April 21, 2011 | General

Composting Essential To City's Waste Diversion (Denmark)

BioCycle April 2011, Vol. 52, No. 4, p. 51
Windrow composting of biosolids, yard trimmings and straw provides a valuable soil amendment to area farms.
Andreas Kunter

ODENSE Environmental Centre (OEC), owned and operated by the City of Odense, Denmark through the Odense Waste Management Company, is an integrated waste management facility that includes composting, sorting and recycling, treatment of oil-contaminated soil and landfilling. OEC started composting yard trimmings from its parks and gardens in 1998. It began composting biosolids from the city’s three sewage treatment plants a year later. Odense has a population of about 190,000 inhabitants.
Finn Andersen is responsible for the windrow composting operation. He uses a computer program to determine the right mixture of biosolids, green waste and straw. “For the calculation, we take into consideration the water content, density, nutrient ratio and even the weather forecast for the first rotting phase,” explains Anderson. In more damp periods, a coarse, shredded material is generally used for bulking, with the mix ratio changed to benefit the drier fractions.
“Straw can make up about six percent by weight of the mix depending on the situation,” adds Anderson. “This allows us to achieve quick starting of the biological activity. We often reach a temperature of 70°C within one day.” After the windrow is constructed with a wheel loader, feedstocks are mixed with a Komptech TOPTURN X67 with side discharge. “Immediately after turning, each windrow is given a ‘hood’ made up of screen residuals for the first two weeks,” he explains. “This performs the function of a biofilter, minimizing odor emissions. We believe we don’t have an odor problem with the neighbors because we take an active approach and adopt an open information policy in regards to what happens on the site. We also conduct all activities in line with weather conditions and keep an accurate time log for this.”
The composting process takes eight weeks, with windrows turned once a week. Andersen monitors the windrows on his computer. Radio sensors in the windrows provide information on temperature and allow corrective action to be taken quickly. Following the cooling phase is a post-maturing phase and then screening and laboratory analysis. All data collected in this time period, from incoming inspection to final analysis log, is stored for every windrow and ensures consistent quality control.

Annual compost production is about 35,000 metric tons, most of which is used as fertilizer for agricultural crops, such as grain. Finished compost is screened to a 1-inch particle size. A small amount is used on golf courses. OEC also makes a green waste only compost for use on gardens.
The Odense Waste Management Company Ltd. operates eight dropoff recycling stations for households and smaller enterprises. Citizens sort material into more than 35 categories, which all have their own container. Bulky waste from industry is processed at a centralized recycling facility. Odense has one of the Denmark’s highest recycling rate (85 percent). Only a small percentage of the city’s waste is landfilled. A significant benefit of Odense’s recycling and composting program is extended landfill life. According to the city’s website, capacity of the landfill at OEC is estimated to be 130 years. The initial estimate was 30 years.

Andreas Kunter is with Komptech GmbH in Austria.

Sign up