United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-91/046 Jan. 1992
i&EPA Project Summary
Bench-Scale Evaluation of
Ammonia Removal from
Wastewater by Steam Stripping
G.B. Wickramanayake, DP. Evers, J.A. Kittel, and A. Gavaskar
The purpose of the study summa-
rized here was to generate laboratory
data to support the development of
wastewater discharge standards for am-
monia in nonferrous metal winning pro-
cesses. Ammonia removal effected by
steam stripping from synthetically com-
pounded "wastewater" samples was
studied to determine the importance of
factors affecting the rate of removal
and the degree of removal.
.The analyses of estimated Henry's
Law constant and changes in ammonia
solubilities indicated that adding caus-
tic, as compared with adding slaked
lime, resulted in higher Henry's Law
constants and lower solubilities for the
three waste streams studied. Although
no significant variation of mass trans-
fer coefficient (K) was observed when
SO4- concentrations were varied from
5,000 to 20,000 mg/L, K was the high-
est for low SO4- wastewaters when pH
was adjusted with NaOH.
Results of the steam stripping study
indicated that varying chemical con-
stituents such as SO - and the molal
strength did not significantly affect the
efficjency of ammonia removal. Pre-
heating wastewater and operating the
stripping tower at high steam-to-waste-
water flowrate ratios such as 4 Ib/gal
achieved higher removals (99.9% or
more). Based on engineering unit pro-
cess and operation requirements, the
cost analysis indicated that lime may
be more economical than caustic for
pH adjustment depending on waste
sludge characteristics and disposal re-
quirement.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that Is fully documented In a separate
report of the same title (see Project
Report ordering Information at back).
Introduction
Extracting metal values from some ores
requires the use of hydrometallurgical tech-
niques that employ ammoniacal lixiviants.
The metal values are recovered from the
pregnant liquors, leaving an aqueous
wastewater residual high in dissolved sol-
ids and ammonia. This wastewater re-
quires treatment for removal of ammonia
as well as dissolved solids to meet the
discharge standards required under the
Effluent Guidelines for the nonferrous met-
als industry.
The practical methods of removing am-
monia-nitrogen from wastewaters include
biological nitrification-denitrification,
breakpoint chlorination, evaporation, re-
verse osmosis, ion exchange, air strip-
ping, and steam stripping. The purpose
of this study was to determine the useful-
ness of steam stripping for the removal of
ammonia from wastewaters generated in
the metal smelting and refining industry.
Approach
The overall objective was pursued as
three discrete tasks. Task 1 involved theo-
retical and laboratory studies to determine
the effects of wastewater composition on
the equilibrium of ammonia (gas) and on
water and gas-liquid mass transfer rates,
especially within the temperature range of
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interest in actual plant operations. In Task
2, laboratory tests were performed in a
bench-scale steam stripping apparatus pro-
cessing two representative synthetic
wastewaters with pH adjustment by either
lime or caustic. Task 3 involved the esti-
mation of the capital and operating costs
associated with the pH adjustment meth-
ods and the handling of the waste sludge
subsequently formed. Disposal costs were
not included in this analysis.
Methods
One objective of the first task was to
investigate how the distribution of inor-
ganic species and changes in tempera-
ture affect the equilibrium of ammonia (gas)
and the wastewaters. Effects of different
electrolytes such as Na*. Mg« Ca«, SO -
and Cl- on the Henry's Law constant (H9)
were evaluated. This theoretical study was
based on information given in the pub-
lished literature.
The second aspect of this task involved
experiments to study the effect of varying
SO." concentration, molal strength, and
pH adjustment method [NaOH or Ca(OH)2]
on the gas transfer rates for ammonia.
Synthetic wastewater stream A (Table 1)
was designed to have a high SO^ level
and molal strength. Waste stream B has a
lower SO/ level than A, whereas both A
and C have the "same SO/ level. Waste
streams B and C have the same molal
strength and were brought to such condi-
tions by adjusting Cl~ concentrations. The
experiments were conducted in identical,
completely mixed batch reactors at tem-
peratures near 90°C. The variation of
ammonia concentration with time was stud-
ied after adjusting solution pH to 12 with
NaOH or Ca(OH)2.
