ion exchange of ammonium in zeolites: a literature review

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ION EXCHANGE OF AMMONIUM IN ZEOLITES: A LITERATURE REVIEW

By Annelie Hedstrom1

ABSTRACT: The objective of this review was to acquire knowledge concerning the ammonium ion exchangetechnique within the field of wastewater treatment. General concepts as well as details concerning the loadingand the regeneration phases were included. Both chemical and biological regeneration processes were reviewed.Concerning ion exchangers, the study focused on different kinds of zeolites. The possibilities of employing theion exchange technique for the recovery of nitrogen was also discussed. The study was carried out as a literaturereview. Conclusions from this study are that full-scale wastewater treatment plants that employ the ammoniumion exchange technique are scarce and few applications have been developed to recover ammonia nitrogen, forexample, for agricultural purposes. Zeolites are somewhat heterogeneous because of natural variations of theminerals. Factors that influence the ammonium adsorption during the loading phase are well known. Biologicalregeneration has primarily been developed to decrease the brine consumption at regeneration or to improve theconventional nitrification-denitrification process. If the ion exchange technique is to be used to recover ammo-nium, both chemical and biological regeneration might be employed.

BACKGROUND

Municipal wastewater contains nitrogen compounds, prin-cipally originating from urine and feces. Urine and feces areorganic compounds, but on their way to the wastewater treat-ment plant, the organic nitrogen is frequently decomposed toammonium. In regions with sensitive recipients (e.g., southernSweden), a nitrogen treatment step (nitrification-denitrifica-tion) is often included as a part of the wastewater treatmentprocess. Nitrogen is biologically transformed from ammoniumto nitrite, nitrate, and further to nitrogen gas. A certain amountof nitrogen becomes assimilated in bacteria and accumulatedin the sludge. This kind of nitrogen management does not aimat reusing wastewater nitrogen as, for example, a fertilizer, butrather to reduce the nitrogen content of the wastewater anddecrease the nitrogen load of the recipient.

An option to recover wastewater ammonium, for example,for agricultural purposes may be to employ ammonium ad-sorption or the ion exchange technique, where ammonium isadsorbed permanently or temporarily to an adsorbent/ex-changer (e.g., zeolite). If the adsorbent is just saturated once,the saturated adsorbent may be applied onto agricultural fieldsas a fertilizer (Perrin et al. 1998). When applying the ionexchange technique where regeneration is included, ammo-nium is first separated from the wastewater flow during theloading phase by filtering it through a column packed with anexchanger (Bolto and Pawlowski 1987). As the exchanger be-comes saturated with ammonium ions, the exchanger is regen-erated chemically by passing a salt solution through the col-umn. The ammonium ions are exchanged by cations such assodium ions, and the ammonium ions thereby become dis-placed (Semmens et al. 1977a). The chemical regeneration canalso be combined with biological regeneration [e.g., Green etal. (1996) and Semmens et al. (1977a)]. The regenerationphase results in a concentrated effluent stream of ammoniumchloride (chemical regeneration) or sodium nitrate (biologicalregeneration). The ion exchange process may be followed byammonia stripping when the brine solution is recovered, andthe ammonia gas may be sorbed in sulfuric (Liberti et al. 1982)or nitric acid (Liberti et al. 1981). Another possibility may beto dry the nitrate solution obtained from the biological regen-

1Div. of Sanitary Engrg., Lulea Univ. of Technol., SE-971 87 Lulea,Sweden. E-mail: [email protected]

Note. Associate Editor: Joseph Flora. Discussion open until January 1,2002. To extend the closing date one month, a written request must befiled with the ASCE Manager of Journals. The manuscript for this paperwas submitted for review and possible publication on June 27, 2000;revised March 7, 2001. This paper is part of the Journal of Environ-mental Engineering, Vol. 127, No. 8, August, 2001. qASCE, ISSN0733-9372/01/0008-0673–0681/$8.00 1 $.50 per page. Paper No. 22361.

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J. Environ. Eng. 200

eration in an evaporation facility. If the nitrogen compound isnot regarded as a valuable resource, the biological regenerationof the brine may then be followed by a denitrification step(Semmens and Goodrich 1977).

Different kinds of natural zeolites are most frequently sug-gested as ammonium exchangers for wastewater treatment ap-plications. Natural zeolites are aluminum silicates with highadsorption capacities.

The ammonium ion exchange technique has not been ex-tensively used on a commercial scale within the field of do-mestic wastewater treatment. The technique has, however,been investigated as an alternative to conventional biologicalnitrogen treatment (nitrification-denitrification) when theBOD/N ratio, wastewater temperature, or nitrogen concentra-tion are low (Ødegaard 1992; Verkerk and van der Graaf1999). It may also be an alternative if the wastewater containsnitrification inhibitors. Applying the ammonium ion exchangetechnique with the aim to recover nitrogen has not been in-vestigated as much. Lahav and Green (1998) mentioned thatthe drained nitrified brine might be used as a fertilizer. Libertiet al. (1986) developed a combined anion and cation exchangetechnique where both ammonium and phosphorus ions wereseparated, desorbed, and then precipitated as magnesium am-monium phosphate. This compound is known as a slow releasefertilizer (Liberti et al. 1986; Dolan et al. 1990). Liberti et al.(1982) also reported that, when ammonia was stripped afteran ion exchange process and then sorbed in sulfuric acid, theformed solution could be used as a fertilizer.

