level (MCL) for N03--N at 10 mg/L and. 11.3 mg/L, re~pectively.~.~ ..... +To convert eq CI-/eq resin to lb NaClicu ft resin, multiply resin capacity (meq/mL) by 2.216. To convert eq Cl-/eq resin .... volume by 72 percent, Le., from 25 to 7 resin BVs.
d
I
RESEARCH I -&
#2j
r
/35-m3
I
I
-wl 1-
P
Ion Exchange for Nitrate Removal Dennis Cliford and Xiaosha Liu A bench-scale ion exchange process with batch biological denitrification of the spent regenerant brine was developed to remove nitrate &om drinking water. This research indicates that the combination procedure results in 50 percent reduction of regenerant consumption and 90 percent reduction in the mass of waste salt discharged. The process features a sequencing batch reactor to accomplish the biological denitrification of a 0.5 N sodium chloride spent regenerant solution. Now being pilot tested in California, the process-which is simple, flexible, and reliable-should be suitable for use by small systems. Contamination of drinking water with nitrate 'presents a health hazard because nitrate (N03-) can be reduced to nitrite (NOz-) ion in the gastrointestinal tract. Nitrite causes methemoglobinemia, a sometimes fatal disease to which infants are particularly susceptible. In addition, nitrate and nitrite have the potential to form carcinogenic N-nitroso compounds that are potent animal carcinogens, although they have not been conclusively shown to cause human cancers.'*2Thus, the US Environmental ProtectionAgency and the European Community have established the maximum contaminant level (MCL) for N03--N at 10 mg/L and 11.3 mg/L, re~pectively.~.~ Although nitrate is not normally significant in pristine groundwaters, a recent survey of US wells found that more than half of them contained nitrate at detectable levels, whereas an estimated 1.2 percent of community wells and 2.4 percent of rural wells exceeded the 10 mg N03-N/L MCL5 In Europe the nitrate problem tends to be worse because of the greater population density. For example, in 1970.60 public water supplies intermittently exceeded 11.3 mg NOB--N/Lin England and Wales; in 1980 the number was 90; in 1987 it had risen to 187.6 Worldwide, the situation is similar; nitrate contamination is ever increasing because it results from human activities, particularly agriculture. As population increases, so does nitrate contamination, because effective nitrate pollution prevention strategies have generally not been implemented. Removing nitrates from drinking water Ion exchange, biological denitrification, and membrane desalting with reverse osmosis hyperfiltration or electrodialysis are the most common methAPRIL
1993
ods reported for the removal of nitrate from water supplies. Although very effective technically, reverse osmosis and electrodialysis are considered too costly for routine nitrate removal unless desalting is also required to reduce the total dissolved solids O S ) level or to remove additional contaminant^.^,^ Ion exchange. Ion exchange (Ix)is the most common process in the United States, although the total number of plants in operation in 1992 for public water supplies probably did not exceed 15. The situation is changing rapidly, however, as many communities that had Rnr wser
I
0.75 4
BPn(n0n" W.CCU.WO,Mm (dmlpobm)
shut down nitrate-contaminatedwells are now finding that those wells must be reactivated to meet water demand. Surface waters have also been affected. Seasonal violations of the nitrate MCL in rivers that supply drinking water in agricultural states such as Iowa and Nebraska are becoming more frequent and more serious. This has resulted in an upsurge of interest in M for nitrate removal. The M process makes use of a packed bed of anion resin in the chloride form. Feedwater anions, including nitrate, are exchanged for chloride on a strong-base anion resin (Figure 1).The nitrate-free effluent is blended with a predetermined fraction of bypass raw water to produce a water of acceptable nitrate concentration, typically 7-8 mg N03--N/L. Just prior to nitrate breakthrough, if this point can be detected, column exhaustion is terminated, and the resin is either completely (e.g., > 95 percent N03- elution) or partially (e.g., 60 percent NOS- eluR.gn*mcM C I
i
-proart-
I
I I
$I Figure 1. Schematicof conventional nitrate-ion exchange process with 25 percent bypass blending and no treatment of spent regenerant DENNIS CLIFFORD & XIAOSHA LIU
135
tion) regenerated with 0.5-2.0 N (3 to 12 percent) sodium chloride (NaC1). Although the TDS of the product water is not changed significantly because ions are being exchanged rather than removed, it is enriched in chloride ion, which is initially the only ion to exit the column. Although technically and economically effective, nitrate removal by chloride M is not without significant problems, the most notable of which is disposal of the spent regenerant brine containing nitrate and excess NaCl?-'' Additional problems, such as increased corrosivity and negative health effects. are associated with the increase in chloride content of the product water. Sulfate ion in the raw water is troublesome because it can result in short exhaustion runs with standard anion resins, which all prefer sulfate to nitrate at the ionic strength ( I s 0.01 N) of typical water supplies. Finally, when the process is applied to surface waters, anion resin fouling can result from the presence of natural organic matter comprising high;molecular-weight anions, which are not easily removed from the resin during a typical NaCl regeneration. Despite the problems with nitrate removal by M,it is still considered the process of choice in the United States because of its simplicity, ruggedness, effectiveness, and relatively low cost. Direct biological denitrificatbn. The nitrate removal process that seems to be favored in Europe is direct biological denitrification by heterotrophic