The Carmel Area Wastewater District was formed as the Carmel Sanitary District in 1908. At that time, the District provided septage facilities for the village of Carmel-by-the-Sea.

Over the years the District has grown to where it now provides collection, treatment and disposal of wastewater for 11,000 people within the District and treatment and disposal for an additional 4,500 people in Del Monte Forest.

The District treats wastewater from Carmel and surrounding areas providing advanced treatment to almost drinking water standards. Almost all treated wastewater is sent to Del Monte Forest where it is used to irrigate seven world famous golf courses including Pebble Beach, Poppy Hills and Spanish Bay.

The District serves an area bounded by Carmel Bay to the west, Carmel Highlands on the south and Del Monte Forest on the north. Service extends as far east as Quail Meadows and Del Mesa Carmel.

The District also maintains about eighty-three miles of sewers within the existing service area. The treatment plant is located one-half mile west of Highway One and adjacent to the Carmel River.

 

The Carmel Area Wastewater District (CAWD) is an independent political entity operating under authority of the California State Health and Safety Code, Division 6, Sections 6400 through 6941.9, and as such is governed by its own 5 member Board of Directors who are elected, at large, for terms of 4 years. The District currently employs 24 full time employees. 4 are in Administration, 5 in Collection System Maintenance and 15 in Treatment and Disposal. [See Figure 3]

The CAWD is located on the Monterey Peninsula in Monterey County, California approximately 125 miles south of San Francisco. The existing CAWD treatment plant is on the south bank of the Carmel River approximately one-third of a mile west of the State Route 1 Bridge. The administration office is located at 3945 Rio Road, Carmel. [See Figure 1]

The service area consists of the city of Carmel-by-the-Sea and outlying County areas including Carmel Woods, Hatton Fields, portions of lower Carmel Valley, Carmel Meadows, Hacienda Carmel, Del Mesa Carmel, Quail Meadows, Pacific Meadows and to the South, Highlands Inn, the Tickle Pink Inn and the Highlands Sanitary Association and several individual lots in the vicinity. The total service area is comprised of approximately 5.5 square miles with a permanent population of approximately 11,000. [See Figure 2]

In addition, CAWD provides treatment and disposal services by contract to the Pebble Beach Community Services District (PBCSD) which owns one-third of the capacity of the CAWD Treatment Facility. The PBCSD is comprised of a service area of approximately 5,300 acres located in the Del Monte Forest with a service population of approximately 4,500.

District Facilities

The existing collection system is one of the oldest in the State with approximately half the manholes being the older brick type and most of the pipelines being of the old vitrified clay type. Deterioration of the mortar holding the manhole bricks together by gases produced in the sewer system and intrusion of roots into the joints of the older pipelines are the major reasons for replacement or rehabilitation.

The CAWD collection system is comprised of approximately 83 miles of gravity sewers ranging in size from 6 inches to 27 inches in diameter together with nearly 5 miles of force mains, 7 pump stations, and over 1,500 manholes.

The major trunk sewers include the Carmel Valley Interim Trunk Sewer which serves Rancho Rio Vista, portions of Carmel Views, the Carmel Rancho Shopping Center, Hacienda Carmel, Del Mesa Carmel, Quail Meadows and outlying areas at the mouth of the Carmel Valley; the Hatton Canyon Sewer (which joins the Carmel Valley Trunk sewer at the intersection of State Route 1 and Rio Road) serves the High Meadows, Carmel Knolls and portions of the Carmel Hills subdivisions; two trunk sewers serve the city of Carmel and the Hatton Fields areas. The Carmel Meadows subdivision is sewered separately.

Pump stations are located at (1) the westerly boundary of Hacienda Carmel, (2) the westerly terminus of 8th Avenue (at Scenic Drive), (3) the intersection of Monte Verde Street and Sixteenth Avenue, (4) west side of Scenic Road approximately 200 feet north of Ocean View Avenue, (5) the end of Calle La Cruz (Carmel Meadows) (6) at the westerly boundary of the Carmel Meadows subdivision approximately 750 feet southwesterly of the Calle La Cruz pump station and (7) on the West side of Highlands Drive approximately 100 yards from Highway 1. All pump stations have been upgraded to allow remote monitoring and have capabilities for standby power and emergency bypass.

The District has an ongoing 5 year collection system capital improvement program and replaces or rehabilitates pipelines at the rate of 1,000 to 4,000 feet annually. Manholes are replaced as necessary.

The five person collection system maintenance crew cleans the entire system a minimum of once annually and performs routine repair work as necessary.

