Effect of Sludge Depth on the
Oxygen Uptake of the Benthal System
ROBERT I. DAVISON, Graduate Student
N.  BRUCE HANES, Associate Professor
Civil Engineering Department
Tufts University
Medford, Massachusetts
INTRODUCTION
The self-purification process of streams involves a complex balance between
many oxygen assets and liabilities. It is essential in the evaluation of a stream
to include all of the factors that significantly contribute or utilize dissolved
oxygen (DO). Without this careful investigation, undoubtedly, in many instances
the stream will behave quite differently than would be predicted.
One such factor than can, under certain circumstances, become a significant
liability to the oxygen balance in a stream is the oxygen requirements of the
bottom sediment. This sediment or benthal deposit undergoes decomposition by
the benthal organisms. Due to the balance between oxygen diffusion and oxygen
comsumption, this decomposition will proceed under both aerobic and anaerobic
conditions. To distinguish the decomposition from the aerobic stabilization of
organic matter and anaerobic decomposition in sludge digestion, Fair et al (5),
defined the stabilization of the bottom environment of a stream as benthal decomposition.
Many factors influence the manner in which the benthal decomposition will
proceed. These factors include: (a) oxygen concentration in the overlying water,
(b) temperature, (c) salinity, (d) sludge pH, and (e) sludge depth.
Baity (1), Edwards and Rolley (4), and McDonnell and Hall (9), have studied
the effect of the oxygen concentration in the overlying water with varying results.
Baity (1), Fair et al (5), Edwards and Rolley (4), McDonnell and Hall (9), and
Hanes and Irvine (6), evaluated the effects caused by temperature changes. The
effects of benthal deposits in sea water have been investigated by Hanes and
White (7), and Baity (1) while the effect of sludge pH on the oxygen demand of
benthal deposits has been studied by Benedict (2) and Lardieri (8).
The question of sludge depth and its effect on the oxygen demand has received considerable investigation with varying results and interpretations. Mohl-
man (11) stated that sludge depth was not the important factor, but that the surface area of the deposit and the rate of re-suspension of solid material due to gas
production in the anaerobic zone were of prime importance. Investigations by
Baity (1) and Fair et al (5) however, demonstrated that deeper deposits exerted a
higher oxygen demand than shallower deposits. Most of the more recent investigations (3,4, 14, 9,10) have reported conclusions that support Mohlman's statement
and indicate that the oxygen requirements of the benthal deposit are independent
on sludge depth. Oldaker et al, (12) noted that only the upper 15 cm of sediment
had any significant effect on the areal oxygen demand.
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