Treatment of Effluents of Fertilizer Industry | Environmental Engineering

In this article we will discuss about the treatment and disposal of effluents of fertilizer industry.

1. Nitrogenous Fertilizer Industry:

a. Ammonia Production:

In the production of nitrogenous fertilizer, ammonia is the basic intermediate product. Ammonia is produced by reaction of hydrogen with nitrogen. This reaction is carried out in a converter in the presence of iron catalyst promoted with metal oxides at elevated pressure, which favours ammonia formation. The raw material source of nitrogen is atmospheric air or pure nitrogen from an air liquefaction plant.

Hydrogen on the other hand is obtained from a variety of sources, namely naphtha, fuel oil coal natural gas, coke-oven gas, hydrogenrich refinery gas, electrolytic hydrogen off-gas, etc.

The production of ammonia from the above feed stock involves three main steps:

1. Preparation of raw synthesis gas,

2. Purification of the gas mixture and

3. Synthesis of ammonia.

The process adopted for synthesis gas preparation depends on the feedstock used. Where a cheap source of electricity is available, electrolysis of water yield hydrogen off-gas with the production of heavy water.

In India only one such unit is operating at present. In the partial oxidation process hydrocarbon feedstock and oxygen or oxygen enriched air are preheated and reacted at high temperature and pressure to form carbon monoxide and hydrogen.

The raw gas is scrubbed with water for removal of the carbon formed during gasification and after desulphurization is sent to the shift conversion unit. In the steam reformation process, desulphurized naphtha or natural gas is subjected to catalytic reforming in a primary reformer in the presence of steam to form carbon monoxide and hydrogen. Since the reaction is incomplete in the primary reformer, a secondary reformer is used for converting the remaining hydrocarbons.

Air is injected into the secondary reformer to burn the unreacted hydrocarbons and supply the nitrogen requirement of the raw gas. Coal gasification process involves pulverized coal gasification in the presence of oxygen and steam. The raw gas produced is cleaned up before it goes for shift reaction for purification.

b. Purification of Raw Gas:

The first step in the purification of raw synthesis gas is the shift conversion of carbon monoxide to carbon dioxide which is accomplished by reacting carbon monoxide with steam over activated iron oxide catalyst; carbon dioxide thus produced with hydrogen is removed by absorption process by use of scrubbing solutions.

The absorbents normally used are hot potash activated with arsenic in Vetrocoke process, hot potash activated with a small quantity of vanadium, arsenic, etc., in the Benfield process, chilled methanol in the Rectisol process, monoethanolamine process, etc. Carbon dioxide is recovered and reused. The residual carbon monoxide is removed by methanation or absorbed by liquid nitrogen wash.

c. Ammonia Synthesis:

Pure hydrogen and nitrogen in the required quantities are made to react under elevated pressure and temperature over activated iron oxide catalyst to produce ammonia. The ammonia produced is cooled so that it condenses and is recovered in a liquid- gas separator.

d. Urea Production:

Urea is the main nitrogenous fertilizer in India. Urea is produced from ammonia and carbon dioxide obtained from ammonia plant normally located at the site of the urea plant. Urea synthesis can be divided into three main sections, namely, synthesis, decomposition/recovery and finishing sections.

In the synthesis sections ammonia and carbon dioxide are compressed in an autoclave at elevated temperature and pressure to form a solution of urea, ammonium carbonate and water. The product stream from the ureas reactor is a mixture of ureas immonium carbonate, water, unreacted ammonia and carbon dioxide.

An excess of ammonia is always maintained, so that carbon dioxide concentration in the exit stream is low. The next section in the urea process is the decomposition section where the solution from the autoclave is heated to decompose ammonium carbonate. The decomposed ammonium carbonate along with excess and unreacted ammonia and carbon dioxide is recycled in the autoclave, while 70 to 75 percent ureas solution recovered.

In the finished section, the ureas solution leaving the decomposition section is further processed. The urea solution is concentrated under vacuum or at atmospheric pressure in a specially designed evaporator of falling film type to raise the urea concentration above 98 percent. The molten urea from the concentrator is pumped to the top of the pilling tower where it is sprayed downward against an upward stream of cold air. The urea prills from the tower are cooled, screened and stored.

e. Ammonium Sulphate Production:

Ammonium sulphate is produced from three sources.

The production of coke from coal results in the production of coke oven gas which contains a significant amount of ammonia. This ammonia is converted into byproduct ammonium sulphate by reacting it with sulphuric acid.

Ammonium sulphate is produced by neutralizing synthetic ammonia with sulphuric acid and the ammonium sulphate crystals formed are separated from the mother liquor by filtration or centrifuging.

Ammonium sulphate is also manufactured from natural of byproduct gypsum. The ground gypsum is reacted with ammonium carbonate producing ammonium sulphate and chalk. The chalk is separated by filtration and the liquor is evaporated and crystallized. The ammonium sulphate crystals are separated by filtration and dried.

