EVALUATION OF TECHNIQUES TO CONTROL FILTER RIPENING TURBIDITY SPIKES AT SOUTH AUSTRALIAN WATER FILTRATION PLANTS

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1 EVALUATION OF TECHNIQUES TO CONTROL FILTER RIPENING TURBIDITY SPIKES AT SOUTH AUSTRALIAN WATER FILTRATION PLANTS Daniel Bonini 1 1. SA Water, Adelaide, SA, Australia ABSTRACT Filter ripening spikes are a critical feature in filtration plants as the period of elevated turbidity represents an increased risk of disinfectionresistant pathogens entering the product water. The effectiveness of three different techniques to control turbidity spikes associated with filter ripening was studied at full-scale. The techniques tested were the delayed (or filter resting), slow and the extended terminal subfluidisation wash (ETSW). The ETSW was found to be the most effective technique at reducing the passage of solids into the filtered water during the filter ripening phase. Use of slow was also shown to be beneficial although not to the extent of ETSW. Filter resting was found to be ineffective in controlling ripening spikes. INTRODUCTION A spike in filtrate turbidity following a backwash is a common occurrence in rapid gravity filters. The duration of the spike is commonly referred to as the filter ripening period. Filter ripening is thought to be a result of particles remaining in the filter after a backwash and newly influent particles penetrating the disturbed filter media. For some drinking water treatment plants, the weak point in the treatment process is filter ripening. An elevated filtrate turbidity can be associated with an increased risk of disinfection-resistant pathogen breakthrough in product water. Cryptosporidium and giardia, for example, are resistant to conventional chlorine/chloramine disinfection practices; hence filtration is critical as it is the last point in the treatment plant that is capable of removing any potential pathogens from the water before distribution. There are several techniques available for controlling the initial filtered water quality after backwashing. Of interest to this study are three techniques that have been published in the water treatment literature. Filter resting involves keeping the filter out of service for a period of time following a backwash. Slow ing a filter involves a controlled rate of flow increase through a filter before reaching the final filtration rate. Extended terminal subfluidisation wash (ETSW) is an advanced backwashing procedure that extends the normal backwash duration at a subfluidisation flow rate (with very little or no bed expansion) for an amount of time sufficient to displace the entire volume of water contained within the filter cell. The aim of this study was to test, compare and evaluate each of these techniques in order to better understand their practicality at full scale. BACKGROUND Filter Resting (Delayed Start) Logsdon & Hess (2002) describe the delayed as a process of allowing the filter media to settle in by holding the filter out of service for a period of time after the conclusion of the backwash. The claim is that this procedure helps minimise the initial turbidity spike by allowing the filter media to consolidate slightly after backwashing. It may also help by allowing floc particles that were not washed out of the filter cell during backwash to settle to the top of the media, as well as allowing floc that wasn t removed from the expanded filter bed to attach to the filter media. In a study by Logsdon and Hess (2002) of plants that were able to consistently control initial turbidity to 0.3 NTU or less, 23 out of 37 plants were using the delayed technique either alone or in a combination with one or more of the other techniques. The time for which a filter should be rested in the delayed technique is debated in the literature. Logsdon & Hess (2002) claim that resting times ranging from 15 minutes to 48 hours have been beneficial with a particular study by Thames Water Utilities showing the most effective resting times between 46 and 144 minutes. Pizzi (1996) on the other hand suggests a resting time of at least four hours before any potential benefit will be realised. Logsdon & Hess (2002) also emphasise that filters cannot be rested indefinitely as microbiological problems can develop. When a filter has been out of service for an excessive period of time, backwashing is recommended to refresh the filter and avoid water quality problems.

