Skip to main content

Wastewater Treatment Plant


Aerial View of Treatment Plant

The CAWD Wastewater Treatment Plant (WWTP) has a permitted capacity of 3.0 million gallons per day (MGD) of dry weather flow.  Current average dry weather flow (ADWF) is approximately 1.1 MGD which represents 37% of the permitted capacity. Of the 1.1 MGD, approximately two-thirds is from CAWD customers and the remaining one-thrid is from Pebble Beach Community Service District customers.

During large storm events the inflows can increase up to eight times the dry-weather rates. This stresses the plant but this can be managed while still meeting environmental water quality discharge standards and Reclaimed Water production.

Influent Pumping

May contain: machine and building

The purpose of influent pumping is to lift the incoming untreated sewage from the terminus of the several interceptor sewers up and into the headworks from where the sewage can flow by gravity through the other treatment processes. Approximately 97% of the influent sewage is pumped at the influent pump station; the remaining 3% is discharged directly into the headworks from the Calle La Cruz pump station in the Carmel Meadows subdivision.

During power outages, a standby generator automatically provides backup power to all three influent pumps. There is always at least a 7-day supply of fuel onsite to power the standby generator.


The headworks structure, together with the adjacent influent manhole, contains essentially all the pretreatment processes of the plant.  Unit processes located within the headworks are: influent flow measuring, mechanical bar screening, and grit removal and washing. 

The mechanical bar screen removes rags and other large solids from the raw sewage and into a hopper which stores the screenings until they are removed and disposed of at the landfill.

Photo of Headworks Facility

Grit (i.e. sand, coffee grounds, etc) is removed from the raw sewage by means of settling in an agitated tank. The agitation keeps the lighter organic solids in suspension and allows the heavier solids to drop out and be removed by pumping the bottom of the tank into a grit classifier and washer. The grit washer further separates the heavier grit particles from the lighter organic matter and the grit is then disposed of into a bin for disposal at the landfill. The lighter organic matter is returned through a pipe overflow back into the grit tank.

Primary Sedimentation Tanks

The Primary Sedimentation tanks remove the majority of suspended solids from the wastewater by gravity settling. CAWD has two primary sedimentation tanks for redundancy. Each tank has a mechanical scraper which rotates around the sloping bottom and pushes settled sludge to a sludge thickening zone and a sludge sump. Sludge is withdrawn from the tanks and is transported to the anaerobic digestion tanks. Floating matter is mechanically skimmed into collecting hoppers and is also transported to the anaerobi digestion tanks.

May contain: water, nature, and outdoors

The primary sedimentation process is basically a physical process utilizing gravitational forces. Settleable and suspended solids, which are the major components of sludge and are heavier than water, settle out of the sewage along with any grit carryover from the headworks. Scum, which is lighter than water, floats to the surface and is removed by skimming. Approximately 60 to 65 percent of the suspended solids will be removed by gravitational forces as part of primary sedimentation.

Effluent from the primary sedimentation tanks overflows into double sided circumferential launders and then flows into the aeration basins.

Aeration (Biological Activatede Sludge Process)

The aeration structures (basins) are designed to promote the growth of helpful bacteria which consume the nutrients in the sewage. This is called the "activated sludge" process. By injecting large amounts of oxygenated air, these bacteria thrive and consume the biological oxygen demand, ammonia, carbohydrates, fats, etc. in the sewage and convert it into bacterial mass or "sludge" for removal in the Secondary Sedimentation Tanks.

May contain: nature, water, river, outdoors, and dam

The aeration structures can hold about 1.15 million gallons total dived into threee seperate basins. The helpful bacteria is maintained in the basins by returning a portion of the sludge separated out of the water downstream in the Secondary Sedimentation Tanks back to the front of the aeration basins. This allows the helpful bacteria to live out their life-cycle in the treatment process, all the while providing a beneficial service to the treatment process. 

CAWD utilizes a different zones in the aeration basins, some with oxygen (aerobic), and some without oxygen (anoxic/anaerobic). The different zones allow for selecting the types of bacteria that are the most beneficial and to enhance the nitrification process.

Aeration in each basin is accomplished by air blowers feeding fine bubble diffusers at the bottom of each basin. Air flow is regulated by automatic dissolved oxygen (DO) control systems.

