BIOLOGICAL NITRIFICATION AND HYDANTOIN REMOVAL IN
COAL GASIFICATION WASTEWATER
Charles D. Turner, Associate Professor
Paula Diepolder, Project Engineer
John Strain, Research Assistant
University of North Dakota
Energy Research Center
Grand Forks, North Dakota 58202
INTRODUCTION
The University of North Dakota Energy Research Center (UNDERC) conducts an extensive low-
rank coal research program. This research includes basic coal science and issues relating to coal
combustion and conversion processes. UNDERC is conducting studies on treatment of coal-derived
wastewaters as part of the coal conversion research program. Wastewater for use in these tests is
generated in UNDERC's slagging, fixed-bed, pilot-scale gasifier (SFBG) operated on Indian Head
lignite from North Dakota. Raw gas liquor from this process is contaminated with dissolved organics
and ammonia and therefore, is not suitable for either discharge or direct reuse within a coal conversion system. Research was undertaken to develop treatment systems for this wastewater.
OBJECTIVE
The initial objective of this research was to determine the feasibility of a two-stage activated sludge
process for the biological oxidation of ammonia in coal gasification wastewater. Hydantoins, a constituent documented as being poorly or non-biodegradable, were also present in the gasification
wastewater [1]. An unexpected occurrence in this research was the biological oxidation of hydantoins
during the nitrification process. The initial research objective was subsequently modified to include
an evaluation of the combined removal of ammonia and hydantoins.
BACKGROUND
The feed wastewater used in this research was first pretreated by a series of physical, chemical
and biological processes. Wastewater condensate from the UNDERC gasifier was treated using oil-
water separation followed by filtration, solvent extraction to remove dissolved organics and steam
stripping to reduce levels of ammonia and acid gases. Following these treatment schemes, the wastewater is termed stripped gas liquor (SGL). The SGL was then frozen and stored for later use. Table
I presents the characteristics of SGL. After thawing and mixing, SGL was fed to an activated sludge
system for carbonaceous biological oxidation. Effluent from this first stage activated sludge system
was frozen and stored for later use as feed to the second stage activated sludge bench scale unit used
in this research.
Hydantoins were first discovered in UNDERC SFBG SGL by Mohr and King [2] and subsequently
characterized by Olson et al. [3]. These highly polar heterocyclic compounds are not removed by
solvent extraction with diisopropyl ether or by steam-stripping. Figure 1 shows the proposed reaction
for their formation. Hydantoins are specific to slagging fixed-bed gasification because the characteristic high hearth temperatures promote cyanide formation. Cyanide combines with carbon dioxide,
ammonia, and a ketone during the product gas quenching process to form hydantoins. If the ketone
is 2-butanone (methylethyl ketone), then 5-methyl, 5-ethyl hydantoin is formed; if it is acetone, then
5,5-dimethyl hydantoin is formed.
43