Start-up of activated sludge process in BNR facilities benefit from dynamic process models and provide basis for acceptance testing and client communication.

By Dwight Houweling, Bruce Johnson and Tim Constantine, CH2M HILL

Dwight Houweling, Bruce Johnson and Tim Constantine presented their paper “Supporting Start-up of BNR Facilities: the role of dynamic modeling” on Tuesday, July 30 during the 2013 WEF/IWA Nutrient Removal and Recovery Conference in Vancouver. Learn more about CH2M HILL’s entire conference presence.

Dynamic process models are effective tools for promoting the start-up of the activated sludge process, particularly BNR facilities where low effluent nutrient limits must be met from the start of operation. The discussion in our paper focuses on practical aspects of using the state-of-the-art dynamic process models for supporting start-up of biological nutrient removal (BNR) processes, the benefits that can be realized and areas where model development should be focused. We believe that model-based support of acceptance testing will become more common in the future and thus strengths and limitations of this approach need to be clearly understood.

Key benefits of these models are:
• Supporting decision making on the quantity of sludge that is required at start-up and the rate at which sewage flow to the facility, as well as bioreactors, can be brought online
• Evaluating operational strategies including sludge wasting and mixed liquor/RAS recycle rates
• Identifying challenges typical of start-ups (e.g. insufficient turndown on blowers and recycle pumps) and simulating mitigation strategies

These process models are important tools for communicating performance to clients, even providing the basis for acceptance testing at times. The paper presents the details of work at The Spokane County Regional Water Reclamation Facility (SCRWRF), which was recently constructed for 8 mgd. Effluent quality acceptance testing was conducted during the summer of 2012 at temperature of 21oC and the flows were 6.8 mgd. To demonstrate that the process could meet its compliance limit under contract loads in the winter, test data was used to calibrate a dynamic process model that could then be used to predict performance under higher loading and colder water conditions. Critical to generating sufficient confidence in the calibrated model was capturing the dynamic conditions encountered during the acceptance test period.

We also discuss using the process model to support start-up at two MBR BNR facilities recently commissioned in the City of Henderson and the City of North Las Vegas, Nevada. Discharging into Lake Mead, it was important that both facilities met effluent phosphorus requirements at the outset. Operating as biological phosphorus removal (Bio-P) processes, the facilities were seeded with sufficient sludge from a local Bio-P facility to allow one train to operate at about half of its capacity. Flow was then increased.

Bioreactors were brought online as mixed liquor concentration increased to remain below design concentrations in the membrane tanks as well as to optimize the biological process. The dilution effect caused the mixed liquor concentrations to drop every time a new bioreactor was brought online.  The model was able to capture this effect because all bioreactor trains were included in the model.

The performance of biological phosphorus removal was effectively captured by the biokinetic model and helped to identify the relationship between deteriorating phosphorus removal and the lack of sludge wasting. Based on these simulation results, the model was used as a support tool in the initiating of sludge wasting with successful results.

Dwight Houweling

Dwight Houweling, Ph.D. is a process engineer and recognized expert in the development and application of process models as tools for design and optimization and the use of bench-scale studies for wastewater and kinetic characterization.

Bruce Johnson

Bruce Johnson is a Senior Principal Technologist and process engineer specializing in the process modeling, design, and sizing of biological treatment systems, solid-liquid separation equipment, and waste sludge reduction.  He has over 20 years of global experience operating, troubleshooting, and designing water and wastewater treatment plants and equipment.

Tim Constantine is a senior process engineer and patent holder recognized for developing innovative solutions for the design of process upgrade and expansion of existing and operational facilities. His experience includes leading all phases of design, coordinating bench- and pilot-scale treatability studies, optimizing plant processes, and performing process modeling for primary, secondary and tertiary treatment trains.