Established in 2001, Northland Forestry Nursery Limited (NFN) supplies quality radiata pine seedlings to the Northland Region. They are a preferred supplier for all the major forestry companies in Northland and are also recognised for quality product outside the region. Currently operating from two sites in Kaikohe, NFN produces up to 13 million seedlings per year, with approximately 20ha of productive land on the main site used solely for an annual crop of bare rooted pine tree seedlings. During abnormally high wet periods throughout the first half of 2023, NFN were experiencing difficulties in achieving compliant discharge standards in relation to the clarity of rainfall leaving site. NFN already had an existing series of sediment ponds, constructed by KCL Civil Construction to GD05 specifications, to assist with the retention of sediment and soil on site during periods of heavy rain, but extreme conditions meant their current measures were proving inadequate. Heavy solids were dropping out, but much of the suspended solids still remained within the water as it entered the receiving environment.
DESIGN
To better understand what was required to achieve compliant water discharge, NFN sent soil samples to Cirtex’s GeoCert Laboratory in Thames. The tests were conducted using the industry standard bench testing methodology as detailed in appendix 1 of the Auckland GD05 Erosion & Sediment Control Guidelines. Incremental amounts of Polyaluminium Chloride were added to a range of test samples. The chemical charges the particles within the water, attracting them to each other so they connect and form larger colloids that gain enough weight to drop out and settle on the pond floor. The testing process ascertained the correct dose rate of chemical needed to reach a minimum requirement of 100mm of visual clarity within the discharged water.
That data was used to implement a full Chemical Treatment Management Plan for the site. Four ponds were designed to cover the 20-hectare site, each receiving runoff from 5-hectare blocks. PortaFloc units were set up as per the Chemical Treatment Management Plan. With dosing directly driven by rainfall displacement and directly related to actual precipitation across the site, simple adjustment of the PortaFloc catchment trays means dose rates can be changed as and when required, depending on ground cover and existing soil moisture levels.
With ever changing weather, ground cover conditions and the nature of their nursery, it was important that NFN had the ability to carry out their own bench testing to ensure they were achieving optimal performance at all times during the growing cycles across the nursery. Cirtex provided the training and support to make this possible.
INSTALLATION
Northland Forest Nursery installed a Large PortaFloc Unit on each SRP, designed to add a PAC coagulant at a prescribed dose rate during significant rainfall events. By utilizing elevated IBCs, NFN can refill the Portfloc units when necessary without the need for either automatic or manual pumps.
As the nature of the site changes, the catchment trays on the PortaFloc units can easily be adjusted to either increase or reduce dosing requirements as required. Set-up for each unit takes less than an hour and ongoing maintenance and routine checks are extremely easy and quick. To further assist with sediment control, Hydroseeding of all drainage in field perimeters and swale drains is also currently being implemented under Cirtex’s recommendation. Since the installation, NFN has been achieving compliance requirements, allowing them more time to focus on producing seedlings in a sustainable way while still meeting customer demand.
The PortaFloc chemical dosing system provided these benefits and features:
- Easy to set up and install
- Minimal maintenance required
- Accurate and easy-to-adjust dosing rates
- Ensures clean water discharge compliance from site
- Re-usable system
