Oro Loma Sanitary District Flow

The Process

The Oro Loma Sanitary District/Castro Valley Sanitary District Water Pollution Control Plant treats wastewater from San Lorenzo, Ashland, Fairview, Cherryland, Castro Valley, and parts of Hayward and San Leandro using primary and secondary treatment. Primary treatment is a simple mechanical process that uses bar screens, grit chambers, and settling tanks to remove trash and settleable solids from the wastewater. Secondary treatment is a complex biological process that uses bacteria to convert dissolved organic matter into suspended matter that can be settled out as sludge. The remaining wastewater is then disinfected and discharged, while the solids are stabilized, dewatered and hauled away. At Oro Loma, energy required to run the treatment plant is generated onsite by two renew- able sources. The first source is methane gas, which is generated by the solids stabilization process and used to produce heat and power to run the treatment plant. The second source is solar power, which is generated by onsite solar panels. Together, these energy resources enable Oro Loma to be self-sustaining now and into the future.

Plant Influent

Wastewater from the Oro Loma service area – the plant’s influent – is collected from homes and businesses and pumped to the treatment plant through a network of 270 miles of sewer pipes.

Bar Screens

Bar screens act as coarse filters to remove rocks, sticks, and other large debris that might clog or damage equipment further downstream.

Grit Chamber

After passing through the bar screens, the wastewater enters the grit cham- ber, where heavy material like sand and gravel settle out by gravity, are washed, and then trucked to a landfill.

Influent Pumps

The wastewater enters the influent pumps and is lifted up to the primary clarifiers. From this point on, the wastewater flows by gravity through the rest of the plant, saving energy and helping keep service rates low.

Primary Clarifiers

The wastewater enters the primary clarifiers where suspended solids gradually settle to the bottom as raw sludge. The sludge, about 70% of the total solids, is mechanically collected and sent to the digesters. Skimming blades remove scum and grease from the surface. The remaining liquid, called primary effluent, flows to the secondary treatment process.

Aeration Basins

The first stage of the secondary treatment process is the aeration basins, where oxygen is introduced to the wastewater for two purposes. First, it creates an oxygen-rich environment to sustain the microorganisms that feed on the suspended and dissolved organic matter in the wastewater. These microorganisms and the biodegradable matter they consume are together called activated sludge. Second, it supplies oxygen that the organic material uses as it decays. When treated wastewater is ultimately released into San Francisco Bay, it won’t demand oxygen from the Bay that fish and wildlife need to thrive.

Secondary Clarifiers

From the aeration basins, the treated wastewater flows to the secondary clarifiers. Here the activated sludge settles out, where it is either sent back to the aeration basins to maintain the microorganism population or thickened and sent to the digesters. The remaining clarified water, called secondary effluent, is sent to the final stage in the treatment process, disinfection.

Disinfection Tanks

Treated effluent from the secondary clarifiers flows into disinfection tanks, where sodium hypochlorite (bleach) is added to kill pathogenic organisms. The disinfected water is then sent to the San Francisco Bay via East Bay Dischargers Authority (EBDA).

EBDA Facility

Prior to leaving the plant, the final effluent meets all requirements set by the Regional Water Quality Control Board. It is then pumped by the East Bay Dischargers Authority for beneficial reuse at Sky West Golf Course, or dechlorinated prior to discharge to protect plants and fish in San Francisco Bay.

Gravity Belt Thickener

Settled activated sludge from the Secondary Clarifiers that is not returned to the Aeration Basins is thickened with a gravity belt thickener. Mixed with polymer, the sludge becomes concentrated as it releases the water, which drains by gravity through a porous horizontal belt.

Grease Receiving Facility

Trucks carrying grease and oils from commercial and industrial users, such as restaurants, offload their waste into the grease receiving facility. The grease receiving facility processes the waste so that it can be treated with the rest of the wastewater solids. Proper disposal of fats, oils and grease from homes and businesses prevents drains and sewers from clogging. This helps keep creeks and the San Francisco Bay clean by preventing sanitary sewer overflows.


Thickened activated sludge from the gravity belt thickener, sludge from the primary clarifiers, and solids from the grease receiving facility all combine in the anaerobic digesters where microorganisms decompose the solids for 15 to 25 days in the absence of oxygen. The digested biosolids become biologically stabilized and reduced in volume.

Belt Filter Press

The stabilized biosolids are mechanically dewatered by a belt filter press. The process removes a significant amount of water and greatly reduces the volume.

Solar Drying Ponds

The dewatered solids are taken to the solar drying ponds, where they are further dried and then hauled away for disposal or beneficial reuse.

Cogeneration System

A byproduct of the anaerobic digestion process is methane gas, which is similar to commercial natural gas with about 2/3 the heating value. Oro Loma generates 100 million cubic feet of methane gas per year. The methane is used by the onsite cogeneration system to produce heat and electricity for operating the treatment plant.

Solar Panel Array

Onsite solar panels generate clean, renewable solar energy, which is used to supplement the electricity needs for operating the treatment plant. When added to the cogeneration system, Oro Loma can meet 100% of its electrical energy needs from renewable energy sources.

Import and Export to PG&E Grid

Excess electricity generated by the cogeneration system and the solar panels is exported back into the PG&E grid. When power is needed it is taken from the grid. The ability for Oro Loma to generate its own power needs onsite makes the treatment plant self-sustaining on an annual basis.

Experimental Levee and Wetland

A special levee and wetland pond mimic nature by filtering treated wastewater to further remove nutrients to protect the Bay. The levee and wetland area enhance the quality of the wastewater leaving the treatment plant. The wetland also provides wet weather storage. The demonstration went live in the Fall of 2015.

Important Numbers

(510) 276-4700 Oro Loma Sanitary District Office 24/7 Emergency Response

(510) 613-8710 Waste Management of Alameda County

(877) STOPWASTE Alameda County Recycling Hotline

(800) 606-6606 Alameda County Household Hazardous Waste

Our Facilities

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