Effective long-channel, pond-based wastewater treatment at Otorohanga

Cliff Boyt (Cliff Boyt Consulting)

The author has been involved with the Otorohanga wastewater treatment plant since 2010 and
has been instrumental in changing it from a traditional oxidation pond to a long-channel,
pond-based plant which performs well. In the process the author has learnt much about how
the wastewater is treated progressively on its journey through the long channel, the keys to
successful treatment and the importance of sludge management in ensuring sound treatment
performance through the pond.
The paper follows on from the paper about the Otorohanga WWTP optimisation that was
presented at the 2019 WaterNZ conference, brings in the role of Advanced Microbial
Digestion (AMD) in long-term management of sludge and the effectiveness of after-pond
coagulant dosing for reduction of Total Phosphorous and other parameters related to pond-based treatment. The plant in this configuration requires only minimal attendance by the
plant operators and is very robust.
The paper is intended to share the author’s learnings and to show how an old oxidation pond
can be modified at reasonable capital cost to provide robust treatment performance outcomes
and to avoid the much higher capital and operating costs and risks associated with moving to
some alternative tank-based secondary treatment system.