Secondary Sedimentation Tanks

The Secondary Sedimentation Tanks are circular tanks equipped with rotating mechanical sludge and scum collectors. Appurtenant systems include spray systems for moving scum and for odor control, and pumps for draining tanks. Sedimentation Tank No. 1 is 75 feet in diameter with side water depth of 9 feet. Sedimentation Tank No. 2, is located just south of Sedimentation Tank No. 1, is 65 feet in diameter with a side water depth of 12 feet. The walls of both tanks extend above the predicted 100 year flood-stage elevation. A flap gate in each tank’s wall allows floodwater inflow so the structure will not become buoyant and prevents wastewater outflow. The effluent from the Aeration Structure enters each tank through the bottom, rises up through the center column, and then is distributed into the sedimentation zone. Settled sludge is removed through collecting pipes located on the submerged collecting rake arms and by means of hydraulic differential flows to the sludge collection chamber near the top of the center column. Adjusting valves on the upper end of each collecting pipe can be set so the sludge removal is equalized throughout the tank. The sludge then flows back to the wet well in the Aeration Structure, where it is pumped and divided into either RAS or WAS. Scum is collected from the surface of the wastewater in each tank and returned to a sump in the Aeration Structure, from which it is then pumped to either the Headworks or the Sludge Thickener. The sludge and scum collectors are driven by ¾ horsepower motors with torque overload protection. The drain pumps are self-priming centrifugal pumps driven by 7 – ½ horsepower motors.

Sewage flow into the Secondary Sedimentation Tanks will continue during power outages. The tanks are on the standby power supply and will continue to operate during power outages.

Secondary Sedimentation Tank No 1 has been designed with a nominal surface loading of approximately 450 gallons per square foot per day, while Secondary Sedimentation Tank No. 2 has been designed with a nominal surface loading of approximately 600 gallons per square foot per day. Secondary Sedimentation Tank No. 2 provides additional secondary sedimentation capacity during peak flows when the design maximum peak overflow rate of Tank No. 1 is exceeded, and provides a standby tank should Secondary Sedimentation Tank No. 1 need servicing.

As outlined above, sewage from the aeration structure flows into each secondary sedimentation tank up through the center column and is distributed into the sedimentation zone. As the sewage passes through the sedimentation zone, the activated sludge settles out and the clarified sewage flows over the weirs of the circumferential launder, and then through the outlet pipe to the chlorination structure.


CAWD uses liquid chlorine for disinfection. The disinfection structure is located along the westerly boundary of the plant. Equipment housed in this structure include:

Chlorine cylinder storage room:
2 – Chlorinators, 2000 lbs/day, automatic control;
2 – Chlorine residual analyzers;
1 – Sodium bisulfite bulk storage tank;
2 – Sodium bisulfite metering pumps;
3 – Sample pumps;
3 – No. 3 water pumps and hydropneumatic tank;
1 – Effluent flow meter;
1 – Chlorine flash mixer;
1 – Chlorine leak detector and an Effluent sampler;

The chlorine contact tanks are located below grade, beneath the Chlorination/Dechlorination Structure.

The Chlorination System is used to disinfect treated effluent, for odor control at the Influent Pumping Station and Headworks, and to control bulking in the Activated Sludge System.

Treated Water Pump Station and Ocean Outfall

The Effluent Pump Station building contains effluent pumps, backup chlorine mixing equipment, backup plant water pumps and return effluent pumps. Accessory equipment associated with the above major equipment items are also located in the Effluent Pump Station Building. Since all effluent discharged to Carmel Bay must be pumped, the entire Effluent Pump Station is connected to the standby electrical power system so that pumping can continue at all times.

Flow enters the Effluent Pump Station from the Chlorination/Dechlorination Structure under normal conditions, or from the Secondary Bypass pipe under abnormal conditions. Under normal conditions, the flow enters an inlet box, flows under a baffle, then over a weir into the wet well which is the sump for the Effluent Pumps. When the secondary bypass is used, the flow enters the inlet box where it is dosed with chlorine added through submerged diffusers. The chlorine solution used for dosing is piped from the Chlorination/Dechlorination Structure. Immediately after chlorine addition, the flow goes under the baffle into a chamber where it is mixed by a Chlorine Mixer. The flow then continues on over the weir into the wet well.

The Effluent Pumps pump out of the wet well and into the discharge manifold leading to the outfall line. The outfall pipeline proceeds to an underground tee with valves which can be used to control the flow in the future when irrigation is used in addition to the discharge to Carmel Bay through the submarine outfall.

The existing submarine outfall is a 24-inch diameter, concrete encased pipe with 10 diffuser ports extending approximately 4.5 feet above the top of the pipe. The risers are 8-inch diameter with a reducing 90-degree elbow at the top. The discharge is through a 3-inch diameter flanged pipe end. Each port has a rubber “duckbill” type valve to prevent debris from entering the outfall pipe during periods of low flow.

The existing point of discharge is just southerly of the mouth of the Carmel River at North 36 degrees, 32 minutes, 02 seconds of latitude, West 121 degrees, 55 minutes, 40 seconds of longitude at a depth of approximately 36 feet, 600 feet off-shore.