bacteria that use methanol, ethanol, or acetic acid as the carbon source and added phosphorus as nutrient. The direct denitrification process is favored because it does not add chloride to the product water and it produces only a small amount of biological sludge to be disposed of. Although this anoxic process efficiently removes nitrate, its application is more costly than IX because of its complexity. In addition to the requirement for continuous controlled addition of a stoichiometric amount of substrate, extensive posttreatment is required to remove microorganisms, excess substrate, turbidity, odor, nitrites, and chlorine demand." Generally this posttreatment includes coagulation, aerated sand or activated carbon filtration, ozonation, and chlorination. In spite of these disadvantages, many pilotscale and a few full-scale drinking water denitrification plants have been built in France12-15and Germany." --Autotrophicdenitrification with hydrogen gas oxidation has also been used for nitrate removal from drinking water in laboratory-scale fured-film reactors17and in full-scale operation.18 Generally autotrophic denitrification with hydrogen is even more complex than the heterotrophic variety because it is a three-phase (gas-liquid-solid) process that requires 136 RESEARCH AND TECHNOLOGY
ymnol
.nd nulrl"
Figure 2. Dutch ion exchange-biological denitrification process with upflow
sludge blanket reactor (IX-USBR process)
-
1
Figure4. Bench-scale laboratory setup to test the IX-SBR process with biological denitrification and reuse of spent brine
TABLE 1 Conditionsforoperation of ion exchange wtm Parameter Exhaustion Flow rate-BV/h Service length-h Feed composition Nitrate-mg NO - N/L SuKate-mg S O F / L Chloride-mg Cr/L Regeneration with 0.5NNaCI' Flow rate--BY/h Volume-BY Slow rinse (discarded)t Flow rate-BV/h Volume-BV
Sulfate-SelectiwResin
Nitrate-Selective Resin
20 20
20 22
20 i 0.5 32-56 14-22
20 f 0.5 32-56 14-22
5.0
5.0
10.0
10.0
5.0 2.5
5.0 2.0
'All the spent regenerant was collected for denitrification. ?All the slow rinse water was wasted. j0CILh;AL AWWA
L
I
c
the dissolution of a relatively insoluble g a s in water. Also, the autotrophic growth rate is slower, a disadvantage that is partially compensated for by reduced sludge production. In spite of these complexities, a full-scale plant that uses the hydrogen oxidation process has been successfully operated in Monchengladbach, Germany."
Makeup
1X combined with biological denitrification. When denitrification of spent nitrate
-+ l
I
WCI
........... ............ . . . . . . .. .. ....... ...... ........... ........... ........... ........... TmW brine *or808
I
Figure 3. American ion exchange-biological denitrification process with sequenc-
ing batch reactor (LX-SBR process)
160
\7
n
4
/J4
.............
. . . . .
,
1w
0
Cal
WA P
mntnfimte
.
.
.
.
l2
...........................................
N U " YCL
,
2w
wo
400
#10
h dv o l v m
Figure 5. Effluent history for cycle 12 of nitrate-selective resin without regenerant recycling (flow rate-20 B V h )
'
R@nf .Sulfateselective Nitrate-selective
Functional Group
1 I
Ttimethvl
NKHh
Triethyl N (C2H5)3
Chpacily mwM,
I I
1.40
1.01
Kwat
1 I
3.22 9.09
KS/CI*
I I
0.29 0.0045
KWd
I
0.31
0.36
I
'These resins are manufactured by Rohm and Haas Co.. Philadelphia, Pa. Similar resins are available from other manufacturers. t&CI is the nitrate-chloride selectivity coefficient +KS/CIis the sulfate-chloride selectivity coefficient 5Kbla is the bicarbonate-chloride selectivity coefficient.
APRIL 1993
brine is incorporated into the M process, the denitrified regenerant can be reused after compensation with NaCl. This significantly reduces the salt consumption and waste brine production while eliminating the eutrophication potential of brine discharged to the environment. Based on the findings of several nitrate removal studies completed in 1985, Clifford et a18 recommended the removal of nitrate by biological denitrification or nitrate-selective membrane transport prior to brine reuse. The first detailed description of an M-biological denitrification process was given by van der Hoek and Klapijk.I9 In their combined process, which is shown in Figure 2, nitrate is removed by chloride M and the resin is regenerated in a closed circuit containing a continuous upflow sludge blanket biological denitrification reactor (USBR). T h e process, it was claimed, reduced brine production by 95 ercent compared with conventional IX.% (Actually, the reduction in brine production by this process is very significant but probably not 95 percent when compared with the efficient use of salt during partial regeneration, as was reported by Guter in full-scale nitrate-removal studies.)" The authors believe that batch biological denitrification of spent brine in a sequencing batch reactor (SBR) is a more appropriate combination with M than is the use of the continuous USBR. A schematic of the proposed E-SBR process is shown in Figure 3. This article describes the successful development of the MSBR process at a bench scale. Thus, the objectives of the research described here were to (1) determine whether an SBR could be used to denitrify the 0.5 N spent brine from a nitrate M process, (2) establish the effects of denitrification and reuse on the performance of the M process with a nitrate-selective resin, and (3) compare the salt consumption and salt discharge performance of the new MSBR process with the conventional nitrate M processes using both standard and nitrate-selective resins. Additional research on the IX-SBR process published elsewherez2examined the influence of (1) the salt concentration on the SBR denitrification rate and (2) the effect of the mass methanol-to-nitrate-N ratio (I?) on the denitrification rate and the total organic carbon UOC) concentration of the reactor effluent. DENNISCLIFFORD & XIAOSHALIU 137
.
