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Thioguard®,
is an alkaline magnesium hydroxide slurry, Mg(OH)2,
that is beginning to be recognized as having many advantages
over the more common alkali sources, such as caustic soda
and hydrated lime. Technical grade magnesium hydroxide
suspensions are akin to the more familiar pharmaceutical
grade "milk of magnesia" used to alleviate acid indigestion,
and are therefore a safe non-hazardous product that will not
cause chemical burns.
Magnesium hydroxide has
found great use in treating industrial metal laden acidic
wastewater, where, compared with caustic soda or lime,
remove metals at lower pH, produce less sludge volume, and a
filtercake that de-waters more readily. However, it is also
beginning to be used in the municipal arena. When comparing
the physical and chemical properties of magnesium hydroxide
with conventional alkalis, hydrated lime and caustic soda,
see Table I, several advantages are noted. The first is that
fewer pounds of magnesium hydroxide are required to
neutralize the same amount of acid, 37% more for caustic
soda and 27% more for hydrated lime. Another unique
characteristic, is the maximum pH obtainable during an
overdosing situation. Excessive additions of caustic soda
and hydrated lime will result in the pH of the waste stream
reaching 14 and 12.5 respectively. However, the pH of a
magnesium hydroxide slurry is 10.5, and when used to
neutralize acidic waste, will only obtain a pH of about 9.0,
even when overdosed. This upper pH limit happens to coincide
with the upper limit under the Clean Water Act, 1976.
|
Property |
50% NaOH |
30% Ca(OH)2 |
58% Mg(OH)2 |
|
% Hydroxide |
42.5 |
45.9 |
58.3 |
|
Solubility in Water g/100 ml |
42 |
0.185 |
0.0009 |
|
Reactive pH |
14 |
12.5 |
9.0 |
|
Freezing Point oF |
61 |
32 |
32 |
|
Weight Equivalency |
1.37 |
1.27 |
1 |
Table 1.
Magnesium
hydroxide is supplied as an easily pumpable "latex paint
like" aqueous suspension, typically ranging from 55 to 65%
solids. It can settle and freezes at 32oF,
therefore should be stored in agitated tanks. It can also be
supplied as a fine powder.
Two new
areas where magnesium hydroxide is being utilized are
municipal sanitary collection systems and treatment plants.
Waste Water Treatment
Plants
Aerobic Processes
Aerobic
processes have been employed by municipal and industrial
wastewater treatment systems for the removal of organics,
the biological conversion of ammonia to nitrates, reduction
of sludge mass and volume, and reduction of pathogenic
organisms. Aerobic digestion consists of two steps; direct
oxidation of biodegradable matter, and subsequent oxidation
of microbial cellular material.
Organic Matter + O2 +
Nutrients CO2 + H2O + NH3 +
Cellular Material (1)
If the
digestion process is provided with sufficient oxygen and
detention time, or a separate nitrification system is
utilized, ammonia will nitrify and form nitrates. The
nitrification process may result in a decrease of both pH
and alkalinity as a result of acid generation during the
process:
NH4+ + 2O2
NO3- + 2H+ + H2O (2)
This
reaction results in the consumption of about 7.1mg
alkalinity per each mg NH4+-N oxidized. Influent
treated with Thioguard® from the collection
system generally arrives at the digester very near pH 7.5,
the optimum pH for bacterial growth in both aerobic and
anaerobic processes. If the wastewater does not have
sufficient alkalinity to compensate for losses in the
nitrification reaction, it will result in a pH drop and
could, if the pH drops too low, result in bacterial
deactivation.
The use of
Thioguard® offers the following advantages over
caustic soda and lime. Magnesium hydroxide buffers to a
controlled pH near 9.0, even when over dosed. This buffering
capacity results in better pH control and makes pH excursion
less likely to occur. Magnesium hydroxide has higher
neutralizing value per dry pound. Therefore, less magnesium
hydroxide is required compared with caustic or lime. Unlike
lime, magnesium hydroxide does not cause large volumes of
sludge and scaling in the collection system or treatment
plant equipment.
Anaerobic Digestion
Anaerobic digestion is
the solubilization and reduction of complex organic
substances by microorganisms in the absence of oxygen. The
products of digestion are methane, carbon dioxide, trace
gases and stabilized biosolids. The microbial population
responsible for this conversion can be divided into three
groups: solubilization, acid formation and methane formation
(methanogens). Proteins, lipids, carbohydrates and complex
organics are solubilized by hydrolysis. These products are
converted into short-chain organic acids, such as, acetic,
propionic and lactic. These acids are then converted into
methane and carbon dioxide. The acid forming bacteria are
tolerant to environmental changes such as pH and
temperature. In contrast, the methane forming bacteria are
intolerant to environmental changes.
C6H12O6
Þ
3CH3COOH (3)
3CH3COOH + 3NH4HCO3
Þ
3CH3COONH4 + 3H2O (4)
3CH3COONH4
+ 3H2O Þ
3CH4 + 3NH4HCO3 (5)
Equation 3
represents acid formation. The acid is then neutralized,
equation 4, by bicarbonate present in the system. The buffer
consumed in equation 4 is then regenerated in the methane
forming step. There is therefore an equilibrium between
buffer formation and consumption. The optimum pH range for
methanogens is also 6.5 to 7.5. In a digester upset, net
consumption of buffer occurs and the process is in danger of
pH failure. When this happens an external source of
alkalinity must be added. Magnesium hydroxide can be added
to the digester to neutralize any excess acid not consumed
by the methanogens. Magnesium hydroxide when used in
anaerobic digesters will have all the benefits apparent in
aerobic processes.
