Better left unsettled: Suspended air flotation for footprint‐optimized management of thin primary and blended solids

The progress in reducing process footprints for primary treatment or sludge thickening has been limited compared to efforts in intensifying secondary treatment. The current focus on primary treatment is on improving settling rates or using advanced primary filters, which have reduced the footprint for solids separation but produce thinner and more variable sludges. Sludge thickening processes, particularly for primary sludge (PS), have received relatively little attention regarding footprint minimization but play a large role in the ability to intensify primary treatment. To minimize process footprints and maximize performance, the present study examined, for the first time, the suitability of a suspended air flotation (SAF) technology for both thin PS and a blend of PS and waste-activated sludge (WAS). A 4-week pilot test was conducted using a trailer-mounted pilot-scale SAF system fed with either thin PS, or a blend of PS and WAS. Solids capture performance was monitored, along with solids (solids loading rate [SLR]) and hydraulic loading rates (HLR), froth, surfactant, and polymer dosage. The pilot validated the SAF technology for thickening of both primary and blended sludges at significantly higher loading rates (and subsequently smaller footprints) than competing thickening technologies. The study also demonstrated that SAF of thin PS can maintain a >90% solids capture rate at roughly twice the SLR (186 kg m^-2 h^-1) and HLR (690 m³ m^-2 day^-1) of blended sludge (90 kg m^-2 h^-1 and 293 m³ m^-2 day^-1, respectively), validating the design values for a pending SAF installation. Results were also used to establish recommended polymer and froth dosing rates for the full-scale installation. Overall, this study demonstrates that SAF represents a viable, footprint-efficient solution for footprint-optimized management of thin primary and blended solids. Further testing of SAF with real primary filter backwash is recommended to accelerate the adoption of primary filtration for more intensified, carbon-efficient, and resilient wastewater treatment. PRACTITIONER POINTS: Pilot study confirms SAF tech effectively thickens primary and blended sludge, with more consistent performance for primary sludge processing. Primary sludge can be processed at 2× higher hydraulic and solids loading rates than blended sludge with over 90% solids capture performance. The study validates proposed SAF design values and demonstrates the importance of maintaining adequate froth and polymer dosage. SAF is an alternative footprint-efficient solution for thickening high-volume waste flows such as backwash from advanced primary treatment processes. Testing and optimizing SAF with primary and blended sludges could inform thickener selection and design at other wastewater treatment plants.