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J. Kosel, et al.
Fig. 5. Hydrodynamic characteristic of the rotation generator equipped with the serrated rotor (A) (for developed cavitation) and the two teeth rotor (C) (for supercavitation). Developed cavitation can be observed under column B and supercavitation under column D.
60 min, which relates to 6 supercavitation passes. After 6 supercavitation passes, the COD for the RW and CRW sam- ples was reduced by 22%, and by 10%, respectively (Fig. 7). However, the redox potential was increased by cavitation. Speci fi cally, the in- crease in the redox potential was much stronger for the CRW samples (77%) than for the RW samples (37%) reaching 160 mV and 107 mV, respectively. Supercavitation treatment increased the content of dis- solved oxygen from 5.3 mgO 2 /L to 7.3 mgO 2 /L for the RW samples and from 4.6 mgO 2 /L to 7.4 mgO 2 /L for the CRW samples. After supercavitation the level of sediments was reduced by 50% and by 95% for the RW and CRW samples, respectively (Fig. 8). Similar results were obtained for the insoluble materials of the RW and CRW samples for which a 67% and a 48% reduction was achieved. Contrary to this, the SAC values increased for both sample types. The SAC 436 , SAC 525 , and the SAC 620 values (m − 1 ) increased by 96%, 93% and by 97% for the RW samples and by 43%, 28% and by 63% for the CRW samples, respectively. Supercavitation treatment strongly reduced the viable count of all the major classes of microorganisms which were found to be present in the RW and CRW samples. During the cavitation treatment the viable count of aerobic bacteria in the RW sample was reduced from 5.9 Log 10 CFU mL − 1 to 3.2 Log 10 CFU mL − 1 . Therefore, a staggering 2.7 logs reduction was achieved after 6 supercavitation passes (a 99.81% de- struction). However, for the CRW samples the reduction of the aerobic bacterial count was smaller (1.2 logs reduction). A 4.2 logs (a 99.99% destruction) and 2.8 logs (a 99.84% destruction) strong reduction of the anaerobic sulphate reducing bacteria was observed for the RW and the CRW samples, respectively. Viable yeast count reduction was also strong, again reaching 4 logs (a 99.99% destruction) and 2.5 logs (a 99.72% destruction) for the RW and CRW samples, respectively. Viability of bacterial spores was reduced from 2.6 Log 10 CFUmL − 1 to 1.3Log 10 CFUmL − 1 for the RW samples and from 2.8 Log 10 CFUmL − 1
on the destruction of bacteria B. subtilis is presented in Fig. 6. In these experiments, the samples were exposed to cavitation for 60 min, which relates to 54 cavitation passes for developed cavitation and to 6 cavi- tation passes for supercavitation (as described in Š arc et al. [42]). When the RGHC device was equipped with the serrated rotor for the formation of multiple zones of developed cavitation, the viable count of the high initial bacterial titer (5.0 Log 10 CFUmL − 1 ) remained relatively una ff ected for the fi rst 27 passes through the treatment zone. After that, the viable count decreased slowly until the end of the experiment when the count was reduced down to 4.6 Log 10 CFU mL − 1 . In all, a slight reduction of 0.4 logs was achieved after 54 cavitation passes. However, when the RGHC was equipped with the two-teeth rotor for the gen- eration of supercavitation, the viable count of the high initial bacterial titer (5.4 Log 10 CFUmL − 1 ) rapidly declined and after 6 supercavitation passes the count was reduced down to only 3.1 Log 10 CFUmL − 1 . In all, a staggering 2.3 logs reduction (a 99.50% destruction) was achieved. Similar trends were observed for low initial bacterial titers. When RGHC was spun with the serrated rotor, the low initial titer (2.6 Log 10 CFUmL − 1 ) was slowly reduced to 2.3 Log 10 CFUmL − 1 after 54 cavi- tation passes. The total reduction of viable count was almost the same as that observed for the high initial titers. However, when super- cavitation was generated (two-teeth rotor), the low initial titer count (2.8 Log 10 CFU mL − 1 ) was again strongly reduced and after 6 super- cavitation passes only 1.6 log 10 CFUmL − 1 remained viable.
3.4. Supercavitation for the recycling of real process waters
The e ff ects of supercavitation (RGHC equipped with the two-teeth rotor), on the destruction of the major classes of microorganisms which were found to be present in the RW and CRW samples and on the chemical and physical characteristics of these samples are presented in Figs. 7 – 9. The RW or CRW samples were exposed to cavitation for
Table 1 Operational characteristics of the rotation generator. RGHC operation Rotor type
Flow rate (L/min)
Revolutions of rotor (rpm)
P L (kPa)
1.8 0.2
9,000
117.2
Developed unsteady cavitation
Serrated rotor Two-teeth rotor
10,000
93.3
Supercavitation
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