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Approches opérationnelles pour favoriser la nitrification dans un biofiltre aéré à flux descendant
Canadian Journal of Civil Engineering
Abstract
With the strengthening of regulatory requirements regarding ammoniacal nitrogen, several water-treatment plants have to reduce their nitrogen load released. In the aim of promoting nitrification in a Biocarbon® biological filter, aeration, washing parameter modification, and flow reduction tests were conducted. Monitoring was carried out at various depths in the filtering medium. The results show that dissolved oxygen has to be at least 6 mg-O2/L to foster nitrification in the deep part of the filter. Removals in chemical oxygen demand (COD), carbonaceous biochemical oxygen demand over five days (CBOD5), and N–NH4⁺ of 89, 95, and 38% were obtained, respectively, whereas the additional energy consumption was estimated between 69 and 407 kWh/day per kg of CBOD5 and N–NH4⁺ removed. Results suggest that aeration is a key parameter that can be controlled in part by the hydraulic retention time, combined with shorter and more frequent washes stimulating more nitrifying bacteria.
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Three different types of internally aerated pilot scale biofilters were operated as tertiary nitrification systems. Long-term performance of the three aerated biofilters was tested under various operating conditions. The maximum volumetric nitrification rates under non NH4-limiting conditions for the three aerated biofilter systems were investigated. Based on measured temperature dependencies, an exponential relationship was established enabling the prediction of the nitrification rates at desired temperatures. Based on a temperature of 10°C, the results allow a comparison between the surface and volume specific nitrification rates in the tested biofilters as a function of the NH4 effluent concentration.
As shown by experiments, nitrification performance depends on water as well as air velocities in the filter. Higher velocities of both air and water increase the nitrification rate. However, they also increase the head loss and thus decrease the filter run time. Therefore, the optimal operating conditions depend also on the filter media and the required effluent quality.
Compared to fully O2-limiting operating conditions, nitrification performance during a period under partially NH4-limiting conditions clearly decreased in all tested biofilters.
Abstract: The work reviewed here was published between 2008 and 2010 and describes research that involved aerobic and anoxic biofilm treatment of water pollutants. Biofilm denitrification systems are covered when appropriate. References catalogued here are divided on the basis of fundamental research area or reactor types. Fundamental research into biofilms is presented in two sections, Biofilm Measurement and Characterization and Growth and Modeling. The reactor types covered are: trickling filters, rotating biological contactors, fluidized bed bioreactors, submerged bed biofilm reactors, biological granular activated carbon, membrane bioreactors, and immobilized cell reactors. Innovative reactors, not easily classified, are then presented, followed by a section on biofilms on sand, soil and sediment.
Keywords: biofilm; wastewater treatment systems; fixed film models; trickling filters; biotowers; rotating biological contactors; biomembrane processes; submerged fixed film; xenobiotics; nutrient removal; nitrification; denitrification; biological phosphorus removal; extracellular polymeric substances
The northeastern United States receives elevated inputs of anthropogenic nitrogen (N) largely from net imports of food and
atmospheric deposition, with lesser inputs from fertilizer, net feed imports, and N fixation associated with leguminous crops.
Ecological consequences of elevated N inputs to the Northeast include tropospheric ozone formation, ozone damage to plants,
the alteration of forest N cycles, acidification of surface waters, and eutrophication in coastal waters. We used two models,
PnET-BGC and WATERSN, to evaluate management strategies for reducing N inputs to forests and estuaries, respectively. Calculations
with PnET-BGC suggest that aggressive reductions in N emissions alone will not result in marked improvements in the acid–base
status of forest streams. WATERSN calculations showed that management scenarios targeting removal of N by wastewater treatment
produce larger reductions in estuarine N loading than scenarios involving reductions in agricultural inputs or atmospheric
emissions. Because N pollution involves multiple sources, management strategies targeting all major pollution sources will
result in the greatest ecological benefits.