OBJECTIVE, SCOPE, AND METHOD

The objective of this review was to study and acquireknowledge concerning the ammonium ion exchange techniquewithin the field of wastewater treatment. General concepts aswell as details concerning the loading and the regenerationphases were included. The review of the regeneration pro-cesses comprised both chemical and biological regeneration.Because zeolites are most frequently suggested within thisfield, this study focused on the various kinds of zeolites asadsorbents—in particular, the characteristics causing the cat-ion exchange properties, adsorption capacity, and occurrenceof zeolites. Also discussed were the possibilities of employingthe ion exchange technique to recover wastewater nitrogen,for example, for agricultural purposes.

The study was done as a literature review and the literaturesearch was performed using Swedish and international scien-tific databases containing references dealing with wastewatertreatment, agriculture, and soil science.

OURNAL OF ENVIRONMENTAL ENGINEERING / AUGUST 2001 / 673

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ZEOLITES

Properties

Natural zeolites are aluminum silicate minerals with highcation exchange capacities (CECs) and high ammonium selec-tive properties (Kithome et al. 1998). More than 50 differentspecies of this mineral group have been identified (Tsitsishviliet al. 1992). Different kinds of zeolites are clinoptilolite (Kith-ome et al. 1998), ferrierite and mordenite (Townsend and Lo-izidou 1984), erionite (Mondale et al. 1995), and chabazite(Green et al. 1996). It has been stressed that the characteristicsof a zeolite mineral depend on its origin because of variationsin natural processes during the genesis (Townsend and Loizi-dou 1984; Mondale et al. 1995). Mondale et al. (1995) men-tioned that, for example, structural imperfections, a variety ofexchangeable cations, and the presence of clay may lead topore blockage and slow diffusion rates. Another relevant as-pect is that the zeolite mineral concentration of an ore sampledepends on the deposit from which the zeolite originates.Mondale et al. (1995) used zeolite samples with 80–100%zeolite and other occurring minerals such as volcanic glass,quartz, and feldspars. However, Curkovic et al. (1997) used aclinoptilolite that was 40–50% pure and Jørgensen et al.(1979) used clinoptilolites with 50–90% purity. Therefore,ammonium exchange experiments may sometimes result in ahigher exchange capacity for clinoptilolite compared to, forexample, chabazite, but similar experiments with the sameminerals, originating from other deposits, might give contra-dictory results.

Clinoptilolite (a commonly suggested zeolite for wastewatertreatment applications) has ion sieving properties, high CEC,and affinity for ammonium. Its chemical formula is(Na4 K4)(Al8 Si40)O96 ?24H2O (Vaughan 1978). Clinoptilolitehas a framework structure consisting of four- and five-tetra-hedral ring channels (Vaughan 1978) that form ion sieve chan-nels (Breck 1974). Channel diameters are within the interval3–8 Aº (Vaughan 1978). The porosity of clinoptilolite is about34% (Ming and Mumpton 1989). Not only does the clinoptil-olite have an ion sieving capability, it also has a CEC that iscaused when silica (Si41) is substituted by aluminum (Al31),thereby raising a negative charge of the mineral lattice. Thisnegative charge is balanced by cations such as sodium, cal-cium, and potassium, which are exchangeable with other cat-ions (Curkovic et al. 1997). Other zeolites that adsorb am-monium do not exactly have the same structure asclinoptilolite, but their ion sieving and ion exchange capacityis governed by the same principles.

Ames (1960, 1967) conducted experiments to rank cationsaccording to their affinity to clinoptilolite and developed thefollowing order:

1 1 1 1 21 21 1 21 31Cs > Rb > K > NH > Ba > Sr > Na > Ca > Fe4

31 21 1> Al > Mg > Li

Howery and Thomas (1965) developed a similar series. Thecorresponding cation affinity sequence for chabazite is as fol-lows (Breck 1974):

1 1 1 1 1 21 1 21 21Ti > K > Ag > Rb > NH > Pb > Na = Ba > Sr4

21 1> Ca > Li

These series show that potassium is adsorbed to the zeolitesto at least the same extent as ammonium. In a practical appli-cation, the separation of both ammonium and potassium couldtherefore be achieved.

The above sequences contain only a few heavy metals.

674 / JOURNAL OF ENVIRONMENTAL ENGINEERING / AUGUST 2001

J. Environ. Eng. 20

Other researchers, however, have stated that several heavymetals such as lead, chromium, cadmium, zinc, and copperhave a high affinity to zeolites (clinoptilolite, chabasite, andphillipsite) (Semmens and Seyfarth 1978; Sherman 1978; Co-lella 1993; Colella et al. 1993; Mondale et al. 1995).

