WO1990003947A1

WO1990003947A1 - Process for removing ammonia and phosphorus from a wastewater

Info

Publication number: WO1990003947A1
Application number: PCT/US1989/004426
Authority: WO
Inventors: W. Wesley Berry; Donn R. Friedman; Gordon J. Rossiter
Original assignee: Mww Limited Partnership
Priority date: 1988-10-13
Filing date: 1989-10-13
Publication date: 1990-04-19
Also published as: AU4501889A

Abstract

A process for removing ammonia and phosphorus from a wastewater is disclosed. Wastewater is directed, preferably through fixed points (12), through a plurality of discrete beds (14) which rotate about a circular path in periodic fluid communication with the fixed points. The discrete beds include a first resin capable of adsorbing ammonium ions and a second strong base anion exchange resin capable of adsorbing phosphate anions, the first and second resins being arranged in a stacked relationship. Ammonia is then loaded onto the first resin and phosphate loaded onto the second resin. The wastewater has a pH low enough such that the ammonia is present in the wastewater as ammonium ions. A regenerant is directed, preferably through fixed points, into the discrete beds containing the resins so as to desorb ammonium ions and phosphate anions from the resin. The regenerant advantageously is directed serially and upwardly through the discrete beds.

Description

PROCESS FOR REMOVING AMMONIA AND PHOSPHORUS FROM A WASTEWATER

BACKGROUND OF THE INVENTION;

The present invention relates to ion exchange processes and, more particularly, to a process for removing ammonium and phosphorus nutrients from a wastewater.

It has long been recognized that the presence of nitrogen and phosphorus compounds in water can be quite harmful to the environment. More specifically, when released into the environment, nitrogen or phosphorus compounds can deplete the dissolved oxygen levels in receiving waters, stimulate aquatic growth (algae blooms) and exhibit toxicity towards aquatic life. Additionally, conventional water treatment processes such as chlorine disinfection are delete- riously affected by the presence of nitrogen compounds.

To date, a number of techniques have been developed to remove harmful levels of ammonia and phosphorus from water prior to discharging the water into the environment. Among the more popular tech- niques are breakpoint chlorination, air stripping, biological nitrification-denitrification, chemical precipitation and coagulation, and selective chemical ion exchange.

In breakpoint chlorination, ammonia is removed from a waste stream by the addition of chlorine in an amount effective to oxidize ammonia into nitrogen gas. More specifically, there is added to the wastewater a quantity of chlorine in excess of that required to oxidize organics in water. Once the oxidation of the organics is complete, the chloride remaining reacts with ammonia to form nitrogen gas and hydrochloric acid. A readily apparent drawback to such process is that large excesses of chlorine will be necessary. Additionally, the increased acidity of the wastewater which results must be chemically neutralized with a base such as lime or a caustic soda. Another problem is that excess chlorine present in the wastewater must be chemically neutralized prior to discharging the wastewater into the environment. Such need for supplemental chemicals increases both the cost of the process and the level of dissolved solids in the effluent stream.' Breakpoint chlorination is also ineffective in reducing the phosphorus content of a wastewater.

Air stripping of ammonia gas from a waste- water stream is accomplished by contracting water in the form of small droplets with large volumes of ammonia-free air. Ammonia, which is in molecular form as a dissolved gas in water, is maintained in solution by the partial pressure of the ammonia in the air adjacent to the water. In air stripping, the ammonia partial pressure is reduced, via introduction of air containing little or no ammonia, thereby causing the dissolved ammonia to leave the water phase and enter the surrounding air.

Air stripping suffers from a number of disadvantages also. In the first place, the process becomes highly inefficient during cold weather due to the diminished capacity of the air for the ammonia and due to the damage which freezing causes to stripping towers. In the second place, scale can form in the towers used in air stripping due to the high pH levels required for the maintenance of ammonia in molecular form. Air stripping also often requires a pH adjust¬ ment of large volumes of feed water. Finally, as with breakpoint chlorination, phosphorus levels are not reduced through air stripping. Biological nitrification-denitrification involves fixation, ammonification, assimilation, and nitrification. Generally, the ammonia-containing wastewater is exposed to particular microorganisms which may be either energy-generating or energy- consuming.

Although microbial processes are often quite efficient, they are not always completely reliable. More specifically, microbial reactions are highly temperature dependent. Thus, as temperatures undergo seasonal fluctuations, especially periods of cold, weather, the rate as well as the extent to which the microbial reactions occur is severely diminished. Additionally, the presence of even small concentrations of many organic contaminants as well as certain metals can destroy an entire population of microbes sensitive to such materials. In such instances, due to the large inventory of microbes typically required to regenerate an activated sludge type system, the time periods for regeneration can be quite lengthy. Additionally, during the interim regeneration period, effluent of poor quality may have to be discharged.

