2022 Excellence in Environmental Engineering and Science™ Awards Competition Winner

Grand Prize - Design and Grand Prize - 50th Anniversary of the Clean Water Act

Regional San EchoWater BNR Project

Entrant: Black & Veatch
Engineer in Charge: James H. Clark, P.E., BCEE
Location: Elk Grove, California
Media Contact: Bruce Moores

Entrant Profile

Black & Veatch (BV) provides integrated engineering, procurement, consulting, and construction services to public and private clients, promoting the resilience and reliability of infrastructure assets that improve the quality of life. The company is a global leader in advanced wastewater treatment, particularly the management of nutrients. It designed the biological nutrient removal facilities added under the EchoWater project by Regional San to improve the effluent discharged by Sacramento’s wastewater treatment plant and promote its reuse for landscaping and agricultural applications.

Project Description

Drivers and Challenges

In 2010, the Sacramento Regional County Sanitation District (Regional San) was issued stringent new treatment requirements to be met by 2023. At the time, the district’s secondary treatment facility used High Purity Oxygen Activated Sludge (HPOAS) to remove of organics and solids only. As a central component of the EchoWater project, which was implemented by the district to comply with the new treatment requirements, new biological nutrient removal (BNR) basins were added to the facility. Their purpose: to achieve ammonia and nitrogen (N) removal in the secondary process, thereby replacing the HPOAS system and reducing N discharge to the Sacramento River. Production of high-quality secondary effluent was also needed to serve as source water for Title 22 reuse and because water from the Sacramento River is intercepted downstream from the plant and pumped to southern California to augment water supplies there.

Specific design challenges were posed by the intersection of extreme influent dynamics and stringent effluent ammonia limits. Storms result in sustained influent flow of 330 million gallons per day (MGD) coinciding with peak influent N loads while the average daily ammonia cannot exceed 2.0 mg/L-N in summer and 3.3 mg/L-N in winter. Other challenges impacted the ability to meet nitrate limits. Influent wastewater was septic, and at the time of design high doses of chlorine were used to combat odors, oxidizing influent organics (COD) required for denitrification. This resulted in 6-7 mgCOD/mgN in the influent, necessitating carbon augmentation to meet the monthly nitrate limit of 10 mgN/L. Chlorination has been replaced by nitrate addition (from centrate nitrification), which somewhat reduced N loading to secondary treatment.

Other challenges were presented by schedule, site, and environmental impact drivers. The schedule required rapid execution, but due to the immense scale many years passed between conceptual design and startup, adding to design uncertainty. A constrained site using the existing primary and secondary clarifiers and the desire to limit energy use and overall environmental impact meant that efficiency in design and operational energy use were critically important.

To address these challenges efficiently without undue risk, a combination of dynamic seasonal modeling and full-scale pilot testing, and the application of cutting-edge applied research were used during design.

Efficiency of Footprint and Energy Use

A primary design objective was the selection of an appropriate solids retention time (SRT) to meet the stringent ammonia requirement while minimizing footprint. The greatest challenge comes when winter storms result in high flow and N loads at low temperature resulting from the collection system and equalization processes. By explicitly modeling these influent dynamics, safety factors typically applied to SRT to address influent variability could be reduced. A sensitivity analysis of SRT impact on peak effluent ammonia was used to select a 6-day aerobic SRT, reducing the required aerobic volume by approximately 20% versus a more traditional approach. Volume requirements were further reduced by incorporating diffused aeration into post-anoxic swing zones and counting this as aerobic volume. Incorporation of ammonia or nitrate-based control of swing zone aeration or carbon dosage ensured that daily ammonia and monthly nitrate limits could be met. Careful analysis of influent data, piloting to estimate nitrification rates, extensive dynamic modeling with incorporation of controls, and the incorporation of online SRT control into the design lent further confidence to this approach.

A high-efficiency diffuser design was also implemented by leveraging applied research into diffuser fouling and aging, and applying that to an evaluated bid process to select an efficient aeration system using tubular silicone diffusers. Selection was based on life-cycle cost and used extensive testing to validate efficiency guarantees that, combined with a basin depth of 26 feet, resulted in a design that reliably exceeds performance demands while also being efficient in terms of construction and operation, and low in environmental impact.

Effluent Quality and Resiliency

Fast-settling and well-flocculating biomass is necessary to achieve high-quality effluent for discharge and reuse while being resilient to increasingly frequent high-flow events. Various bulking and foam forming organisms are detrimental to this aim, and the construction of an entirely greenfield BNR basin presented the opportunity to incorporate comprehensive industry knowledge of growth pressures into the design to out-select these organisms. This is achieved by surface wasting of lighter foam or scum from various hang-up points in the process, resulting in free flow across the surface of BNR basins. Surface waste from the process is then discharged to a surface wasting classifying selector that receives return sludge and where lighter foam-forming sludge is ultimately wasted from the process.

