Relationship Between and Effect of Intermittent Ultra-Sonication and Specific Energy on Degree of Disintegration of Secondary Sludge

Introduction:
The disintegration of excess sludge from activated sludge units for production of a carbon source in post-anoxic processes in order to remove nutrients from wastewater streams has been widely used in recent years. For this purpose, ultra-sonication is one of the most applicable methods for the disintegration of cell membranes to provide carbon the source from disruption of microorganisms. Such substances extracted from disintegrated sludge can be used as readily bio-degradable sources of carbon to be used by other microorganisms in following process units. The amount of power (P) per volume (V) applied to the samples in specific durations (t) as well as the concentration of sludge are the most dominant factors in the determination of the disintegration process. The main hypothesis of this research is that if the sonication of samples is intermittent during the total time of ultrasonic radiation, the defensive mechanism of cells will be weakened due to irregular forces applied to the cell membrane. This will lead to degradation of microorganisms at lower Specific Energy (Es) while the energy consumption in the whole process will be reduced accordingly. The effect of this parameter has been investigated through this research, whilst no research has been focused upon this issue earlier in previous studies.
In this research, samples of secondary sludge have been disintegrated at the frequency of 24 kHz while the applied power, time of sonication and sludge concentration were changed for each set of experiment. As an extra independent variable, intermittent sonication was altered from 0.3 to 0.9 second. The Degree of Disintegration (DD) was monitored as a dependent variable to evaluate the efficiency of the sludge disintegration process.

Materials and Methods:
The sludge for disintegration was sampled at the outlet of an anaerobic tank fed by a continuous stream of wastewater. Prior to the anaerobic phase, the sludge was aerated in an aerobic tank at a retention time of 12 hours. Raw wastewater with the average COD content of 358 mg/lit was daily supplied from a municipal wastewater treatment plant named Mahalati and located at north-east of Tehran, Iran. The type ultrasonic set was Sonotrode with a five mm diameter probe manufactured by FAPAN (Iran). The generated frequency and maximum power generated by the set were 24 kHz and 300 watts, respectively. The degree of disintegration has been calculated by the method introduced by Muller (2000) and one Molar NaOH was used to determine the total COD of the sludge. To separate the supernatant of the sludge from its solid contents, a centrifuge set (SDN- United Kingdom) was used along with and ultra-filtration by a 0.45-micron glass filter (CHMlab, Spain). For experiment design, a Response Surface Method and Central Composite Design were developed using Design Expert software. The power, sonication duration, the solid content of the sludge samples and intermittent sonication were independent variables and degree of disintegration was considered as dependent parameters. For standardizing the independent variables, Specific Energy (Es) was calculated at each stage. Determinations of COD and solid concentration were performed in accordance with Standard Methods for Water and Wastewater Experiments. For COD measurement, a colorimetric method was used by Hach DR 1900 (Germany) as Spectrophotometer.

Discussion of Results:
Considering the aforementioned independent parameters, a relationship between these variables has been proposed as the formula no.1. The desirability of the model has been evaluated through ANNOVA variance analyzes for the proposed logarithmic model. The R2 parameter for this model was calculated as 0.9797.

Formula no.1: Log10 (DDCOD) = -3.27179 +1.78725E-003× Watt +5.28686× Seq +0.16406× Time -0.098290× Density -3.82414× Seq^2 -8.14053E-003× Time^2

Considering response surfaces generated by the software for interactions of parameters and their effects on DD, it was figured out that when the power increases, the DD rises up to the maximum of 38% too (under circumstances of this study). Moreover, decreasing sonication sequence from 1 to 0.6 second, leads to increasing the DD and afterward, this parameter decreases significantly when the sequence reaches 0.4 second. Emitted energy from the probe of the ultrasound generator produces little bubbles of water due to acoustic cavitation. When such bubbles collapse, some radicals such as OH., OOH. and H. are formed, which attack the cells in their vicinity. By increasing the power and sonication time, production of such bubbles increases and consequently, the efficiency of the disintegration process significantly improves. Additionally, in the higher concentration of sludge solid contents, a portion of applied energy to the aqueous systems transfers to the solids instead of making cavitation. In the other words, parts of the ultrasound waves are damped by the microorganisms’ body. As a result, the number of collisions between radicals and cells are declined, which leads to a decrease in the DD parameter. Hence the more sludge is concentrated the less sludge disintegration occurs.
In order to the comprehensive understanding of the interactive between the parameters, Specific Energy (Es) was calculated and a relationship between Es and intermittent sonication with the degree of disintegration has been developed as the formula no.2:

Formula no.2: DDCOD = -0.30799 +8.59092E-006× Es +1.21280× Seq -1.00445×Seq^2

The desirability of the model has been verified by ANNOVA analyzes with an R2=0.9633. According to the respond surface, increase in Es leads to higher efficiency of sludge disintegration, while reduction of sonication sequences from 1 to 0.5 second, affects the DD parameter in such a way that the maximum DD takes place in sequences between 0.6 to 0.7 second. In such circumstances, it can be argued that little bobbles of acoustic cavitation thoroughly dispersed in the sample during the sonication period. These bubbles cause breaking of sludge flocs at the first stage and collapsing of cell tissues in next step. If the radiation is continuous, the resistance of cell walls is strengthened because of the uniform collision of bubbles to biomass and its defensive mechanism toward external forces. Such conditions will lead to decrease in the degree of disintegration. On the other hand, if the bubbles are not produced in an extremely short period of time (a portion of a second), cell’s discipline of defensive mechanism will be devastated and accordingly, they will be more vulnerable to external forces. This will lead to higher values of sludge disintegration.
Another phenomenon that was observed in the experimental phase is that when the specific energy increases the optimum point for intermittent sonication approaches from 0.6 to 0.7 second. The reason is that in higher Es values, applied energy to the sludge is high enough to destroy cell tissue. Thus, as soon as sonication starts, the cell membrane disrupts and the theory of alteration in the defensive mechanism of microorganisms takes place in the smaller scale.

Conclusion:
Through a comprehensive literature review associated with ultrasonic sludge disintegration, it was found that no study puts its emphasis on intermittent sonication and its effects on microorganisms. While it is stated that precise microscopic survey is needed to understand the exact mechanism, which has not been performed in this research, macroscopic observations confirm that intermittent sonication can improve the efficiency of the disintegration process. Accordingly, a hypothesis of destructive resonance in cells’ structure due to the irregular emission of waves is proposed in this study, which should be verified with other extensive microscopic observations. Assuming this theory, when radiation of waves is cut off for a very short period of time, defensive mechanism of microorganisms suddenly interrupts and the cells show a less resistance against external forces. This will lead to disruption of cells in rather lower energy levels.