Two-Stage Conversion of Land and Marine Biomass for Biogas and Biohydrogen Production

Sammanfattning: The replacement of fossil fuels by renewable fuels such as biogas and biohydrogen will require efficient and economically competitive process technologies together with new kinds of biomass. A two-stage system for biogas production has several advantages over the widely used one-stage continuous stirred tank reactor (CSTR). However, it has not yet been widely implemented on a large scale. Biohydrogen can be produced in the anaerobic two-stage system. It is considered to be a useful fuel for the future due to its high energy density and clean combustion with the emission of only water vapour. Anaerobic digestion can be used to treat wastewater and for energy production, leading to a reduction in eutrophication and greenhouse gases. The material remaining after treatment can also be used as a fertiliser as long as certain standards are met. The production of biogas and biohydrogen from a range of land and marine biomasses was studied in this work. The reduction of the heavy metal content of seaweed was also studied in order to improve fertiliser quality. Two-stage, dry anaerobic digestion of mussels, reeds, seaweed, solid cow manure, and a mixture of seaweed and manure was studied. The system consisted of a leach bed reactor for hydrolysis and an upflow anaerobic sludge blanket (UASB) reactor for methane production. The results showed that mussels with shells, seaweed, and the seaweed and manure mixture were efficiently digested in the two-stage system; 68 to 83% of the methane being produced in the UASB reactor. The manure by itself, and reeds, which are slowly degradable, were efficiently digested in the one-stage dry leach bed process, in which most of the biogas was produced. Seaweed and manure can also be co-digested in the one-stage dry digestion process, since methano¬genic conditions prevailed in the leach bed reactor, thus reducing the cost of operating two biogas reactors. Technically, both the new feedstocks and the one- and two-stage dry anaerobic systems have great potential for biogas production. However, economic evaluations are needed to validate practical applicability. The removal of heavy metals from seaweed hydrolysate was studied in the two-stage system. The heavy metals Cd, Cu, Ni and Zn were adsorbed using iminodiacetic acid Cryogel® carriers. However, removal of the heavy metals resulted in low methane yields, possibly due to the removal of micro¬nutrients needed for anaerobic digestion. It is therefore suggested that the metals be removed after methane production in a UASB reactor. Alkaline and autoclave post-treatment of the seaweed digestate resulted in 86% organic matter solubilisation and the leachate may be treated in a UASB reactor, providing a means of handling digestate with high heavy metal content. Co-digestion of leachates from the leach bed reactor and the post-treatment resulted in a high methane yield, 0.34 l/gVSadded in a batch test. Subsequent treatment of the leachate from the leach bed reactor resulted in a high methane productivity at a loading rate of 20.6 g COD/l.day in a UASB reactor. Treatment of the seaweed leachate in the UASB reactor resulted in a stable process without the need for additional nutrients or buffer. As the seaweed leachate was rich in nutrients and buffer capacity, its co-digestion with wheat straw hydrolysate in the UASB reactor resulted in a stable process. Biohydrogen and biogas were co-produced from wheat straw hydrolysate in a two-stage system consisting of a CSTR and a UASB reactor, employing the thermophile, Caldicellulosiruptor saccharolyticus in the first H2 reactor. Straw hydrolysate was efficiently produced by acid-catalysed steam and enzyme pretreatment, giving a 95% sugar yield of the theoretical yield. High biofuel production rates of 1.8 to 3.5 l H2/l.day and 2.6 to 4.0 l CH4/l.day were obtained under stable operational conditions and treatment efficiencies. However, the cost of nutrient supplementation was high, and cheaper nutrient sources will be required to make the production cost economically competitive. This research has demonstrated the versatility of a two-stage system that allowed the digestion of new kinds of biomass such as seaweed with sand, mussels with shells, reeds, manure and wheat straw. It has also been shown to be possible to remove heavy metal from seaweed to improve fertiliser quality. High hydrogen and methane production rates were also demonstrated, and the two-stage anaerobic system is thus, technically, a promising reactor configuration for the production of biofuels.