THE SYNERGISTIC EFFECTS OF DESIGNING OZONE WITH A BIOLOGICALLY ACTIVE FILTER

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1 THE SYNERGISTIC EFFECTS OF DESIGNING OZONE WITH A BIOLOGICALLY ACTIVE FILTER Ivan Zhu, PhD, Senior Process Engineer 1 Thomas Getting, P. E., BCEE, Principal Engineer-Filtration 1 Achim Ried, PhD, Chief Engineer, Higher Order Solutions 2 Samik Mukherjee, Market Manager-Municipal Treatment 3 1. Xylem Water Solutions Zelienople LLC, PA, USA 2. Xylem Services GmbH, Herford, Germany 3. Xylem Water Solutions Australia Ltd, Sydney, NSW, Australia INTRODUCTION Municipalities and industry are now faced with the prospect of further treating, or deep polishing water and wastewater prior to use as potable water, direct or indirect reuse water or for effluent discharge to sensitive surface waters. Prior research has shown that there are few inexpensive methods that can remove or destroy all of the required micro-pollutants such as Endocrine Disrupting Compounds (EDC), Pharmaceutical and Personal Care Products (PPCP), and industrial recalcitrant Chemical Oxygen Demand (COD) that must be deep polished. Reverse Osmosis has commonly been used but is expensive and concentrates these micro- pollutants into a side stream that must be further treated. Designers are now turning to both physical-chemical and biological systems to provide treatment that can destroy the micro-pollutants before further use. In most cases, ozone alone can destroy most of the micro-pollutants. However, the ozone dosing rates can sometimes be very high increasing both the capital and operating costs while also possibly forming by-products such as assimilable organic carbon (AOC), bromate, and Nitrosodimethylamine (NDMA). Designers have found that reducing the ozone dose, followed by a biologically active filter can reduce capital and operating costs and reduce byproducts with bio-filtration, while producing a minimal non-toxic biological side stream that can be easily treated or returned to the headworks. Ozonation followed with biologically active filtration (BAF) has synergistic effects of reduced operating cost in terms of performance, media replacement, and ozone dosage, and that potentially the combined ozone and BAF can be used to replace reverse osmosis membrane process which was usually applied for indirect or direct water reuse. However, a literature survey suggests that few studies have focussed on industrial design parameters especially for the selection of media type, retention times, and effect of temperature and the optimized pre-treatment in the wastewater matrix. METHODOLOGY To investigate the operating parameters of the combined ozonation and BAF process for guiding future process design, a comprehensive pilot study was planned and started in October 2012, including ozonation and biologically active filters (BAF) at Hammarby Sjöstadsverk Wastewater Treatment Plant, Stockholm, Sweden. The pilot study lasted for two years. The ozone pilot consisted of two columns operated in series. The first column was operated in a downstream mode, and the second column was operated in an upstream mode. Each column had a water filling level of 11.8 feet (3.6 m) with an inner diameter of about 0.62 feet (0.19 m). The ozone gas was continuously bubbled into the water through the ceramic diffusor built in the bottom of each 1

