Development of Nitrification in Biological Filters · Development of Nitrification in Biological ... optimize the development of nitrification in biological filters. ... Hach DR/890

Development of Nitrification in Biological Filters

Samantha S. Stoughtenger and Eric C. Wert Southern Nevada Water Authority, P.O. Box 99955, Las Vegas, NV 89193

ABSTRACT

Ammonia-based strategies are becoming more popular for mitigating bromate formation during the ozonation of drinking water. Biofiltration presents a cost effective alternative to remove biodegradable organic matter and residual ammonia for utilities targeting a free chlorine residual for secondary disinfection. In this study, we investigated the development of nitrification with pilot-scale biological filters using anthracite/sand and GAC/sand filter media. Sampling ports on the column walls allowed us to examine removal of various water quality constituents over the media depth. During these experiments, sample flow did not exceed 5% of the total flow rate through the filter. Ammonia, nitrite, nitrate, carboxylic acids, bromate, and trihalomethanes were monitored throughout the depth of media. Results showed that CAAs were removed in the upper portion of the media bed while the removal of ammonia was more distributed throughout the bed. Full conversion of ammonia to nitrate was observed in each of the filters. Bromate removal was witnessed within the filters but ambient influent concentrations were less than 15 µg/L. THMs removal was experienced within the filter; however, the total THMs concentration was less than 16 µg/L. Ammonia removal was not impacted during filter ripening.

INTRODUCTION

Biofiltration provides utilities with a cost effective tool to remove biodegradable organic

matter (BOM) from the drinking water supply. BOM removal results in the production of biologically stable water and can also reduce the formation of chlorinated disinfection byproducts. Ozonation is known to produce aldehydes and carboxylic acids, which increase the quantity of BOM. Therefore, biofiltration is commonly applied after ozonation to remove these byproducts prior to distribution. Furthermore, the development of a biologically active nitrifying filter (BAF) can remove residual ammonia used for bromate control during ozonation (Wert et al. 2008). Ammonia removal eliminates the need for breakpoint chlorination when targeting a free chlorine residual for secondary disinfection. Furthermore, the cometabolism of ammonia and THMs (in concentrations ranging from 25 to 800 µg/L) has been documented during biofiltration. This cometabolism favors the more bromine-substituted species which degrade faster in the following order bromoform (TBM) > chlorodibromomethane (DBCM) > bromodichloromethane (BDCM) > chloroform (TCM) (Wahman et al. 2004; Wahman et al. 2005; Wahman et al. 2006a; Wahman et al. 2006b). Additional research is needed to further optimize the development of nitrification in biological filters.

2009 © American Water Works Association WQTC Conference Proceedings All Rights Reserved2009 © American Water Works Association WQTC Conference Proceedings All Rights Reserved

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Several studies have investigated the importance of media type, specifically anthracite and granular activated carbon (GAC) (Maloney et al. 1982; van der Kooij 1987; LeChevallier et al. 1992; Krasner et al. 1993; Hacker et al. 1994; Urfer et al. 1997). These studies have found that GAC filters removed more total organic carbon (TOC) than anthracite filters. This can be attributed to the fact that GAC can support three to eight times more biomass than anthracite (Wang et al. 1995). (Krasner et al. 1993) found that aldehyde removal is slightly better with GAC/sand when compared with anthracite/sand media filters. Researchers have also documented that bromate can be removed by use of GAC in BAF (Siddiqui et al. 1994; Bao et al. 1999). Further, (Kirisits and Snoeyink 1999) reported through batch experiments that the microorganisms in the BAC filters are responsible for a majority of bromate removal.

Several studies have also investigated the impact of backwashing on biofiltration

performance (Urfer et al. 1997; Ahmad et al. 1998; Liu et al. 2001). In general, dechlorinated backwash water can minimize effects on biomass stability (Miltner et al. 1995). Bromate removal was unaffected by backwashing when GAC was used as the filter media for BAF (Kirisits et al. 2002). Air scour had little impact on the removal AOC in one study (Ahmad et al. 1998).