~~
Materials and methods IX runs. The bench-scale apparatus used to test the combined IX-batch biological denitrification process is shown schematicallyin Figure 4. To exhaust the resin, Houston tap water spiked with 20 mg NOB--N/Lwas pumped downflow at a rate of 30 mL/min through 90 mL of resin contained in a 2.2-cm-ID glass column. Other constituents of the Houston tap water averaged 44 mg sulfate (SO&/L, 18 mg chloride (Cl-)/L, 71 mg bicarbonate (HCOs-)/L, and 4 mg TOC/L. Effluent samples were collected at approximately 30-bed-volume (BV) intervals until the termination of a run at nitrate breakthrough, which occurred at 400-440 BV throughput. At the exhaustion flow of 20 BV/h, a run typically lasted from 20 to 22 h. Generally one exhaustion-regeneration cycle was conducted daily. Further information on the operation of the IX system is given in Table 1. Two different strong-base anion resins* were tested: resin 1, a conventional sulfate-selective, type 1 resin, and resin 2, an experimental nitrate-selective resin. Resin 2 has ethyl as opposed to methyl functional groups on the quaternary nitrogen to (1) separate the charged sites further and (2) make the resin more hydrophobic. Separating the exchange sites diminishes sulfate selectivity because divalent sulfate requires two closely spaced sites for attachment, and making the resin more hydrophobic improves its affinity for nitrate.7.'0*23Further information on these resins is given in Table 2. The IX column experiments were conducted in three phases: (1) exhaustion-regeneration of resin 1, (2) exhaustion-regeneration of resin 2, and (3) exhaustion-regeneration of resin 2 with regenerant recycling. In each phase, regenerant consumption, defined as equivalents of chloride consumed to remove one equivalent of nitrate from the resin, was calculated based on the measurement of anion concentrations in the spent regenerant. Regenerant, which for the experiments conducted in this study was always fresh 0.5 N NaCl or denitrified spent regenerant reconstituted to 0.5 N NaCl, was pumped downflow through the exhausted bed and collected in the SBR for subsequent denitrification. For the recycle experiments, the denitrified regenerant was reused after compensation with NaCl. For the nonrecycle r u m ; the spent regenerant was discarded after denitrification. SBR operations. Prior to and during the IX experiments, three amber-glass bench-top 1.5-L SBRs were operated daily in a temperaturecontrolled- (22 f
6
5 4
2
0
8.d volumn
Figure 6. Effluent history for cycle 12 of nitrate-selective resin with regenerant recycling @ow rate-20 BV/h)
RunE40
-$RunE-11
......9...., Run E-12 ......... ..a WE-13
--)-.
-----
RWIE-14 RunE-13
Figure 8. Sulfate breakthrough curves for runs 9-15 during regenerant reuse (raw water sulfate-32-56 mg/L)
'Resin 1-hberlite IRA-400. resin 2-Amberlite IRA-996 Rohm and Haas Co.. Philadelphia, P a
138 RESEARCH AND TECHNOLOGY
JOURNAL AWWA
I L
x
10
- 6
A. RunEQO .
+
RunE.11
I
RunE.12
+.
-+,
RunE-13
4
--)- Run E-14
+Run E-15 2
I
I
0
I
loo
I
I
2w
I
I
so0
I
I
4m
I
I
600
Bod Volumes
Figure 7. Nitrate breakthrough curves for runs 9-15 during regenerant reuse
*ant
(raw water nitrate-90 mg/L)
R.pwwnnk 0.5 N 11.21
figure 9. Performance of an SBR denitrifying spent regenerant from the nitrate-
selective resin, with and without recycle (to obtain mg NO3--N/L, multiply mg NOdL by 0.226)
APRIL 1993
3OC) laboratory over a period of 18 months. A typical cycle consisted of fill, react, settle, and drawdown. Each reactor was carefully sealed to exclude air and was connected to a displacement gas collector through a carbon dioxide(C02) absorbing bottle, as shown in Figure 4. (The absorbent was only a precaution, because COSwas not released at the typical reactor pH of 9.1.) Reactor 1,the focus of this article, was fed with spentregenerant brine, whereas reactor 3 received synthetic brine similar in composition and concentration to the spent regenerant. Reactor 2 was a control reactor to study the effect of salt on denitrification; no additional salt except nitrate was added to this reactor. The length of the stirred reaction phase was controlled by an electronic timer. The seed sludge for the denitrifying reactors came from a final clarifier in the 69th Street Wastewater Treatment Plant, Houston, Texas. This plant consists of a four-stage pure oxygen activated sludge process with nitrification in the final two stages. An acclimation procedure, which gradually increased salinity over a period of two months, was applied to allow the bacteria in the reactors receiving spent regenerant and artificial brine to adapt to the saline environment. The salt concentrations were set at 0.5 N for spent regenerant and 0.25 Nand 0.5 N for artificial brine. The composition of the water fed to each reactor is listed in Table 3. Filling a reactor and charging it with methanol took about 10 min. Reaction time varied in the range of 8-20 h, depending on the mass methanol-to-nitratenitrogen