Even when operation normally, biological nitrification processes can generate large quantities of sludges which require further processing prior to their disposal. The microbial activity.- can also adversely affect the levels of suspended solids in the effluent. As with the other processes, biological nitrification also does not reduce phosphorus levels. Chemical precipitation and coagulation is generally employed to remove phosphorus from waste- waters in combination with methods which also reduce the content of ammonia in the wastewater. Typically, the phosphate species are precipitated in a downstream operation via the addition of a precipitant such as lime or alum. The major drawback of such method is the necessity of processing the entire wastewater stream and the operation of large solid-liquid separation circuits which produce massive quantities of sludge which must be landfilled or disposed of by other means.

In selective chemical ion exchange, ammonia is removed from a wastewater by passing the wastewater through a bed of clinoptilolite, a natural zeolite mineral that has a high selectivity_a for ammonium ions. Once the clinoptilolite is chemically exhausted, the zeolite bed is regenerated with sodium salt and^' washed to remove excess regenerant. Such process is disadvan¬ tageous with respect to the limited extent to which the regenerant may be recycled, difficulties in the disposal of spent regenerant, inherent chemical costs, and disposal or processing of the concentrated ammonia obtained from the sorbent during regeneration. The process does, however, remove both ammonium and phosphorus type contaminants.

A particular ion exchange process for removing contaminants from a wastewater is disclosed in U.S. Patent No. 4,477,355 issued to Liberti et al. The adsorbents utilized are clinoptilolite and a strong base anion exchange resin. After loading the contami¬ nants onto the resins, the nutrient species are recovered utilizing a sodium chloride regenerant. The nutrient species are then recovered from the regenerant solution via precipitation with magnesium. One problem encountered with the Liberti et al process is that of salt precipitation within the resin bed. More specifically, in addition to nutrient ionic species such as ammonium and phosphates, wastewaters often contain other ionic species which are absorbed onto a resin and which can form salts which precipitate upon exposure of the resin to a regenerant. To overcome this problem, Liberti et al (1) pretreats the wastewater, such as with an alkaline material, to precipitate hardness and heavy metal components therefrom prior to contacting the wastewater with the resins and (2) contacts the resins with a regenerant in parallel using separate regeneration solutions which are combined only after such solutions have passed through the resins.

Quite clearly, the need to carry out the additional pre-treatment step to remove hardness detracts from the desirability of Liberti et al's process since, optimally, one would want to be able to feed the wastewater solution directly into the ion exchange medium. Additionally, in requiring the feeding of multiple regenerant streams in parallel to regenerate the nutrient-loaded resins, increased material costs for the large amounts of regenerant required as well as for the additional equipment required to handle such multiple streams are encoun¬ tered. It is also noted that the Liberti et al process is not continuous but rather, requires stopping of the process in order to effect the necessary operations.

It is apparent from the above that the art has failed to provide a satisfactory method for simultaneously removing both ammonium and phosphate nutrient ions from a wastewater and recovering such nutrient ions as commercially valuable end products. Thus, a number of processes, while effective in the removal of either ammonium ions or phosphate anions from a wastewater, are ineffective in the removal of both ammonium ions and phosphate anions simultaneously. Typically, therefore, removal of the second ionic species is accomplished only by subjecting the wastewater to yet another costly and time consuming process. Additionally, even among processes which are able to simultaneously remove ammonium and phosphate ions from a wastewater, it has proven difficult to run such processes smoothly and efficiently. To the contrary, a number of sacrifices in terms of the number of operations which have to be carried out and the amount of materials which have to be employed are required in order to enable the two separate ion exchange processes to proceed simultaneously.

SUMMARY AND OBJECTS OF THE INVENTION:

In view of the foregoing limitations and shortcomings of prior art methods for removing ammonium and phosphate ions from a wastewater by ion exchange as well as other disadvantages not specifically mentioned above, it should be apparent that there still exists a need in the art for a process for the removal of ammonium and phosphate ions from a wastewater wherein both the ammonium and the phosphate ions can be removed simultaneousT.y without having to otherwise sacrifice process efficiency. It is, therefore, a primary objective of the present invention to fulfill that need by providing a process for simultaneously removing ammonia and phosphate nutrients from a wastewater which is not adversely affected by precipitation of salts in the ion exchange resin during regeneration thereof and thus, can be carried out with a common regenerant stream and without pretreating to remove hardness. It is a further object of the present invention to provide a process for removing ammonium and phosphorus nutrients from a wastewater which can be carried out continuously.

