4.2.1 Cyanobacteria

In contrast to many algae, the main component of cyanobacteria is proteins. They also lack a hard polysaccharide-based cell wall. These properties explain the higher digestibility of cyanobacterium species. Two genera, Arthrospira (Spirulina) and Anabaena, have been studied as a potential feedstock for the ADP. The BMP assay for a cyanobacterium mixture collected from Lake Dian resulted in a higher meth­ane yield of 0.37 L/gVS (HRT 35 days) with a methane fraction of 60-65% in the biogas [155]. The methane yield during batch digestion of Arthrospiraplatensis and Arthrospira maxima species varied from 0.29 to 0.33 L/gVS corresponding to 68-77% of the theoretical methane yield [156-158].

Samson and LeDuy studied the digestion of A. maxima in a continuous reactor and concluded that A. maxima can be the sole substrate for stable methane produc­tion. Municipal anaerobic sewage sludge can easily adapt to the cyanobacterium feedstock, and the observed methane yield was 0.26 L/gVS at an OLR of 0.97 gVS/ L-day, HRT of 33 days, and T at 30°C [159]. Despite high ammonia and fatty acids concentrations (2.5 and 2 g/L respectively), methane production was stable possibly due to high alkalinity (8 g/L) and pH of 7.55.

The incoming VS concentration, OLR, and HRT have a large influence on AD stability and methane yield with A. maxima (Fig. 9) [160]. The methane yield and

Tabic 18 Characteristics of AD of brown seaweeds

M. pyrifera (21.5% mannitol content from TS)

M. pyrifera (8.3% mannitol content from TS)

M. pyrifera (ground)

M. pyrifera (ground, desalted)

M. pyrifera (ground control) M. pyrifera (ground. N and P added)

M. pyrifera M. pyrifera M. pyrifera M. pyrifera added)

M. pyrifera adopted sludge)

Table 18 (continued I

M. pyrifera (ground, inoculum D: Л + marine sediments)

M. pyrifera (ground, inoculum E: analogous to D. developed at room T)

pyrifera (ground, control) pyrifera (ground) pyrifera (ground) pyrifera (21.5% mannitol content from TS >

M. pyrifera S. fluitans

Sargassum tenerrimum

L. hyperborea L. saccharina A. nodosum

18

IS

BMP biomethane potential; CSTR continuous stirred-tank reactor; SCSTR semi-continuous stirred-tank reactor; NMVFR non-mixed vertical flow reactor; USR upflow solids reactor; BFR baffle-flow reactor; ASBR anaerobic sequencing batch reactor; UFA upflow anaerobic filter “Estimated from data given in the paper

bVS reduction estimated from data given in the paper using an equation VS red. = [L(CH4 / gVS)] / [L(CH4 / gVS)]^^

“Methane yield estimated from biogas yield given in the paper using a CH4/biogas ratio of 0.5 dEstimated from data given in the paper using total work volume 6 L and VS/TS ratio of 0.6

methane volumetric production rate were 0.04-0.36 L/gVS and 0.17-0.8 L/L-day, respectively. The maximum methane yield was obtained at HRT equal to 30 days, VS concentration of 20 gVS/L, and the OLR of 0.67 gVS/L-day. Despite a high concentration of ammonia (1.9-7.1 g/L) and volatile acids (up to 23.2 g/L), the methane production was stable with the exception of operation with HRT of 5 days and high feed concentration at HRT of 10 days. The average methane content of the biogas was in range of 69-71%. At high OLRs, it dropped to 46-60%, which is evidence for inhibition of methanogens. A stable ADP occurred when the alkalinity was high (7.2-29 g CaCO3/L) (Table 19).