Ammonium Nitrate and Calcium Ammonium Nitrate Production – Ammonia reacts with nitric acid in a neutralizer producing ammonium nitrate. Ammonia and nitric acid are preheated with the vapours of the neutralizer. In the neutralizer, concentrated ammonium nitrate solution is produced which is further concentrated in vacuum concentrators.

In ammonium nitrate production, the concentration is carried out up to molten nitrate which is then sprayed from a prilling tower against an upward stream of air to produce prilled ammonium nitrate. In the case of calcium ammonium nitrate (CAN), the concentrated liquor is pumped and sprayed into the granulator which is also fed with a measured quantity of limestone powder and recycle fines. The hot granules are dried, screened, cooled and coated with soapstone dust in a coating drum stored.

f. Nitric Acid and Sulphuric Acid Production:

In the industries where ammonium nitrate and ammonium sulphate are produced, nitric acid and sulphuric acid production plants are also installed. Sulphuric acid and nitric acid are also required for the production of phosphatic fertilizers. Nitric acid is produced by oxidation of ammonia over a noble matal catalyst and absorbing in water.

Sulphuric acid is normally produced by burning sulphur to form sulphur dioxide which is then oxidized to sulphur trioxide over vanadium catalyst; sulphur trioxide is then absorbed in concentrated sulphuric acid.

g. Ammonium Chloride Production:

Ammonium chloride is normally obtained as a byproduct in the production of soda ash. Sodium chloride is reacted with ammonium bicarbonate producing ammonium chloride and sodium bicarbonate. The ammonium chloride solution is filtered, evaporated and crystallized. Ammonium chloride is also manufactured by direct neutralization of ammonia with hydrochloric acid gas.

2. Phosphatic Fertilizer Industry:

a. Phosphoric Acid:

In the manufacture of phosphatic fertilizers, the production of phosphoric acid is the basic building block. The first step involved in phosphoric acid production is grinding of rock phosphate. Ground rock phosphate is mixed with sulphuric acid after the acid has first been diluted with water to 55 to 70 percent sulphuric acid concentration.

The acidulated rock is digested and retained for several hours in attack vessels. The rock phosphate is converted into gypsum and phosphoric acid vessels. Some of the fluorine contained in the rock phosphate is evolved from the attack vessels as silicon tetrafluoride and hydrofluoric acid. Both silicon fluorine and P₂O₅ remains along with the byproduct gypsum which poses disposal problems.

After the reaction in the digester, the mixture of phosphoric acid and gypsum is pumped to filter where gypsum is separated from phosphoric acid. Dilute phosphoric acid, thus produced is further concentrated to 40 to 54 percent phosphorus pentoxide under reduced pressure. During concentration, the evolved fluorine together with minor quantities of phosphoric acid passes to the barometric condensers and these contaminate the condenser water.

b. Single Superphosphate:

Single superphosphate is produce by the reaction of sulphuric acid with ground rock phosphate. After reaction, the mixture is transferred to a den where sufficient retention time is provided for solidification. At the end, it is taken to storage for curing.

c. Triple Superphosphate:

Ground rock phosphate and phosphoric acid are mixed in a tank with agitation. After reaction the slurry is distributed on to the recycled dry product. It is dried in rotary driers and sized in vibrating screens before storage.

d. Ammonium Phosphated:

Two primary raw materials for the production of ammonium phosphated are ammonia and phosphoric acid. Different grades of ammonium phosphate vary only in the nitrogen and phosphate contents.

Therefore, by controlling the degree of ammoniation during the neutralization of phosphoric acid, different grades of ammonium phosphate can be obtained; Ammonia is reacted with phosphoric acid in vertical cylindrical vessels with or without agitation. The resultant slurry is then distributed on to dry recycled product. The product is them discharged into rotary driers from where it passes to storage.

e. Nitrophosphated:

Nitric acid acidulation differs from sulphuric acid acidulation in that phosphoric acid is not separated as a product from the acidulation reaction mixture. Nitric acid and rock phosphate are mixed in a series of reaction vessels with agitation.

In the first few vessels, the reaction products – calcium nitrate and phosphoric or sulphuric acid is added together with ammonia to produce a specific mix of calcium compounds, ammonium nitrate and phosphoric acid. This then converted into a dry product.

Sources, Volume and Characteristics of Effluents:

1. Nitrogen Fertilizer Industry (Sources of Effluents):

a. Ammonia Plant:

From raw material handling, storage and preparation sections normally a small stream of effluent containing mainly some coal dust, fuel oil or naphtha is discharged, depending on the feed stock used.

Where coal is used as feedstock, a considerable quantity of quenched ash is discharged continuously from the coal gasification section. The ash slurry from the direct scrubber recirculating water settling system containing some cyanides is also discharged to the ash pond.

