Tertiary Treatment Quality For A Secondary
System Utilizing the Zurn-Attisholz Process
RUDY VAN SOEST, Vice President
GUATAM DAS, Process Engineer
Van Luven Consultants, Limited
Montreal, Quebec, Canaca
INTRODUCTION
It is becoming more obvious each year that the world's social and environmental
problems are increasing and the cost of correcting these problems is becoming more and
more difficult to bear. The treatment of domestic and industrial waste has become one
of the most urgent problems of our time.
For several years, scientists in Europe and elsewhere have sought to reduce the cost of
biological treatment processes by employing very high loading rates (400 to 800 pounds of
BOD versus 50-75 pounds per 1000 cubic feet of aeration tank capacity, employed in
conventional activated sludge plants).
To date, it is our feeling that certain modifications of this process offer not only low
operational and capital costs, but also can produce effluent quality capable of meeting
requirements of the regulatory agencies. With reduced land requirements and construction
costs through reduction in tankage requirements, the high rate system can compete
economically with most every alternative means of treatment, perhaps with the exception of
lagoons.
In general, for high rate systems, the reduction in volume results in a reduction in
sludge age and hence, a worsening in settling characteristics.
A preliminary consideration pf high rate systems then indicates a feasible decrease in
aeration tankage, but only at the expense of larger secondary clarification tankage and/ or
chemical addition. U nless the environment is then altered in some way to produce a settling
floe (e.g. usage of oxygen and/or other means of selective acclimatization of cultures).
Thus, settling properties alone can become a limitation.
Despite predictions of poor settleability, several high rate processes have been
developed and reviewed (1,2): (Step Aeration, Kraus Process, Biosorption, Contact
Stabilization, Unox Process and the Zurn-Attisholz Process). Combinations of high-rate
trickling filters and activated sludge has also been considered (3,4). Each is capable of
providing BOD removals in excess of 85 percent. Others providing slightly less efficient
treatment include Activated Aeration, Modified Aeration and Supra-Activation.
The Zurn-Attisholz Process makes use of an interesting application of the two-stage
activated sludge process. Two high-rate stages are employed, the first utilizing a mixed
culture predominant in bacteria and lower fungi, and the second stage, a culture
predominant in protozoan microorganisms. Although the first stage bacteria may form
dispersed growths that cannot be easily settled from water, solids carried over from the first
stage clarifier are readily consumed as food by the protozoa. The protozoa, in turn, are
about 1000 times larger than bacteria and can be readily settled from the water in a second
stage clarifier.
With the definite application of multiple stage systems for removal of color,
phosphorous and nitrogen (5,6,7,8,9) and the stability of the system relative to other
activated sludge systems, Van Luven Consultants Limited have taken considerable interest
in this process.
483