Nitrifying biofilms were cultured in laboratory scale rotating drum biofilm reactors. Biofilm growth, substrate removal and product formation were monitored using microelectrodes with tip diameters less than 5 µm throughout the experiment. Three stages were observed during the formation of the biofilm: establishment of heterotrophic bacteria, establishment of nitrifiers, and possible establishment of denitrifiers. At each stage of the biofilm formation, microelectrode measurements were made to obtain the microprofiles of dissolved oxygen, ammonium, nitrate, pH, and redox potential within the biofilms. The results of this study provide validation of the heterogeneous nature of biofilm and the stratification of different microbial processes in biofilms. Key words: nitrification, denitrification, biofilm, microelectrode, biofilm formation, mass transport.
Biological aerated filters (BAFs) can combine ammonia, carbonaceous matter, and solids removal in a single-unit process, Biological nutrient removal (N and P) can also be accomplished. Removal rates based on reactor volume for carbonaceous BOD, ammonia, and nitrates of up to 4.1 kg BOD m(-3) day(-1), 1.27 kg NH3-N m-3 day(-1) and 5 kg NO3-N m(-3) day(-1), respectively, are normally reported, The small footprint and adaptability of BAFs allows them to be used in upgrading established works, especially those in built up areas where space is at a premium. Although many configurations are available, including upflow and downflow, sunken, and floating media, the process still requires some optimization, especially with regard to media type, backwashing rates, and aeration control to decrease power consumption.
Since the start-up of the first full scale plant in the North of Paris in 1982, Biological Aerated Filters (BAF) have been installed in more than 500 treatment plants worldwide, treating the effluents from more than 50 Million population equivalents. More than ten different technologies are available, although most of the plants have been built with a few dominant systems:
downflow countercurrent aeration (Biocarbone)
upflow co-current aeration (Biofor)
upflow floating filters (Biostyr and Biobead)
a sequence of corrugated plastic and granular media (Biopur)
Similarly, most plants are implemented in only four countries with high population densities and advanced wastewater treatment requirements, which have more than 50 plants each: France, Germany, Japan and United Kingdom. Denmark and Switzerland also have a high occurrence relative to the number of population, as coastal or mountain locations favour compact plants.
Initially BAF application was mainly carbon removal in compact plants, often following chemically enhanced primary treatment on lamella settlers for reduced footprint. This allowed a whole new concept of covered, deodorized wastewater treatment plants, located in sensitive or space-restricted locations on the coast, in mountains or in city centers.
As biofilm reactors have shown a particular advantage in the attachment of nitrifiers, in the last ten years, as nitrogen standards were tightened, BAF were recognized as a powerful tool for nitrification. The flexible arrangement of BAFs, integrating aerobic and anoxic reactors, or combinations in one vessel, allowed upgrades of existing plants with limited footprints.
Nitrospira are barely studied and mostly uncultured nitrite-oxidizing bacteria, which are, according to molecular data, among the most diverse and widespread nitrifiers in natural ecosystems and biological wastewater treatment. Here, environmental genomics was used to reconstruct the complete genome of "Candidatus Nitrospira defluvii" from an activated sludge enrichment culture. On the basis of this first-deciphered Nitrospira genome and of experimental data, we show that Ca. N. defluvii differs dramatically from other known nitrite oxidizers in the key enzyme nitrite oxidoreductase (NXR), in the composition of the respiratory chain, and in the pathway used for autotrophic carbon fixation, suggesting multiple independent evolution of chemolithoautotrophic nitrite oxidation. Adaptations of Ca. N. defluvii to substrate-limited conditions include an unusual periplasmic NXR, which is constitutively expressed, and pathways for the transport, oxidation, and assimilation of simple organic compounds that allow a mixotrophic lifestyle. The reverse tricarboxylic acid cycle as the pathway for CO2 fixation and the lack of most classical defense mechanisms against oxidative stress suggest that Nitrospira evolved from microaerophilic or even anaerobic ancestors. Unexpectedly, comparative genomic analyses indicate functionally significant lateral gene-transfer events between the genus Nitrospira and anaerobic ammonium-oxidizing planctomycetes, which share highly similar forms of NXR and other proteins reflecting that two key processes of the nitrogen cycle are evolutionarily connected.
