How to reduce or remove ammoniacal ammonia

VOL. 62, 2017

A publication of

The Italian Association

of Chemical Engineering

Online at www.aidic.it/cet

Guest Editors: Fei Song, Haibo Wang, Fang He

Copyright © 2017, AIDIC Servizi S.r.l.

ISBN 978-88-95608- 60-0; ISSN 2283-9216

Research on Mechanism of Air Stripping Enabled Ammonia

Removal from Industrial Wastewater and Its Application

Dan Jiaa, Wenlong Lua, Yanfu Zhangb

aJilin Institute of Chemical Technology, Jilin 132022, China

bPetrochina Jilin Petrochemical Company Wastewater Treatment Plant, Jilin 132000, China

jiadan@126.com

In this paper, the stripping method was used to deal with the wastewater with high concentration of ammonia

generated in the industrial production process, and the effects of blowing time, waste water pH, stripping

temperature and gas-liquid volume ratio on the maximum removal rate of ammonia were investigated by

experiments. The experimental results show that the gas - liquid ratio is linear to the stripping efficiency at 50

°C and strongly alkaline. And it can be deduced that the effect of different air flow rates on the blowout

efficiency at gas-liquid ratio below 1000 is not significant. In this paper, the maximum air flow rate is 2L/min,

and the maximum reaction time (stripping time) is 120min, so the maximum gas-liquid ratio is only 800. At the

same time, this paper also explored the reaction mechanism of ammonia removal via the stripping method

and its application in ammonia removal from industrial wastewater produced by monosodium glutamate.

1. Introduction

With the development of industry and agriculture and the improvement of people's living standards, the

emission of nitrogenous compound wastewater has increased dramatically, which has attracted much

attention as the main source of the environment pollution. Ammonia is an important pollutant to the water

eutrophication and environmental pollution. Upon entering water, the ammonia can lead to water hypoxia,

breeding harmful aquatic organisms that poison fish, and humans who eat such fish tend to be slightly

poisonous or even dead (Hu et al., 2003). In addition, ammonia also affects the oxygen transport by fish gills,

and the high concentration of ammonia will even kill fish. A large amount of ammonia wastewater discharged

into the rivers and lakes has brought difficulties in the industrial wastewater treatment. When the chlorine

disinfection is adopted, ammonia and chlorine will produce chloramine, which clearly have increases chlorine

consumption rate and further drives great demands for chlorine. Ammonia is converted to nitric acid, and

nitrates are further converted to ammonium nitrite with a serious three-pronged effect that directly affects

human health, for which the treatment of ammonia nitrogen wastewater has become a hot issue in today's

water treatment (Huang et al., 2014). At present, ammonia nitrogen wastewater treatment methods are as

follows: air stripping, ion exchange, chemical precipitation, membrane separation, chemical oxidation,

biological method, etc. As for the treatment of ammonia wastewater, domestic researches mainly focus on the

materialization method, while biological methods are rarely used. It is of great practical significance to seek

practical treatment technology with small investment, reliable operations and high efficiency for the processing

of industrial ammonia wastewater (Wilfried and Chakkrit, 2015).

1.1 Classification of industrial ammonia wastewater

According to the different concentrations, industrial ammonia wastewater can be divided into three categories,

namely (1) low concentration of ammonia nitrogen wastewater: NH3 <50mg/L; (2) moderate concentration of

ammonia nitrogen wastewater: NH3 is 50-500mg/L; (3) low concentration of ammonia nitrogen wastewater:

NH3> 500mg/L. High ammonia concentration wastewater is generally derived from the production process of

coke, ferroalloy, coal gasification, hydrometallurgy, oil refining, animal husbandry, chemical fertilizers, man-

made fibers and incandescent lamps that are all extremely complex and have different types of industrial

wastewater ammonia concentration in the ever-changing state (Xiang H, et al., 2011). Moderate concentration

DOI: 10.3303/CET1762020

Please cite this article as: Dan Jia, Wenlong Lu, Yanfu Zhang, 2017, Research on mechanism of air stripping enabled ammonia removal from

industrial wastewater and its application, Chemical Engineering Transactions, 62, 115-120 DOI:10.3303/CET1762020

115

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of ammonia wastewater is generally derived from fertilizer, oil refining, rubber, synthetic rubber, dairy

production, pharmaceutical, farming, paper, leather and other industries; Low concentration of ammonia

wastewater generally comes from canned food, vegetables, starch, grain food processing industry and anti-

corrosion, paint ink, and other industries.

