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How accurate are greenhouse gas emission estimates for the wastewater and sour gas sectors?

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photo of a biogas digester

By  Kurt Hansen

Various Canadian regulatory jurisdictions, as well as institutions such as the International Panel on Climate Change (IPCC) and the Western Climate Initiative (WCI), have published extensive guidelines on how to calculate reportable emissions of greenhouse gas (GHG). They contain optional methods of calculation approaches depending on industry sector, government jurisdiction and adopted calculation approaches. This means that reported GHG emissions are rarely based on actual direct measurement of these gases. Rather, they are just estimates based on “best conformance” with various dictated jurisdictional guidelines.

There are pitfalls associated with conforming to jurisdictional guidelines versus assuring the utmost jurisdictional trust in the correct scientific estimation approaches, regardless of guidelines, protocols, etc. Examples of these are a wastewater treatment plant and a sour gas processing plant.

Anaerobic wastewater treatment plant
Diagram of food processing plant wastewater plant
Figure 1. A basic process diagram of a modified anaerobic treatment plant for food processing wastewater.

Figure 1 shows a basic process diagram of a modified anaerobic treatment plant for food processing wastewater. Modifications involved covering a receiving wastewater lagoon to thermally enhance anaerobic treatment and capture the resulting methane gas for moisture removal. The gas is then used as supplemental fuel at the food processing plant.

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The modifications were made to obtain jurisdictional status as “a GHG emission offset project”. This status was granted on the principle of “best conformance” with a provincial guideline on offset emission project quantification protocol. However, many feel that the IPCC guideline is too broad and simplistic when applied to anaerobic wastewater treatment.

The IPCC principal approach is to use wastewater COD to estimate the methane mass generation rate. It factors in continuously measured wastewater flow rate (Q) and periodic COD (C) sample data.

This approach is scientifically illogical as anaerobic wastewater treatment is a biological process best monitored by means of the BOD characteristic. One provincial protocol uses a conservative default COD/BOD mass ratio of 2.4 to circumvent this IPCC COD approach. Yet, a review of a wastewater engineering handbook reveals that the 2.4 ratio is only typical for domestic wastewater. The ratio for various types of food processing wastewater ranges from 0.15 to 1.69.

This means that the IPCC-related protocol default ratio of 2.4 and other material balance calculations will result in over-estimated methane emissions captured and used as supplemental fuel.

An alternative to better quantify captured methane emissions is to measure the biogas flow rate and biogas methane content and use it to estimate the captured methane rate. The general problem with this approach is that it requires additional monitoring equipment and periodic sampling and analysis. These are resources that a wastewater treatment plant operator may be hesitant to invest in, unless made a mandatory requirement by the regulating agency.

Also, the regulatory agency might not accept the calculated avoided GHG emissions based on this approach if they feel that baseline emissions should be subtracted from current measured and calculated methane emissions. The rational for this is that the average temperature of the previously uncovered lagoon was likely lower than the covered and insulated wastewater lagoon. A warmer temperature means increased biologic activity and more methane generation.

The GHC calculation protocol uses a baseline lagoon fluid temperature. But, in reality, the specified value can only be approximate, because lagoon temperatures are not normally measured. Also, unless the wet biogas is periodically sampled and analyzed for moisture content, the calculated annual methane emission rate will be over-estimated regarding captured methane emissions.

Overall, the regulatory options for calculating offset GHG emissions from an anaerobic wastewater treatment plant lead to great uncertainties.

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