Recently developed techniques involving opposed, gel-stabilized gradients of O(2) and H(2)S permit cultivation of a marine Beggiatoa strain as a chemolithoautotroph which uses gliding motility to precisely track the interface between H(2)S and O(2). In the current study with microelectrodes, vertical profiles of H(2), O(2), and pH were measured in replicate cultures grown for various intervals. After an initial period of exponential biomass increase (doubling time, 11 h), linear growth prevailed throughout much of the time course. This H(2)S-limited growth was followed by a transition to stationary phase when the declining H(2)S flux was sufficient only to supply maintenance energy. During late-exponential and linear growth phases, the Beggiatoa sp. consumed a constant 0.6 mol of H(2)S for each 1.0 mol of O(2), the ratio anticipated for balanced lithoautotrophic growth at the expense of complete oxidation of H(2)S to SO(4). Over the entire range of conditions studied, this consumption ratio varied by approximately twofold. By measuring the extent to which the presence of the bacterial plate diminished the overlap of O(2) and H(2)S, we demonstrated that oxidation of H(2)S by Beggiatoa sp. is approximately 3 orders of magnitude faster than spontaneous chemical oxidation. By integrating sulfide profiles and comparing sulfide consumed with biomass produced, a growth yield of 8.4 g (dry weight) mol of H(2)S was computed. This is higher than that found for sulfide-grown thiobacilli, indicating very efficient growth of Beggiatoa sp. as a chemoautotroph. The methods used here offer a unique opportunity to determine the yield of H(2)S-oxidizing chemolithoautotrophs while avoiding several problems inherent in the use of homogeneous liquid culture. Finally, by monitoring time-dependent formation of H(2)S profiles under anoxic conditions, we demonstrate a method for calculating the molecular diffusion coefficient of soluble substrates in gel-stabilized media.
The concept of solids retention time (SRT) was used for describing the growth of biofilm in a biological aerated filter (BAF) system. The SRT profile was obtained from the change in solids accumulation, estimated from the head loss profile data before and after backwash using the Carmen-Kozeny equation. The SRT profile along the filter bed depth showed the SRT of about 2 days for the lower layer and about 6 days for the upper layer. The overall SRT was determined by the direct estimation of excess solids mass during backwash and of solids retained in the filter bed. The ideal characteristic SRT distribution was maintained by regular backwash, for organic removal and nitrification. The SRT for high organic removal and nitrification is demonstrated by the SRT distribution along the filter bed in this BAF process.
In order to estimate N(2)O emissions from immersed biofilters during nitrogen removal in tertiary treatments at urban wastewater treatment plants (WWTPs), a fixed culture from the WWTP of "Seine Centre" (Paris conurbation) was subjected to lab-scale batch experiments under various conditions of oxygenation and a gradient of methanol addition. The results show that during nitrification, N(2)O emissions are positively related to oxygenation (R (2) = 0.99). However, compared to the rates of ammonium oxidation, the percentage of emitted N(2)O is greater when oxygenation is low (0.5-1 mgO(2) L(-1)), representing up to 1% of the oxidized ammonium (0.4% on average). During denitrification, the N(2)O emission reaches a significant peak when the quantity of methanol allows denitrification of between 66% and 88%. When methanol concentrations lead to a denitrification of close to 100%, the flows of N(2)O are much lower and represent on average 0.2% of the reduced nitrate. By considering these results, we can estimate, the emissions of N(2)O during nitrogen removal, at the "Seine Centre" WWTP, to approximately 38 kgN-N(2)O day(-1).
To comply with new effluent discharge standards of 10 mg TKN/l, different upgrading methods for a highly loaded activated sludge plant were explored. As a conclusion, demonstration units were tested to assess process feasibility and performance data of an innovative technology. The Achères Treatment plant of the city of Paris is currently being extended to purify a flow of about 2 700 000 m3/d, corresponding to 8 Million population equivalents. Conventional activated sludge, loaded at about 0.6 kg BOD/kg SS d, delivers an effluent of 30 mg/l for both BOD and SS.
To achieve nitrification, a considerable multiplication of basin volume and clarifier area would be required. In the densely urbanised Paris area, insufficient space is available for a such an extension. Therefore, new technology for plant upgrading was tested on industrial scale. Biological aerated filters combine aerobic degradation of pollutants with physical retention of suspended solids in one reactor. A high concentration of active biomass can be retained in the packed bed, and nitrifying bacteria can be attached to the filter media. Removal efficiency becomes thus independent of clarification and sludge settling, and ammonia oxidation can be achieved without sludge age requirements.
