Austin Water seeks variances and code amendments to expand Walnut Creek wastewater plant to 100 million gallons per day

Austin Water presented plans and a request for administrative variances to facilitate the expansion and enhancement of the Walnut Creek Wastewater Treatment Plant during a committee briefing. The utility said it plans to add 25 million gallons per day to increase the plant’s rated capacity from 75 million to 100 million gallons per day, convert the existing treatment process to biological nutrient removal, install ultraviolet disinfection, and construct a flood wall to protect the site from updated flood-plain modeling.

The project team described the expansion as a generational capital program, noting that the City Council previously approved design and construction contracts of roughly $1 billion for work at Walnut Creek. Austin Water said construction will be delivered in multiple packages using a construction-manager-at-risk (CMAR) model to allow earlier procurement of long-lead equipment and to coordinate complex sequencing needed to maintain continuous plant operations during construction.

Why it matters: Austin Water reported that plant flows exceeded 90% of treatment capacity for three months in 2024 (January, February and May), a level that the utility said triggers a Texas Commission on Environmental Quality (TCEQ) requirement to begin construction. City staff warned that failing to expand while over capacity can expose the utility to enforcement and fines from TCEQ.

Key technical and environmental components described by Austin Water staff included: - Capacity: add 25 million gallons per day to take the plant from 75 to 100 MGD. - Treatment conversion: change from conventional activated sludge to biological nutrient removal (BNR). Staff estimated BNR would reduce phosphorus loading to the Colorado River by about 60%, from roughly 2,000 pounds per day under current conditions to an estimated 860 pounds per day (staff called the 860 figure a minimum estimate based on modeling). - Disinfection: replace chemical disinfection with ultraviolet (UV) disinfection to reduce reliance on chemical reagents. - Flood protection: build a flood wall around the plant footprint following updated Atlas 14 rainfall modeling; the plant sits in the 100-year floodplain. - Odor control: install bio-trickling filters, activated carbon filters and basin covers with controlled venting.

Austin Water said the site already contains man-made berms and deep basins from earlier configurations that create the need for cuts and fills beyond usual thresholds. As part of the request, staff asked for administrative variances addressing construction on slopes, cuts and fills that exceed four feet, and for relief related to two heritage pecan trees on the site; staff said they propose to relocate those trees subject to an arborist feasibility study.

Staff also described a proposed adjustment to critical water quality zone boundaries tied to the flood wall and plant footprint; the presentation included mitigation commitments but offered inconsistent figures in the briefing transcript (staff at one point described a 150% mitigation parcel proposal and later referred to 50% mitigation). The project presentation said staff will work with city planners and watershed protection on mitigation and final designs.

Procurement and schedule: Austin Water said the CMAR contract (contractor named in presentation) is on board and that guaranteed maximum price (GMP) packages will be developed as design progresses; the utility stressed that long lead items may require ordering early to avoid supply-chain delays. Staff emphasized phasing to maintain the plant’s treatment capacity during construction, including building new capacity on-site before taking existing basins offline for conversion.

Regulatory context and next steps: staff reported the project has been before the Environmental Commission (April 2) and the Planning Commission (presentation the prior day). Austin Water said the project is scheduled for City Council consideration on May 22. The utility told the committee it will continue to coordinate with watershed protection, planning staff and city arborists as designs mature.

Questions from committee members focused on asset life cycles and maintenance of mechanical equipment (staff said concrete basins typically have a long life but mechanical components commonly require replacement on multi‑decadal cycles), reasons for using CMAR rather than design-bid-build (staff cited complex sequencing, need to maintain operations and to secure long-lead equipment), and community impacts during construction (site access, staging, truck routing and coordination with adjacent capital projects on MLK Boulevard).

What the presentation did not include: no formal vote was taken during the briefing. The project team discussed seeking administrative variances and code amendments; final decisions on variances and any ordinance would be determined later by permitting staff and, where applicable, City Council.

Next steps: Austin Water will refine design packages and mitigation proposals, respond to Environmental Commission recommendations, finalize GMPs with the CMAR, and return to council on the planned May 22 agenda for action on required code amendments and project approvals.