In July 2006, the Pismo Beach (Calif.) Wastewater Treatment Plant celebrated the completion of a $12 million upgrade and expansion project, which will ensure that the plant can provide advanced secondary treatment for its service base of local residents and tourists. The upgraded plant was designed to accommodate and treat flows expected at full buildout of Pismo Beach. Because of this, the city anticipates that no construction will be necessary within the next 25 to 30 years.
Before the upgrade project was undertaken, the plant had overflow problems at its headworks structure, as well as at some lift stations and its effluent pump station. The old plant did not have the capacity to treat the flows that the city was generating.
Influent to the treatment plant is primarily from residential sources. However, an appreciable amount of flow is tourism-related. The second-highest source of flow is from hotel rooms, followed by commercial sources. The flow to the plant, therefore, fluctuates greatly, depending on the time of year and day of the week. Flow increases during the warmer months and on weekends when more tourists are attracted to the area.
To provide reliability and redundancy and meet discharge requirements, the City of Pismo Beach, Calif., upgraded and expanded its wastewater treatment plant to include two oxidation ditches with a combined capacity of 1.9 mgd (7200 m3/d) and nutrient removal capabilities. (Photo: Pam Goshgarian)
Prior to the upgrade, the plant consisted of a headworks structure with a manual bar screen and an aerated grit chamber, primary treatment via two clarifiers, and secondary treatment via three aeration tanks and secondary clarifiers. Solids handling relied on two anaerobic digesters, a dissolved-air flotation thickener, and a belt filter press. The effluent was disinfected with sodium hypochlorite.
As part of the upgrade and expansion project, the headworks structure, primary clarifiers, secondary clarifiers, and chlorine contact basin were all demolished. A new headworks structure, operations center–laboratory building, oxidation ditches, secondary clarifiers, and sludge pumping station were built. In addition, the existing aeration basins were converted to the new chlorine contact basin, and the effluent pump station was upgraded.
Now, a single mechanical bar screen with a 0.625-in. (16-mm) bar spacing and a 6-mgd (23,000-m3/d) capacity captures large debris, such as rags and sticks, from the raw wastewater in the headworks. From the headworks, an influent splitter box divides the flow between two oxidation ditches, each with a side water depth of 12 ft (3.7 m) and a volume of 0.89 million gal (3.4 million L). Aeration is provided by mechanical aerators; about 12% (0.11 million gal [0.42 million L]) of each tank is anoxic, and 88% is aerobic (0.78 million gal [3.0 million L]).
Biological nutrient removal is utilized through the plant’s oxidation ditches. This process promotes removal of nitrogen from the wastewater without chemical treatment. Future wastewater reuse is achievable with this type of treatment, should the city ever decide to pursue it.
Effluent from the oxidation ditches passes through the mixed liquor splitter box to be evenly distributed between the plant’s two 65-ft-diameter (20-m-diameter) secondary clarifiers. These clarifiers each have a side water depth of 14 ft (4.3 m) and a volume of 0.35 million gal (1.3 million L).
From the secondary clarifier, the flow travels through a sodium hypochlorite mixing box to the chlorine contact basin. An adequate detention time and environment is provided in this basin to disinfect the water. To neutralize the toxic effects of the chlorine, the effluent is dechlorinated with sodium bisulfite. The disinfected and dechlorinated effluent is discharged 1 mi (1.6 km) off the coast of the Pacific Ocean under about 60 ft (18 m) of water.
Settled solids from the secondary clarifiers are either returned to the oxidation ditches or wasted to the dissolved-air flotation thickener. The return activated sludge–waste activated sludge pump station contains three centrifugal pumps, each with a capacity of 785 gal/min (2970 L/min) and 15 hp (11 kW). The firm capacity of the pump station is 3.4 mgd (12,900 m3/d), and the waste activated sludge capacity is 120 gal/min (450 L/min).
In the dissolved-air flotation thickener, air bubbles are infused into the wastewater solids. This process carries biosolids to the upper portion of the tank, where they can be collected and pumped to the biosolids holding tank. The biosolids are stored in the holding tank until they can be dewatered by a belt filter press.
Dewatered biosolids are hauled to a composting facility. After composting, the material will be used
for farmland application to provide soil conditioning and nutrient sources for crops.