Inverkan av torrsubstanshalt och temperatur på kompostens syrabildning
Composting is a process in which organic matter is mineralized by microorganisms. In January 2005 landfilling of organic waste was prohibited in Sweden. With this prohibition composting has become an important alternative to handle the produced organic wastes. One problem with continuous small-scale composting is the production of organic acids that will inhibit a further composting process. The aim of these theses was to study what effects temperature and dry matter content had on the production of organics acids, and what effect the organic acids had on the mineralisation of the substrate as well as the stability during storage of the product.
Laboratory experiments were performed, with seven trails where the temperature was set to 37°C, 55°C or 70°C. The rate of drying was varied by letting water either evaporate, or condensate and return to the container, or by adding water. The experiments were started with a mix of starter culture, already existing well-going compost, and substrate made of dog food, bread and apple. During the experiment the same amount of substrate was added to the containers daily. Furthermore, two trails were performed without adding any starter culture; in one of those the substrate was hygienised in 70°C before added to the container.
The highest concentration of organic acids was measured where the dry matter content was low, as this benefited the organic acids producing bacteria and inhibited the aerobic bacteria. When dry matter content was high the concentration of organic acids was low, as the growth of both organic acids producing bacteria and aerobic bacteria was inhibited by the dry environment.
Mineralisation of the substrate was quantified through different analyses. A rising temperature during the first days of the experiment shows the existence of microbial activity. The emission of carbon dioxide was high at first and declined as the concentration of organic acids increased. However, the experiments without starter culture showed the opposite development, where materialization of substrate increased over time. Mineralisation was greatest where temperature was set to 37°C and 55°C, and very low at 70°C.
Although pH was lowest in the containers with 37°C, the stability during storage was very bad, with appearance of insects and growth of mold. Best stability during storage was obtained in the containers with 70°C, where the substrate had the highest dry matter content. Here the matter had the same appearance both before and after storage.
Higher temperatures resulted in higher dry matter content than lower temperatures, and the drying depended on how hard the water was bond to the particles of the substrate. The energy removed with aeration from the containers became higher as temperature rose; however, the fastest drying was obtained at higher temperatures. The amount of energy used to evaporate one gram of water was approximately the same at any temperature.
When working with the organic waste it was apparent that the matter from the containers with 37°C had an acid smell and a good structure. The stability during storage was poor, as the material was still moist it attracted insects and molds grew. At 55°C the material smelled very bad and the structure was poor with large aggregates. The stability during storage was good when the dry matter content reached above 65 %. At 70°C the material had an acid smell and the structure was good as well as was the stability during storage.