Secondary Facilities

The CAWD Water Pollution Control Plant is a secondary type plant utilizing the activated sludge process for secondary treatment. The plant has been designed to treat 4.0 million gallons per day (MGD) of primarily domestic wastewater. At the present time the plant has a permitted capacity of 3.0 MGD. The general layout of the plant is shown in Figures 4A & 4B. [See Figure 4A] [See Figure 4B]

Current average dry weather flow (ADWF) is approximately 1.8 MGD which represents 60% of the permitted capacity or 45% of design capacity. Of the1.8 MGD approximately1.2 MGD is from CAWD and 0.6 MGD from PBCSD.

A schematic diagram and Hydraulic Profile of the treatment facility is shown on Figures 5 and 6 and a basic description of the individual unit processes follows: [See Figure 5] [See Figure 6]

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.

Five major interceptor or trunk sewers (two Carmel trunk sewers, the Hatton Fields sewer, the Carmel Valley sewer and the Pebble Beach Interceptor) join together at a point on the northerly bank of the Carmel River directly across from the Treatment Facility Influent Pump Station. The sewage then flows through a 24-inch sewer line that runs under the Carmel River to the influent pump station. The influent pump station is a wet-well dry pit type pump installation with the incoming sewage sump (wet well) and pump room (dry pit) located below grade. Three influent sewage pumps are located in the station along with a sump pump for the dry pit, in an air compressor for the level sensing bubbler, ventilation fans, and a standby engine-generator. The influent sewage pumps are driven by 50 horsepower adjustable frequency drive electric motors which are controlled by the sewage level in the wet well. Each pump is rated at capacities ranging from 800 gpm (1.2 MGD) to 3,500 gpm (5.0 MGD) at speeds ranging from 575 to 1,200 RPM. The influent pump station is designed to pump at flow rates up to 8.0 MGD when two pumps are operating. The third pump serves as a standby and is activated when one of the duty pumps fails or when peak wet weather flows exceed the capacity of the two duty pumps operating at maximum speed. Provision for standby power at the influent pump station during power outages is by a standby generator located in the pump station.

The headworks structure, together with the adjacent influent manhole, contains essentially all the pretreatment processes of the plant. The facilities have been designed to enable expansion to handle the projected ultimate flow, principally by adding additional mechanical equipment and a parallel influent pipe and flowmeter. The present increment is capable of handling peak flows of approximately 8.0 MGD. Unit processes located within the headworks are: influent flow measuring, mechanical bar screening, grit removal and washing, and communication. Prechlorination may be accomplished at the influent manhole. The primary sludge and scum pumps are located within the headworks along with controls for the primary sedimentation tanks. Flow splitting weirs to divide the flow between the primary sedimentation tanks are also located in the headworks. The headworks is designed for continuous operation during projected flooding resulting from a 100 year storm.

The influent flow meter is a 14-inch diameter magnetic flow meter designed to measure flows ranging up to 8.0 MGD with an accuracy of plus or minus 1% of range.

The mechanically cleaned bar screen has been designed to pass sewage flows up to 8.0 MGD. The bar screen can be controlled to operate either fully automatically according to a preset cycle which is variable from 1 minute to 60 minutes or manually. The material removed on the screen is mechanically lifted out of the sewage flow, deposited onto a screenings conveyor, and then dropped into a hopper which stores the screenings until they are removed and disposed of.

The grit chamber is a circular, air agitated and mechanically scraped tank. Grit and other settled matter is removed from the bottom of the tank by pumping the grit slurry from a sump up and into the grit washer located on the headworks upper deck. The grit washer separates the heavier grit particles from the lighter organic matter and the grit is then lifted by a screw conveyor-classifier into a hopper where it is stored until removal and disposal. The lighter organic matter is returned through a pipe overflow back into the grit tank.

The barminutors are mounted in separate channels leading to the flow splitting weirs and each is designed to comminute (grind) sewage screenings not removed by the bar screen at flows up to 8.0 MGD. The ground screenings are not removed from the sewage but pass through the machine to be removed in the primary sedimentation tanks. The barminutors are automatically controlled and will initiate a grinding cycle when the water surface differential across the machine exceeds a preset amount. A bar rack with 1-1 ½ inch clear openings is installed in a channel parallel to the barminutor channels for use when the barminutor is temporarily out of service.

