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270 Lawrence K. Wang et al.

3. The situations when two-stage or three-stage scrubbing process may be used for
hydrogen sulfide removal are described in the following discussion. If ammonia
is produced in normal operations of a WWTP, then a two-stage scrubbing system
is required. The first stage will remove the ammonia with dilute acid scrubbing
liquor and the second stage uses a caustic/oxidizing step for removal of hydrogen
sulfide.

Additionally, as this text is being prepared, increasing concern with other malodor-
ous reduced sulfur compounds is being noted throughout North America.
Historically, waste-treatment plants as well as other odor producing industries (such
as rendering) have been located in remote areas to minimize odor complaints from
neighbors. As urban growth has accelerated, such plants often find their location
surrounded by new development, where previously only open fields or forests had
been neighbors (40).
As a result, more concern must be given to control of malodorous-reduced sulfur
compounds such as methyl disulfide (MDS), dimethyl disulfide (DMDS), and mer-
captans that previously were allowed to simply disperse into the atmosphere.
Although commonly present in very low concentrations, typically ppb rather than
ppm as with hydrogen sulfide, these other reduced sulfur compounds have extremely
low odor thresholds.

When present in an airstream, a three-stage odor scrubbing system may be called for:
(a) first stage to remove NH with dilute acid scrubbing; (b) second stage to remove
3
DMS, DMDS, and mercaptans with oxidative scrubbing at neutral pH; and (c) third
stage “traditional” oxidative scrubbing at high pH to control H S (41–43). This type
2
of three-stage odor control scrubbing is common in Europe, where urban congestion
around industrial facilities has long been a problem.
When hydrogen sulfide concentrations are approx 100 ppm and higher, a two-stage
scrubber system to control hydrogen sulfide will be justified based on chemical costs.
In such a system, approx 80% of the H S present is solubolized in a first-stage caustic-
2
only scrub, then the remainder of the H S present is oxidized in a second
2
caustic/oxidation scrubber such as the scrubber described in this example. The advan-
tage of this two-stage system is that all the oxidizing chemical (the most expensive
chemical consumed) will be utilized as the blowdown from the second stage is directed
to the sump of the first stage.

Additionally, when using sodium hypochlorite as the oxidizing agent, a competitive
chemical reaction is present in the scrubber:

H S + NaOCl → S + NaCl + H O
2 2
Although this reaction accounts for only about 1% of the chemistry in a
caustic/hypochlorite scrubbing system, at higher concentrations of H S, the elemental
2
sulfur formed can form deposits on the tower packing. An open packing with all
rounded surfaces and high void space, such as Q-PAC from Lantec Products, may
minimize fouling problems in a scrubber. Nevertheless, this reaction will consume
additional chemicals.
4. Other alternative technologies for odor control at wastewater-treatment plants
(WWTPs) are presented in the following discussion. There are many alternative
technologies for removing hydrogen sulfide and other malodorous substances from