OPTIMISATION OF GRANULAR MEDIA FILTRATION: IMPACT OF CHEMICALCONDITIONING

Con Pelekani 1, Neil Crossing 2, Loreline Kerlidou 3, Sam Hancock 1, Jean-Francois Robin 4

1. SA Water Corporation, Adelaide, South Australia2. Allwater, Adelaide, South Australia

3. Inside Infrastructure, Adelaide, South Australia4. Department of Process Integration and Optimisation, CIRSEE, Suez Environnement, France

ABSTRACT

Addition of chlorine to the influent of pilot granular media filters at Happy Valley water treatment plant (WTP) was found to result in significant reductionsin filtered water particle counts compared with standard turbidity measurement. When the WTP is operated in enhanced coagulation mode, the limiting factor influencing chlorine dose is the presence of soluble manganese in the clarified water. Increasing filter influent pH improved kinetics of manganese oxidation by chlorine, with filtered water aluminium residuals below 0.2 mg/l. Polymer dosing was also found to be beneficial, with no adverse interaction with free chlorine. The use of a control filter was necessary to assess the impact of different treatments. Operating a filter with no chemicals to enable biofilm development was achieved reasonably quickly. However, filtered water turbidity and particle counts were generally higher than those from chemically conditioned filters. The ability of the biofilter to degrade algal metabolites (MIB and geosmin) was poor,confirming that alternative chemical conditioning practices are necessary.

INTRODUCTION

In 2011, in response to identified underdrain structural failures, a capital works project was initiated under the Cryptosporidium risk mitigation strategy to initially refurbish 8 of 16 filters at SA Water’s largest water treatment plant at Happy Valley, to restore functionality to this critical treatment barrier. The project was completed in 2014. In 2012, a pilot plant was constructed to establish the optimum new sand media configuration (size, depth) to achieve the required filtered water quality requirements to maximise filtration barrier performance against protozoan pathogens, in particular, Cryptosporidium.Combined filtered water turbidity requirement for metropolitan Adelaide WTPs is <0.15 NTU (95% of the time per month), with individual filter turbidity target of < 0.15 NTU within 15 min and less than 0.3 NTU (100% of time per month). The pilot plant was also used to establish optimum filtration rate and backwash sequence conditions (Pilot Plant investigation Phase 1).

Phase 2 of the investigation, the subject of this paper, involved characterisation of the impact of different chemical conditions applied to the clarified water upon filtration performance, as well as operational requirements and treatment performance of a non-chlorinated biofilter.

MATERIALS AND METHODS

Figures 1 and 2 illustrate a process schematic andphotograph of the filtration pilot plant, respectively. Clarified water could be accessed from the settled water duct (which directly feeds the full-scale sand filters) or directly from the on-line sedimentation tanks (during periods when the full-scale plant is off-line). The rig is equipped with four filter columns (clear PVC construction; 250 mm diameter; 1.3 mof 1.1-1.2 mm effective size sand with graded gravel support). The common supply line to the filters had provision for pH adjustment with sodium hydroxide. Filter 1 was designated as the control filter and was filtered/chlorinated water. Filter 2 was designated as the ‘biological’ column. No chemicals were dosed into this filter and the backwash water did not contain chlorine. Filters 3 and 4 were designated as the chemically dosed filters and operated under the same treatment conditions (duplicate columns).

Chemical dosing systems were initially provided for columns 1, 3 and 4. During the latter stages of the testing program, a dosing point was installed for column 2 (biological filter) to enable the MIB/Geosmin spiking trial. A filter loading rate of 13 m/h was employed for columns 1, 3 and 4. This value was found to provide optimum filtration conditions based on the work undertaken during Phase 1. For column 2 (biological), the loading rate was 5 m/h for the first month, after which it was increased to 10 m/h and maintained at this rate. Outlet flow control capability was provided for each column.

Figure 1. Pilot filter process schematic

Figure 2. Photo of filter pilot plant

On-line turbidity (Hach 1720E) and particle counters (Chemtrac PC3400; 2-15 m range) were installed for each filter column for real time water quality analysis. Data from on-line sensors (flow, differential pressure) was stored on a dedicated PC located on-site. Remote access capability was enabled to view system status. An Excel macro was created to extract operational data into CSV type files for analysis. Table 1 summarises the backwash conditions for columns 1, 3 and 4, established from Phase 1.

