Effectiveness analysis of a wastewater treatment plant retrofitted by MBBR in a cold plateau region

To further implement environmental protection policies on the Qinghai-Tibet Plateau, a wastewater treatment plant at an altitude of 2 200 m with a capacity of 85 000 tons per day was retrofitted by moving bed biofilm reactor(MBBR) process. After retrofitting, the effluent could stably meet the designed standards even the water temperature dropped below 9 ℃ during winter. The newly established suspended carrier zone and anoxic zone primarily facilitated the removal of NH4+-N and total nitrogen, respectively. Microbial richness and diversity on the suspended carriers were significantly higher than those in the activated sludge. Additionally, the extracellular polymeric substances on the suspended carriers exceeded those of the activated sludge. The relative abundances of nitrifying microbes and nitrification-related genes on suspended carriers were markedly higher those in activated sludge, which established a microbial foundation for the enhanced low temperature nitrification via suspended carriers in high-altitude cold regions. Conversely, the sums of relative abundances of denitrifying microbes and denitrification-related genes in activated sludge surpassed those of suspended carriers. The successful implementation of this large-scale project provides replicable strategies for the construction and upgrading of wastewater treatment plants in cold plateau regions.