Wastewater Treatment

Learn more about Moulton Niguel’s wastewater treatment facilities and processes. 

Moulton Niguel Water District has been providing wastewater collection and treatment services to its customers since 1964.  In the late 1960s, the District, in partnership with our local community, was one of the first agencies in the State of California to begin recycling treated wastewater to use for irrigation purposes.  Today, recycled water accounts for nearly 25% of the District’s total water demand and directly reduces the amount of drinking water we need to import from the Colorado River or Northern California.  The District continues to evaluate opportunities to expand our use of highly treated wastewater to improve local water supply reliability and reduce the amount of treated wastewater discharged to the ocean.

As the region experiences an increasing need for cost-effective and reliable sources of water, the District’s Board of Directors has established goals to identify opportunities to maximize the reuse of water while ensuring cost-effective and reliable wastewater treatment services.  As stewards of the environment, the District is focused on ensuring a high-performing wastewater system and reducing the amount of treated wastewater discharged to the ocean.  We continue to partner with the County and local cities to develop projects and programs that will reduce unwanted urban runoff into our local creeks and streams by promoting the efficient use of water while exploring the potential to capture urban runoff as an additional water source.

Wastewater Collection and Treatment


Currently, the District collects and treats more than 10 million gallons of wastewater per day from residential and commercial customers throughout our service area.  The District utilizes three different treatment plants to appropriately treat the wastewater with two of those treatment plants equipped to produce recycled water.  The 3A Treatment Plant in Mission Viejo is owned and operated by the District.  The Regional Treatment Plant in Laguna Niguel is currently operated by a regional operator, the South Orange County Wastewater Authority, but is also home to the largest recycled water facility that is owned and operated by the District.  Finally, the J.B. Latham Treatment Plant in Dana Point, which is also operated by SOCWA, provides an additional facility for treatment of wastewater.  Any treated wastewater that cannot be utilized for irrigation purposes is then discharged to the ocean through one of two ocean outfalls, the Aliso Creek Ocean Outfall and the San Juan Creek Ocean Outfall.  The District works closely with SOCWA, as the regional permitting entity, and the appropriate regulatory agencies to ensure that all treated wastewater, whether used for irrigation purposes or discharged to the ocean, is compliant with all regulatory requirements.

MNWD Site Map

3A Treatment Plant


The 3A Treatment Plant is a six million gallons per day (MGD) treatment facility consisting of preliminary, primary, secondary, and tertiary treatment. These core treatment processes produce undisinfected secondary effluent, which is conveyed to the San Juan Creak Ocean Outfall. The plant also has sand media tertiary filters and chlorine disinfection facilities to produce disinfected tertiary recycled water, as defined by Title 22 of the California Code of Regulations. Tertiary-treated effluent is used for irrigation at a local golf course and other landscape areas. Recycled water is stored in the recycled water reservoir system when irrigation demand is low. 3A Treatment Plant’s solids stream includes sludge thickening, anaerobic digestion, and sludge dewatering. Dewatered sludge, also known as “biosolids,” are trucked off site for landfill disposal.

3 A

Wastewater Treatment Process

Preliminary Treatment involves:

  • Ferric Chloride: Used to remove sulfur from influent wastewater
  • Bar Screens: Removes large debris that would otherwise cause damage to or interfere with downstream equipment and process operation
  • Compactors: Removes excess organics and moisture from screenings
  • Raw Sludge Pumps: Lift influent wastewater
  • Grit Removal System: Removes heavy inorganic material that would otherwise damage downstream equipment
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Preliminary Treatment 1

Preliminary Treatment

Mechanical Bar Screens

Preliminary Treatment 2

Preliminary Treatment

Grit Chambers

Preliminary Treatment 3

Preliminary Treatment

Grit Classifiers

Primary Treatment involves the removal of settleable materials in the primary sludge and floatable materials in the skimmings. The primary sedimentation tanks are large basins that allow heavy material to settle out and light material to be removed. Each tank contains flight and chain system, scum skimmers, effluent weirs, primary sludge pumps, and primary scum pumps.

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Primary Treatment 1

Primary Treatment 1

Primary Clarifiers

Primary Treatment 2

Primary-Treatment-2

Primary Gallery

Primary Treatment 3

Primary Treatment 3

Primary Clarifiers

Secondary Treatment is focused on the biological treatment of wastewater which comprises of aeration Tanks, aeration blowers, secondary tanks, RAS Pumps, WAS Pumps, and equalization basin. The 3A Treatment Plant conventional activated sludge process is the most complex, temperamental, and critical component of wastewater treatment.  During this process, primary effluent (“food”) enters the aeration tank and is mixed with microorganisms to form Mixed Liquor Suspended Solids to biologically remove substances from the wastewater.  Aeration blowers are used to supply dissolved oxygen to the aerobic microorganism, which facilitates the oxidation of substances in the wastewater. Microorganisms settle out in secondary sedimentation tanks and are returned to the aeration tanks as Returned Activated Sludge (RAS). Excess microorganisms are removed from the process as Waste Activated Sludge (WAS) for solids treatment. The secondary effluent liquid steam is either discharge to the San Juan Ocean Outfall or pumped to the tertiary treatment system for recycled water production.

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Secondary Treatment 1

Aeration Basin

Secondary Treatment 2

Aeration Blowers

Secondary Treatment 3

Secondary-Treatment-3

Secondary Clarifier

Tertiary treatment is the final step of the wastewater treatment process and involves chemical and physical treatment. It further treats secondary effluent to meet Title 22 recycle water permits, which is achieved by a combination of advanced wastewater treatment and disinfection. Advanced wastewater treatment, more commonly known as sand filtration, remove suspended particles by facilitating the gravity flow of secondary effluent through a media of sand. Trapped particles are moved from the flow stream and sent back to preliminary treatment. Filtered effluent continues to the disinfection stage, where sodium hypochlorite is used as the disinfection agent of filtered water before entering the Galivan Storage Reservoir.

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Tertiary Treatment 1

Tertiary Treatment 1

Sand Media Filtration System

Tertiary Treatment 2

Tertiary Treatment 2

Tertiary Pump Station

Tertiary Treatment 3

Tertiary Treatment 3

Galivan Pump Station

The Dissolved Air Floatation Thickener (DAFT) helps treat solids by using a combination of saturated air and polymer to float light solids from the Waste Activated Sludge (WAS) stream. Concentrated solids leave as Thickened Waste Activated Sludge (TWAS). The liquid overflow returns to process. The digester provides an environment for anaerobic microorganisms to reduce volatile solids content within the digester sludge. Centrifuges are used to capture as much solids as possible and remove excess water from digested sludge; the product is called “cake.” This biosolid is removed offsite for landfill disposal. The boiler uses digester biogas as a fuel source for heating the digester to maintain a mesophilic environment. Excess biogas is burned at the flare.

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Solids Treatment 1

Solids Treatment 1

Anaerobic Digesters

Solids Treatment 2

Solids Treatment 2

Centrifuges

Solids Treatment 3

Solids Treatment 3

Biosolids Bin Room