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All about our bioreactors


Our amazing volunteers have been working hard to shift mulch from the Te Hōnonga a Iwi Restoring Rosedale Park site into our bioreactors.


So let’s find out why these simple but incredibly effective pieces of equipment are so integral to the work we are doing to improve soil health on the site.


A bioreactor is a method of aerobically composting organic matter which is suitable when you have plenty of time available, but not much scope for intensive management, says Matt Cummings from Untangled Landscapes.


“We're using them to create an appropriate habitat for all the microorganisms we need to cultivate to kickstart the soil's natural process of becoming a medium capable of supporting native forest.”


The organic material will spend at least a year in the bioreactor (although the compost will keep improving for up to two years). For the process to work properly, the compost needs a habitat that’s aerobic and about 60 per cent moisture. The design of the bioreactors facilitates this by allowing airflow and drainage to all parts of the compost, Matt says.


The main benefit of the bioreactors over other composting methods is that they don’t need much management once they are set up.


“There are also diversity benefits for the microorganism community by having a system that doesn't require disturbance - turning - for aeration.”


“And we can compost using the reactors without worrying about our recipe. ‘Traditional’ thermophilic composting requires a finely tuned ratio of ingredients to produce heat without losing texture, if you care about the organisms you're growing. The bioreactors were originally designed to deal with manure lagoons on dairy farms in the US, and their design plus the extended time frame allows us to create good aerobic compost regardless of the ingredients used. This is useful at Rosedale where we have an abundance of wood and leaf chip but no manure, or high-nitrogen ingredients.”




Our 20 bioreactors have been built with capable assistance from local secondary school students and each reactor holds approximately two cubic metres of uncompacted feedstock. The organic material will be ready for next year’s planting season – this year we will use compost donated to the project by Untangled Landscapes.


“Biology is great because it self-replicates in the right environment,” Matt says. “So actually our 20 bioreactors would be sufficient to inoculate a farm the size of the entire North Shore if certain techniques were employed to mobilise the organisms in water. Unfortunately, most of them would die currently because the prevailing soil conditions do not allow them to make themselves at home (low organic matter, compacted, agrichemical residues). That’s why we will use cover plants at Rosedale to look after and provide energy for the organisms as they transform the soil on the site.


“A one-time application of compost with cover plants, followed by keeping the site devoid of agrichemicals should enable things to start sorting themselves out, and we'll be checking on things regularly with the microscope to make sure.”


Matt says planting methods may be revised for future projects, depending on the outcomes of this pilot project. Therefore the amount of compost needed for future sites could vary.

But for the next 12 months, our bioreactors will be quietly working their magic growing a richly diverse microbial ecosystem, ready to go out and build our soils.





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