1.2 Introduction to the blowout method

The air stripping method is currently used for the treatment of ammonia waste. The blown off ammonia can be

recycled, but most is directly discharged into the air. Therefore, when using the air stripping method, we

should not only pay attention to how to improve the efficiency of ammonia bleed, but also take the initiative to

prevent secondary pollution. If the emitted ammonia is discharged directly into the atmosphere, it is necessary

to consider whether the total amount of free ammonia emitted is in line with the ammonia emission standards

(Lee et al., 2013). Ammonia returns to the earth in the atmosphere by gas deposition (60%), aerosol

deposition (22%), rainfall (18%), etc. After the set of animals and animals have a stimulating and toxic effect;

When there are sulfur dioxide emissions near the point, the atmosphere of ammonia, the atmosphere of sulfur

dioxide and water generate ammonium sulfate aerosol. The air stripping process of ammonia refers to that

adjusting the PH value of wastewater to transform the ammonia in the ion state into molecular ammonia, and

then blowing the air into the wastewater (Zhang et al., 2009). For the ammonia in the wastewater is usually

present in equilibrium with the state of ammonium ions (NH4

+) and free ammonia (NH3), the equilibrium

relationship between NH4

+ and NH3 in water is as follows:

4

3

2

NH

OH

NH H O

+

-

+

+

ƒ

(1)

The percentage distribution between ammonia and ammonia ions can be calculated by using the following

formula:

3

4

a

/

(

. )/

W

b

NH

H

NH

K

K

K

C

C

C

+

+

=

=

(2)

Where: Ka - ionization constant; Kw - water ionization constant; Kb - Ionization of ammonia; C - substance

concentration

This equilibrium is related to pH and temperature. At pH 7, only NH4

+ ions are present in the solution, and

when pH is 12, the solution is fully soluble NH3. Ammonia stripping is to raise the pH and temperature of the

wastewater, and then provide sufficient gas and water to blow the ammonia from the solution. The air stripping

ammonia removal method is stable and easy to control, with simple operations. But this method is also easy to

fill the fouling, affecting the operations of the equipment. When the water temperature is low, the air stripping

efficiency is low, blowing off after the completion of the callback value. In addition, the wastewater after

stripping treatment still contains a small amount of ammonia and cannot meet the discharge standard. So the

air stripping method is often used for the pretreatment of high concentration of ammonia nitrogen wastewater

(Ma H R, et al., 2010). How to improve the efficiency of air stripping, how to avoid secondary pollution and

how to control the production process and scale production in the process of industrialization call for much

attention.

1.3 The ammonia removal process by air stripping

The air stripping method has the advantages of high denitrification rate, flexible operations and small

footprints. The stripping method is often domestically used for treating high concentration ammonia nitrogen

wastewater, and the ammonia can be recycled. In order to ensure the continuous operations of the production

line, the sewage treatment system must be implemented in three stages. (Wu et al., 2010).

When the water level reaches the high level, the industrial wastewater is discharged into the accident pool.

When the regulation tank’s water level reaches the low level, the accident pool lift pump will lead wastewater

in the accident pool to the adjustment pool. Adjust the pool lift pump to transfer the wastewater from the

conditioning tank to the primary stripping tower, which is controlled by the regulating valve of the lift pump

outlet. The wastewater is added to the primary stripping tower before adding NaOH to adjust the pH of the

wastewater to 11. The wastewater enters the tower from the top of the first-stage stripping tower and flows

from the top of the tower along the gap in the packing. The air is entrained with steam from the bottom of the

tower into the tower, forming convection with the wastewater. Air will waste water in the ammonia nitrogen

blown off the wastewater into the bottom of the bottom of the collection tank. Similarly, the wastewater flows

through the secondary stripping tower and the tertiary stripping tower. Three-stage stripping tower lift pump

will strip the bottom of the wastewater to the mixing reaction tank, with HCl added to adjust the pH between 6

to 9 (Song et al., 2008). Ammonia content in wastewater is detected by the ammonia nitrogen analyzer. If the

discharge standard is reached, it will be transported to the park sewage treatment plant for further treatment. If

