Temperature-phased AD project gets underway at California wastewater plant

The Orange County Sanitation District (OCSD) in California, US, has contracted engineering and consulting firm Brown and Caldwell and Kansas-based engineering company Black & Veatch to design a new temperature-phased anaerobic digestion (AD) facility at its Plant No.2 in Huntington Beach.

With the sixth-largest wastewater collection, treatment, and recycling infrastructure system in the US, OCSD provides wastewater services for 2.6 million people in central and northern Orange County.

The 144 million gallons-per-day capacity rated Plant No.2 consists of 18 digesters, built from 1959-1979. Due to their age, the digesters require ‘significant rehabilitation’ to ensure protection against seismic hazards, comply with code, and be resilient to sea-level rise.

OCSD’s 2017 Biosolids Master Plan (BMP) was developed to evaluate future biosolids management options and recommend capital improvement projects to upgrade solids handling facilities at the plant. The new Class A TPAD Facility is the largest of several projects to come from the BMP and will be capable of processing all wastewater solids generated from Plant No.2 until 2045.

The TPAD system will result in significant improvements in biogas and methane production; solid and organic removal; pathogen reduction; and dewaterability over conventional digestion practices, providing long-term resource recovery and operational benefits to OCSD.

OCSD’s director of engineering, Kathy Millea, said: “This important project supports OCSD’s commitment to providing reliable service to our ratepayers, expanding resource recovery, and prolonging the life of our assets, or replacing them with improved technology and innovative solutions.”

The $300 million (€255 million) project will see the construction of six new thermophilic digesters and six new Class A batch tanks at Plant No.2, allowing the facility to generate Class A biosolids that meet US Environmental Protection Agency (EPA) guidelines for land applications including fertiliser on farms, vegetable gardens, and residential use as compost or fertiliser.

Digested sludge from the thermophilic digesters or Class A batch tanks will then be cooled and pumped into the existing mesophilic digesters, before dewatering. The OCSD said the project will offer operational flexibility to feed the existing mesophilic digesters while future projects replace and demolish other digesters as identified in the BMP.

Brown and Caldwell, in association with Black & Veatch, will develop and provide OCSD with a preliminary and final design for the new TPAD Facility at Plant No.2.

Dan Bunce, principal-in-charge at Brown and Caldwell, said: “Our highly qualified team is thrilled to have been chosen for this project which represents a unique opportunity for OCSD to implement a next-generation biosolids facility in sync with its long-term goals.”

After the design phase concludes in summer 2024, construction and commissioning is expected to start in 2025 and be completed by 2030.

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