66    AUTOTROPHIC BIOOXIDATION PROCESS FOR
TREATMENT OF COAL GASIFICATION
WASTEWATERS
Frank J. Castaldi, Senior Engineer
Radian Corporation
Austin, Texas 78766
INTRODUCTION
Biological treatability studies were conducted on wastewaters from a non-tar producing coal gasification process as part of a characterization program to develop design and environmental data for
synthetic fuels plants based on this technology [1,2]. The results of this experimentation produced a
series of wastewater characterization and treatability documents which established basis of design
data for the following treatment unit operations/processes: 1) coagulation/clarification for removal
of small diameter (<2 microns) carbon particles (fines) present in the wastewaters after bulk settling
of dense solids suspensions found in the quench liquors/gas cooling condensates [3]; 2) chemical
conversion and fixation of cyanide to less toxic forms [3]; 3) steam stripping for removal of dissolved
acid gases and ammonia [4]; and 4) activated sludge for ammonia, thiocyanate, and organic carbon
removal [5,6].
This paper presents the results of a study which developed basis of design data for an autotrophic
activated sludge process to treat process wastewaters from an ash agglomerating fluidized-bed gasification process. The technology also is generally applicable to the treatment of process wastewaters
from entrained-bed gasifiers and should be viewed as a technology innovation for treatment of low
tar, oil, and phenol coal gasification quench liquors and gas cooling condensates. Previous research
has indicated that a variety of aerobic bacteria are capable of degrading thiocyanates during the
biooxidation of coke oven wastewaters and fixed-bed coal gasification raw product gas quench
condensate. One of the purposes of this study was to examine the feasibility of developing an obligate
autotrophic population of bacteria that could perform similar metabolic functions in order to achieve
thiocyanate degradation and attain simultaneous nitrification in a single bioreactor.
CHEMOAUTOTROPHIC BIOOXIDATION
Chemoautotrophic organisms are dependent on chemical energy sources and employ carbon dioxide as the principal carbon source. The use of C02 as a carbon source by chemotrophs is always
associated with the ability to use reduced inorganic compounds as energy sources. This ability is
confined to the bacteria and occurs in a number of specialized groups that can use reduced nitrogen
compounds (NH3, N02~), ferrous iron, reduced sulfur compounds (H2S, S = , S203 = ), or H2 as
oxidizable energy sources.
By definition, a chemoautotroph must possess two special biochemical capacities: the ability to
derive Adenosine Triphosphate (ATP) and reducing power from the oxidation of a reduced inorganic
compound and the ability to use CO, as its principal or sole source of carbon, an attribute that implies
possession of specialized enzymatic machinery. The two groups of chemoautotrophic bacteria examined during the study were the nitrifying bacteria and sulfur oxidizing bacteria.
The nitrifying organisms fall into two separate physiological groups: bacteria that oxidize ammonia
to nitrite and bacteria that oxidize nitrite to nitrate. The most common ammonia oxidizer in soil is
Nitrosomonas and among the nitrite oxidizers the most common soil form is Nitrobacter. The
chemoautotrophic sulfur-oxidizing bacteria are placed in a single genus, Thiobacillus. These organisms can grow at the expense of elemental sulfur, and many can use thiosulfate as well.
The process of converting ammonia to nitrates involves the destruction of alkalinity, which, in turn,
means a drop of pH, and it follows, an inhibition of the nitrification reaction. From the equations
presented below, it can be seen that two equivalents of alkalinity are consumed per equivalent of
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