Ammonia removal and recovery from urea fertilizer plant waste

It was found that one litre of commercially available raw grade phosphoric acid was sufficient to absorb the ammonia stripped out from more than 275 litres of wastewater having an influe

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Ammonia removal and recovery from urea fertilizer plant waste

Vijay K Minocha a & A.V.S Prabhakar Rao b

a

Centre of Environmental Engineering, Department of Civil Engineering, University of Roorkee, Roorkee, 247667, India

b

Civil Engineering Department, Indian Institute of Technology, Kanpur, 208016, India Published online: 17 Dec 2008

To cite this article: Vijay K Minocha & A.V.S Prabhakar Rao (1988): Ammonia removal and recovery from urea fertilizer plant

waste, Environmental Technology Letters, 9:7, 655-664

To link to this article: http://dx.doi.org/10.1080/09593338809384616

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Environmental Technology Letters, Vol 9, pp 655-664

© Publications Division Selper Ltd., 1988

AMMONIA REMOVAL AND RECOVERY FROM UREA

FERTILIZER PLANT WASTE

Vijay K Minocha * and A.V.S Prabhakar Rao

* Centre of Environmental Engineering, Department of Civil Engineering, University of Roorkee, Roorkee - 247667, India

and Civil Engineering Department, Indian Institute of Technology, Kanpur - 208016, India

(Received 10 July 1987; in final form 8 March 1988)

ABSTRACT Air stripping is the most simple method of ammonia removal from fertilizer wastewaters Herein, studies were carried out for the removal and recovery of ammonia With experimental data, a nomogram was generated relating the three contrblable variables, namely waste-water flow rate, air flow rate and percentage ammonia removal It was found that one litre of commercially available raw grade phosphoric acid was sufficient to absorb the ammonia stripped out from more than

275 litres of wastewater having an influent ammonia concentration

2000 mg 1-1

INTRODUCTION The liquid wastes coming out of a urea fertilizer industry generally contain nitrogen in the form of ammonia and urea, approximately 800 mg I"1 and 1800 mg 1" respectively Presence of urea by itself can be tolerated by fish at much higher levels, but even a small quantity of ammonia, either directly or through the hydrolysis of urea, is toxic to aquatic life (1, 2 )

Various compounds containing the element nitrogen are becoming increasingly important in wastewater management programmes because

of the many effects that nitrogenous materials can have on the environment Nitrogen, in its various reduced forms, depletes dissolved oxygen levels in receiving waters, stimulates algal growth, exhibits toxicity towards fish life, affects chlorine disinfection efficiency and presents a public health hazard

Ammonia concentrations of 280-490 mg I"1 in, air have been reported

to cause eye, nose and throat irritation (3) Concentration of 1700-4500 mg m~3 must be reached before human or animal toxicities begin to occur (2)

Biological and chemical processes which function in wastewater treatment and occur in the natural environment can change the chemical form of nitrogen Such change may eliminate the deleterious effects

of nitrogen while producing, or leaving unchanged, another effect For example, by converting ammonia in raw wastewater'to nitrate, the

oxygen depleting and toxic effects of ammonia are eliminated, but the biostimulatory effects may not be significantly changed (2)

Nitrogen control techniques are divided into two broad categories namely physico-chemical arid biological The first group involves selective ion-exchange, ammonia stripping, electrodialysis, reverse osmosis, electrochemical precipitation and break point chlorination The latter group involves nitrification, denitrification, intensive algal culturing, active flocculating algal-bacterial system and simple lagdoning

Nitrification processes are involved with the conversion of organic and ammonia nitrogen to nitrate nitrogen, a less objection-able form for aquatic life The other processes are involved not merely with conversion of one form of nitrogen into another, but result in the partial or complete removal of nitrogen from the wastewater

In determining the method which is most suitable for a particular application, consideration must be given to six principal aspects(3) namely form and concentration of nitrogen compounds in the process influent, required effluent quality, other treatment processes to be employed, cost, reliability and flexibility Great care must be taken in developing and evaluating alternatives