However, little research has investigated filter removal efficiencies throughout the filter

depth by media type with respect to water quality parameters. Some research has shown that microorganisms vary as a function of filter depth (Moll et al. 1998). Little information could be found in the literature regarding the effect of media depth or empty bed contact time (EBCT) on the simultaneous removal of BOM and ammonia. As such, the current project examined the following with respect to biofiltration performance: (1) media depth on the removal of water quality constituents (i.e., ammonia, carboxylic acids, bromate, and trihalomethanes), (2) backwashing with respect to media depth, and (3) changes to the hydraulic loading rates with respect to media depth.

MATERIALS AND METHODS Pilot Plant Description

Pilot scale tests were completed on a pilot scale filter module which consisted of three filters, real-time instrumentation and data acquisition capabilities, and manual backwash components. The filter module consisted of a shallow bed anthracite (SBA) filter, a deep bed anthracite (DBA) filter, and a shallow bed GAC during this investigation (Table 1).

Table 1. – Media characteristics for the filters operated during the study.

Filter Media Type Effective Size (mm)

Uniformity Coefficient

Media Depth (in) L/d Total

L/d anthracite 1.2 <1.7 21 445 SBA

sand 0.50 <1.5 10 508 953

anthracite 1.2 <1.4 50 1058 DBA sand 0.65 <1.4 10 391

1449

GAC F816 1.4 <1.4 32 581 SB GAC sand 0.50 <1.5 13 660 1241


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Each filter has been constructed with sample taps numbered A through G, respectively from the bottom to the top of the filter. These samples were about 10” apart. Sample tap depths were measured from the center of the tap to the top of the media, designated as depth from top of media (DfToM).

Filter source water was fed from the SNWS full scale plant filters #21 and #24 (Figure 1).

To dechlorinate the source water, calcium thiosulfate (CTS) was dosed by assuming a chlorine residual of 4.0 mg/L at pH of 8.0. Ammonia was added to promote growth of biological nitrifiers. Each filter was operated until the maximum terminal head loss of 8 feet was reached. At such time, the filter was backwashed (with calcium thiosulfate treated dechlorinated water) and immediately placed back in service.

Figure 1. – Schematic of pilot scale filter modules source water.

Testing Approach

Nitrification was developed in the pilot filters by adding 0.1 mg/L of NH3-N from April 2007 to June 2008 and 0.5 mg/L of NH3-N from June 2008 to December 2008. Testing for this project was conducted during December 2008 and January 2009. For a majority of the tests, each filter was operated at a constant hydraulic loading rate of 2.0 gpm/sf. One series of testing was conducted at hydraulic loading rates ranging from 2.0 to 7.5 gpm/sf. Steady-state flow from each sampling tap was established by very slowly opening each of tap. Quiescent, steady-state flow rates were approximately 0.02 gpm or less than 5 percent of the total filter flow rate. A one minute equilibration period was provided before sample collection. Samples were collected sequentially from the bottom to the top of each filter.

The first part of the study investigated the removal of various water quality constituents

(i.e., ammonia, nitrate, nitrite, total organic carbon (TOC), trihalomethanes (THMs), bromate, and carboxylic acids (CAAs)) over the depth of media. Three ammonia doses (0.927, 3.80, and 1.06 mg/L NH3-N) were applied to the filters as separate investigations. The filters were given a conservative 2.5 hours (as the calculated travel time was less than 30 minutes) to equilibrate after a change in concentration. Temperature and ammonia concentrations were measured at each sample tap during all three dosage investigations. Samples were analyzed weekly for nitrite, nitrate, carboxylic acids, and total organic carbon (TOC) from each of the sampling taps at the ammonia dose of 1.06 mg/L. At the ammonia dose of 0.927 mg/L NH3-N, samples were analyzed for THMs and bromate in the source water, at the top of each filter, and at each filter effluent. All DfToM = 0 samples were collected from the filter skid sample tap and not at the top of each filter for this portion of the project; therefore, the DfToM = 0 samples have a frequency of N = 1.