The circular primary sedimentation tanks have been installed and are arranged to operate in parallel. Either tank may be isolated for maintenance or repair purposes. The tanks are 60 feet in diameter with normal side water depths of 9 feet. Each tank has a mechanical scraper which rotates around the sloping bottom and pushes the sludge to a sludge thickening zone and a sludge sump. Sludge is withdrawn from the tanks by positive displacement plunger pumps located in the headworks and is transported to the anaerobic digestion tanks. Floating matter is mechanically skimmed into collecting hoppers in each tank and is then piped to a single scum sump located on the south edge of Sedimentation Tank No. 1. The scum is pumped from the sump to anaerobic digestion by a positive displacement plunger pump located in the headworks. Plant effluent can be returned from the effluent pump station to the primary sedimentation tanks to assist in sewage sludge separation.

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 effluent junction boxes. The effluent then flows to the aeration structure, where it is directed to the aeration basins.

The original aeration structure consists of four square basins, a return sludge pump room, a control room and appurtenant channels and inlet and outlet chambers. Each basin is 50 feet square with a nominal water depth of 14 feet. Each has a volume of approximately 260,000 gallons. The Northerly basins (# 1 and #2) have been converted to primary influent flow equalization basins. The Southerly basins (#3 and #4) are currently operated in series and a single large basin with an anoxic zone at the head of the tank.

Aeration basins 5 and 6, of rectangular configuration measuring 110 feet long by 25 wide by 14.3 feet deep for a total volume of 308,000 gallons each, were added in 1994. Both have anoxic zones at the head of the tanks.

All aeration tanks were modified with the anoxic zones in 2008 and part of the MF/RO Project.

Aeration in each basin is accomplished by air blowers feeding fine bubble diffusers that are regulated by automatic dissolved oxygen (DO) control.

Activated sludge is returned from the secondary sedimentation tank to the aeration basins and waste activated sludge can be discharged into either the Sludge Thickener or to the Headworks. The activated sludge process can be operated in several modes including complete mix or variations approaching conventional, step aeration or contact stabilization.

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.

The effluent flow meter is an 18-inch diameter magnetic flow meter located in the influent line from the secondary sedimentation tanks to the chlorine contact tanks. The meter utilizes the principle of electromagnetic induction to produce an AC voltage proportional to the rate of fluid flow. A signal converter transmits an analog 4-20 mA DC signal proportional to flow rate to the data logger, where the flow is recorded. The flow meter signal is also used to control the Composite Effluent Sampler in the Analyzer Room of the Chlorination Building, and to control chlorination and dechlorination.

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.

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.

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 disposed of by trucking to Kings county where it is composted and 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.

The original tertiary plant, constructed in 1994, consisted of an influent flow equalization basin, influent pumps, chemical contact chambers for the addition of coagulants and flocculants, a sand filter, chlorine contact channels and effluent pumps. With the addition of the microfiltration and reverse osmosis (MF/RO) facilities in 2008, the chemical contact chambers and sand filters were “mothballed”. The flow equalization basin, chlorine contact channels and effluent pumps continue to be used with the MF/RO facility.

The new MF/RO Facility (Also referred to as the Salinity Management Project”), placed into service in the fall of 2008 consists of microfilters with a capacity to produce 1.9 MGD and reverse osmosis membranes capable of producing 1.2 MGD. With a design “blend” of 80% RO and 20% MF, 1.5 million gallons of blended recycled wastewater can be produced. Based on average flows, the average output of recycled water is 1.0 MGD.

Financial Data

Revenues for the operation and maintenance (O&M) of the District are primarily from collection of user fees assessed at the rate of $30.88 per month per equivalent residential unit. This amount 4,887,761 together with revenue from interest on cash reserves; connection and permit fees; and tax subventions comprise the total revenues required for O&M costs chargeable to CAWD.

As discussed earlier, the PBCSD contracts with the CAWD to treat and dispose of wastewater from Del Monte Forest. PBCSD’s payment for wastewater treatment is calculated based upon the flow of sewage from their district. PBCSD’s slow currently amounts to 34.45% of the total annual flow. A 7.50% administrative fee is added to the flow percentage and applied to the costs for treatment and disposal. This amount currently is $805,000 leaving $ 4,082,761 to be paid by the rate payers of CAWD.

The District budget for Fiscal Year 20010/11 called for expenditures of $1,010,900 for O&M and an additional $4,910,190 for capital asset projects/purchases. A graphic breakdown of O&M expenditure for FY 2010/11 is shown in Figure 7. [See Figure 7]

Small to moderate capital expenditures are funded by cash reserves while larger expenditures may be funded by bond issues approved by the voters. The District, at present, does not have any bonded indebtedness.