Urea Hydrolysis

In urea plant wastewaters, along with ammoniacal • nitrogen, substantial quantities of the highly soluble urea nitrogen is also present which is a stable compound and requires considerable energy

to hydrolyse chemically However, biological systems achieve this without any seeming effort There are about 200 species of bacteria that can hydrolyse urea into ammonia and carbon dioxide Micrococcus ureae Proteus vulqaris and Bacillus pasteurii are among the major types of organisms which actively hydrolyse urea(4) A microbial reactor consisting of Bacillus pasteurii grown on coal was proposed for the hydrolysis of urea rich fertilizer waste effluents(5) An active composite obtained from soil, chicken manure and a source

of organic carbon in the form of sweet potato flour has been used for continuous conversion of urea to ammonia(6)

An actively immobilized microbial reactor with raschig rings as support medium was developed in our laboratory using the enriched culture prepared from chicken manure(7) for the hydrolysis of urea

It was observed that a source of organic carbon was essential for the maintenance of microbial activity With simulated urea plant wastewater, even up to an influent concentration of 6600 mg I"1 urea,

no inhibition was observed Studies also indicated that presence

of ammonia in the influent up to 1000 mg 1" did not inhibit urea hydrolysis(7)

Experiments were also conducted(8) with urea plant effluent, hydrolysis reduced from 85 to 65% when hydraulic load was increased from 1 57 m3 m"2 h'1 to 6.32 m3 m2 fi A Bioreactor was used continuously for a period of three months without any significant loss in activity indicating that no other nutrients apart from a source of organic carbon are required for the maintenance of active attached mixed culture microbes

Ammonia Stripping Ammonia in the molecular form is a gas which dissolves in water

to an extent controlled by the partial pressure of the ammonia in the air adjacent to the water Reducing the partial pressure causes ammonia to leave the water phase and enter the air Ammonia' removal from wastewater can be effected by bringing small drops of water

in to contact with a large amount of ammonia free air This physical' process is termed desorption, but the common name is "ammonia stripping" In a given time, the quantity of ammonia desorbed from

a solution depends on (a) concentration of unprotonated ammonia, (b) gas-liquid surface area, (c) mass transfer coefficient, and (d) partial pressure exerted by ammonia in the gas phase These.,

in turn, are influenced by environmental and process conditions(9) The effect of pH on the ammonia concentration can be readily seen(IO) The equilibrium product of ammonia-ammonium ion varies with temperature( 10) At pH = 7, only ammonium ions in true solution are present, while at pH = 12, only dissolved ammonia gas is present, and this gas can be liberated from wastewater under proper conditions The combined effect of temperature and pH on the liquid phase ammonia concentration and effect of temperature on the Henry's law constant for ammonia is shown diagrammatically (10)

Two major factors affect the rate of transfer of ammonia gas from water to the atmosphere(11) , (a) surface tension at the air-water interface, and (b) difference in concentration of ammonia in the water and air Surface tension is at a minimum in water droplets when the surface film is being formed, and ammonia release is greatest

at this instant Little additional gas transfer takes place once

a water droplet is completely formed Therefore, repeated droplet formation and coalescing of the water assists ammonia stripping The ammonia stripping process, then, consists of (a) raising the

pH of the water to values in the range of 10.8-11.5, (b) formation and reformation of water droplets in a stripping unit, and (c) providing air-water contact and droplet agitation by circulation

of large quantities of air through the unit Unfortunately ammonia

is highly soluble in water, and its volatility decreases markedly with decreasing temperatures For these reasons (12), effective ai.r stripping requires warm temperatures and the use of large volumes

of air per unit volume of wastewater

Ammonia being transferred from the liquid to the gas phase must pass through the interface It is difficult to measure accurately either the length of the transfer path or the time of

contact-It may be assumed(9) that both phases are separated and that transfer resistance layers are formed on either side of the boundary It

is in these layers, or films, that the greatest amount of resistance for mass transfer is encountered During the removal of ammonia from an aqueous solution by air-stripping, the greatest resistance

to mass transfer occurs in the transfer from the liquid to the gas phase because of the high solubility and ionization of ammonia in water

