The City is designing a replacement Sourdough Water Treatment Plant.
Demand is rapidly approaching the 15 million gallon per day capacity of the current plant, we have experienced some 13 million gallon days. Under spring runoff conditions and other high turbidity events we are not likely to have the ability to process much more than 8 million gallons per day. Even with moderate population growth the current plant will not be able to meet demand in a few years.
The foundation and major concrete tankage was constructed in 1955 and is in poor condition. The aluminum filter and flocculation tankage is reaching the end of its useful life. It is highly pitted and leakage may become problematic soon. Mechanical, electrical, and control systems are dated; in some cases replacement parts are no longer available. The metal building was of poor design for a Water Treatment Plant; it is in poor condition.
The current plant was designed under a far less stringent regulatory environment. For example the effluent turbidity requirement in 1984 when the current plant was put online was 5.0 NTUs; current regulations require an effluent of 0.3 NTUs.
The current Direct Filtration plant would not be able to effectively process water in the aftermath of a large fire in the watershed. The current plant lacks an essential treatment step for high turbidity water; settling. The popularity of the watershed for recreational use causes an increased biological threat, particularly for Cryptosporidium and Giardia outbreaks. Currently available technology provides a positive barrier to bacteriological contaminates.
The removal of sticks, twigs, leaves, grass, pine needles and other large debris from the raw water.
The removal of sand and similar material.
The operators will have the option of utilizing oxidants to change the chemical composition of the water by converting undesirable constituents in the raw water to forms that may be filtered out. For example the seasonal removal of manganese for taste control may be desirable.
This process converts very small particulates in the water to larger, more readily removable particles through the addition of coagulants.
High energy mixing of raw water with coagulants so as to promote contact between the coagulant and small particles in the water.
Gentle mixing of water containing coagulants so as to allow collision of particles. This aids in the process of creating larger, more removable particles called floc.
The passage of coagulated, flocculated water through large tanks at a slow rate in order to promote the settling and removal of floc. The proposed replacement plant will utilize plate settlers allowing a smaller settling basin footprint. Also included in the same tankage is equipment to remove the sludge (settled floc).
Mechanical strainers will remove any remaining particles and debris that may damage the membranes.
The proposed plant will utilize low pressure membrane modules that will provide a positive barrier to particles larger than 0.1 micron. Giardia cyst range between 8 and 24 microns in size, Cryptosporidium Oocyst range between 3.5 and 6 microns in size (a micron is equivalent to 0.00004 inches). The membranes will also remove some viruses (a typical intestinal virus is 0.026 microns in size)
As a second barrier, the proposed plant will utilize sodium hypochlorite to kill or damage any remaining pathogens. Enough disinfectant is added to provide protection from contamination until the water comes out of the consumers tap.
Fluoride will be added to promote dental health, especially in children. This is required by City ordinance dating back to 1953.
The operators will have the ability to prevent, or lessen, the leaching of lead and copper ions from domestic plumbing systems into the water. This is accomplished by the addition of sodium hydroxide to raise the pH, making the water less aggressive.
The treatment process will result in a waste stream that will be about 5% of the raw water entering the plant. Some of the waste will be treated on site through the use of a dissolved air flotation process, settling ponds, and dechlorination. Some of the waste will be trucked to the City Water Reclamation Plant for further processing.