Diagnostics tools for understanding and prevention of process problems in biogas plants

Project 1. In Denmark several joint large-scale biogas plants combine treatment of manure together with organic wastes from slaughterhouses and food processing industries. Some of three admixtures contain a high concentration of easily degradable lipids, which make up a large biogas potential. During the anaerobic digestion process lipids are initially hydrolysed to long chain fatty acids, which may be inhibitory to the anaerobic microbial activity at even low concentration. In project 1 the effect of oleate on methane production and VFA-dynamics in thermophilic anaerobic lab-scale reactors was investigated. Two continuously stirred tank reactors fed with cattle/pig manure mixtures diverging in TS/VS content (2.9 and 5.4% respectively) were subject to increasing pulses of oleate. Following pulses of 0.5 and 1.0 g oleate/1 the most distinct increases in VFA-concentrations were observed in the reactor with the lowest TS/VS content. This suggests, that a higher adsorption of oleate onto the surfaces of biofibers in the reactor With the highest TS/VS moderated the inhibition of the anaerobic digestion process. Addition of 2.0 g oleate/1 caused severe process inhibition in both reactors illustrated by significant increases in all VFA concentrations and immediate drops in methane production. However, increased methane production values and lower VFA concentrations in both reactors at the end of the experiment showed that the oleate addition had a stimulating effect on the overall process. The improved acetogenic and methanogenic activity in the reactors was confirmed by batch activity tests. Toxicity tests in the beginning and end of the experiments revealed that the sudden additions of oleate induced an increase in the tolerance level of aceto trophic methanogens towards oleate. Project 2. Ammonia may be inhibitory to the anaerobic digestion process. For unadapted methanogens ammonia inhibition has been observed to begin at 1.5-2.0 g-N/1 while for an adapted biogas process an ammonia tolerance of 3-4 g-N/1 was reported. The main feedstock of centralized biogas plants in Denmark is livestock waste, which may have an ammonia concentration higher than 4 g-N/1. For most Danish biogas plants the ammonia concentration in the reactors is between 2.5 and 4.2 g-N/1 and the plants are therefore operated close to the inhibitory level of ammonia. Several biogas plants combine the treatment of manure together with organic wastes (from slaughterhouses and food processing industries) containing a high concentration of proteins that readily release further amounts of ammonia when degraded. A sudden addition of large amounts of protein waste may, therefore, cause a temporary increase in ammonia concentration, which might lead to an inhibition of the biogas process. In project 2 the effect of partly digested protein (peptone) on methane production and VFA-dynamics in themophilic anaerobic lab-scale reactors was investigated. Two continuously stirred tank reactors fed cattle manure only diverging in ammonia content (1.5 and 3.0 g N/1 respectively) was subject to consecutive pulses (10 g/l, 10 g/1 and 15 g/1) of peptone. In all three cases the addition of easily degradable peptone had an immediate stimulating effect on the biogas process in both reactors illustrated by significant increases in methane production and VFA concentration. However, approximately three days after the peptone addition a second peak in acetate was observed. At the same time showed the reactor with the lowest ammonia content three days after puls number 2 and 3 a lower methane production than before the peptone-pulses, while the reactor with the highest ammonia content showed a lower methane production three days after puls nr 3. These patterns indicates an imbalance between the acetate producing and acetate consuming microorganisms which could be a result of a periodically ammoniainhibition of the acetoclastic methanogens caused by an increase in the ammonia concentration during the degradation of peptone. During the first two peptone-pulses the increases in the VFA levels was most distinct in the reactor with the highest ammonia content while the methane production was 15-30 % higher in the reactor with the lowest ammonia content. An explanation of these differences could be that although the biogas process of R1 was adapted to an ammonia tolerance of 3.0 g-N/1 the microbes in the reactor was under a 'higher daily stress' and therefore had less capacity to withstand a sudden pulse of extra ammonia