Organic waste as a biomass resource - Biomass as Energy Source: Resources, Systems and Applications - page 111

Figure 6.2. Microbiological pathway for biogas production (Lens et al. , 2004).

Table 6.2. Comparison of theoretical and practical methane yields. FM = Fresh mass, oDM = organic dry
matter.

Methane yield from wet digestion Methane yield from dry digestion

batch test full scale plant

(Nm 3 /10 3 kg FM) (Nm 3 /10 3 kg oDM) (Nm 3 /10 3 kg FM) (Nm 3 /10 3 kg oDM) System and Substrate

Plug flow digester 1 157 482 126 388

Energy crops
Plug flow digester 2 99 352 69 243

Biowaste
Tower digester 1 142 459 108 349

Energy crops
Tower digester 2 77 291 65 246

Household waste
Garage digester 49 374 33 256

Biowaste

Considering waste streams, several additional aspects become a matter of interest, for example
(Ahrens and Weiland, 2007):
• waste pre-treatment;

• digestate quality (e.g. heavy metal content) and its suitability to be used as fertilizer;

• process technology (e.g. disturbing material or accumulation effects);

• suitable plant locations (e.g. waste infrastructure, digestate utilization, etc.).

6.3.2 Technical background for waste-to-biogas utilization strategies
According to substrate characteristics, waste as biogas substrate often requires different digestion
strategies in comparison to energy crops. A matter of discussion is the so-called dry digestion,
which might have a huge potential for waste-to-biogas utilization strategies. Reasons for this
are the significantly lower demand of water for moisturizing the substrate and furthermore (as a
parallel effect) the reduced amount of digestate to be treated afterwards. One basic question is the
effectiveness of such applications in terms of methane productivity. This has been investigated
by comparing examples of full-scale plant applications with parallel batch tests using the same
substrate mixtures. Table 6.2 shows the results of this comparison.