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bioplasticsMAGAZINE_0902

Basics Industrial

Basics Industrial Picture 1: Household biowaste as delivered to the composting plant Picture 2: Manual sorting station MT - When talking about end-of-life recovery options ‘industrial composting’ is a term that is mentioned again and again. Now, what exactly is ‘industrial composting’ and how do the operators of such industrial composting facilities feel about biodegradable plastics in their input-streams? In order to find some answers to these questions bioplastics MAGAZINE visited two industrial composting plants and spoke to the operators. One of the plants is located in Moerdijk, in the Netherlands. It has an annual capacity of 100,000 tonnes and is one of 22 facilities in the Netherlands for household biowaste. The total capacity of the more than 100 plants for composting biowaste from households and public gardens is 2.5 million tonnes/a in the Netherlands. The other composting plant that we visited is in Dortmund, Germany, and has an annual capacity of 24,000 tonnes. First of all it should be mentioned that for example in the Netherlands about 50% of all municipal waste is biogenic material, as Tim Brethouwer of the Essent Milieu composting plant in Moerdijk points out. In practice about 35 % is collected separately (1.5 million tonnes of household biowaste). In a large number of countries this biogenic waste, consisting of lawn cuttings and leaves, as well as kitchen waste such as potato peelings or in some cases even food leftovers, is collected and transported to composting facilities. Picture 1 shows a pile of such biowaste. “You see, even if people are asked to put their biowaste only in biodegradable bags there are a lot of conventional bags in the heap”, says Ludger Lammers of the Dortmund composting plant. Tim Brethouwer explains that the incoming waste consists of 60% water, 20% sand or soil and 20% of biodegradable material (plus the plastic, metal cans and other impurities). In both plants that were visited, this so called bio-waste is screened, as a first stage, using an 80mm (or, as in Moerdijk, 150 mm) sieve. The smaller fraction, after passing a metal detector, can go to the next stage right away. In Dortmund the bigger parts, for example plastic bags filled with biowaste, go to a manual sorting station. The staff in Dortmund are well trained, as Mr. Lammers explains, so they can recognize the ‘Seedling’ logo (picture 2). Such bags can pass to the next stage. All other bags are torn apart, the biowaste is fed into the system and the empty bags dumped in a bin to be compressed and taken for incineration. Mr. Lammers emphasized that the staff in Dortmund are well educated and there is not a high staff turnover. “Our people work on the manual sorting today and 34 bioplastics MAGAZINE [02/09] Vol. 4

Basics Composting tomorrow they drive the wheel loader. Then perhaps they work in the office, helping customers. The wheel loader (picture 3) is equipped with a perfect air conditioning, particle filter, noise reduction and even a CD player. “30% of the investment in such a wheel loader is for the safety of our workers,” he proudly points out. Picture 3: Wheel loader Next, the sieved and sorted biowaste is tumbled in a huge drum for about an hour to homogenize the waste in terms of size and moisture. After this stage the material is left in the rotting hall (picture 4). While in Moerdijk the rotting phase is about 3 weeks, the Dortmund facility allows 6 weeks for biodegradation. As this kind of composting is also called aerobic biodegradation, it is important to constantly aerate the windrows. In the Dutch plant fresh air is blown through the windrows through perforated pipes in a bed of stones while in Germany air is sucked from the top through a multitude of holes in the concrete floor. In order to assist good aeration and to constantly mix the maturing compost in Dortmund a big screw (Picture 5) moves down the rows of material, turning them as it goes. In Moerdijk a big turningwheel is used for homogenizing and mixing. The necessary temperature of about 58-60°C is reached automatically by the action of the microorganisms when assimilationg the biowaste as a food or energy source, thereby producing CO 2 , water and biomass. The flow of air is used to control the temperature. Tim Brethouwer said that the heat in the piles could sometimes reach 90°C, so that by blowing more or less air through the compost the temperature is kept constant at around 60°C. This temperature is enough to kill pathogens as well as weed seeds, as Ludger Lammers pointed out. In both plants the fetid air is led though a big biofilter (picture 6), consisting of a thick bed of special burl wood that has been mechanically treated to become fibrous. The air exiting this biofilter is clean and no longer smells. The humidity in the windrows is also measured and controlled by irrigating the whole system with a mix from collected rainwater, the water that drains through the perforated floor, and the water that comes from the biowaste. “All natural, or a complete closed-loop,” says Ludger Lammers. Tim Brethouwer explains that about 120 m³ of water is extracted from the biowaste per day. And around 4-4.5 tonnes per hour of water is sprayed back onto the windrows. Picture 4: Rotting hall Picture 5: Aeration screw bioplastics MAGAZINE [02/09] Vol. 4 35

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