The ion exchange capacity of zeolites may be given as thetotal CEC, which is defined by the number of equivalents offixed charges per amount of exchanger (Bolto and Pawlowski1987). The CEC indicates the theoretical amount of cationsthat can be accommodated by the zeolites; however, the CECdoes not correspond to the operating exchange capacity, whichis lower (Haralambous et al. 1992). Townsend and Loizidou(1984), Haralambous et al. (1992), and Nguyen (1997) haveall given values of CEC within the interval 1–2.27 meq/g(corresponding to 14–32 mg NH4-N/g) for different zeolites.Nguyen (1997) found that in practical applications it was dif-ficult to obtain an operating exchange capacity that exceeded50% of CEC. Townsend and Loizidou (1984) could achievean ammonium exchange capacity of 76% of CEC for clinop-tilolite. Other researchers have performed column and batchexperiments to determine the operating ammonium exchangecapacity, which were all within the range 1–7 mg NH4-N/g(Jørgensen et al. 1979; Semmens and Porter 1979; Hlavay etal. 1982; Townsend and Loizidou 1984; Chmielewska-Hor-vathova et al. 1992; Haralambous et al. 1992; Booker et al.1996; Beler-Baykal and Guven 1997; Nguyen 1997). In prac-tical applications, it would not be relevant to discuss ‘‘totaloperating exchange capacity’’ because too much ammoniumwould leach out when trying to saturate the exchanger. Sem-mens and Porter (1979) put the results of their pilot-scale ex-periment in relation to a 10% breakthrough. Furthermore, thereis a competition in the wastewater for exchange sites betweenammonium ions and other cations, resulting in a lower am-monium exchange compared to if only an ammonium solutionwould have been applied (Koon and Kaufmann 1975).

Different researchers have discussed what happens to theammonium exchange capacity when the exchanger is loadedand regenerated repeatedly. Liberti et al. (1981) revealed that,although they had treated 40,000 bed volume (BV) of waste-water in their laboratory plant, no obvious decrease in am-monium exchange was noted and no replacement or restora-tion of the clinoptilolite had been required. Booker et al.(1996) did not observe any capacity decrease. Beler-Baykaland Guven (1997) found that there was a decrease in ammo-nium exchange capacity for a clinoptilolite that had been inoperation and regenerated 10 times, compared to a fresh cli-noptilolite. Koon and Kaufmann (1975) found that caustic re-generation solutions seemed to cause an attrition of zeolitesbecause the caustic solution attacked the clinoptilolite framestructure. There was also a significant decrease in zeoliteweight when the pH of the regeneration solution was increasedfrom 11.5 to 12.5.

Occurrence

Zeolite deposits are found in many parts of the world. Thezeolites used in experiments referred to in reviewed researchpapers originated from southeastern Europe (Jørgensen et al.1979; Chmielewska-Horvathova et al. 1992; Beler-Baykal etal. 1996), the United States (Koon and Kaufmann 1975; Sem-mens and Goodrich 1977; Jørgensen et al. 1979; Liberti et al.1981; Townsend and Loizidou 1984; Chmielewska-Horva-thova et al. 1992; Perrin et al. 1998), Australia (Booker et al.1996), and New Zealand (Nguyen 1997). Zeolite deposits havealso been found in Russia, Mongolia, Korea, China, Mexico,Cuba, South Africa, Tanzania, and Kenya (Tsitsishvili et al.1992).

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FACTORS HAVING IMPACT ON AMMONIUMEXCHANGE ON ZEOLITES

Homoionic Form of Zeolite

Chemical pretreatment of the zeolite to transform it to ahomoionic form and increase its ammonium exchange capacityhas been discussed by many authors (Koon and Kaufmann1975; Semmens and Goodrich 1977; Jørgensen et al. 1979;Hlavay et al. 1982; Townsend and Loizidou 1984; Haralam-bous et al. 1992; Beler-Baykal et al. 1996; Booker et al. 1996;Beler-Baykal and Guven 1997). When preparing a zeolite tobe of a homoionic form, the zeolite is loaded with a solutioncontaining, for example, a sodium, magnesium, or calciumsalt. Once saturated, the zeolite is then rinsed with distilledwater. Pretreatment with sodium ions has been shown to im-prove the selectivity and ammonium exchange capacity of thezeolite more than pretreatment with magnesium and calciumions (Jørgensen et al. 1979; Booker et al. 1996). The calciumform of the clinoptilolite had an even lower ammoniumexchange capacity than untreated clinoptilolite (Hlavay et al.1982), which was explained in terms of cation size. Becausethe calcium ion is larger than the sodium ion, it cannot ap-proach the exchange sites as closely as sodium, which issmaller and freer to migrate through the zeolite channels(Koon and Kaufmann 1975). Haralambous et al. (1992) sug-gested, however, that pretreatment may not be necessary if thezeolite is regenerated with a sodium solution because the ze-olite turns more homoionic during regeneration. Jørgensen etal. (1976) observed that the three first regenerations increasedthe ammonium exchange capacity.

Grain Size

The grain size distribution of the zeolite has an impact onthe operating ammonium ion exchange capacity. Hlavay et al.(1982) investigated grain sizes in the intervals of 0.5–1.0,0.3–1.6, and 1.6–4.0 mm. The smallest fraction resulted inthe highest ammonium exchange capacity. The investigationby Jørgensen et al. (1976) gave analogous results in experi-ments with zeolites in the interval 1.4–5.0 mm. Ames (1960)concluded that grain sizes >1.0 mm drastically decreased theammonium exchange capacity. However, the head loss in-creases with smaller grain sizes. Hlavay et al. (1982) and Øde-gaard (1992) recommended minimum grain sizes of 0.4–0.5mm. Nguyen (1997) showed that, with a low surface loading,the ammonium exchange capacity was similar for the smaller(0.25–0.5 mm) and larger (2.0–2.8 mm) grain size distribu-tion. When the loading was higher, a higher ammoniumexchange capacity was obtained for the smaller grain sizes.When using smaller grain sizes, the higher ammoniumexchange capacity is probably caused by a higher mass trans-fer into the zeolite.

