68    USING ROTATING BIOLOGICAL CONTACTORS FOR THE
TREATMENT OF COAL GASIFICATION
WASTEWATERS
Charles D. Turner, Associate Professor
Department of Civil Engineering
University of North Dakota
Grand Forks, North Dakota 58202
Kevin Wernberg, Project Engineer
Widseth, Smith & Nolting Consulting Engineers
Brainerd, Minnesota 56401
INTRODUCTION
The University of North Dakota Energy Research Center (UNDERC) is involved in an extensive
low-rank coal research program. This research includes basic coal science and topics relating to coal
combustion and conversion processes. UNDERC is conducting studies on the treatment of coal-
derived wastewaters as part of the coal conversion research program. Wastewaters for these tests were
generated by two sources: UNDERC's slagging, fixed-bed, pilot scale gasifier operated on Indian
Head lignite from North Dakota and the Lurgi Mark IV gasifiers at the Great Plains (GP) gasification
plant located north of Beulah, North Dakota. Raw gas condensate from these processes is contaminated with dissolved organics and ammonia. This liquor is not suitable for either discharge or direct
reuse within a coal conversion system and must be pretreated prior to biological processing.
OBJECTIVE
The objective of this research was to determine the treatability of UNDERC's and Great Plains'coal
gasification wastewaters using a bench scale four-stage rotating biological contactor (RBC). The
treatability testing included an evaluation of organic removal rates in the first stage and the overall
rates in the last three stages using the Slover-Kincannon model. Nitrification was evaluated at various
loading rates.
BACKGROUND
The feed wastewaters used in this research were first pretreated by a series of physical/chemical
processes. Wastewater condensate from the UNDERC gasifier was treated using oil/water separation
followed by filtration to remove tars, solvent extraction to remove dissolved organics, and steam
stripping to reduce levels of ammonia and acid gases. The Great Plains raw gas liquor was treated by
tar oil water separation followed by the Phenosolvan process to remove phenolic compounds and then
by the Phosam process reduce ammonia concentrations. Following these treatment schemes, the
wastewater is termed stripped gas liquor (SGL).
The SGL was then frozen and stored for later use. Tables I and II present the characteristics of each
SGL.
A comparison of the two wastewaters shows that analytical differences in organic compounds
exerting biochemical oxygen demand (BOD) are due to the 99 mg/L of phenol and 148 mg/L of
thiocyanate in the UNDERC wastewater while the GP SGL contained 14 mg/L and 8 mg/L of the
same compounds. The UNDERC wastewater contained 27 mg/L of cyanide as compared to 5 mg/L
for the GP wastewater. The mean BOD5 of the UNDERC wastewater was, however, only 690 mg/L
while the mean BOD5 for GP SGL was 1300 mg/L. All BOD5 analyses were run on filtered samples.
EQUIPMENT
The four-stage RBC system was constructed of plexiglass and PVC pipe. Each stage contained 39
plexiglass discs 4.83 inches in diameter and 0.125 inches thick. Discs in the first stage were separated
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