Pond Upgrade or Plant Rebuild - A Guide to Navigating the Pros and Cons

John Crawford, Reuben Bouman, Claire Scrimgeour – Beca Ltd

Due to higher environmental performance expectations from the public, and the added rigour applied to consenting processes through a matured RMA (including guidance such as the National Policy Statement for Freshwater Management, MfE 2020), plus additional capacity required due to municipal and industrial growth, many pond systems have been unable to comply with treatment requirements, and in particular nitrogen limits. 

To provide for likely future Capacity and Levels of Service requirements and to upgrade 'tired', end of life assets, the owners, typically local authorities, are faced with the dilemma of whether to persevere with the ponds as core treatment process at a particular site or to largely start again with new technology.  This paper considers the pros and cons of each approach from a number of different perspectives related to future management of the asset or site. While these may seem obvious to some, the answers are not necessarily clear cut and we note that there are still a lot of asset owners in the wastewater sector for whom the best way forward is still unclear and confusing. 

There are many different possibilities for upgrading of oxidation pond systems to improve capacity and performance. Hugh Ratsey dealt with many of these in his excellent 2016 paper This paper will not revisit the 'options' part of the equation, except to briefly mention the newer- options that have subsequently arrived on the scene. These include the 'Return Stream MBBR' (Moving Bed Biofilm Reactor) as employed at Hāwea, and the newer side stream MABR (Membrane Aerated Bioreactor) technology being employed at Helensville and Te Kauwhata.