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failing to meet the discharge standard, the wastewater will be transported to the regulating tank and be re-

stripped (Wang et al., 2013).

2. Object of study, purpose and experimental program

2.1 Research objects

The research object of this paper is to simulate the industrial high concentration ammonia wastewater

(concentration> 500m/L) produced by the fertilizer industry. This kind of waste water generally has the

characteristics of high temperature, high ammonia, high flow rate, low organic matter and it is difficult to

conduct biochemical treatment.

2.2 Research purposes

High concentration of ammonia wastewater treatment in domestic is always difficult to solve. Different regions,

different seasons, different industrial enterprises’ emissions of ammonia nitrogen wastewater conditions are

different, and the existing governance methods are difficult to achieve a high degree of unity of social benefits,

environmental benefits and economic benefits of (Min and Yi, 2016). Therefore, the purpose of this paper is to

try a new method for the treatment of high concentration of industrial ammonia wastewater, to explore its

application feasibility and reaction mechanism, which lays the foundation for further researches.

2.3 Experimental program

Based on the physical and chemical properties of ammonia volatilization and oxidation, and through the

comparison, this study adopts the method of gas-phase ammonium oxidation as the follow-up method based

on the stripping method. Under the action of the catalyst and a certain temperature range, we will strip the

ammonia for oxidation, and the possible oxidation products are NOx, N2, H2O, hydrazine and so on. And the

oxidation products are mainly N2 and H2O by temperature control. The wastewater treated by the experiment

was made by artificial dispensing, and the NH3 concentration was 1024 m/L. The effects of pH value, water

temperature, stripping time and addition amount of No. 1 denitrification on the denitrification rate were

obtained by the static test, and the reasonable design parameters were provided for the engineering design.

The experimental protocol is as follows:

(1) In the atmospheric pressure, strong alkaline conditions, by heating and drumming into the excess air, the

NH3 in the waste water is transformed to the gas phase;

(2) with air as the carrier gas, through its carrying NH3 into the catalyst with quartz catalyst tube, heating with

electric heating, digital temperature control equipment to control the reaction temperature, in a certain

temperature range, the oxygen and ammonia in the air produce oxidation reaction in a series of catalytic; (3)

The concentration of NH3 and NO2 in the reaction tube was measured by the atmospheric sampler, and the

NH3 concentration in the water samples before and after the reaction was measured.

(3) Study on Treatment of High Concentration Ammonia Wastewater by Stripping Method under Normal

Pressure and Gas - phase Ammonia Catalytic Oxidation.

According to the change of NH3 concentration in the gas phase, the conversion rate of NH3 was calculated,

and the removal rate of ammonia in ammonia-containing wastewater was calculated according to the change

of NH3 concentration in the liquid phase and the amount of NH3 in the gas phase. The formation rate of N2

was calculated according to the concentration of NO2 in the gas phase. The experimental flow diagram is

shown in Figure 2.

Air

pump

Ammonia Wastewater

Water Sample Constant

Temperature Water Bath

Device

Reactor

Atmospheric

Collector

Liquid phase

extraction

analysis

Heating and

temperature

control

equipment

Gas sampling

analysis

Figure 2: Analysis of Ammonia Catalytic Oxidation Process for Treatment of Ammonia Wastewater at Normal

Pressure

117

---

Page 4

3 Test results and discussion

3.1 Effect of temperature and pH on stripping effect

Take the amount of ammonia wastewater, and control the amount of NaOH dosage for different pH. The gas-

liquid ratio is 3000:1; stripping time is 9h; and select the different temperature and solution pH to carry out the

experiment. The experimental results are shown in Figure 3 and Figure 4.

