BIOLOGICAL HYDROLYSIS OF UREA
IN A CONTINUOUS-FLOW STIRRED-TANK REACTOR
UNDER LABORATORY CONDITIONS-
A BENCH-SCALE STUDY
T. Chakrabarti, Professor
P. V. R. Subrahmanynam, Assistant Director
National Environmental Engineering Research Institute
Nagpur, India 440020
A number of nitrogenous and phosphatic fertilizer factories in India are trying to meet
the increasing demand of nitrogen and P205 for agricultural purposes. It is anticipated that
by 1984-85, the requirement will be 6.0 million tonnes of nitrogen and 2.3 million tonnes
of P205 of which 5.12 million tonnes of nitrogen and 1.3 million tonnes of P2Os will be
supplied by the country's existing fertilizer factories together with the fertilizer projects
which are currently under implementation [1]. In the next few years, 19 new fertilizer
plants (8 nitrogenous and 11 phosphatic) will be set up to meet the growing demand and reduce dependence on imports. The nitrogenous fertilizer industry is mainly concerned with
the production of urea, ammonium sulfate, ammonium nitrate, calcium ammonium nitrate
and ammonium chloride. Phosphatic fertilizer industries, on the other hand, manufacture
mainly single superphosphates, triple superphosphates, ammonium phosphates and nitro-
phosphates.
Urea is the major nitrogenous fertilizer used in India. Considerable quantities of urea and
ammonia find their way into the effluent as filter washings, vacuum condensates, spillages,
floor washings, leakages of pumps, glands, joints, etc. and scrubber washings. In addition
loss of urea occurs in the prilling tower. The wet scrubber, used following the dry cyclone
separator, also contributes significantly to the volume and strength of the waste from the
urea synthesis section.
In modern urea manufacturing plants the quantity of urea and ammonia in the wastewater is appreciably reduced by process modification, incorporation of thermal urea hydrolysis technology with means for recovery of ammonia present in the wastewater and by
collecting all spillages, leakages, floor washings and overflows of urea bearing waste, concentrating and recycling back in the urea process. In older plants, however, incorporation of
these facilities is difficult since it requires large investments.
Ammonia, in very small concentrations, is toxic to fish. Southgate [2] has reported that
1.2 to 3.0 mg/l of ammonia could be lethal to fish. The toxicity of ammonium salts such as
ammonium chloride, ammonium sulfate and ammonium nitrate is low in water having high
bicarbonate alkalinity but is very high in soft waters and under high pH conditions. Wastewater from a nitrogenous fertilizer unit is alkaline due to the presence of ammonia, caustic
soda, urea and potassium carbonate [3]. Indian Standard Institution (ISI) has prescribed a
tolerance limit of 50 mg/l of ammoniacal nitrogen in an industrial waste meant for discharge
into a surface water [4].
Urea may be tolerated by fish at higher concentration in rivers; the critical range being
16,000-30,000 mg/l in the Detroit River [5]. However under facultative anaerobic conditions, a small quantity of urea may create an extremely toxic condition because of its hydrolysis into free ammonia and carbon dioxide; free ammonia being solely responsible for
toxicity. In river water, biodegradation of urea is reported at a concentration of 1-15 mg/l;
the degradation at 20 C is complete within 4-6 days and is negligible below 8 C for up to 14
days[6].
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