Redox Potential - A Guide to Oxidation Pond Performance
477
stability; the relative significance between biological activity in pond liquor, at the
sludge/water interface, in the sludge, and the site of maximum conversion of the
pollution load.
Redox potential measurements may provide a better understanding of the conditions
for biological activity in the different regions of the ponds. The theory and application of
oxidation-reduction potential or redox potential has been discussed by many workers of
whom Hewitt (1950) is the most authoritative. Redox potential has been used as a
monitoring device for anaerobic digestion by Grune (1965) and by Dirisian, Molof and
Borchardt (1963). There appears to have been no record of redox potential measurements
in oxidation ponds, nor has there been any attempt to relate these potentials to biological
activity, or to oxidation pond performance.
MATERIAL AND METHODS
It was found possible to measure the redox potentials of various depths in the
oxidation ponds, using a Beckman submersible Assembly (727-100) which contained a
palladium junction large reservoir reference electrode and a platinum inlay glass electrode.
This system was standardized with a 0.05M phthalate buffer to which a pinch of
quinhydrone was added. The assembly was connected to a Beckman model 900 ORP
Analyser. Continuous readings were noted by a Varian G200 recorder.
An area was selected which contained a thick layer of sludge and that was also close to
power supply. Such a site was found at the front of pond No. 3 where there is a channel
of actively digesting sludge close to the bank which divides it from pond No. 4. The
location of the electrode assembly is marked (x) in Fig. 1. A jetty was built at the
selected site and the electrode assembly fixed to the end of it. This assembly could be
raised or lowered to any desired depth. Three important zones were selected for study.
These were sludge, sludge/liquor interface and pond top liquor 10-15 cm. below the
surface. Sufficient results have been obtained during the 1970-71 period to show that
redox potential measurements could become a useful guide to oxidation pond
performance. Collection of data was restricted to two seasons, winter when oxidation
pond activity is low and summer when activity is at a maximum.
No regular measurements were made of the pH of sludge and interface between pond
liquor and sludge. However, pH measurements were made frequently as opportunity arose
during sampling over the two-year period. Values were always within the range 6.5 — 6.8
for sludge and 6.9 - 7.2 for sludge/liquor interface. Pond liquor pH values, obtained from
routine records on the Manukau Purification Works ranged from 7.1 - 8.3.
RESULTS AND DISCUSSION
Redox Potential of Sludge
Fig. 2 (a) and (b) shows the redox potentials recorded when the electrode was
positioned in the sludge 10 cm. below the sludge/liquor interface. There was no variation
from day to day, but in winter the Eh was -250 mV compared with a summer value of
Eh -190 mV. At both times the pH range was 6.5 - 6.8. The steady state of the redox
potential of the sludge indicates the stability of this region, which does not appear to be
affected by the changing conditions of the regions above. Hence its importance in the
477
stability; the relative significance between biological activity in pond liquor, at the
sludge/water interface, in the sludge, and the site of maximum conversion of the
pollution load.
Redox potential measurements may provide a better understanding of the conditions
for biological activity in the different regions of the ponds. The theory and application of
oxidation-reduction potential or redox potential has been discussed by many workers of
whom Hewitt (1950) is the most authoritative. Redox potential has been used as a
monitoring device for anaerobic digestion by Grune (1965) and by Dirisian, Molof and
Borchardt (1963). There appears to have been no record of redox potential measurements
in oxidation ponds, nor has there been any attempt to relate these potentials to biological
activity, or to oxidation pond performance.
MATERIAL AND METHODS
It was found possible to measure the redox potentials of various depths in the
oxidation ponds, using a Beckman submersible Assembly (727-100) which contained a
palladium junction large reservoir reference electrode and a platinum inlay glass electrode.
This system was standardized with a 0.05M phthalate buffer to which a pinch of
quinhydrone was added. The assembly was connected to a Beckman model 900 ORP
Analyser. Continuous readings were noted by a Varian G200 recorder.
An area was selected which contained a thick layer of sludge and that was also close to
power supply. Such a site was found at the front of pond No. 3 where there is a channel
of actively digesting sludge close to the bank which divides it from pond No. 4. The
location of the electrode assembly is marked (x) in Fig. 1. A jetty was built at the
selected site and the electrode assembly fixed to the end of it. This assembly could be
raised or lowered to any desired depth. Three important zones were selected for study.
These were sludge, sludge/liquor interface and pond top liquor 10-15 cm. below the
surface. Sufficient results have been obtained during the 1970-71 period to show that
redox potential measurements could become a useful guide to oxidation pond
performance. Collection of data was restricted to two seasons, winter when oxidation
pond activity is low and summer when activity is at a maximum.
No regular measurements were made of the pH of sludge and interface between pond
liquor and sludge. However, pH measurements were made frequently as opportunity arose
during sampling over the two-year period. Values were always within the range 6.5 — 6.8
for sludge and 6.9 - 7.2 for sludge/liquor interface. Pond liquor pH values, obtained from
routine records on the Manukau Purification Works ranged from 7.1 - 8.3.
RESULTS AND DISCUSSION
Redox Potential of Sludge
Fig. 2 (a) and (b) shows the redox potentials recorded when the electrode was
positioned in the sludge 10 cm. below the sludge/liquor interface. There was no variation
from day to day, but in winter the Eh was -250 mV compared with a summer value of
Eh -190 mV. At both times the pH range was 6.5 - 6.8. The steady state of the redox
potential of the sludge indicates the stability of this region, which does not appear to be
affected by the changing conditions of the regions above. Hence its importance in the