When naphtha is used as feed stock, the effluents from the oil gasification section and carbon recycle section contain high concentration of oil, in addition to the carbon particles and sulphide impurities. Catalytic steam reformation process is mostly adopted when naphtha is used as feedstock. No liquid effluents are produced in this process.

In the partial oxidation process finely divided carbon is produced. Some built-in facility in the plant exits for recycle and reuse of this carbon in the process itself, but due to unforeseen accidental failure of the system, some carbon slurry may be discharged for a short period. This carbon slurry may also contain some cyanides and sulphides.

Depending on the absorbent used for the purification of raw gas, some toxic chemicals, namely, arsenic, MEA, vanadium, methanol and some alkali are discharged in a small stream.

b. From the CO:

Conversion unit, some quantity of condensate containing ammonia and catalyst dust is discharged.

During the commissioning of the plant and initial start-up some quantity of ammonia is discharged when the catalyst reduction operation is carried out. Normally, this effluent emanated once every 2 to 3 years.

From the ammonia synthesis section, a stream of condensate containing oil is discharged.

Some effluent containing ammonia is sometimes discharged from the storage and recovery sections of some plants.

A continuous purge from recirculating cooling water is discharged which contains conditioning chemicals and biocides.

c. Urea Plant:

From the carbon dioxide compression section some effluent containing oil is discharged.

Considerable quantities of ammonia and urea are discharged continuously along with the vacuum condensate. In modern urea plants, the quantities of ammonia and urea discharged has been reduced appreciably be process modification. When urea solution is concentrated at atmospheric pressure, no liquid effluent is produced in the urea plant, as no barometric condenser is needed for vacuum generation.

Some urea and ammonia are occasionally discharged which originate from spillage, leakage of glands, flanges, joints, etc., floor washings and also from draining during shutdown and startup of plants. In modern plants these discharges are collected and recycled.

A stream of cooling water containing conditioning chemicals and biocides is discharged from the cooling tower continuously.

d. Ammonium Nitrate arid Calcium Ammonium Nitrate Plant:

The scrubber liquor from the neutralization section contains ammonia and nitric acid which may or may not be recycled.

Some ammonium nitrate is discharged from the vacuum concentration section.

Occasional spillage and leakage from process may give rise to an effluent containing ammonium nitrate.

The cooling water blow-down containing some conditioning chemicals and biocides is discharged continuously.

e. Ammonium Sulphate Plant:

From the reaction and filtration section of the gypsum process, some effluents are discharged which contain ammonium sulphate, ammonia, chalk, etc.

Where direct neutralization is done, a small quantity of ammonia may be released in the effluent.

From the concentration, evaporation and crystallization section, an effluent containing ammonium and ammonia sulphate is discharged.

Spillage and leakage also form another effluent stream effluent containing mainly ammonium sulphate.

Cooling tower blow down containing conditioning chemicals and biocides is discharged continuously.

f. Ammonium Choride Plant:

The effluents are mixed up with soda ash plant effluent and contain ammonia and ammonium chloride. However, this effluent is discharged in a limited quantity.

In the direct process, the main effluent is the wash water used to wash the gases before they are let out. This will be of considerable volume and will contain in ammonia.

2. Phosphatic Fertilizer Industry (Sources of Effluents):

a. Phosphoric Acid Plant:

During the digestion of rock phosphate with acid, silica, fluorine and other impurities present in it are evolved as silicon fluoride, hydrofluoric acid, dust, etc. These off-gases are scrubbed with water. A part of the scrubber liquor is discharged continuously.

In the phosphoric acid concentration section, fluorine together with minor quantity of phosphoric acid passes to the barometric condenser. The condenser discharged contains 2 to 3 percent H₂SiF₆.

From the gypsum filtration section also, some quantity of effluent is discharged which contains suspended matter, phosphorus pentoxide and fluorine.

Normally, the gypsum obtained as a by-product is collected in a pond; the overflow from this pond contains suspended matter, phosphate, fluorine, etc.

Single Superphosphate – During the production of single superphosphate, dust, fluorine, phosphate bearing waste water is discharged from scrubbers of the digestion section and scrubber liquor of the exit off-gases from the dens.

b. Triple Superphosphate:

In the manufacture of triple superphosphate dust, fluorine, phosphate bearing off-gases from the reaction ressels, granulator and dryer are scrubbed with water. A part of this scrubber liquor is put into the reactor.

c. Ammonium Phosphate:

The main effluent normally discharged from ammonium phosphate plant contains ammonia, phosphate, fluorine etc. The contaminates indicated above are evolved during the neutralization reaction, and granulation, drying and sizing operations. These off-gases are scrubbed with phosphoric acid and the entire scrubber liquor is put into the reactors.

d. Nitro-Phosphate:

In nitro-phosphate production also, dust, fluorine, phosphate, ammonia, etc., containing off-gases from digestion and ammoniation section and also from drying, granulation and sizing section are scrubbed with water for reduction of the pollutants in the emissions of nitro-phosphate plant. A portion of the scrubber liquor is discharged as effluent continuously.

