Innovative Process Piping Modifications to Enhance Carbon Management in Primary Treatment

INTRODUCTION
Conventional WRRF process design provides utilities with limited process control for active management of what type and how much carbon is routed to which process operation to balance competing treatment goals and maximize overall treatment objectives. As WRRFs are retrofitted for biological nutrient removal (BNR), carbon deficiency can lead to poor or inconsistent nutrient removal. Effective process configurations for so-called 'carbon diversion' have been discussed in recent years and include primary sludge fermentation, in-situ mixed liquor fermentation (Barnard et al. 2010), activated primary clarifier, A-stage configurations (Wett et al., 2020; Wett et al., 2021), sidestream enhanced biological phosphorus removal (S2EBPR) (Wang et al., 2019) or simply addition of carbon-rich substrates. These approaches have been shown to result in significant treatment benefits and cost savings. This paper introduces three innovative process arrangements that have not been implemented widely to date in North America. Implementation is non-proprietary and cost-effective. Individually or in combination, these configurations provide a higher degree of operational flexibility for balancing treatment needs with respect to secondary treatment capacity, nutrient removal, energy conservation through reduced aeration, and biogas generation for energy recovery.
METHODS
The three innovative process configurations presented in this paper include:
1. Integration of Anaerobic or Anoxic Selectors within Existing Primary Clarifier Footprint
2. A2B Process (Anaerobic A-Stage Integration in Primary Clarifier)
3. Partial Primary Clarifier Flow Bypass The three configurations are introduced, and process modifications discussed and illustrated through process flow schematics. Their individual performance and benefits are compared to conventional process designs using the following criteria:
1. BOD and TSS Removal efficiency
2. Impact on nutrient (N and P) removal efficiency
3. Potential for energy recovery through biogas generation in anaerobic digestion
For comparison of the process performance, two baseline process configurations are used in this paper:
1. Conventional activated sludge: Primary treatment followed by secondary treatment for BOD removal
2. Conventional BNR treatment: Primary treatment followed by secondary BNR treatment (A2O treatment configuration)
RESULTS: INNOVATIVE PROCESS ARRANGEMENTS AND PERFORMANCE Configuration 1: Integration of Anaerobic or Anoxic Selectors within Existing Primary Clarifier Footprint This first innovative process arrangement involves a reconfiguration of existing primary clarifiers such that a portion of the clarifier footprint is repurposed as an anaerobic or anoxic reactor. This is schematically shown in Figure 1 for the example of a circular or rectangular clarifier. To our knowledge, this process arrangement has not been implemented in the US to date but has successfully been retrofitted into several German BNR retrofits.
Main Benefits: - The treatment capacity of the secondary process (aerated basin volume) remains unchanged while unaerated selectors for N and P removal are retrofitted into the existing facility footprint. - The footprint of the existing primary clarifiers is reduced allowing more colloidal carbon to pass into the anoxic/anaerobic selectors for improved BNR. - The selectors are designed in a beneficial plug flow configuration (high length:width ratio) which benefits nitrate and phosphorus removal efficiencies. - Continued primary clarifier operation and removal of grit and flotsam. Required process modifications: - Construction of an internal baffle wall and new weir to separate the clarifier footprint from the selector zone in all or some clarifiers. - RAS pipe extension from head of aeration basins to new selector zone in primary clarifiers. - Modifications to the primary sludge and float collection mechanisms - As-needed modifications to accommodate additional flow between primaries and aeration basins. - If needed, supplemental mixing in the selector zone. (Need should be verified through CFD modeling. Some existing European facilities successfully operate without supplemental mixing.) - Process flexibility: Anaerobic or anoxic zone volume in the primary clarifiers can replace or reduce the required anaerobic zone volume in the aeration basins or may be utilized to extend anaerobic retention time to improve fermentation. Table 1 includes estimated performance benefits of this configuration in comparison to traditional CAS and BNR process configurations.
Configuration 2: A2B Process (Anaerobic A-Stage Integration into Primary Clarification) In this configuration, primary clarifiers are used as biologically active tanks while also separating solids upstream of secondary treatment. The A2B process concept was first introduced by Fraser and Dapcic (WEFTEC 2016) as a modification to the WASACâ„¢ process (Dapcic, 2015), Narayanan (2004)). A process schematic is shown in Figure 2. This process configuration is similar to facilities that use cosettling of primary sludge and waste activated sludges in primary clarifiers. Here, a portion (or all) of the RAS is blended with the influent in the primary clarifiers for carbon diversion. This process involves recycling RAS from the EBPR process recycled to the primary clarifiers for anaerobic contact with raw influent wastewater under very short HRTs (less than 30 minutes).
Main Benefits: This configuration achieves an additional 30% soluble BOD uptake in the primaries through phosphorus accumulating organisms (PAO) and a high rate of phosphorus release which allows for improved P uptake in the aeration basins (Dapcic, 2015). A portion of the PAOs enriched with stored carbon are wasted from the primaries to digestion. As a result, additional carbon is diverted for energy recovery while downstream nutrient removal remains effective. Similar to AAA process but using a non-aerobic contact basin, thereby more energy efficient. Required process modifications: - RAS pipe extension from head of aeration basins to new selector zone in primary clarifiers. - As-needed modifications to accommodate additional hydraulic flow between primaries and aeration basins. - As-needed primary sludge pump capacity upgrades
Configuration 3: Partial Primary Clarifier Flow Bypass Reducing the HRT in primary clarifiers can enhance passing additional carbon to the downstream BNR process, specifically the slowly settleable colloidal organic carbon fraction. Carollo has implemented this configuration in a recent BNR retrofit for the City of Boulder, Colorado. Figure 3 shows the process configuration.
Main Benefits:</> The bypass piping can be designed to allow plant staff to route anywhere between 0% to 100% influent flow directly to secondary treatment, providing a vast range of flexibility to balance various treatment objectives. Operational flexibility allows utilities to balance this operation with secondary capacity impacts. Required process modifications: - Bypass pumping and pipeline to route plant influent to the primary effluent splitter box; automated flow control valves. - Bypass of primary clarification may trigger National Fire Protection Association (NFPA) 820 code classifications. Figure 4 shows performance results from a recent optimization study at the City of Longmont, Colorado, illustrating the benefit of an alternative approach, taking primary clarifiers out of service for enhanced nitrogen removal downstream.
RELEVANCE
This paper presents innovative primary treatment process configurations to actively manage carbon for enhanced BNR performance and energy recovery. These non-proprietary and cost-effective configurations can provide operational flexibility to meet competing treatment needs of WRRFs.