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Backgrounds and Objectives: Large quantities of sludge are produced in
biological wastewater treatment. Because this sludge is highly rotten, it
should be stabilized before its disposal. Aerobic and anaerobic digestion is
widely considered as stabilization techniques. Because of high retention time
and sludge dewatering difficulties, reduction in retention time, operation and
maintenance should be given into consideration. Ultrasonic process increases
the enzymatic activity, so decreases the hydrolysis time, a limiting factor in
digestion process, and contributes to the decrease of the detention time. The
objective of this investigation is to determine the effect of ultrasound in
improving dewatering and stabilization of aerobic and anaerobic digested
sludge. In addition, the impact of ultrasonic treatment on improvement of
sludge dewatering and aerobic and anaerobic digestion is compared.
Materials and Methods: In this survey, samples of aerobic and
anaerobic digestion were collected from local full-scale Garb Town
and Tehran South wastewater treatment plant, respectively. The grab samples
were collected for 4 month from July to October 2010. Total numbers of 20
samples were collected biweekly for each type of digestion. Each sample was
sonicated for 15, 30, 60, and 90 min under 35 and 131 kHz frequencies
separately. Total solids, volatile solids , pH, temperature , total COD,
dissolved COD and settle able solids were measured. Ultrasound bath of the
solution in a 300 mL glass reactor was performed as a bath reactor with power
of 500 W.
Result: The results showed that the application of ultrasonic
wave increased dissolved COD and temperature and decreased volatile solid, pH
and settle able solids. Application of ultrasonic wave with frequency of 131
kHz decreased the VS and increased the dewatering of sludge more effective than
the 35 kHz frequency and the highest performance was at 15 min of time and 131
kHz of frequency. Also sonication method showed better efficiency for anaerobic
sludge samples compared to the aerobic sludge samples.
Conclusion: The results obtained showed that digestion
and dewatering properties of sludge improved by ultrasonic application.
Therefore it can be used as an alternative method for the sludge treatment.
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