December 22, 2010 | General

Composting Garden Waste On Innovative Pad (Denmark)

BioCycle December 2010, Vol. 51, No. 12, p. 56
Facility in Roskilde processes more than 220,000 tons/year of green waste using a compost pad made from interconnected concrete pavers.
Craig Coker and Martin Wittrup Hansen

ROSKILDE, Denmark, the oldest city in Northern Europe, lies about 35 km (22 miles) west of Copenhagen, the Danish capital, and is the ancient capital of this southern Nordic country. During the Viking time around 1000 A.D., the people of Roskilde decided to sink a number of their ships in the fjord at Skuldelev in order to prevent the Vikings from coming in and raiding (a successful tactic). Some 986 years later, Christian B.S. Christensen and a partner opened a composting facility in Roskilde. Their company, Solum Gruppen, started life as a nursery and landscape contractor. Christensen found that mixing compost with peat was a good soil amendment for his nursery plants, so they started the composting facility in 1986 to handle garden waste from the city’s population of about 50,000.
Green waste is collected in and around Roskilde by both the municipality and private businesses. All suppliers with a Value-Added Tax (VAT) number can deliver material to the facility. The composting facility processes high-moisture garden waste while a 660 tons/day waste-to-energy (WTE) plant handles brushy and dry woody materials. Solum uses an open-air static pile composting method. In addition to green waste, it has started processing construction debris. Woody debris goes into either composting or to the WTE plant (depending on particle size), while concrete is ground into aggregate. Roskilde takes in 220,000 tons/yr of green waste and 77,000 tons/yr of construction debris.
Incoming green waste is ground using a Doppstadt slow-speed shredder. Front-end loaders form material into piles about 325-feet long by 20- to 26-feet high. Piles are watered periodically to maintain moisture at optimum levels. The process takes about one year, during which temperatures are monitored frequently, routinely reaching 60°C (140°F). When finished, compost is screened to one-quarter to three-eighth inch sizes using Doppstadt trommel screens. The primary markets are nearby agriculture (as a soil amendment), golf courses and soccer stadiums (for turfgrass). Three customized compost-sand blends are made using sand quarried from onsite sources.

Unique Compost Pad
One unique aspect of the Roskilde composting facility is that the compost pad is made out of concrete paving blocks. These pavers, which are common in Denmark, were chosen over a poured concrete slab because the blocks lock together and are easier to repair. An asphalt pad was not considered because of the risk of polyaromatic hydrocarbon (PAH) contamination of the groundwater. (Denmark is a very low-lying country and groundwater resources are not very deep.)
The pad was constructed as a connected net of pavers; fine gravel was used to fill the spaces in between. The surface – with a 2 percent slope – is considered impermeable. Runoff from the paver pad is collected in open channels and flows to sand-filled capture vessels invented by Solum.
Since it began in 1986, Solum Gruppen has grown to now include three composting facilities handling more than 330,700 tons/year, making it the largest composter in Denmark. One of the facilities (in Holbæk) also handles biowaste (source separated kitchen organics). It has a percolating reactor anaerobic digester on the front end, which generates biogas for electricity. A similar facility is operating in Elverum, Norway.

Craig Coker is a Contributing Editor to BioCycle and a Principal in the firm of Coker Composting & Consulting near Roanoke, Virginia ( Martin Wittrup Hansen is the Project Development Manager for Solum Gruppen in Hedehusene, Denmark (

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