Still another object of the present invention is to provide a process for removing ammonium and phosphorus nutrients from a wastewater employing a common regenerant stream which can be recycled and from which nutrient species are recovered as potentially valuable agronomic products.

In a first aspect, the present invention relates to a process for removing ammonia and phos¬ phorus nutrients from a wastewater including the steps of:

(i) rotating discrete beds including a first resin capable of adsorbing ammonium ions and a second strong base anion exchange resin capable of adsorbing phosphate anions, the first and second resins being arranged in a stacked relationship;

(ii) directing the wastewater through an adsorption feed zone defined by one or more fixed points in periodic fluid communication with said discrete beds and loading ammonium ions onto said first resin and phosphate anions onto said second resin, the wastewater having a pH low enough such that the ammonia is present in the wastewater as ammonium ions; (iϋ) directing a regenerant capable of desorbing said nutrients from the resin, through a regeneration zone defined by one or more fixed feed points in periodic fluid communication with the discrete beds, and desorbing the ammonium ions and phosphate anions from the resin.

Advantageously, the process further includes feeding a common regenerant stream, i.e., a regenerant stream derived from a common source, through one of the fixed points defining the regeneration zone and passing said common stream, via additional fixed points of the regeneration zone, in series one or more additional times through the resin. Preferably, the regenerant stream is flowed upwardly through the discrete beds. In a second aspect, the present invention relates to a process for removing ammonia and phos¬ phorus from a wastewater comprising the steps of: directing the wastewater, including ammonia and phosphorus, through a plurality of discrete beds including a first resin capable of adsorbing ammonium ions and a second strong base anion exchange resin capable of adsorbing phosphate anions, the first and second resins being arranged in a stacked relationship; and loading ammonium ions said first resin and phosphate anions onto said second resin, the wastewater having a pH low enough such that the ammonia is present in the wastewater as ammonium ions; direction a regenerant, capable of desorbing the nutrients from the resin, through said discrete beds to desorb ammonium ions and phosphate anions from the resins, the regenerant being directed serially and upwardly through said discrete beds.

With the foregoing and other objects, advantages, and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of preferred embodiments, the appended claims, and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1 is a perspective view of an Advanced Separation Device suitable for carrying out the process of the invention; Figure 2 is a schematic illustration of the overall process wherein the regenerant is circulated through the stacked sorbent beds in upflow, split stream manner; and Figure 3 is a schematic illustration of the overall process wherein the regenerant is circulated through the stacked sorbent beds in a combination of upflow and countercurrent contacting.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS: The process of the present invention effectively removes ammonia and phosphorus nutrients from a wastewater and enables their subsequent recovery as economically valuable products. Wastewaters which are typically treated in accordance with the present invention can contain 5-50 mg/L of ammonia and 5-30 mg/L of phosphorus. Typical sources of the wastewaters include municipal effluents as well as waste streams generated in various industrial processes, such as fertilizer manufacturing, which must be depleted of ammonia and phosphorus prior to discharge.

In order to remove the ammonia and phosphorus nutrients from the wastewater, they are contacted with a first ion exchange resin capable of adsorbing ammonium ions and a second strong base anion exchange resin capable of adsorbing phosphate anions. The removal of ammonia in the nutrient removal process is based upon two factors namely, the weak base properties exhibited by ammonia in aqueous solution and the selectivity shown by the resin capable of adsorbing ammonium ions. Thus, the equilibrium relationship between ammonia (NH3) and its protonated conjugate acid ammonium (NH₄ ⁺) is represented by the following equation: NH₃ (aq) + H⁺ _{( aq)} ^=__^ NH₄ ⁺ _{( aq)} ( 1)

The above relationship is pH dependent. Thus, at a pH less than about 8, the NH₄ ⁺ species predominates whereas at a pH greater than about 10, the NH₃ species predominates. Between a pH of 8 and 10, a gradual change in the distribution between the two species is observed.

The adsorption of ammonia by a resin capable of adsorbing ammonium ions proceeds as follows when the resin is, for example, in the sodium form:

R-Na + NH₄ ⁺ _(aq) => R-NH₄ + Na⁺ _(aq) (2)

wherein R represents the exchange site on the resin. It is by virtue of the selectivity of the resin for ammonium in preference to the exchangeable cation such as sodium which allows the NH₄ ⁺ to displace the Na⁺ on the exchange site. Quite clearly, there¬ fore, since it is NH₄ ⁺ and not NH₃ which is adsorbed, the pH of the wastewater should be below 8 to ensure predominance of the NH₄ ⁺ species. In this regard, acid must be added to wastewaters having a pH level above 8 or alternatively, the exchange medium can be put into the H⁺ form (instead of Na⁺ form) to force a pH decrease, as the initial NH₄ ⁺ is adsorbed onto the medium. The lower limit on the pH of the wastewater is generally determined by the nature of the ammonium- adsorbing resin employed.