During the process of manufacture of phosphoric acid and phosphatic fertilizers considerable quantity of recirculating, cooling water is used. A continuous stream of cooling water blow down containing conditioning chemicals and biocides is discharged from the cooling towers.

e. Sulphuric Acid Plants:

When there are leakages, the cooling water gets contaminated with sulphuric acid.

f. Nitric Acid Plant:

When there are leakages, the cooling water gets contaminated with nitric acid.

3. Quantity of Effluent:

The total quantity of finally treated effluent 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 effluents.

While a phosphatic fertilizer plant with recirculating cooling water system and auxiliary facilities and having a production capacity of about 100 tonnes of P₂O₅ per day as fertilizer generally discharges 3000 to 6000 m³/day effluents.

4. Characteristics of Effluents:

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;

(d) Oil;

(e) Arsenic, ME A and methanol;

(f) Nitrates;

(g) Urea;

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

(i) Cyanides and sulphides;

(J) Biochemical oxygen demand;

(k) Fluorides; and

(l) Phosphates, etc.

Nitrogenous Fertilizer:

Typical ranges of contaminant concentrations from various operations are given below:

a. Cooling Tower Blow Down:

b. Water Treatment Plant:

The effluents from the water treatment plant of a nitrogenous fertilizer varies from 380 1/tonne of urea to 2000 1/tonne of urea, depending upon the quantity of raw water used. The dominants contaminants in a water treatment plant effluent 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. Besides these, when a process condensate is treated for use as boiler feed water, ammonia finds its way into the water treatment plant effluent.

c. Boiler Blow-Down:

d. Ammonia Plant:

e. Urea Plant:

f. Phosphatic Fertilizer:

Typical ranges of contaminant concentrations from various operations are given below:

g. Cooling Tower Blowdown:

h. Bioler Blowdown:

i. Superphosphate Plant:

j. Blending Unit:

Methods of Treatment, Utilization and Disposal:

1. General:

In the preparation of any scheme of treatment for effluents it is essential that source of effluent be studied regarding its flow over a 24-hour period for several days and the maximum, minimum and average flow be ascertained. Installation of flow meter or weir of continuous recording type is useful. Otherwise, readings of flow have to be recorded at hourly intervals normally.

In the case no measurement device can be installed, the effluents should flow to a holding tank where the level has to be recorded hourly. While, locating the source of effluent, due consideration should be given to occasional discharges due to leakage and floor washings, etc., and also the effluents which may be discharged during manufacturing of the plants and during the start-up or shutdown of the plant.

Each effluent source has to be analysed individually over a 24-hours period with samples drawn hourly. The samples may be collected hourly and made into 3 to 6 composite samples, depending on the variation of flow and composition.

2. Segregation of Effluents:

The effluents streams have to be segregated according to the nature of pollutants present in them and their concentration. As a general practice, all effluents containing high concentration of total ammonia nitrogen should be combined. Normally effluent containing ammonia nitrogen above 100 mg/l should fall in this category. However, effluents with 50 to 100 mg/l ammonia nitrogen may also be collected in this stream if the volume is large.

The following steps should be followed, wherever applicable:

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

(b) Oil bearing effluents should be combined as for as possible;

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

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

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

(f) Ash slurry should be separated from the rest of the effluents;

(g) Effluents containing carbon slurry should be stored separately;

(h) Arsenic and cyanide bearing effluents should be stored separately;

(i) Effluents containing fluorides and phosphates are to be segregated from other effluents;

(j) Sewage effluents should be treated separately as far as possible and

(k) 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.

After assessment of the individual effluent streams regarding their volume, pollutant content, frequency of discharge etc., the volume and concentration of various pollutants in the final effluent discharged beyond the factory boundary limit have to be ascertained.

These figures along with the prevailing standard of the effluents and the receiving water and also the local regulation will indicate the degree of specific type of treatment of the individual segregated effluents that will be necessary for adoption for treatment of the effluent.

Accordingly, various methods of treatment available are to be studied to suit the requirements for individual pollutants. Once the treatments for the pollutants are finalized, a broad scheme is developed and in the same scheme integration of all the treated effluents is made.

While studying the different treatment schemes, preference should always be given to such schemes where some recovery of waste products for reuse in the process of recovery for direct marketing can be made from the wastes. Sometimes the effluent water after adequate treatment can be recycled in the process. This reduces water consumption as well as the final effluent volume discharged.

Sometimes the segregated effluents can be combined in such a way that one can be utilized for the treatment of the other. This type of judicious combination reduces the cost of chemicals and also increases the efficiency of treatment rendered.