It is reported (9) that the value of desorption rate constant (K,) obtained in both laboratory batch and continuous flow units are comparable with those obtained in the large scale studies So the data obtained from small scale units can be • used to estimate the

K, values in large scale batch and continuous flow systems Ample work has been documented on air-stripping for ammonia removal from sewage effluents However, comparatively less work has been done

on air stripping of high nitrogenous wastes Reeves(13) quoted Kuhn

to have found out that the optimum pH for stripping was 11.0 The loading that yielded the best results was 1.46 to 1.54 m3 min1 of

air and 0.63 1 s"1 of wastewater effluent Also, he stated that

3 1 X

air and 0.63 1 s of wastewater effluent Also, he stated that the best air/liquid ratio was 205 m3 min'1 X s Rao et al (14)

observed that air-stripping is effective for the removal of ammonia from nitrogenous fertilizer.wastes In another study, when the waste-water was passed through a closely packed aeration tower with 3.6 m

of air supplied per litre, ammonia nitrogen removal was very effective

at any pH above 9.0 The removal fell to 91- percent at pH 8.9 and

to 58 percent at pH 8.8 (11)

The presence of urea in wastewater, which is usually present

in urea fertilizer wastewater does not affect the release of ammonia (15) The treatment with lime to raise the pH is not as effective

as the treatment with sodium hydroxide (16)

Ammonia Recovery

Since ammonia is highly soluble in water, and very reactive with acids, these can be used to trap ammonia which is coming out from the stripper Recovered ammonia will be a very useful product because of the economic importance of nitrogen compounds Different methods of ammonia stripping are reported in the literature, few

of which describe ammonia removal and recovery systems._ Ammonia recovery from wastewaters in packed columns was studied (17) It was found that the concentration-of ammonium salt in phosphoric acid solution at a pH of about 6.5 was increased continuously upto 25 percent salt by weight Trulsson(17) quoted Dankwerts and Onda to have found that an increase ir ionic strength of the solution will decrease the solubility, so absorption is favoured by low pH and high ionic strengths

MATERIALS AND METHODS

Removal Studies

Experiments for removal of ammonia were conducted in a column packed with raschig rings • The influent was either a solution of ammonium chloride in tap water or effluent from the fixed film micro-bial reactor used for hydrolysing urea factory wastewater Performance

of the fixed film microbial reactor is described under "Urea hydrolysis1 in the introduction A concentration of 2000 mg 1" of ammoniacal nitrogen was used throughout the experiments Sodium hydroxide was used to raise the pH of wastewater to 11.0 The effective height of the packed column was 140 cm The wastewater was uniformly distributed over the randomly packed bed, exposing

a large surface for continuous contact of liquid and gas in counter-current flow With this arrangement it was possible to examine the effect of rate of aeration, pH, influent concentration of ammonia and wastewater flow rate on the stripping of ammonia from the waste-water These experiments were conducted at room temperature which varied between 24°C and 28°C The characteristics of the packing material used in the packed column were as follows

(i) Surface area of packing material

per litre of sample volume = (ii) Porosity

(iii) Number of raschig rings

per litre of sample volume

3000 cm* (approximately) 0.67

300 (approximately)

(iv) Weight of raschig rings per litre of sample volume = 670 g

Recovery Studies

A reactor vessel of 6 litre capacity containing 4 litres of

a very strong ammonium chloride solution- (40 g I"1 ) was placed in

a constant temperature bath The pH was adjusted to 11.0 just before starting the run The air containing ammonia was absorbed in two columns (2.5 cm internal diameter) connected in series containing

400 ml of phosphoric acid each The strength of phosphoric acid taken was 30 percent of P, Og , which is the strength of commercially available raw phosphoric acid (18) The air flow rate was kept cons-tant at 6 1 miri1