Table 1. Filter backwash conditions

The following chemicals were employed: MnCl2solution (for manganese spiking trials); NaOH solution (for pH adjustment); LT22S solution (cationic polyacrylamide for filter aid polymer trials);and NaOCl solution (12.5%w/w; for chlorine addition). For Mn spiking trials, chemical was dosed for the first 6 hours of run time.

Test conditions are summarised in Tables 2 and 3.Itis important to note that the stated polymer doses were five times higher than originally intended, due to a miscalculation of the actual batch polymer stock concentration produced for full-scale plant dosing (0.5%w/v versus 0.1%w/v). Unfortunately, this error was only identified near the end of the experimental period. In practice, filter aid polymer doses above 0.1 mg/L are impractical as they significantly increase the risk of filter clogging via mudballing. However, short-term testing of high doses provides an upper limit on the particle reduction capability afforded by use of high molecular weight polymers.

Each test was normally operated for a period of 24 hours, after which the test was repeated. Turbidity and particle count results were the average of the four tests corresponding to duplication of columns 3 and 4. Data analysis excludes the ripening period, typically designated as the first 30 minutes after filter start-up. Due to the start-stop nature of full-scale plant operation there was variation in feed water quality between different trials, the impact of which is described later.

Table 2. Test conditions for chlorine and Mn dosingTrial Manganese dose

(mg/l) Chlorine dose

(mg/l) pH

Low dose Mn 0.05 0.5 7.2 Low dose Mn 0.05 1 7.2 High dose Mn 0.1 0 7.2 High dose Mn 0.1 0.5 7.2 High dose Mn 0.1 1 7.2 High dose Mn 0.1 2 7.2

High pH 0.1 2 7.6 High pH 0.1 3 7.6

Biological Column 1 0.1 0 7.2 Biological Column 2 0.02 0 7.2

Table 3. Test conditions for polymer dosing

RESULTS AND DISCUSSIONTurbidity and particle count profiles for duplicate columns 3 and 4 were found to be consistent for all test conditions. For this reason results presented herein are predominantly associated with column 3.Clarified water turbidity was normally very low (<0.5NTU), with short-term spikes of up to 4 NTU observed after start-up of the full-scale WTP. The low turbidity supports the effectiveness of the sedimentation basins to achieve effective floc removal.

Trial Number Polymer Concentration

(mg/L)

Chlorine Concentration

(mg/L)

pH

1 0.1 0.0 7.2 2 0.4 0.0 7.2 3 0.1 2.0 7.2 4 0.4 2.0 7.2 5 0.4 0.0 7.6 6 0.08 2.0 7.6 7 0.02 2.0 7.2

Impact of chlorination

Figure 3 illustrates the filtered water turbidity and particle count results for the reference column (#1; no chemical dosing) and column 3, for 3 different chlorine dose conditions (0.5, 1, 1.5 mg/L). The colour coding allows visual comparison of columns 1 (control) and 3 for the same test period. Analysis of the steady state profiles is summarised in Table 4. The chlorine demand of columns 3 and 4 was in the range of 1 mg/L (0.9 - 1.1 mg/L). The comparative reduction in particle counts was much greater than for turbidity. The findings are consistent with those of Roder et al. (2000), who evaluated the impact of filter pre-chlorination using pilot filters operated on settled water from Hope Valley WTP. In that study, filtered water particle counts decreased significantly during pre-chlorination (dosed to achieve a filter effluent residual of 0.3 mg/L). The effect was immediate and reversible once dosing ceased.

Figure 3. Impact of filter pre-chlorination

Table 4. Measured pre-chlorination improvement

Becker et al. (2004) conducted a comprehensive investigation of the impact of a range of oxidants for enhancement of filtration performance, including chlorine. A range of mechanisms were proposed, including reduction in the adsorbed hydrodynamic layer of NOM on influent particles associated with chemical structural changes. Batch tests were conducted using model surrogate Hematite particles (64 nm diameter) for a range of oxidant doses and water quality conditions (pH, ionic strength, calcium concentration). Dynamic laser light scattering equipment was used to measure the thickness of the adsorbed layer, while static light scattering was used to calculate particle attachment efficiency. The results did show a reduction in layer thickness with increasing oxidant dose, with a concomitant increase in attachment efficiency. Pilot testing with a range of surface waters showed significant and rapid decrease in filtered water particle counts once pre-oxidation was initiated. Performance deteriorated once oxidant dosing was ceased. The beneficial impact was dependent upon the oxidant used, with chlorine, ozone and chlorine dioxide all performing very well, while permanganate displayed only modest improvement. As with this study, the beneficial impact of filter pre-oxidation was far greater with particle counts than turbidity. Furthermore, comparison between filters exposed to oxidant and those operated in biological mode, consistently demonstrated higher particle counts from the biological filter.