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The second part of the study investigated the effect of backwashing on ammonia removal within the BAF. All three filters were backwashed and started simultaneously. Ammonia was continuously dosed to all filters at a concentration of between 0.44 mg/L and 0.72 mg/L NH3-N. Samples were collected from every sample tap of each filter for ammonia analysis on hours 2, 4, 6, 22, 26, and 70. Due to the number of samples, the grab sample/Hach method was used to determine ammonia concentrations for this portion of testing. Temperature and ammonia concentrations were monitored for each sample tap at every time frame on all three filters. Samples were collected from every tap at time 22 hours for bromate analysis.

The third part of the study investigated the effect of different hydraulic loading rates on ammonia removal within the BAF. Hydraulic loading of 2.0, 4.0, 6.0, and 7.5 gpm/sf were tested. All three filters were backwashed and start simultaneously. Ammonia was continuously dosed to all filters at a concentration of approximately 0.30 mg/L. An acclimation period of 2.5 hours was observed between adjusting the hydraulic loading rate and water sampling. Due to the number of samples, the grab sample/Hach method was used to determine ammonia concentrations for this portion of testing. Analytical Methods

All chemical analyses were completed by the SNWA laboratory. Nitrate and nitrite were analyzed on the Dionex ICS3000 Ion Chromatograph by EPA Method 300. Carboxylic acids samples were preserved with benzalkonium chloride and analyzed on the Dionex ICS3000 Ion Chromatograph by EPA Method 300. Ammonia samples were preserved with sulfuric acid and were analyzed on the Seal AQ2+ discrete analyzer by method SM4500NH3 F. Bromate samples were preserved with ethylenediamine and analyzed on the Dionex DX600 Ion Chromatograph with post column reactor and UV/VIS detection by EPA Method 317. THM samples were preserved with ascorbic acid and hydrochloric acid and analyzed on the Agilent 6890/5973 GC/MS by EPA Method 524. TOC samples were preserved with hydrochloric acid and analyzed on the Shimadzu TOC-Vcsh combustion analyzer via method SM5310B.

A certified VWR Traceable Thermometer was used to record temperature readings. A

Hach DR/890 Colorimeter was used to measure the free chlorine residual with Hach Method 8021. To expedite evaluation of filter performance, grab samples were collected and tested for ammonia concentrations via Hach Method 8155 on the Hach DR/890 Colorimeter. In previous work, pilot plant staff used the grab-sample/Hach method to analyze the samples for ammonia and sent duplicate samples to the SNWA laboratory for testing. Results from this duplicate testing are shown in Figure 2; a correlation of 0.956 was shown between the laboratory and grab-sample/Hach method for ammonia concentrations less than 0.60 mg/L NH3-N.


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y = 0.9138x - 0.0018R2 = 0.956

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Ammonia (mg/L) Hach Method 8155

Am

mon

ia (m

g/L)

Lab

Figure 2. – Laboratory versus grab-sample/Hach method correlation curve for ammonia.

RESULTS

Ammonia Removal throughout the Media Bed

Shown in Figure 3 are the ammonia removal profiles for each of the SBA, DBA, and SB GAC filters at an initial ammonia concentration of 0.927 mg/L NH3-N and hydraulic loading rate of 2.0 gpm/sf. DfToM = 0 samples have a frequency of N = 1. Ammonia removal appears to take place gradually over the entire filter for the anthracite filters and over the first 28 inches of the GAC filter. The temperature was relatively stable throughout the filter varying no more than 0.3ºC from the influent temperature of 17.6ºC.

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Depth from Top of Media (inches)

Am

mon

ia (m

g/L

NH

3-N

)

SBA

DBA

SB GAC

Figure 3. – Ammonia removal with a 0.927 mg/L NH3-N dose, HLR of 2.0 gpm/sf,

SBA filter run time (FRT) of 113 hr, DBA FRT of 330 hr, and SB GAC FRT of 143 hr.