Hydraulic Retention Time

The hydraulic retention time influences the operating am-monium exchange capacity when filtering wastewater througha zeolite packed column. Beler-Baykal et al. (1996) investi-gated the effect of the hydraulic retention time within the range0.5–12 min. They did not recommend a hydraulic retentiontime of <3 min because the breakthrough would occur too fast,and they eventually selected a retention time of 5 min for theirexperiments. Beler-Baykal and Guven (1997) found that alonger hydraulic retention time (within the interval 3–10 min)delayed the breakthrough but that most ammonium was ad-sorbed after 5 min. Booker et al. (1996) conducted batch ex-periments, which indicated that the ammonium adsorption tothe zeolite was a fast process that occurred within 10 min.Their following experiment showed that, if the hydraulic re-

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FIG. 1. Impact of pH on Ammonium-Ammonia Equilibrium [fromØdegaard (1992)]

tention time was <6 min (10 BV/h), then the breakthroughoccurred significantly faster.

Influent Concentration of Ammonium

Hlavay et al. (1982) conducted experiments to investigatethe effects of influent concentrations of ammonium in the in-terval 17–45 mg NH4-N/L and found that a higher influentconcentration resulted in a faster breakthrough. The exchangedamount of ammonium was rather similar for the different con-centrations, although slightly higher for the influent concen-tration 45 mg NH4-N/L. Kithome et al. (1998) and Jørgensenet al. (1979) performed similar experiments and found thatgreater influent concentrations of ammonium resulted in alarger amount of exchanged ammonium.

Ionic Strength—Competition

Although some zeolites (e.g., clinoptilolite) have a high af-finity and selectivity for ammonium ions, other ions in thesolution have a negative impact on the ammonium exchange.Koon and Kaufmann (1975) performed experiments where theammonium exchange was investigated in relation to the totalcation concentration of the influent. The exchange capacitydecreased significantly up to a cation concentration of 0.01mol/L. Increases of cationic strength above this value contin-ued to decrease the exchange capacity but to a much lesserdegree. Jørgensen et al. (1976) observed that the ammoniumexchange capacity was lower when the influent was based ontap water instead of distilled water. Simply measuring the ionicstrength to estimate the competition of exchange sites betweendifferent ion species may, however, be of limited value, be-cause they do not have the same affinity for the zeolite (seethe affinity sequences described above).

pH

Koon and Kaufmann (1975) investigated the impact of pHon the ammonium exchange in zeolites by varying the pHbetween 4 and 10 and obtained the highest ammoniumexchange at pH 6.

They explained that at low pH, the ammonium ions had tocompete with hydrogen ions among the exchange sites; how-ever, when the pH was high, the ammonium ions were trans-formed to ammonia gas (Fig. 1). For practical applications,they recommended a pH value within the interval 4–8 duringthe loading phase (Koon and Kaufmann 1975). Kithome et al.(1998) found that, when they performed experiments where


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the pH was varied between 4 and 7, most ammonium wasadsorbed at pH 7. They gave the same explanation for low pHvalues as Koon and Kaufmann (1975), but Kithome et al.(1998) assumed that new sorption sites were formed at ahigher pH.

Temperature

Koon and Kaufmann (1975) discussed the temperature ef-fect on the ammonium exchange process and claimed that tem-peratures between 107 and 207C did not impact on the pro-cesses. Atkins and Scherger (1997) mentioned that oneadvantage of employing ammonium exchange for nitrogentreatment was the temperature independence of this method.

Scaling Up

When scaling up an ion exchange system, the larger systemwill probably not achieve the same operating ammoniumexchange capacity as one of a smaller laboratory scale. Hlavayet al. (1982) observed that, when scaling up a system 100-fold, just 60% of the ammonium breakthrough capacity couldbe reached compared to that of the smaller system. The ex-planation given was that channeling occurred in the ionexchange column in the larger system because of larger di-mensions of the column.

REGENERATION

Chemical Regeneration of Ammonium SaturatedZeolites

Chemical regeneration aims at desorbing ammonium ions,thus making the exchange sites of the exchanger available fornew cations. The regeneration phase is often followed by airstripping, whereby the ammonium ions are turned into am-monia gas and absorbed to an acid.

When the zeolite is exhausted, the loading is interrupted andbrine is pumped through the column. Both upflow (Koon andKaufmann 1975; Liberti et al. 1981) and downflow (Hlavayet al. 1982) applications of the brine have been described.

Many authors have used sodium chloride with the concen-tration 0.1–0.6 M NaCl as the regeneration brine (Koon andKaufmann 1975; Liberti et al. 1981; Hlavay et al. 1982;Chmielewska-Horvathova et al. 1992). The time needed forsatisfactory regeneration of the exchanger depends on the con-centration and pH of the brine. Koon and Kaufmann (1975)investigated the impact of pH on the regeneration perfor-mance. At pH 11.5 and 2% regeneration brine, 20 BV of re-generation brine were needed, corresponding to 1.3 h of re-generation. When pH was increased to 12 and 12.5, 20 and10 BV of brine were needed, respectively. In both cases, 1.2%regeneration brine was sufficient. Ødegaard (1992) recom-mended a mixture of sodium chloride and sodium hydroxide


J. Environ. Eng. 20

as a regeneration brine. This would decrease the need for brineby 90% compared to using only sodium chloride. However,the advantage of using caustic regeneration brine must be putin relation to the disadvantage of possible zeolite attrition(Koon and Kaufmann 1975).

