There have been some very valid questions from multiple parties about the lifespan of one filter unit. Below is a paper I cited in both my EPA and thesis research that explores this topic.
An experimental study on the water-purification properties
of porous concrete
Sung-Bum Parka,*, Mang Tiab
aDepartment of Civil Engineering, Chungnam National University, Daejeon, Republic of Korea
bDepartment of Civil and Coastal Engineering, University of Florida, Gainesville, FL, USA
Received 19 November 2002; accepted 1 July 2003
Units of pervious concrete were tested in fairly neutral waters to test the strength of the concrete after constant immersion as well as the ability of bacteria housed in the concrete to remove/reduce Total Phosphorous and Total Nitrogen.
The pervious units were immersed in neutral pH water for 90 days (1 unit per tank). The pH after immersion (day 1) began at 12 and settled to 9 at 90 days. The pervious concrete units did not use standard type II portland cement, instead they used varying types of fly ash to lower the pH to allow the bacteria to be able to survive.
Based on comparing the pervious concrete units in the above paper and the units in my experiments and factoring in the use of unaltered type II portland cement in my experiments, I can comfortably say that I would expect one unit to have a lifespan in excess of 7 months in waters with a pH greater than 6. Less in more acidic waters of course.
But also consider that in all cases except for humanitarian, more than one unit would be in use. And as the units initially exposed to the influent would began to expire, units further down the array, which have been exposed to the elevated pH waters coming from the initial units, would pick up the slack.
And the units tested in the above paper were completely immersed. In natural conditions, high influent volume would only occur during spring and possibly a monsoon season (which would most likely not be as high as during simultaneous snow melt and spring precipitation). So much of the time, I would estimate that an array would only be 50% saturated at most outside of the high flow season(s).