During Task 2, a laboratory-scale steam
stripping unit was designed and con-
structed (Figure 1). The pH of two differ-
ent synthetic waste streams, B and C
(Table 1), was adjusted to 11.5 or greater
with either NaOH or CaO. Ammonia re-
moval was studied for the different pH
adjustment methods and different steam-
to-wastewater flowrates.
The system was designed to pump a
high pH synthetic wastewater influent to
the top of the packing material in the col-
umn casing. As shown in Figure 1, the
wastewater flowed down over the packing
material and exited through the effluent
line at the column base. Steam was in-
jected into the column at the base of the
packing material. As the wastewater
passed through the jet of steam, it was
heated to 100°C. Flowrates for the steam
and/or the influent wastewater were var-
ied to determine the differences in ammo-
nia removal efficiency. The steam flowrate
was measured by condensing steam en-
tering the column and measuring changes
in the flowrate of column effluent. The
steam flow entering the column was con-
trolled using a valve/orifice unit.
A total of nine steam stripping experi-
ments were performed to explore the ef-
fects of sulfate concentration and total
molal strength and to compare the use of
NaOH (caustic) or CaO (lime) for pH ad-
justment of the solutions. The extent of
ammonia removal was studied for differ-
ent steam-to-wastewater flowrates.
In Task 3, the costs for chemical addi-
tion, sludge removal, and thickening were
estimated for four different representative
waste streams and for a variety of waste-
water flowrates. Included in these esti-
mates were costs for mixing facilities,
chemicals, and sludge removal concen-
trations, handling, and transport. Not in-
cluded were costs for chemical storage
facilities, buildings, land, or sludge dis-
posal—all of which are likely to vary con-
siderably from site to site.
Results and Discussion
Taskl: Theoretical and
Laboratory Studies on the
Equilibrium and Mass Transfer
of Ammonia in Wastewater
The analyses of the estimated Henry's
Law constant and changes in solubilities
of ammonia indicated that adding caustic,
as compared with adding slaked lime, re-
suited in higher Henry's Law constants
Tabls 1. Wastewater Characteristics for Gas Transfer Rate Experiments
Concentration. mg/L
Chemical Species
Wastewater A
Wastewater B
Wastewater C
NH3-N
Mg"
A/a-
SO/
ci-
5,000
200
19,127
20,000
28,000
5,000
200
11,939
5,000
28,000
5,000
200
11,939
20,000
16,906
and lower solubilities for the three waste
streams considered. These effects can
be attributed to the relatively high ionic
strength found in wastewaters when pH
was adjusted with NaOH. For solutions B
and C, where the sulfate concentration
was varied without changing the solution
strength (total number of moles), no sig-
nificant difference in Henry's Law con-
stant or solubility could be seen for either
of the two pH adjustment methods. The
highest Henry's Law constant and corre-
sponding lowest ammonia solubility were
observed in solution A, which had the
highest molal strength.
Experimental studies to determine the
effects of dissolved species and pH ad-
justment method on the mass transfer co-
efficient (K) indicated that those effects
were relatively low and the' maximum
changes in mass transfer coefficient did
not exceed 25%. In two of the three
different solutions, however, the mass
transfer coefficient was higher for the waste
sream where pH was adjusted using caus-
tic. For solutions with approximately the
same SO/ concentration but different mo-
lal strength, the mass transfer coefficients
were comparable when NaOH was used
as the pH adjustment method. For
Ca(OH)2, however, the K values were com-
parable for those where both the molal
strength and SO / level were different.
The overall analysis of data indicated that,
when compared with lime, addition of
NaOH promoted ammonia removal for so-
lutions with relatively low SO- levels (5,000
mg/L).
Task 2: Ammonia Removal
Studies Using Steam Stripping
Unit
The results of the steam stripping study
are summarized in Table 2. The data
show that when the steam-to-wastewater
flowrates were low (1.3 Ib/gal), the ammo-
nia removal efficiency was as low as 93%.
By increasing steam-to-wastewater
flowrates to 3.8 Ib/gal, removals of more
than 99.9% were observed. The removal
efficiency in one study was improved by 2
percentage points when the temperature
of the influent waste stream was raised by
about 20°C. The addition of lime increased
the temperature more than 10°C because
hydration of lime is an exothermic pro-
cess.
For wastewater with low initial SO/
level (5,000 mg/L), ammonia removal was
3 percentage points higher when pH was
adjusted with caustic rather than with lime.