Four parallel units were installed on the Colombes research platform, handling a total flow of 3000 m3/d. An extensive demonstration test program was carried out over a period of five years to assess the feasibility and performances of the process in line with a conventional activated sludge plant. The limits of loading to achieve different residual ammonia concentrations were studied, and the influence of temperature on biological and hydraulic parameters was verified. Backwash requirements and residual values of carbonaceous and suspended matter were explored in dependence on influent values and filtration velocity.
At 13 °C, an ammonia load of 0.5 kg N/m3 d was completely oxidized. A concentration of 20 mg/l N-NH4 can thus be totally converted with an empty bed contact time of 1 hour. The Arrhenius temperature coefficient for nitrification was measured as 1.05. Biodegradable carbonaceous and suspended matter was completely removed at filtration velocities higher than 4 m/h, yielding an effluent of less than 5 mg/l for both SS and BOD. Backwash frequency was less than once per day, and a maximum of 5 % of the filter flowrate was used for backwashing.
A bench-scale biological aerated filter (BAF) was operated under a range of substrate loadings and C/N influent ratios. Start-up with a substrate loading of 4.8 COD kg m−3 d−1 and an influent C/N ratio of 1.5 gave no nitrification, due to the high biofilm detachment rate caused by the daily backwashing. Subsequent runs with lower substrate loadings and backwashing rates allowed stable nitrification. A final run with the initial substrate loading and backwash rate gave stable nitrification. Oligonucleotide probes targeted to the 16S rRNA of the nitrifiers, as well as to other types of microorganisms, were used to characterize how the changes in loading and detachment affected the microbial structure in the biofilms. The proportion of nitrifiers decreased with increasing C/N ratio. The biofilm remaining after backwashing also contained a higher fraction of nitrifiers than did the backwash solids stripped from the outer layers of biofilm, which consisted mainly of active heterotrophs. Thus, stable nitrification was possible only when the nitrifiers became established deeper inside the biofilm.
The wastewater treatment plant Seine Aval at Achères is the largest plant in France, with an average flowrate of 25 m3/s. For the moment, this plant consists of an activated sludge process for organic carbon removal. However, due to the recent European water regulations, it will soon be necessary to remove nitrogen pollution. For this reason, tertiary nitrification prototype reactors were installed and tested, in order to examine a future upgrading of the plant. This article presents the results obtained, during the last two years, with an upflow BAF reactor utilizing a granular media with a density > 1. The objective will be to examine a key parameter of the process: the water velocity (part of the applied loading rate. The removal rate depends directly and linearly on the applied loading rate, as long as the nitrification conditions (aeration, temperature, etc …) are not limiting. The highest loading rates for nitrification, as well as for BOD5 removal and SS retention, were obtained at high water velocity (10 m3/m2.h). These results demonstrate that the hydraulic retention time, even when extremely short (10 minutes), does not influence the reactor performance. The high water velocities in this kind of filter positively influence and improve the mass transfer between the liquid environment and the bacteria.
In response to strong growth in energy intensive wastewater treatment, public agencies and industry began to explore and implement measures to ensure achievement of the targets indicated in the 2020 Climate and Energy Package. However, in the absence of fundamental and globally recognized approach evaluating wastewater treatment plant (WWTP) energy performance, these policies could be economically wasteful. This paper gives an overview of the literature of WWTP energy-use performance and of the state of the art methods for energy benchmarking. The literature review revealed three main benchmarking approaches: normalization, statistical techniques and programming techniques, and advantages and disadvantages were identified for each one. While these methods can be used for comparison, the diagnosis of the energy performance remains an unsolved issue. Besides, a large dataset of WWTP energy consumption data, together with the methods for synthesizing the information, are presented and discussed. It was found that no single key performance indicators (KPIs) used to characterize the energy performance could be used universally. The assessment of a large data sample provided some evidence about the effect of the plant size, dilution factor and flowrate. The technology choice, plant layout and country of location were seen as important elements that contributed to the large variability observed.