In general, any resin known to be capable of adsorbing ammonium ions may be employed in accordance with the present invention. Especially preferred are the zeolites such as clinoptilolite due to its high selectivity for ammonium ions in preference to other cations normally found in the wastewater such as Ca²⁺, Na⁺, and Mg ⁺. Where clinoptilolite is employed, the lower pH limit is between 3 and 5 since below this range degradation of clinoptilolite can occur.

The removal of the phosphate is based upon an anion exchange with a strong base anion exchange resin and the affinity of the resin for the monovalent and divalent phosphate species (H₂P0₄~ and HP0₄ ²"") . The equilibrium relationship for the dissociation of phosphoric acid to the various phosphate anions is represented by the following chemical equations, and, like the ammonia-ammonium relationship, is pH depen¬ dent:

A characteristic of this relationship is that, at the typically used pH range of the adsorption step, i.e., pH 7-8, the monovalent and divalent forms are approxi¬ mately evenly distributed.

The adsorption of the phosphate anions by the strong-base anion exchange resin probably occurs as a combination of the following chemical equations, where the exchangeable anion, in this case Cl~, is replaced on the resin by the phosphate species:

R-Cl + H₂P0₄~ (aq) R-H₂P0₄ + Cl" (aq) (6)

2R-C1 + HP0₄ ²" (aq) ■> R₂-HP0₄ + 2C1" (aq) (7)

wherein R represents an exchange site on the anion exchange resin. It should be noted that the other anionic species present in various wastewaters, e.g., sulfate (S0 ²~) , carbonate (C0₃ ²~) and bicarbonate (HC0₃~) , probably participate in other, simultaneously occurring, anion exchange reactions similar in nature to the above equations (6) and (7) .

In general, any of the strong base anion exchange resins known to adsorb monovalent and divalent phosphates may be employed in accordance with the present invention. Included among such resins are Dowex 21k, TG550A, Duolite A101D and Purolite A400. A preferred resin is Dowex 2IK.

After the ammonium and phosphate ions have been removed from the wastewater using the first resin capable of adsorbing ammonium ions and the second strong base anion exchange resin capable of adsorbing phosphate anions, the first and second resins are regenerated by desorbing the ionic species therefrom.

The ammonium is desorbed from the first resin either at a neutral pH or at an alkaline pH. The use of a neutral pH regenerant stream to remove the ammonium from the first resin such as clinoptilolite relies upon a mass action mechanism in which a relatively High concentration of an exchangeable ion, such as Na⁺, is used to displace the ammonium ions from the exchange sites on the resin. This displacement is represented by the following chemical equation:

R-NH₄ + Na⁺ (aq)- R-Na + NH ⁺ (aq) (8)

The use of an elevated pH regenerant enhances the desorption of the ammonium ion from the resin by taking advantage of the equilibrium relationship between NH₄ ⁺ and NH₃ and the fact that, at pH levels above ten, the NH₃ species predominates. The elevated pH desorption of NH₄ ⁺ occurs essentially as a two-step sequence illustrated by the following chemical equations:

R-NH₄ + Na⁺ (aq) ■> R-Na + NH₄ ⁺ (aq) (8) NH₄ ⁺ (aq) + OH~ =» NH₃ (aq) + H₂0 (9)

In the first step, the mass action mechanism utilized in the neutral pH regeneration scheme is again employed to displace the NH ⁺ from the exchange site. In the second step, the NH₄ ⁺ is converted to NH₃ via reaction with hydroxyl ions. The overall desorption step at an elevated pH, therefore, is a combination of phase equilibrium accompanied by a chemical reaction. The presence of hydroxyl ions drives the neutralization reaction, equation (9) , to the right, which disturbs the phase equilibrium by depleting the aqueous phase ammonium ions. This depletion of NH₄ ⁺ from the aqueous phase drives the desorption reaction, equation (8) , also to the right, resulting in further desorption of NH₄ ⁺ from the sorbent. The desorption of the phosphate species from the anion exchange resin is relatively unaffected by pH variations and is essentially the reverse of the absorption reactions. The exchangeable anion, such as Cl~, present in relatively high concentration, replaces the phosphate species on the resin according to the following chemical equations:

