Environmental Engineering Reference
In-Depth Information
is combined with bioremediation where microbes break down the environmental pollutants into
harmless substances, the overall effect of the waste treatment process is achieving a remark-
ably higher level of sustainability. In order to further improve the ecological (and most often
also economical) value of the waste treatment process, it is worth developing actual biotech-
nological processes based on the understanding of the metabolic activities and diversity of the
microorganisms. In practice, this means often increased understanding of the mixed cultures,
and mixed substrate utilization. In such conditions, the outcome of the reactions is depending
on many parameters. Obviously, the conditions with versatile substances as raw materials in rel-
atively low concentrations of various raw materials favor so-called mixotrophs, organisms with
versatile capabilities (Harder and Dijkhuizen, 1982). On the other hand, in the presence of higher
concentrations of any particular substrate running the process can lead to the dominance of the
microorganisms most furnished for the exploitation of this substrate. In this case, the ecological
succession is then determining the selection of other strains further metabolizing the materials. If
the waste mixtures contain recalcitrant compounds, which are not easily degraded by any single
species, the microbes often develop so called co-metabolism, which extends the capabilities of
the mixed cultures with respect to the degradation of several compounds (Dalton and Stirling,
1982). This is a valuable option for the removal of toxic compounds from the wastes. In some
environments, it is also possible that the microbes are interacting in a deliberate manner in order to
achieve best possible results for their survival as a community. Such interactions, and avoidance
of direct competition, could be important in the human digestive tract, in regions where nutrients
are quickly absorbed by the host (Hakalehto, 2012).
The integrated waste utilization process could consist of, for example, mushroom production
(food protein for human consumption), feeding material (combined solid-state fermentation of
agricultural and forest wastes by white rot fungi), biogas and biofertilizers (Chang, 1987). In the
case of animal feed, decomposing with fungi increases the rumen digestibility up to 30-60% from
only 3% of the undecomposed wood. In case of biogas (methane) production by mixed bacterial
cultures, the tendency to produce CH 4 and H 2 O is somewhat an alternative to the production
of H 2 and CO 2 . Besides these processes where the microbial cell mass is either used as single
cell protein, and the gases emitted by them are exploited in energy generation, the fermentative
cultures produce liquid phase products into the solution or suspension. The usage of this potential
has been limited by the view that these anaerobic fermentations are slow reactions in which lack
of speed increase the cost of e.g. product recovery. In the PMEU research, we have documented
that the capacity of the microbial cells to carry out anaerobic reactions is in fact not slow, but it
equals the aerobic reactions (Hakalehto et al. , 2007). This is true also with many facultative anaer-
obic strains. We have applied the principles of the PMEU for the production of 2,3-butanediol,
which is valuable compound for the production of synthetic rubber and plastic monomers (after
conversion to butadiene). In this experimentation, we achieved the productivity of 8-10 g/L/hour
(Fig. 13.2).
Even though the animal manure is containing high nitrogen levels in comparison with the
carbon content, it is possible to microbiologically liberate the carbon from the cellulose in the
sludge. In the case of piggery sludge, a system with endoglucanase, exoglucanase and cellobiase
activities were constructed using five isolated bacterial strains (Ping et al. , 2008). The strains
were identified as Pseudomonas citromellolis , Sternotrophomorus maltophilia , Flavobacterium
mizutaii , and two strains of Pseudomonas aeruginosa . In our research experiments in Sotkamo,
Finland, with the bioprocess pilot plant (designed by Finnoflag Oy for Biometa Finland Oy),
the cow manure mixture (with dry weight of a few percent only) was producing relatively high
yields of hydrogen and methane gases, which were resulting from the carbon polymers in the
waste material. The hydrogen generation has been shown to increase the energy efficiency of
waste treatments. For example, in case of cereal or food processing wastewater, the treatment
process was linked with a microbial fuel cell (MFC) (Oh and Logan, 2005). The maximum power
density was above 50mA per a liter of wastewater in this experiment. It is also noteworthy that
in an advanced bioprocess system the aliquots are mixed with untreated new wastewater flow in
order to maintain adequate substrate levels. It has been also shown that in the PMEU conditions
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