Urban Organic Waste

April 2009

A Plant & Food Research urban compost trial for agricultural crops

A large plant composting municipal organic wastes has been operating in Timaru for three years, taking in around 13 16,000 tonnes of organic waste each year and producing around 8000 tonnes of matured compost, mainly for application onto farm land. Trials by Plant & Food Research with kale, a typical forage brassica crop grown in the region and the greatest acreage of any crop type in NZ, have shown considerable increases in yield and other benefits on the areas treated with compost. This shows that applying compost is one way of improving soil productivity and condition as an adjunct to (but not at replacement for) conventional fertiliser regimes.

A much larger plant with a 65,000 tonne feedstock intake capacity is currently being commissioned at Bromley, Christchurch, and the official opening is on 6th March 2009.

Transpacific Industries Ltd owns and operates the Timaru plant, and Living Earth has designed and will operate the Bromley plant on behalf of the Christchurch City Council.

At both plants:

Incoming material reception and sorting


Composting material, covered at Timaru using a large mechanical cover winder

Loading out heated material

Windrowing for maturing

Screening to produce fine, evenly sized final product

Karina Downs

Plot replicates showing differences between those with and without compost applied.

Overseas research has shown that there can be significant long-term yield and soilquality benefits from the application of compost to agricultural crops. Small scale applications to crops and pasture have been made in New Zealand, but there is not much material available in agricultural quantities.

In 2006 a medium sized plant in Timaru using the Gore Cover compost system was established, and this has produced 8,000 tonnes each year since then. Household organic waste from Timaru city and also rural locations is collected in green bins and delivered to the Redruth site. (The plant was built by Envirowaste in 2006 and was subsequently taken over by Transpacific Industries.)

The material is ground into smaller pieces using a large Crambo shredder that is powerful enough to break down large tree stumps. Water is added to the shredded material if required. A loader then places this into a 50m long windrow 8m wide and 3.8m high. Each windrow contains 750 cu.m, and this reduces in volume by 30% during the composting process.

The windrows are covered with Gore covers using a large cover winding machine. The covers help to speed up the compost process, maintain moisture and aerobic conditions in the windrow, and reduce odours.

The composting site accommodates eight windrows. New material is covered for 4 weeks. The cover is then removed and the windrow is then moved and aerated, and rests for 2 more weeks. This process is then repeated.

Covers weigh nearly a tonne so a custom built cover winder machine is used to allow easy laying and removal of covers. The machine will only operate in calm conditions.

Oxygen levels, temperature and moisture are keys to the composting process. Oxygen and temperature probes keep track of conditions in the windrows. The material needs to be kept at a minimum temperature of 55 deg C for at least 3 days to kill weed seeds and pathogens.

Underneath the windrows are two air channels that blow air into the compost to ensure that the compost process is aerobic and high temperatures are maintained.

After 8-10 weeks, the material is sieved through the Trommel to produce fine, nutrient rich compost.

Non-organic contaminants create problems for the processors. Metal, plastic and glass are all present in small quantities and glass especially is impossible to eliminate entirely. For this reason the final product cant be used on potato crops a great pity because potatoes are nutrient demanding, have planting and harvesting techniques that are typically detrimental to soil quality, and have one of the highest crop gross margins.

The other problem areas are vegetation treated with herbicides e.g. clippings from lawns treated with Versatil to get rid of broadleaf weeds, and ashes from treated timber which is high in arsenic and copper. Each batch of compost has to be tested for heavy metals and any that exceed allowable limits are not sold to the public but can be used as landfill cover or to landscape on top of old landfill areas.

Avoidance of contamination is the main reason that many commercial compost producers use pine bark as the raw material for composting. Unlike the Timaru compost, this material has a low nutritive content and nutrients have to be added.

Karina Downs:

Two South Canterbury trial sites were established in 2007 on kale and ryegrass crops

to investigate the effects of applying different rates of compost. For the kale, rates of 25, 50 and 100 t/ha were topdressed onto the soil forming a layer up to 16mm thick, and for the ryegrass 50 t/ha. Both crops were direct drilled into the soil leaving the compost on top.

The main findings from the first year have been:

Significant increases in dry matter yields of both kale and ryegrass with

increasing rates of compost application.

Some benefits have been measured in soil properties, many of which are expected to increase in subsequent years.

Nutrient concentrations of kale plant matter increased with rate of compost.

Soil nutrient levels increased with rate of compost.

Measurements of feed quality, such as ash and lipid fat content, increased with rate of compost.

Higher earthworm numbers

Increase in water holding capacity of the soil

No negative environmental impacts

Results from the first year are thus very promising, suggesting that agricultural

crops in Canterbury could certainly benefit widely by utilising compost in their rotations.

These trials have clearly shown that a significant increase in crop productivity can be achieved from the application of compost.

In addition there is the potential for a continuation of increased yield responses for many years to come as the remaining compost continues to decompose and release organic matter and nutrients a considerable amount of compost remains on the kale trial plots, despite a year of decomposition since being applied, some mixing with the soil from stock treading, and some minor incorporation from the initial drilling of the kale, recent deep ripping, and earthworm activity. Trials overseas have shown increased yield responses over a period of at least 34 years and up to 7 years from single applications of similar compost.

Further research is required to confirm if similar yield benefits can be achieved in other

crops and to determine effects of the compost on soil and environmental parameters in

different locations. It is proposed that soil and plant measurements will continue on these two trials for a further 2-4 years.

In 2007 Living Earth was awarded a major contract to design, build and operate a $20m Indoor Tunnel system to process 65,000 tonnes/year of kitchen waste and green waste for the Christchurch City Council. The plant opened in March 2009.

The facility operates in a similar sequence to the Timaru plant, but is fully enclosed from reception through to the end of the first stage of composting at which time the material is pathogen and weed free. The compost is then transferred outside for maturation.

Incoming material will include kitchen waste and greenwaste from a new kerbside collection service being provided to Christchurch residents. Finished products including premium quality composts, mulches and mixes, both packaged and in bulk, will be distributed through a variety of channels to farmers, horticulturalists, landscapers and home gardeners.