Turning food scraps into food production

A climate action enterprise uses bokashi and no-turn composting methods to create carbon-rich, nutrient-dense soil. I talked to City to Farm who are diverting landfill to profitable crops.

Betsy and David Kettle of City to Farm collect food scraps from commercial kitchens in Auckland and ferment them in bins by the Bokashi method. Bokashi is an anaerobic method of reducing food-scraps by fermenting rather than rotting, which generates microorganisms to break the scraps down faster. Regenerative farmers Phil and Jenny Grainger compost the results to grow organic bananas and pumpkins. It’s a win-win for them and a big win for the environment.

“How we grow food and dispose of food-waste is a huge contributor to climate change,” says Betsy.

More than 150 tonnes of food scraps have been diverted from landfill into high-value food productions on the Grainger’s re-hectare farm at Waitoki, Rodney, simultaneously preventing greenhouse gases escaping into the atmosphere and sequestering carbon into the soil.

Sourcing scraps

City to Farm meets with commercial kitchens to assess the quality and quantity of scrap output. Raw dough, liquids, fats, oils, large bones, compostable serve-ware, plastics and bioplastics are off the menu.

City to Farm supplies 15L buckets or large wheelie bins with a layer of biochar and mulch in the bottom. The kitchens layer the food scraps with a sprinkle of Zing, a microbial inoculant to create a bokashi fermentation system.

The woody mulch keeps the scraps out of the liquid generated by the fermenting process which drains to the bottom of the bin or bucket, reducing the odours.

Scraps to Soil

The scraps are collected weekly and held in bins to ferment for two weeks before being spread between layers of biochar and woody mulch into swales on the Grainger’s land. Swales are landforms with a ditch and bund (Hump) at the edge of a terrace that catches and traps rainwater so it soaks into the ground without running off.

The bokashi and mulch begin to compost, with the layer of biochar beneath being critical to the process. Biochar is a charcoal of untreated timber or plant matter created by ab oxygen-excluded burning process known as pyrolysis.

Biochar absorbs water and nutrients and improves soil structure and drainage. Once inoculated with microorganisms and charged with minerals, biochar is thought to be a permanent form of compost in the soil and is often used as a mode of carbon-fixing. It also tends to be alkaline, offsetting the acidic bokashi process.

Soil vs Compost

Phil Grainger is a member of the Kaipara Regenerative Farming Group and explains the process has multiple benefits.

“Food scraps require low inputs so chemical costs don’t exist.

“Combining anaerobically bokashi-fermented food-scraps with biochar and placing it in an aerated system kick-starts the microbial diversity necessary to begin rapid plant growth, and as a result, soil transformation occurs. The rapid soil transformation creates an extremely stable form of carbon and establishes diverse fungal species, including arbuscular mycorrhizal fungi.

“The combination of swales, improved water filtration, and groundwater storage enables the bananas to be grown with minimal irrigation, and the enhanced biology of the soil and ability of the fungi to unlock the minerals which feed the plants, which the cows then eat, is all symbiotic. You can afford to run fewer cattle, and the land is more drought resistant. We’ve also used the Johnson Su and SPIC compost systems to inoculate the land.”

Conventional compost-making laws

When looking to make conventional compost on a commercial scale, resource consent could be a limiting factor. In Auckland in a Rural Production Zone, on-site composting is limited to 50m³ at one time. Amounts beyond 50m³ require an Assessment of Environmental Effects (AEE) due to potential overflows and odours.

David Kettle says that City to Farm moves their compost through constantly creating an enriched mulch that worms work into the soil. The food scraps are completely broken down after six to ten weeks and the swales can then be reloaded. Bananas grow on the edge of the bund drawing some of their nutrition from the swale. The swale compost is never turned, thus enabling carbon sequestration.

Costs and benefits

“Keeping food-scraps out of landfill,” says Betsy, “and using them to build topsoil, creates a carbon sink that improves the soil and enables horticultural crops instead of pine trees and cattle.”

Vermin on the farm is controlled by bait stations and traps, but Phil and Jenny say it’s no worse than on other farms.

The City to Farm enterprise has been assessed carbon-neutral by Massey University and will be carbon-negative once the banana crop is sold, as it will replace imported bananas.

Phil Grainger adds that it makes economic as well as environmental sense. “I can run one cow on a hectare which will return between $500-$1000, but the 0.2ha on which I currently have 150 banana plants with a theoretical crop of 2,250kgs will sell at the market for $5/kg or wholesale for $2/kg, resulting in an income between $22,000 and $56,000 per hectare.

“We still need to determine if the banana sheltering will be sufficient to produce that crop. In the last five years we have seen a fixed predominant SW wind become more NW and NE and have had to adjust shelter belts accordingly, but the plants are growing well. After Cyclone Gabrielle the bananas were fine because they originate in cyclone-prone environments. The leaves are shredded but as soon as the wind stops a new leaf appears.”

Other crops can be grown using this system, but it is especially suited to bananas as they are luxury feeders requiring lots of nutrients, and new Zealanders love them, eating the most bananas per capita in the world. And unlike imported bananas which have to be fumigated, these bananas are organic.

Phil has also noticed a substantial improvement to the water flow. “Food-scraps have changed the soil structure which started as hard clay and is now more porous with 25-30 times the infiltration rate, and with far more groundwater storage on previously hydrophobic land. The little stream that dried up all the time now flows all year round.”

City to Farm currently pays $300 for 12m³ of untreated wood mulch from a local mill and $200 per 1.5m³ of raw untreated biochar. Storage, on-farm fermenting, and maintenance of the swales are covered by the Graingers, while collection costs are borne by City to Farm. However, it is expected that as the climate-challenging enterprise grows, some costs will be handled differently.

Limits to expansion

Betsy and Dave seek more carbon farmers to work with but have yet to exhaust the capacity of the swales on the Grainger’s farm.

“we can see farmers in the future getting carbon credits and becoming the champions of climate action,” says Betsy. “Our biggest limitation really is the small number of people who understand the value of growing soil carbon and are willing to support the system in these early stages.”

She says banning food-scraps from landfill has been talked about by government but hasn’t yet been implemented.

Currently food-scrap contributors to City to Farm include retirement villages, cafes, brewers, and early childcare centres and schools which incorporate the project as part of their education.



© Theresa Sjoquist – January 2023

First published in Organic NZ Magazine – May/Jun 2023 – Vol.82 No.3