Gerald E. Speitel Jr. 1, James M. Symons 2, Marlo M. E. Wanielista 1, and Julie M. Mialaret 2
1 Department of Civil Engineering
University of Texas at Austin
Austin, Texas 78712
2 Department of Civil and Environmental
University of Houston
Houston, Texas 77204-4791
American Water Works Association
666 W. Quincy Avenue
Denver, Colorado 80235-3098
EXECUTIVE SUMMARY OF FINAL REPORT
The drinking water industry is continually challenged to provide its consumers with drinking water that is free from microbial pathogens. The disinfectant of choice historically was chlorine, until the discovery in 1974 that the addition of chlorine to drinking water resulted in the formation of trihalomethanes (THMs). Additionally, other potentially dangerous disinfection by-products (DBPs) can result from the reaction of chlorine with natural organic matter (NOM), for example, haloacetic acids (HAAs). The U.S. Environmental Protection Agency (USEPA) currently regulates THMs and is developing new regulations for microbials (M) such as Cryptosporidium and Giardia, disinfectants (D), and DBPs. The 1996 Safe Drinking Water Act (SDWA) Amendments require the USEPA to meet aggressive deadlines for regulation of M/D/DBPs. Promulgation of the Stage 1 M/DBP rule and the Interim Enhanced Surface Water Treatment Rule (IESWTR) is anticipated in November, 1998.
In response to current regulations and projected long-term regulatory trends,
significant changes in treatment practices are taking place. Changes in treatment practice
are occurring especially in two areas: the use of alternative disinfectants and the
removal of DBP precursor material, typically NOM, which generally is measured by the total
organic carbon (TOC) analysis. One group of alternative disinfectants of specific interest
to this research is the nonhalogen-based disinfectants and oxidants, such as ozone and
ultraviolet (UV) irradiation.
Ozonation has at least one disadvantage, however, in that it produces biodegradable organic matter (BOM) that can lead to microbial regrowth problems when BOM is biodegraded in the water distribution system. Removal of BOM through a biological treatment step would produce a biologically stable water that would minimize microorganism regrowth and reduce disinfectant demand. Therefore, to achieve acceptable water quality, the introduction of a biological treatment process will most likely be necessary to facilitate the widespread application of ozonation as a viable treatment process. For smaller water utilities, however, the complexity and cost of ozone based systems make UV irradiation a more attractive disinfection alternative. The production of DBPs is not eliminated with either disinfectant, however, because both ozone and UV require a secondary disinfectant, typically chlorine or chloramines, to maintain a residual in the distribution system and ozone produces DBPs as it reacts with NOM.
Precursor removal, the second area of change in treatment practice, can be achieved in
several ways. The M/D/DBP rule proposes enhanced coagulation as the standard precursor
removal process, using TOC as the surrogate measurement. Precursor removal also can be
achieved by using nonhalogen-based disinfectants in combination with biodegradation. In
particular, considerable research has been performed on ozonation followed by
biodegradation. The combination is potentially attractive because the two processes
together have shown a better ability to remove TOC and DBP precursors than either alone,
although effective removal may require larger ozone doses than used in practice today or
needed for disinfection alone. Advanced oxidation processes (AOPs) (e.g.,
ozone/hydrogen peroxide [O3/H2O2] and UV/hydrogen peroxide [UV/H2O2])
represent alternatives than may be more effective and less costly than ozone alone as an
oxidation step before biodegradation. The addition of hydrogen peroxide to the ozonation
process might allow the same degree of oxidation at a lower ozone dose, while the addition
of hydrogen peroxide to the UV disinfection process greatly increases the TOC oxidation
capability relative to UV alone, which has essentially none.
Unfortunately, little information is available on the performance of advanced oxidation processes in combination with biodegradation for TOC and DBP precursor removal. Therefore, this project was undertaken as a broad feasibility study of O3/H2O2/biodegradation and UV/H2O2/biodegradation for the control of TOC and DBP precursors. The two treatment processes were compared to one another, to ozone (O3)/biodegradation, and to enhanced coagulation and softening (conventional and enhanced) in treated surface water of different qualities. The participants in this project were the University of Texas at Austin, the University of Houston, the City of Austin, and the City of Houston.
