large concentrations of short-chain fatty acids. For drainage with low solid concentrations (5% or less), an upflow anaerobic sludge blanket (USAB) was developed to allow treatment with retention times of one to two days (Fig. 9.5d). Also
developed and commercialized were high-speed methane fermentation processes
such as the expanded granular sludge bed (EGSB) approach and the upflow
anaerobic filter process (UAFP) approach. These have been widely used since the
1990s, especially in the domestic and overseas food processing industries.
A UASB takes advantage of the way that the microorganisms involved in
methane fermentation naturally come together to form microorganism granules. By
maintaining a high density of bacterial bodies in the reactor, substantially larger
treatment speeds can be achieved than with the previous methane fermentation
approach. Hopefully, high-speed processing will become available in which a
high-speed methane fermentation tank is connected to a methane production tank,
so that organic acids accumulate at high concentrations and can be swiftly converted to methane by methane fermentation bacteria even when the organic matter
load is increased.
Fannin et al. used a NMVFR at high load conditions (11.2 kg-VS/m
3 /day) as a
hydrolysis and organic acid formation tank for the giant kelp Macrocystis. The
supernatant was then placed in a methane production task to produce methane with
a yield rate of 0.29 m
3 /kg-added VS. While lower than the maximum methane yield
for giant kelp as shown in Table 9.2, this was the result of a low solid hydrolysis
rate in terms of hydrolysis and organic acid production; achieving higher methane
yields would require a different pre-treatment process instead of crushing
(Nakashimada and Nishio 2011).
9.3.7 Issues with Algae Methane Fermentation
A rather troublesome problem area remains with anaerobic decomposition of algae,
namely the fact that seaweed bodies contain high conditions of sulfate ions, salt
(NaCl), and heavy metals. Among these, heavy metals are a particularly serious
issue (Cecchi et al. 1996).
The high molecular weight polysaccharides in algae cell walls contain large
amounts of ionic groups such as carbonyl and sulfate groups, which adsorb heavy
metals. As a result, algae that grow in back bay regions with little seawater
movement accumulate high concentrations of heavy metals such as cadmium. (In
Sweden, algae are classified as toxic waste.) This limits the ability to use residue
from anaerobic decomposition as a biofertilizer.
But anaerobic decomposition can also be helpful in removing heavy metals. As
previously noted, it is best to apply a two-stage process for anaerobic composition of
algae (Omil et al. 1995). In a two-stage process, solid organic matter is solubilized
and hydrolyzed in the first reactor, after which chiefly organic acids are produced by
anaerobic fermentation using microorganisms. The resulting organic acids can be
subjected to high-speed methane fermentation with the UASB method. The heavy
9.3 Methane Production from Algae
313
developed and commercialized were high-speed methane fermentation processes
such as the expanded granular sludge bed (EGSB) approach and the upflow
anaerobic filter process (UAFP) approach. These have been widely used since the
1990s, especially in the domestic and overseas food processing industries.
A UASB takes advantage of the way that the microorganisms involved in
methane fermentation naturally come together to form microorganism granules. By
maintaining a high density of bacterial bodies in the reactor, substantially larger
treatment speeds can be achieved than with the previous methane fermentation
approach. Hopefully, high-speed processing will become available in which a
high-speed methane fermentation tank is connected to a methane production tank,
so that organic acids accumulate at high concentrations and can be swiftly converted to methane by methane fermentation bacteria even when the organic matter
load is increased.
Fannin et al. used a NMVFR at high load conditions (11.2 kg-VS/m
3 /day) as a
hydrolysis and organic acid formation tank for the giant kelp Macrocystis. The
supernatant was then placed in a methane production task to produce methane with
a yield rate of 0.29 m
3 /kg-added VS. While lower than the maximum methane yield
for giant kelp as shown in Table 9.2, this was the result of a low solid hydrolysis
rate in terms of hydrolysis and organic acid production; achieving higher methane
yields would require a different pre-treatment process instead of crushing
(Nakashimada and Nishio 2011).
9.3.7 Issues with Algae Methane Fermentation
A rather troublesome problem area remains with anaerobic decomposition of algae,
namely the fact that seaweed bodies contain high conditions of sulfate ions, salt
(NaCl), and heavy metals. Among these, heavy metals are a particularly serious
issue (Cecchi et al. 1996).
The high molecular weight polysaccharides in algae cell walls contain large
amounts of ionic groups such as carbonyl and sulfate groups, which adsorb heavy
metals. As a result, algae that grow in back bay regions with little seawater
movement accumulate high concentrations of heavy metals such as cadmium. (In
Sweden, algae are classified as toxic waste.) This limits the ability to use residue
from anaerobic decomposition as a biofertilizer.
But anaerobic decomposition can also be helpful in removing heavy metals. As
previously noted, it is best to apply a two-stage process for anaerobic composition of
algae (Omil et al. 1995). In a two-stage process, solid organic matter is solubilized
and hydrolyzed in the first reactor, after which chiefly organic acids are produced by
anaerobic fermentation using microorganisms. The resulting organic acids can be
subjected to high-speed methane fermentation with the UASB method. The heavy
9.3 Methane Production from Algae
313