2 column. Then, the ozone effluent was fed to the downstream filters. The heart of the ozone treatment system was a WEDECO MODULAR HC8 (nominal ozone production 8 g/h). The filter pilot unit was a self-sustained and self-controlled automatic system. It was mounted on a welded stainless steel skid and support structure with an instrument panel on one side. The unit was equipped with two tertiary filters 8 inches (20.3 cm) diameter and 12 feet (3.66 m) tall with a cross section area of 0.35 ft 2 ( m 2 ), with independent control systems. The online data acquisition included: influent turbidity, effluent turbidity, temperature, ph, differential pressure of the filter in the forward flow mode, differential pressure of the slotted cap (media support plate) in the forward flow mode, backwash air rate and water rate, water level during backwash, differential pressure of the cap during backwash, and influent flow rate. Table 1 shows the media configuration and sizes. Table 1 Media Configuration and Sizes Top layer Bottom layer Filter 1 Filter m (1 feet) anthracite 0.61 m (2 feet) GAC (ES mm UC (ES 1.0 mm UC 1.4) 1.5) m (1 foot) sand (ES m (1 foot) sand (ES 0.5 mm UC 1.4) 0.5 mm UC 1.4) The source water was secondary effluent after biological treatment. The period from January 16, 2014 to March 10, 2014 was for microbial acclimation. The ozone dose was maintained at 0.6 mg/l O3/ mg/ltoc. The period from March 10, 2014 to April 13, 2014 was dedicated for ozone enhanced biofiltration. In this period, there was no pre-filtration in front of ozone. The system received secondary effluent right after the secondary biological process. Ozone doses were varied from 0.6 O3/TOC to 1.2 O3/TOC. Results and Discussion Ozone Enhanced Biological Filtration During the study period, ozone dose was varied from 0.6 O3/TOC, up to 1.2 O3/TOC. The Total Suspended Solids (TSS) in the ozone influent varied between 7.91±1.92 mg/l. It was interesting to see that the average TSS concentrations were reduced to 4.03±1.01 mg/l after ozone with about 50% reduction. Figure 1 shows the actual ozone dosages and the COD changes before and after ozonation. The COD concentrations varied between 40.7 ±6.84 mg/l in the ozone inlet and 31.7±6.18 mg/l in the ozone outlet. Both filters were maintained at a flow rate of 1.96 L/min, corresponding to 15 minutes EBCT, and maintained at this contact time until the end of the study. Generally, COD removal efficiencies were similar with 0.6 and 0.8 O3/TOC, and were increased when O3/TOC was increased to 1.0 and 1.2 (Figure 2). Both BAFs showed similar COD removal efficiencies at a fixed ozone dose. When considering the overall efficiencies of the combined ozone and BAF, about 40% COD removal on an average basis was achieved at the O3/TOC ratio 0.6, and about 50% removal was achieved at the O3/TOC ratios of 0.8, 1.0, and 1.2 on an average basis. At the ratio of 0.8, the COD removal appeared to reach a plateau. 2

3 The removal of UV 254 adsorption did not follow the trending of COD removal. The removal efficiencies were averaged at around 20%, and seemed not dependent on the ozone dose (Table 2). But the removal efficiency of UV 254 was increased at the ozonation step with an increasing ozone dose (Table 2). 3

4 Figure 1 COD Reduction by ozone from March 10, 2014 to April 13, 2014 Figure 2 COD removal efficiencies of the combined O3 and BAF from March 10, 2014 to April 13, 2014 (no prefiltration) 4

5 O3/TOC Ozonation UVA Removal BAF 1 UVA Removal BAF 2 UVA Removal % 20% 26% % 15% 21% % 18% 24% % 22% 26% Table 2 UV 254 Adsorption removal of BAF (no prefiltration) from March 10, 2014 to April 13, 2014 CEC Gr. B Figure 3 Principle effects on absorbance measurements by increased ozone dosing and the corresponding effects on CECs reduction in municipal waste water The Figure 3 illustrates the key findings and correlations relating to the removal of contaminants of emerging concern (CEC) or micropollutants. Important here is to find an easy to measure signal (e.g. absorbance single wavelength or spectra) which describes the ozone reaction in a wastewater matrix. The signal change needs to be significant enough to get a good measurable delta signal. A certain part of the applied ozone to a wastewater is consumed relative quickly and no residual ozone is observed. By further increasing the ozone dose, a measurable dissolved ozone concentration will appear. One finding is that the majority of the CEC reduction has taken place in this first part of the ozone dosing. As a result the on-line signal needs to be measured describing this part of the ozonation process. For example the measurement of the UVT-254nm signal corresponds relatively well with the ozone consumption of the water. Plotting the UVT-254nm signal against the ozone dose will give the typical curve shown in Figure 3. With increased dose the absorbance is reduced. However, the curve of absorbance flattens out at the end, so that the increase of the difference is lower than that at the 5

