Takahiro Kuba

Phosphorus and nitrogen removal with minimal cod requirement by integration of denitrifying dephosphatation and nitrification in a two-sludge system

Abstract Denitrifying dephosphatation enables the removal of phosphorus and nitrogen with minimal use of COD, minimal oxygen consumption and minimal surplus sludge production. Moreover it would make aeration only necessary for nitrification. Therefore we have studied an anaerobic-anoxic (A2) sequencing batch reactor (SBR) coupled to a nitrification SBR. Denitrifying phosphorus removing bacteria (DPB) and nitrifiers were completely separated in two sludges in these two SBRs. The nitrified supernatant was recirculated from the nitrification SBR to the A2 SBR where nitrate was utilized by DPB as an electron acceptor for phosphorus removal. The technical feasibility for simultaneous phosphorus and nitrogen removal in the proposed two-sludge system was evaluated. The benefits of two-sludge systems over single-sludge systems were also discussed. It could be concluded that the separation of the nitrification step leads to an optimal process design for the application of denitrifying dephosphatation. The two-sludge system showed stable phosphorus and nitrogen removal, and enabled the removal of 15 mg-P/1 and 105 mg N/1 at the expense of only 400 mg-COD/1 acetic acid. Stoichiometric calculations showed that, in the two-sludge system the required COD can be up to 50% less than for conventional aerobic phosphorus and nitrogen removal systems. Moreover oxygen requirements and sludge production can be decreased in significant amounts of about 30 and 50%, respectively.

An integrated metabolic model for the aerobic and denitrifying biological phosphorus removal

In this work, an integrated metabolic model for biological phosphorus removal is presented. Using a previously proposed mathematical model it was shown to be possible to describe the two known biological phosphorus removal processes, under aerobic and denitrifying conditions, with the same biochemical reactions, where only the difference in electron acceptor (oxygen and nitrate) is taken into account. Though, apart from the ATP/NADH ratio, the stoichiometry in those models is identical, different kinetic parameters were found. Therefore, a new kinetic structure is proposed that adequately describes phosphorus removal under denitrifying and aerobic conditions with the same kinetic equations and parameters. The ATP/NADH ratio (delta) is the only model parameter that is different for aerobic and denitrifying growth. With the new model, simulations of anaerobic/aerobic and anaerobic/denitrifying sequencing batch reactors (A(2) SBR and A/O SBR) were made for verification of the model. Not only short-term behavior, but also steady state, was simulated. The results showed very good agreement between model predictions and experimental results for a wide range of dynamic conditions and sludge retention times. Sensitivity analysis shows the influence of the model parameters and the feed substrate concentrations on both systems.

A metabolic model for biological phosphorus removal by denitrifying organisms

A metabolic model for biological phosphorus removal under denitrifying conditions has been established. The model is based on previous work with aerobic phosphorus removal. The form of the kinetic equations used is the same as for the aerobic model. The main difference is the value of P/NADH2 ratio in the electron transport phosphorylation with nitrate (δN). This value was determined independently from batch tests with an enriched culture of denitrifying phosphorus‐removing bacteria. The measured δN was approximately 1.0 mol ATP/mol NADH2. This indicates that the energy production efficiency with nitrate compared to oxygen is approximately 40% lower. These batch tests were also used to identify a proper set of kinetic parameters. The obtained model was subsequently applied for the simulation of cyclic behavior in an anaerobic‐anoxic sequencing batch reactor at different biomass retention times. The simulation results showed that the metabolic model can be used successfully for the denitrifying dephosphatation process. The obtained kinetic parameters for denitrifying enrichment cultures, however, deviated from those obtained for the aerobic enrichment cultures.

A sludge characterization assay for aerobic and denitrifying phosphorus removing sludge

Abstract It was recently recognized that denitrifying bacteria, as well as aerobic bacteria, contribute to the biological phosphorus removal process. For evaluation of the phosphorus removal process it is necessary to have tests enabling a proper characterization of the microbial population in the activated sludge. In this study we evaluated the possibility of anoxic phosphorus release or aerobic/anoxic phosphorus uptake tests for sludge characterization. Hereto we employed sludge mixtures of enrichment cultures from laboratory sequencing batch reactor (SBR) systems operated under aanaerobic-aerobic ( A O ) or anaerobic-anoxic (A2) conditions. It is shown that interpretation of phosphorus release tests, with or without nitrate present, is difficult. On the other hand a comparison of phosphorus uptake under aerobic and anoxic conditions leads to a straightforward characterization of the microbial population in the phosphorus removing organisms, with respect to their phosphorus removal activity under aerobic and anoxic conditions.

