The influence of hydraulic retention time on nitrification rates in the treatment of reject water with the addition of calcium carbonate.

Sammanfattning

Human activities have disturbed the global nitrogen cycle, especially because of excessive use of inorganic nitrogen in agriculture. This has resulted in extensive water pollution and eutrophication, creating greater demands on wastewater treatment works for accurate and efficient removal of nitrogen before discharging the environment. To reduce these impacts, modern wastewater treatment is moving towards a model of circular economy, in which wastewater treatment plants become 'biorefineries' for nutrient recovery. One of the biggest problems when moving to this new technology is the treatment of 'reject water' which is a very high concentration side stream generated during sludge dewatering. The efficiency and success of this circular valorization approach rely on high nutrient extraction efficiency and on keeping the balance between water input and output. Thus, it is necessary that extraction be effective at certain Hydraulic Retention Times (HRTs) that would be representative of the operational flow requirements of full-scale treatment plants. This study aimed to study the effects of Hydraulic Retention Time (HRT) on the nitrification rates and resource recovery efficiency in a pilot-scale, two-stage Moving Bed Biofilm Reactor (MBBR) facility at VIVAB (Vatten och Miljö i Väst AB), located in Varberg, Sweden. A self-regulating buffering system was used to neutralize acids formed by biological nitrification by using calcium carbonate. The experiment was conducted in three phases; 15-day, 10-day and 5-day HRT. Volumetric Nitrification Rate (VNR) and effluent nitrogen speciation (spectrophotometric analysis) were used to assess the performance. A significant difference was found for the VNR with HRT determined by a One-way ANOVA (p=0.00428). A Tukey HSD test after the post-hoc analysis showed that the system was stable after 15 days and 10 days (p=0.77), but was significantly increased after 5 days (p=0.003) of HRT compared to 15 days (p=0.01) and 10 days (p=0.003). Biologically, the reduction of HRT initiated a "Nitrite Shunt" where nitrite concentrations reached above 400mg/L and the pH remained stable (6.5-7.0). The results agree with the hypothesis that the 5-day HRT is best to maximize nitrogen throughput without achieving a kinetic breaking point. This high-rate phase is favorable for the production of nitrite instead of nitrate, but both are acceptable for the circular recovery of resources, since nitrite will naturally convert to the more stable nitrate over time. Accordingly, the present study offers sustainable biological solution of wastewater treatment and mineral resource recovery at a scalable level.