R-H₂P0₄ + Cl" (aq) ^• R-Cl + H₂P0₄" (aq) (10)

R₂-HP0₄ + 2C1^" (aq) 2R-C1 + HP0₄ ²" (aq) (11)

If an elevated pH regeneration scheme is employed to enhance the desorption of ammonium from the ammonium-adsorbing resin, such resin should then be the first sorbent contacted with the regenerant. If the anion exchange resin is contacted first, the hydroxyl content of the solution could be reduced and the pH lowered, due to ion exchange reactions such as the following taking place:

R-H₂P0₄ + OH" (aq) -pR-OH + H₂P0₄" (aq) (12)

Contacting the ammonium-adsorbing resin first, therefore, serves to maximize the NH4⁺ desorption enhancement provided by the hydroxyl ions.

As indicated above, the regenerant may be either a neutral or an alkaline salt solution having a high enough concentration of salt to effect desorption of the nutrients. The concentration of salt in the regenerant is selected based on the desorption mechanism to be employed in the particular application. The desorption by mass action only, using a neutral regenerant, requires a higher salt concentration than desorption employing a combination of mass action and phase equilibrium, conducted with a regenerant at an elevated pH. Typically, the salt should be present in a concentration between 0.5 and 2.ON and preferably 0.75 to 1.0N. When an alkaline regenerant is employed, the pH is preferably greater than 10. Regenerant salts are selected which desorb the nutrient species from the resins. In this regard sodium chloride is suitable. A preferred regenerant is a sodium chloride/sodium hydroxide solution containing about 0.75N - 1.5N of sodium chloride with sodium hydroxide added as a pH modifier.

It will be appreciated that a strong anion exchange resin such as Dowex 2IK will adsorb other anionic species present in the wastewater in addition to phosphate, e.g., sulfate and carbonate. Accord¬ ingly, upon exposure of the resin to the regenerant solution, both carbonate and sulfate anions can accumulate in the regenerant along with the phosphate anions. Indeed, testwork utilizing a regenerant stream with a salt (NaCl) concentration of 0.75 to 1.5N, of which the cation content was predominantly sodium, contained a corresponding anionic content comprised primarily of chloride, carbonate-bicarbonate and sulfate. These species were present in the proportions of 50-60% chloride, 20-25% carbonate-bicarbonate and 20-25% sulfate.

Similarly, but to a lesser extent, probably due to the selectivity of the clinoptilolite for ammonium, cationic species other than ammonium, e.g. magnesium and calcium, may also enter the regenerant. The relatively high concentration of sulfate, carbonate- bicarbonate, as well as phosphate in the recirculating stream, is conducive to the formation of hardness precipitates. This environment for precipitation exists as a result of the relatively low solubility products (K_Sp*s) for the magnesium and calcium salts of phosphate, sulfate, and carbonate. Consequently, small concentrations of these metals entering the regenerant solution cause the K_sp's of insoluble alkaline-earth salts to be exceeded, resulting in the formation of compounds such as CaC0₃, MgC0 , Ca₃(P0₄)₂, CaS0₄, etc. The actual location of the phase change resulting from the K^sP's being exceeded is a function of the precipitation reaction kinetics. In most precipitation and crystallization operations, one of two mechanisms are usually rate determining, namely, nucleation rate or particle growth rate. In the case of rapid nucleation rates, precipita¬ tion will occur immediately and solid particles will accumulate and begin to grow in the sorbent beds. These accumulating particles must be removed to avoid a loss of hydraulic capacity in the sorbent beds which could occur if the particles were allowed to remain in- situ for extended periods of time. As is described later, this removal of solid particles is achieved according to the present invention by operating at least one stage of the regeneration zone in the upflow mode. This upflow operation allows any particles to be washed from the sorbent beds and ultimately recovered in the solid-liquid separation circuit.

In the case of slow nucleation kinetics, post- precipitation in the downstream solid-liquid separation circuit is favored. This case is more amenable for controlling the size of the particles in order to optimize the settleability and filterability of the solid products. After the nutrients, as well as any other anionic or cationic species have been desorbed from the resin, the regenerant, including the desorbed nutrient species can be treated and ultimately recycled to the resin beds. The nutrient-rich regenerant solution discharged from the resin beds, which may contain various suspended solids including the products of spontaneous precipitations, may be treated with a specific precipitant, e.g. Ca²⁺ or Mg²⁺, to reduce the ammonia and/or phosphorus concentrations to lower levels and recover the nutrient values. A solid-liquid separation step can then be employed to remove all solids associated with the regenerant prior to recycling the stream to the beds. Developmental testwork has indicated that by carefully controlling the dosage of the precipitant, ammonia and phosphorus may be selectively and incremen¬ tally removed from the solution as precipitated calcium or magnesium ammonium phosphate compounds. Alterna¬ tively, in the case of complete ammonia removal via air stripping,, the phosphorus remaining in the solution may be removed as a calcium or magnesium phosphate compound. Where the strippers have a low solids tolerance, however, it might be necessary to remove solids from the stream prior to its introduction into the stripper. The. following reactions illustrate this concept:

