Pinellas County solves H2S problem with magnesium
hydroxide
Pinellas County has fought with H2S-related odor complaints at
PS #016 for years. Even with iron salts dosing, gas levels typically averaged
623 ppm inside the incoming manhole serving PS #016.
WW Utilities Director James Rolston and his staff were looking
for an alternative. In 2005, they were introduced to Premier Chemicals and its
product line known as Thioguard TST.
Thioguard® is an alkaline magnesium hydroxide slurry, Mg(OH)2.
Considered a “technical grade” magnesium hydroxide suspension, it is similar to
the more familiar pharmaceutical grade “milk of magnesia” used to alleviate acid
indigestion. It is a non-hazardous product that is not harmful to humans or the
environment.
Odor and corrosion problems in sanitary sewer systems 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
surface pH, frequently to values below 2.0.
Adding magnesium hydroxide to municipal wastewater suppresses
hydrogen sulfide gas formation by increasing and stabilizing pH. At near neutral
pH, relatively small adjustments in pH results in large changes in hydrogen
sulfide production. At pH 7.0 approximately 50% of the hydrogen sulfide remains
dissociated in solution. Nearing 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.
Thioguard is typically supplied as a pumpable “latex paint
like” aqueous suspension, ranging from 53 to 65% solids. To control H2S buildup,
the product is proportionally added to the collection system at pump stations at
a rate varying from 25 - 100 gallons of product per million gallons of
wastewater.
When Thioguard was added to the Pinellas County system, the
H2S gas levels dropped to an average of 48 ppm. This >91% reduction of H2S gas
coming into PS #016 has resulted in the reduction of scrubber operation and
subsequent cost at this pump station. Also, the surface pH inside the incoming
manhole to PS #016 increased from a pH of 2 to a near neutral pH of 7 within
four months.