow the performance of the filter material to be assessed as required. It will be necessary to repeat the trial to quantify the load reduction of P from the filter.

Nitrogen and Phosphorus filter trial documents