The main objectives of the research were:
1. to define the potential of the two advanced oxidation/biodegradation processes for TOC and DBP precursor removal in two typical surface waters,
2. to demonstrate successful performance of both advanced oxidation/biodegradation processes in laboratory-scale, continuous-flow treatment units, and
3. to compare process performance with that of enhanced coagulation and conventional and enhanced softening for TOC and halogen-substituted DBP precursor removal.
The principal water quality parameters measured in this research to assess process performance were TOC, BDOC, UV254 absorbance, and DOXFP4. Bromate and bromide concentrations also were measured in LAW when ozone was used.
The project consisted of three phases, each involving advanced oxidation by O3/H2O2 and UV/H2O2 in combination with biodegradation. All three phases were conducted in Lake Austin water (LAW) and Lake Houston water (LHW). In Phases I and II, the performance of the AOPs were compared to each other, as well as to O3/biodegradation.
In Phase I, a series of bench-scale, batch, exhaustive biodegradation experiments (i.e., biodegradable dissolved organic carbon [BDOC] determinations) were performed on water oxidized under a wide variety of conditions to broadly characterize the potential of the AOP/biodegradation processes. In each experiment water was first oxidized, then subjected to complete biodegradation. TOC, UV254 absorbance, and halogen-substituted DBP precursor removal, as determined by a four-day dissolved organic halogen formation potential (DOXFP4) test, were measured after oxidation and after biodegradation, as the primary indicators of treatment performance. Other analyses, including bromate concentration and disinfectant demand, also were performed. Initial screening with batch experiments permitted evaluation of a broad range of potential operating conditions within a reasonable time, although a detailed analysis of any one condition was not possible. The results of the Phase I work are reported in Chapter 5.
The most promising operating condition for each AOP identified in the first phase was further investigated in Phase II. Water underwent oxidation periodically in semi-batch (O3/H2O2) or continuous-flow (UV/H2O2) reactors, while biodegradation occurred continuously in laboratory-scale columns packed with granular activated carbon (GAC) exhausted with respect to TOC. Two columns were operated in series with the first designed to simulate a filter sorber and the second a post-filter sorber. The results of the Phase II work are reported in Chapter 6.
In Phase III, pilot-scale experiments with enhanced coagulation (LHW) and conventional
and enhanced softening (LAW) also were performed to provide comparative performance data
to assist in evaluating the potential of the advanced oxidation/biodegradation processes
relative to other treatment technologies. The results of the Phase III work are reported
in Chapter 7.
1. Of the batch conditions tested with O3/H2O2, the most promising operating condition for both LAW and LHW was the one with the greatest amount of H2O2 added, 1.3 mg H2O2/mg O3. The ozone dose was 1 mg O3/mg TOC. In LAW, the addition of H2O2 in combination with a larger ozone dose (2 mg O3/mg TOC) offered no benefits relative to ozone alone, aside from the possibility of controlling bromate formation.
2. Of the batch conditions tested with UV/H2O2, the most promising operating condition for both LAW and LHW was judged to be a normalized energy input, called the driving force (DF), of 0.5 W-min/Ámol TOC and a H2O2/TOC molar ratio of 5:1. Additional oxidation occurred at larger DF values, but the incremental increase steadily diminished and the additional energy investment was not judged to be worthwhile. Also, the goal was not to directly oxidize TOC, but rather to make it more biodegradable, and this goal was achieved at the DF value selected.
3. The O3/H2O2 process performed similarly in both water sources. Substantial removal of TOC, UV254 absorbance, DOXFP4, and chlorine demand occurred, as well as substantial formation of BDOC in the oxidation step. The chloramine demand unexpectedly increased after treatment, which is undesirable. Although the O3/H2O2 process performed well on both water sources, its performance, in general, was not superior to ozone alone at the identical ozone dose. Two notable exceptions were the markedly lower formation of bromate in LAW with O3/H2O2 and the greater removal of DOXFP4 in LHW. The DOXFP4 removal in LHW and the general trend of decreasing DOXFP4 concentration with increasing H2O2 dose in LAW suggest that, at a sufficiently large H2O2 dose, the O3/H2O2 process would have performed better than ozone alone in both water sources for DBP precursor removal.