These observations agreed with the trends
predicted from the estimated Henry's Law
constant and mass transfer coefficient. In
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Heated v
Wastewater In
Mixing
Device
Filter
0
Synthetic
Wastewater
100 gal. Wastewater
Tank
Flowmeter
Steam Out
Steam
Stripping
Column
Packing Materials
Steam In
Water
Pump
Wastewater
Out
Orifice
T= Thermocouple
P= Pressure Gauge
V= Valve
Figure 1. Experimental setup for ammonia removal by steam stripping.
Table 2. Summary Results of Laboratory Study on Steam Stripping
Wastewater Temp. °C
Wastewater
B (low SO"4)
C(high SOJ
pH
Adjustment
Method
NaOH
NaOH
CaO
CaO
NaOH
NaOH
NaOH
CaO
CaO
Column
Influent
26
26
38
39
26
26
47
39
39
Steam to
Column Wastewater Ratio
Effluent (Ib/gal)
101
101
101
101
101
101
101
101
101
1.9
1.3
1.9
3.8
1.9
3.8
1.9
1.9
3.8
NH3 - N cone., mg/L
Influent
5,200
5,200
5,100
5,000
4,750
4,700
3,950
3,950
3,825
Effluent
32-102
255-420
188-288
12-16
121-198
3.9-4.5
2.0-80
87-92
1.1-2.2
Average NH3
Removal, %
99.1
93.1
95.3
99.7
96.9
99.91
98.8
97.7
99.96
•&U.S. GOVERNMENT PRINTING OFFICE: 1992 - 648-080/40129
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the experiments conducted with wastewa-
ters using higher inhial SO/ levels (20,000
mg/L), ammonia removal was slightly
higher when pH was adjusted with lime
instead of w'rth caustic. These observa-
tions agreed w'rth the conclusions reached
from the corresponding mass transfer rate
studies, which did not agree with the theo-
retical estimates of solubilities based on
Henry's Law constant.
In summary, more than 99.9% of am-
monia was removed by introducing high
steam-to-wastewaterflowrates, such as 3.8
to/gal. Varying chemical constituents such
as SO4" and the molal strength had only a
little effect on net NH3 removal. Preheat-
ing wastewaters and operating the strip-
ping tower at high temperatures by in-
creasing the steam-to-wastewater flowrate
ratio resulted in higher ferhovarefficierices.
Task 3: Engineering Cost
Estimates
The cost estimates for the chemicals
and equipment to adjust the pH of an
ammonia-bearing, metal-winning wastewa-
ter before stripping showed that lime can
be more economical than caustic. In ad-
dition, the most cost-effective method for
disposal of the sludge solids generated is
dewatering in a lagoon followed by landfill
disposal of the solids. There may be a
different set of cost-effective processes,
however, when costs for land, transport,
and handling of large quantities of sludge
are high.
Summary and Conclusions
The analyses of estimated Henry's Law
jcorjstanL,and_ changes in ammonia solu-
bilities indicated that adding caustic, rather
G. B. Wickramanayake, D.P. Evers, J.A. Kittel, andA.R. Gavaskarare withBattelle
Memorial Inshitue, Columbus, OH 43201-2693.
John Burckle is the EPA Project Officer (see below).
The complete report, entitled "Bench-Scale Evaluation of Ammonia Removal from
Wastewater by Steam Stripping," (Order No. PB91- 234 633/AS; Cost: $26.00;
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
than slaked lime, would result in higher
Henry's Law constants and lower solubili-
ties for the three waste streams studied.
Although no significant variation of the
mass transfer coefficient (K) was observed
when SO4' concentrations were varied
from 5,000 to 20,000 mg/L, K was the
highest for low SO - wastewaters when
pH was adjusted with NaOH.
Results of the steam stripping study
indicated that varying the chemical con-
stituents such as SO • and the molal
strength did significantly affect the effi-
ciency of ammonia removal. More ammo-
nia was removed (99.9% or more) by pre-
heating wastewater and by operating the
stripping tower at high steam-to-wastewa-
ter flowrates such as 4 Ib/gal. Based on
engineering unit process and operation
requirements, the cost analysis indicated"
that lime may be more economical than
caustic for pH adjustment.
The full report was submitted in fulfill-
ment of Contract No. 68-03-3248 by the
Columbus Division of Battelle Memorial
Institute under the sponsorship of the U.S.
Environmental Protection Agency.
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-91/046
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