Manganese removal

Chlorine is dosed into the clarified water of most WTPs serving metropolitan Adelaide. The primary reason is to oxidise dissolved manganese present in source waters to insoluble form (typically MnO2/Mn3O4) for removal by filtration. Figure 4

Chlorine Dose Turbidity removal C3&C4 compared to C1

Particle counts removal C3&C4 compared to C1

mg/l % Removal % Removal 0.5 +13% +37% 1 +16% +50%

1.5 +15% +52%

compares the performance of different levels of chlorine upon removal of designated high Mn (0.1 mg/L) dosed filter influent for columns 3 and 4. Mn removal was poor for chlorine doses less than 2 mg/L. At 2 mg/L, Mn removal was consistently poor at short time scales (30 min), reflecting the kinetic limitation associated with oxidation by chlorine.

Figure 4. Impact of filter pre-chlorination on Mn removal.

Comparison against the control filter (#1) yielded surprising results. Mn removal was found to be significantly better for Filter 1, although did

deteriorate rapidly at longer run times (4-6 hours of dosing) (Figure 5). Biological removal is possible for Mn2+, as it can be used in co-metabolism for microbial energy production. However, it appears that the active biofilm’s capacity (if present) is limited.

Figure 5. Mn removal: control versus chlorinated filters

Figure 6 shows the impact of increasing filter influent pH from ambient (7.2) to ~7.6. As expected Mn removal was improved at short time scales. However, the greatest overall improvement was observed when increasing the chlorine dose to 3 mg/L. In the full-scale WTP, chlorine is typically dosed to achieve a residual in the filtered water (0.3- 0.6 mg/L). The accumulation of MnO2 on the sand filter grains with time (volume of water processed) is most likely responsible for the very good removal of Mn under ambient pH conditions. MnO2 acts catalytically to accelerate the rate of dissolved Mn oxidation (Knocke et al. 1991). The Mn spiking trials were short and specific events, with no long term dosing to generate a coating of MnO2 on sand grains. Irrespective, the results do confirm the relative importance of pH and chlorine dose. The latter strongly influences the redox potential (ORP),which needs to be high (>700 mV) to drive effective oxidation of dissolved Mn to MnO2 precipitate.Increasing pH does shift Mn speciation towards insoluble species, but ORP is the controlling parameter.

Figure 6. Impact of pH on Mn removal.

Impact of filter aid polymer

LT22S (BASF, Australia), a polyacrylamide polymer is dosed as a filter aid at the Happy Valley WTP.Figure 7 illustrates the filtered water turbidity and particle counts for the reference column (#1; nochemical dosing) and filter column 3, for 2 different polymer doses (0.1 and 0.4 mg/L). An analysis of the steady state filtrate profiles is summarised in Table 5. A comparative reduction in filtered water turbidity was observed for both applied polymer

doses, however poor particle count removal was observed at 0.1 mg/L.

Figure 7. Impact of filter aid

Table 5. Measured water quality improvement from polymer dosing

Figure 8 illustrates the filtered water turbidity and particle count profiles for the reference column (#1; with 2 mg/L of chlorine dosing) and column 3, for 3different LT22S doses (0.02, 0.1 and 0.4 mg/L) and 2 mg/L of chlorine. Analysis of the steady state profiles is summarised in Table 6. Increased turbidity and particle count removals were observed with pre-chlorination, consistent with previous findings. No apparent negative interaction between chlorine and LT22S was observed.

Figure 8. Impact of filter aid (with pre-chlorination)

Table 6. Measured filter aid improvement with pre-chlorination

The Happy Valley WTP was offline due to maintenance for some of the filter aid dosing trials. This led to significant variations in residence time in the sedimentation basins and ultimately changes inthe quality of settled water used to feed the pilot plant. This variation in settled water quality can be seen to have an impact on filtered water quality in the reference filter (Figure 7) making a direct comparison of filtered water turbidity and particle counts for these trials inconclusive. Comparative reductions in particle counts and turbidity allowed for trial results to still be utilised despite the varying settled water quality conditions.