Figure 3: Effect of Temperature on the Stripping Effect

Figure 4: Effect of pH on the Stripping Effect

It can be seen from Fig.3 that the stripping efficiency increases with the stripping time, and the temperature

change has obvious effect on the stripping efficiency. Stripping at 25 °C and 40 °C can only reach about 44 %,

and when the water temperature rose to above 50 °C, after stripping 6h, the efficiency can reach 54%. From

the effect of pH change on the stripping efficiency in Fig. 4, it can be seen that the pH is above 9.5 and

stripping after 6h can guarantee more than 50% of the blowing efficiency.

3.2 Effect of gas - liquid ratio on stripping experiment of ammonia - nitrogen wastewater

According to the results of the stripping method for the treatment of high concentrations of ammonia

wastewater, pH and temperature are two key factors in the removal of NH3 from water. The higher the water

temperature and the pH value, the more favorable the NH3 escape. When the temperature and pH value are

constant, the gas-liquid ratio also has a certain impact. Therefore, we conduct experiment to study the

relationship between gas-liquid ratio and blow-off effect. Experimental conditions: simulated ammonia waste

water 300ml, pH = 14, water temperature 50 °C, temperature 25 °C, stripping time 120min. The measured

water sample is diluted by 100 times, so the calculated NH3 concentration multiplied by 100 is the actual water

sample concentration, and the experimental results are shown in Table 1.

Table 1: Gas-liquid Ratio Test Data

Test number Gas-liquid ratio Absorbance Concentration of NH3 in wastewater

Stripping

efficiency

1

2000

0.576

366

66.8

2

2200

0.481

322

71.2

3

2500

0.434

289

76.7

4

2800

0.339

190

81.4

5

3000

0.256

123

88.3

The graph shows that the gas-liquid ratio is linear with the stripping efficiency at 50 °C and strongly alkaline,

and it can be deduced that the effect of different air flow rates below 1000 on the stripping-out efficiency is not

significant. In this experiment, the waste water volume is 300ml; the maximum air flow rate is 2L/min during

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stripping and catalytic oxidation; the maximum reaction time (stripping time) is 120min, so the maximum gas-

liquid ratio is only 800. It can be argued that, for the purposes of this experiment, the total amount and volume

change of NH3 are negligible under different conditions of flow (<2 L/min) of air at the same time, and

ammonia is actually "boiled out." Different air flows only bring about different concentration of gas phase NH3.

4 Study on the Reaction Mechanism of Ammonia by Stripping

It is generally believed that the reaction of NH3 and 02 can be expressed as a stoichiometric equation:

3

2

2

3

2

2

2

3

2

2

2

:4

5

4

6

:4

4

4

6

:4

3

2

6

A NH

O

NO

H O

B

NH

O

N O

H O

C

NH

O

N

H O

+

=

+

+

=

+

+

=

+

(3)

The reaction of NH3 with NO can be expressed as:

3

2

2

4

6

5

6

NH

NO

N

H O

+

=

+

(4)

The reaction of A, B, and C can occur at 900 °C. The equilibrium constant is K1 = 1053; K2 = 1061; K3 = 1063.

The equilibrium constant of the three reactions is large, so the reaction to the left can be regarded as

irreversible. From K3> K2> K1, it is known that the reaction of NH3 oxidation to N2 is the most complete, and the

final product of NH3 oxidation can be all N2 (Hao et al., 2008). When there is a catalyst, the reaction of NH3

with O2 has the mechanism of "selective catalytic oxidation(SCO)". Denote NH3 (a) and O (a) to represent the

ammonia and oxygen in the adsorbed state, and the two reaction mechanisms are described below:

3

3

2

3

2

2

3

2

2

2

2

2

2

2

( )

2

2 ( )

( )

( )

( )

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a)

(a) (

)

X

Y

X Y

X Y

NH

s

NH a

O

O a

NH a

NH a

OH a

NH

O

NH

OH

NH

OH

NH

H O

NH

OH

NH

H O

NH

OH

N

H O

NH

O

N

OH

NH

NH

N H

N H

X Y OH

*

+

+

+ ↔

+

↔

→

+

+

→

+

+

→

+

+

→

+

+

→

+

+

→

+

+

→

+

+

2

2

( )

( ) (

)

a

N g

X Y HO

→

+

+

(5)

Foreign scholars tend to believe that, under the condition of excessive oxygen oxidation, the SCO mechanism

is suitable for describing the way of ammonia catalytic oxidation. This experiment detects the presence of NO

and confirms this view (Shaji, et al., 2012).

