Municipal wastewater treatment indicator: data sources and methods, chapter 5

5. Caveats and limitations

The representativeness of the survey sample with respect to community size has changed over time. From 1983 to 1999, the Municipal Water Use and Pricing (MUD) survey collected data only from municipalities with a population greater than 1000 residents. In 2001, the Municipal Water and Wastewater Survey (MWWS) was expanded to sample municipalities with populations less than 1000. Although improvements have been made, municipalities with small populations remain under-represented in the MWWS, while municipalities with large populations are over-represented. It is likely the percentage of the population on private septic systems is underestimated.

Both the MUD survey and the MWWS exclude municipalities on federal lands and First Nations communities.

Changes in the survey methodology likely account for the decrease from 2004 to 2006 in the percentage of the responding population reported as receiving secondary treatment. In 2004, a portion of the data was imputed from 1999 values, at which time municipalities were placed in the secondary treatment category if they used a combination of primary and tertiary treatment. In 2006, secondary treatment was taken to mean only mechanical or waste stabilization processes, while tertiary treatment was reserved for municipalities using at least one nutrient removal process.

Changes in the survey methodology likely account for the decrease from 2006 to 2009 in the percentage of the population receiving tertiary treatment. In 2006, municipalities were considered to have tertiary or advanced treatment if they used just one nutrient removal process, regardless of the treatment that the effluent had received. In 2009, the question was revised to reflect treatment levels rather than treatment processes. As a result, some municipalities that fell into the tertiary treatment category in 2006 were classified in the secondary or primary treatment category in 2009.

In 2009, the methodology to calculate the percentage of the population served by sewers changed so as to be based on the entire municipal population. The MWWS data for 2004 and 2006 are based only on the municipal population on sewer systems. The indicator results have been corrected so that the 2004 and 2006 results are calculated with the 2009 methodology. Small differences may exist between the data used for the CESI indicator and the MWWS 2004 and 2006 data.

Although this indicator assumes municipal wastewater plants are functioning at their design level, equipment failure and weather conditions may prevent them from doing so. Severe storms can cause overflows in combined sanitary and stormwater sewer systems. During such events, the everyday treatment level is not applied and raw sewage is released directly to surface waters.

No treatment information is available for sewage treated by private septic systems. Treatment levels depend on the efficiency and maintenance of the septic system. Similarly, no information is collected on the destination of hauled sewage or its treatment level.

This indicator is not a measure of compliance with municipal, provincial or federal wastewater regulations or treatment standards. For more information on the Government of Canada's role in wastewater management, see Environment Canada's Wastewater Management website. For more information on national efforts to develop a harmonized regulatory framework for municipal wastewater treatment, see the Canadian Council of Ministers of the Environment's Canada-wide Strategy for the Management of Municipal Wastewater Effluent.