A comparison of the turbidity and particle count reductions at ambient pH (7.2) and elevated pH (7.6) yielded inconsistent results (Table 7).Decreased turbidity and particle count removals were observed for a low polymer dose with pre-chlorination but increased removals were observed at a high polymer dose without pre-chlorination. It is hypothesised that the turbidity and particle count removal may decrease with elevated pH, due to an increased anionic particle charge. Variable settled water quality during these tests possibly contributed to these results. Settled water and filtrate samples were collected during the high polymer dose trial to assess aluminium breakthrough when operating the filters at an elevated pH of 7.6. All samples were found to comply with the Australian Drinking Water Guideline value of 0.2 mg/L (acid-soluble Al).

Table 7. Measured filter aid improvement at elevated pH

Biological filtration

To encourage biological activity, column 2 was backwashed with non-chlorinated filtrate produced from the filter. Figure 9 illustrates the particle count and turbidity profiles for column 2 (blue line) and the control column (pink line) over a 4 month period (May-August 2014). The control column was subject to chlorinated backwash conditions. Raw water flow for the full-scale WTP is also included(green line). In general, trends were similar, with most data in the 0.1-0.2 NTU range. Particle count trends were also similar, converging towards the end of the period.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.00

2.00

4.00

6.00

8.00

10.0

0

12.0

0

14.0

0

16.0

0

18.0

0

20.0

0

22.0

0

24.0

0

Turb

idity

(NTU

)

Column 1 - ReferenceSummary Turbidity - Polymer & Chlorine

Rep 1 & 2

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.00

2.00

4.00

6.00

8.00

10.0

0

12.0

0

14.0

0

16.0

0

18.0

0

20.0

0

22.0

0

24.0

0

Turb

idity

(NTU

)

Column 3Summary Turbidity - Polymer & Chlorine

Rep 1 & 2

Poly = 0.02 mg/L

Poly = 0.10 mg/L

Poly = 0.40 mg/L

0

500

1000

1500

2000

2500

3000

0.00

2.00

4.00

6.00

8.00

10.0

0

12.0

0

14.0

0

16.0

0

18.0

0

20.0

0

22.0

0

24.0

0

Tota

l Par

ticle

Cou

nts (

cts/

mL)

Column 1 - ReferenceSummary Particle Counts - Polymer & Chlorine

Rep 1 & 2

0

500

1000

1500

2000

2500

3000

3500

0.00

2.00

4.00

6.00

8.00

10.0

0

12.0

0

14.0

0

16.0

0

18.0

0

20.0

0

22.0

0

24.0

0

Tota

l Par

ticle

Cou

nts (

cts/

mL)

Column 3Summary Particle Counts - Polymer & Chlorine

Rep 1 & 2

Poly = 0.02 mg/L

Poly = 0.10 mg/L

Poly = 0.40 mg/L

Figure 9. Turbidity profiles for biological and reference filter columns.

A contributing factor to the transient variability in filtered water quality is the backwash sequence and interval. Figure 10 illustrates the headloss profiles for the biological and reference filters. Numerical designations 1-3 refer to the type of backwash sequence. Type 1 involved only a low rate water wash (20 m/h for 5 min). This was employed to prevent potential loss of active biofilms associated with turbulent backwash. Type 2 involved addition of separate air scour (50 Nm/h for 1 min) before the water wash. Type 3 involved air scour, combined air and water sequence, equivalent to that employed for chemically treated filter columns (refer to Table 1). Backwash of the biological filter was normally triggered when the headloss exceeded 2m, with a maximum of 3 m. An important indicator of success was the ability of the backwash to restore headloss to less than 0.5 m. Water only (Type 1) was not able to achieve this requirement on a consistent basis. Intermediate Type 2 backwash did assist. However, the addition of a combined air/water scour (Type 3) was necessary to achieve operational stability. Under these conditions the rate of headloss development was not significantly different between the biological and reference filters.

Figure 10. Headloss profiles for biological and reference columns.

Measurements of dissolved oxygen, oxidation reduction potential, UV254 absorbance and true colour (@ 456 nm) were taken for the inlet and outlet of the control and biological filters (Figure 11). Surprisingly, no significant changes were observed between the inlet and outlet of the biological filter for any of these parameters. It is hypothesised that the empty bed contact time of 7.8 min was not long enough to encourage appropriate microbiological growth on the sand media surface.

