How is my sewer bill calculated?

Sewer bills are generated from the total water consumption per the readings from Pembroke Water Works.  

The Sewer Rate per 1,000 gallons are as follows:

2007$6.50
2008$6.83
2009$6.83
2010$7.05
2011$7.60
2012$7.60
2013$7.60
2014$7.59
2015$7.58
2016$7.57
2021 -$7.56