The City of Calistoga’s Dunaweal Wastewater Treatment Plant (WWTP) is a 0.84 million gallon per day (mgd) average dry weather flow activated sludge tertiary treatment plant. The treatment processes consist of the following; primary treatment by coarse bar screening at the headworks structure, secondary treatment by aeration and clarification, tertiary treatment by coagulation, filtration and disinfection. After tertiary treatment, effluent may be discharged to the Napa River from October 1st through May 15th (as per NPDES Permit No. CA0037966, Order 00-1312). During the remainder of the year, effluent is distributed for recycled water use or stored for future use in effluent storage ponds. The following sections include detailed descriptions of the treatment processes and the equipment used.
The headworks structure receives raw wastewater from the City of Calistoga’s collection system through an 18 inch diameter gravity trunk line. Although the treatment plant is capable of treating a peak wet weather flow of 4 mgd, the headworks structure is designed for flows up to 7 mgd.
Aeration Basins and Secondary Clarifiers
The aeration basins receive primary effluent from the headworks structure. This is an aerobic biological process, where microorganisms (activated sludge) stabilize the organics in the water. Aeration occurs from fine-bubble diffusers, suspended near the bottom of the aeration basins; displace the activated sludge as the air rises to the surface. The slow, continuous oscillation created by the diffusers results in high-efficiency mixing and maximizes oxygen uptake by the microorganisms. The activated sludge flows from the aeration basins into the secondary clarifiers where settling can occur. This step is required in the process to separate activated sludge from the treated water. The settled water flows over a weir as secondary clarifier effluent into the filter influent wet pit. The microorganisms are returned to the aeration basin to continue stabilizing incoming organics.
Two variable speed filter pumps are designed to pump up to 1 mgd of secondary effluent from the clarifiers to the filters. Alum and polymer are injected at the filter pumps and mixed with the effluent in order to achieve coagulation prior to filtration. Parkson DynaSand Filters are a continuous backwashing sand filter used to achieve tertiary treatment.
Tertiary Effluent Chlorine Contact Basin
The tertiary effluent chlorine contact basin receives up to 1 mgd of filter effluent. Effluent from the filters is injected with a chlorine solution for disinfection purposes. During the dry season, the chlorinated effluent flows to the Utility Water Clearwell and the Irrigation Pump Station. During the wet season, the disinfected effluent is diverted to the Riverside Ponds prior to river discharge.
Effluent Storage Pond
The effluent storage pond is a storage basin used to hold tertiary treated effluent before it is recycled or discharged from the plant. The total volume available in the storage pond is 20 million gallons.