Koon and Kaufmann (1975) found that the regeneration per-formance was independent of the flow rate within the interval4–20 BV/h and chose 15 BV/h. Similar results were observedby Semmens and Porter (1979), who varied the regenerationflow between 12 and 20 BV/h and continued with the regen-eration flow 12 BV/h for 1 h. Liberti et al. (1981) investigatedthe performance of a pilot plant and applied a regenerationbrine with the concentration 0.6 M, flow of 24 BV/h, andregeneration period of 40 min. Hlavay et al. (1982) found that,if the regeneration flow rate was 5 BV/h, 4 h of regenerationwas needed. However, if they increased the flow rate to 7BV/h, the regeneration period decreased to 1.4 h. Sodiumbrine with the concentration 0.34 M was used.

The frequency of the regeneration periods depends on theamount of exchanger used, loading rate, influent concentration,and acceptable effluent concentration of ammonium. Koon andKaufmann (1975) suggested a regeneration interval of about12 h, corresponding to a loading of 150–180 BV. Liberti etal. (1981) regenerated the exchangers every third hour, after80 BV of loading, and Semmens and Porter (1979) suggesteda single regeneration per day. Metcalf and Eddy (1991) rec-ommended calcium hydroxide to be used as a regenerationsolution. Hlavay et al. (1982) wrote, however, that the sodiumions regenerated the zeolite faster and more efficiently com-pared to calcium ions because the calcium ions may be per-manently sorbed to the exchanger and permanently decreaseits ion exchange capacity. It could, however, be practical touse calcium hydroxide if ammonia stripping follows the ionexchange. Thus, the need of a caustic chemical addition in thefollowing step would be decreased. Buday (1994) used nitricacid as a regenerant agent.

Liberti et al. (1981) and Semmens and Porter (1979) ob-tained regeneration effluents with ammonium concentrationsof about 280 and 100–200 NH4-N/L, respectively. Verkerkand van der Graaf (1999) wrote, however, that ammoniumconcentrations in the interval 300–1,000 mg NH4-N/L couldbe achieved with chemical regeneration.

Table 1 gives a summary of data for chemical regeneration.

Biological Regeneration of Ammonium SaturatedZeolites

An alternative to chemical regeneration is biological regen-eration, which actually is a combination of chemical regener-ation and nitrification (Semmens et al. 1977a,b; Semmens andGoodrich 1977; Semmens and Porter 1979; Green et al. 1996;Nguyen 1997; Lahav and Green 1998). One reason given forthe development of biological regeneration was to decrease

TABLE 1. Summary Table of Chemical Regeneration

ParameterKoon and Kaufmann

(1975)Liberti et al.

(1981)Semmens and Porter

(1979)Hlavay et al.

(1982)

Zeolite type Clinoptilolite Clinoptilolite Clinoptilolite ClinoptiloliteGrain size of zeolite (mm) 0.84–0.3 — 0.3–1.0 —Exhaustion flow (BV/h) 15 24 5.5–6.9 5–15Regeneration mode Upflow mode Upflow mode Upflow mode Downflow modeRegeneration interval 2 times/day 6 times/day 1 time/day —Regeneration brine NaCl NaCl NaCl/NaNO3 NaClBrine concentration (M) 0.34, 0.21, 0.21 0.6 0.3 0.34, 0.34, 0.34Regeneration flow (BV/h) 15, 15, 15 30 12–20 5, 7, 7.5Duration of regeneration (h) 1.3, 1.3, 0.7 0.7 1 4, 1.4, 1.7Brine volume needed (BV) 20, 20, 10 20 12–20 20, 10, 12.5Regeneration pH 11.5, 12, 12.5 >7 7–8.4 12.3, 12.3, 12.3Regeneration efficiency (%) >100, >100, >100 >100 — 98.7, 99.2, 98.4

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TABLE 2. Summary Table of Biological Regeneration

ParameterBiological regeneration

of zeolite by nitrifying sludgeBiological regeneration

of regenerationBiological regeneration

of zeolite in single reactor

Regeneration principle Nitrifying sludge with sodium ionsis pumped through zeolite col-umn and ammonium ions aredesorbed

Nitrified brine solution is pumpedthrough zeolite column andammonium ions are desorbed

Nitrified brine solution is pumped throughzeolite column and ammonium ions aredesorbed

Nitrification process Takes place in aeration tank lo-cated after zeolite column; nitri-fying bacteria are not separatedfrom regeneration brine

Takes place in aeration tank lo-cated after zeolite column; ni-trifying bacteria are separatedfrom regeneration brine

Takes place in zeolite column; nitrifica-tion bacteria are attached on zeolitegrains

Duration of regeneration Limited by nitrification process Limited by ion exchange process Limited by nitrification processRegeneration flow Fluidization needed Fluidization needed

brine consumption (Semmens et al. 1977b). A second reasonwas to achieve a more efficient nitrification process in a smallvolume with minimal competition between autotrophic nitri-fying bacteria and heterotrophic bacteria caused by a low con-centration of organic compounds (Lahav and Green 1998).

