Sidestream granular sludge bioaugmentation for year-round mainstream anammox under seasonal low-temperature constraints: A critical review and quantitative assessment.

Anaerobic ammonium oxidation (anammox) offers an energy-efficient route for mainstream nitrogen removal, but its application in temperate regions remains constrained by low winter wastewater temperatures. Low temperatures suppress the metabolic activity of anammox bacteria and severely limit their net growth. This review assesses low-temperature mainstream anammox from the perspective of biomass maintenance and evaluates continuous sidestream granular sludge bioaugmentation as a potential practical option for year-round support. The specific anammox activity declines to approximately 21% at 15 °C and to 5% at 10 °C relative to that at 30 °C, while net biomass growth becomes strongly constrained below about 15 °C. Winter biomass maintenance thus becomes increasingly unreliable. Sidestream two-stage partial nitritation/anammox systems treating anaerobic digestion reject water can serve as a year-round source of surplus granular sludge for sustained bioaugmentation. Continuous biomass supply may also promote progressive cold adaptation during seasonal cooling in mainstream reactors. Quantitative assessment suggests that continuous sidestream bioaugmentation could increase the anammox contribution to nitrogen removal from 58.5% to 68.4% on an annual average and from 43.0% to 54.6% in winter. When coupled with wastewater source heat pumps, the corresponding values further increase to 72.9% and 67.0%, which may allow anammox to remain the primary nitrogen removal pathway throughout the year. Overall, this review provides a quantitative basis for supporting more reliable mainstream anammox under seasonal low-temperature conditions in temperate climates.