The objective of this study is to investigate to what extent the nitrification capacity of a pilot-plant fixed-film reactor changes during extensive periods of nutrient supply deficiency. The examined pilot-plant was an upflow reactor filled with swelling clay of medium grain size (6 to 8 mm).
The experiments revealed that the maximum nitrification rate remained practically constant during the first weeks after the onset of unregulated ammonium supply. The capacity declined slowly, dropping to approximately 66% of the initial capacity after about ten weeks. Still ammonium peaks of up to 8 mg/l were readily nitrified throughout the entire period of the experiment. The reduction in nitrification capacity during the observation period did not result from decay processes of biomass but from the reactor becoming blocked and thus hampering transfer processes. It could be observed that the detached organisms attached again further up.
This semi-industrial project demonstrated that a plug-flow fixed-film reactor can be used as effective means of tertiary nitrification.
This paper presents the results of a two-year pilot evaluation of floating media biological aerated filters (BAFs). The pilot consists of two stages - tertiary nitrification (N) filter followed by post denitrification (DN) filter. The focus of the pilot study was to determine the BAFs’ capacity to meet stringent tertiary treatment requirements in treating the effluent from a conventional activated sludge process. It was demonstrated that the two-stage BAF system, on a 30-day average basis, is capable of achieving 1) a total nitrogen (TN) requirement less than 3 mg/L, 2) a total phosphorus (TP) requirement less than 0.3 mg/L with chemical addition, 3) a zero net COD increase under methanol addition conditions, and 4) a TSS concentration less than 5 mg/L. Biological growth in the DN stage was not limited with an influent ortho-P less than 0.3 mg/L. Complete nitrification was achieved at loading rates up to 2.0 kg/m³/d with an oxygen-to-ammonia mass ratio of 30 g/g or above. High rate denitrification was observed at methanol-to-NOx ratios between 2.8 to 3.0 g/g. Methanol dosages greater than 3.4 g/g resulted in increased effluent NOx concentrations, and the increase probably resulted from unstable methanol dosing control. Water temperature down to 80C was shown to minimally affect both nitrification and denitrification performance.
The wastewater plant Seine Aval (located in Acheres, France) treats 6 million PE (25 m3/s average flow, 570 MGD) with activated sludge. Following a research program called “Azote Acheres” which demonstrated that biofilters are an economical solution, the Paris Metropolitan Area Sewage Service (SIAPP) has tested 3 industrial prototypes during a period of more than 4 years. One of these is an up-flow biofilter using a floating filter bed made of expanded polystyrene beads. This specific process is called BIOSTYR and is used here as a nitrifying tertiary biofilter.
The purpose of this paper is to show that the biofilter process achieves very low, steady state, effluent ammonia concentrations with different applied nitrogen loads (0.3 to 2.7 kg NTK/m3.d), even with low temperature and high applied carbonaceous load.
During this 4 years period, a model has been developed to explain the hydraulic behavior of the filter. By measuring the operating conditions (temperature, loads), the head loss of the filter can be predicted.
After this test period, the prototype has been operated by Seine Aval plant staff. The results obtained have confirmed the previous test results.
The variables affecting nitrification in biofilms are identified on a theoretical basis. The influence of various design and operational parameters which affect the resulting substance fluxes into and out of the biofilm and the biomass activity regarding nitrification are illustrated with the help of experimental investigations with plastic media trickling filters, rotating biological contactors and different aerated biofilters. The results of experiments with these systems in tertiary nitrification applications reveal process limitations and technical measures to enhance nitrification performance in each system. In a case study, a comparison between the different biofilm processes and activated sludge alternatives showed that biofilm systems may lead to remarkably smaller reactor volumes, but high energy consumption due to unfavorable oxygen utilization.
Although, it has been recognized that the existence of organic matter in a recirculating system tends to reduce the nitrification efficiency of biofilters, quantitative information is still lacking. In this study, the effect of sucrose carbon on the nitrification rate of biofilters was evaluated under steady-state conditions using a reactor series experimental system. The experiential solution with a carbon/nitrogen ratio of C/N=1.0 or 2.0 resulted in approximately a 70% reduction of total ammonia nitrogen removal rate as compared with a solution that has a similar nitrogen level, but without carbon (C/N=0). The data showed that the nitrification rate decreased with an increase in the organic concentration, but the impact became less pronounced when the carbon concentration became sufficiently high. The results show the benefits of removing organic matter for improving nitrification in recirculating systems.