The combined chemical and biological regeneration mech-anisms were derived by Semmens et al. (1977a). Their exper-iments showed that the nitrification rate correlated to the dis-solved ammonium concentration in the solution and not to theamount of ammonium adsorbed to the zeolite that was sus-pended in the water. These observations agreed with investi-gations performed by Goldberg and Gainey (1955). The am-monium concentration was, in turn, correlated to the amountof ammonium displaced by ion exchange (Semmens et al.1977a,b). The equilibrium and reaction formulas for chemicaland biological regeneration suggested by Semmens et al.(1977a) were

1 1 1 2[Z]NH 1 NaHCO ↔ [Z]Na 1 NH 1 HCO (ion exchange)4 3 4 3

1 2 1NH 1 2O → NO 1 2H 1 H O (total nitrification reaction)4 2 3 2

The combination of these two reactions is

1 1 2 1[Z]NH 1 2O 1 2NaHCO ↔ [Z]Na 1 NO 1 Na 1 3H O4 2 3 3 2

1 2CO (combined ion exchange and nitrification reaction)2

Because the nitrification reaction causes a consumption ofalkalinity, sodium bicarbonate can be added as a chemical re-generant instead of sodium chloride. The bicarbonate ionswould neutralize the hydrogen ions formed during the nitrifi-cation process and half the sodium ions would contribute tothe displacement of the adsorbed ammonium. The other halfof the added sodium ions would remain in the solution,thereby resulting in a buildup of sodium nitrate in the solution.

Three different methods of biological regeneration are pre-sented below. Table 2 presents a summary of the differentmethods.

Biological Regeneration of Zeolite by NitrifyingSludge

A few applications on biological regeneration were foundupon researching the literature. Semmens et al. (1977b) inves-tigated one application (Fig. 2) where zeolite was used as anion exchanger during the loading phase as ammonium ionswere being adsorbed. When the zeolite was exhausted, it wasregenerated by pumping a nitrifying sludge up through thezeolite. The sludge fluidized the bed and drained back to thesame aeration tank from which the nitrifying sludge wasdrawn. The fluidization corresponded to about 50% expansionof the bed. The concentration of sodium nitrate in the sludgewas about 0.3 M, and the sodium ions were there to displacethe ammonium ions from the zeolite. The ammonium ionswere then to be oxidized in the aeration tank. During nitrifi-cation, sodium carbonate was added to compensate for the

J. Environ. Eng. 20

FIG. 2. Process Scheme for Regeneration of Ammonium Exchangerwith Nitrifying Sludge [from Semmens and Porter (1979)]

alkalinity consumption. After the zeolite was completely re-generated, it was backwashed to remove any excess sludge.The first portion of the backflush water was collected to re-cover the nitrifying bacteria of the sludge and was returned tothe aeration tank. The remaining backwash water was drainedout of the system.

Of note from this experiment was that the regeneration timewas limited by the nitrification rate and not by the ionexchange, a much faster process (Semmens et al. 1977b). Theregeneration period seemed, however, to not exceed 2 h. Thebiological regeneration was not as efficient as the chemicalregeneration, performed at the same conditions pertaining tobrine concentration (0.3 M), flow, and temperature. The reasongiven by Semmens et al. (1977b) was an accumulation of cal-cium and magnesium ions in the recycled regenerant duringthe biological regeneration. However, they neither noticed anysignificant fouling during 40 cycles nor observed any loss inzeolite capacity or deterioration in column performance.

Semmens and Porter (1979) wrote that the advantage with

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FIG. 3. Process Scheme for Biological Regeneration of RegenerationSolution [from Semmens and Porter (1979)]

the above application was its simplicity. However, they re-ported that the method had several problems such as an un-steady ammonium concentration in the aeration tank that fol-lowed the ion exchange column. In the beginning of theregeneration phase, a peak of ammonium ions would appearin the aeration tank, ions that may pass through the tank with-out being oxidized and be adsorbed to the zeolite again duringthe regeneration phase. This problem would link the regener-ation time to the nitrification rate. Furthermore, a high flowrate would be needed to fluidize the zeolite, ensuring that thenitrifying bacteria are flushed through the column and returnedto the aeration tank.

Biological Regeneration of Regenerant

Because of the problems associated with the above concept,Semmens and Porter (1979) suggested an alternative applica-tion that was very similar in nature except that the nitrificationand the ion exchange steps were separated (Fig. 3). The ex-hausted zeolite would be regenerated by using a neutral pHbrine containing 0.3–0.4 M sodium nitrate (initially a 0.3 MNaCl was to be used). When displaced, the ammonium ionswould flow to a feed tank from which they would be fed to anitrification tank, preferably with a constant flow. During ni-trification, sodium carbonate would be added to compensatefor the alkalinity consumption. The nitrification sludge wouldbe separated from the nitrified brine in a separation/storagetank, and the brine would be stored in the storage tank untilthe next regeneration. This process is more complex but hassome advantages (Semmens et al. 1977b). First, it would elim-inate the contact between the nitrifying bacteria and the zeo-lite, hence reducing the possibility of biological fouling of the



FIG. 4. Biological Regeneration of Ammonium Exchanger in SingleReactor [from Lahav and Green (1998)]

zeolite. Second, this process would reduce the time needed forregeneration to that of the ion exchange, which is shorter thanthe required time for nitrification. The separation of the ionexchange and nitrification mechanisms would also allow eachstep to be optimized and the volume of the nitrification tankwould be smaller (Semmens and Porter 1979). Furthermore, alower regeneration flow rate may be used and a higher am-monium concentration of the spent regenerant may beachieved.

