As early as the 1980s, in The Netherlands a reduction in CH4 production in animal manure digesters was observed as a result of digestion of the manure in pre-storage (Zeeman et al, 1985). It is difficult to prevent such CH4 emissions from taking place in many manure storage systems, especially where large storage capacities, able to hold the manure production of more than 100 days, are compulsory. Storage temperature, storage time and the presence of an inoculum (i.e. manure leftovers after emptying the storage) are three crucial factors that determine the amount of CH4 emitted from a manure storage (Zeeman, 1994). Manure has to be entirely fresh to ensure the highest possible biogas production when it is digested as the sole substrate. Today, co-digestion is more often used and the co-products may be responsible for the largest part of the produced biogas, reducing the attention on fresh manure input. Aside from emissions in pre-storage, CH4 emissions in post-storage may also occur. After digestion of fresh pig manure in a CSTR at 30°C and 20 days detention time, the additional CH4 gas production in the post-storage at 10-15°C can amount to 23 per cent of total production (Zeeman, 1994). Co-digestion may result in a variation in composition and concentration of input material, resulting in variations in the organic load to the anaerobic digester. These variations could negatively influence the stability of the digestate and so lead to increased CH4 emissions during post-storage (depending on the hydraulic retention time (HRT) applied in the digester).
Post-storage CH4 emissions can be avoided with the application of gas-tight covers on digestate storage tanks. Lately, many biogas plants have installed gas collection systems in after-storage tanks (Angelidaki et al, 2005). Extensive research in the 1980s in The Netherlands and Switzerland into low (ambient) temperature digestion using combined storage/digestion (accumulation) systems could help the optimization of such systems for (co-)digestion elsewhere, and solve emission problems from both pre- and after-storage (Wellinger and Kaufmann, 1982; Zeeman, 1991).
The use of post-digestion for substrate mixtures is reported to improve biogas production (Boea et al, 2009). Applying a mixture of cow and pig manure and industrial wastes during post-digestion resulted in an increase in the biogas production of 11.7 per cent, 8.4 per cent and 1.2 per cent at a temperature of 55°C, 37°C and 15°C, respectively, and an HRT of 5.3 days. The main reactor was operated at 55°C with an HRT of 15 days. For on-site digestion, covered post-storages might be more efficient than post-digestion, as long-term storage is frequently compulsory. For central digestion, as applied in Denmark, short-term post-storage may be applied as land prices are often high. To prevent CH4 emissions during subsequent long-term on-farm storage (Figure 10.4) and to increase the CH4 recovery, post-digestion at an elevated temperature - as suggested by Boea et al (2009) - may be an efficient option.
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