Dry weather diversions (also called low flow diversions) are being increasingly used to divert storm drain flows into the sanitary sewer system where they can be treated to remove pollutants. To understand how dry weather diversion works, you first need to understand that most cities have two separate sewer systems a sanitary sewer system to treat what goes down your toilets, sinks, shower drain and other building drains, and a storm sewer system to directly drain rainwater and other surface water runoff to the ocean. It is not practical to have one system to continuously collect and treat both types of flow. A treatment plant large enough to handle both sanitary wastewater and storm water would be enormous and prohibitively expensive. Some cities do combine the two sewer systems, but when it rains, the treatment plant gets overloaded and massive spills of untreated sewage, called combined sewer overflows occur.
A compromise position between two separate sewer systems and a combined sewer is to divert the storm drain flow to the sanitary sewer during dry weather when the storm drain flows are low. This can be done in a variety of ways. Small dams can be installed in storm channels and the water that collects behind the dam can be pumped or, in some cases, drained by gravity to the sanitary sewer. Sumps with installed pumps (also called "lift stations" or "pump stations") can also be constructed within or adjacent to storm drains to collect runoff flow. These systems are also often built with some type of screening device to separate out trash and debris to keep from clogging the sewer lines. In either case, the sumps, pumps, and pipelines are sized to handle the low flows that occur during dry weather. When significant rainfall occurs, the sumps or dams in the dry weather diversion systems overflow and the majority of rainfall runoff drains to the ocean.
The practice of collecting and treating dry weather flows has the potential of significantly improving ocean water quality in the immediate vicinity of the storm drain outlet. If there is no water coming out of the storm drain, there's no pollution. This "end of pipe solution" is not without its drawbacks. The sewage treatment plant accepting the diverted flow from the storm sewers must have the capacity to handle the flows and must be able to effectively treat or remove the pollutants. Another potential pitfall with dry weather diversions is that concerns about discharges to the storm sewer system (pollutants and excessive water flow) may be ignored since "the sewage treatment plant will take care of it." Despite the benefits offered by dry weather diversions, we should continue to strive to keep all pollutants out of the storm drain system and minimize water discharges through water conservation, infiltration of runoff into the ground, and recycling.
In a paper Measuring the effects of stormwater mitigation on beach attendance published in Marine Pollution Bulletin, Volume 72, Issue 1, 15 July 2013, Pages 87–93, the authors state that data collected at 26 beaches in Southern California over a 10-year period indicate that beach attendance increased at beaches with diversions compared to those that did not have diversions.
Also see: Dry Weather Diversions in California - Diverted by Diversions?