4. The relative comparison between O3/H2O2 and ozone in the treatment of LAW and LHW does not fully agree with other research. Karpel et al. (1996) showed distinct advantages of O3/H2O2 over ozone for TOC removal and BDOC formation in waters having a greater humic content (i.e., larger SUVA values) than LAW and LHW, which have low to moderate SUVA values.
5. In general, the performance of both the oxidation and biodegradation steps with UV/H2O2 was similar for the two waters. The most
notable exception is in the removal of DOXFP4. Significantly more
removal occurred through oxidation in LAW, but significantly less removal occurred through
biodegradation in comparison to LHW.
6. In LAW, the TOC removal achieved through oxidation with UV/H2O2 was comparable to that with ozone alone and better than that with O3/H2O2, while in LHW the TOC removal achieved through oxidation with UV/H2O2 was better than that with either ozone or O3/H2O2. In both waters, UV/H2O2 did not produce as much BDOC as ozone; therefore, TOC removal during biodegradation was less than with ozone. In general, the effectiveness of UV/H2O2 in producing BDOC was comparable to that of O3/H2O2
7. In LAW, UV/H2O2 provided the best removal of DOXFP4 through oxidation; unfortunately, little additional removal of DOXFP4 occurred in the biodegradation step. Because of the small removal of DOXFP4 through biodegradation, the overall performance of UV/H2O2 was similar to that of O3/H2O2 and poorer than that of ozone. In LHW, UV/H2O2 clearly provided the best removal of DOXFP4. The DOXFP4 concentration after biodegradation was substantially less than that for both ozone alone and O3/H2O2, demonstrating that the UV/H2O2 process does have an inherent ability to convert DBP precursors to a biodegradable form.
8. In LAW and LHW, UV/H2O2 oxidation destroyed UV-absorbing chemicals to a smaller extent than ozone and O3/H2O2 oxidation; however, significant removal of UV-absorbing chemicals occurred during biodegradation.
9. Increases in both UV254 absorbance and SUVA were observed in the batch biodegradation experiments in both waters studied. Accumulation of UV-absorbing metabolic products is advanced as a possible explanation of these results. The absence of such increases during the "milder" biodegradation conditions in the continuous-flow experiments (see below) provides support for the concept of accumulation of metabolic products in the batch system. This occurred with all three oxidation/biodegradation schemes studied.
10. The batch experiments demonstrated that, in general, the three oxidation/biodegradation processes performed similarly. The performance of the UV/H2O2 process when compared to ozonation was very encouraging, however, and suggested that benefits similar to ozonation for TOC and DBP precursor control could be obtained using less complicated technology. The use of less complicated technology may be especially attractive for small systems. An additional benefit of the UV/H2O2 process relative to ozonation is the absence of bromate formation in bromide-containing waters.
1. In LAW, TOC removal in the biofilters was about the same in all three processes, but DOXFP4 removal in the biofilters was greater with the two AOPs in comparison to ozonation. Because of differences in DOXFP4 removal in oxidation, overall removal with UV/H2O2 was slightly better than with ozone. Overall removal of DOXFP4 with O3/H2O2 was poorer than with the other two processes.
2. In LHW, TOC removal in the biofilters was the same with ozone or O3/H2O2, and somewhat greater with UV/H2O2. DOXFP4 removal in the biofilters was about the same for all three processes. Because of differences in DOXFP4 removal in oxidation, overall removal of DOXFP4 was best with ozone.
3. Although the differences among the treatment processes were not great in both waters, an overall assessment does point to UV/H2O2 as performing better than O3/H2O2, which agrees with the findings of the batch experiments. O3/H2O2 performance was, in general, inferior to ozone, while UV/H2O2 performance was comparable to ozone. The similar performance of the three processes can be viewed positively for the UV/H2O2 process, but negatively for the O3/H2O2 process. The comparable performance of the UV/H2O2 process and ozonation demonstrates that the UV/H2O2 process is a technically-viable alternative to ozonation, while the performance of O3/H2O2 suggests it offers little beyond what can be achieved with ozone alone.
4. Except for a few isolated parameters studied, only a small change in cumulative percent biodegradation occurred in the second biofilter for both waters, indicating that only a relatively short biodegradation contact time was needed to achieve substantial biodegradation.
