The resilience of microbial communities to press disturbances and whether ecosystem function is governed by microbial composition or by the environment have not been empirically tested. To address these issues, a whole-ecosystem manipulation was performed in a full-scale activated sludge wastewater treatment plant. The parameter solids retention time (SRT) was used to manipulate microbial composition, which started at 30 days, then decreased to 12 and 3 days, before operation was restored to starting conditions (30-day SRT). Activated sludge samples were collected throughout the 313-day time series in parallel with bioreactor performance ('ecosystem function'). Bacterial small subunit (SSU) rRNA genes were surveyed from sludge samples resulting in a sequence library of 4417 000 SSU rRNA genes. A shift in community composition was observed for 12-and 3-day SRTs. The composition was altered such that r-strategists were enriched in the system during the 3-day SRT, whereas K-strategists were only present at SRTsX12 days. This shift corresponded to loss of ecosystem functions (nitrification, denitrification and biological phosphorus removal) for SRTsp12 days. Upon return to a 30-day SRT, complete recovery of the bioreactor performance was observed after 54 days despite an incomplete recovery of bacterial diversity. In addition, a different, yet phylogenetically related, community with fewer of its original rare members displaced the pre-disturbance community. Our results support the hypothesis that microbial ecosystems harbor functionally redundant phylotypes with regard to general ecosystem functions (carbon oxidation, nitrification, denitrification and phosphorus accumulation). However, the impacts of decreased rare phylotype membership on ecosystem stability and micropollutant removal remain unknown.

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Broader application of ultrafiltration (UF) membranes in drinking water treatment is limited by particulate and organic fouling. This pilot‐scale study demonstrated the ability of rapid biofiltration (without prior coagulation or ozonation) to reduce fouling of a UF membrane treating impacted river water over a two‐year period. Three biofilter empty bed contact times (EBCTs; 5, 10, and 15 min) were investigated, using parallel dual‐media filters followed by a hollow‐fiber membrane unit. Utilizing essentially conventional rapid filtration operating conditions, biofiltration pretreatment enabled stable membrane operation with respect to hydraulically reversible and irreversible fouling. Increased biofiltration EBCTs led to lower rates of irreversible fouling. The results were reproducible, and biofiltration was able to provide effective pretreatment during all seasons (T = 1‐25°C). The biofilters also contributed to effective turbidity removal. Overall, biofiltration pretreatment for UF is a promising chemical‐free alternative to coagulation pretreatment. A particular niche application would be in small or remote systems.