Biorefineries using waste – production of energy and chemicals from biomasses by micro-organisms - Biomass as Energy Source: Resources, Systems and Applications - page 245

water microbes are recovered fast emitting gases in a short time from appropriate substances
(Hakalehto et al. , 2011a). This approach of wastewater treatment also links the microbiology
with direct electricity generation.
The hydrogen production reactions can be divided into photobiological processes and dark
hydrogen fermentation. In the latter ones, many bacteria reduce protons to hydrogen in order
to get rid of reducing substances resulting from their primary metabolism. The hydrogen thus
replaces the oxygen as an electron acceptor in anaerobiosis. In some reactions, organic substances
can act as electron acceptors. Such a case is the conversion of butyrate into butanol. This reaction
is also resulting in hydrogen production, and is an important potential route for the treatment of
municipal sludges and animal manures, which contain high concentrations of butyrate as a result
from activities of the butyric acid bacteria. These strains are important members of the microflora
of the cecum and the colon (Hakalehto, 2012). They are also provoked by the CO 2 produced by
other bacteria (Hakalehto and Hänninen, 2012). This illustrates the potential power of the mixed
cultures in the waste treatment. The members of the Enterobacteriacae are not inhibited by high
H 2 pressures, which could make them a potential production strain for the energy gas (Tanisho
et al. , 1987; Kumar and Das, 2000).
In our own experiments in the Finnoflag Oy laboratory we were able to convert the butyric
acid produced by some other clostridia strains further into butanol by Clostridium acetobutylicum
in about four hours in an enhanced mixed fermentation with simultaneous hydrogen production.
In fact, in these mixed cultures the sole products are CO 2 ,H 2 , butyric acid and butanol. The
typical products for a conventional ABE fermentation result also in acetic, lactic and propionic
acids as well as ethanol.
The direct production of electricity from cellulose was demonstrated by a microbial fuel cell
(MFC) using a defined binary culture (Ren et al. , 2007). In this experiment, the cathode was built
from the insoluble substrate cellulose, which was hydrolyzed by a Clostridium cellulolyticum
strain. This bacterium alone did not produce electricity, but combined with an electrochemically
active bacterium, Geobacter sulfurreducens , the mixed culture generated remarkable amounts
of electricity. Hydrogen, ethanol and acetate were the main residual metabolites of the binary
culture, all of which could be further exploited in the energy generation from the cellulose
wastes.
Municipal wastewater is a combination of liquid and water-carried solid wastes. Treatment
of wastewaters produces massive volumes of excess sludge and other organic material, which
require proper treatment. Even one third of operating costs in a wastewater treatment plant
may be caused by the treatment of this excess sludge (Laaksonen, 2011). It should be taken
into account that this excess sludge is not only waste, but it can also offer many economically
feasible possibilities, such as nutrients for agriculture and a source of energy e.g. via biogas
production or combustion. Moreover, excess sludge contains valuable organic material, such
as proteins and enzymes. According to Metcalf and Eddy Inc. (2003), approximately 75% of
total suspended solids in wastewater are organic. The organic matter is typically composed of
proteins (40-60%), carbohydrates (25-50%) and lipids (8-12%), but the composition may vary
very much depending on the wastewater sources. Sludge utilization is of great interest (LeBlanc
et al. , 2008).
13.7.6 Removal of harmful substances
The chemicalized industrial production in many fields, as well as in the agriculture and
forestry is creating a chain of unbalanced and distorted microenvironments and biomass build-
up. Chemical contaminants then accumulate into the organic matrices and matter, such as
waste materials and sludges. During combustion, for example, some of them are concen-
trated into the ashes, whose deposition can become an environmental problem. Microbial
biotechnologies, such as bioleaching, offer solutions for the removal of toxic elements (Fanga
et al. , 2011).