Biogas an all-rounder

“Green waste offers immense potential”

Interview with Michael Buchheit, Technical Director of Biokompost Betriebsgesellschaft Donau-Wald mbH

In the Bavarian Forest, 250–270 kilogrammes of organic waste and green waste per inhabitant are collected each year – more than twice the average volume for Germany. This biomass is utilised in eight composting and two fermentation plants. Commissioned by the technology company Sius GmbH, chemical and process engineer Michael Buchheit is responsible for these plants as well as eleven green waste collection points.

Mr Buchheit, in 2004, you integrated a biogas plant into the Passau-Hellersberg composting plant, a pioneering project for the combination of recycling and energy production. What is your experience with the plant?

We try to emulate with technology something that already happens in nature. To give you a simple comparison: before we had a simple composting plant akin to a horse that would eat hay and produce fertiliser. The biogas plant is more like a cow, which not only supplies us with fertiliser, but also with milk. The second product, in our case, is electricity: we generate eleven mil lion kilowatt hours per year, enough to supply 4,000 households. The plant is absolutely reliable, and the fixed feed-in tariff also makes it profitable.

All the organic waste is fermented in a first step. In the past, the capacity of the composting plant was insufficient and organic waste had to be transported elsewhere for utilisation. By adding the fermentation step to the start of the process, we have been able to double the capacity. The operation of the plant ensures that part of the biomass is converted into biogas, while the other part is pre-digested, so to speak.

So the positive experience with the fermentation of organic waste then led to the construction of a second plant in Regen?

Yes, though the Regen plant does not ferment organic household waste, but green waste. It is the first large plant of its type in Germany. First the material must be chopped, and subsequently

sieved to separate out the wood fraction, which is then incinerated in a biomass heating plant. Portions of the material containing fine particles below 15 mm contain a lot of sand, and are therefore composted directly. The rest, around 50–60 mass percent, consists of a herbaceous, grassy material, which is very well-suited to fermentation.

What is your conclusion after three years of operation?

At the beginning there were some teething problems. The plant and our materials management had to be optimised. Since then, things have been perking up: we are already exceeding the goals we have set ourselves. In addition to 14,000 metric tons of processed green waste, we also use 4,000 metric tons of grass and maize silage feedstock per year in order to keep gas production at a constant level. Further to this we have integrated farmers into our project.

For organic waste, the acceptance price has an effect on the profitability of the fermentation plant. This is not the case for green waste. Is the Regen plant still profitable regardless?

Of course it is. First of all, we have a good heating concept: a small village and a hotel with a swimming pool are heated with the waste heat from the biogas CHP plant. Also, the green waste is regarded as material from landscape conservation, which has an effect on the statutory feed-in tariff for the electricity. Without these subsidies, the fermentation of green waste would not be profitable.

You have been working in waste management for 20 years. How do you see the prospects for waste fermentation in the context of the many different processing options?

Fermentation has a great future ahead. Green waste in particular offers immense untapped potential. Hedge and grass cuttings from private gardens, mowed grass from parks, playgrounds and sports fields, cuttings from roadsides, water bodies or biotopes – all of this waste can be used to generate energy! According to estimates, in addition to the current volume of 4.4 million metric tons, another four million metric tons could be mobilised in Germany. As part of our work for Sius GmbH we are already considering the construction of more fermentation plants for green waste.

I strongly believe that combined fermentation and composting is the right way to go, and I would like to see many composting plants being retrofitted with fermentation units. Once an old composting plant has been written off and expensive investments become necessary, the time is ripe for this.