Because of the low COD/N ratio, carbon management within the secondary process is also critically important to effluent quality and resiliency. The carbon feed system is flexible and allows for feed of different carbon sources to several locations and, further, the carbon supplementation requirements are minimized by providing sufficient unaerated volume at the front of the BNR process. This volume, which the process flexibility provided, and high mixed liquor recycle capacity allow for different operational strategies, depending on influent characteristics and N removal or carbon management goals. By accumulating mixed liquor and fermenting it within unaerated zones, endogenous carbon can be used to reduce the need for external supplementation. Together, these design features reduce the impact of supply chain disruptions and allow excess nitrate to be removed without external carbon addition under ideal circumstances.

Results

Based on the first year of full-scale operation, the design approach relying on extensive dynamic modeling, testing and the application of cutting-edge process understanding has addressed many project-specific challenges. Effluent quality is stable with lower aeration and carbon requirements than would be expected for the level of treatment achieved. Further, scum and foam have been almost non-existent on the basin and settling rates in the final clarifiers have been stable while staying well below design values. A record storm in October 2021 during the first year of full-scale operation tested the design beyond what was planned for at the outset of the program. Despite the challenge, the process was stable. No upsets in operation or effluent quality resulted.

Click images to enlarge in separate window.


While the site appears relatively open, the area that was practical and available for the BNR upgrade was constrained and hemmed in by: key assets (Flow Equalization and Primary Treatment); facilities that needed to operate continually during construction (existing High Purity Oxygen Activated Sludge, or HPOAS, basins); plus an area necessary for construction staging. Note: the HPOAS facilities are no longer used.	The eight BNR basins are arranged into four groups of two mirrored basins. Running down the middle of BNR basins are two RAS reactors each with a classifying selector zone and a preanoxic zone. These reactors convey RAS to the influent piping gallery where PE and RAS are split between BNR basins and between zones within basins in different operating modes. Mixed liquor discharges at the opposite side of the three-pass BNR basins to the mixed liquor channel, which conveys BNR basin effluent to the mixed liquor splitter to evenly divide flows between three secondary clarifier batteries.

This conceptual process flow diagram shows one of two RAS reactors and one of eight mainstream reactors. Unaerated zones are highlighted in dark blue, aerated zones in light blue, and anoxic/aerobic swing zones in purple. Grey arrows indicate open channel flow through the reactors, and colored arrows indicate major process inputs, outputs, and internal recycles. Generally, the process operates as a 4-stage nitrification/denitrification process, but upfront bypasses and a mixed liquor fermenter recycle allows for operation with an increased anoxic inventory to supplement carbon for denitrification through mixed liquor fermentation processes.

The processes have started up and run stably without any scum or foaming and with consistent settling characteristics that are well below design assumptions. The stable performance allows for high effluent quality during low-flow conditions and affords the resiliency necessary to manage high flow conditions. The stable operation is owed to the effects of free surface flow and surface wasting of scum and foam forming biology from the BNR basins.	RAS and PE piping galleries keep these flow streams separate to allow process flexibility to operate in several nutrient removal configurations depending on influent characteristics and drivers; all while making the most of the available influent carbon regardless of the form the carbon is in when it arrives at the BNR process.

BNR basins are deep to reduce footprint but with the added benefit of increasing oxygen transfer efficiency (OTE) and reducing energy usage. Further increasing OTE is a high-density diffuser design that was selected through an evaluated bid with extensive shop and field testing to validate transfer performance. Other energy-saving features include mixing chimneys that mix influent streams of different densities and allow for reduced mixing energy input to unaerated zones.	Both average and peak aeration requirements were substantially reduced based on both basin depth and diffuser density. Based on evaluated bid performance testing, the higher diffuser density design resulted in specific standard oxygen transfer efficiencies (sSOTE) of 2.3%/ft. or more, where typical designs result in averages of 2.0%/ft. or lower, resulting in a reduction in aeration requirement of 10-15%. Similarly, the 26 ft. side water depth versus a more typical 20 ft. side water depth resulted in improvements in transfer efficiency of 25% that, when accounting for the increased pressure, still reduces aeration energy a further 15%.

Long-term dynamic modeling was used to evaluate the impact of the seasonal, wet weather, and typical daily variability of process loading on performance for different designs. This approach allowed for a reduction in safety factors associated with irreducible risks and ultimately a more efficient design. It also allowed for more accurate life-cycle evaluations based on actual system variability and, therefore, improved decision making and equipment selection.	Long-term dynamic simulations with integrated ammonia-based control of swing zone aeration were used to predict the distribution of nitrogen species for various aerobic solids residence times (aSRT). An aSRT of 6 days was selected, which was further simulated with different control inputs to assess the risk of ammonia-based aeration control failure. As a result, a 6-day aSRT (inclusive of swing zones) was selected that reduced process volume by 30% from what would have resulted from more conservative traditional approaches. Reducible risk associated with modeling uncertainty was reduced through full-scale testing.

Long-term dynamic simulations with integrated controls also helped to assess BNR configuration alternatives in a carbon management framework. This showed a substantial reduction in carbon dosage and carbon storage requirements by using mixed liquor fermentation under carbon limited conditions.