ratio (R) applied. The reaction phase was followed by a quiescent settling phase of 2.5-12 h. To complete a cycle, the drawdown phase took about 10 min. This sequence was repeated daily. During drawdown, the volume of decanted effluent was 0.9 L; thus, the displacement ratio of the 1.5-L reactor was 0.6/d. The mass of solids in the reactors was controlled by siphoning sludge to maintain a predetermined sludge height after settling. This resulted in a mixed liquor volatile suspended solids (MLVSS) concentration range in the reactors of 9.2-4.4 g/L, which is very low by comparison with the 150-200 g/L in the sludge zone of the USBR." Chemical analyses. All the samples were filtered through 0.45-pm membrane filters* prior to analysis. Nitrate, nitrite, sulfate, and chloride were measured using an ion chromatographt equipped with a 25(lmm anion separation column*. TOC was measured using a carbon analyzers. pH was determined by glass electrodes using a pH-mV meter.** Al'Gelman Sciences Inc.. Ann Arbor, Mich. tModel 16, Dionex Corp., Sunnyvale. Calif. $HPIC. Dionex Corp.. Sunnyvale. Cali. gDC-80. Dohrmann. Sanka Clara, Calif. "Model 4380000, Hach Co..Loveland, Colo
DENNIS CLIFFORD & XlAOSHA LIU
139
3.300
0 0
3.000
0
0
0
0
-
0
Results and discussion
U
0
theoretically expected. Thus, it was concluded that gas production could be used to monitor the rogress of the denitrification reaction. 21)
0
0
IX column performance. T h e breakthrough curves (or effluent histories) for chloride, bicarbonate, sulfate, and nitrate . o from the nitrate-selective resin (resin 2) bed without regenerant reuse are shown in Figure 5. As expected for the nitrateselective resin, sulfate broke through earlier than nitrate. (This was not the case with the sulfate-selective resin 1,000 [resin 11 which allowed nitrate to break through before sulfate.) Nearly identical 1 effluent histories for these ions were observed when the regenerant was denitrified in the SBR and reused (Figure 6). Based on these and similar curves for other exhaustions of the two columns, the authors concluded that reusing the denitrified regenerant after NaCl compensation to 0.5 N did not negatively inFigure 10. Nitrate in spent regenerant during 15 cycles of denitrification and fluence the column performance. This reuse (regenerant4.5 N NaC1; regenerant Pow rate-5 BV/h; regeneration lack of influence from regenerant reuse time-2 h; raw water nitrate-89 mg/L 120 mg/NO3--N/LI; to obtain mg No3-can also be seen in Figure 7, which deN/L, multiply mg N O d L by 0.226) picts only the nitrate breakthroughs for the ninth through fifteenth runs during regenerant reuse. The breakthroughs are nearly identical and do not suggest any trend to shorter or longer nitrate removal runs. The sulfate breakthrough curves for the same runs were not nearly as tightly grouped, as can be seen in Figure 8. In spite of the variation in sulfate breakthrough, no consistent trend toward shorter or longer run length with successive exhaustion numbers was observed. SBR performance. The performance of reactor 1,which was fed a batch of spent regenerant daily for 40 days, is shown in Figure 9. During the first 25 days of the successful 40-day run, the denitrified regenerant was simply wasted after treatment. Then, for the remaining 15 days, the treated regenerant was recycled and reused. Table 3 summarizes the quality of the denitrified brine from reactor 1 during the 40-day run and allows comparison with the control reactor. The first important observation made was that the nitrate removal performance of the IX column did not deteriorate as a Figure 11. Sulfate buildup in regenerant during 15 cycles of denitrification and result of reusing the denitrified regenerreuse (regenerant-O.5 N NaC1; regenerant Pow rate-5 BV/h; regeneration ant. Nitrate removal remained close to time-2 h; raw water sulfate-32-56 mg/L) 100 percent, and the IX run length was maintained at approximately 400 BV prior to nitrate breakthrough. In spite of kalinity and bicarbonate were deterMeasuring gas production. The temper- a nonoptimum R value of 2.2 and fluctuamined-by potential titration using the pH ature, volume, and rate of production of tions in influent nitrate concentration meter and a digital titrator.* The MLVSS the collected gas were measured and re- from 610 to 835 mg N03--N/L (2,700 to concentration was measured using the corded. Because the COZ produced dur- 3,700 mg NOdL), the SBR nitrate reprocedures that are described in Stan- ing denitrification was converted to moval efficiency averaged 99 percent and dard Methods.26In all cases, duplicate or HC03-in the reactor (the reactor pH was was always > 95 percent. An R value of 2.2 triplicate measurements were made and 8.8-9.3), only nitrogen and water vapor was chosen for the 40-day extended run the results were averaged. Reagent- accumulated in the gas collector. The ac- in an effort to conserve methanol and grade chemicals were used throughout tual N2 gas production after correction 'Model 1690001. Hach Co.. Loveland. Colo. the study. for water vapor content was equal to that 0