Figure 11. Impact of biological filtration on DO, ORP, UV254 and colour

After more than 6 months of operation, a taste and odour spiking test was undertaken to assess whether the non-chlorinated ‘biological’ filter could remove selected algal metabolites of interest: geosmin and 2-methyl-isoborneol (MIB). The reference and biological columns were spiked with a laboratory prepared solution of MIB and geosminto achieve approximately 250 ng/L at the inlet to the filters. The stock solution was kept chilled to minimise losses by volatilisation. Solution was dosed for a period of 4.5 hours, with samples collected at regular intervals for GCMS analysis in-house. No significant decrease in MIB or geosmin was observed for either the control or biological filter (Figure 12). Sampling of the filter effluent one week after the spiking test did not reveal any measurable quantities of either compound, confirming that the biological filter was not producing MIB or geosmin. The findings confirm that simply passing non-chlorinated water that has been subjected to coagulation, flocculation and clarification does not guarantee effective

biofiltration for rapid gravity filters. Sawade et. al.(2011) found that at least 10 months of filtration at an empty bed contact time of 15 min was necessary before biological removal of MIB and geosmin would become significant on filter sand media. To achieve reliable and consistent removal of target compounds using biofiltration is likely to require optimisation of influent nutrient conditions to encourage appropriate biological activity and to minimise production of extracellular polymeric substances (EPS) that foul the media. Also referred to as ‘engineered biofiltration’, this has the potential to significantly enhance the treatment capability of traditional media filters (Lauderdale et al., 2012). This is the subject of current research activity at SA Water.

Figure 12. MIB and geosmin spiking trial results

CONCLUSIONS

In this study, the benefits of chlorine and/or polymer addition to filter influent were demonstrated. Filter pre-chlorination resulted in a significant reduction in filtered water turbidity and particle counts (2-15 m), when a dose above 0.5 mg/L was applied.

Chlorine doses in excess of 2 mg/L were necessaryto achieve acceptable reductions in soluble manganese at ambient pH (~7.2). Filtration at an elevated pH of 7.6 was found to improve the oxidation kinetics of manganese using chlorine without exceeding the regulatory limit for soluble aluminium (0.2 mg/l). Filter aid polymer addition afforded improvements in filtered water turbidity and particle counts with no apparent negative interaction when dosed simultaneously with chlorine. The water quality benefits, as measured via turbidity and particle count analysis, confirm that the filter pre-chlorination and polymer dosing can be employed to reduce the risk of pathogen (e.g. Cryptosporidium) breakthrough, thus improving barrier effectiveness.

Operating a filter without chlorine to encourage biological activity for the purposes of enhancing taste and odour removal capability was undertaken. Optimisation of the physical backwash sequence was necessary to achieve stable filter runs. Filtered water quality (turbidity and particle counts) were generally higher than for chemically dosed filters. Even after 6 months of operation no effective removal of MIB and geosmin was observed,

confirming the importance of conditioning the filter influent to select for appropriate biological conditions.

ACKNOWLEDGMENTS

Loreline Kerlidou was the Allwater project engineer for Phase 1 and 2 pilot investigations from May 2013 to August 2014. Emma Sawade, Rolando Fabris and Mary Drikas (Water Treatment, Research & Innovation Services group, SA Water) for managing the MIB/Geosmin spiking trial. Uwe Kaeding (Water Treatment Manager, Allwater) and Dr. Jeremy Lucas (Senior Manager Water Quality & Treatment Strategy, SA Water) for sponsoring this applied research initiative.

REFERENCES

Becker W.C., O’Melia C.R., Au K. and Young Jr. J.S. (2004), Using Oxidants to Enhance Filter Performance. American Water Works Association Research Foundation. Report #90998.

Knocke W.R., Occiano S.C. and Hungate R. (1991), Removal of Soluble Manganese by Oxide-Coated Filter Media: Sorption Rate and Removal Mechanism Issues. Journal AmericanWater Works Association, 83:8:64-69.

Lauderdale C., Chadik P., Kirisits M.J. and Brown J. (2012), Engineered biofiltration: Enhanced biofilter performance through nutrient and peroxide addition. Journal American Water Works Association, 104(5):298-309.

Roder E., Holmes M. and Chipps M. (2000), The Application of Particle Counters in the Optimisation of Water Filtration. Proceedings of the Water Quality Technology Conference,American Water Works Association, USA.

Sawade E., Slyman N., Dixon M., Ho, L. Hoefel D., McDowall B. and Newcombe G. (2011), Biological filtration for the treatment of cyanobacterial metabolites. Proceedings of the Australian Water Association Ozwater Conference, Adelaide, South Australia.