Principal Investigator: Rasheed Ahmad (Georgia Institute of Technology)
Principal Investigator: A. Amirtharajah (Georgia Institute of Technology)
Sponsor: GWRI
Start Date: 1994-04-01; Completion Date: 1995-03-01;
Keywords: Water Treatment, Biofilms, Microbial Detachment, Hydrophobicity, Hydrophobic Interaction, Biological Filtration, Backwashing, Bed Expansions, Air Scour
Description:
Biological Filtration may become an essential part of drinking water treatment in the United States during the next several decades. Since biological granular activated carbon (GAC) filters accumulate both biological and non-biological particles on the filter bed, the differences in the detachment of these groups of particles during backwashing will be an important aspect of operation of these filters. Among mechanisms controlling attached biofilm reactor performance, biofilm detachment is one of the aspects of the process that is least understood and studied. This study presents the results of an analysis of the detachment of particles from biofilms in terms of a microscopic force model and the evaluation of this model with results from operation of a pilot filter. Experiments on attachment and detachment were completed in a packed glass column with GAC/sand media and an indigenous bacterial population. The operation of a biological filter was simulated by using filter influent (nonchlorinated) water from the nearby Water Treatment Plant with the addition of a carbon source. Bacterial hydrophobicity is an important factor in the study of microbial attachment and detachment from surfaces. To develop a valid microscopic force model for microbial detachment, the hydrophobic force must be included in the total force calculations. The microscopic force model for detachment includes electrostatic, van der Waals, Born repulsion and hydrophobic forces. In these calculations, hydrophobic colloids produce a deeper primary minimum as compared to hydrophilic colloids. Consequently, hydrophobic bacteria are far more strongly attached to the GAC surface as compared to hydrophilic clay particles. During backwash of a biological filter, the maximum turbidity occurs close to zero time in contrast to the maximum microbial detachment which occurs later. In biological filters, the location and degree of attachment of biological (bacteria) and non-biological (clay) particles to the GAC media surface are different. Hence, their mechanics of removal and times of detachment are different. The use of different bed expansions during backwash has a bearing on microbial detachment. A sixty percent bed expansion produced maximum bacterial removal. The optimum porosity expression for traditional filter backwash can be applied to biological filters in drinking water treatment. Water washed filters even at optimum porosity build up excessive headloss over successive filter runs. Hence, fluidization water wash alone is not an effective method to clean biological filters. Biological filters for drinking water treatment need air scour. The biological filters backwashed with a combination of air plus subfluidization water flow at collapse-pulsing followed by water wash with 20% bed expansion produced not only lower headloss but also little headloss build up over successive runs. The chemistry of the backwash water has an impact on biofilm detachment. Limited experiments suggest that chlorinated backwash is detrimental to biological filtration. The biofilm detachment model developed in terms of forces at the microscopic level, explains the detachment mechanism in biological filtration and is consistent with the experimental results.