2 Slow (Gradual) Start The theory behind the slow procedure is that by using multiple rate increases and limiting the magnitude of each increase, the shock on the particles remaining in the filter bed after a backwash can be minimised (Logsdon et al. 2005). The theory has similar reasoning to the philosophy that discourages stop- operation at water filtration plants. Large changes in flow rates can cause hydraulic shock to the media and associated particles due to the forces exerted by a rapid change in fluid velocity. Logsdon et al. (2005) has given minutes as a reasonable period over which filter flow can be gradually increased in a number of small steps. In this method, the steps should be as small as possible to replicate a linear ramp up of filtration rate, although this can be limited by the precision of filter outlet control valves. A common criticism of the slow method is that while the magnitude of the ripening peak may be trimmed during a throttled -up, the overall ripening period becomes longer meaning that in total, a similar amount of solids pass into the filtered water. Whilst the trimming of ripening peak may help a water treatment plant to achieve its performance goals, it may not necessarily reduce the potential for pathogens to pass into product water which ultimately is the goal of optimising filter operation. Logsdon et al. (2005) reports on a number of plants trialling this method using particles counters, some of which recorded a reduction in total particle breakthrough while others found a prolonged ripening period with little or no difference in total particle breakthrough. Extended Terminal Subfluidisation Wash (ETSW) ETSW is an additional washing stage at the end of the backwash sequence with the intended purpose of rinsing out the majority of the particles sheared from the filter media during the fluidisation stage of the backwash. This is achieved through a washwater flow rate below the minimum fluidisation velocity of the media for a duration sufficient to displace one entire filter volume of particle-laden water. ETSW has been described as a rinse-towaste as it essentially acts as a filter-to-waste procedure in reverse. The water that remains in the filter after a standard backwash would usually become the initial part of the filtrate. With ETSW, this water is flushed out of the filter before the filtration cycle commences so that the initial filtrate is of much higher quality. Amburgey (2005) explains that the lower flow rates associated with the ETSW procedure produce smaller shear forces at the surface of media grains resulting in fewer additional particles being detatched from the media than the higher flow fluidisation stage. At the same time the particles already detached during fluidisation (backwash remnant particles) are transported out of the filter. The result of the process is a filter cell with a very low concentration of solids. Based on the mechanics of filter ripening discussed in detail by Amburgey (2005), this should alleviate much of the ripening effects associated with backwash remnant particles. Amburgey s (2005a) work found ETSW was able to reduce turbidity passage by up to 50% in the first 22 minutes of filter operation following a backwash and in some cases was able to almost eliminate the ripening spike. Amburgey (2005) has also attempted to explain the apparent conflict of theories between the ETSW theory and the commonly accepted additional collector theory of media filtration. The latter suggests a level of particle attachment to the media is actually beneficial to filtration through an additional collector effect in which the attached particles act to provide an added barrier to influent particles. If this is the case then it seems counterintuitive to aim for such a low concentration of remnant backwash particles. However, Amburgey (2005) found that the collector particles sheared off the media during a backwash tend to revert back to their original raw water zeta potential. Consequently any particles remaining in the filter following a backwash do not readily reattach themselves to the media. This concept explains why ETSW can be implemented without risk of over-cleaning the filter and reducing performance. MATERIALS AND METHODS Full-scale Treatment Plant The Morgan Water Treatment Plant (WTP) was the primary test site for the study. The plant employs a conventional treatment process extracting water from the River Murray and serving approximately 130,000 regional SA customers. The plant has a design capacity of 200 ML/d. The process consists of alum coagulation, flocculation, gravity sedimentation, filtration, and chloramine disinfection. Settled water from the two sedimentation tanks flows into a filter inlet channel where it is directed into a maximum of eight rapid gravity filters. The filters utilise dual media with a 750mm layer of mm effective size (ES) anthracite (uniformity coefficient UC < 1.5) covering a 300mm bed of mm ES sand (UC < 1.5). The dual media is supported by a 200mm gravel base. Each filter has a nominal filtration area of 72 m 2 and a maximum design loading rate of 14.5 m/h. The outlet flow from each filter is programmed in the control system in the range L/s which corresponds to a filtration rate of m/h. Each of the filters is cleaned regularly with a backwash process that includes an air scour, followed by a three stage water wash including subfluidising and fluidising rates. When a backwash