Solids Treatment and Disposal

Waste activated sludge and scum from the Secondary Sedimentation Tanks are pumped to the Sludge Thickener. The purpose of this unit is to thicken the influent sludge from 0.5% – 0.75% solids density to 3.0% – 4.0% solids density. The thickened sludge is then pumped directly to the Digestion Control Building and into the digestion tanks. The Sludge Thickener is the dissolved air flotation type with four basic component parts: The thickener tank (including the mechanism for sludge skimming and raking); the pressurization system (including pressurization pump and air injection system); the sludge and scum pumping; and the polyelectrolyte feed system. The unit has been designed to handle the normal waste activated sludge flows and solids loadings from an average daily plant influent flow of 4.0 million gallons.

Thickening is achieved by continuously returning approximately 325 gallons per minute of air pressurized sludge thickener effluent to the center of the Thickener Tank. The air pressurized effluent is produced in a multi-step process. First, compressed air is injected to the discharge of the second stage of the two-stage pressurization pump or directly into the retention tank. Then this aerated effluent is directed to a retention tank where the air is more fully absorbed into this effluent flow stream. Finally, the retention tank effluent is piped to the center of the Thickener Tank where it mixes with the influent (WAS) to the Sludge Thickener and enters the tank through ports in the center column near the tank’s liquid surface. As the air pressurized effluent enters the center column, its high pressure is reduced to atmospheric pressure conditions. This pressure change releases entrained air from the WAS-recycled effluent mixture. The rising air bubbles suspend or float solids to the tank water surface where the solids are removed by a skimmer. This floating material generally constitutes the thickened sludge, with only nominal amounts settling to the tank bottom. The settled sludge is raked to a hopper in the tank bottom for periodic withdrawal by the Sludge and Sump Pump. Sludge skimmed from the tank surface is collected in a scum sump for periodic withdrawal by the Sludge and Scum Pump. Polymers can be added to the influent sludge piping to aid in coagulating the influent WAS just prior to entrance into the Thickener Tank. Effluent from the Sludge Thickener flows through the tank launder to the effluent recirculation sump, then by pipeline to the Aeration Tank influent. The pressurization pump also draws effluent from the effluent recirculation sump.

The Sludge Thickener is located just north of the Aeration Unit, with the pressurization system, sludge and scum pumping, and polyelectrolyte feed equipment located on a concrete slab-on-grade adjacent to the Thickener Tank. Operation of the Sludge Thickener must be suspended during flooding conditions, as the equipment is not protected from flood waters. During flooding, the Thickener Tank must be full of water to resist damage from buoyant forces. Depending on the extent of flooding, some of the electrical and mechanical equipment may be damaged, since this facility was not designed as a critical facility to be located above projected flood levels.

Sludge digestion is accomplished by the anaerobic process. This anaerobic digestion occurs in the three covered digesters located southeast of the operations building. Sewage solids (sludge and scum) are collected in the primary sedimentation tanks and pumped to the digesters. Similarly, waste activated sludge and scum collected in the secondary sedimentation tanks and thickened in the sludge thickener is also pumped to the digesters. Waste activated sludge and scum from the secondary sedimentation tanks may be directed to the headworks or the digesters when the sludge thickener is not in operation.

After the solids have been stabilized sufficiently by the digestion process and are no longer offensive in odor, the solids are held in a sludge holding tank until subsequent dewatering by the belt press.

Dewatering is a physical/mechanical process used to reduce the moisture in digested sludge (biosolids). There are several reasons for dewatering sludge. In general, it is more economical to dispose of the dewatered sludge than it is to pump or haul liquid sludge to disposal sites because by reducing the moisture content, the sludge volume and weight are reduced.

The CAWD plant uses a belt filter press to dewater the digested sludge, with sludge drying beds for temporary storage in the event of emergency belt filter shutdown. The belt filter press consists of two endless belts that travel over a series of rollers. The sludge is pre-conditioned with polymers, then applied to the “free water drainage zone” of the filter belt where most of the free water is allowed to drain through the filter. The partially dewatered solids are then carried to a point where they are trapped between the two endless belts and dewatered further. This part is known as the “press” or “dewatering zone.” As the solids travel through this zone, they are subjected to shearing forces, and the water is forced from between the belts into filtrate trays. The retained solids are scraped from the belts when they separate at the discharge end of the press. The two endless belts then travel through washing chambers for removal of fine solids that may increase the chance of plugging.

The original belt press installed in 1984 was capable of dewatering 80 gpm of 2.5% digested sludge. A newer belt press capable of dewatering 80 gpm was installed in 1999 and the older unit is now on standby.

The dewatered sludge in the form of a “cake”, with a solids content of approximately 18%, is hauled by truck to Kern County where it is used as a compost amendment. The compost is land applied to non-food crops. Total annual biosolids production is approximately 2,000 to 2,200 wet tons.

The District continues to explore alternative biosolids disposal options