0
2300-
t"
il_i -t
140
RESEARCH AND TECHNOLOGY
s con-
TABLE 3
? used
Summary of SBR results
:rifica-
IreakLS) for itrate sin 2) hown trate-ough bt the resin xeak mtical -e obenitrir e 6). ts for mns, the
Parameter Reaction t i m e ' 4 Settling t i m e 4 MLVSWL Influent Nitrate-mg NOj--N/L Chloride-mg Cl -/L Sulfate-mg SO~'-/L CHsOH/NOrN (J?) Effluent Nitrate-mg NOa--N/L Nitrite-mg NOz--N/L Nitrate removakpercent ToC-mg/L PH Alkalinity-mg CaCOdL
Reactor 1 Spent Brine
Reactor 2 Conhol
Reactor 3 Synthetic Brine
la20 2.5 3.54.2
19-20 2.5 3.2-3.9
19-20 2.5 3.3-4.4
610-835 8,400-11.200 750-2,250 2.2
680120 14-22 32-56 2.2
680Y20
12,000 2,070f60 2.2 0-2.7 NDt 99.6100 44-90 8.ag.o 5,=150
0.5-4.0
0.1-42 0-8.1 95-99.9 32-168 8.ag.3 5,800-9,100
NDt 99.4-100 25-90 8.ag.i 4,800i150
'These long reaction times correspond to an R value of 2.2; at the optimum R value of 2.7, the denitrification reaction time is approximately 8 h. tND-not detected
TABLE 4 Summary of ion a c h a n g e results Process 1
2
3
Resin 2 Nosselective Complete, with brine reuse 0.5 440 4.95
Resin 2 Nosselective Complete, without brine reuse 0.5 440 4.95
Resin 1 Standard type 1 Complete, without brine reuse 0.5 400 3.57
3.75
8.47
10.5
Parameter Resin Resin description Regeneration method7 NaCl concentration-N Length run-BV eq Cl-supplied/eq resin+ eq Cl- consumed/eq Nosremoved5 NaCl discharge** lb/ldgal blended water Bypass-percent of feedwater Volume of wastewater-BV Wastewater-percent of blended water$+
317 ( W t t 30 2.0
2,890 30 12
3,170 30 12.5 2.3
2.1
0.4
Resin 3* Standard Partial, without brine reuse
I
1
4.84 673 t.6
I
1.1
'Resin 3 is a type 2 strong-base anion resin (ASB-2) manufactured by Ionac Inc, Philadelphia, Pa. Equivalent resins are available from other manufacturers. ?Complete regeneration is removal of >95 percent of the nitrate from the exhausted resin, followed by exhaustion with bypass blending. Partial regeneration is removal of about 60 percent of the nitrate followed by high-nitrateleakage exhaustion without bypass blending. +To convert eq CI-/eq resin to lb NaClicu ft resin, multiply resin capacity (meq/mL) by 2.216. To convert eq Cl-/eq resin to kg NaCl/m3 resin, multiply resin capacity (meq/mL) by 58.5. Wis is the key parameter with respect to salt consumption. 'This is the key parameter with respect to salt wastage. To obtain g/m3, multiply lb/1O6 gal by 0.120. ttThe salt discharge value in parentheses was calculated based on continuous reuse of regenerant without discardinn it after 15 cvcles.
TABLE 5 Comparison of IX-SBR with IX-USBR under ideal conditions PlOlXSS
Parameter Process configuration Operational mode Regeneration technique Hydraulic retention t i m e 4 Regeneiant concentration-N NaCl Regenerant volume-BV Biomass concentration-g VSS/L$ CH30H/NOrN (R) Equalization tank volume-BVt Reactor volume-BVt Treated brine storage-BV Total reactor plus storage volume-BVt
r 1 American E-SBR Batch Complete 8 0.5 10 3.34.4 2.7 0 15 10
25
2
American E-SBR Batch Partial
a
0.5 3 3.3-4.4 2.7 0 4 3 7
'Conditions for the Dutch process am based on references 19.20, 24, and 25. flank volumes are given in terms of bed volumes (BV)of resin. +VSS-volatile suspended solids
APRIL 1993
3 Dutch E-USBR' Continuous Complete 0.5 0.17 35 122-138 3.0 10 3 35 48
minimize TOC in the denitrified effluent. This low R value led to minimal substrate availability with its subsequent potential for nitrite prod~ction.'~ In fact, nitrite production was occasionally observed in this reactor, whereas its production was not detected in either the control reactor or the synthetic brine reactor. Two reasons are proposed for nitrite production in reactor 1. First, owing to the variability of influent nitrate, it was difficult to maintain an R value of exactly 2.2, which led to a shortage of methanol. Second, the alkalinity and pH of this reactor were higher than those of the other two reactors because of accumulation of bicarbonate eluted from the resin and generated during denitrification. No attempt was made in these experiments to control the pH of the SBR by acid addition before, during, or after the denitrification reaction. The authors adopted the no-pH-control strategy to keep the IX-biological denitrification system as simple as possible so that it could be used by small water systems. The recycle-reuse experiment demonstrated the importance of pH increase and alkalinity buildup in the brine, and the authors intend to study these variables in a planned pilot study of the IXSBR process in McFarland, Calif. When nitrate is in short supply, the high sulfate content of the regenerant brine can lead to unwanted sulfide production. Although the authors did not monitor the denitrified effluent for sulfide, its odor was detected in reactor 1on one occasion. Because this can be a serious problem, possible sulfide production will be carefully monitored