Section 1. COAL, COKE, AND POWER PLANT WASTES
PROCESS PERFORMANCE OF PILOT-SCALE ACTIVATED SLUDGE
TREATMENT OF PRETREATED COAL GASIFICATION WASTEWATER
John R. Gallagher, Research Associate
Gale G. Mayer, Research Supervisor
University of North Dakota Energy Research Center
Grand Forks, North Dakota 58202
INTRODUCTION
The University of North Dakota Energy Research Center (UNDERC) is assessing the applicability
of conventional wastewater treatment technology for use with coal gasification wastewater. This pilot-
scale research is being conducted to demonstrate various treatment options for commercial gasification operations. Gasification facilities located in arid sections of the United States will require
wastewater reuse, such as in cooling water systems, to meet water needs and to attain compliance
with environmental regulations. Problems center on the level of wastewater treatment required to
produce acceptable reuse water. Ideally, cooling water should result in low rates of corrosion and
fouling (including biological fouling). Additionally, in an open cooling system, if drift and stripping
rates are high, undesirable pollutants and odorants become a concern.
The first commercial gasification plant in the United States, the Great Plains Gasification Associates' (GPGA) plant, located near Beulah, North Dakota, is a model for these investigations. This
plant was designed and constructed as a zero-discharge facility using Lurgi dry-ash gasifiers and
processes with North Dakota lignite as the feedstock. The gasification condensate is very high in
COD, phenols and ammonia. This wastewater is solvent-extracted and steam-stripped to reduce phenols, ammonia and acid gases (H2S, C02, and HCN). This pretreated wastewater, known as stripped
gas liquor (SGL) is then used as cooling water in an open circulating cooling system operated at 10
cycles of concentration.
Research at UNDERC was initiated to test three levels of SGL pretreatment for makeup wastewater to a pilot cooling tower. This testing program used wastewater produced in UNDERC's slagging fixed-bed gasifier. The feedstock coal was the same as that used at GPGA and gas clean-up
operations and wastewater pretreatment simulated GPGA's processes as much as possible. An initial
cooling tower test using UNDERC SGL without biotreatment showed: 1) unacceptably high rates of
corrosion of mild steel (12 to 15 MPY); 2) high levels of biological fouling; 3) vigorous foaming;
and 4) very high levels of stripping of phenol (91%) [1,2]. Biological treatment was then investigated
as a means to reduce the biochemical oxygen demand (BOD), thereby reducing phenol stripping,
foaming and biological fouling in the cooling water system.
This paper will present data on the process performance of the pilot-scale activated sludge treatment system using UNDERC SGL. This treatment system operated for over 300 days and more than
1.89 x 10' liters (50,000 gallons) of wastewater were treated.
MATERIALS AND METHODS
Wastewater Production
The UNDERC gasifier, which has been described previously [3,4,5], is a pilot-scale slagging version of the commercial dry-ash, fixed-bed process. In the Lurgi dry-ash process, gasification temperatures are maintained sufficiently low by excess steam so that the ash is removed dry. In the
slagging process only steam required for the gasification reaction is supplied, and temperatures in
the combustion zone of the gasifier are high enough to melt the ash for discharge as molten slag.
Because of the lower steam requirement, the slagging gasifier produces less wastewater than the dry-
ash version.