Innovative Carbon-based Advanced Treatment in Action: Unveiling Key Lessons Learned from Pilot Demonstrations

Introduction
As population growth, urbanization, and climate change place burdens on existing water supplies, water resource recovery facilities (WRRFs) are required to meet increasingly stringent discharge requirements, that are beyond the capabilities of conventional treatment, prompting the adoption of advanced treatment. In parallel, utilities are increasingly faced with low quality and limited quantity of potable water supplies. Recovery and reuse of treated wastewater effluent (i.e., water reuse) has emerged as a key strategy for meeting future water demand and water management needs. Treatment to achieve reuse requirements and/or nutrient reductions can be achieved using membrane based advanced treatment (mBAT), e.g., reverse osmosis; however, mBAT can be capitally and energetically intensive. Alternative carbon-based advanced treatment (CBAT) schemes employing multiple processes like coagulation/flocculation/sedimentation, ozonation, biofiltration and granular activated carbon (GAC) continue to be explored as an alternative to mBAT since they can reduce capital costs and energy requirements. However, data documenting the ability of these alternative treatment trains to concurrently and effectively remove organics, nutrients, and CECs to meet stringent treated water targets is limited. This work showcases two distinct case studies that demonstrate the effective removal of organics, nutrients, and CECs through the integration of multiple unit processes within multi-barrier CBAT trains. The following are some key learning objectives:
Treatment Process Requirements to achieve specific end-use goals in two distinct cases: a drinking water and a wastewater perspective.
Comparison of Organic and Contaminants of Emerging Concerns (CECs) Removal Performance: Understand capability of a CBAT train to concurrently treat and remove organics and CECs.
Summary of key lessons learned from each study to offer insights and guidance for future full-scale implementations.

Materials and methods
Case study 1
A CBAT train was piloted downstream of a WRRF in Tennessee to meet multiple fit-for-purpose objectives, that were primarily driven by a need to treat the wastewater effluent to meet stringent nutrient standards. This objective aligned well and paved path for other applications such as river and reservoir augmentation. The WRRF employed a three-stage biological nutrient removal (BNR) configuration with denitrification filters. The CBAT train (Figure 1) consists of membrane filtration (Suez ZW500D, Veolia, Boston, MA), ozonation (Intuitech Z400), Intuitech F1200 filtration unit filled with acclimated GAC media (F820) for BAC filtration, and Calgon GAC skid filled with Filtrasorb 300 (F300) type media. The membrane filtration unit was installed to remove total suspended solids (TSS) and simulate the performance of a full scale MBR system. The Ozone skid (Z400) included two, over-under baffled ozone contactors with a nominal hydraulic retention time of 20 minutes each. The F1200 BAC filters were operated in aerobic and anoxic modes to investigate the impacts on nitrogen removal. The pilot was operated for 9 months.

Case study 2
In this study, a CBAT train was piloted for 6 months to evaluate the level of treatment needed to treat groundwater impacted by rapid infiltration basins with and without ozonation (Intuitech Z400) prior to carbon treatment (Intuitech F1200), as well as testing carbon in both adsorptive (GAC) and biological mode (BAC). The configuration for the CBAT is shown in Figure 2.

Results and discussion
Total organic carbon (TOC) removal. Figure 3 illustrates the evolution of TOC concentrations along the different stages of the CBAT process in Case Study 1. The TOC concentration remains relatively stable after ozonation but begins to decline remarkably after passing through the aerobic BAC and GAC, demonstrating its effectiveness in adsorbing organic compounds. Figure 4 illustrates the TOC concentration trends across multiple CBAT scenarios over time in case study 2. Across all scenarios, a general downward trend in TOC concentration is observed. Ozonation provides a moderate reduction, while BAC and GAC stages contribute further TOC removal. The GAC consistently demonstrates significant TOC adsorption capacity, helping achieve TOC levels below the treatment target. To sum up, these results underscore the multi-barrier CBAT system's ability to incrementally and effectively remove organic contaminants. Constituents of Emerging Concern (CECs. Figure 5 presents the concentrations of PFAS various PFAS compounds (e.g., PFBS, PFHxS, PFNA, PFOA, and PFOS) across different scenarios for BAC and GAC stages. The BAC effluent (Figure 6a) shows moderate removal of PFAS compounds. The GAC effluent (Figure 6b) demonstrates more substantial reductions in PFAS concentrations compared to BAC. To sum up, these results also emphasize the need for multi-stage treatment to address both short- and long-chain PFAS effectively.