The various processes available at present for the treatment of individual pollutant parameters relevant to the fertilizer industry have been compiled below for study before final adoption according to the suitability of a particular process depending on the degree of treatment considered necessary.

3. Treatment of Effluents for Specific Pollutants:

Ammonia Nitrogen:

Various processes have been developed for removal/recovery of ammonia nitrogen from effluents.

These processes basically fall in two categories:

(a) Physico-chemical, and

(b) Biological.

Physico-Chemical Processes:

(a) Air Stripping:

The concentration of ammonia nitrogen in effluent can be reduced considerably by adopting air stripping of ammonia from the effluent at an elevated pH value.

Ammonium ions (NH₄₊) in water exist NH₃, as follows:

NH₄ + NH

At pH level above 7.0 the equilibrium is shifted progressively towards the right, so that ammonia is liberated as gas. This dissolved gaseous ammonia in the effluent is stripped off by flowing air through the effluent.

In actual operation, the pH, of the waste is brought to a pH level between 10.0 and 11.0 by adding alkali; the waste is then pumped to the top of the cooling tower type packed tower and distributed evenly to cover the full surface of the packing. The waste water moves down through the packing counter-current with the air flow.

The tower for ammonia stripping may be either cross flow or counter flow type with induced or forced air circulation. The ammonia present in the waste water is stripped off before it leaves at the bottom of the tower.

The extent of ammonia removal depends on many factors of which pH, temperature, ammonia concentration, contact time with air and water-air-water ratio, etc., are very important and these factors are to be considered adequately while designing an air stripper for ammonia removal. In a well-designed plant, the concentration of ammonia in the effluent can be reduced to 50 mg/l adopting this process.

(b) Steam Stripping:

Steam stripping of ammonia is a well-established process. The process is adopted by the oke-oven industries for the recovery of by product ammonia. Here also stripping of ammonise from waste water depends on how the ammonia exists in the water. In neutral solution, ammonia does not exist as dissolved NH₃ gas at ambient temperature.

Therefore, the pH and the temperature are increased, so that the reaction proceeds progressively further to the right, namely, in favour of the formation of NH₃. In a suitably designed distillation unit, the ammonia can be stripped off by steam with or without raising the pH as the case may be and the resultant ammonia can be covered by condensing as dilute ammonia solution or as ammonium sulphate solution after neutralizing it with sulphuric acid. Under ideal operating conditions, 90 to 99 percent ammonia removal efficiency can be obtained.

(c) Ion Exchange:

Ion exchange is a unique effluent waste water treatment method. Ion exchange can accomplish purification of the waste water to a quality that could comply with zero pollutant discharge criteria or that would permit complete recycle of waste waters. The ion exchange process can also accomplish complete recovery of waste products being lost along with the recovered products into the plant processes.

This may be represented as follows:

When the recovery of ammonia by ion exchange is aimed at from ammoniacal waste waters and no recovery of waste water is envisaged, a simple process based on absorption of ammonium ion by hydrogen from of a cation exchanger is incorporated. The clarified ammoniacal waste water is passed through the exchange column where ammonium ion would be absorbed in the exchanger replacing hydrogen ion.

When the exchanger approaches exhaustion (indicated by residual ammonium ion in the treated effluent at the outlet of the exchanger), it is regenerated to the hydrogen from with a suitable concentration of sulphuric nitric acid.

The regeneration process is adopted to get minimum regenerate use and maximum concentration of product solution. The product ammonium sulphate or ammonium nitrate solution is concentrated and processed in the process plant for the production of fertilizer and the waste water with very low concentration of ammonia is neutralized before discharged along with the other effluent streams.

When the recovery of waste water is also envisaged, in addition to a cation exchanger, an anion exchanger is incorporated. The unit can be used for the treatment of waste waters containing both ammonium ions and other acidic ions. The ammonium salt contaminated waste water after proper clarification first flows through a bed of strongly acidic cation, resin operating in the hydrogen form.

The ammonium ion combines with the cation, while the hydrogen ion combines with the nitrate/sulphate ion to from nitric/sulphuric acid. The acidic water then passes through the bed of anion resin in base form where the acidic ions are absorbed. The effluent water from the second bed is very low in ammonium salts, and can be reused in the process as make up water in boiler feed water treatment plant and may be used in the boilers after polishing in mixed bed ion exchange system.

The cation exchange resin holding the ammonium ion can be regenerated using sulphuric or nitric acid to form ammonium sulphate or nitrate solution. The anion resin holding the acidic ion is regenerated using a solution of ammonium hydroxide to form more ammonium sulphate or nitrate solution.

The ammonium salt solution thus produced may be used in the process for the production of ammonium sulphate or nitrate, provided such facilities are available at site. It may be noted that soluble inorganic contaminants in the waste water will also find their way into the product.