_Ca²⁺ ₍aq) + NH^ aq) + P0₄ ³~(aq) >CaNH₄P0₄(s) (13)

Mg²⁺(aq) + NH₄ ⁺(aq) + P0₄ ³"(aq) >MgNH₄P0₄(s) (14)

₃Ca²⁺ ₍aq) + 2P0₄ ³""(aq) ^* Ca₃(P0₄)₂(s) (15)

3Mg²⁺(aq) + 2P0₄ ³"(aq) >Mg₃(P0₄)₂(s) (16)

_Ca²⁺(aq) + HP0₄ ²"(aq) >CaHP0₄(s) (17)

Mg²⁺(aq) + HP0₄ ²"(aq) >MgHP0₄(s) (18)

Addition of a slight stoichiometric deficiency of Ca²⁺ and/or Mg²⁺, based on equations (13) and (14) , for example, essentially ^■■starves" the system for the precipitant and allows the incremental decrease of the ammonia and phosphorus content to be controlled. This incremental decrease in nutrient concentration is desirable to 1) prevent the introduction of excessive calcium and magnesium values into the sorbent beds via the recycling of the regenerant, which could cause obvious operating problems, and 2) to minimize the precipitation of other insoluble salts, such as carbonate and sulfate, which would tend to decrease the potential commercial value of the product. Quite clearly, any compounds capable of precipi¬ tating the nutrient ions from the regenerant may be employed in accordance with the present invention. Preferred precipitants include CaCl₂ and MgCl₂. In a preferred embodiment, the regenerant first enters an air stripper wherein the ammonia is partially, or even completely removed. The removed ammonia may then be absorbed by sulfuric acid to produce ammonium sulfate. The phosphorus and any remaining ammonia is then precipitated from the solution as, for example, a calcium or magnesium ammonium phosphate compound. Where ammonia was completely removed in the air stripper, the phosphate nutrients can be precipitated as a calcium or magnesium phosphate compound. In this regard, the extent of ammonia removal in the air stripper is dependent upon the characteristics of the particular installation and the desired stoichiometry of the nutrient solid products.

As discussed above, the concentration of the precipitant can be selected such that there is a slight stoichiometric deficiency of, for example, Ca²⁺ and/or Mg²⁺, thereby enabling a controlled incremental decrease of the ammonium and phosphate content. In general, a salt concentration corresponding to about 75 to 90 percent of the mass of the nutrients to be precipitated is employed in the precipitant.

After the resin has been stripped of the ammonium and phosphate ions with the regenerant, it is generally desirable to pretreat the resin prior to reloading it with additional ionic species from a wastewater. The pretreatment can be carried out by exposing the resin to a slightly acidic water wash, e.g., having a pH between 4 and 6 using acids such as sulfuric or hydrochloric. The acidic water wash cleanses the resins of entrained regenerant and lowers the pH of liquid entrained, for example, in the pores of the clinoptilolite to a pH conducive to ammonium adsorp¬ tion, i.e., a pH between 7 and 8. The wash water discharged from the resin is advantageously used as make-up water for the regeneration circuit. If necessary, additional acid may be added to the acid water as the water travels between multiple beds.

The process is now described in conjunction with the Advanced Separation Device (ASD) which enables such process to be carried out continuously and with a high degree of control over the various processing streams. The ASD is described in detail in U.S. Patent No. 4,522,726. For convenience, a brief description of the ASD, illustrated at Fig. 1, is provided.

The ASD comprises a plurality of fixed ports 12, to each of which may be supplied the various feed materials. In the case of the present invention, those materials include the wastewater feed, the regenerant, and the acid water wash.