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Shown in Figure 4 are the ammonia removal profiles for each of the SBA, DBA, and SB GAC filters at an initial high ammonia concentration of 3.80 mg/L NH3-N and hydraulic loading rate of 2.0 gpm/sf. DfToM = 0 samples have a frequency of N = 1. The ammonia removal trend is similar to that at a lower concentration. One interesting observation is that the shallow bed anthracite filter removed more ammonia than the other two filters. Perhaps microorganism competition for the ammonia food source is much greater in the 31 inches of the SBA filter media verses the 45 inches in the SB GAC filter or 60 inches in the DBA filter. Therefore when a greater ammonia dose is applied to the filter, the organisms are available to digest the increased food source. Another observation drawn from this data is that an ammonia concentration of these high levels is not completely converted to nitrate or nitrite by the biologically active filter. The temperature was stable throughout the filter varying no more than 0.3ºC from the influent temperature of 17.3ºC.

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mon

ia (m

g/L

NH

3-N

)

SBA

DBA

SB GAC

Figure 4. – Ammonia removal with a 3.80 mg/L NH3-N dose, HLR of 2.0 gpm/sf,

SBA FRT of 117 hr, DBA FRT of 334 hr, and SB GAC FRT of 147 hr.

Shown in Figure 5 are the ammonia removal profiles for each of the SBA, DBA, and SB GAC filters at an initial ammonia concentration of 1.06 mg/L NH3-N and hydraulic loading rate of 2.0 gpm/sf. DfToM = 0 samples have a frequency of N = 1. The ammonia removal takes place gradually over the entire filter for the anthracite filters and over the first 28 inches of the GAC filter. The temperature was stable throughout the filter varying no more than 0.3ºC from the influent temperature of 17.3ºC.


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mon

ia (m

g/L

NH

3-N

)SBA

DBA

SB GAC

Figure 5. – Ammonia removal with an initial 1.06 mg/L NH3-N dose, HLR of 2.0 gpm/sf, SBA

FRT of 141 hr, DBA FRT of 358 hr, and SB GAC FRT of 171 hr.

Shown in Figures 6, 7, and 8 are nitrification performances of the SBA, DBA, and SB GAC filters, respectively. The initial ammonia concentration was 1.06 mg/L NH3-N and hydraulic loading rate was 2.0 gpm/sf. DfToM = 0 samples have a frequency of N = 1; therefore, some caution must be exercised when assuming this one value equally represents all three filters.

The SBA filter removed approximately 69% of the ammonia feed. Evident in Figure 6,

ammonia is being converted to nitrite and nitrite is being converted to nitrate. But with the high residual concentration of nitrite, the filter is not a 100% efficient nitrifying biofilter. To gain a better perspective on the filter conversion efficiency, the nitrogen mass balance has been examined. The total nitrogen loaded onto the filter is 2.010 mg/L and total nitrogen in the effluent is 2.001 mg/L; therefore, only 0.009 mg/L or 0.45% of the nitrogen is unaccounted for in the SBA filter effluent. Given the N = 1 frequency for the ammonia, nitrate, and nitrite samples loaded onto the filter, all the ammonia appears to be well accounted for in the SBA filter.


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Con

cent

ratio

n (m

g/L)

AmmoniaAmmonia RLNitriteNitrate

Figure 6. – Shallow bed anthracite (SBA) filter with an initial 1.06 mg/L NH3-N dose,

HLR of 2.0 gpm/sf, and filter run time of 141 hr.

The DBA filter removed approximately 90% of the ammonia dose. Evident in Figure 7, ammonia is being converted to nitrite and nitrite is being converted to nitrate. With a residual nitrite concentration of 0.25 mg/L, the filter is not a 100% efficient nitrifying biofilter. The nitrogen mass balance shows that the total nitrogen loaded onto the filter is 2.010 mg/L and total nitrogen in the effluent is 1.890 mg/L; therefore, only 0.120 mg/L or 5.97% of the nitrogen is unaccounted for in the DBA filter effluent. Given the N = 1 loaded samples frequency, the ammonia appears to be well accounted for in the DBA filter.