Biological Processes:

(a) Biological Nitrification and Denitrification:

Biological nitrification and denitrification can reduce ammoniacal nitrogen content of the final effluent to a very low level. This process is being adopted in municipal waste treatment for years. In the treatment of industrial waste, this treatment may be adopted as a secondary or tertiary treatment where the ammonia nitrogen content of the influent is comparatively nitrogen is desired.

The treatment is based on the reaction of ammonia nitrogen with oxygen in aerated pond or lagoon to form nitrites and finally to the nitrate nitrogen form in the presence of a specialized group of nitrifying organisms.

The nitrate in turn reacted in another anaerobic pond in the presence of biodegradable carbon compound employing may be represented as follows:

The first step nitrification takes place in the presence of aerobic, bacteria which converts the ammonia nitrogen into nitrates. This reaction is affected by degree of aeration, water temperature, initial ammonia nitrogen content, bacterial population, pH of solution, etc.

As destruction of alkalinity is associated with the reaction, sufficient alkalinity should be present in the waste in the nitrification tank; otherwise alkalinity should be supplemented to the waste water.

Similar supplementation may be required for other bacterial nutrients like phosphate, potassium, magnesium, iron, etc. if these are not originally present adequately in the waste water. This step can be carried out in tank, pond, lagoon, trickling filter, etc.

The denitrification step is an anaerobic process which occurs when the biological micro-organisms cause the nitrates and the organic carbon to be broken down into nitrogen gas and carbon dioxide.

As the organisms responsible for denitrification can utilize only organic carbon as their carbon source, a supplement of a readily biodegradable soluble organic compound is required to be added to the nitrified effluent prior to its entry into the denitrification unit. The organic carbon used for such a process is methanol, sewage effluent of organic waste from industries.

In case methanol is used as the organic carbon source, 2 to 2.5g of methanol is required for denitrification of nitrate nitrogen. This reaction is carried out in a tank, pond or lagoon under anaerobic conditions. The reaction requires very low or nil dissolved oxygen in the effluent, neutral pH range, proper supply of organic carbon, suitable detention time, etc.

(b) Algal Uptake:

Since ammonia nitrogen is an algal nutrient, algae are capable of extracting this nutrient from the waste water. Algae growing in waste water stabilization ponds utilize ammonia nitrogen of the waste water to form cell tissue in the presence of sunlight. Adequate carbon dioxide and some nutrients are also required in this process. For fixing up 1 g of nitrogen into algal cell material 10 to 12g of carbon as carbon dioxide gas is normally required.

(c) Oxidation Pond:

Like ponds may be used for the culture of algae. Carbon dioxide may be supplied by biodegradation of organic matter through a network of carbon dioxide diffusers in the pond. Other necessary nutrients for algal cultures may be supplemented in the pond.

With suitable detention time, depth of the pond, concentration of algae, concentration of ammonia nitrogen, sunlight, etc., the uptake of ammonia nitrogen in the cell formation of algal cells is quite appreciable. Algae thus produced may be harvested using a suitable process and utilised as manure.

(d) Urea and Nitrate Nitrogen:

In modern urea manufacturing technology, thermal urea hydrolysis with recovery of ammonia of the waste water is being incorporated in the plant itself. This system, if provided, is expected to reduce the quantity of urea in the effluent appreciably. The use of hydrolyser stripper should be considered as an alternate arrangement.

Urea nitrogen can be removed from effluents by hydrolyzing urea 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.

NH₂CONH₂ + 2 H₂O → (NH₄)₂CO₃

The pH increases with the progress of hydrolysis; under properly maintained conditions, over 95 percent of urea can be hydrolyzed in 24 h. The hydrolyzed solution containing ammonia can be treated by any of the methods described under ammonia removal.

(e) Suspended Solids:

Suspended solids originate from various sources in the fertilizer industry. The process water clarification plant suldge, ash slurry from coal gasification plants, steam generation plants or phosphoric acid plant effluent during neutralization of effluent. These effluents containing suspended solids are settled in a suitably designed settling basin 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 wastes as required.

(f) pH:

Sometimes the effluents are highly acidic or alkaline in nature. When both acidic and alkaline waste water are found, they may be mixed suitable for neutralization. Otherwise for neutralization of acidic effluent, lime or soda ash may be used and for neutralization of alkaline effluent sulpnuric 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.

(g) Oils and Greases:

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

(h) Arsenic:

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

The quantity of this arsenical solution can be controlled within reasonable limits by good housekeeping. The arsenic solution which is discharged even after taking all the precautions is completely separated from other waste waters.

The waste water containing arsenic is then filters, 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 the deep sea far away from the coastline.

(i) Chromate and Phosphate:

Fertilizer industry requires a high 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. Various inhibitors are used depending on the local conditions.

