| Summary: | 碩士 === 國立臺灣大學 === 食品科技研究所 === 85 === Acid-soluble and water-soluble chitosan prepared from
commercial β-form chitin were utilized in coagulating
aquacultural wastewater. The effects were estimated by
analyzing some important water-quality parameters:
turbidity, suspended solid (SS), chemical oxygen demand
(COD), biochemical oxygen demand (BOD), ammonia-
nitrogen, nitrite-nitrogen and fish pathogens (Aeromonas &
Pseudomonas).
On the acid-soluble chitosan test, closed-system cultural
sewage was treated with chitosan coagulant with various
degrees of deacetylation (DD) including DD 2.78 (chitin),
30.81, 48.73, 73.10, 86.19, combined with various
concentrations- 0.1, 1, 5, 10, 50, 100 mg/ L wastewater. The
range of optimum concentration for reducing turbidity and
SS was very narrow, ca. 5~10 ppm. At the optimum
concentration, cationic chitosan could destabilize negative
colloidal suspensions by charge neutralization as well as
subsequent formation of particle-polymer-particle bridge. In
contrast, overdosage of chitosan would re-stabilize
colloids/solids from the coagulated to suspended phase and
made the condition worse instead of improving water quality,
which occurred especially on more polycationic chitosan
with higher degree of deacetylation. Because acetic acid and
chitosan were both organic compounds, the COD and BOD
value of treated wastewater widely rose. On the other hand,
the BOD value in this study could not accurately stand for
the content of organic compound due to the antimicrobial
potential of chitosan and nitrification of aerobic
microorganism. The addition of chitosan resulted in cations
exchange between colloids and water, so the ammonia-
nitrogen value would slightly increase. By forming ion pairs
with chitosan, nitrite was kept from supernatant aliquots
during colloids'' settling. In addition, the suspended
chitosan-nitrite complex would decrease the active
coefficient of nitrite due to electrostatic interaction. Above
causes made the nitrite-nitrogen value slightly decreased. At
the concentration of chitosan higher than 50 ppm, there was
a high correlation between the ability of pathogen inhibition
and the degree of deacetylation of chitosan; at 5~10 ppm
chitosan, complete coagulation and sedimentation reduced
more pathogens than using highly deacetylated chitosan.
That was, with high addition of chitosan, the effect of
pathogen inhibition depended on the degree of deacetylation
of chitosan; when less chitosan was added, the effect of
clarification was the key to determine the amount of survival
pathogens.
Effectiveness of treatment was evaluated for the acid-
soluable chitosan solutions and they were stirred at room
temperature for 2 or 14 days. The results indicated chitosan
stirred for 14 days showed lower coagulating ability with
higher turbidity than that for 2 days. The reason was probably
due to degradation of chitosan by acetic acid which made it
difficult to form bridge during coagulation.
Finally, water-soluble chitosan with DD 68.31 and MW
3.88×106 was also used for the wastewater treatment. The
results showed the tendencies of the effects to water-soluble
chitosan were similar to those of acid-soluble one with
different parameters of water quality. Furthermore, shaken
by sonication might enhance expanded chitosan
conformation and made it easier to form bridge to clarify
wastewater than acid-soluble one did. To sum up, water-
soluble chitosan possessed some advantages which acid-
soluble one was short of: (1) the chitosan solution could be
preserved for longer, and there was no fear for acid
degradation, (2) avoiding to increase COD and BOD value
by acetic acid as solvent, (3) to avoid fish-like odor.
Therefore, except for the concern of cost, water-soluble
chitosan was superior to acid-soluble one for improving
water quality.
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