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Cost for Current Sewer Users

The usage fee for the South Acton sewer has risen 70% since its inception in 2002. This can be seen the plot below, which shows the usage rate (in dollars/gallon), as well as the number of hookups across the sewer district, over time:

The trend of increasing sewer cost is not unique to Acton, as can be seen in the following plot, which shows annual sewer cost for a family of four using 60 gallons/person/day in several towns in Metrowest:

It has been suggested that if the sewer expands into West Acton, the usage fee will decrease. This phenomenon is called economy of scale. To find out if this is the case for sewer expansions, we looked at state-wide surveys of sewer fees conducted by consulting firm Tighe and Bond.

We asked the question, Does sewer district expansion result in lower fees? Unfortunately, the data indicates that economy of scale is only assured for large expansions encompassing at least 4000 new users. This is illustrated below.

The plot below shows the change in sewer fee vs. change in user base between 2002 and 2012 in districts across Massachusetts. (The last year that district population was surveyed is 2012.) Red dots represent districts. Next to each red dot is the district name and a dollar amount representing the 2012 sewer fee. This fee was calculated for a family of four using 60 gallons/person/day.

The first thing to note is that fees haven't decreased anywhere (except in one district, Williamstown). So what we're looking for is not where fees have decreased, but where smaller increases have occurred. Districts in roughly the top half of the plot had higher than average fee increases, while districts in the lower half had lower than average fee increases.

First, note that districts which have not expanded are shown just to the left of the orange box, along the y-axis. Acton's sewer district hasn't expanded since 2002, although hookups in the district continue. Hence, Acton reported a district size of 2000 people for every year of the Tighe & Bond survey. Note that the average fee increase for districts along the y-axis is $308.

Now consider districts in the orange box. These districts expanded in size, but by less than 4000 people. Their average fee increase is $339.

Finally, in the green box, we see districts which expanded by more than 4000 people. Their fee increases were far below average, with an average of just $180. We can summarize this behavior in a bar plot:

The bar plot shows that lower fee increases are associated with districts that expand by 4000 or more people, while higher fee increases are associated with districts that expand by less than 4000 people.

If the South Acton sewer is extended into West Acton, the district size will increase by approximately 500 people, which is far too few to ensure economy of scale. Moreover, we would expect a West Acton sewer expansion to result in a greater increase in sewer fees for South Acton users, reflecting the behavior of the orange bar in the above plot.


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