REDOX POTENTIAL -
A GUIDE TO OXIDATION
POND PERFORMANCE
O.D. BROCKETT* and P. WELSBY
*Post Doctoral Fellow, c/o Botany Department, Auckland University
Auckland, New Zealand
INTRODUCTION
The Manukau Purification Works are designed to treat the sewage from an ultimate
population equivalent to 1.5 million (800,000 people, 700,000 trades waste equivalent).
Primary treatment employs screening, sedimentation and heated sludge digestion with
power generation. Secondary treatment occurs in four oxidation ponds, approximately
equal in area, totalling about 1,400 acres, 4 ft. 6 in. deep. Commissioning took place in
November 1960, and a consistently satisfactory effluent has been produced and
discharged into the Manukau Harbour since then. However, despite the overall success of
the Plant, problems were encountered in the operation of the oxidation ponds, which
highlighted the need to develop careful control measures. In particular it was felt that a
system of short term forecasting of biological changes in the ponds would be an
advantage.
Midge nuisance
The ponds were formed by constructing embankments between the foreshore and
Puketutu Island (Fig. 1), thus impounding a tidal flat. This work was completed about a
year before the Works started up, and during this time the salt water was leached out and
replace by fresh water, midge bloodworms began to thrive in the bottom of the ponds,
midge (Chironomus zealandicus) on the wing reached nuisance proportions and extensive
spraying of insecticides was required. This midge problem persisted for three more years
but then declined quite dramatically until about 1965, and since then very few
bloodworms have been observed. It was considered likely that increasing loading, together
with ageing of the ponds had progressively increased the amount of sludge at the bottom,
creating an anaerobic environment inimical to bloodworms.
Biological variations
There is, not unexpectedly, a tendency for the types of flora and fauna to vary with
weather conditions, causing wide fluctuations in dissolved oxygen concentrations. For
example a prolific growth of green algae, promoted by long hours of sunlight, with
Chlorella spp. generally predominating is often arrested by increasing numbers of
predators such as rotifers or Daphnia spp. However, particularly in the late summer and
autumn blooms of blue-green algae have occurred under circumstances which cannot be
simgly explained. These blooms occurred more regularly during the earlier years of
operation. In 1960 and 1961 there were rapid and troublesome blooms of Nodularia spp.
and later Anacystis spp. In the following few years blooms of varying intensity of
Anacystis spp., or Oscillatoria spp. were observed, and typically, the algae tended to
agglomerate into floating mats, the surfaces of which desiccated and caused unpleasant
odours. Also the dead and moribund algae increased the oxygen demand. Fortunately the
blooms, though causing severe problems, were of relatively short duration, and the
475
A GUIDE TO OXIDATION
POND PERFORMANCE
O.D. BROCKETT* and P. WELSBY
*Post Doctoral Fellow, c/o Botany Department, Auckland University
Auckland, New Zealand
INTRODUCTION
The Manukau Purification Works are designed to treat the sewage from an ultimate
population equivalent to 1.5 million (800,000 people, 700,000 trades waste equivalent).
Primary treatment employs screening, sedimentation and heated sludge digestion with
power generation. Secondary treatment occurs in four oxidation ponds, approximately
equal in area, totalling about 1,400 acres, 4 ft. 6 in. deep. Commissioning took place in
November 1960, and a consistently satisfactory effluent has been produced and
discharged into the Manukau Harbour since then. However, despite the overall success of
the Plant, problems were encountered in the operation of the oxidation ponds, which
highlighted the need to develop careful control measures. In particular it was felt that a
system of short term forecasting of biological changes in the ponds would be an
advantage.
Midge nuisance
The ponds were formed by constructing embankments between the foreshore and
Puketutu Island (Fig. 1), thus impounding a tidal flat. This work was completed about a
year before the Works started up, and during this time the salt water was leached out and
replace by fresh water, midge bloodworms began to thrive in the bottom of the ponds,
midge (Chironomus zealandicus) on the wing reached nuisance proportions and extensive
spraying of insecticides was required. This midge problem persisted for three more years
but then declined quite dramatically until about 1965, and since then very few
bloodworms have been observed. It was considered likely that increasing loading, together
with ageing of the ponds had progressively increased the amount of sludge at the bottom,
creating an anaerobic environment inimical to bloodworms.
Biological variations
There is, not unexpectedly, a tendency for the types of flora and fauna to vary with
weather conditions, causing wide fluctuations in dissolved oxygen concentrations. For
example a prolific growth of green algae, promoted by long hours of sunlight, with
Chlorella spp. generally predominating is often arrested by increasing numbers of
predators such as rotifers or Daphnia spp. However, particularly in the late summer and
autumn blooms of blue-green algae have occurred under circumstances which cannot be
simgly explained. These blooms occurred more regularly during the earlier years of
operation. In 1960 and 1961 there were rapid and troublesome blooms of Nodularia spp.
and later Anacystis spp. In the following few years blooms of varying intensity of
Anacystis spp., or Oscillatoria spp. were observed, and typically, the algae tended to
agglomerate into floating mats, the surfaces of which desiccated and caused unpleasant
odours. Also the dead and moribund algae increased the oxygen demand. Fortunately the
blooms, though causing severe problems, were of relatively short duration, and the
475