Most of the present-day complex fertilizer plants, which discharge effluents containing phosphate, fluorine and nitrogen, are based on modern technology, which has been designed on the basis of a proper heat balance and water balances, in which most of the effluents are recycled back into the system, and the very limited quantity which does emerge is suitably treated and, then, discharged.
This has become necessary with the very stringent pollution legislation, in most countries, today, which aims at reduction of the contaminants in effluents. Such reduction can only be achieved when the following requirements are fulfilled:
1. Recovery of nutrients in vapours and gases, from each process step, with subsequent recycling to the process.
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2. Recycling of water used for scrubbing and cleaning. To avoid build up and process upsets, when recycling the water, a water treatment unit must be installed.
Vapours and gases are acid or alkaline and must be treated according to their nature. Acid gases, i.e., gases containing nitric acid, normally with small quantities of fluorine, can be treated with clean water.
Ammoniacal gases, mainly from the neutralization and conversion section, can be absorbed in nitric acid or any other absorption media.
As for vapours from the evaporators, these contain fluorine and must be treated in a fluorine scrubber before they are treated with nitric acid to absorb the ammonia.
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Waste water is treated in a separate system, where P205is removed and returned to the digestion section.
In a complex fertilizer plant, producing about 1,000 tonnes per day of NP fertilizer of the composition NP 28-14-0, the system has been designed to meet the following effluent requirements:
1. Phosphate, kg/day as P =12
2. Nitrogen, tonne/day = 3.
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Such systems are, in operation, based on modern advances and, it is claimed, that this has reduced the P2O, effluent to 60 percent of the above limits, resulting in a P2O5, efficiency of about 99 percent, whereas the nitrogen removal has been reduced to about 50 percent of the above mentioned limits.
Choice of the Method of Treatment:
The choice of the method of treatment is determined by the conditions prevailing in each situation such as (1) The character of waste (2) Circumstances pertaining to the body of the water, being used for final disposal (3) State Health Department requirement (4) The cost of the plant and its operation. In general, waste treatment and disposal practices may be classified into three categories:
1. Disposal of untreated waste by dilution
2. Primary treatment, which removes part of the suspended, floating and suitable solids.
3. Secondary treatment (complete) usually proceeds by primary treatment, designed to provide some means of satisfying the oxygen demand (reduction of BOD). In this treatment further removal (after primary treatment) of solids is with the aid of living organisms.
This treatment is also known as biological treatment. Treatment Operations
Treatment operations are divided into three basic groups:
1. Physical
2. Chemical
3. Biological
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Physical Treatment:
It includes those operations which do not involve chemical changes in the waste.
Mechanical operations:
1. Screening (separation of course, suspended and floating matter) 2 Skimming or floatation (separation of grease and oil)
3. Sedimentation (separation of finely divided suspended matter)
4. Filtration (separation of finely divided suspended matter).
Physical operations:
1. Heating
2. Cooling
3. Boiling
4. Evaporation.
Chemical Treatment:
The treatment involves the addition of chemicals to the waste and is concerned with chemical changes or reactions in the constituents of the waste. Topical of chemical treatment processes are:
1. Neutralization of acid or alkalies
2. Precipitation of metal hydroxide
3. Oxidation with air, ozone, chlorine and other agents Biological Treatment. Separation of stabilization of purtrescible matter in suspension, colloidal state it includes
1. Aerobic biological treatment
2. Anaerobic biological treatment.
Aerobic treatment:
1. Lagooning
2. Biological sludge processes
3. Activated sludge process
4. Contact aerators
5. Oxidation ponds.
Anaerobic treatment:
It is usually thought of as sludge digestion, but it also includes the fermentation of certain organic waste.
Sludge Treatment:
1. Digestion
2. Concentration or thickening by stirring or centrifuging
3. Coagulation by heating, chemical treatment of freezing
4. Elutriation
5. Biological floatation (more or less experimental)
6. Air drying on sand beds
7. Filtration through cloth, under vacuum
8. Heat drying
9. Incineration.
Solid sludge can be disposed of without or with treatment. Sludge could be treated by the septic tank, Inhofe tank and separate sludge digester methods. Final disposal of treated sludge can be done by incineration, as land fill and as fertilizer. Similarly, liquid waste can be disposed of by an intermittent sand filter, trickling filter, contact bed, activated sludge process or chlorination methods.