Section Seven
BIOLOGICAL:
PHENOLIC WASTE TREATMENT
25    FATE OF AMMONIA, THIOCYANATE, PHENOLICS,
AND CYCLIC AROMATICS WITHIN ACTIVATED
SLUDGE COKING PLANT WASTE TREATMENT
Clifford R. Lange, Assistant Professor
David E. Chambers, Graduate Student
Department of Civil Engineering, Auburn University
Auburn, Alabama 36849-5337
INTRODUCTION
Coal distillation processes, such as coal gasification and coal coking produce complex wastewaters. The coking process involves the destructive distillation of coal in an anoxic environment.
The volatile compounds released during the coking process are either condensed or scrubbed out
in the gas main or primary coolers. Tar is decanted from the condensate and the remaining aqueous waste, known as the flushing liquor or weak ammonia liquor (WAL), is the major source of
coking plant wastewater.
Coke plant wastewaters include a complex mixture of organic and inorganic compounds, including: ammonia, cyanide, thiocyanate, monohydric phenols (70% phenol, 25% m-cresol, and
5% o-cresol), heterocyclic nitrogenous compounds (HNCs), and polyaromatic hydrocarbons
(PAHs). The concentrations of these wastewater constituents are a function of: coal type; coking
temperature; and process technology. The ammonia and cyanide concentrations of the WAL are
typically diminished by air stripping processes to make the wastewater more amenable to biological treatment. Dilution water may be added to the waste to reduce the concentrations of other inhibitory compounds prior to biological treatment.
The goal of biological treatment of coking process wastewater is to mineralize organic contaminants and to oxidize the ammonia and thiocyanate to nitrate. One of the most commonly employed biological treatment processes for coking plant wastwaters is activated sludge. These
processes are typically operated with long mean cell residence times (Ocs) and can achieve high
degrees of ammonia nitrification, thiocyanate conversion, and organic removals. However, the
presence of inhibitory compounds in the wastewater may cause process upsets.
Nitrification is the biological oxidation of ammonia (NH4+) to nitrite (N02"), and ultimately to
nitrate (NO,-). The specialized population of bacteria which mediate this oxidation are known as
nitrifiers. Although there are seven known genera of nitrifiers, the species Nitrosomonas and Ni-
trobacter perform most of the nitrification that occurs in wastewater treatment. Nitrosomonas eu-
ropea has been identified as the only organism capable of oxidizing ammonia to nitrite in coking
plant biological processes, while several species of Nitrobacter are known to oxidize nitrite to
nitrate. Nitrifiers have low growth rates and cell yields and are easily out-competed by het-
erotrophs under high growth rate conditions. Consequently, nitrification occurs only in low
growth rate processes and is slow to recover from process upsets.
Nitrifiers are inhibited by a number of compounds present in coking plant wastewater. Excessive levels of free ammonia were reported by Wong-Chong and Loehr1 to be inhibitory to Nitrosomonas (10 mg/L) and Nitrobacter (0.1 mg/L). Similarly, free nitrous acid (HN02) concentrations in excess of 0.22 mg/L are toxic to nitrifying populations. Thiocyanate concentrations of
52nd Purdue Industrial Waste Conference Proceedings, 1997. Ann Arbor Press, Chelsea, Michigan 48118. Printed in
U.S.A.
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