in the pilot reactor during the field study in McFarland where methods to prevent its formation will be developed. Composition of recycled mgenerant. A major concern when a spent regenerant is reused is the accumulation of ions stripped from the resin during regeneration. These include nitrate, sulfate, and bicarbonate. Nitrate should be eliminated by denitrification, but sulfate and bicarbonate can accumulate during reuse when there is a net removal of these ions during exhaustion. Figure 10 illustrates that the average nitrate concentration in t h e spent regenerant did not continuously increase during 15 cycles of reuse following denitrification; its average value was about 3,000 mg/L (680 mg N03--N/L). Sulfate, on the other hand, started out at 750 mg/L and increased gradually following each reuse cycle until it stabilized at around 2,200 mg/Lafter the eighth reuse cycle (Figure 11). Fortunately, it did not increase without limit, which it could have done had sulfate not been dumped from the resin during exhaustion. In Figure 8, it can be seen that sulfate was typically dumped from the nitrate-selective resin beginning at about 280 BV and DENNISCLIFFORD & XlAOSHALIU
141
(Do-
7m-
ow-
Fipre.12. The influence of methanol-twnitrate-N ratio (R) on the rate of gas
production in reactor 3 (NaCl concentration4.5 N ) lasting until the end of the run at 440 BV. (The area under a breakthrough curve represents the elution or dumping of ions from the resin.) Apparently the process reaches a steady state in which the sulfate dumped during the sulfate peak beginning at about 300 BV equals the sulfate removed from the water during the early part of the run. The authors suspect that sulfate would build up to a much higher value with a standard sulfate-selective (trimethyl functionality) resin, but they did not test the recycle process with this kind of resin. Figures 8 and 11 illustrate the importance of the potential sulfate buildup in the process. This is always to be avoided because of the previously mentioned potential for sulfate reduction to hydrogen sulfide when all the nitrate has been consumed in the SBR. Salt consumption. Table 4 summarizes the IX run results and allows a comparison of the ideal nitrate-selective resin, brine-reuse IX-SBR process with the more conventional approaches, including a nitrate-selective resin process without regenerant reuse, a standard sulfate-selective resin process without regenerant reuse, and the type 2 sulfateselective resin process with partial regeneration and no brine reuse. The data for the partial regeneration process were not obtained during this study but were taken from a previous study using similar water in Glendale, Ariz.' The key economic parameter in the comparison is the equivalents chloride consumed per equivalent nitrate removed from the water. Here lower is bet142 RESEARCH AND TECHNOLOGY
ter. The M-SBR process required only 3.75 C1- ions, whereas standard M required 10.5 Cl- ions for each Nos-) ion removed from the raw water. Another important process consideration is waste discharge. Here the key parameter is the mass of NaCl wasted per volume of blended product water. Again, the IXSBR process delivered the best performance: 317 lb NaCl wasted/106 g a l blended water (38 g NaCl wasted/m3 blended water) compared with conventional E , which wasted 10 times as much salt (3,170 lb/106 gal), and even partial regeneration, which wasted more than twice as much salt (673 lb/106 gal). When the volume of wastewater discharged from the process during each cycle is considered, the M-SBR process again delivered the best performance compared with the other alternatives. Of course all these advantages come at the price of increased capital cost and process complexity. IX-SBR process optimization. All the data for the recycle-reuse experiments with brine denitrified in an SBR were obtained at a mass methanol-to-nitratenitrogen ratio (R) of 2.2, which was used in an attempt to conserve methanol. This was lower than the optimum R of 2.7." The time for >95 percent nitrate removal from the brine can be reduced from 20 h to < 8 h when more methanol is used and the R value is increased to 2.7. This is shown graphically in Figure 12, a plot of nitrogen gas production as a function of time for R values of 2.2 and 2.7. (Increasing R above 2.7 did not increase the denitrification rate.)22Further development