3 is initiated, the filter inlet valve is closed and the filter drains to a level just above the filter media. The flow modulating outlet valve is then closed and the backwash outlet valve opened. Air is then blown up through the filter. Following completion of the air scour, non-chlorinated, filtered water is pumped up through the filter bed. Dirty backwash water overflows into launders, and is directed to the waste wash water tank. The duration of the backwash process is approximately 6 minutes, with the filter cleaning cycle, including draining and refilling, taking approximately 20 minutes. A small number of proof of concept trials were also carried out at the Anstey Hill and Mt Pleasant WTPs. Anstey Hill is an Adelaide metropolitan filtration plant very similar in size and technology to Morgan WTP. Mt Pleasant is a much smaller plant serving a small area in the Adelaide Hills. Both plants utilise dual media filters. Filter Resting Full-Scale Trials Filters were trialled at resting times of 15 minutes, 30 minutes, 45 minutes, 1 hour, 2 hours, 3 hours, and 4 hours in no particular order and as time and plant operation permitted. The study aimed to replicate typical operating conditions as closely as possible and hence backwashes were not forced partway through a filtration cycle for the purpose of the trial. A trial was only commenced once the filter reached its run time limit or head loss became excessive. Slow Start Full-Scale Trials At Morgan WTP, the filtration rate is controlled by the filter outlet valves. The valves are pneumatically controlled butterfly-type. At the conclusion of the backwash, the filter to be tested was briefly set to standby mode whilst changes were made to the outlet valve control setting. The outlet valve control mode was set to local (i.e. manual flow control) so that the filtration rate could be manually entered. The trial filter was then brought on-line at the first rate specified by the slow flow schedules (Table 1 for the 15-minute trial and Table 2 for the 30 minute trial). A stop watch was used to monitor elapsed time during the procedure and every 3 minutes, the flow rate was adjusted in the supervisory control and data aquistion (SCADA) system according to the trial schedule. At the conclusion of the trial, the filter outlet control valve was returned to automatic control. Table 1: 15 minute slow flow schedule Time since filter ed (minutes) Filtration Flow (L/s) Filtration rate (m/h) 0 to to to to to and on Auto (>200) Auto (>10) Table 2: 30 minute slow flow schedule Time since filter ed (minutes) Filtration Flow (L/s) Filtration rate (m/h) 0 to to to to to to to to to to and on Auto (>200) Auto (>10) ETSW Full-Scale Trials The two main requirements of an ETSW are subfluidisation wash water flow rates and a sufficient volume of water to displace one entire filter cell s water content. These two requirements necesscitated a number of calculations. A simplified method described in Amburgey and Brouckaert (2005) was used to determine the appropriate ETSW backwash settings for Morgan WTP. The existing Morgan WTP backwash sequence and the modified ETSW sequence are detailed in Tables 3 and 4 respectively. Table 3: Standard backwash sequence Morgan WTP Flow (L/s) Rate (m/h) Duration (sec) Volume (L) Low Flow ,000 High Flow ,000 Medium Flow ,500 Total ,500

4 Area under the curve (NTU.min) Table 4: ETSW backwash sequence Morgan WTP Flow (L/s) Rate (m/h) Duration (sec) Volume (L) Low Flow ,000 High Flow ,000 decreasing turbidity peak with longer resting times as suggested in the literature. As the trials took place on a working water treatment plant subject to variable raw water quality, there are some notable offsets in ripened filtrate turbidity between trials, but this is corrected in the numerial analyis by the normalisation procedure previously described. ETSW ,782 Total ,782 Analysis Method A common method was required to compare the efficacy of each of the techniques trialled in the study. Whilst peak turbidity is a good ing point, it is not a robust measure of total solids (and potentially pathogen) passage into filtered water. Turbidity provides an on-line surrogate measure of suspended solids concentration and as such must be analysed alongside the quantity of water in question. For example, a turbidity spike of 0.70 NTU for 30 seconds would be of less concern than if the same 0.70 NTU level of turbidity persisted for 15 minutes, as the quantity of solids passing into the filtered water would be significantly higher. Figure 1: Ripening spikes from initial filter resting trials A more meaningful analysis involves the integration method employed by Amburgey (2003). In this method, the area under the curve of the turbiditytime plot is estimated using the trapezoidal method of numerical integration. Over a given time period a single value (NTU.min) is obtained that provides a measure of solids passage in the specified interval. For this work, the interval was set between 0 and 45 minutes following a filter coming back online as this was deemed to cover initial filter ripening period in each of the analyses. In situations where the filtration rate is not constant, namely the slow technique, a slightly different method must be adopted where every step in the integration is multiplied by the filtration rate, as this influences the particle breakthrough. A mathematical procedure was applied to each result to provide some normalisation of the baseline turbidity to allow for appropriate comparison between different tests. This involved subtracting the minimum turbidity in the sample period (i.e. the ripened turbidity) from each data point in the sample to account for differences in final turbidity that could have arisen from external variables (coagulation conditions, raw water quality etc.). DISCUSSION AND RESULT ANALYSIS Filter Resting The results from the Morgan WTP resting trials