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cent

ratio

n (m

g/L)

AmmoniaAmmonia RLNitriteNitrate

Figure 7. – Deep bed anthracite (DBA) filter with an initial 1.06 mg/L NH3-N dose,

HLR of 2.0 gpm/sf, and filter run time of 358 hr.

The SB GAC filter removed 99% of the ammonia dose. Evident in Figure 8, ammonia is being converted to nitrite and nitrite is being converted to nitrate. As the residual ammonia and nitrite concentrations are less than reporting limits, the filter is a 100% efficient nitrifying biofilter. The nitrogen mass balance shows that the total nitrogen loaded onto the filter is 2.010 mg/L and total nitrogen in the effluent is 1.870 mg/L; therefore, only 0.140 mg/L or 6.97% of the nitrogen is unaccounted for in the SB GAC filter effluent. Given the N = 1 loaded samples frequency, the ammonia appears to be well accounted for in the SB GAC filter. An item to note is that ammonia is removed throughout depth of the filter and the nitrate is generated throughout the depth of the filter. Furthermore, nitrification clearly takes place over the first 28 inches of the filter or throughout 82.5% of the GAC media.


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cent

ratio

n (m

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AmmoniaAmmonia RLNitriteNitrite RLNitrate

Figure 8. – Shallow bed GAC (SB GAC) filter with an initial 1.06 mg/L NH3-N dose,

HLR of 2.0 gpm/sf, and filter run time of 171 hr. Organics Removal throughout the Media Bed

Shown in Figures 9 and 10 are the total organic carbon (TOC), formic acid, and oxalic

acid removal profiles throughout the filters. The initial ammonia concentration was 1.06 mg/L NH3-N and hydraulic loading rate was 2.0 gpm/sf. DfToM = 0 samples have a frequency of N = 1. Displayed in Figure 9 is a 16% TOC reduction by the SB GAC filter and 12% TOC reduction by the SBA and DBA filters. Further notice the removal occurs throughout the SBA and DBA filters; whereas the removal in the SB GAC occurs within the first 18 inches. As displayed in Figure 10 more than 60% of the formic acid introduced to the filters is removed by the first sampling tap. Further, both the DBA and SB GAC filters remove the formic acid to non-detectable levels by the filter effluent location. Also displayed in Figure 10 is more than 70% of the oxalic acid introduced to the filters is removed by the first sampling tap within the SBA filter. By the second sampling tap, the SBA filter removes the oxalic acid to non-detectable levels. The DBA and SB GAC filters remove the oxalic acid to non-detectable levels by the first sampling tap.


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Tota

l Org

anic

Car

bon

(mg/

L)SBA

DBA

SB GAC

Figure 9. – TOC with a 1.06 mg/L NH3-N dose, HLR of 2.0 gpm/sf,

SBA FRT of 141 hr, DBA FRT of 358 hr, and SB GAC FRT of 171 hr.

Figure 10. – Formic acid and oxalic acid concentrations with a 1.06 mg/L NH3-N dose, HLR of 2.0 gpm/sf, SBA FRT of 141 hr, DBA FRT of 358 hr, and SB GAC FRT of 171 hr.

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ic A

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lic A

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Bromate Removal throughout the Media Bed Removal efficiency within the SBA, DBA, and SB GAC filters was examined for

bromate. The influent concentration, first sampling tap, and effluent sampling tap were examined with an initial ammonia dosing of 0.927 mg/L NH3-N and hydraulic loading rate of 2.0 gpm/sf. Shown in Figure 12 are the bromate profiles within the SBA, DBA, and SB GAC filters. DfToM = 0 samples have a frequency of N = 1.