Most of the plants use combinations of chromate, phosphate and zinc in different proportions. Normally, zinc is used in very low concentration, therefore, any specific treatment for the removal of zinc is not considered necessary. In the treatment for removal of chromate from waste water, phosphate is also simultaneously removed, so specific treatment for removal of phosphates 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. 17.10).

The cooling tower blowdown which contains chromate is collected in a tank and the pH of the water is lowered to the range 2 to 4 by adding sulphuric acid. After mixing with the acid, ferrous sulphate, sodium sulphite, sodium metabisulphite of sulphur dioxide is added to reduce hexavalent chromium.

For removal of 1g of CrO₄ about 10 g of ferrous sulphate, 2.5 g of sodium sulphite of 1.5 g of sulphur dioxide is required. After reduction, lime is added to the effluent for raising the pH and precipitation of chromium. The settled effluent is allowed to be discharged along with other effluents of the factory.

The reactions which take place during the above operations are as follows:

Reduction of Chromate:

1. When ferrous sulphate is used for reduction:

Na₂Cr₂O₇+ 6FeSO₄ + 7H₂SO₄ – – – Cr₂(SO₄)₃ + 3Fe₂ (SO₄)₃ + 7H₂O + Na₂SO₄

2. When sulphur used for reduction:

Na₂Cr₂O₇ + 3Na₂SO₃ + 4H₂SO₄ – – – 4Na₂SO₄ + Cr₂(SO₄)₃ + 4H₂O

3. When sulphur dioxide is used for reduction:

Na₂Cr₂O₇ + 3SO₂ + H₂SO₄ – – – Cr₂(SO₄)₃ + H₂O + Na₂SO₄

Precipitation with Lime:

Cr(SO₄)₃ + 3Ca(OH)₂ – – 2Cr(OH)₃ + 3CaSO₄

Fe₂(SO₄)₃ + 3Ca(OH)₂ – – 2Fe(OH)₃ + 3CaSO₄

Lime treatment for precipitation of chromium also partially precipitates out phosphate which is added to the cooling towers as sodium hexametaphosphate.

Recently, another ion process based on reduction with ferrous ion provided by electrolysis using iron electrode has been developed. This process can operate at pH 6 to 8. The chromium hydroxide and iron hydroxide are precipitated together and can be separated as a sludge by clarification. It consumes only electricity and metallic iron.

Many fertilizer units use furnace oils containing about 4 percent sulphur in their boilers; the boiler stack contains around 0.2 percent SO₂ which is a reducing agent.

The chemistry of the process is:

Cr₂O_– + 3SO₂ + 2H⁺ – – – 2Cr ^{+ + +} + 3SO₄ – – + H₂O

The reaction takes place quite rapidly at low pH (2 to 3). The SO₃ present in the flue gas helps in attaining the low pH of this order; under this condition even a small percentage of SO₂, is able to reduce the hexavalent chromium. In this arrangement the problem of air contamination is also reduced due to utilization of SO₂ and SO₃

The resulting trivalent chromium as chromium sulphate is much less toxic. To fully overcome the toxicity problem, it is necessary to convert soluble chromium sulphate into chromium hydroxide at pH 10 to 11 through the addition of alkali as suggested in 3.7.

However, to further reduce the cost of disposal, the ammonia containing waste water itself may be utilizes as an alkali to bring about the precipitation of chromium hydroxide. Effluents from fertilizer plants happen to be rich in plant nutrients and can be a secondary source of fertilizer.

These effluents can, therefore, after suitable treatment, be applied on land for irrigation with the prior permission of local authorities. Experiments have shown that the effluents from fertilizer plants can be usefully employed to raise various crops and vegetables due to their high nitrogen and phosphorus contents.

Sampling and Analytical Control:

In order to observe the performance of the effluent treatment units and also to control the plant operating system effectively, suitable instrumentation for recording pollutants and other physical characteristics (namely temperature, pressure, flow of effluent, quantity of treatment chemicals, etc.) are required to be incorporated into the effluent treatment process design, so that input and output conditions of effluent treatment units can be assessed properly.

Where suitable automatic continuous monitoring of pollutants in the effluents cannot be provided, regular sampling and analysis of the pollutants in the effluents cannot be provided, regular sampling and analysis of the pollutants necessary for the control of operation are to be conducted. In such a case, the frequency of sampling and analysis will depend on the process plant operating conditions but a minimum of two composite samples should be analysed daily.

In the case of final effluent discharged beyond the factory boundary limit, a suitable arrangement for recording the volume and proper sampling of the final effluent is to be made. Installation of an automatic pollutant monitoring and recording system for final effluent of the factory is very advantageous and an endeavour should be made to install these instruments wherever possible.

Similarly, a composite sample of the receiving water should also be analysed daily. In case some other industries are located on the upstream of the river and they also discharge some effluents to the same river. Sampling and analysis of the receiving water should be done, both from the upstream and downstream of the effluent outfall. This will indicate the contribution to pollution by the fertilizer industry concerned.