5 Application of Stripping Method

MSG production process can be divided into high, medium and low concentrations of organic wastewater.

High concentration of wastewater, which is glutamic acid mother liquor, has a strong acid, high COD,

ammonia nitrogen, and sulfate and a series of characteristics, so it brought more difficulties for the

management work especially that the high concentration of ammonia nitrogen will produce a strong inhibition

of biological activity. In this experiment, the ammonia removal experiment of high concentration monosodium

glutamate wastewater was carried out by stripping method, which provided the basis for its application in

actual production.

5.1 Primary ammonia bleaching purification process

Because of the low cost of running, mature process, high stripping efficiency, stable operation and suitable for

the treatment of high concentration ammonia nitrogen, it is widely used at home and abroad. Therefore, this

method uses stripping method to remove high concentration ammonia in wastewater (Wu et al., 2001). Due to

the high ammonia content in alkaline solution, it will react with heavy metal ions to produce complexes that is

difficult to deal with, and will play a buffer in the regulation of the time.

5.2 Secondary ammonia bleaching purification process

Ammonia stripping tower selects polypropylene ladder ring as packing tower. The best pH value is 11, and the

best temperature is room temperature. It directs high gas to water than the free ammonia in the waste water to

the air, and then uses dilute acid on ammonia gas to purification absorption. Absorbent solution is dilute

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sulfuric acid solution, and after cleaning the tail gas to the high-altitude discharge, the absorption of sulfuric

acid solution can be made semi-finished products, which can also be applied to the monosodium glutamate

plant area of green trees. The pH value of the secondary stripping tower is 11, and the stripping efficiency is

about 90% at room temperature. After the secondary stripping, the concentration of ammonia can reach the

biochemical pool for the treatment.

6. Conclusion

The effect of blowing time, waste water pH, stripping temperature and gas-liquid volume ratio on ammonia

removal rate was investigated by using stripping method to treat high concentration ammonia wastewater

produced in industrial production. At 50 °C and strongly alkaline condition, the gas-liquid ratio is linear with the

stripping efficiency. And it can be deduced that the effect of different air flow rates below 1000 on the blowout

efficiency is not significant. The maximum air flow rate of this experiment is 2L/min, and the maximum reaction

time (stripping time) is 120min, so the maximum gas-liquid ratio is only 800. At the same time, this paper also

explores the reaction mechanism of ammonia removal in stripping method and the application of ammonia in

industrial wastewater produced by monosodium glutamate.

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923-925, DOI: 10.16581/j.cnki.issn1671-3206.2014.05.036.

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Niobium by Dissolved Gas Stripping Method, Enterprise Technology Development, (7), 3-4+7, DOI:

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120

How to Treat Sewage from Fertilizer Industry? | Waste Management

Article shared by : 

The following article will guide you about how to treat sewage from fertilizer industry.

Sewage from Fertilizer Industry:

The fertilizer industries may be classified as nitrogenous fertilizer industries and phosphatic fertilizer industries. The nitrogenous fertilizer industries are mainly concerned with the production of fertilizers such as urea, ammonium sulphate, ammonium nitrate, calcium ammonium nitrate (CAN), ammonium chloride, etc. Phosphatic fertilizer industries manufacture mainly single superphosphates, triple superphosphates, nitrophosphates, ammonium phosphates, etc.

Sources, Quantity and Characteristics of Sewage:

ADVERTISEMENTS:

i. Sources:

In the case of nitrogenous fertilizer industry sewage is contributed by ammonia plant, urea plant, ammonium nitrate and calcium ammonium nitrate plant, ammonium sulphate plant and ammonium chloride plant.