do not appear to yield any benefit from filter resting. Graphical analysis of the ripening profiles from the trials (Figures 1 and 2) reveals no clear pattern of Figure 2: Ripening spikes from second filter resting trial run y = x R² = Resting Time (hrs) Figure 3: Solids passage after resting (first 45 minutes) The data from the first two trial runs appears to show a positive correlation between filter resting time and solids passage in terms of NTU.min (Figure 3). This finding is contradictory to literature as it suggests resting can be detrimental to filter performance. Whilst the trend is clear, it is difficult to explain why this occurs. The published literature does acknowledge that not all plants will see a benefit from filter resting but there are no published results that exhibit a deteriorating ripening profile from resting. A possible explanation is that when a filter is rested post-backwash, the remaining particles in the dirty

5 back wash water have time to settle on top of the media and form a slug of material which can then pass through the bed rapidly upon -up. This theory is supported by Amburgey s (2005) findings that during a backwash, the collector particles that are sheared off the media during a backwash tend to revert back to their original raw water zeta potential. Consequently any particles remaining in the filter following a backwash do not readily reattach themselves to the media. This may help explain why the particles that settle on the media during filter resting are of little value to filtration as they will not become additional collectors. A critical point from this discussion is the turbidity spikes generally present at Morgan WTP are quite low and short in duration when compared to other filtration plants in the published literature. Logsdon and Hess (2002) used the criteria of plants consistently able to control spikes to below 0.3 NTU in an evaluation of turbidity spike reducing techniques. Morgan WTP generally meets this criterion without employing any of the published techniques. This is possibly associated with the relatively high coagulant doses employed for optimised turbidity removal. The consequence of this already high level of performance is that the work performed in this study has tested the limits of what filter resting can achieve. This study does not necessarily prove that filter resting is an ineffective technique at reducing ripening spikes, yet it demonstrates a limitation in reducing already low ripening peaks. A limited number of filter resting trials were also undertaken at the Anstey Hill and Mount Pleasant WTPs. These trials did not exhibit any benefit from filter resting, with ripening profiles from an immediate filter -up being similar to delayed s; however the number of trials undertaken was not sufficient to conclude that resting actually worsened ripening performance as was the case at Morgan. Slow Start Slow shows promise as a method to control filter ripening spikes. The turbidity profiles for each test in Figures 4 to 7 show improvement in ripening performance over a normal up. This improvement is quantified using the numerical integration method to approximate the solids passage, the results of which are supplied in Table 5. The analysis method is slightly different for slow as the filtration rate must be taken into account. As the filtration rate is not constant for the duration of the slow procedure, there is an additional influence on the solids passage as the lower flow rate results in less overall particle breakthrough. Hence each data point has been multiplied by the flow rate at that time point yielding the unit NTU.L. Each trial is compared to the previous instantaneous up on the same filter. Figure 4: Ripening spike of 30-minute slow on Filter 4 vs. standard Figure 5: Ripening spike of 30-minute slow on Filter 1 vs. standard Figure 6: Ripening spike of 15-minute slow on Filter 3 vs. standard Figure 7: Ripening spike of 15-minute slow on Filter 5 vs. standard

6 Table 5: Solids passage through filters during first 45 minutes slow trials entering the filter rather than the water remaining after a backwash. Trial Solids passage (NTU.L) instant up Solids passage (NTU.L) slow Percent reduction in solids using slow Filter 3 15 minute slow Filter 4 30 minute slow Filter 1 30 minute slow Filter 5 15 minute slow % % % % Figure 8: Ripening spike following ETSW vs. standard backwash on Filter 2 - Morgan WTP In each slow trial there is a reduction in solids passage into the filtered water in the first 45 minutes when compared to a normal instantaneous up. There is certainly evidence to support the implementation of slow at treatment plants on a regular basis. Practically, this would require programmed changes to the filter control system to allow the process to be automated. ETSW Of the techniques trialled in the study, the extended terminal subfluidisation wash was most effective at reducing the magnitude and duration of filter ripening. Figures 8 and 9 show examples of the ripening spike following an ETSW trial on a particular filter compared with the most recent up following a standard backwash on the same filter at Morgan WTP. Figure 10 shows the result for the single trial at Anstey Hill WTP. Table 6 shows the complete set of results. It would seem the success of the ETSW method in this study is a consequence of the sound principles of the technique. From what is known about filter ripening, it can be seen that ETSW addresses several of the root causes of the problem. The intramedia remnant particles are flushed out during the ETSW stage, essentially eliminating their role in filter ripening. The upper filter remnant particles