A Method for Measuring Sediment Oxygen Demand
Using a Bench Model Benthic Respirometer
by
Peter M. Nolan
Arthur F. Johnson
May 1979
U.S. ENVIRONMENTAL PROTECTION AGENCY
REGION I
NEW ENGLAND REGIONAL LABORATORY
60 WESTVIEW STREET
LEXINGTON, MASSACHUSETTS 02173
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A Method for Measuring Sediment Oxygen Demand
Using a Bench Model Benthic Respirometer
Introduction
The apparatus described has been in use at the New England
Regional Laboratory since 1973 and was designed and constructed to
facilitate the measurement of Sediment Oxygen Demand (SOD) in the
laboratory without the expense and time required for in situ techniques
which involve the careful placement of large chambers on the bottom
sediment using SCUBA or other techniques for positioning the equipment
from a boat. Advantages of the "bench" technique include the ability
to conduct several rate measurements for a given sample for prolonged
periods of time, samples taken from environments with low dissolved
oxygen can be measured, and a variety of substrate types can be used.
Design
The bench model SOD chamber is constructed of clear 1/4 inch
acrylic plastic and has uniform inside dimensions of 30.5 cm. The top
is larger (33.0 cm square) and is fitted with a rubber gasket. Water
tightness is perfected using a lock down arrangement constructed from
wing nuts, and threaded rod connected in two places on each side of
the chamber with 2.5 cm thickness blocks of acrylic plastic.
A hole is drilled centrally in the top to accommodate a dissolved
oxygen probe. A small reservoir 5 cm h X 10 cm 1 X 10 cm w is built
on the top around the hole so that when the probe is in place a water
seal is achieved and air leakage prevented. Large rubber O rings to
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fit the probe on either side of the top gives added protection
against air leakage.
Two holes are drilled, one each on opposite sides of the
chamber and threaded to accommodate a 5/8" pipe thread. 'The influent
port is centrally located 7.5 cm from the bottom. The effluent port
is centrally located 7.5 cm from the top.
All seams are "glued" using butt joints where the ends to be
joined are soaked in a mixture of 70% Methylene chloride - 30%
Ethylene dichloride until slightly softened and then joined under
pressure. A bead of silicon rubber cement run along the inside
seams adds strength and water tightness to the chamber. Figure I is
a schematic of system design.
Operation
The basic principle of operation of the bench model respirometer
is to circulate a confined volume of water at a controlled rate within
a chamber into which an environmental sediment sample has been placed
to a uniform depth and area. Oxygen depletion in the water is
monitored for a specified period of. time in order to estimate the
sediment oxygen demand which is the sum total of processes in the
sediment utilizing oxygen.
In the field, replicate sediment samples are collected by grab -
placed in plastic bags with a minimal amount of disturbance - iced and
returned to the laboratory for analysis. The sediment is carefully
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placed in the chamber to a uniform depth of 2.5 cm. Aerated aged
tap water or water from the sample source (min D.O. 70% saturation)
is gently layered over the sediment to prevent roiling. A flat
acrylic plastic paddle or stainless steel spatula works well to
disburse water over the sediment and with experience a minimum amount
of disturbance can be achieved. Before a run, any suspended sediments
are allowed to settle.
Circulation in the chamber is maintained at a constant rate with
a variable speed peristaltic pump using a closed loop principle. The
entry port is split into two diffusion nozzles so that mixing is
rapid and total. The exit port is a single fitting. Both ports are
connected to the pump by one continuous piece of tygon or equivalent
pump tubing.
Tests can be conducted at ambient room temperature 20+2°C or at
environmental temperatures found at the sample source providing
temperatures can be held constant.
Dissolved oxygen is monitored by probe calibrated daily (more
often if required) for periods of time usually ranging from 4-24 hours
depending on the rate of uptake. This normally includes a stabiliza-
tion period of 15-60 minutes. If possible, the D.O. information should
be continuously recorded.
Light and Dark bottles should be set using incubation water from
the chamber to determine if respiration or photosynthesis attributable
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'to the water has a significant influence on the rate.
Given the change in dissolved oxygen, time, water volume,
sediment surface area, and correction for respiration or production
if any, SOD results are derived from the following equation:
SOD gm O2/m2/day = Rbj - Of) - (B^ - BfT| V
(SA) (t)
O^ = DO initial mg/1
Of = DO final mg/1
BI = DO bottle initial mg/1
Bf = DO dark bottle final mg/1
V = Volume confined water (.0255 m^)
SA = Sediment area (.093 m2)
t = time/days
For the final determination of SOD, a rate is obtained by
plotting time vs. oxygen depletion. The portion of each rate,
where oxygen consumption versus time is constant is used in calculation
of the rate. Appropriate corrections are made if respiration or
production is significant in the li^ht and dark bottles.
Whenever possible, analysis on a minimum of three sample
replicates should be conducted for use statistically and as a means
to determine natural variability among the sediments tested.
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FIGURE I
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Schematic and Design of Bench Model Sediment
Oxygen Demand Apparatus
1. Recorder
2. Dissolved Oxygen Meter & Probe
3. Bench Model Respirometer
4. Variable Speed Peristaltic
Pump with Closed Loop and
Tubing
Single pf flnpnt-.ypnr
Lid with rubber gasket
Water reservoir and D.O. probe port
Lock down 2 each side
• Sediment 2.5
Influent port split into 2-way diffuser
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I
Not To Scale
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