Dewatering Characteristics of Sludge
From Industrial Oxygen Activated Sludge Systems
RONALD J. ROMAGNOLI, Environmental Engineer
Environmental Systems
Union Carbide Corporation
South Charleston, West Virginia 25303
INTRODUCTION
One of the major results of a biological wastewater treatment process is the net
generation of excess biological organisms or sludge. It has been stated that the more one has
of this sludge, the worse off one is. This is due to the fact that facilities require to stabilize,
dewater and dispose of this material represent a very substantial portion of the cost of
secondary wastewater treatment plants. The cost of sludge handling and ultimatedisposal,
which some say is as much as fifty percent of the total wastewater treatment cost (1),
depends not only upon the quantity of excess sludge produced but also on the physical and
chemical characteristics of the sludge. Wastewater solids handling remains a very big
problem in terms of cost, environmental impact, and public acceptance. Many treatment
facilities in the past have tended towards the less expensive means of solids disposal such as
land application or ocean dumping, and have not considered the more sophisticated solids
disposal handling systems because of the economics of the situation.
Wastewater treatment processes have been improving in the past few years to the point
where tertiary treatment is now being considered at many sites; however, this has not been
the case for the solids handling area. Unlike wastewater treatment process designs, which
are based on widely studied wastewater characteristics, very little fundamental data has
been collected to permit rational designs of handling systems for the wide variety of
wastewater sludge solids that are produced today. Because data gaps exist in the field of
wastewater solids handling, and the costs for disposing of the solids generated from waste
treatment facilities are high, it is desirable to investigate the solids handling properties of
each individual sludge. The Union Carbide Sludge Dewatering Laboratory provides for
bench or pilot scale simulation of various waste solids treatment processes. From the data,
design parameters can be determined, costs can be assigned to each unit process and the
overall system can be optimized. Tests with sludges produced by the UNOX System
indicate that each industrial UNOX waste activated sludge is unique in its own physical and
chemical properties. The data obtained from these pilot studies may be used to directly scale
up to full scale unit processes for sludge handling and disposal.
Oxygen Activated Sludge
Based on twenty-two side by side UNOX and air activated sludge studies, the following
statements can be made pertaining to the UNOX or oxygen activated sludge: 1) Settles more
rapidly and yields 2-3 percent solids concentration; 2) Is significantly less in quantity; and 3)
Dewaters easier in processes such as gravity thickening, vacuum filtration and
centrifugation (2). These factors indicate that capital and operating costs for sludge
handling will be significantly reduced with an oxygen activated sludge system.
The main hypothesis for improved settling and decreased net sludge production for
oxygen-activated sludge is the maintenance of high dissolved oxygen tension in the bulk
liquid. This provides the concentration gradient necessary to overcome the diffusional
resistance of oxygen through the bulk floe (3). Because each floe particle is totally aerobic
the entire biological mass is participating in the aerobic process. Due to the absence of
any oxygen-limiting conditions, the floe can assume a more spherical and compact shape
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