A two year pilot-scale study was conducted using a biological aerated filter (BAF) to evaluate performance and establish full-scale operating parameters for Tahoe-Truckee Sanitation Agency (T-TSA). It was demonstrated through pilot and full-scale testing, that the system could achieve effluent total inorganic nitrogen (TIN) concentrations of less than 2 mg/L and 3 mg/L for summer and winter periods, respectively. Furthermore, it was demonstrated that both systems could achieve moderate total suspended solids (TSS) and orthophosphate (PO4-P) removals. It was established that the optimum oxygen-to-ammonium (NH4-N) and optimum methanol-tonitrate/ nitrite (NOX-N) ratios were 30 g/g and 2.9 g/g, respectively. The nitrification stage was not sensitive to temperature for either the pilot or full-scale systems. The denitrification stage exhibited temperature sensitivity during full-scale testing. The PO4-P removal was greater during full-scale testing and removal was inversely proportional to temperature.
Biofiltration is distinguished from other biological waste treatments by the fact that there is a separation between the microorganisms and the treated waste. In biofiltration systems the microorganisms are immobilized to the bedding material, while the treated fluid flows through it. In recent decades, a vast amount of literature has been written on single experiments involving the treatment of fluids by immobilized microorganisms. Several artificial immobilization methods have been examined and impressive results have been achieved in the treatment of fluids with one of the artificial immobilization methods – the entrapment of microorganisms within polymer beads. This method, even though it needs to be improved, seems to have a future potential in commercial biofiltration systems. The methods of artificial immobilization of microorganisms within biofiltration systems have several advantages, but also suffer from several disadvantages in comparison to the treatment of fluids by naturally attached microorganisms. Understanding the mechanisms and forces responsible for the attachment of microbes to the bedding material, in attempt to improve this attachment, is of the utmost importance. Further improvement of the artificial entrapment of microorganisms within polymers will allow the exploitation of the advantages of this method in the treatment of fluids. The aim of this review essay is to introduce the main principles of two immobilization processes – the self-attachment of microorganisms to the bedding material and the artificial entrapment of microorganisms within polymer beads. Both treatments of liquids and gases with each immobilization process are discussed. The advantages and disadvantages of each immobilization process are pointed out and different aspects of the fluid treatment with the two immobilization processes are compared.
Technical and economical approaches of municipal wastewater treatment by biofiltration are dealt with in this paper. The treatment outcome, which was assessed from the skill acquired through the operation of the Seine Centre plant (SIAAP, Colombes), evidenced the efficiency of both biological abatement of carbonaceous pollution and nitrification (NH4+ → NO3-). The concentrations of oxidizable organic matter and ammonium in discharged water are far below the regulatory thresholds. On the other hand, the efficiency of the denitrification step (NO3- → N2) depends on the operating mode being adopted. That reduction is comprehensive when the denitrification step occurs downstream from the treatment (downstream denitrification – exogenous carbonaceous substrate), whereas it is only partial when being integrated upstream from the treatment (upstream denitrification – endogenous carbonaceous substrate). Thus, the plant operation in a layout only including an upstream denitrification can be contemplated and the treatment should then be supplemented with a post-denitrification step (combination of upstream and downstream denitrifications). Investigating the operating costs revealed the economical feature of the treatment process combining upstream and downstream denitrifications. The plant performance in that configuration results in operating costs that are lower by 5-10% than the conventional system only integrating a downstream denitrification.
