Greenhouse gas emissions from wastewater treatment schemes - New Zealand case study examples

Both public and private sector organisations are increasingly being required to report on, manage and (where possible) reduce their greenhouse gas (GHG) emissions. Key drivers in this area include central and local government level initiatives (such as the Carbon Neutral Public Service pro gram and New Zealand’s national commitment to the Kyoto Protocol).

The assessment and management of greenhouse gas emissions is of interest for wastewater treatment and conveyance facilities as these facilities can be relatively energy intensive, and can contribute to the production of greenhouse gasses such as methane, carbon dioxide and nitrous oxide.

For wastewater treatment plants GHG management typically becomes a balance between energy intensive process routes such as high rate, or forced aeration systems which use more electricity but produce a higher quality effluent and avoid methanogenic process routes and low energy process methods such as passive pond systems which use significantly less power but may produce lower quality effluent and use methanogenic process routes.

This paper discusses two recent case studies undertaken to assess GHG emissions from wastewater treatment systems within New Zealand. The first addresses a waste water treatment plant upgrading from a facultative pond system to a (more power-intensive) Sequencing Batch Reactor (SBR) activated sludge system with the key finding that (by avoiding methanogenic process routes) the more energy-intensive SBR plant may have a smaller GHG footprint than the less energy-intensive pond system.

The second discusses GHG emissions for a land disposal scheme and identifies environmental nitrogen enrichment, and on site power use as the main contributors to the system’s GHG footprint. Tree planting in the site’s buffer zones was also assessed in terms of its capacity to sequester carbon and thereby offset emissions.