Impact on nitrifiers of full-scale bioaugmentation.

Nitrifiers are the slowest growing bacteria used in conventional biological wastewater treatment. Furthermore, their growth rate is seriously hampered by low temperature. As a result, the volume needed for nitrification dominates the volume of the biological reactors at a wastewater treatment plant. As a way of enhancing nitrification and reducing this volume, bioaugmentation can be used. Nitrifiers from a side-stream plant can be inoculated to the mainstream process, which is thereby boosted. The effect of bioaugmentation can be measured in different ways. This full-scale study focuses on the effect of bioaugmentation from a microbial point of view by using 16S rRNA amplicon sequencing. The study reveals how bioaugmentation increases the diversity of nitrifiers in the mainstream process and in the side-stream plant; that the abundance of nitrifiers is increased in the mainstream process; the interaction between nitrifiers from the side-stream plant and mainstream process; and the effect of bioaugmentation on nitrifying genera and species over time. To our knowledge, this detailed microbial information on nitrifying species during a full-scale bioaugmentation study has not been presented before.

Promotion of nitrifiers through side-stream bioaugmentation: a full-scale study.

Bioaugmentation of nitrifiers from a side-stream treatment is an efficient method for boosting the mainstream process at a wastewater treatment plant (WWTP). Although this technology has been known for several years, the number of full-scale applications for it is limited. For a WWTP approaching its critical nitrogen load capacity, the benefits are doubled if the introduced side-stream treatment for digester supernatant is combined with bioaugmentation. Not only is the nitrogen load to the mainstream process decreased by 10-25%, but the mainstream process is also boosted with nitrifiers, increasing the nitrifying capacity. In this full-scale study, the increment of the nitrification rate is examined in the mainstream process at different temperatures and at different flow rates of returned activated sludge to the side-stream treatment. Our results show that the nitrification rate in the mainstream process was increased by 41% during the coldest period of the study, implying that the examined WWTP could treat considerably higher nitrogen loads if bioaugmentation were permanently installed.

Dynamic modelling of nitrous oxide emissions from three Swedish sludge liquor treatment systems.

The objective of this paper is to model the dynamics and validate the results of nitrous oxide (N2O) emissions from three Swedish nitrifying/denitrifying, nitritation and anammox systems treating real anaerobic digester sludge liquor. The Activated Sludge Model No. 1 is extended to describe N2O production by both heterotrophic and autotrophic denitrification. In addition, mass transfer equations are implemented to characterize the dynamics of N2O in the water and the gas phases. The biochemical model is simulated and validated for two hydraulic patterns: (1) a sequencing batch reactor; and (2) a moving-bed biofilm reactor. Results show that the calibrated model is partly capable of reproducing the behaviour of N2O as well as the nitritation/nitrification/denitrification dynamics. However, the results emphasize that additional work is required before N2O emissions from sludge liquor treatment plants can be generally predicted with high certainty by simulations. Continued efforts should focus on determining the switching conditions for different N2O formation pathways and, if full-scale data are used, more detailed modelling of the measurement devices might improve the conclusions that can be drawn.

Oxygen-induced dynamics of nitrous oxide in water and off-gas during the treatment of digester supernatant.

Nitrous oxide (N(2)O) is a potent greenhouse gas and of special concern in wastewater treatment. It is formed in biological wastewater treatment under both aerobic and anaerobic conditions. A major reason for high N(2)O emissions is low oxygen concentration during nitrification. In this full-scale study of N(2)O emissions from a sequencing batch reactor for treating digester supernatant, the oxygen concentration was reduced stepwise to investigate how N(2)O emissions were influenced. N(2)O concentrations were measured online in water and off-gas. A distinct relationship was found between low oxygen concentration and high N(2)O emissions. N(2)O was formed in water during both nitrification and denitrification. Decreased oxygen concentration during nitrification led to increased nitrite concentration, which in turn led to increased N(2)O concentration in the subsequent denitrification phase. When the nitrification resumed, accumulated N(2)O was stripped off to the atmosphere. Very high concentrations of N(2)O, over 56,000 ppmv, were measured in the off-gas. Furthermore, the maximum amount of N(2)O emitted during one cycle corresponded to 107.6% of the total nitrogen load (21.9% of total nitrogen present in the bulk liquid at the beginning of the cycle). This is among the highest emission levels ever measured from a full-scale municipal plant for digester supernatant.