Gas Scrubbing for H2S Removal and
Methane Enrichment
A.M. BUSWELL, Consultant
Gainesville, Florida
INTRODUCTION
Soon after the sewage treatment plant at Miami, Florida, was put in operation the digester gas was found to contain up to 600 grains of H2S per 100
cf or nearly one per cent by volume. This was a serious matter since the gas
was to be used for power production and 60 grains per 100 cf was the limit set
by the gas engine manufacturers for use in internal combustion engines. The
value of the gas was approximately $100 per day. The high H2S content of
the gas was not anticipated since the city water was low in sulfates which are
in general the source of H2S by reduction under anaerobic conditions (1).
Analyses of the sewage, however, showed chlorides and sulfates in high
amounts which on further investigation were traced to infiltration of ground
water of high salinity due to salt water intrusion (2). Two remedial measures were taken by the city personnel. The infiltration was found to be heaviest in certain old collectors, and these were repaired as far as was practicable.
Second, the operation of the sludge thickener was improved so that the water
content of the sludge entering the digesters was decreased by about one-half.
This reduced the amount of sulfates entering the digesters by about one-half
since the sulfates were carried in solution in the water. The results of these
changes was to reduce the H2S to about 300 grains/100 cf.
The question as to whether the high H2S would be a permanent problem
was naturally raised. A review of the literature and consultation with persons having experience with a similar problem revealed conflicting opinions.
Several operators reported that H2S was acute during the first months of operation but gradually was reduced or disappeared.
In many cases these conditions were based on casual observations and
were hard to explain. In one instance at Peoria, Illinois, the decrease in HoS
in the gas may be explained by changes in certain industrial processes which
decreased the amount of combined sulfur entering the digester. Norris (1) reported on conditions at Peru, Indiana, where H2S was attributed to the reduction by bacteria of the high sulfates in the deep well water which supplies the
town.
Bacteriologists at Purdue University, the University of Illinois, the Ohio
State University and Iowa State College discouraged any attempt to solve the
problem by bacterial control. The plant was provided with the conventional
type of "iron sponge" purifier (iron oxide on hard wood shavings) but it was
found inadequate and was costly to maintain. The problem was solved by the
construction of a tower filled with 2-1/2 to five in. stone. Water is introduced at the top and the gas at the bottom thus obtaining counter current
flow. A water to gas ratio of approximately 1:1 was used. The overall
dimensions of the device gave a volume to rate ratio of two cf per one cf/m
gas treated.
After weighing all the evidence, it appeared probable that in the Miami
situation H2S in the digester gas would be a continuing problem. This conclusion seems confirmed by three years experience.   Since early 1957, when
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