The experiences from the investigations of Semmens andPorter (1979) showed that this regeneration was stable andeffective. Ninety to ninety-seven percent ammonium was dis-placed during the 43 regeneration cycles despite not havingoptimum conditions. No fouling or permanent loss ofexchange capacity occurred. The magnesium and calcium con-centrations were, however, built up in the regeneration solutionat the expense of sodium and caused an earlier breakthroughof ammonium. Reported factors that affected the cation con-centration of the solution were wastage rate of brine, sludgewastage rate, chemical precipitation, salinity of brine, cationconcentration in the wastewater, and zeolite characteristics(Semmens and Porter 1979). They also experienced problemswith maintaining the pH at an optimum level for nitrificationand simultaneously avoiding carbonate precipitation. This re-generation method may be more expensive compared to themethod presented by Semmens et al. (1977b) because of theneed for more reactors (Semmens and Porter 1979).

Biological Regeneration of Zeolite in Single Reactor

Green et al. (1996) and Lahav and Green (1998) investi-gated an application of biological regeneration where the entireprocess (loading and regeneration) was to be carried out in asingle reactor (Fig. 4). The reactor was to be operated in twosequential modes. In the first mode, the reactor would workas an ion exchanger for ammonium capture. In the secondmode, the same column would operate as a fluidizing bed re-actor for biological regeneration of the saturated zeolite, whichwould also act as the carrier for the nitrifying biofilm. Duringthe regeneration phase, the reactor was operated batchwise andoxygen and sodium bicarbonate were supplied for the nitrifi-cation process and to maintain a constant pH. Sodium salt wasadded to the recirculating solution to facilitate the ammoniumdesorption. Because the ammonium was oxidized to nitrate,the regeneration solution could be used during several cycleswith no addition of external regenerant. The addition of ex-ternal cations was limited only to the amount of sodium bi-carbonate buffer added. At the end of both the adsorption andthe regeneration phases, the column was backwashed. Afterthe regeneration phase, it was suggested that the nitrate-richbackwash water could be used for agricultural purposes orconveyed to a denitrification reactor (Lahav and Green 1998).

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One of the experiences from the investigations presentedabove was that ammonium removal efficiency during the load-ing phase was >95% (2-min retention time) and that a highnitrification population had been established (Lahav and Green1998). Green et al. (1996) did not detect any significant dif-ferences in the ammonium exchange capacity between cleanand biofilm covered zeolite. They also found that the cationconcentration seemed to reach equilibrium; therefore, no so-dium salt would have to be added except for buffer purposes.Ammonium desorption experiments showed that the regener-ation with the concentration 10,000 mg Na/L was more effi-cient compared to a solution with 2,440 mg Na/L. However,the nitrification step seemed to limit the time of the regener-ation process and the lower sodium concentration appeared tobe enough (Green et al. 1996). Further investigations showedthat a regenerant concentration as low as 0.05 M NaNO3

(1,150 mg Na/L) was sufficient to establish maximum nitrifi-cation rates within the system’s operational parameters. Higherconcentrations did not decrease the regeneration period (Lahavand Green 1998). The oxygen supply during the nitrificationwas the limiting factor even though pressurized oxygen wasused.

Verkerk and van der Graaf (1999) wrote that the brine pro-duced during various kinds of biological regeneration pro-cesses could be defined as a high nitrate solution with theconcentration 300–1,500 mg NO3-N/L and a pH within theinterval 5.5–8.5.

Ion Exchange and Nitrification for Peak Loads ofAmmonium

Another application where ammonium exchange in zeolitesand nitrification are combined has been investigated by Beler-Baykal et al. (1994, 1996) and Beler-Baykal and Guven(1997). Its purpose was to use the zeolite as an adsorbent whenpeak loads of ammonium occurred in a trickling filter. Theynoticed that the ammonium, earlier adsorbed to the zeolite,was desorbed during periods of very low ammonium concen-trations. Beler-Baykal et al. (1996) performed experimentswhere 90% of adsorbed ammonium could be desorbed withtap water in the first half-hour. Nguyen (1997) showed, how-ever, that ammonium held tightly to the zeolites and was notextracted after 116 h of shaking with distilled water.

DISCUSSION

The reviewed ion exchange applications for the treatmentof ammonium appeared to be rather complicated and to requireextensive operation and control. If the regeneration consists ofa combination of chemical and biological regeneration, thesystem will be even more complicated because both stepsshould be optimized.

The loading phase within the ion exchange technique ap-pears to have been investigated to a high extent, and the fac-tors that influence the exchange of ammonium are well known.Because the characteristics of similar zeolites may differ be-cause of their origins, laboratory experiments will always beneeded prior to new full-scale applications to optimize the ap-plication to the present conditions.

