Media selection for anammox‐based polishing filters: Balancing anammox enrichment and retention with filtration function

Abstract

Retrofitting conventional denitrification filters into partial denitrification‐anammox (PdNA)‐ or anammox (AnAOB)‐based filters will reduce the needs for external carbon addition. The success of AnAOB‐based filters depends on anammox growth and retention within such filters. Studies have overlooked the importance of media selection and its impact on AnAOB capacity, head loss progression dynamics, and shear conditions applied onto the AnAOB biofilm. The objective of this study was to evaluate viable media types (10 types) that can enhance AnAOB rates for efficient nitrogen removal in filters. Given the higher backwash requirement and lower AnAOB capacity of the conventionally used sand, expanded clay (3–5 mm) was recommended for AnAOB‐based filters in this study. Owing to its surface characteristics, expanded clay had higher AnAOB activity (304‐ vs. 104‐g NH4+‐N/m2/day) and higher AnAOB retention (43% more) than sand. Increasing the iron content of expanded clay to 37% resulted in an increase in zeta potential, which led to 56% more anammox capacity compared to expanded clay with 7% iron content. This work provides insight into the importance of media types in the growth and retention of AnAOB in filters, and this knowledge could be used as basis in the development of PdNA filters. Practitioner Points Expanded clay showed the lowest head loss buildup and most likely will result in longer runtime for full‐scale PdNA applications The highest AnAOB rates were achieved in expanded clay types and sand compared with smaller media typically used in biofiltration Expanded clay resulted in better AnAOB retention under shear, whereas sand could not withstand shear and required more frequent backwashing Expanded clay iron coating enhanced AnAOB enrichment and retention, most likely due to increased surface roughness and/or positive charge

Mainstream partial denitrification‐anammox in sand and expanded clay deep‐bed polishing filters under practical loading rates and backwashing conditions

“…Given expanded clay higher surface roughness and higher iron content, it was hypothesized that increased PdNA rates would be achieved in the expanded clay due to high AnAOB retention efficiency previously observed in bench‐scale testing (Fofana et al, 2022). However, in this study, we observed similar PdNA rates of 840 ± 158 and 843 ± 196 g N/m 3 /d, and total nitrogen removal rates of 960 ± 125 and 964 ± 159 g N/m 3 /d for the sand and expanded clay filters, respectively (Figure 4, Table 2).…”

“…No significant difference in performance was observed between the two media types although expanded clay was expected to have higher AnAOB growth and retention based on a previous bench-scale study (Fofana et al, 2022). The bumping and backwash applied in the pilot filters were automatically controlled based on head loss buildup, and the settings were similar for both media types.…”

“…Aquabacterium was however significantly more abundant in the expanded clay filter than in the sand filter (p < 0.005) (Figure 3). This might be related to expanded clay high iron content compared with sand (Fofana et al, 2022) because Aquabacterium has been found in iron-rich environments (Li et al, 2020(Li et al, , 2021Yang et al, 2021).…”

“…As AnAOB are slow-growing organisms (Strous et al, 1999), media with increased porosity and/or depressions might protect AnAOB from external shear forces and potential washout during backwash, which as a result might benefit PdNA applications. Expanded clay with its higher surface roughness and higher iron content showed higher AnAOB growth and retention in comparison to sand in a bench-scale study (Fofana et al, 2022). In addition, expanded clay resulted in increased runtimes and hydraulic throughput in biofiltration applications (Chipps, 2017; Water Research Foundation [WRF], 2018), thus showing potential for lower backwash frequencies.…”

Full‐scale transition from denitrification to partial denitrification–anammox (PdNA) in deep‐bed filters: Operational strategies for and benefits of PdNA implementation

“…For the latter carbon sources, it was hypothesized that having a good NO 2 À -N sink by efficient AnAOB retention is critical to provide efficient PdNA (Ladipo-Obasa et al, 2020;Riffat et al, 2021). For filters, it was shown that sand media was effective in retaining AnAOB under pilot scale conditions (Fofana, Peng et al, 2022).…”