RESULTS AND DISCUSSIONS

Ammonia removal by air stripping is not new Earlier experiments have established various conditions regarding air stripping Further work in removing ammonia from the air by " catching it in sulphuric acid solution indicated the feasibility of preventing air pollution during the process However, recovering ammonia from the ammonium sulphate solution is still a practical problem due to the high solubi-lities of the ammonium salts of sulphuric acid The experimental results discussed hereafter include some studies on the possibility

of not only ammonia removal but also recovery

Ammonia Removal Simulated as well as microbially hydrolysed urea plant waste-water was used as the influent, and was allowed to trickle down

in the column The two basic variables, wastewater flow rate and air flow rate were varied experimentally, and the percentage removals

of ammonia were obtained These three variables were interplayed to arrive at optimum conditions for the stripping process Fig.(1)•shows percentage ammonia removal with varying wastewater flow rate at different specific air flow rates From the figure it can be observed that the maximum attainable extrapolated removals of ammonia were

81, 91 and 99% with simulated wastewater and 60, 80 and 91% with hydrolysed urea plant wastewater at 26±2 C with air flow rates of

180, 300 and 420 m? m2 h"1 respectively Maximum attainable percentage removal is less with lower air flow rates because of the fact that the ammonia content of the air is relatively high in this case and

so the driving force for desorption of ammonia is reduced

Fig.(2) was prepared to illustrate the patterns of percentage ammonia removal that could result from different combinations of simulated wastewater flow rate and air flow rate Thus many options can be made to obtain a specific ammonia removal, each requiring different desorption times to accomplish the desired results It was also possible to generate a nomogram (19) based on the experimental data Fig.(3) shows the nomogram relating the three variables, simu-lated wastewater flow rate, air flow rate and percentage ammonia removal This nomogram is valid only for prevailing experimental conditions as described in the figure For microbially hydrolysed wastewater, the same nomogram can be used indirectly For that matter the following nonlinear relationship was developed to fit the experi-mental data

Microbially hydrolysed _ Simulated waste- _ r 50 ,0.95 wastewater flow rate water flow rate Air flow rate

Where all flow rates are in m m h

Since dimensionless parameter can be obtained by taking the ratio of simulated wastewater flow rate to air flow rate, all the experimental data were made dimensionless and were plotted against percentage ammonia removal Three different straight lines, inter-secting at a point, were obtained, depicting the relationship between the dimensionless parameter and percentage ammonia removal at various specific air flow rates, as shown in Fig (4) At the pivoted point, the value of the dimensionless parameter was 10"3 , and the removal

of ammonia was 68% which remained unchanged with variable air flow rate in the range of 180 m3 m"2 h*1 to 420 m3 m"2 h"1 for the prevailing experimental conditions From the figure it can be observed that for maximum utilization of supplied air, if the required ammonia removals are less than 6 8 % , air flow rate should be 180 irr3 mz h"1

or less On the other hand, if the required ammonia removals are more than 68% air flow rate should be higher

Ammonia Recovery

Since ammonium phosphates are less soluble, in comparison to ammonium sulphates, phosphoric acid was tried as a recovery medium

as it will facilitate comparatively easy crystallization The solubi-lities of relevant compounds in g per TOO ml with temperature are

as follows (20)

Ammoinum salt

of acid

Solubility

Solubility

(0°C) (20°C)

NH4H

22

36

2P O4

7 8

( N H4)2

42 68

HPO4

.9 6

NH4HSO4

100.0

(NH4

70 75

)2so4

.6 4 For practical recovery, the strength of phosphoric acid used was equal to the commercially available raw grade phosphoric acid Since large amounts of ammonia will be required to neutralize this acid, higher initial concentration of ammonia (40 g 1" ) in influent were used for this experimental study It was found that initially all the ammonia was absorbed in the first of the two recovery columns kept in series containing 400 ml of phosphoric acid each When the concentration of ammonia in the first column reached the value 200

g I'1 , pH of the acid solution was about 6, and formation of crystals was noticed in the solution Since the concentration of ammonia in

the reactor vessel by now had come down to about 20 g 1 , and the

quantity of ammonia being released had also decreased, 40 g of ammonia (126 g NH^ Cl> was added to the reactor vessel after six hours of experiment The released ammonia • from the reactor thereafter seems

to have distributed in both the column containing acid solutions

as shown in Fig (5) When the concentration of ammonia reached

230 g 1~1 in the first column, the concentration of ammonia in the second column was 50 g T1 , which would have otherwise gone to the atmosphere in the absence of the second column Though the first column was not fully saturated with ammonia it was not possible

to take a sample of acid solution from this column because of clogging

of sample ports with crystals of ammonium salts of phosphoric acid After stopping the air supply, it was found that the first acid column was choked with the formed thick slurry If the' above series

