Search

Municipal food waste is a unique source of biomass but with great variances in occurrence and properties. Economic, cultural and biophysical factors influence its composition. Therefore, investigation of novel applications requires experiments with real materials obtained from industrial processings, rather than tests with self-composed waste samples having a highly controlled composition. In this study, cultivation of the heterotrophic algae Chlorella sorokiniana on enzymatically hydrolyzed food waste was investigated. Municipal food waste of industrial origin hydrolyzed in a 1000-l stirred tank reactor led to an increase in glucose content from approximately 10 g l−1 to 50 g l−1 within 6 h. Chlorella sorokiniana grown on this medium exhibited an optical density of 32 corresponding to algae biomass concentration of approx. 9.5 g l−1 at a glucose concentration of 20 g l−1. The maximum biomass concentration was reached within four days under significant consumption of carbon and nitrogen. Approximately 30% of the carbon and 25% of the nitrogen were used, whereby glucose and acetic acid were the main carbon compounds consumed by C. sorokiniana in a diauxic manner. Other molecules like lactic acid were not metabolized, suggesting their conversion to biogas in a cascade approach. Nutrients present in the cultivation medium of C. sorokiniana are beneficial for subsequent anaerobic digestion. With a protein content of 37.8% and a lipid content of 26.4%, the composition of algae biomass was comparable to biomass grown on synthetic and other waste derived media.

Anaerobic co-digestion of organic wastes from households, slaughterhouses and meat processing industries was optimised in a half technical scale plant. The plant was operated for 130 days using two different substrates under organic loading rates of 10 and 12 kgCOD.m−3.d−1. Since the substrates were rich in fat and protein components (TKN: 12 g.kg−1) the treatment was challenging. The process was monitored on-line and in the laboratory. It was demonstrated that an intensive and stable co-digestion of partly hydrolysed organic waste and protein rich slaughterhouse waste can be achieved in the balance of inconsistent pH and buffering NH4-N. In the first experimental period the reduction of the substrate COD was almost complete in an overall stable process (COD reduction >82%). In the second period methane productivity increased, but certain intermediate products accumulated constantly. Process design options for a second digestion phase for advanced degradation were investigated. Potential causes for slow and reduced propionic and valeric acid degradation were assessed. Recommendations for full-scale process implementation can be made from the experimental results reported. The highly loaded and stable co-digestion of these substrates may be a good technical and economic treatment alternative.

Organic wastes are increasingly collected source separated, thus requiring additional treatment or recovery capacities for municipal biowastes, organic industrial wastes, as well as agroindustrial byproducts. In this study, we demonstrate that anaerobic digestion is preferentially suited for highwater- containing liquid or pasty waste materials. We also evaluate the suitability of various organic wastes and byproducts as substrates for anaerobic digestion and provide a current status survey of codigestion. Biodegradation tests and estimations of the biogas yield were carried out with semisolid and pasty proteins and lipids containing byproducts from slaughterhouses; pharmaceutical, food, and beverage industries; distilleries; and municipal biowastes. Biogas yields in batch tests ranged from 0.3 to 1.36 L/g of volatile solidsadded. In continuous fermentation tests, hydraulic retention times (HRTs) between 12 and 60 d, at a fermentation temperature of 35 degrees C, were required for stable operation and maximum gas yield. Laboratory experiments were scaled up to full-scale codigestion trials in municipal and agricultural digestion plants. Up to 30% cosubstrate addition was investigated, using municipal sewage sludge as well as cattle manure as basic substrate. Depending on addition rate and cosubstrate composition, the digester biogas productivity could be increased by 80-400%. About 5-15% cosubstrate addition proved to be best suited, without causing any detrimental effects on the digestion process or on the further use of the digestate.