In case of phosphatic fertilizer industry sewage is contributed by phosphoric acid plant, single superphosphate plant, triple superphosphate plant, ammonium phosphate plant, nitrophosphate plant, sulphuric acid plant and nitric acid plant.

ii. Quantity:

The quantity of sewage discharged from fertilizer industries varies widely, depending on the raw material used, the end product obtained and the process adopted for the production of fertilizers. A 1000 tonnes per day urea plant having recirculating cooling water system and all the auxiliary facilities required for production, generally discharges 8000 to 12000 m3/day sewage, while a phosphatic fertilize plant with recirculating cooling water system and auxiliary facilities and having a production of about 100 tonnes of phosphorus pentoxide (P2O5) per day as fertilizer discharges 3000 to 6000 m3/day sewage.

iii. Characteristics:

ADVERTISEMENTS:

The main pollutants from the nitrogenous and phosphatic fertilizer industry along with the auxiliary facilities are indicated below:

(a) Ammonia and ammonium salt;

(b) Suspended solids and ash;

(c) Acids and alkalis;

ADVERTISEMENTS:

(d) Oil;

(e) Arsenic,

(f) Nitrates;

(g) Urea;

ADVERTISEMENTS:

(h) Cooling water conditioning chemicals like chromate, phosphates, biocides, etc.

(i) Cyanides and sulphides;

(j) Biochemical Oxygen Demand (BOD);

(k) Fluorides; and

(l) Phosphates, etc.

Nitrogenous Fertilizer Industry:

Typical ranges of contaminant concentrations in the effluents from various operations in nitrogenous fertilizer industry are as given below:

(i) Cooling Tower Blowdown:

(ii) Water Treatment Plant:

The effluents from the water treatment plant of a nitrogenous fertilizer unit vary from 380 l/tonne of urea to 2000 l/tonne of urea, depending on the quantity of raw water used. The dominant contaminants in a water treatment plant effluents are anions and cations.

In a typical nitrogenous fertilizer unit manufacturing urea the amount of sodium hydroxide in the water treatment plant effluent is 11.6 kg/tonne of urea manufactured. The total sulphate ion quantity is 18.2 kg/tonne of urea manufactured. Besides these, when a process condensate is treated for use as boiler feed water, ammonia finds its way into the water treatment effluent.

(iii) Boiler Blow-Down:

(iv) Ammonia Plant:

(v) Urea Plant:

Phosphate Fertilizer Industry:

Typical ranges of contaminant concentrations in the effluents from various operations in phosphatic fertilizer industry are as given below:

(i) Cooling Tower Blowdown:

(ii) Boiler Blowdown:

(iii) Superphosphate Plant:

(iv) Blending Unit:

Methods of Treatment of Sewage from Fertilizer Industry:

The treatment of the sewage from fertilizer industry may consist of the following:

(a) Segregation of effluents

(b) Treatment of effluents for specific pollutants

(a) Segregation of Effluents:

The effluent streams are segregated according to the nature of pollutants present in them and their concentration.

The following steps should be taken wherever applicable:

(a) Effluents containing high concentration of total ammonia nitrogen, say above 100 mg/l, should be combined. However, effluents with 50 to 100 mg/l ammonia nitrogen may also be collected in this stream if the volume is large.

(b) Effluents containing suspended solids above 100 mg/l should be combined together as far as practicable.

(c) Oil bearing effluents should be combined as far as possible.

(d) Highly acidic and alkaline effluents should be separated from the rest of the effluent streams.

(e) Urea bearing effluents which also contain high concentration of ammonia should be separated from ammonia bearing effluent.

(f) All cooling tower purge water containing chromate, phosphate and biocides should be separated from the rest of the effluents.

(g) Ash slurry should be separated from the rest of the effluents.

(h) Effluents containing carbon slurry should be stored separately.

(i) Arsenic and cyanide bearing effluents should be stored separately.

(j) Effluents containing fluorides and phosphates are to be segregated from other effluents.