6 beginning. A large part of the reaction of CECs takes place in the part of the curve where the slope is at its highest. When the curve flattens, the reaction of CECs is usually completed. As a result, the targeted ozone dosage to reduce the majority of CECs is achieved when the flat portion of the curve begins. A further increase of ozone dosing will generate measurable dissolved ozone which is important to achieve a stricter disinfection result. The UVT-254nm will change if residual dissolved ozone appears. For fast reacting compounds the targeted ozone dosage should be in the range of the steep part of the curve. By finding the right on-line signals which are describing the ozone consumption in the wastewater allows the operator to control the ozone dosing and gives in parallel a correlation to the possible reduction rates of specific groups of CECs. From the above discussion, it seems important to combine ozonation and biofiltration in an integrated control platform so that ozone dosage can be controlled in an efficient way to maintain effluent concentrations within targets. Both filters behaved similarly for turbidity removal. Turbidity was reduced from an average of 3.03 NTU (no pre-filtration) down to less 0.58 NTU and 0.53 NTU (average) for BAF 1 and BAF 2 (Figure 4), respectively, according to online turbidity sensors. Figure 4 Turbidity trending of BAF 1 and BAF 2 from March 10, 2014 to April 13, 2014 (no prefiltration) Assimilable organic compounds (AOC) were usually used as a parameter to evaluate the biodegradability of organic constituents. Ozonation increased biodegradability of organic constituents, and AOC increased with an increasing O3/TOC ratio (Figure 5). After biofiltration, AOC were reduced by biological activity. 6

7 Figure 5 AOC increase at different O3/TOC ratios from March 10, 2014 to April 13, 2014 (no prefiltration) The Occurrence and Mitigation of NDMA N-nitrosodimethylamine (NDMA) is a potent probable human carcinogen. Dimethylamine (DMA) is the simplest organic nitrogen precursor for NDMA, and other low molecular weight precursors include aliphatic tertiary amines and molecules with dimethylamine functional groups, which react with monochloromine to form NDMA. The NDMA occurrence seems site sepcifc. Ozone-induced NDMA formation ranging from <10 to 143 ng/l was observed at all but one site, but the reasons for the variation in formation remain unclear (Figure 6) (Gerrity et al. 2014). In this study, NDMA in the secondary effluent was under detection and so it was in the ozonation effluent. In another study, it was reported that NDMA increased by about 3-5 fold after ozonation (Figure 7) and reduced to the similar levels (of the ozone influent) after biofiltration (with biologically activated carbon or BAC) (Robinson et al. 2014). Additionally, the removal effciencies of BAF seemed to increase with the increasing empty bed contact time (EBCT) (Figure 8). 7

8 Figure 6 NDMA occurrence in a full scale survey Figure 7 NDMA formation after ozonation and reduction after biofiltration 8

9 Figure 8 NDMA reduction effciencies with biofiltration System Design Considerations As discussed above, ozone and biofiltration complemented each other in terms of COD reduction, NDMA removal, and the removal of other micropollutants, it is important to design the whole system on an integrated control platform in order to maximize the system benefits. By designing an integrated system, one master PLC is used to control the entire system. All of the system components such as the analysers and system components feed into this master unit. Control algorithms are contained within the master unit along with acquired data. The master unit then controls all of the sub-units within the system such as ozone generation, chemical feed, bio-filtration, etc. Figure 9 shows a general arrangement of a typical control system. 9

10 Figure 9 Typical System Control Scheme As shown in Figure 9, typical water sensors used for the influent, intermediate and effluent flows incorporate an advanced sensor network for multi-parameter monitoring in a simple design, for instance utilizing the Xylem Total IQ Sensor Net and the UV-Vis Sensor. The UV-Vis sensor scans 256 wavelengths to find absorption coefficients with specific water quality parameters. These coefficients are used to develop algorithms for a specific water matrix that accurately provide water quality values for variables such as TOC, UVT, TSS, and more. The Total IQ Sensor Net provides a platform to easily integrate multiple sensors with a single point of interface for both sensors and the master unit. These signals are then transmitted to the master unit for determining optimum system performance along with data acquisition. Other sensors in the ozone effluent are used to optimize ozone feed as a secondary feedback loop. Individual PLCs on the ozone generator and bio-filtration units provide signals to the master controller to provide unit status control and receive control signals from the master controller to optimize the operation of the overall as well as individual systems. The ozone-to-toc ratio drives the design of most ozone systems used to destruct micro-pollutants. High ozone to TOC ratios can completely oxidize micro-pollutants but at the expense of high electrical costs and production of by-products as mentioned before. Lowering the ratio partially destructs the micro-pollutants and reduces the ozone required. Lowering the ratio also reduces electrical consumption and by-product generation. The remaining partially oxidized compounds are further removed by bio-filtration. By measuring the influent TOC, the ozone can be fed based on an operator selectable ratio. The optimum ratio is determined by either bench scale testing, pilot testing or prior experience with the desired micro-pollutants involved. Most other systems are set at the rare, high TOC influent condition. An integrated control and sensor network allows for optimal dosing of ozone to suit the influent. The electricity required for ozone production is the single largest operational cost in an ozone system, and in most systems ozone is over-produced to suit rare, high TOC influent conditions. By using a system to match the influent requirement and determine the optimum ozone generator production, the system can adjust its ozone output to the most efficient, safe levels. Overall, this can lead to Operational 10