The bromate removal displayed in Figure 11 is 9% by the SBA filter, 12% by the DBA filter, and 16% by the SB GAC filter at the first sampling tap. At the effluent, bromate removal is 25% by the SBA filter, 16% by the DBA filter, and 25% by the SB GAC filter. Nearly 25% reduction is a significant decrease in bromate; however, the concentration amounts to less than 4 µg/L. While this is an interesting phenomenon, further bench-scale work is recommended to validate these findings. These results were not unexpected as an anaerobic condition is needed to reduce bromate. The results indicated that these conditions are unlikely to exist after ozonation, which usually has an elevated oxygen content.

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Bro

mat

e (µ

g/L)

SBA

DBA

SB GAC

Figure 11. – Bromate concentrations with a 0.927 mg/L NH3-N dose,

HLR of 2.0 gpm/sf, SBA FRT of 113 hr, DBA FRT of 330 hr, and SB GAC FRT of 143 hr.

Shown in Figure 12 are bromate removal profiles for each of the SBA, DBA, and SB GAC filters following backwashing. Bromate samples were collected 22 hours after filter backwashing. Only the SBA filter showed a slight bromate removal of 2.0 µg/L or 17%. While bromate concentration within the DBA and SB GAC decreased throughout the media depth, the effluent concentrations were unchanged from the source water values. At the low ambient concentrations, it is difficult to conclude the filters successfully remove bromate.


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Bro

mat

e (µ

g/L)

SBA

DBA

SB GAC

Figure 12. – Bromate concentrations with an approximate 0.50 mg/L NH3-N dose,

HLR of 2.0 gpm/sf, and FRT of 22 hours. Trihalomethanes Degradation throughout the Media Bed

Removal efficiency within the SBA, DBA, and SB GAC filters was examined for total

trihalomethanes (Figure 13) and the speciated THM (Figure 14) compounds. The influent concentration, first sampling tap, and effluent sampling tap were examined with initial ammonia dosing of 0.927 mg/L NH3-N and hydraulic loading rate of 2.0 gpm/sf. DfToM = 0 samples have a frequency of N = 1.

TTHMs in the effluent were 4 µg/L or 29% lower for the SBA filter, 2 µg/L or 14%

lower for the DBA filter, and unchanged for the SB GAC filter. TBM concentrations in the effluent were 0.3 µg/L or 24% lower for the SBA filter, 0.2 µg/L or 19% lower for the DBA filter, and 0.3 µg/L or 24% lower for the SB GAC filter. DBCM concentrations in the effluent were 1.7 µg/L or 30% lower for the SBA filter, 1.2 µg/L or 21% lower for the DBA filter, and 0.8 µg/L or 14% lower for the SB GAC filter. BDCM concentrations in the effluent were higher than the influent for the SB GAC filter, 0.7 µg/L or 25% lower than the influent in the SBA filter, and 0.4 µg/L or 9% lower than the influent in the DBA filter. TCM concentrations in the effluent increased in both the DBA and SB GAC filters. The SBA filter showed a 0.4 µg/L or 15% reduction in chloroform. Similar to bromate, it is difficult to decisively conclusion that TTHMs have been successfully removed due to the extremely low concentrations experienced during ambient operating conditions.


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TTH

M (µ

g/L)

SBA

DBA

SB GAC

Figure 13. – Total THM concentrations with a 0.927 mg/L NH3-N dose,

HLR of 2.0 gpm/sf, SBA FRT of 113 hr, DBA FRT of 330 hr, and SB GAC FRT of 143 hr.

Figure 14. – Speciated THM concentrations with a 0.927 mg/L NH3-N dose, HLR of 2.0 gpm/sf, SBA FRT of 113 hr, DBA FRT of 330 hr, and SB GAC FRT of 143 hr.

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15

Effect of Backwashing on BAF Performance

Shown in Figures 15, 16, and 17 are the SBA, DBA, and SB GAC filters normalized ammonia removal data immediately following a dechlorinated backwash. For each filter, little to no difference is evident between the 2 hour and 70 hour removal. Therefore backwashing had little to no affect on BAF performance of the filters. Further, these results show that FRT does not impact nitrification. Since previous work was conducted at different FRTs, these graphs should be representative and comparable among the filters tested.

