Rising to the cardboard composting challenge
Adding cardboard to your composting regime is not for the faint-hearted, but one firm has proved that with careful planning and investment, it can be done successfully. Maxine Perella reportsCardboard is a tricky material to process when it comes to composting. It is hard to break down, and its dry nature doesn't bode well for moisture retention. It can also be a heavily contaminated waste stream - tape, plastic film and bubble wrap are often attached to cardboard packaging - and the ubiquitous nature of tetra pak, with its fusion of polyethylene, aluminium foil and paper, is especially difficult to handle.
However, one company has managed to overcome these obstacles to turn over a quality product from the material. Agrivert's Showell Farm site in Oxfordshire is taking 50-60 tonnes of cardboard a day, sourced from commingled collections in the Bristol area, and mixing it with green waste collections from three councils in nearby regions - the county of Oxfordshire plus the districts of West Oxfordshire and Cherwell.
The open windrow site, which is primarily an on-farm operation, has been operating for five years and converted to take cardboard two-and-a-half years ago. Harry Waters, sales & marketing director for Agrivert, says that proper investment needs to be made if a composting site is to process cardboard efficiently and safely.
"Cardboard is a difficult material and no-one really wants to handle it. The biggest issue is contamination - there needs to be pre-picking before shredding and we employ two permanent litter pickers on site to do this. The cardboard comes in contaminated with a lot of plastic and this needs to be removed as much as possible before it goes into the shredder," he explains.
Plastic can be problematic
Waters adds that around two tonnes of plastic a week is shifted out from the composting process by a windshifter on the screen - that's in addition to what's already been removed from the pre-picking stage. Another element which must be factored into the equation is wind factor. Due to its light nature, cardboard can be easily blown away if the wind picks up and so once it is mixed in with the green waste and shredded, thatch or horse manure is mixed into the piles to keep it fixed.
Cardboard must also contain adequate moisture to be broken down, and the piles are constantly re-wetted by water stored in underground tanks beneath the site. The use of a side turner is also important to consistently reblend, aerate and break up the cardboard. Agrivert employs trapezoidal windrow turners as with cardboard, it exposes less surface area which reduces odour and also the risk of wind blow.
Once the piles have matured, it's onto the screening process. "The main challenge is to screen it to a point where you get a marketable product," says Waters. "A lot of the material will have to go through the process two or three times, which can take between 10 to 36 weeks. But in terms of nutrients, the product we produce is almost identical to green waste."
However, he acknowledges: "There are industry concerns over the inks in some cardboard dyes which may pose a risk in terms of heavy metals but so far, we've seen no evidence of this through our testing procedures. But it is something we definitely keep our eye on."
Agrivert produces a 40mm coarse screen fraction from its green/cardboard mix - "farmers like this as they can get a slow release of nutrients from it," explains Waters - but this can also be screened further for a finer, bagged product. Some 90% of the composted product goes back onto the local farm and Waters reckons that the farmer saves about £33,000 a year in fertiliser costs from this. The farmer also benefits from the organic matter in the soil and gets higher yields from his crops as a result.
As for the remaining 10%, Waters says that Agrivert has established a network of local landscape gardeners to sell to, who tend to take 20 tonnes at a time and will pay £20-25 per tonne for it. The company also sells to turf growers who take 1,000 tonnes at a time and will pay £5-8 per tonne. Meanwhile, the farmer pays about £2 a tonne.
"At least we gat a token value for it, which is very important. Often, it is just given away," points out Waters.
Despite it being a complex and costly operation, the site does pay for itself through the higher gate fees charged for cardboard - between £28-38 per tonne, compared to £18-28 for green waste. It is also running at capacity, operating 5.5 days a week producing around 16,000tpa of compost.
As Waters points out: "We've proved you can operate a successful operation and produce a quality product at the end of it. But it is labour-intensive and you do need to put the investment in - it's certainly not for the faint-hearted."
Maxine Perella is editor of LAWR