43    NITRITE ACCUMULATION IN THE EFFLUENTS FROM
HIGH-STRENGTH DENITRIFICATION OF INDUSTRIAL
WASTEWATERS
Srikanth Krishnamachari, Environmental Engineer
Engineering-Science, Inc.
Pasadena, California 91101
William W. Clarkson, Associate Professor
School of Civil Engineering
Oklahoma State University
Stillwater, Oklahoma 74078
INTRODUCTION
Biological removal of nitrogen compounds from highly nitrogenous industrial wastewaters has been
the focus of several studies due to its economies over physical/chemical processes. High nitrogen
effluent streams from industries such as fertilizer, semiconductor, and munitions contain nitrogen
concentrations higher than 1000 mg N/L. A two-stage biological nitrogen removal system consists of
aerobic nitrification followed by anaerobic denitrification to ensure complete conversion from ammonia nitrogen to gaseous nitrogen. Such a treatment scheme is also the basis for the nutrient removal of
low-strength (<50 mg N/L) wastewater in municipal treatment plants.
One of the several parameters that affects the treatment performance of a biological nitrogen
removal system is nitrite (N02-N) in the treated effluent. In the two-stage system mentioned above,
effluent nitrite could result due to partial ammonia oxidation or incomplete nitrate reduction or both.
Nitrite in the effluent increases the chemical oxygen demand (COD), and is indicative of incomplete
nitrogen removal. Besides, nitrite in water presents potentially harmful health effects.
SCOPE OF WORK
A feasibility study for high-rate denitrification was conducted using bench-scale anaerobic attached
film expanded bed (AFEB) and upflow sludge blanket (USB) reactors. The reactors were continuously
operated with simulated industrial wastewater containing nitrate, methanol, and other micronutri-
ents. The experiments were carried out in two series. The feed nitrate concentrations were incrementally increased in the first series of the study while the hydraulic residence times (HRT) were kept
constant. This was done primarily to identify the optimum influent nitrate concentrations for satisfactory reactor performance at a given HRT. The second series was performed by varying the HRTs to
determine the optimum HRT, at the optimum nitrate feed concentrations found from the first series.
The treatment performance of both reactors up to the point of failure was studied and compared, and
the causes of failure were investigated. Failure conditions were defined by a significant (> 15-20 %)
decrease in nitrogen conversion efficiency from one loading condition to the next.
BACKGROUND
Suspended growth and attached growth reactors have been used for high-rate denitrification. As
the name implies, microorganisms in the suspended growth reactors are held in suspension within the
reactor fluid. There is no supporting media for microbial attachment. Examples of suspended growth
reactors are completely mixed reactors (CMR with and without clarifiers), and upflow sludge blanket
reactors (USB). USBs are used for high-rate anaerobic processes due to higher removal capacity per
unit volume of reactor at high sludge concentrations and good settling characteristics. In addition,
recycling may not be necessary due to low linear velocity required for fluidization of sludge particles,
ah economical advantage for the USB reactor.1 However, an appropriate mixing arrangement to
maintain a good sludge blanket is a critical design parameter for USB. Improper mixing may lead to
relative instability, sludge bed separation, and poor effluent quality. These problems are largely
47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
383