Wastewater Treatment Selection and Operation to Benefit Downstream Resource Recovery
Last Modified: Jun 06, 2018
- Stephanie Ishii, Wendell Khunjar, Phill Yi, Enrique Vadiveloo, Buddy Boysen, Chris Owen, Troy Walker - Hazen and Sawyer
Effective integrated water management and resource recovery requires an acknowledgement of the interconnectivity between upstream wastewater treatment and downstream processes and applications. The purpose of this presentation is to discuss drivers, treatment, and cyclical relationships specific to different reuse scenarios using three case studies. Ultimately, the need to treat source control and wastewater treatment as part of an integrated water supply plan will be highlighted.
Case Study #1: Resource Recovery Impacts Resource Recovery
Plant A is a is an 11 mgd advanced treatment facility that employs a 5-Stage biological nutrient removal (BNR) process, followed by membrane filtration and granular activated carbon (GAC) and UV disinfection. Effluent from the facility must meet enhanced nutrient removal requirements, as well monthly COD and TKN limits of 10 mg/L and 1 mg/L, respectively, that were developed to protect downstream potable water supplies.
Planning is underway for a treatment capacity expansion at Plant A. Through the planning process, three key themes were identified that could impact Plant A’s ability to comply with effluent discharge and reuse criteria:
• Theme 1 – The utility aimed to maximize the use of non-potable reclaimed water at industrial cooling operations. During the cooling process, evaporative losses reduce the volume of flow while the mass of recalcitrant constituents (e.g., total dissolved solids and dissolved organic carbon and nitrogen) is conserved. This type of reuse can result in the delivery of highly concentrated return flows back to the wastewater treatment facility, where removal mechanisms for these recalcitrant constituents are limited. It was determined that Plant A must limit their reclaimed water use in cooling to 33% of the influent municipal flow to comply with various water quality thresholds of importance, such as those related to surface water discharge, reclaimed water economic incentives, and in-plant operations.
• Theme 2 – Implementation of thermal hydrolysis process (THP) was considered as pretreatment to anaerobic digestion of biosolids to support energy neutrality and minimize solids disposal; however, THP would result in the production of non-biodegradable organic nutrient fractions that would increase the effluent COD and TKN concentrations above the current limits. To resolve this issue, Plant A would either need to direct the sidestream generated from THP to another plant with less stringent COD and TKN limits or implement an enhanced tertiary treatment process (e.g., ozonebiofiltration).
• Theme 3 – Plant A could expand and avoid the construction of new GAC contactors if the minimum acceptable empty bed contact time (EBCT) were reduced by 25%. Rapid small-scale column tests (RSSCT) confirmed that the existing GAC treatment process at Plant A can maintain compliance with COD and TKN limits if the EBCT were reduced by 25%; however, re-rating the GAC contactors in this manner is contingent on Plant A operating the contactors to promote sorption as the primary mode of contaminant removal. Operation in the sorption mode requires frequent media regeneration, thus impacting operating costs and reducing the benefits that Plant A can achieve by allowing biofilm to grow in the contactors.
Case Study #2: Nitrogen Impacts Downstream Water Quality Plant
Plant B is a 24 mgd pure oxygen biological treatment process with chlorine disinfection that currently discharges to an open ocean outfall for secondary effluent disposal. Plant B is required to eliminate use of the outfall by 2025 and perform recharge of a saline aquifer. To meet aquifer recharge water quality requirements and minimize the carbon footprint of treatment, Plant B conducted a pilot study of a non-membrane reuse train. The pilot system included deep bed filtration, ion exchange for organic carbon and nutrient removal, ozonation, biofiltration and UV advanced oxidation.
An interesting outcome of the pilot was observed N-nitrosodimethyl amine (NDMA) production in biofilter columns at EBCTs greater than 14 minutes. Further testing indicated that NDMA production was generally limited to zones in which denitrification was the primary biological process in the columns. This finding is important because Plant B does not perform nitrification; effluent provides reduced nitrogen for nitrification in the aerobic zones of the biofilter immediately downstream ozonation, which results in the production of nitrate that then drives denitrification in the anoxic regions of the biofilter. This is an unintended consequence of active nitrogen metabolism that must be considered when adopting ozonation and biofiltration downstream of a wastewater treatment facility with limited nitrogen removal.
Case Study #3: Nitrogen Impacts Downstream Asset Management
Membrane-based treatment may employ the use of chloramines to minimize biological fouling of membranes, thus supporting the lifespan and performance of the equipment. For example, secondary effluent is delivered to Plant A, where ammonia is dosed based on residual chlorine and a ratio controller. If influent chlorine and ammonia concentrations are outside the capabilities of the controller, inadequate protection of membranes, downstream ammonia-related impacts, and/or membrane oxidation may occur. Achieving the target chlorine to ammonia ratio for intended membrane protection is based on consistent upstream wastewater treatment. Furthermore, clear communication pathways between the upstream and downstream operators, as well as known triggers for communication, are critical for membrane system in the event of an upset.
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