Estimating GHG Emissions from Wastewater Treatment

Introduction

As part of its commitment to quantifying its organisational greenhouse gas (GHG) emissions, North Shore City Council (NSCC) commissioned Carbon Group to develop algorithms to estimate on-site fugitive emissions from the Council’s wastewater treatment plant (the Rosedale WWTP).  In addition, fugitive emissions for the year to June 2009 were estimated, using the algorithms.

This paper provides an overview of the results of the project.  Please contact Carbon Group (details below) for further details on the project.

The estimation algorithms will be incorporated into the NSCC’s GHG system (FoundationFootprint™), enabling the fugitive GHG emissions to be estimated on an ongoing, periodic basis.  These fugitive emissions are classified as Scope 1 (Direct) emissions under the GHG accounting rules.

Fugitive GHG Emissions from Wastewater Treatment

Wastewater treatment results in the emission of all three of the main GHGs – carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O).  However, for wastewater treatment, CO2 emissions do not need to be considered, in line with GHG accounting standards.
The emissions of methane and nitrous oxide take place at various stages of the treatment process; in addition, there are emissions during pre-treatment (i.e. raw wastewater collection and transfer) and post treatment (i.e. biosolid disposal and treated effluent dispersal). 

Methane emissions result from the metabolism of organic matter by microorganisms under anaerobic conditions. This mainly occurs in the treatment of sludge.  (Trace amounts of methane are also generated in the sewer and variable levels produced from biosolid disposal.)  Advanced treatment plants, including Rosedale, have biogas collection.  The collected gas is either used for power generation (as at Rosedale) or flared.

Nitrous oxide emissions occur as a by-product during the conversion of organic nitrogen and ammonium into nitrogen gas, via nitrification and denitrification.  These emissions mainly occur in the treatment process, and in the aquatic receiving environment (post discharge).  Technologically advanced treatment plants (like the Rosedale WWTP) generally result in the removal of more nitrogen during the treatment process (when compared to other treatment process utilized within NZ, such as oxidation ponds), thereby limiting the amount of residual nitrogen in the effluent being discharged into the receiving environment.

Whilst there is certainty over the conditions under which emissions of the different gases occur, there is some uncertainty regarding quantifying these emissions, especially for nitrous oxide.  For this project, Carbon Group has used output from some of the latest global research, although it is widely acknowledged that further work is required to reduce levels of uncertainty.
The relative amounts of GHGs emitted depend on a wide range of factors, including:

• The method, and extent, of anaerobic treatment (methane)
• The collection of methane (for power generation and/or flaring)
• The method, and extent, of aerobic agitation/aeration of the wastewater (nitrous oxide)
• The composition of the input wastewater (methane and nitrous oxide)

One of the main areas of uncertainty, recognised internationally, is the quantification of nitrous oxide emissions from the nitrification and de-nitrification processes, which mainly occur in the activated sludge reactor.  The levels of nitrous oxide emissions are dependent on a range of factors (such as carbon substrate and dissolved oxygen levels).  As a consequence, and as a result of methodological differences, the international literature provides a range of emission factors for this type of process.  For this study we have used a median value from these studies.
 

Estimated Fugitive Emissions

The estimated fugitive GHG emissions from the Rosedale WWTP for the period July 2008 to June 2009 were 4651 tonnes of CO2 equivalent.  For the reasons noted above, there is a high level of uncertainty associated with these estimates.
The GHG breakdown, together with the main input variables, is shown in Table 1.

Table 1: Estimated fugitive emissions (2008/2009)

Estimated Avoided Emissions

The capture and use of biogas for on-site power generation avoids two sources of GHG emissions:
1. Avoidance of methane release, from the capture of the biogas
2. Avoidance of fossil fuel combustion associated with the reduced need for grid provided electricity, from the use of biogas for power generation
Using this approach, the estimated avoided GHG emissions associated with the capture and use of methane at the Rosedale WWTP for the period July 2008 to June 2009 were 30,915 tonnes of CO2 equivalent.

The breakdown, together with the main input variables, is shown in Table 2.

Table 2: Estimated avoided emissions (2008/2009)

Report Authors

This summary and the project report have been prepared by Carbon Group, specialists in GHG emissions management and clean technology.  Input on wastewater treatment was provided by Auckland-based environmental services company, and Carbon Group sister company, Andrew.Stewart Limited.  Carbon Group is grateful for the assistance from the Rosedale WWTP manager during the project.
For further details contact Phil Jones  (09 984 7751).

September 2009