112 Manure treatment makes it possible to close mineral, heat, CO2 and water cycles in combination with the cultivation of algae. This is already possible with a combination of existing techniques, but the digestion of the thick fraction prior to fermentation is new. As a result of this, about fifty per cent more biogas is extracted from the manure at comparatively low costs. From a technical and financial perspective, the manure treatment process can best take place at a central location, with several companies supplying upwards from 30,000 tonnes of manure. A decentralized approach is also possible for certain parts of the process, particularly manure pre-treatment and inverted osmosis. This, however, leads to higher costs and a longer payback time. Note, incidentally, that to clean the air in the stalls by means of algae pre-cultivation, the decentralized approach is not an option but a must. Algae cultivation The cultivation of algae traditionally takes place in large open ponds of a quarter metre deep. Water and algae are kept in motion using a paddle wheel. In the Dutch climate the production volume is about 25 tonnes of dry matter per hectare per year. The algae cultivation company Ingrepro in Borculo has developed a hybrid variant specially for ECOFERM!. With this method, the algae are initially cultivated in a closed photo bioreactor before being fed as a host flow to the open algae pond. Warm air in stalls containing CO2, water vapour and ammonia is used at decentralized level to heat the photo bioreactor and provide it with nutrients, so that the algae can be rapidly pre-cultivated. Any surplus heat in the summer can be used to heat a local algae pond (which is either open or covered with transparent foil). This also helps to increase the production somewhat. At a central location (where the fermentation/CHP facility is located) both nutrients and heat are available from the pigs as well as from the gas engine. Here, the algae can be put out and cultivated on a large scale. The advantage of the hybrid variant is that the large host flow of fresh and pure algae prevents the open pond from being polluted with other undesirable types of algae. The experiments show that the hybrid system is an extremely attractive way of increasing the production levels of algae ponds. Productivity increases of 55 to 65% can be achieved compared to – heated or unheated – algae ponds supplied with extra CO2. Compared to commonly used algae ponds, the yield per hectare is even doubled. Extrapolating small-scale experiments over a limited period of the year to a production volume per hectare over a full year is not simple. However, the results seem to suggest that a yield of 40-60 tonnes of dry matter per hectare should be feasible. In practical terms the expected yield is between 42 and 53 tonnes/ha/year. For comparison: the yields for maize and sugar beet are, respectively, about 12 tonnes and 16 tonnes per hectare. So the yield for algae is many times greater than for ‘classic’ crops. Pagina 120
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