Assessing and optimizing biofilter performance in drinking water treatment
Doctoral thesis, 2020
A full-scale study involving experimental manipulation of parallel biofilters with non-adsorptive media allowed the study of DOM removal as a function of empty bed contact time (EBCT). By continuously monitoring effluent turbidity from the filters and measuring DOM removal via fluorescence spectroscopy, it was shown that turbidity and protein-like DOM removal increased linearly with increasing EBCT up until at least 80 min EBCT. Removal of refractory humic-like DOM removal improved, although to a smaller extent. This was contrary to the prevailing view that there is a negligible improvement in DOM removal efficiency at contact times longer than 30 min. Striking a good balance between DOM removal by biofiltration and the cost of longer EBCT can in turn result reduced operational costs while improving finished water quality.
This research was also carried out to distinguish biotic (biological degradation) and abiotic (adsorption and desorption) processes occurring within biofilter media. To distinguish these requires a suitable abiotic control, i.e. filter media with the same chemical properties but no biology. To identify abiotic controls for BAC filter experiments, a batch-scale study was conducted using gamma irradiation as a sterilization method. However, by measuring DOM removal via fluorescence spectroscopy, it was possible to observe that the chemical properties of biofilter materials changed even at low gamma doses (2.5 kGy) and a dose-related release of protein-like fluorophores occurred, possibly from the biofilm. The gamma-irradiation method was therefore deemed to be unsuitable for producing abiotic controls for BAC studies.
In a further attempt to identify abiotic controls for BAC filter experiments, the temperature was utilized as an alternative control strategy. Depending on responses to temperature in batch experiments, it was deduced whether DOM removal predominantly occurred via adsorption (chemisorption/physisorption) or biological degradation. Under the particular experimental conditions, there was little evidence of biological removal; instead, removal of DOM fractions emitting at longer wavelengths (“humic-like”, >430 nm) was consistent with chemisorption,
removal of DOM emitting at intermediate wavelengths (“humic-like”, 390-420 nm) was consistent with physisorption, and multiple mechanisms were indicated for “protein-like” (<380 nm) DOM. Abiotic mechanisms like adsorption are often assumed to be unimportant for aged BAC filters; however, these results suggest that abiotic processes may be important for some DOM fractions.
Ultimately, this research aims to inform the design and operation of full-scale biological filters under Nordic climate conditions. To that end, a simple and cost-effective operational strategy was investigated for improving short-term DOM removal in full-scale biological filters. The strategy involved replacing a small fraction of saturated filter media with new media. Relative to replacing the entire media, this approach required lower capital cost and shorter downtime and maintained conditions for biological filter functioning. The modified biological filters showed improved DOM removal lasting for several weeks.
The results of this thesis demonstrate that fluorescence spectroscopy, due to high analytical precision and sensitivity, is a sensitive method for tracking DOM removal via biological filters. Additionally, it suggests there are opportunities to improve drinking water treatment by promoting one or other of the removal mechanisms depending on the incoming water quality. For example, allowing longer contact time in summer when there is elevated biodegradable DOM removal or performing partial renewal of biofilter media after heavy rains when incoming water has relatively high organic pollutants. Overall, these results are relevant to water producers that aim to optimize biofilters performance under strained operating conditions.
dissolved organic matter
Chalmers, Architecture and Civil Engineering, Water Environment Technology
Moona, N., Holmes, A., Wünsch, U.J., Pettersson, T.J. and Murphy, K.R (2020). Full-scale manipulation of empty bed contact time to optimize dissolved organic matter removal by drinking water biofilters.
Temperature-dependent mechanisms of DOM removal by biological activated carbon filters
Environmental Science: Water Research and Technology,; Vol. 5(2019)p. 2232-2241
Moona, N., Wünsch, U.J., Simonsson, I., Bondelind, M., Bergstedt, O., Pettersson, T.J. and Murphy, K.R. Effects of gamma irradiation as a sterilization method on biological activated carbon
Partial renewal of granular activated carbon filters for improved drinking water treatment
Environmental Science: Water Research and Technology,; Vol. 4(2018)p. 529-538
One of the common treatment steps in DWTPs is water filtration systems. These filtration systems remove organic and inorganic materials such as microbiological contaminants and particulates like sand, clay or silt from source water. Water filters are packed with filtration material of different kinds and remove impurities from water by working like a sieve. The biological filtration system is one of the widely used treatment steps in Swedish DWTPs that uses the naturally occurring bacteria to remove organics from water. Research on biofilters is difficult at full-scale, where they are influenced by many factors that vary over time. Additionally, multiple processes occur simultaneously within biofilters including adsorption (adhesion of organic pollutants within the pores of the filter media), desorption (a release of previously adsorbed organic pollutants from the pores of the filter media) and biodegradation (degradation of organic substances by micro-organisms attached on the filter media). This research examined how the optical properties (i.e., sensitivity to light) of the organic matter can be used to provide novel insights into the performance and fundamental mechanisms of drinking water treatment via biofilters. The main outcome of this thesis is that the treatment by the biofilters in Swedish DWTPs can be improved by promoting one or other removal mechanisms in biofilters depending on the incoming water quality. For instance, lowering the flow through the biofilters may be advantageous in summer when the biodegradation rate is high whereas promoting adsorption onto biofilters is effective after periods of heavy rains when source water has relatively high organic pollutants.
Other Chemical Engineering
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4885
Chalmers University of Technology
Online i Zoom (Password: 774666)
Opponent: Riku Vahala, Department of Built Environment, Aalto University, Finland