As the quality of raw water deteriorates, a number of additional treatment techniques have been developed and adapted to water treatment for producing safe and aesthetically satisfactory drinking water. One of various alternatives is to use a biological process, such as a submerged bed filter, as a first process in the treatment train followed by conventional treatment processes. Biologically stable water promote relatively low growth of microorganisms during its journey in the distribution network. Biofilm system seems suitable for the pretreatment of water because biofilm can act as niches where slow-growing degraders can survive at low organic concentrations. This thesis describes the experimental study of biological treatment of drinking water. Biofilm processes to remove the primary causes of biological regrowth including ammonia and biodegradable organic matter are examined.
Effects of hydraulic retention time (HRT) and temperature on the efficiency of submerged biofilm reactors were investigated. Removal efficiency of total organic carbon (TOC), turbidity, and algae increased with HRT. It is considered that over one hr HRT is needed to ensure the removal of higher than 30% TOC. In comparison to honey comb type media, fibrous media were more favorable for the attachment and growth of active microorganisms. The close relationship with chlorophyll-α concentration and particulate organic carbon (POC) concentration make POC a surrogate parameter for estimating the removal efficiency of algae.
The presence of ammonia, usually in the form of ammonium ion ($NH_4^+$), could make the production of drinking water more costly if ammonium must be removed to ensure good disinfection. Removal of ammonia by biological oxidation could be economical which prevents excess chlorine dosage. The biofilm reactors are effective in efficiency for biological solids capture and hydraulic control. The results indicate that biofilm can remove over 77 percent of ammonia with HRT of longer than 2 hr even at low temperature ranging from 14.6 to 16.6℃. The HRT has a significant effect on nitrification. The overall efficiency of nitrification and ammonia removal increases with increasing HRT. It has been also observed that when the fibrous media was used, the ammonia removal, nitrification rate, and endurance to shock were improved.
The submerged bioreactors were found to remove the taste and odor compounds by the mechanism of air stripping and secondary substrate utilization. The bioreactor did not effectively remove the trihalomethane (THM) precursors when the reactor had the clogging problems. A submerged biofilm, however, which had no clogging problems could be recommended as a primary treatment process for the removal of taste and odor compounds as well as THM precursors. The measured turbidity and TOC might be able to be used as surrogate parameters to evaluate the THM precursor content in water treatment plant.
Pilot-scale biofilm reactors were evaluated with various contaminants that were frequently encountered in water supplies. It was found that the water was more biologically stable when treated in the submerged biofilm process even though the reduction of organics was relatively low. Biofilm grown on natural water supplies was effective in removing THM precursors as well as taste and odor compounds such as Geosmin.
As humic substances left in treated water tend to form THMs during chlorination, their removal in water treatment processes is a significant concern for drinking water supplies. One of the removal technologies, biofilm reactor is studied for the microbial decomposition of aquatic fulvic acid (AFA) that is characterized by elemental analysis, UV/Vis, $^{13}C$-NMR, and IR spectroscopic methods. Biological process showed the potential in removing AFA. The spectroscopic investigation appears to be useful means in characterizing the microbial decomposition of AFA. As the influent FA was the only electron donor in the feed, its oxidation should provide carbon and energy sources for biofilm growth and maintenance. Biologically treated fulvic acid was in more oxidized state whose spectra displayed higher degree of condensation of aromatic constituents than influent fulvic acid. Microbial degradation of AFA was more active in lower molecular weight fractions and was intensively occurred in aliphatic fraction.
Another study was carried out to evaluate the quantitative characteristics of phenol biodegradation and to establish the effective control techniques using biofilm process. The result from experiments of biofilm process showed that acetate and ammonia as well as fulvic acid took a role as primary substrates to form steady-state biofilm. Biofilm grown on each substrate could use phenol as a secondary substrate so as to kept phenol concentration in effluent at low level. Although the three columns had different potential electron donors for supporting heterotrophic growth, the extent of phenol removal was not significantly different among three columns. Thus, N-column, fed with ammonia, achieved almost as much removal as A- and F-column, which were fed acetate and fulvic acid as a primary substrate, respectively. This result suggested that the primary substrate for the heterotrophic biomass that used the aromatic compounds was comprised of traces of unidentified organic material present in the feed solution and soluble microbial products produced by the nitrifiers.