of the E-SBR process in McFarland will be carried out at the optimum R value of 2.7. This will result in a higher TOC concentration in the denitrification reaction effluent, the effect of which will also be studied in McFarland. Roceu design considerations. The size of the denitrification reactor will have a major influence on the capital cost of the E-SBR process. This size will be dictated by the number of BVs of regenerant required to completely regenerate the ion exchanger. In the experiments described here, 10 BV of regenerant were used. Although the authors did not experiment with the partial regeneration technique, it is a possibility for the MSBR process. It is not expected that partial regeneration will further reduce salt consumption or wastewater discharge, but it could dramatically reduce the size of the denitrification reactor and the treated brine storage tank. Table 5 is a comparison of the E-SBR complete regeneration process the authors tested with a hypothetical E-SBR partial regeneration process and with the Dutch M/USBR process described by van der Hoek et a1." If partial regeneration can be made to work, it will potentially reduce the total reactor plus brine storage volume by 72 percent, Le., from 25 to 7 resin BVs. These approximate calculations indicate that the total volume required by the Dutch WUSBR process will be considerably larger. These volumes, however, are based on their weak regenerant (0.17 N . If this could be increased to 0.5 N, the E-SBR and IXUSBR processes would require similar total volumes. Even if the continuous process could be operated at 0.5 N, it would still be much more difficult to control than a sequencing batch reactor. MeFarland pilot study. McFarland, Calif., was chosen as the field site to pilot test the E-SBR process because it has been the location of several previous research, development, and demonstration pro'ects related to nitrate removal by A 1-gpm version of the IX-SBR process has been built into the University of Houston-USEPA Mobile Drinking Water Treatment Research Facility.31 This mobile inorganics pilot plant, which also contains reverse osmosis and electrodialysis pilot plants, has been moved to McFarland where a year-long pilot study is under way. The plan is to operate the IX-SBR pilot process using a 0.5 N (3 percent) NaCl regenerant and a methanol-to-nitratenitrogen (R) value of 2.7. The objectives of the McFarland pilot study are to minimize the alkalinity buildup and consequent pH increase in the denitrification reactor, maximize the cycles of denitrified regenerant reuse, minimize the waste salt discharge, minimize the bacterial contamina-
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tion of the M product water by controlled disinfection of the denitrified brine, develop operational strategies to minimize nitrite and sulfide production during denitrification, and determine whether the process can be operated using partial as opposed to complete regeneration. Summary and conclusions A nitrate removal pt;ocess that drastically reduces salt consumption and waste discharge has been developed on a bench scale. It consists of chloride M and 0.5 N (3 percent) NaCl regeneration followed by batch denitrification and reuse of the spent brine. This process is referred to as the American or IX-SBR ion exchange-biological denitrification process because it makes use of an SBR as opposed to the USBR used in the Dutch (E-USBR) process. During the recycle-reuse experiments, essentially complete (>99 percent) denitrification of spent 0.5 N NaCl brine was achieved in 20 h using a nonoptimum methanol-to-nitrate-nitrogen ratio (Ry of 2.2. At the optimum R value of 2.7, the time for >95 percent denitrification was 8 h. Compared with a complete regeneration nitrate M process, the M-SBR process with 15 cycles of limited regenerant reuse consumed 50 percent less NaCl and resulted in 90 percent less salt discharge. Calculations indicate that if the regenerant is not discarded after 15 cycles but reused indefinitely, the reduction in salt discharge will be 95 percent. Compared with a partial-regeneration nitrateM process, the IX-SBR process with 15 cycles of limited reuse produced 70 percent more product water per run, consumed 23 percent less salt, and resulted in 50 percent less salt discharge. If the regenerant can be reused indefinitely, the reduction in salt discharge will be 84 percent. Compared with a USBR, the SBR has several advantages that are pertinent to small systems: simplicity, flexibility of operation, stability of effluent quality, and built-in equalization. Although the higher-rate USBR requires less reactor volume, when equalization and brine storage are taken into account, the space occupied by the processes is not much different.
Acknowledgment The authors thank Tom Sorg, USEPA research engineer, and Sarah Liehr, University of Houston engineering microbiologist, for technical advice. Louis Si"s is recognized for assistance with the laboratory work, and Ann Marie Martone for assistance with editing and typing. This work was funded by the United Nations Industrial Development Organization, the USEPA, and the University of Houston. APRL 1993