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C/C

o A

mm

onia

TIME = 2 HR

TIME = 4 HR

TIME = 6 HR

TIME = 22 HR

TIME = 26 HR

TIME = 70 HR

Figure 15. – Shallow bed anthracite filter performance following backwashing

with an approximate 0.50 mg/L NH3-N dose and HLR of 2.0 gpm/sf.


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onia

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TIME = 6 HR

TIME = 22 HR

TIME = 26 HR

TIME = 70 HR

Figure 16. – Deep bed anthracite filter performance following backwashing


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Figure 17. – Shallow bed GAC filter performance following backwashing



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Effect of Hydraulic Loading Rate on BAF Performance Ammonia removal efficiency within the filters was examined for varying hydraulic

loading rates (i.e., 2.0, 4.0, 6.0, and 7.5 gpm/sf). Shown in Figures 18, 19, and 20 are ammonia removal performances of the SBA, DBA, and SB GAC filters, respectively. The initial ammonia concentration was 0.3 mg/L NH3-N. As the hydraulic loading rate increased the removal efficiency decreased for each of the filters. Under the highest hydraulic loading rate of 7.5 gpm/sf, the SB GAC and SBA filters performed the best by removing 95% and 80% of the ammonia. While under the lowest hydraulic loading rate of 2.0 gpm/sf, all filters performed equally at removing between 96 and 100% of the ammonia.

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o A

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onia

2.0 gpm/sf4.0 gpm/sf6.0 gpm/sf7.5 gpm/sf

Figure 18. – Shallow bed anthracite filter performance at varying hydraulic loading rates with an

initial ammonia dose of 0.3 mg/L NH3-N.


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Figure 19. – Deep bed anthracite filter performance at varying hydraulic loading rates with an


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Figure 20. – Shallow bed GAC filter performance at varying hydraulic loading rates with an



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CONCLUSIONS

CAAs, particularly oxalic acid, were removed more readily than residual ammonia. The removal of CAAs occurred within the first 13 inches of filter media, whereas nitrification took place over 28 inches or more of filter media. Both the anthracite and GAC filters removed formic acid to below detectable limits by the filter effluent. Ammonia conversion occurs throughout the media depth. The GAC filter was the most efficient filter media at converting ammonia to nitrate. This conversion goes to completion within the first 28 inches of media depth. There was little difference between the GAC and anthracite filters from a TOC perspective. This is contrary to previously documented literature which states that GAC filters are more affective at removing TOC (Maloney et al. 1982; van der Kooij 1987; LeChevallier et al. 1992; Urfer et al. 1997). This is likely due to age of the SNWS GAC media which has been in service for more than 1.5 years. Overall, the filters removed between 14 and 16% of the influent TOC. THMs removal in the shallow bed anthracite filter ranged between 15 and 30 percent. Furthermore, the shallow bed anthracite filter removed between 25 and 17 percent of bromate. The speciated THM degradation results appear similar to trends reported in other published literature (Wahman et al. 2004; Wahman et al. 2005; Wahman et al. 2006a; Wahman et al. 2006b). Backwashing has little to no affect on the biological activity of the filters which is in alignment with previously reported research (Urfer et al. 1997; Ahmad et al. 1998; Liu et al. 2001). As the hydraulic loading rate increased, the filter removal efficiency for ammonia decreased.

RECOMMENDATIONS

It is recommended that the report findings be used to develop methods to promote the growth of the indigenous nitrifiers within biologically active filters.

It is recommended that further investigation should be undertaken to evaluate if the removal efficiency could be increased at higher hydraulic loading rates, if the system were operated under such conditions for extended periods of time.

It is recommended to investigate the bromate removal under higher source water concentrations.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the SNWS laboratory and Julia Lew, Pilot Treatment Plant Specialist, for their assistance on this project.


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Wert, E. C., J. J. Neemann, D. J. Rexing and R. E. Zegers (2008). "Biofiltration for removal of BOM and residual ammonia following bromate control." Water Research 42(2008): 372-378.


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