are also addressed as the ETSW, by its inherent design, ensures that the headspace above the media is flushed of most remaining particles. This is evident visually in the images presented in Figure 11 The only stage of filter ripening that is not addressed by the ETSW is the filter conditioning stage in which newly influent particles attach themselves to the filter media to become additional collectors. This process takes some time and during this period higher than usual turbidity may be experienced until the media is conditioned. ETSW has no effect on this part of ripening, as additional collectors are sourced from the new settled water Figure 9: Ripening spike following ETSW vs. standard backwash on Filter 5 - Morgan WTP Figure 10: Ripening spike following ETSW vs. standard backwash on Filter 8 Anstey Hill WTP Table 6: Solids passage through filters during first 45 minutes - ETSW trials Trial Solids passage (NTU.min) standard backwash Solids passage (NTU.min) ETSW Percent reduction in solids using slow Filter 7 Morgan % Filter 8 Morgan % Filter 2 Morgan % Filter 5 Morgan % Filter 2 (rpt) Morgan % Filter 8 - Anstey Hill % Filter 1 Mt Pleasant % The main obstacles to ETSW being implemented on a permanent basis are related mostly to the low flow rates required, as well as the higher volume of backwash water used (see Tables 3 and 4). Operationally, ETSW flow rates can present a challenge when the existing backwash equipment is not designed for very low throughput. For example

7 at Morgan WTP, each of the two backwash pumps operate at 600 L/s without a variable speed drive (VSD). Flow is controlled via throttling through a modulating butterfly valve. The age and size of these valves made the control of the low flow difficult and some tuning of the controller was required to achieve a stable flow. 5 min Pre-ETSW 6 min 1 min 7 min 2 min 8 min 3 min 9 min Figure 11: Photography of ETSW sequence at Morgan WTP 4 min The issue of water consumption with ETSW is acknowledged by Amburgey and Brouckaert (2005). At many WTPs, including Morgan, waste wash water is recycled to the head of the plant meaning that additional backwash water generated by ETSW is not strictly wasted. Additional costs of the procedure do exist in the form of increased pumping and re-treatment though this is likely to be negligible. A cost-benefit analyis may be required on a plant-by-plant basis but in most configurations

8 it would be expected that the water quality benefits would outweigh the pumping costs. There is scope to optimise the ETSW procedure given that the calucations to determine the filter cell volume are somewhat approximate. Visual observation of the Morgan filters during an ETSW backwash seem to indicate that the water in the filter cell does not become any clearer after the 5-6 minute point. This was confirmed by collecting backwash water samples every minute during the backwash sequence and generating a turbidity profile (Figure 12). The wash water profile supports that the ETSW could be shortened without detriment. Recommended as a viable technique to control turbidity spikes in locations where filter valves are able to provide a suitable degree of stable flow control. Extended terminal subfluidisation wash (ETSW) Found to be the most effective technique at reducing the passage of solids into the filtered water during the filter ripening phase The ETSW procedure is based on addressing the mechanisms associated with filter ripening, giving it a sound scientific basis Solids reduction between 47% and 84% was achieved during the first 45 minutes of filter operation ACKNOWLEDGMENT Figure 12: Wash water profile from ETSW trial at Morgan WTP CONCLUSION This study evaluated three published techniques to control turbidity spikes associated with filter ripening to reduce the risk of pathogen breakthrough. Key findings for each method are summarised as follows: Filter resting Slow Found to be ineffective in controlling ripening spikes at Morgan WTP in dual media filters Insufficient peer reviewed literature of demonstrated benefit or sound theory of operation Displayed evidence of actually worsening the initial turbidity performance of filters for resting periods greater than one hour Showed promise as a method to alleviate some of the turbidity spikes associated with filter ripening Sound theory of operation that aims to minimise shock on filter media. Solids breakthrough reduction between 9% and 25% was achieved during the first 45 minutes of filter operation The author wishes to thank Werner Mobius, Con Pelekani and Michael Holmes for their guidance during this study. The author also wishes to thank Andrew Prosser and John Knoblauch for facilitating the Morgan WTP trials. REFERENCES Amburgey J.E. (2004). Effect of Washwater Chemistry and Delayed Start on Filter Ripening. American Water Works Association Journal (96): 1. Amburgey J.E. (2005). Optimisation of the extended terminal sub-fluidisation wash (ETSW) filter backwashing procedure. Water Research (39): Amburgey J.E. and Brouckaert B.M. (2005). Practical and theoretical guidelines for implementing the extended terminal subfluidisation wash (ETSW) backwashing procedure. Journal of Water Supply: Research and Technology AQUA; 54.5: Logsdon G.S. and Hess A.F. (2002). Filter Operations & Guidance Manual. American Water Works Association Research Foundation. Logsdon G.S., Hess A.F., Chipps M.J., Gavre J., Locklair J., Hidahl C. and Wierenga J. (2005). After Backwash: Controlling the Initial Turbidity Spike. American Water Works Association: Opflow (31): No. 10. Pizzi N. (1996). Optimizing Your Plant s Filter Performance. American Water Works Association: Opflow (22): No. 5