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D Pushing Nutrient Removal Frontiers By JB Neethling, PE., Ph.D. utrient removal requirements have steadily tightened since the early 1990s. Before then, only facilities in sensitive watersheds were required to remove nutrients. Nitrogen removal is typically required to 10 mg/L and phosphorus to 1 mg/L. In the late 1980s, Clean Water Services (then Unified Sewage Agency) in Tigard, Ore., was challenged with the nation's most restrictive phosphorus limit -0.07 mg/L total phosphorus. Soon after, the city of Las Vegas' was also challenged with new phosphorus limits — initially 1 mg/L, quickly reduced to 0.3 and currently 0.17 mg/L. At the CWS Durham Wastewater Treatment Plant, a systematic research program started with a survey of treatment technologies and facilities achieving low phosphorus limits. The survey identified both chemical and biological phos- phorus removal as potential solutions. However, two stages of removal were deemed necessary to meet these low limits. Pilot studies were conducted to test filter options, media selection and operational parameters. Based on this study, the system was designed to provide flexibility for future optimization plus reliability to meet permit requirements. A two -stage chemical addition process added alum to both the primary clarifiers and tertiary filters. The activated sludge process was reconfigured in an enhanced biological phosphorus (EBPR) configuration that accommodated both the Anaerobic - Anoxic -Oxic (A20) and modified University of Cape Town (UCT) modes of operation. The plant has been in compliance since the 1990s when modifications were implemented. Subsequent investigations and full -scale studies achieved the following: • Optimizing the chemical dose and switching to EBPR mode reduced chemical usage. • Implementing the Unified Fermentation and Thickening fermenter process optimized EBPR performance and further reduced chemical usage. • Side stream treatment reduced shock loading to the plant and flattened the loading. • Research to control struvite formation led to selection of appropriate materials and design /operational conditions to avoid scale formation in pipes. Work continues on the fate of phosphorus and nitrogen during digestion. • Studies on uptake and release of phosphorus and metals were used to modify biological treatment models for phosphorus removal. • Full -scale performance evaluation and modeling showed that the biological nutrient removal plant has 20 percent additional capacity. • Bench, pilot and full -scale studies compared efficiency of plug flow and complete mixed reactors to achieve low effluent ammonia limits. While plug flow provided better performance, aeration control is more compli- cated and requires special considerations. In the last 20 years, the city of Las Vegas water reclamation facility grew from 41 to 91 mgd capacity while phosphorus limits continued to decrease. The original trickling filter treatment plant now includes nitrification activated sludge, effluent filtration, and EBPR processes. Pilot and full -scale studies were conducted to optimize process performance and capacity. Benchmark events included: • Pilot studies to determine nitrification kinetics and performance of trickling filters and activated sludge, filter efficiencies and filter media design. 'The city of Las Vegas permit based on an allowable mass discharge. Concentrations are based on design flow values to meet the mass loading limits. a a a `E a 2t.50.;i, _ . _..._............ .-------- L- WAy43 .. .. .- ..---.- ---- --.-.- .- .. - ._ -_.-.., recorded oP 0.02 mg/L 'd 1.5 f-..... . . . . .. ....._........- ---------------- ...... ....... _. _. ........ _..... _........= .. ._. ..._........a 3AY43 ; i . J-03 A,9-0 Ocbo3 �ecJ Pob-03 AprvO{ JcnM ALPS-04 -g -B M E0, W Figure 1. Continued performance improvements at city of Las Vegas through operator vigilance and optimization. Offgas testing and modeling of nitrification facilities, resulting in increasing rated capacity of the 49.5 mgd facilities by more than 25 percent, eliminating the need to build the eighth train. Studying performance of the EBPR trains to determine the impact of dissolved oxygen (DO), sludge age and reactor configuration on the performance of the process. Operating in UCT mode proved more stable than the A20 mode. Pilot testing of filtration technologies and modes to meet even lower phosphorus limits in the future. Nitrogen and phosphorus limits are presently being pushed to limits that may be below the limits of technology. Phosphorus concentrations as low as 0.02 mg/L are being proposed for some areas in the Pacific Northwest and Colorado River basin, which is below the reported equilibrium precipitation concentration of 0.02 -0.04 mg/L for ferric and alum salts. Nitrogen limits of 3 mg/L total nitrogen are proposed for facilities discharging to Chesapeake Bay. This low TN limit is very difficult to meet using a classic biological process, since it approaches the nonbiodegradable fraction of organic nitrogen in typical effluents. These new challenges call for new technologies. HDR is working with utilities to identify treatment options by testing new techniques, investigating new scientific principles, and looking toward removing limitations that form the boundary condition for the technology. Projects under development include: • Using membrane bioreactors to achieve low phosphorus limits. • Tertiary filtration and microfiltration for phosphorus removal. • Aeration basin configurations, including aeration modulation (on -off) to achieve simultaneous nitrification and denitrifi cation, utilizing NADH probes to control the biological process, and adding fixed media to improve capacity. • Side stream management of recycle loads, including struvite formation as a control strategy, prenitrifi cation and seeding, and load attenuation. • Adsorption processes for phosphorus removal, using fixed bed adsorbents or formed metal hydroxides to adsorb phosphates. The focus is on finding new and innovative solutions, while participating in the national search for a better, more efficient way to be good stewards of the environment. l8 Neethling, P.E., Ph.D., is HDR's technical director of wastewater treatment and disposal. He can be reached in HDR's Folsom, Calif., office at (916) 817 -4830 or jb. neethling @hdrinc. com.