11 Expense savings of anywhere from 15-20% as compared to other systems. However the ozone generator system must be designed to have turn-down capabilities. Cost Savings In addition to operational cost savings compared to other O3-BAF systems, there are savings in the capital costs. By designing the two unit operations together, common wall construction can be used between the ozone contactors and the bio-filtration filters thereby saving concrete construction costs. Compared to other processes such as Reverse Osmosis, both the Opex and Capex for Ozone- Biofiltration are much less. In a major study comparing various processes for water reclamation with flows between 1 and 80 mgd (158 m3/hr to 12,640 m3/hr), the capital construction costs of MF-RO systems compared to the O3-BAC systems were between 2 and 4.6 times higher for MF-RO systems. In the same study, the operation costs of the MF-RO systems were between 6.75 and 7.2 times higher than the O3-BAC systems. In another comparison for drinking water, Figure 10 compares various features. This shows that Ozone- Biologically-active filtration treatment is 40% less expensive in terms of capital cost. It is also 50% less expensive in operation costs. This is largely due to the fact that energy consumption in O3-BAF is very low as compared to RO which requires a lot of energy to maintain the pressure necessary to operate. O3-BAF also has a far lower cost in terms of consumables. Ozone has very few consumables and the the media in the biofilter need only be replaced once a decade or longer. RO by comparison requires frequent membrane replacement. In terms of residual management, O3-BAF has a minimal waste stream, which can be returned into the system influent, while RO creates a 20% toxic brine stream that must be disposed of, which can be very costly and can render some reuse projects infeasible. However, in high TDS water, RO is the only method for removing salts. In that case Ozone-BAF can be an effective addition to the treatment train because it is a destructive technology. This means that the O3-BAF process actually destroys organic contaminants, organic carbon, and pathogens as opposed to RO which merely separates and concentrates them into side streams which must then be dealt with. NDMA is a disinfectant byproduct that is a regulated carcinogen. Ozone can create NDMA in certain cases. UV photolysis has been found to be an effective treatment for NDMA as needed, following an O3-BAF system. The BAF is also effective at removing NDMA. 11

12 Figure 10 Treatment train Comparison: O3-BAF versus RO CONCLUSION Ozone plays an important role in oxidizing chemical oxygen demand, micro-pollutants, and in increasing assimilable organic compounds, which can be removed subsequently with bio-filtration. The ozone enhanced bio-filtration system provides a physical barrier for multiple parameters including TOC, ammonia, micro-pollutants and oxidation by-products. Synergistic effects were obtained by designing the two process units as an integrated system under one control platform. REFERENCES Gerrity, D., Pisarenko, A.N., Marti, E., Trenholm, R.A., Gerringer, F., Ruengoat, J., Dickenson, E.R.V. Nitrosamines in pilot-scale and full-scale wastewater treatment plants with ozonation. Water Research, DOI: /j.watres Plumlee, M., Stanford, B., Debroux, J., Hopkins, D., and Snyder, S. (2014) Costs of Advanced Treatment in Water Reclamation, Ozone: Science & Engineering: The Journal of the International Ozone Association, 36:5, , DOI: / Robinson, K., Csalovszki, D., Berkebile, D., Gerringer, F., Venezia, T., and Trussell, S. Evaluation of Ozone to Enable Multi-Barrier Treatment for Potable Reuse IOA Montreal Wieland, A., Ried, A., and Csalovszki, D. How to control the operation of oxidation processes in waste water with the aim to remove Chemicals of Emerging Concern? A review of concepts under discussion IOA Montreal. 12