5. The biodegradation results from the batch and the continuous-flow experi-ments were in remarkable agreement, considering the very large differences in overall biodegradation contact times. Therefore, simple batch biodegradation experiments can provide an initial estimate of process performance, although such estimates will clearly overstate to some extent the level of performance that can be achieved in typical continuous-flow biofilters.
1. In the pilot-plant comparisons with LAW, in almost all respects the oxidation/biodegradation processes clearly outperformed both softening and enhanced softening.
2. In the pilot-plant comparisons with LHW, if the second biofilter is considered, oxidation/biodegradation provided a small performance improvement over enhanced coagulation. Overall, the oxidation/biodegradation (second biofilter) processes performed comparably to enhanced coagulation.
Water utilities faced with excess concentrations of DBP precursors in their finished waters, in spite of adding additional coagulant to maximize TOC removal during clarification or softening, have had few treatment options in the past, e.g., adsorption or membrane treatment. Past research has shown that higher than usual doses of ozone followed by biofiltration was a viable option that had the advantage of actually removing DBP precursors, not just concentrating them for future treatment as adsorption and membrane treatment do.
The disadvantage of ozonation/biodegradation is twofold: (1) although ozonation is gaining in popularity, many utilities find it difficult to manage, and (2) although ozonation/biodegradation is effective, it is not completely effective. The purpose of this study, therefore, was: (1) to determine if ozone were to be used, could its performance be enhanced by the addition of hydrogen peroxide, and (2) would an oxidation scheme that avoids ozonation altogether (UV/H2O2) be possible.
The results of this broad feasibility study showed that the addition of hydrogen peroxide during ozonation did not enhance performance in the two low-SUVA waters studied, in spite of the reports of others to the contrary in waters having larger SUVA values. Because of conflicting results and the limited amount of data on the O3/H2O2 process for DBP control, limited testing to assess process performance may be worthwhile for source waters where ozone alone proves unsatisfactory. In this research, hydrogen peroxide addition did limit bromate formation in the bromide-containing water studied. Therefore, the O3/H2O2 process may be attractive for controlling bromate, although it must compete with other approaches for bromate control. Also, utilities practicing ozone disinfection must be certain that hydrogen peroxide addition does not decrease aqueous ozone concentrations below acceptable levels.
The UV/H2O2/biodegradation process, in general, slightly outperformed ozone/ biodegradation for control of most of the parameters measured, and is a much simpler process (no mass transfer, in situ oxidant generation, or toxicity problems, for example). Water utilities considering advanced DBP precursor removal strategies should study the UV/H2O2/biodegradation process for applicability in their particular water, especially utilities interested in avoiding the operational complexities of ozonation. Another advantage of UV/H2O2 over ozonation for bromide-containing waters is the absence of bromate formation during oxidation.
Batch oxidation and biodegradation screening tests of the type performed in this research are recommended as a first step in assessing the attractiveness of advanced oxidation/biodegradation. In this research, DBP precursors were characterized by DOXFP4. Evaluation of specific DBPs in addition to DOX is recommended in screening studies of this process. Formation potential and simulated distribution system data for THMs and HAAs would provide the basis for a more comprehensive assessment of the UV/H2O2/ biodegradation process for a particular source water.
The benefits of biological processes, in general, in drinking water treatment are also worth noting. Ozonation and advanced oxidation processes form BDOC, as either an intended or unintended consequence of the oxidation step. This research showed that biological filtration effectively removes BDOC for each of the three oxidation processes studied. In fact, the kinetics of biodegradation are rapid enough that one biofilter should normally be sufficient to the remove the bulk of the BDOC. Removal of BDOC also represents removal of DBP precursors and a decreased disinfectant demand in the treated water. Improvements in distribution system water quality accrue because of less potential for microbial regrowth, formation of lower concentrations of DBPs, and longer lasting disinfectant residual concentrations.
Speitel, G. E. Jr., Symons, J. M., Mialaret, J.
M., and Wanielista, M. M. E., "AOP/Biofilm Processes for DOX Precursors",
Journal of the American Water Works
Association, 92, (10), 59-73 (October, 2000).