Abstract:
Biofilms are important in nature and in engineered processes. Because of this, a fundamental understanding of their growth and behaviour is required. This work aimed at monitoring biofilm growth using a biological rotating reactor and the Rotoscope biofilm monitor. Both methods worked on the principle of a rotating circular disc that was semi-submerged in water and the light reflected of the area that was outside of the water. Light reflectance on the disc was taken three times a day and the average recorded as the daily reading. It was noticed that in both systems, growth of biofilms on the discs caused a decrease in the amount of light reflected. A decrease in light reflectance indicated an increase in biofilm thickness. The growth of biofilm was confirmed by scanning electron microscopy analysis. The addition of a biocide caused a slight increase in light reflectance indicating partial biofilm removal. The Rotoscope was very sensitive to changes in biofilm characteristics. Rotoscope met the requirements needed for an on-line, real-time and non-destructive biofilm monitoring system. The aged anolyte was effective in killing both suspended and biofilm bacteria at a concentration of 1:10 irrespective of its age and storage conditions. Exposure of aerobic bacteria to different concentrations of sodium nitrite at different time intervals indicated that sodium nitrite had a limited, or no biocidal effect on these bacteria mostly encountered in biofilms. The ready to use chlorine dioxide was also used as the means of controlling biofilms. MIC for RTU ClO2 was found to be 80ppm, which in certain instances killed all bacteria immediately upon exposure while in other cases an exposure time of 1h was required. It was indicated that at this concentration, biofilms were removed. This was confirmed by scanning electron microscopy analysis. Proteins of suspended bacteria treated with 1:10 and 1:100 anolyte dilutions and the control were extracted and compared using SDS-PAGE. Protein bands of bacteria treated with 1:10 NaCl derived anolyte were fewer and fainter as compared to those from untreated cells. More bands were produced in cells treated with 1:100 NaCl derived anolyte as compared to the untreated cells. Cells treated with the non-halide anolyte, both 1:10 and 1:100 dilutions, produced more bands than in the untreated cells. Anolyte destroyed vital proteins for bacterial survival causing cell death or it caused fragmentation of proteins to small peptides, reducing the number of viable cells. NaNO2 was ineffective as biocide while aged anolyte and RTU liquid ClO2 were effective as biocides. SDS-PAGE indicated that anolyte killed bacteria by affecting their proteins.