Moving about a circular path in periodic fluid communication with each of the above-described fixed feed ports are a plurality of chambers 14 filled with a sorbent which interacts with the various feed fluids. In the process of the present invention, the sorbent includes a first resin capable of adsorbing ammonium ions and a second strong base anion exchange resin capable of adsorbing phosphate anions, the first and second resins being arranged in a stacked relationship. It will be appreciated that the various feed materials are supplied continuously to their respective feed ports 12 for periodic interaction with the sorbent in each of the chambers 14. In similar fashion, a plurality of fixed discharge ports 16 are provided at an end of the chambers opposite to that of the fixed feed ports 12. Each feed port 12 has a corresponding discharge port 16. Although the feed ports 12 are illustrated in Fig. 1 as being on top, it is noted that each of the feed ports could as easily be provided at the bottom of the device and each of the corresponding discharge ports at the top of the device.

In Fig. 2, there is illustrated the basic flow concept wherein the regenerant is circulated through the stacked sorbent beds in an upflow, split stream manner. The ASD device is divided into three separate zones, namely, an adsorption zone, a regeneration zone, and a sorbent washing and conditioning zone. Each zone is defined by a number of fixed feed ports and corresponding fixed discharge ports through which a treating fluid is continuously supplied.

The rotating chambers of the ASD are filled with an anion exchange resin and with clinoptilolite, in a stacked relationship. The particular order in which the sorbents are placed in the chambers depends upon the specific process application. For example, in an application using an elevated pH regenerant passed through the unit in upflow, the anion exchange resin would be placed on the top. In the application where a neutral pH regenerant solution is used, the anion exchange resin may be placed on the bottom without adverse consequences. The depth of the sorbent beds is a function of the characteristics of the wastewater to be heated and the level of nutrients present in the water. As is apparent from Fig. 2, the chambers rotate clockwise when viewed from the top.

The wastewater stream (I) , including ammonium and phosphate, is fed to one of the fixed ports defining the adsorption zone of the ASD. If necessary, the pH of the wastewater feed is adjusted via the pH adjust¬ ment stream (II) . The wastewater is passed through the sorbent system for the number of times required to lower the nutrient concentration of the wastewater to the desired levels. In the embodiment illustrated in Fig. 2, the wastewater passes through the ASD three times in countercurrent fashion, i.e., in a direction opposition to that in which the chambers rotate. An essentially nutrient-free wastewater stream (III) is collected.

The rotating sorbent chambers exit the adsorption zone and enter the regeneration zone, defined by the fixed feed ports through which the regenerant is fed. A nutrient or alkaline salt solution regenerant stream (IV) is circulated through the rotating chambers in an upflow, split-stream manner. Alternatively, as illustrated in Fig. 3, the regenerant stream (IV) may be circulated through the rotating chambers using a combination of upflow and countercurrent contacting. The exact nature of the arrangement is dependent upon the specific application and facility characteristics. The regenerant solution removes the nutrients from the sorbents and, via the upflow contacting, any solids accumulated in the sorbent beds. The accumulated solids could originate in the wastewater, as suspended materials. Alternatively, as described previously, the solids could be precipitates formed in the regenerant solution as a result of hardness cations being present in a medium containing phosphates, carbonates and sulfates.

The spent regenerant solution stream (V) is then routed to a regenerant treatment circuit where the nutrients are recovered and the stream recycled. The regenerant first enters an air stripper where the ammonia is partially or completely removed by a stream (VI) of air and the stream (VII) of ammonia and air subsequently absorbed by an incoming stream (VIII) of sulfuric acid to produce ammonium sulfate (IX) and a stream of air (X) .

The phosphorus and any remaining ammonia, stream (XI) , is then combined with a stream (XII) of precipi¬ tant salts and the ammonia and phosphorus precipitates from the solution as, for example, a₌ calcium or magnesium ammonium phosphate compound or, in the case of complete ammonia removal in the air stripper, as a calcium or magnesium phosphate compound stream (XIII) . The extent of ammonia removal in the air stripper is dependent upon the characteristics of the particular installation and the desired stoichiometry of the nutrient solid products. Following a solid/liquid separation step, the regenerant solution (XIV) is recycled to the regeneration zone. Prior to entering the zone, provision is made to add the necessary make¬ up chemicals (XV) to the solution.

The regenerant solution used to recover the ammonia and phosphorus from the sorbents is passed through the ASD as a recirculating stream with a much lower net flow rate through the system than the influent wastewater. This low net flow rate results from the recirculation of the regenerant which allows the nutrient concentrations to build up to signifi¬ cantly higher levels than in the wastewater. The net throughput flow which is removed from the system, therefore, is the flow of product slurry taken from the solid-liquid separation circuit.

