On November 30, 1891, the City of Seattle adopts a plan to build a combined sewer system to handle both sewage and stormwater. The city has used cesspools and wooden-box sewers for sewage, but those measures are no longer adequate for the growing population. Although a plan developed in 1889 called for separating sewage from stormwater, it was not adopted, likely because of the cost associated with building a separate system for stormwater. Two years later, the city hired civil engineer Benezette Williams (1844-1914) to develop a system handling sewage and stormwater in the same pipes. City Engineer R. H. Thomson (1856-1949) will begin implementing a combination of the two plans in 1893. Sewer trunk lines will carry wastewater and storm drainage to the Duwamish River and Puget Sound. When heavy rains overwhelm the system with stormwater, the overflow, which includes untreated sewage, will discharge into Lake Washington, Lake Union, Puget Sound, and the Duwamish River. These "combined sewer overflows" will involve millions of gallons of water and add significant pollution to local waterways. Beginning in the 1980s, the city will develop solutions to prevent combined sewer overflows, using tanks to store sewage until treatment plants can handle the wastewater.
As Seattle's population grew in the 1880s and 1890s, the city began developing a sewer system. For several decades residents had relied on a rudimentary system of wooden-box sewers and cesspools, but these posed health hazards to the people living and working around them. Many of them utilized open ditches or contaminated nearby water sources, such as springs or lakes.
A large diphtheria epidemic in the mid-1870s killed numerous children and brought attention to the problem, but it was not until 1885 that the city council required that all residential properties connect to sewer lines. Most of the lines discharged untreated sewage into Elliott Bay, Lake Washington, and Lake Union. In the bay, tides, currents, and the large volume of water in Puget Sound carried sewage away and diluted it. Lake Union and Lake Washington did not have strong currents, however, and the sewage contaminated their waters.
According to historian Matthew Klingle, the 1889 fire that leveled a large part of the city provided an opportunity to rework the existing sewer system and improve it. City officials considered an 1889 plan developed by George E. Waring Jr. (1833-1898), an expert on sewer systems. The plan itself is not in city records, but other documents refer to Waring's plan and criticize his contention that the city only needed to build a sewer system for wastewater. According to an article about the Memphis sewer system that Waring co-wrote in 1881, he believed that stormwater could be handled on the surface and did not need to be routed into pipes. Benezette Williams, another civil engineer who would do extensive work on Seattle's water and sewer systems, described Waring's 1889 plan in his own sewer-system plan. According to Williams, Waring had concerns that sewer gases would build up in the sewer system during the summer and fall dry season, so that stormwater should be excluded entirely and water for flushing the system regularly should be stored in tanks to be released as needed.
Waring's plan was not adopted. It is likely that city officials realized his recommendation for a sewage-only system would require the additional construction of a stormwater system. Williams stated, in his discussion of Waring's plan, that Seattle's rain falls primarily in six months and during that time the ground is regularly saturated with water and so runoff would have to be managed. Allowing it to run downhill in gutters, as Waring recommended, would result in puddles and the erosion of street surfaces due to the volume of water that would be moving across the ground.
A Single System
The city then hired Benezette Williams to design a sewer system. He quickly dispensed with Waring's ideas:
"In short, the accumulated experience of all populous cities and towns leaves no chance for being mistaken in the assertion that the underground removal of storm water is a necessity in a modern city. Any attempt to dispense with it is a retrograde movement, and one not to be tolerated at the present day" (Williams, 4).
Williams submitted his sewer-system plan to the city in 1891. It relied on a single system of pipes to take both sewage and stormwater to outfalls in local waterways. These pipes would carry sewage from buildings and stormwater from storm drains and gutters connected to underground drainage pipes.
The city council adopted the plan on November 30, 1891, but it was not immediately implemented. When City Engineer R. H. Thomson did construct the system, he used elements of both plans. Following Williams's plan, the city built two sewer trunk lines that began carrying wastewater in 1894. One, on the north side of downtown, carried sewage to Elliott Bay, at the base of Denny Hill (before it was regraded). The other, on the south end of downtown, discharged onto the tide flats south of King Street until they were filled and the outfall pipe was extended to the Duwamish River in 1910. Thomson did not direct any of the sewage to Lake Washington, as Williams had recommended, considering it too close to town.
Dealing with Overflows
Tides and currents carried the sewage away from the city and, although water-treatment facilities would not be developed until the 1920s, the city saw a marked decrease in illnesses and deaths attributed to polluted water. Unfortunately, the system was not designed to handle the volumes of wastewater and drainage that would flow into it during heavy storms once the city was fully developed. It relied on overflow outlets along the lakes, the Duwamish River, and Puget Sound that released untreated sewage along with stormwater when the water volume overwhelmed the system.
Treatment plants, built beginning in the 1920s, eased the pollution of waterways, but storm-related overflows continued to release millions of gallons of untreated wastewater into waterways each year. In the 1960s and 1970s the city worked to separate sewer and storm-drainage lines in the parts of the city that had been developed before the 1950s with combined sewer systems.
In the 1980s, the city began to address pollution caused by combined sewer overflows (releases of untreated wastewater during storms) and untreated drainage water that did not go to water treatment plants. Since 2001, the city has been making changes to limit combined sewer overflows, the ones that come from parts of the city that still carry sewage and stormwater in the same pipes, to one overflow per outfall per year, by building inline storage tanks that hold excess water until the treatment plants can handle the volume.