Chemical Engineering October 2013 - 40

Cover Story
COD
1,600
1,800
2,000
2,200
2,400
2,600
1,400
1,200
1,000
600
800
400
200
30,000
Before
treatment
After
treatment
25,000
20,000
15,000
10,000
5,000
Before
treatment
After
treatment
TOXICITY
100
120
140
160
180
200
20
40
60
80
COD
2,000
Before
treatment
After
treatment
1,500
Before
treatment
After
treatment
TOXICITY
1,000
500
FIGURE 6. These charts show COD and toxicity before and
after biological treatment of primary plant wastewater [1]
rate of wastewater from the primary
plant was 260,000 gallons per day
(gal/d) with chemical oxygen demand
(COD) of 3,000-4,000 milligrams/liter
(mg/L). The flowrate of wastewater
from the secondary plant was 26,000
gal/d with COD of 80-1,600 mg/L.
These two sites are connected by a
sewage line. The primary plant used
activated sludge to treat its wastewater;
the secondary plant used activated
carbon for the removal of toxicity.
A Swedish Environmental Protection
Agency (EPA) investigation
showed that these wastewaters were
very toxic and contained hard-todegrade
organics and phosphorus,
and stricter limits were about to be
issued. To ensure compliance with
the anticipated regulations,
AstraZeneca
decided to build a new wastewater
treatment plant to meet the
requirements for discharge of the
treated wastewater to Lake Mälaren.
This lake neighbors several cities,
including Stockholm.
AstraZeneca sought alternative
methods to obtain a solution for the
new treatment plant. To be able to
discharge the wastewater into Lake
Mälaren, the treatment plant effluent
had to be free of toxic chemicals
and, at the same time, at least 95%
of the organic content had to be removed.
The wastewater was evaluated
and an optimal biological treatment
FIGURE 7. These charts show COD and toxicity before and
after biological treatment of secondary plant wastewater [1]
approach was developed to meet the
discharge requirements. Below is the
step-by-step process used to evaluate
the alternatives and develop the best
treatment option.
Step 1: Obtain wastewater characteristics
and site information.
Flowrates, wastewater characteristics,
discharge limits and local environmental
conditions were among
the data obtained from the sites. The
space at the primary plant was limited,
but there was space for the new
wastewater-treatment facility at the
secondary plant.
Step 2: Evaluate and summarize
the information. According to the information
collected, the toxicity of the
wastewater in question was a major
problem. Toxic substances typically
inhibit biological treatment. In some
cases, the bacteria can become acclimated
to the toxic wastewater, and
sometimes toxic wastewater can be
pre-treated to become biodegradable.
If no economical pre-treatment can
make the toxic wastewater biodegradable,
a non-biological treatment must
be considered.
Step 3: Examine the wastewater's
biodegradablility. If the wastewater
has common characteristics and
information can be drawn from project
experience, laboratory testing is
not necessary. However,
laboratory
testing is recommended for complex
40 CHEMICAL ENGINEERING WWW.CHE.COM OCTOBER 2013
industrial wastewaters such as these.
An initial evaluation was conducted in
the laboratory to determine whether
the wastewater was biodegradable
and whether the toxicity of the wastewater
would be reduced after the biological
treatment. Various wastewater
streams (labelled A through I) were
collected from both the primary and
secondary plants. A continuous laboratory-scale
activated-sludge process
was used for this work. The temperature
for the study was 20°C and the
pH was 7. Influent and effluent COD
and toxicity were measured. The contribution
of toxicity from each stream
was calculated as toxicity units, as
measured by a Microtox test system,
which uses luminescent bacteria to
determine the toxicity of a sample.
When exposed to a toxic sample, the
amount of light emitted by the bacteria
is decreased. The more toxic a
sample, the less light will be produced
by the bacteria. Here, toxicity is calculated
as flowrate in m3/day × 100/
EC 50, 15 min, where EC50, 15 min
represents the effective concentration
(EC) of a sample that will cause a 50%
reduction in light emission after 15
minutes of exposure of a sample to the
test bacteria[1]. The laboratory study
results are shown in Figure 6 (for the
primary plant) and Figure 7 (for the
secondary plant).
Based on the results shown in Figure
COD, lb/d
A.89,800 gal/d
B. 14,500 gal/d
C.113,600 gal/d
E.17,170 gal/d
D. 2,640 gal/d
F.26,400 gal/d
Toxicity units, Microtox
A.89,800 gal/d
B. 14,500 gal/d
C.113,600 gal/d
E.17,170 gal/d
D. 2,640 gal/d
F.26,400 gal/d
COD lb/d
G. 130 gal/d
H. 120 gal/d
I. 395 gal/d
Toxicity units, Microtox
G. 130 gal/d
H. 120 gal/d
I. 395 gal/d
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Chemical Engineering October 2013

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