By virtue of the above-described regeneration scheme carried out in the ASD, the problem of precipi- tation of salts in the resin bed is avoided. More specifically, the upflow operation of the regeneration zones allows the sorbent beds to be flushed and backwashed during each cycle of operation, whereby particles, which otherwise could cause a loss of hydraulic capacity in the system, are washed from the sorbent beds and ultimately recovered in the solid- liquid separation circuit described above. This further makes it possible to use a common regenerant stream (IV) . More specifically, since precipitation in the sorbent beds does not disrupt the present system, it is not necessary to provide more dilute regenerant streams, using larger amounts of regenerant. The sorbent chambers, after having been regen- erated, enter the acid wash zone of the ASD. The acid wash water stream is fed into one of the fixed feed ports of the ASD and circulated countercurrently through the ASD a plurality of times. If necessary, the recirculating acid wash water can be pH adjusted prior to re-entering the ASD via the acid stream

(XVII) . The stream (XVIII) of wash water discharged from the ASD can be used to provide make-up water to the regeneration circuit.

An exemplary material balance of the above- described system is as follows:

IV Add XIV and XV

V 48 L/min

VII 1.34 g/min NH₃

VIII 4.2 g/min H₂S0₄ (9₃% basis)

IX 5.2 g/min (NH₄)₂S0₄

XII 10.7 g/min CaCl

XIII 14.7 g/min CaNH₄P0₄ 2.97 g/min P 1.6₃ g/min NH₃ ₃.86 g/min Ca

XIV 48 L/min XV 1.2 g/min NaCl 7.7 g/min NaOH

XVI 3.1 L/min XVIII 3.1 L/min

Thus, a truly continuous process for removing ammonia and phosphorus from a wastewater is provided using multiple sorbents arranged in a stacked bed configuration. Regeneration is achieved simultaneously to adsorption using a common regenerant stream and ammonia and phosphorus are recovered from the regen¬ erant as potentially valuable agronomic products and the regenerant stream recycled. Although only preferred embodiments are specifi¬ cally illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A process for removing ammonia and phosphorus nutrients from a wastewater comprising the steps of:

(ii) directing the wastewater through an adsorption feed zone defined by one or more fixed points in periodic fluid communication with said discrete beds and loading ammonium ions onto said first resin and phosphate anions onto said second resin, the wastewater having a pH low enough such that the ammonia is present in the wastewater as ammonium ions;

(iii) directing a regenerant, capable of desorbing said nutrients from the resin, through a regeneration zone defined by one or more fixed feed points in periodic fluid communication with said discrete beds, and desorbing the ammonium ions and phosphate anions from the resin.

2. The process of Claim 1 further including feeding a single wastewater stream through one of the fixed points defining the adsorption zone and passing said single stream, via additional fixed points of the adsorption zone, in series one or more additional times through the resin.

3. The process of Claim 2 wherein the wastewater stream is fed through the additional fixed points in a direction countercurrent to the rotation of the discrete beds.

4. The process of Claim 1 further including feeding a single regenerant stream through one of the fixed points defining the regeneration zone and passing said single stream, via additional fixed points of the regeneration zone, in series one or more times through the resin.

5. The process of Claim 4 wherein said regenerant stream is flowed upwardly through said discrete beds.

6. The process of Claim 1 wherein said regenerant is an alkaline salt solution and wherein said regenerant contacts the first resin capable of adsorbing ammonium ions before contacting the second strong base anion exchange resin.

7. The process of Claim 1 further including the step of removing said nutrients from the regen¬ erant.

8. The process of Claim 7 wherein the ammonium is removed by gas stripping.

9. The process of Claim 8 wherein phosphate anions and remaining ammonium are removed by precipi¬ tation with a salt.

10. The process of Claim 7 further including the step of recycling the regenerant from which said nutrients were removed.

11. The process of Claim 10 wherein the recycled regenerant is fed, as a single stream in series, through one or more of the fixed points defining the regeneration zone.

12. The process of Claim 11 wherein said recycled regenerant stream is flowed upwardly through said discrete beds.

13. The process of Claim 1 further including the step (iii) of directing an acidic water wash solution through a wash zone defined by one or more fixed points in periodic fluid communication with said discrete beds.

14. A process for removing ammonia and phosphorus from a wastewater comprising the steps of: directing the wastewater including ammonia and phosphorus through a plurality of discrete beds including a first resin capable of adsorbing ammonium ions and a second strong base anion exchange resin capable of adsorbing phosphate anions, the first and second resins being arranged in a stacked relationship; and loading ammonium ions onto said first resin and phosphate anions onto said second resin, the wastewater having a pH low enough such that the ammonia is present in the wastewater as ammonium ions; directing a regenerant, capable of desorbing the nutrients from the resin, through said discrete beds to desorb ammonium ions and phosphate anions from the resins, the regenerant being directed serially and upwardly through said discrete beds.