An analysis using daily water and wastewater records can provide the answer to the question, "how much treated water ends up in the sewer?" while helping municipalities to anticipate the impact of water conservation on servicing requirements.

To the uninitiated, the answer seems straightforward enough. The ratio of wastewater flow to water demand should roughly be 1.0, given the volume of water consumed is approximately equal to the sewage generated.

A 1993 Water Environment Association of Ontario (WEAO) survey revealed a surprising amount of deviation from this. Of fifty municipalities surveyed, forty-five reported ratios of annual wastewater treated to water produced ranging from 0.5 to 2.0, and five from 2.0 to 4.0.

Those familiar with water and wastewater servicing know that simple ratios such as these fail to provide a perspective on real life. Ratios tend to obscure the reality of our less than perfect world - a world with leaky sewers, water loss, inaccuracies in metering and the need for large volumes of water for irrigation - and should be largely ignored. The path from water treatment plant to sewage treatment plant (STP) is far from predictable.

It is only by disaggregating daily water demand and daily wastewater flow that their relationship can be understood. In a desk top analysis, CH2M Gore & Storrie disaggregated several years of daily water production and wastewater treatment records for three Ontario communities, referred to as C1, C2 and C3.

In order to compare water use to sewage flows under normal conditions, extreme daily water demand and sewage flow figures needed to be tagged and excluded. Extremes were experienced on the water side on days when demand for irrigation elevated water demand, and on the wastewater side, when precipitation occurred, resulting in excess flows to the STP.

By estimating dry weather wastewater flow and normal water use, and comparing these with yearly averages (i.e. average over 365 days, including the "extremes"), first cut estimates on the contribution of inflow/infiltration (I/I) to the STP and the amount of water used for irrigation could be made.

In the sample communities, the ratio of annual sewage flow to water produced was 0.88, 1.09 and 1.22 (Table 1). Superficially, the relationship between water and wastewater appeared reasonably close to 1:1 in all communities.

However, when the statistics on these three municipalities were analyzed, it was found that C1, instead of having what appeared to be a good water to sewage ratio, in fact, had a system of sewers that contributed an excess flow of 39% from groundwater and stormwater sources (I/I).

C2 was found to have little flow from wet weather events but contributed groundwater flows to the STP to the tune of 22% of total flows over the year.

Wet weather I/I was certainly a problem for C1 and C3. These communities, however, have some combined sewers which partially account for their high wet weather flows. Age of their sewer systems is another factor.

The Figure provides a profile of the water users and wastewater contributors in C2 and C3. Note the major differences in the per capita water and wastewater figures. Evidently, each municipality has unique characteristics that affect servicing more than is immediately apparent to the consumer.

For example, it is commonly assumed that water conservation in homes will have a direct influence on flows at the STP. However, if I/I comprises 50% of flows to the STP (as in C3), and domestic and industrial/commercial water consumption equally split the remaining 50%, then the impact of conservation of domestic water would be seriously blunted by the 75% of annual wastewater from industrial and I/I sources. The real reduction in wastewater will be 1/4% for every gain of 1% in reduction of domestic sewage flows. This community may get a bigger "bang for its buck" by attacking its I/I problem. In communities with other characteristics, residential conservation programs could be the most effective measure to reduce water demand or wastewater flow.

It is important to appreciate that the methodology presented is intended to generally characterize water and wastewater systems for planning purposes. Meter errors, service area differences, the influence of wet industries and other variables point to the need to temper results with judgement and system-specific experience. In fact, the extreme sewage to water ratios derived from the WEAO survey likely reveal more about the existence of such data problems or local anomalies than about the actual relationship between water and wastewater.

Nevertheless, with some tailoring to reflect local conditions, the community profiles generated through this exercise provide a valuable perspective for evaluating future servicing options, establishing water conservation policies, setting water and sewer rates and undertaking capital planning. The profiles are also valuable educational tools for demonstrating to the public how servicing priorities are set and how public funds can be most wisely directed.

Furthermore, in considering privatization of water and wastewater facilities, municipalities and private bidders need to understand the major influences on water demand and wastewater flow that could affect the economics of a project. For instance, inflow and infiltration into sewers is beyond the control of private plant operators, yet may greatly impact plant operations. Privatization contracts must consider, and be accountable for such influences. Even municipalities owning their own treatment plants need to know whether they should invest in reducing I/I or in expanding plants to meet future requirements. Profiling water and wastewater systems using the system described can provide the information required to help make these decisions.

The proposed analysis can be done with minimal effort and cost. Once an initial program is developed for a community, municipalities can maintain and update the analysis inhouse in the future.

The answer to the question "how much treated water ends up in the sewer" is within the notebooks and spreadsheets of each individual municipality - and cannot be deduced from any studies, trends or literature. Suffice it to say that rarely is the relationship between water and wastewater as close as it appears.

Reference: Water Environment Association of Ontario. Survey of Municipal Sewage Systems in Ontario - 1993.

Note: The full version of this article includes tables on Community Population Profiles and unaccounted for water usage.