Section 15. REFINERY WASTES
NITRIFICATION IN REFINERY WASTEWATER TREATMENT
James A. Mueller, Director
Kuen-Bing VVu, Graduate Student
Environmental Engineering & Science Graduate Program
Manhattan College
Bronx, New York 10471
Stan Kaczmarek, Senior Engineer
Exxon Research and Engineering Company
Florham Park, New Jersey 07932
INTRODUCTION
Many petroleum refineries biologically treat their wastewater to remove soluble contaminants.
Given the variety of food sources available in refinery wastewaters, the biological reactors sustain
mixed populations of microorganisms. Two groups of microorganisms, Nitrosomonas and Nitro-
bacter, convert ammonia to nitrite and nitrate in a specific wastewater treatment process called nitrification. At many petroleum refineries, nitrification is essential to obtain low effluent ammonia
concentrations. This is because ammonia is a regulated wastewater parameter that is also sometimes
implicated in cases of aquatic toxicity. In an activated sludge biological oxidation system with a long
sludge age, the consistent presence of nitrification gives an indication of a healthy system. However,
nitrification is a sensitive process and a rise in effluent ammonia may signal problems for process
performance . . . from a long-term inability to oxidize ammonia to the beginning of problems with
other wastewater quality parameters, such as Biochemical Oxygen Demand and Total Suspended
Solids.
Previous research within and outside of Exxon has identified and validated the importance of
several factors in maintaining the nitrification process. Dissolved oxygen concentrations above 2 mg/
1, pH between 6 and 8, and sludge ages (or sludge retention times) greater than 10 days are now
accepted as essential operating criteria for nitrification [1]. In this research, 5 laboratory activated
sludge reactors were operated with these essentials, but under various hydraulic retention times, SRT's,
and feed organic nitrogen conditions. The reactors were then "shocked" with simulated 4 hour spills
of chemicals that are sometimes associated with nitrification upset in petroleum refineries. The purposes were to compare the nitrification performance of different activated sludge systems, to identify
the dynamic responses of nitrification under feed quality upsets, and to examine the resiliency of the
process.
BACKGROUND
In completely mixed refinery wastewater treatment systems designed for nitrification, organic
carbon oxidation and nitrification occur simultaneously. Figure 1 shows a schematic of these processes.
In organic carbon oxidation, chemical energy and carbon from the organic compounds are used
by heterotrophic organisms for respiration and growth. The quantity of energy available to the het-
erotrophs depends on the oxidation state of the organic carbon. Energy released by the reaction
typically varies from 90 to 170 kcal per mole of organic carbon oxidized [2].
Nitrification is a two stage process carried out by autotrophic organisms. Nitrifiers do not require
organic compounds for growth. Rather, they typically use carbon dioxide as their sole carbon source
and energy from ammonia and nitrite for respiration and growth. Nitrosomonas convert ammonia
to nitrite releasing 66.5 kcal of energy per mole of ammonia oxidized. Nitrobacters oxidize the nitrite
to nitrate releasing 17.5 kcal of energy per mole of nitrite oxidized [3].
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