Nitrification pilot plant
The increasing demand for efficient nitrification in wastewater nitrogen removal emphasizes the need for in depth studies of nitrifying systems. The foundations of these are the communities of nitrifying bacteria present. In order to go beyond the “black box” approach to these systems, we thus have to get a better understanding of the composition and activities of the nitrifying communities, and how these react to environmental stimuli, such as changes in substrate composition. To facilitate such studies, a large-scale two-step pilot plant was constructed at the Rya wastewater treatment plant (WWTP), the regional sewage works for the Göteborg region in Sweden.
Nitrification at Rya WWTP is achieved using fixed biofilms on nitrifying trickling filters (NTFs). Receiving the same water as the full-scale NTFs, the pilot plant consisted of a 12,8m3 moving bed reactor (MBR) separated into four aerated tank-compartments connected in series and containing suspended biofilm carriers of either of two types (Figure 1). This resulted in the formation of an ammonium gradient in the tanks, with decreasing concentrations from tank 1 to tank 4. Grids between the compartments allowed water to pass but made sure no exchange of carriers would occur.
Fig.1 Schematic overview of the pilot plant
In addition, a second step of four small NTFs was constructed. Owing to a versatile construction, these filters could be fed with the same water as incoming to tank 1 or water from either of the tanks themselves. Furthermore, the water was distributed over the pilot plant NTFs in short, even pulses in order to mimic the water distribution over the full-scale biofilms.
The design of the pilot plant, together with continuous monitoring of several physical parameters, made a wide variety of different experimental approaches possible. Examples are in situ studies of ammonium-concentration as a structuring force for microbial nitrifying communities, as well as a factor affecting the nitrification potential of these populations. Furthermore, by performing Fluorescence In Situ Hybridization (FISH) on cryosectioned biofilm samples, comparative studies of biofilm structure are made possible on the pilot-plant NTFs and the carriers of the MBR. These results should thereafter be compared to similar studies from the full-scale system, such as in Lydmark et al (2006).
Before the onset of sampling, this pilot-plant was allowed to run for one year, allowing biofilm establishment and stable levels of nitrification to be formed.
Characterization of the nitrifying populations in the compartments of the MBR, using FISH, revealed at least five populations of ammonia-oxidizing bacteria (AOB) and two nitrite-oxidizing bacteria (NOB) populations (Lydmark et al 2007). The distribution of these populations varied between the compartments, most probably due to differences in substrate concentration. In addition, total biomass was estimated and specific nitrification rates calculated. When compared to studies from the full-scale NTFs (Lydmark et al 2006), the population pattern of the two systems was similar, supporting the relevance of the pilot-plant. Through batch-transfer of carriers between the compartments of the MBR and thereby between different ammonium concentrations, changes in the composition of these nitrifying communities can be monitored, using FISH, over desired periods of time.
One drawback of the pilot plant design and its indoor-placement was the aerosol-production from the aerated MBR, a potential hazard to persons in frequent contact with the pilot-plant. Moreover, the NTFs were subjected to heavy grazing by predators such as snails and filter fly larvae. This could however be controlled by periodical drowning of the biofilms. Although a problem, this is quite similar to the actual situation in the full-scale trickling filters and needs to be further investigated from an operational point of view.
In summary, this pilot plant made studies of how environmental changes affected nitrifying populations possible without impairing the operation of the WWTP. By using the same wastewater as the full-scale NTFs, these observations can result in direct applications for this treatment plant.
Lydmark P., Lind M., Sörensson F. and Hermansson M. (2006). Vertical distribution of nitrifying populations in bacterial biofilms from a fullscale nitrifying trickling filter. Environ. Microbiol. 8, 2036-2049
Lydmark P., Almstrand R., Samuelsson K., Mattsson A., Sörensson F., Lindgren P-E., Hermansson M. (2007). Effects of environmental conditions on the nitrifying population dynamics in a pilot wastewater treatment plant. Environ. Microbiol. 9(9):2220-33.