(k) Sewage effluents should be treated separately as far as possible.

(l) Storm water and drain water should not mix with individual plant effluents.

However, many of the above effluents may be combined, depending on their characteristics, flow and type of treatment to be adopted.

(b) Treatment of Effluents for Specific Pollutants:

Various methods are available for the treatment of individual pollutant parameters relevant to the fertilizer industry and the same are indicated below:

(i) Ammonia nitrogen:

The processes for removal recovery of ammonia nitrogen basically fall in two categories:

(1) Physio-chemical processes, and

(2) Biological processes.

(1) Physio-Chemical Processes:

These include air stripping, steam stripping and ion exchange. Fig. 19.9 shows a flow diagram for the treatment of ammoniacal effluent by air/steam stripping process, and Fig. 19.10 shows a flow diagram for the treatment of ammoniacal effluent by ion exchange process.

(2) Biological Processes:

These include biological nitrification and denitrification, and algal uptake.

Biological nitrification and denitrification can reduce ammonia nitrogen content of the effluent to a very low level. This process is being adopted in municipal sewage treatment for years. In the treatment of industrial sewage, this process may be adopted as a secondary or tertiary treatment where the ammonia nitrogen content of the sewage is comparatively low and also a high degree of treatment for the removal of ammonia nitrogen is desired. Fig. 19.11 shows a flow diagram for the treatment of dilute ammonia and urea bearing effluent by biological nitrification and denitrification.

Algal uptake is another biological process used for the removal of ammonia nitrogen from the sewage. Since ammonia nitrogen is an algal nutrient, algae are capable of extracting this nutrient from the sewage. Algae growing in sewage stabilization ponds utilize ammonia nitrogen of the sewage to form cell tissue in the presence of sunlight. Adequate carbon dioxide and some other nutrients are also required in this process. For the culture of algae ponds similar to oxidation ponds may be used. Fig. 19.12 shows a flow diagram for the treatment of ammoniacal effluent by algal uptake process.

(ii) Urea and Nitrate Nitrogen:

Urea nitrogen can be removed from the effluent by using a process called thermal urea hydrolysis in which urea is hydrolyzed in the presence of enzyme urease secreted by some bacteria formed in the soil. The dilute urea solution is hydrolyzed by the above bacteria in the presence of organic carbon compounds to give ammonia and carbon dioxide.

Fig. 19.13 shows a flow diagram for the treatment of ammonia and urea bearing effluent by thermal urea hydrolysis. The hydrolyzed solution containing ammonia can be treated by any of the methods described under ammonia removal. Further reduction of nitrates can be effected by the denitrification process.

(iii) Suspended Solids:

Suspended solids originate from various sources in the fertilizer industry. The process water clarification plant sludge, ash slurry from coal gasification plants, steam generation plants or phosphoric acid plant effluent during neutralization of effluent with lime, etc., suspended solids in different particle sizes find their way into the effluent.

These effluents containing suspended solids are settled in a suitably designed settling tanks and the clear overflow passes out. In some cases, particularly where the particle size is comparatively small, mechanical clarifiers having proper arrangements of dosing coagulants or polyelectrolytes are required for quick settling. The sludge discharged from the bottom of the clarifier may be drawn out mechanically, dewatered and disposed of as solid waste as required.

(iv) pH:

Sometimes the effluents are highly acidic or alkaline in nature. When both acidic and alkaline effluents are found, they may be mixed suitably for neutralization. Otherwise for neutralization of acidic effluent, lime or soda ash may be used and for neutralization of alkaline effluent sulphuric acid may be used. In the process of neutralization proper mixing is very important. This can be effected by flash mixing or mixing by agitation or recirculation.

(v) Oils and Greases:

Oils and greases normally discharged in fertilizer industry effluents are mostly in non-emulsified form. Moreover, a majority of these insoluble oils are lighter than water and therefore they will float on its surface. Thus insoluble oils lighter than water are usually separated in settling tanks provided with an adjustable skimming weir as shown in Fig. 19.14.