Waste Utilization:

Apart from the utilization of waste waters and reuse of treated effluents for conservation of water as well as for other purposes, recovery of usable products present in this waste water has gained importance in recent days. The main recoverable products from waste water of fertilizer industries are ammonia, urea, carbon, fluoride, gypsum, phosphate, chalk, etc., depending on the product manufactured and the process adopted.

Ammonia:

The processes commonly used for the recovery of ammonia from ammoniacal waste waters are steam stripping and ion exchange system. Steam stripping of ammonia is suitable for ammoniacal effluent containing high concentration of ammonia with comparatively low volume. The stripped ammonia gas is either absorbed in acid to form ammonium salts or condensed to form ammonia liquor which is recycled in the process itself.

In the case of ammoniacal waste waters containing low concentration of ammonia, ammonia can be recovered using a cation exchange system regenerated with acid to produce ammonium salt solution. This process is more suitable where already a secondary ammonium sale manufacturing facility exist.

Urea:

In spite of improvement in the design of the urea manufacturing process, substantial amount of urea along with ammonia finds its way into the waste water of urea plant.

The different methods of recovery in the urea effluents are as follows:

(a) Thermal hydrolysis of urea present in the condensate followed by stripping of the ammonia produced and recycling of the ammonia in the urea process itself;

(b) Collecting of all spillages, leakages and overflows of urea bearing waste, concentrating and recycled them in the urea process; and

(c) Scrubbing urea dust from pilling tower exhaust vapours and recovering this urea as per process mentioned in (b) above.

In modern plants all or some of the above processed form an integral part of the urea plant itself and the effluent which comes out from urea plant contains practically a negligible quantity of urea. In older plants installation of the above facilities is difficult as it requires large investment.

Also, there are constraints in accommodating this additional load in the process. In any case installation of these facilities for the recovery of urea improves process efficiency by reducing the specific ammonia consumption. The cost of ammonia recovered by this process is enough to pay back the capital invested in a short time.

Carbon:

In the partial oxidation process of ammonia manufacture, the carbon formed in the process is normally thrown out as carbon slurry. This carbon can be recovered either by pelleting with a suitable petroleum distillate followed by filtration and drying.

The recovered carbon has very low particle diameter, large surface area, high covering power and adsorption capacity. It can be used as carbon black suitable for printing ink, rubber, battery and other industries. It can also be further processed into active carbon.

Flouride:

The effluents from phosphoric acid plants contain varying concentration of fluoride which pollutes the water course seriously if not removed prior to its discharge. Flouride is now recovered from the flouride bearing effluents by treating them with lime to recover calcium flouride with aluminium salts to recover aluminium flouride and with sodium salts to recover sodium flouride. Various processes are available for the recovery of fluorides that serve as raw material for the manufacture of a wide range of fluoride chemicals.

Gypsum:

Gypsum obtained as a byproduct during the production of phosphoric acid used to be dumped in low lying areas. This gypsum can be processed for various products like ammonium sulphate by Mersburg process, plaster boards, and building blocks; it can also be used for land reclamation and recovery of sulphur with simultaneous manufacture of cement.

Chalk:

The chalk is obtained as a byproduct in ammonium sulphate production using Merburg process utilising gypsum. This chalk is used as raw material in the manufacture of cement. It is also used to a large extent in neutralizing acidic effluents in industry.

Phosphate:

Substantial amounts of phosphates are present in waste waters of phosphatic industries; these are normally removed during the removal of fluorides. This phosphate can be used in phosphoric acid manufacture after blending with rich rock phosphate. The phosphate bearing can also be used as low nutrient value cheap fertilizer in some cases.

Disposal:

The final disposal of the treated effluents beyond the factory boundary limit is an important step. Normally, effluents originating from individual effluent treatment units are led to a mixing pond. The uncontaminated effluents which do not require any treatment also flow to this mixing pond. It is preferable to give sufficient detention time in this mixing pond for equalization and also to effect secondary settling of suspended matter.

The overflow from this final mixing pond passes to the effluent drain leading to the receiving waters. It may be clearly understood that the treated effluents in the effluent drain conform to IS: 2490 and therefore cannot normally be used as raw water source.

In case the drain passes through a locality where there is possibility of use of this water as raw water source by the inhabitants and cattle, suitable protection of the drain from the approach of the people and cattle with proper warnings has to be made. In some cases it is preferable to discharge the treated effluents through a pipe-line to the receiving water.

When all the characteristics of the individual effluent streams of the process plants are properly assessed, the effluents discharged from effluent treatment units also can be evaluated with respect to the extent treatment plants. The final effluent characteristics can be predicted and made to comply with the requirements prescribed by the regulatory authorities.