Adapting Combined Sewer-Stormwater Systems to Climate Change

"Using combined systems was common practice for urban sewers built before the turn of the 20th century, said [San Francisco Public Utilities Commission] PUC General Manager Harlan Kelly Jr., when wastewater and rainfall both were typically dumped into nearby bodies of water," reports Dominic Fracassa for the San Francisco Chronicle on Oct. 21.

Because the city's sewer system can be easily overwhelmed during heavy downpours, and is vulnerable due to king tides and sea level rise, all of which will become more intense due to climate change, the San Francisco Public Utilities Commission, which provides retail drinking water and wastewater services to the city of San Francisco, is at the forefront of climate adaptation efforts.

The PUC’s improvement program will be rolled out in phases through 2032. The improvements themselves — everything from replacing outdated sewer pipes to raising critical electrical systems in treatment plants in anticipation of the higher sea waters — are designed to respond to how San Francisco’s climate could look in the year 2100. 

Another critical element of the PUC plan is an ambitious program of installing bioretention or rain gardens to divert stormwater from the sewer system and allow it to be absorbed into the ground. "[B]y allowing natural processes to take over the work we've been building infrastructure to handle, operations and maintenance costs also fall," according to a 2012 study on green infrastructure co-authored by the American Society of Landscape Architects.

In August, Fracassa reported on a ribbon-cutting by the PUC for the first of the eight rain gardens planned for the Ingleside district, an investment of $7.3 million.