ological Denitrification Process for Nitrate Removal From Ground Water Under Different Process Conditions. Water Res., That N-Nitroso Compounds Contribute to 22:6:679 (1988). the Causation of Certain Human Cancers. NATO AS1 Series, vol. G 30, Nitrate Con- 21. GUTER,G.A Nitrate Removal From Contaminated Water Supplies: Volume I. Detamination (I. Bogardi and R.D. Kuzelka, sign and Initial Performance of a Nitrate editors). Springer-Verlag, Germany Removal P l a n t . P r o j . S u m m a r y , (1991). EPA/600/S2-86/ 115. USEPA. Cincinnati, 2. FORMAN, D. Nitrate Exposure and Human Ohio (1987). Cancer. NATO AS1 Series, vol. G 30, NiD.A. & Lru. X. Bioloaical Denitrate Contamination (I. Bogardi and RD. 22. CUFFORD. trificatio'n of Spent'Regenerint Brine Kuzelka, editors). Springer-Verlag, GerUsing a Sequencing Batch Reactor. Water many (1991). Res. (in press). 3. Fed. Reg., 54:124:27486 Uune 29, 1989). 23. CUFFORD, D A &WEBER, W.J. JR.The De4. Official Directive No. L229/11-L229/23. terminants of Divalent/Monovalent SelecOficial Jour. of the European Communitivity in Anion Exchangers. Reactive Polyties,vol. 23 (Aug. 30,1980). mers, 1:77 (1983). 5. BRISKIN,J.S. Pesticides, Nitrates Found in U S . Wells. Chem. & Eng~g.News, p. 46 24. VANDERHOEK,J.P.;VANDERHOEK,w.F.; & KIAPWIJK, A. Nitrate Removal From (May 6, 1991). Groundwater-Use of a Nitrate-Selec6. HISCOCK,K.M.; LLOYD,J.W.; & LERNER, tive Resin and a Low Concentrated D.N. Review of Natural and Artificial DeniRegenerant. Water, A i r & Soil Poll., trification of Groundwater. Water Res., 37:41 (1988). 25:9:1099 (1991). 7. GLITER, G.A. Removal of Nitrate From Con- 25. VAN DER HOEK,J.P. Combined Ion Exchange/Biological Denitrification for Nitaminated Water Supplies for Public Use. trate Removal From Ground Water. DocRept. EPA/600/2-82/042. USEPA, Cincintoral dissertation, Waningen Agricultural nati, Ohio (Aug. 1982). Univ., Wageningen, the Netherlands 8. CUFFORD, D.A. ET AL. Nitrate Removal (1988). From Drinking Water in Glendale, Arizona. Summary Rept. EPA/600/52- 26. Standard Methods for the Examination of Water and Wastewater. APHA, AWWA, 86/107. USEPA, Cincinnati, Ohio (1987). and WPCF, Washington, D.C. (16th ed., 9. GAUNTIHT,R.V. Nitrate Removal From 1985). Water by Ion Exchange. Water Trtmnt. & 27. BALDERSTON, W.L. & SIEBURTH, J.McN. NiExam. 24:172 (1975). trate Removal in a Closed-SystemAquacul10. CUFFORD, D A & WEBER,W. JR. Nitrate ture by Columnar Denitrification. Appl. Removal From Water Supplies by Ion ExEnvir. Microbiol., 32808 (1976). change. Rept. EPA/600/2-78/052. 28. GLITER, G A Operation, Performance, and USEPA, Cincinnati, Ohio (1978). Cost of the McFarland, California, Nitrate 11. GAYLE,B.P.;BOARDMAN,G.D.;&SHERRARD, Removal Plant. 1983 AWWA Ann. Conf., J.H. Biological Denitrification of Water. Las Vegas, Nev. Jour. of Enuir. EngYg., 115:5:930 (1989). 29. GLITER, G.A. Nitrate Removal From Con12. ROGALIA, F. ET AL Experience With Nitaminated Water Supplies: Volume 11. Proj. trate-Removal Methods From Drinking Summary, EPA/600/S2-87/034. USEPA, Water. NATO AS1 Series, vol. G 30, Nitrate Cincinnati, Ohio (1987). Contamination (I. Bogardi and R.D. 30. LAUCH, R.P. & GUTER, GA. Ion Exchange Kuzelka, editors). Springer-Verlag. Gerfor the Removal of Nitrate From Well many (1991). Water. Jour. AWWA. 78583 (May 1986). 13. PHILLIPOT, J.M. Biological Techniques 31. CUFFORD, DA. & BILIMORIA, M. A Mobile Used in the Preparation of Drinking Drinking Water Treatment Research FaWater: Nitrate, Iron, and Manganese Recility for Inorganic Contaminants Removal. USEPA Res. Symp.. Cincinnati, moval: Design, Construction, and OperaOhio, Aug. 1985. tion. Proj. Summary, EPA/GOO/ S2-84/ 14. RUCK,B.R & RICHARD, Y. Ergebnisse und 018. USEPA, Cincinnati, Ohio (1984). Erfahrungen mit der Biologischen Denitrifikation in Einem Wasserwerk. Vom Wasser, 64:145 (1985). About the authors: 15. RICHARD, Y.R. Operating Experiences of Dennis Clifford is proFull-Scale Biological and Ion-Exchange fessor of environmenDenitrificationPlants in France./our. Znst. Water Enuir. Mgmt., 3:lM (1989). tal engineering in the 16. ROENNEFAHRT, KW. Nitrate Elimination Department of Civil With Heterotrophic Aquatic Microorgaand Environmental nisms in Fixed-Bed Reactors With BuoyEngineering, Univerant Carriers. Aqua, 5283 (1986). sity of Houston, Hous17. KURT,M.; DUNN,IJ.; & BOURNE, J.R. Bioton, TX 77204-4791. logical Denitrification of Drinking Water Using Autotrophic Organisms With Hy- He has 30 years of experience in water drogen in a Fluidized Bed Biofilm Reactor. treatment, including several years conductBiotechnol. & Bwengrg., 29493 (1987). ing research related to nitrate removal. 18. GROS,H.; SCHNOOR, G.; & RLTITEN, G. Ni- Clifford is a graduate of Michigan Technotrate Removal From Groundwater by Auto- logical University, Houghton (BS)and the trophic Microorganisms. Water Supply, University of Michigan, Ann Arbor (MS 4:11 (1986). and PhD), and is a member of A W A , 19. VAN DER Hom,PJ. & KLWWJJK, k Nitrate Removal From Groundwater. Water Res., ACS, and AIChE. Xiaosha,Liu is a graduate sfrrdent research assistant in the De21:989 (1987). 20. VANDERHOEK,J.P.;VANDERVEN, P.J.M.; & partment of Civil and Environmental EnKIAPWUK, k Combined Ion-Exchange/Bi. gineering, University of Houston.
References 1. CRESPI,M . & RAMAZZO~TI, V. Evidence
DENNISCLIFFORD & XLAOSHALlU 143