80

a o

c o

E a a

20

With simulated wastewater With hydrolysed urea plant wastewater

pH of the influent =11-0 Temperature =(26±2)"c Effective height of

packed column = 140 cm Influent concen- -1 trationof NH 3 -N =2000mgl

00 0-2 0-4 0-6 Wastewater flow rate (m 3 rff 2 h~1)

0-8

Fig 1—Variation of Percentage Removal of Ammonia

with Wastewater Flow Rate at Various Specific Air Flow Rates in Continuous Flow Packed Column

600 540 480

T 420

360-1 300

240-18 12 6C

pH of the Influent =11-0 Temperature =(26±2)°c Effective height of

packed column =140cm Influent concen- ri trdtion of NH3-N =2000 mg I

90 Percent eOPercert 70Pzrcent 60 Percent 50Percent 40 Percent removal removal removal removal removal removal

0 0 0-2 0-4 0-6 0-8 Simulated wastewater flow rate(m 3 rr7 z h~ 1 )

Fig.2-Patterns of Percentage Ammonia Removal for Various Combinations of Air Flow and Simulated Wastewater Flow Rates for Continuous Flow Packed Column

ua

to l

p 6

Simulated wastewater flow rate(nrm h~ )

to

cn

5* m -< •a

o

\

p

S o

3- 5"

cn o

3 5

n

Us

a

So-2 o

i J

c

§

Q ro

o o

Percentage ammonia removal

Percentage ammonia removal

3i o

o

3 n

>

^ o

3

X

o

ro

ro

O

3

3 ro

I

O

3

3 ro

I

CO

Temperature = ( 3 8 ± 1 ) ' c Rate of aeration =6 litres per minute

Total NH3-N Present in 1st recovery column Reactor vessel

Und recovery column

3 4 5 6 7 8 9 Aeration time in hours

Fig.5-Variation of Ammoniacal Nitrogen with Aeration Time in

Reactor Vessel and Recovery Columns.

of two columns is used for recovery along with the continuous flow packed column for removal of ammonia, and if 90% removal of ammonia

is achieved in wastewater having influent ammonia concentration of

2000 mg I'1 , it can be calculated with the help of Fig (5) that one litre of commercially available phosphoric acid will be sufficient

to absorb the ammonia stripped out from more than 275 litres of wastewater

SUMMARY AND CONCLUSIONS The results of the laboratory investigation demonstrated the feasibility of air stripping of ammonia from microbially hydrolysed urea plant wastewater and its subsequent recovery as ammonium phos-phate crystals using phosphoric acid as collector media One litre

of commercially available raw grade phosphoric acid was sufficient

to absorb the ammonia stripped out from more than 275 litres of wastewater having influent ammonia concentration of 2000 mg I assuming 90% ammonia removal When influent ammonia concentration was 2000 mg I , maximum removals of ammonia obtained in continuous flow packed column were 81, 91 and 99% with simulated wastewater, and 60, 81 and 91% with microbially hydrolysed wastewater at 26+2 C with air flow rates of 180, 300 and 420 m3 m2 h"1 respectively and

at pH 11 In the continuous flow packed column when the ratio of air to simulated wastewater was 1000, the removal of ammonia was 68%, and remained unchanged with variable air flow rate in the range

of 180 to 420 m3 m"2 K"1 A nomogram was generated for prevailing experimental conditions relating the three variables, namely simulated wastewater flow rate, air flow rate and percentage ammonia removal for continuous flow packed column The nomogram was found to correlate well with the experimental data obtained using microbially hydrolysed wastewater