Novel media-sludge-fly ash ceramic particles (SFCP) employed in an upflow lab-scale A/O BAF were investigated for synthetic wastewater treatment. The influences of hydraulic retention time (HRT), air-liquid ratio (A/L) and recirculation on the removals of chemical oxygen demand (CODcr), ammonia (NH(4)(+)-N) and total nitrogen (TN) were discussed. The optimum operation conditions were obtained as HRT of 2.0 h, A/L of 15:1 and 200% recirculation. Under the optimal conditions, 90% CODcr, more than 98% NH(3)-N and approximately 70% TN were removed. The average consumed volumetric loading rates for CODcr, NH(4)(+)-N and TN with 200% recirculation were 4.06, 0.36 and 0.29 kg(m(3)d)(-1), respectively. The CODcr and TN removal mainly occurred in the anoxic zone, while nitrification was completed at the height of 70 cm from the inlet of the bottom due to a suitable column layout of biological aerated filter (BAF). The characteristics of wastewater and backwashing affected TN removal to a large degree. In addition, the features of media (SFCP) and synthetic wastewater contributed to a strong buffer capacity in the BAF system so that the effluent pH at different media height fluctuated slightly and was insensitive to recirculation.
Two biofilm reactors operated with hydraulic retention times of 0.8 and 5.0 h were used to study the links between population dynamics and reactor operation performance during a shift in process operation from pure nitrification to combined nitrification and organic carbon removal. The ammonium and the organic carbon loads were identical for both reactors. The composition and dynamics of the microbial consortia were quantified by fluorescence in situ hybridization (FISH) with rRNA-targeted oligonucleotide probes combined with confocal laser scanning microscopy, and digital image analysis. In contrast to past research, after addition of acetate as organic carbon nitrification performance decreased more drastically in the reactor with longer hydraulic retention time. FISH analysis showed that this effect was caused by the unexpected formation of a heterotrophic microorganism layer on top of the nitrifying biofilm that limited nitrifiers oxygen supply. Our results demonstrate that extension of the hydraulic retention time might be insufficient to improve combined nitrification and organic carbon removal in biofilm reactors.
The evolution of European legislation has led to the rehabilitation of many wastewater treatment plants, sometimes through the installation of a biological complementary treatment stage. Among these sites, some plants in mountain areas are considering a biofiltration process. The design of such plants, especially for winter, appears to be tricky because of the very low influent temperature, the high performance requested for ammonia removal and the important and short term variations of the influent loads. The monitoring of a site during two consecutive winters has allowed us to study some aspects of the treatment. The major results are: a maximal nitrification capacity of about 0.59 kg of formed N-NO3- x m(-3) of material x d(-1) with an influent temperature around 7 degrees C at the plant inlet; a nitrifying biomass growth rate, expressed as nitrifying capacity increase, of 0.03 kg of N-NO3- x m(-3) of material x d(-2); quick and short terms load variations require a specific operation of the filters prior to the load increase, in order to grow enough active biomass to be able to treat the peak load immediately.
For some years, bio-filtration has been used in municipal wastewater treatment as a relatively new method. Within a German DWA research project, the method of benchmarking was used to compile and scientifically evaluate performance data and operation experiences with this technology on municipal wastewater treatment plants in Germany. This report presents selected partial results from the project. Generally, it becomes apparent that bio-filters as supplementary technology allow for additional improvements in regard to the COD and nitrogen effluent values. Bio-filtration is a compact method which combines biological purification processes with the filtration process. Apart from sound results in regard to the operation costs, operational problems (MSR technology, filter material losses) are described. In terms of energy balances, dimensions of the biological main stages can be compared to other biological purification methods. In regard to the sludge production, further research is necessary.
In this paper, three identical membrane bioreactors (MBRs) were operated in parallel in order to specify the influence mechanism of hydraulic retention time (HRT) on MBR. The results showed that the removal efficiency of chemical oxygen demand (COD) was stable though it decreased slightly as HRT decreased, but biomass activity and dissolved oxygen (DO) concentration in sludge suspension decreased as HRT decreased. The filamentous bacteria grew easily with decreasing HRT. The extracellular polymeric substances (EPS) concentration and sludge viscosity increased significantly as filamentous bacteria excessively grew. The over growth of filamentous bacteria, the increase of EPS and the decrease of shear stress led to the formation of large and irregular flocs. Furthermore, the mixed liquid suspended solids (MLSS) concentration and sludge viscosity increased significantly as HRT decreased. The results also indicated that sludge viscosity was the predominant factor that affecting hydrodynamic conditions of MBR systems.
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