43    DENITRIFICATION IN ANAEROBIC DIGESTERS:
A REVIEW OF RECENT STUDIES
J. C. Akunna, Lecturer
Department of Civil Engineering
Surveying and Building
University of Abertay Dundee
United Kingdom
INTRODUCTION
During the past few years, there has been an increasing demand from both the public and from
governments to protect the environment against the increasing pollution generated by human activities. These demands have led to the setting up of standards for wastes discharged into the environment, thus obliging the polluters to treat their wastes products before disposal to the environment.
Wastewaters from food processing industries (and domestic activities) are usually treated prin-,
cipally for organic carbon removal. But recent standards have generated interests in nitrogen and
phosphorus removal. This has led to the addition of nitrification, denitrification and phosphorus
removal units in the existing treatment plants, thus increasing the cost of treatment operations.
Anaerobic treatment for carbon removal has proved to be an economic operation over the aerobic process for the treatment of high strength wastewaters because of the following reasons:
• low energy consumption
• low sludge production
• production of energy source (methane)
But this method does not remove nitrogen from the wastewater; instead, during digestion organic nitrogen is transformed to ammonia nitrogen which is discharged in the effluents. Post-
treatment for ammonia removal involved nitrification and denitrification processes, which necessitates two separate treatment units thus increasing treatment costs.
The need to reduce treatment costs has led to research on ways to carry out many treatment
processes in a single system. One of these systems is a single unit denitrification/anaerobic digestion system with a downstream nitrification unit equipped with recycling facilities which enable the nitrified effluent to be sent to the denitrification/anaerobic digestion unit.1 The process
configuration is shown is Figure la. It consists of anaerobic and aerobic units in series with effluent recycle. In the anaerobic unit, anaerobic digestion and denitrification take place simultaneously producing methane and nitrogen gas while in the aerobic unit, ammonia oxidation to nitrate (nitrification) takes place. This type of treatment can be of interest in the treatment of a
wide range of food-processing wastewaters which apart from having high concentrations of organic carbon can contain in certain cases high concentrations of nitrogen in the form of ammonia
and/or organic nitrogen. This process configuration appears to give lesser problems associated
with operations such as the addition of raw wastewater or external organic carbon to ensure complete denitrification as shown in Figure lb. There is also a substantial economic gain if two biological reactors are used instead of three as shown in Figure lc.
Anaerobic microbial conversion of organic substrates is a complex biochemical process involving a large number of microbial populations, often linked by their individual substrates and
product specificities. It is widely believed that the by-product of nitrate removal in such a system
is ammonia as shown in Figure 2.2 If this assumption (or observation) is correct, the process con-
50th Purdue Industrial Waste Conference Proceedings. 1995, Ann Arbor Press, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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