Some Benefits of Thioguard®
Slurry in Biological Treatment Processes
Thioguard®
magnesium hydroxide slurry offers the following
advantages for biologic processes:
Provides
Alkalinity and nutrient. Magnesium hydroxide not only
supplies alkalinity, but also supplies magnesium, an
essential micro-nutrient for controlled bacterial growth.
Buffers
at the pH maximum. Magnesium hydroxide buffers to a
maximum pH of 9.0, even when over-dosed. This buffering
effect provides better pH control in the critical pH
operating ranges between 7.0 and 9.0. Caustic and lime are
very easily overdosed with a rapid and dramatic change in pH
that can reach pH 14 and 12.5 respectively. This results in
bacterial kills and can, in severe cases, deactivate the
process. Even when not overdosed, the point of addition for
caustic or lime will often produce localized hot spots that
kill bacteria.
Although
magnesium hydroxide buffers at a maximum of 9.0, it provides
more alkalinity than caustic and lime. Because its
solubility is substantially lower, it acts like a slow
release agent contributing needed hydroxyl ions only when
required.
Safe to
handle. Magnesium hydroxide is safe to handle, is
non-toxic and non-corrosive, unlike caustic soda and lime.
Caustic and lime react exothermically when added to water.
Has a
greater neutralizing capacity per pound. Magnesium
hydroxide has a higher neutralizing value per dry pound when
compared with caustic (which requires 37% more) and lime
(which requires 27% more).
Other
benefits include improved clarifier and DAF performance,
reduced sludge volume index (SVI), more dense filter cake
with lower press cycle times, enhanced activated sludge
performance, lower effluent TSS, phosphorus removal and
sludge conditioning, metals precipitation and reduced
effluent BOD5. Thioguard® can also be used for
lagoon odor control, and suspended growth nitrification.
Preventing Corrosion in
Sanitary Collection Systems
Odor and
corrosion problems in concrete sanitary sewer systems is a
wide spread problem. The corrosion is the result of a two
step biological process. Sulfate reducing bacteria present
in the collection system convert sulfates into hydrogen
sulfide gas. A series of oxidizing bacteria that reside on
the sewer crown convert hydrogen sulfide gas into sulfuric
acid which will eventually result in a substantial lowering
of pH, frequently to values below 2.0, see Figure 1. Low pH
conditions and corrosion problems are also similarly
experienced in maintenance holes. This acid attacks the
concrete, reducing it to a soft putty like gypsum. According
to a 1992 EPA report, maintaining a surface pH of 4 or
higher is sufficient to prevent an unacceptable corrosion
rate.
An effective
method to combat the corrosion problem is to spray the
corroded sewer crown with a modified magnesium hydroxide
suspension, that is formulated to resist surcharging. Field
studies conducted in the City of Los Angeles demonstrated
that a single application provided enough alkalinity to
protect concrete surfaces for over a year.
The
treatment process involves spray painting the magnesium
hydroxide suspension onto the concrete surface requiring
protection, to achieve a coating thickness of 100-125 mils.
The magnesium hydroxide neutralizes any sulfuric acid
present on the surface and raises the pH up to about 10.0.
This high pH has the added benefit of deactivating the
bacteria responsible for the acid generation, since they
cannot tolerate high pH conditions. The magnesium hydroxide
coating will also react with hydrogen sulfide gas thus
helping to reduce potential odor problems. The coating is
intended to be sacrificial, slowly being consumed by
hydrogen sulfide, and will need to be replenished on a
regular maintenance schedule. It has been estimated that
annual treatment of sewers using magnesium hydroxide may
extend sewer life by 20 years. Typical spray treatment will
cost approximately 200 times less than rehabilitation.
Representative field results from spray treatment of
concrete surfaces using Thioguard® are presented
in Figure 1
.
Figure 1
Odor
Prevention with Continuous Thioguard®
Addition
Adding
magnesium hydroxide to municipal wastewater suppresses
hydrogen sulfide gas formation via an increase in pH. At
near neutral pH, relatively small adjustments in pH results
in large changes in hydrogen sulfide dissociated in
solution, via the equilibrium reaction:
H2S
Û H+
+ HS- Û
H+ + S2- (6)
At pH 7.0
approximately 50% of the hydrogen sulfide remains
dissociated. However, at pH 8.0, only 8.3% is present as
hydrogen sulfide, and at pH 9.0 hydrogen sulfide levels drop
to less than 1.0%. Small additions of magnesium hydroxide
sufficient to raise wastewater pH in this range can
significantly reduce hydrogen sulfide gas emissions, and
thus help control odor/corrosion problems.
Field
results using continuous addition of Thioguard®
for hydrogen sulfide odor control are presented in Figure 2.
Figure 2
Summary
Although
magnesium hydroxide has traditionally been utilized in
industrial wastewater treatment, it is now being recognized
as having many uses in Municipal wastewater treatment. It
has may advantages over more common alkalis, and is
considerably safer to handle.
Glossary:
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NO3 -
Nitrate anion
NO2 - Nitrite anion
H+ Hydrogen ion (acid) |
NaOH - Caustic soda
Ca(OH)2 - Hydrated lime |
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