If the nitrogen treatment process is to be used to recoverammonium nitrogen from the wastewater, for example, for ag-ricultural purposes, ion exchange with chemical or biologicalregeneration may be possible solutions. This is because withboth concepts a concentration of wastewater nitrogen is ob-tained, facilitating the following treatment steps that areneeded. The choice of regeneration process depends on theactual processes following the ion exchange step. If dryingwastewater in an evaporation facility is considered to be analternative after the ion exchange step, it may be suitable to

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employ biological regeneration to transform the ammoniumions to nitrate. In that way, nitrogen losses through ammoniagas release could be prevented (Johansson 1999). Conversely,if the ion exchange process is to be followed by air stripping,it would be more appropriate to regenerate the zeolite chem-ically. The ammonium ions are then concentrated in the ionexchange step and converted to ammonia gas during the strip-ping process. The ammonia nitrogen may, thereafter, be re-covered by sorption in, for example, sulfuric acid (Liberti etal. 1982) or peat (Witter and Kirchmann 1989). Liberti et al.(1986) examined another ammonium recovery method whereammonia nitrogen, concentrated in an ion exchange process,was precipitated together with phosphate and magnesium ions.Here, chemical regeneration was needed.

Most of the investigations referred to in this review did notattempt to recover wastewater nitrogen. Thus, further labora-tory investigations are needed if those methods are to be usedfor ammonium recovery applications. The biological regener-ation method investigated by Green et al. (1996) and Lahavand Green (1998), where the ion exchange and biological re-generation processes were conducted in the same reactor,seems to be the simplest biological regeneration process.Green et al. (1996) wrote that they had not observed any de-crease of ammonium adsorption capacity, although a biofilmhad developed on the zeolite grains. This was somewhat un-expected because the adsorption capacity of, for example,granulated activated carbon, an adsorbent with a high specificsurface, was reduced rather rapidly when a film of nitrifyingbacteria had developed on the grain surfaces (Servais et al.1992). Therefore, the ammonium exchange processes of thesingle reactor may be further investigated in more detail.

CONCLUSIONS

Conclusions from this literature review were that not manyfull-scale wastewater treatment plants employ the ammoniumion exchange technique and only a few applications were de-veloped to recover ammonia nitrogen, for example, for agri-cultural purposes. Liberti et al. (1982) investigated a systemwhere ammonium ions were concentrated by the ammoniumexchange technique, followed by air stripping of ammonia gas,which was subsequently absorbed in sulfuric acid. Anotherinvestigated nitrogen recovery method was the use of a com-bination of anion and cation exchangers to concentrate bothammonium and phosphorus ions and, thereafter, precipitate thenutrients as magnesium ammonium phosphate (Liberti et al.1986). Different kinds of zeolites, such as clinoptilolite andchabazite, have been used as exchangers in cation exchangeexperiments. Zeolites are heterogeneous materials because ofnatural variations of the minerals. Therefore, ammoniumexchange experiments may sometimes result in a higherexchange capacity for clinoptilolite compared to, for example,chabazite. However, similar experiments with the same min-erals, originating from other deposits, may yield contradictoryresults. The loading phase of the ammonium ion exchangeprocess has been investigated to a great extent, and the factorsinfluencing ammonium adsorption on zeolites are quite wellknown. To obtain a high ammonium adsorption, the grain sizeof the zeolites should be <1 mm in diameter and the hydrauliccontact time >5 min. A greater influent ammonium concentra-tion increased the adsorption of ammonium, but a competitionof other cations caused a decrease of adsorbed ammonium.Optimum pH for ammonium adsorption on zeolites was be-tween 6 and 7. Ammonium adsorption was rather temperatureindependent when operating in the interval 107–207C. TotalCEC was reported to be 1–2.27 meq/g (corresponding to 14–32 mg NH4-N/g), but the operating ammonium exchange ca-pacity seemed to be just about 1–7 mg NH4-N/g. When re-generating ammonium saturated zeolites chemically, a sodium


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chloride brine with the concentration 0.1–0.6 M was mostlyused. A high pH (>11) improved the regeneration. The biolog-ical regeneration, which actually is a combination of chemicaland biological regeneration, has primarily been developed todecrease the brine solution at regeneration or to improve theconventional nitrification-denitrification process. Regenerationwith nitrifying sludge, nitrification of regeneration brine, andfinally, ammonium adsorption and regeneration in a single re-actor are different biological regeneration methods that weresuggested. If the ion exchange technique is to be used forammonium recovery, both chemical and biological regenera-tion may be used. The treatment steps that follow the ionexchange determine which regeneration process would bemost suitable.

FURTHER STUDIES

Because there are only a few full-scale applications em-ploying the ammonium ion exchange technique, further de-velopment and evaluation of the technique is needed. Fromthis review the following areas are suggested:

• A further development of biological regeneration to im-prove nitrogen recovery

• Further studies of the biological regeneration in a singlereactor to investigate the impact of the biofilm, attachedon the zeolite, on the ammonium exchange capacity

• Because many of the experiments referred to were per-formed at lab-scale, it would be interesting to investigatethe operation efficiency in a pilot-scale experiment

ACKNOWLEDGMENTS

The Stockholm Water Co. is gratefully acknowledged for supportingthis project. Further, the writer is very thankful to Prof. Jorgen Hanæusat the Division of Sanitary Engineering for his academic support duringthis work. The interesting comments by Dr. Daniel Hellstrom, StockholmWater Co., were also highly appreciated. Finally, the writer would like toexpress her gratitude to Wayne Chan for his help in proofreading theEnglish language of this manuscript.

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