Municipal wastewater treatment indicator: data sources and methods, chapter 4

4. Methods

The Municipal Water and Wastewater Survey (MWWS) categorizes the sample population based on whether it is served by municipal sewer systems. Municipalities with sewer systems are asked to classify their sewage treatment level as no, preliminary, primary, secondary - mechanical, secondary - waste stabilization or tertiary treatment. For the purposes of this indicator, the population not served by municipal sewer systems is assumed to have septic systems or private haulage for their sanitary waste.

Wastewater treatment levels for this indicator were categorized based on the definitions used in the MWWSFootnote [1]:

No treatment/preliminary treatment: Wastewater receives no treatment or preliminary treatment only. Preliminary treatment removes relatively large solids from wastewater and refers to processes such as grit removal or skimming, or the use of screens or bar racks.

Primary treatment: treatment processes that allow undissolved solids in raw sewage to settle out of suspension, forming sludge. Some common techniques include primary sedimentation/clarification, plate/tube settlers, and chemical precipitation/flocculation.

Secondary treatment: treatment processes for the removal of organic matter from sewage to reduce biochemical oxygen demand and suspended solids. This result may be achieved using mechanical systems or lagoons and waste stabilization ponds (WSPs). Mechanical systems include conventional activated sludge, extended aeration activated sludge, pure oxygen activated sludge, other types of activated sludge, oxidation ditches, trickling filters, rotating biological contactors, and sequencing batch reactors. Lagoons and WSPs may be aerated, aerobic, anaerobic, facultative or storage ponds.

Tertiary treatment: enhanced treatment to remove specific substances of concern to achieve a particular level of desired effluent quality. It can be accomplished using a number of physical, chemical or biological processes. In 2006 and prior surveys, plants with any nutrient removal process were classified as tertiary; in 2009, only municipalities identifying their treatment processes as tertiary were classified as tertiary.

Septic system: an on-site wastewater disposal consisting of a buried concrete septic tank that holds the solid waste from a home's plumbing waste drains, and a septic drain field that distributes the wastewater to the ground where it disperses through the soil or evaporates.

Haulage: systems where wastewater is pumped from a collection tank and taken to a disposal site. Sewage haulage mostly occurs in Northern communities.

To calculate the Municipal Wastewater Treatment indicator, the percentage of the population served by sewers is compiled using data from the MWWS. The population served by sewers is divided by the responding population for the survey question. Population percentages served by no, primary, secondary and tertiary treatment are calculated by dividing the population per wastewater treatment level by the responding population for that survey question. The population per wastewater treatment level is then applied to the percentage of the population served by sewers to obtain the final data for this indicator.

Not all municipalities report the treatment level of their population served by sewers, creating a small population served by sewers for which there is no information about wastewater treatment. For example, in 2009, the MWWS reports data for 24 498 272 Canadians living in municipalities served by sewers. Out of that population, the survey reports wastewater treatment level results for 23 618 481 Canadians. Given that information on treatment levels is missing for less than 8% of the total survey population, the distribution of wastewater treatment levels is applied directly to the population served by sewers.

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