Using qualitative indications for the effects of the wastewater mixing, a mathematical optimization model was developed for the optimal design (necessary reactors number and sizes) and operation (feeding pattern) of the centralized codigestion plant. Software was developed incorporating the full mathematical model. This is a handy tool to aid the designer of centralised codigestion plants.
A new Bioreactor concept, the Periodic Anaerobic baffled Reactor, based on the anaerobic baffled reactor has been conceived, developed and tested. The idea is to have an annular bioreactor, with the space in the annular region separated in upflow and downflow compartments. The reactor has the additional advantage that it is best suited for energetically efficient operation, since its cylindrical shape leads to the least possible heat losses, whereas the inner tube may be used as a heat exchanger. A lab-scale PABR was designed based on available mathematical models for anaerobic digestion.
Special batch and UASB experiments were carried out, with dairy wastewater in the feed, so that a comparison between suspended growth and granular systems could be made for high organic strength wastewaters. It was found that little is to be gained with using a UASB reactor for a high COD wastewater. Consequently, a CSTR type digester rather than a UASB (or a PABR) should be used at the pilot scale. In order to properly design experiments at the pilot scale, a version of the dynamic model developed earlier was used in order to simulate codigestion performance under steady-state and transition conditions, between one mixture and another. Assuming a fixed amount of piggery wastewater fed, CSTR simulations show an increased efficiency as dairy and olive-mill wastewater are codigested. Dynamic simulations of the CSTR show that for low values of the dilution rate (high HRT) following an initial adaptation period, a new stable steady-state operation is reached, and the digester is capable to withstand the imposed feed changes. For high values of the dilution rate, failure is predicted unless a reservoir is used.
The required systems as determined earlier were designed in pilot-scale. The designed pilot scale plants were constructed. The pilot-scale plants (in Denmark at Beauvais and in Greece at ICEHT) were operated for a period of time. The criteria for codigestion from the experiments in the other tasks were used to ensure a stable and effective treatment of the seasonal wastewaters. It was possible to make a start-up of both pilot plants within a short period of time without any significant problems. The UASB system was very stable, ie the COD reduction did not change even though the OLR changed significantly. The data indicate that the start-up procedure could be even quicker because it seems that the UASB-system can withstand a much higher OLR, without this having any influence on the overall performance of the system. The CSTR pilot-plant verified secure codigestion of piggery with olive-mill and dairy wastewaters.
Food processing industry is generating wastewaters with a relatively high organic content in a seasonal manner. These wastewaters can essentially only be treated through anaerobic digestion. The seasonal nature of the wastewater, coupled with the frequently small company size, suggests that anaerobic codigestion of multiple seasonal source wastewaters should be substantially more economical and effective.
The proposed work will examine the characteristics of seasonal produced wastewaters in a Greek region and those of a food processing industry receiving different raw materials around the year in Denmark, will investigate the codigestibility of these wastes from a kinetic and a microbial point of view (developing appropriate criteria), will determine the optimal planning and scheduling for digestion processes, will investigate alternative digester types, configurations and designs (especially as far as mechanical and control aspects are concerned) for flexibility, will develop overall processes that exploit fully the byproducts of anaerobic digestion (by appropriate re-use and recycle) and will build, test and optimise two pilot scale operations, one in Greece and one in Denmark.
Funding SchemeCSC - Cost-sharing contracts
SW7 2BY London