These settlers are usually termed as gravity type mechanical oil separators. The oils readily float on these separators and the depth of the weir is adjusted according to the amount of oil present in the sewage. The collected oil is skimmed by mechanical means periodically.

A properly designed oil separator can reduce the oil content of the effluent below 50 mg/l. If a greater degree of oil removal is desired, the effluent from the oil separator may be passed through active carbon or a porous coke bed by which the oil and grease content of the effluent is reduced to 2 to 10 mg/l.

(vi) Arsenic:

In fertilizer industry, arsenic is a constituent of absorbent liquids used for the removal of carbon dioxide. Normally adequate arrangements are provided in the plant so that arsenic does not find its way out in the effluent. But in actual practice, due to leakages in pump glands, flanges, joints, etc., and also from spillages, some arsenical solution is discharged.

The arsenic solution which is discharged even after taking all precautions is completely separated from other effluents. The effluent containing arsenic is then filtered, concentrated, further filtered through active carbon filter if necessary and recycled in the process. When it is not possible to take it into the process, the arsenical solution is evaporated to dryness and the solids are placed in concrete drums, sealed properly and buried underground or disposed of into deep sea far away from the coast line.

(vii) Chromate and Phosphate:

Fertilizer industry requires large quantity of cooling water during processing of fertilizers. In most of the fertilizer factories, cooling water is recycled through cooling towers. Suitable inhibitors for control of scaling/corrosion properties of circulating water are dosed into the cooling water system.

Most of the fertilizer plants use combinations of chromate, phosphate and zinc in different proportions as inhibitors. Normally zinc is used in a very low concentration, therefore, any specific treatment for the removal of zinc is not considered necessary. In the treatment for removal of chromate from the sewage, phosphate is also simultaneously removed, so specific treatment for removal of phosphate is not considered necessary.

The basic principle of chromate removal is the reduction of hexavalent chromium to trivalent form and precipitation of chromium as chromium hydroxide. Fig. 19.15 shows a flow diagram for the treatment of chromate bearing effluent.

(vii) Cyanide:

Depending on the process and raw material, cyanides are sometimes encountered in the fertilizer factory effluent. Usually cyanide containing effluents are completely segregated from other effluents and are treated or disposed of separately. When the cyanide content is low the effluent can be discharged at a controlled rate along with the other effluents, so that cyanide content of the final effluent does not go beyond specified limits.

When the cyanide content is high and the effluent volume is low it can be removed by stripping with steam and acidic gas. The residual cyanide after stripping may be treated further if required. Fig. 19.16 shows a flow diagram for the treatment of cyanide bearing effluent.

(ix) Sulphides:

Sulphides are sometimes present in small quantities in fertilizer factory effluents. However, the sulphides present in fertilizer factory effluent normally do not require any special treatment. Natural dilution by the other effluent from the factory is sufficient to bring down the level of sulphides within specified limits.

(x) Fluorides and Phosphates:

The main source of fluorides and phosphates in the phosphatic fertilizer industry are scrubber liquors from various unit operations involving scrubbing of the off-gases, floor washings and gypsum and water. In the effluent, fluorides are present as fluosilicic acid with small amounts of soluble salts as sodium and potassium fluosilicates and hydrofluoric acid.

Phosphorus is present principally as phosphoric acid with minor amounts of soluble calcium phosphates. For the removal of fluorides and phosphates two-stage treatment with chalk followed by lime or double lime treatment is adopted. Fig. 19.17 shows a flow diagram for the treatment of fluoride and phosphate bearing effluent.

(xi) Sewage Effluent:

The sewage from toilets and other sanitary facilities in the factory area has high biochemical oxygen demand (BOD) and contains suspended solids. The sewage effluent is segregated from other industrial sewage and treated for removal of BOD and suspended solids. The volume of the sewage effluent is normally comparatively low.

The general practice is to treat this effluent in oxidation ponds or by aeration processes. However, depending on the level of BOD, the sewage effluent may be subjected to partial BOD removal by any conventional method and discharge along with the other treated industrial sewage so that the BOD value of the final effluent does not go beyond specified limits.