Section Four
BIOLOGICAL SYSTEMS -
D. SLUDGE TREATMENT
53    IMPROVED SLUDGE DEWATERING
Edwin R. Hendricks, Senior Engineer
BBS Corporation
Columbus, Ohio 43229
INTRODUCTION
Overview of Solids Disposal
Ultimate disposal of wastewater sludge is and has been an increasingly complex and expensive
proposition for the City of Columbus, Ohio.
Sludge incinerators have been in operation for over thirty years. The incineration capacity is
initially limited by the amount of water contained in the sludge. Historically, the two multihearth
incinerators at the Southerly WWTP have been limited to about 5.5 wt/hr. Two new incinerators have
recently come on line and they provide about 7.0 wt/hr each. Total incineration capacity at the
SoWWTP is about 25 wt/hr and, allowing 3 of 4 units on line continuously, the reliable capacity is
about 430 wt/d.
When it is possible to obtain sludge cake dry enough such that minimal auxiliary fuel is required to
sustain burning, the condition is described as autogenous combustion. Analysis of the incinerators
indicates that autogenous combustion will be obtained when the sludge cake is about 24% to 28%
solids. At that point, fuel requirements necessary to maintain control of the incinerators should fall to
less than 0.10 gallon, per wt. of sludge burned.
The projection of changes in incinerator performance as feed sludge cake dryness is increased from
18% to 30% solids is inexact due to the number of critical variables. However, the projection of near
autogenous combustion is reliable. Recent operations where 26-28% cake was burned with greatly
reduced fuel indicates verification of the projections.
Contrarily, projection of quantitative changes in composting performance as feed sludge cake is
varied from 18% to 30% is not practical, given the limited scope of studies and time allotted.
However, initial surveys of the composting situation clearly indicate that the impact of improved cake
solids would be even greater on composting than on incineration. For instance, it is projected that
doubling the cake solids would result in using about 70% less wood chips. The combined effect of
decreased wood chip addition, improved mixing and reduced handling are expected to combine
synergistically to provide significant improvements at composting.
There does not seem to be any overwhelming advantages or disadvantages to ongoing land application programs vis-a-vis cake solids improvement. The City currently practices surface application of
cake, and injection of thickened sludge is being piloted on a full scale basis. Analysis to date indicates
similar costs for both methods. In other works, the reduced cost of hauling sludge cake is offset by
dewatering costs. However, there may be some advantage to the drier filter press sludge in that
handling and storing the 28% cake is expected to be much easier than the 18%cake.
Systematic Analysis
In order to provide guidance for selection of new solids handling facilities and/or expansion of
existing facilities, a systematic cost comparison of selected solids handling models was undertaken.
Key assumptions of that analysis were as follows:
1. Incinerator capital costs was treated as sunk cost (capital cost not included in the
analysis).
45th Purdue Industrial Waste Conference Proceedings, © 1991 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Primed in U.S.A.
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