Paramètres opérationnels favorisant le développement d'un biofilm nitrifiant au sein d'un procédé par biomasse fixée : évaluation par qPCR, amorces dégénérées et paramètres intégrateurs en eaux usées

Abstract

In order to meet Canadian regulations or Quebec requirements for ammonia nitrogen discharges and effluent toxicity, an operational approach favoring the development of a nitrifying biofilm is proposed. This low-cost approach would allow biofiltration-type treatment plants to upgrade only on an operational basis rather than on a complete upgrade of the existing process. These parameters are aeration, backwashing and flow control. Existing studies have not yet been able to evaluate whether these parameters can promote the development of nitrifying flora in a process that is not originally designed to support nitrification. In addition, existing studies do not demonstrate the link between bacterial flora and treatment performance in the case of fixed biomass. The general objective of this thesis is to define the link between the state of the relative populations of nitrifying bacteria in a "fixed biomass" type biofilm from an aerated biological filter, the characteristics of wastewater, the operating conditions and the treatment capacity of organic matter or ammonia nitrogen. During different experimental campaigns in the treatment plant, samples of filter medium and wastewater were taken at different depths of an existing biofilter (0.3, 0.6, 1.2 and 1.5 m). to characterize the state of the biofilm as well as the purification performance throughout the passage of water in the biological filter, and this, according to the operational parameters studied. Biofilm samples were studied using an original approach using qPCR with degenerate primers to estimate the relative presence of the main bacteria responsible for nitrification: ammonium oxidizing bacteria (AOB) and nitrites oxidizing bacteria (NOB). These new molecular markers were designed on the basis of the partial genetic sequences of the amoA genes (alpha subunit of the enzyme "ammonium monooxygenase") and nxrB (subunit beta of the enzyme "nitrite oxidoreductase"). A threedimensional characterization of the biofilm by FISH analysis and confocal imaging was also performed to confirm the trend observed by the qPCR results. The integrating parameters of CBOD5, COD, total and ammonia nitrogen, nitrate, alkalinity and sulphides were analyzed on the wastewater samples taken to estimate the treatment efficiency at the different depths of the treatment cell and to confirm the presence of nitrification. The results on the effects of the operational parameters showed an increase in the relative population of AOB and NOB compared to the bacterial population (Article 1). The relative presence in the test cells, with the primers amoA and nxrB, were evaluated at 0.50 % and 0.033 % respectively during the aeration tests, at 0.21 % and 0.046 % during the backwashes modification tests and 0.58 % and 6.3 % in the wastewater inflow tests. FISH analyzes have shown that there does not appear to be any mandatory colocation between AOB and NOB, thus raising the assumption that NOB are not totally dependent on AOB for their growth. The development of the relative population of AOB and NOB was also observed when considered in relation to the entire biofilm (Article 2). The results of the integrating parameters of water analyses revealed significant improvement in the treatment of the carbon load. The highest rate of ammonia nitrogen removal observed was 49 % in the test cell during the modified backwash parameter assay, while the relative nitrifying population within the entire biofilm was evaluated at 0.032 % and 0.008 % for the AOB and NOB respectively. The highest observed values of AOB and NOB come from the inflow tests with values of 0.034 % and 0.18 % respectively. However, observed nitrification was evaluated at 21 % of ammonia removal. The additional energy consumption for the removal of carbon and nitrogen load from the test cell compared to the control cell was evaluated at 407 kWh/kg·d-1 during the aeration condition test, at 69 kWh/kg/d during the modified backwash parameter assay and 152 kWh/kg/d during the inflow adjustment test (Article 3). It appears that dissolved oxygen is the key parameter to ensure a good nitrification performance, despite the nitrifying bacterial population present. Thus, dissolved oxygen should be maintained at least 6 mg-O2/L in the case of fixed biomass, and in depth if biofiltration bed is used. Finally, it is possible, through some operational changes, to promote the development of a nitrifying bacterial flora in a process that is not originally designed for this purpose. Aeration, backwashing and inflow all have a positive impact on the development of both AOB and NOB. However, to ensure nitrification, maintaining aeration at 6 mg-O2/L and less intensive backwashes appear to be promising solutions. Increasing hydraulic residence time did not improve nitrification, but it increased the concentration of dissolved oxygen in the interstitial fluid. This parameter promotes the diffusion of dissolved and thus reduces the associated energy demand. These results represent an advance in the understanding of the behavior of fixed biomass biofilm in the field of municipal purification and the main conclusions can easily be applied to all processes of this type, thus broadening their remediation potential.

Date	26 Oct 2018
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Frédéric Monette (Supervisor)