Sustainability and Stability - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 847

etc.) produced from the hydrolysis of starch. Single-cell protein (SCP) is produced simulta-
neously with potato waste treatment.
Corn and pea wastes are also treated by a mixed culture of Trichoderma viride and Geo-
trichium sp. Trichoderma viride produces cellulase to break down cellulose into glucose, and
Geotrichium produces amylases to break down starch into glucose. Both organisms utilize
glucose for growth.
A mixed culture of Candida lipolytica and Candida tropicalis has been grown on hydrocar-
bons, n-paraffins, or gas oil for SCP production purposes in both laboratory and pilot-scale
operations. The utilization of a mixed culture of yeasts was proved to yield better product
quality as compared to pure yeast strains.
Gaseous hydrocarbon substrates like methane can be utilized by certain bacteria to
produce SCP. Several experimental studies have shown that mixed cultures of methane
utilizing organisms grow faster than pure cultures.
Certain methane-utilizing species of Pseudomonas oxidize methane to methanol. However,
Pseudomonas is inhibited by methanol. Inclusion of a methanol-utilizing bacteria such as
Hyphomicrobium into the growth medium eliminates the problem of methanol inhibition.
This relationship is mutualistic in the sense that Pseudomonas supplies carbon source
(CH 3 OH) for Hyphomicrobium, and Hyphomicrobium removes the growth inhibitor (methanol)
of Pseudomonas.
(16.106)
A schematic of a typical wastewater treatment flowsheet is shown in Fig. 12.8 and redrawn
in Fig. 16.25 . Activated-sludge processes include a well-agitated and aerated continuous-flow
reactor and a settling tank. Depending on the physical design of the tank and how feed is intro-
duced into the tank, it may approximate either a PFR (plug flow reactor) or CSTR. A long
narrow tank with single-feed approaches PFR behavior: circular tanks approach CSTR. The
concentrated cells from the settling tank are recycled back to the stirred-tank reactor. Usually,
amixed culture of organisms is utilized in the bioreactor. Some of these organismsmayproduce
polymeric materials (polysaccharides), which help the organisms to agglomerate. Floc forma-
tion is a common phenomenon encountered in activated-sludge processes, whichmay impose
some mass transfer limitations on the biological oxidation of soluble organics; but good floc
formation is essential to good performance of the system, since large dense floes are required
in the sedimentation step. Cell recycle from the sedimentation unit improves the volumetric
rate of biological oxidation (i.e. high-density culture) and therefore reduces the residence
time or volume of the sludge reactor for a given feed rate. The recycle maintains the integrity
of biocatalysts (microbials) in the process in case of minor upsets or fluctuations. The recycle
ratio needs to be controlled to maximize BOD (biological oxygen demand) removal rate.
16.6.7. Mixed Culture in Nature
Mixed cultures of organisms are common in natural ecological systems. Microorganisms
are involved in the natural cycles of most elements (e.g. carbon, hydrogen, nitrogen, oxygen,
and sulfur). Simplified diagrams of the carbon and oxygen cycles are presented in Fig. 15.1 or