Occurrence of denitrifying phosphorus removing bacteria in modified UCT-type wastewater treatment plants

Abstract The occurrence of denitrifying phosphorus removing bacteria (DPB) and the contribution of DPB to phosphorus removal in full-scale wastewater treatment plants (WWTPs) has been investigated by batch tests with the activated sludge from two WWTPs (WWTP-Genemuiden and -Holten). Both WWTPs are operated as UCT-type processes. Batch tests to evaluate the biomass composition of the sludges were developed. These batch tests showed a clear difference of denitrifying dephosphatation activity between the WWTP-Genemuiden and -Holten sludge. In the sludge of the WWTP-Genemuiden the (denitrifying) dephosphatation activity was rather low, whereas a high activity was formed in the WWTP-Holten sludge. The following possible reasons for the lower proportion of (denitrifying) dephosphatation activity in the WWTP-Genemuiden sludge in comparison with the WWTP-Holten sludge, were suggested; (a) nitrate/oxygen transfer to the anaerobic/anoxic zone due to the internal recirculation inside the treatment plant, (b) lower amounts of fatty acids in the influent due to shorter retention time of waste water in sewer lines, and (c) lower amount of nitrate recycled to the anoxic zone.

Effect of cyclic oxygen exposure on the activity of denitrifying phosphorus removing bacteria

The effect of oxygen on the activity of denitrifying phosphorus removing bacteria has been studied. Denitrifying phosphorus removing bacteria were enriched without oxygen in an anaerobic-anoxic SBR (sequencing batch reactor) over a long time, after which an aerobic phase was introduced into the SBR. The performance on phosphorus and nitrogen removal was examined for an anaerobic-aerobic-anoxic SBR. It could be concluded that oxygen has no detrimental effect on the denitrifying dephosphatation activity. The maximum phosphorus uptake rate by the enriched denitrifying sludge was almost equal for anoxic and aerobic conditions, and the anoxic phosphorus removal activity was kept within at least 5 months after the introduction of the aerobic phase. From the experiments it followed that the advantage of applying denitrifying phosphorus removing bacteria can only be obtained in a pre-denitrification process like the UCT-type of process.

Biological dephosphatation by activated sludge under denitrifying conditions pH influence and occurrence of denitrifying dephosphatation in a full-scale waste water treatment plant

The effect of pH on phosphorus release under anaerobic conditions was examined for denitrifying phosphorus removing bacteria (DPB) cultivated in an anaerobic-anoxic sequencing batch reactor. Also batch tests were conducted with activated sludge from a full-scale waste water treatment plant (WWTP) in order to investigate occurrence and contribution of DPB in phosphorus removal processes. In the experiments for the pH effect, enriched DPB sludge was maintained under anaerobic conditions with acetic acid (HAc) present at 5 different pH conditions (6.0∼8.0), and released phosphorus and consumed HAc concentrations were measured. When the biomass concentration was around 2.7 g-VSS/ l , the observed P/C (released-P/consumed-HAc) ratios were 0.7, 1.1 and 1.2 g-P/g-C at pH=6, 7 and 8. At 4.2 g-VSS/ l , the observed P/C ratios were 0.9, 1.3 and 1.2 g-P/g-C, respectively. The difference between the two experiments resulted from the endogenous phosphorus release. The same pH effect as observed for conventional anaerobic-aerobic SBR sludge, was obtained for the DPB sludge in the range of pH=6.0∼7.5. However due to precipitates formation at pH=8.0, the apparent P/C ratio was approximately 20% less than the ratio calculated from the biological released phosphorus concentration by DPB. From the results of the batch tests with activated sludge and observations on the full-scale WWTP, it was also shown that clearly denitrifying dephosphatation occurs and approximately 50% of the phosphorus removal occurs via denitrifying activities in the WWTP.

Metabolism of micro-organisms responsible for enhanced biological phosphorus removal from wastewater, Use of dynamic enrichment cultures

The removal of phosphorus from wastewater is already widely applied. In many cases use is made of micro organisms capable of accumulating phosphorus as polyphosphate inside the cell. The main characteristic providing the competitive advantage to these polyphosphate accumulating bacteria is the capability to use polyphosphate, in the absence of external electron acceptors, as energy source for the uptake and storage of acetic acid in the form of polyhydroxybutyrate (PHB). The reduction equivalents for the formation of PHB are derived from the conversion of glycogen to PHB. Despite the widespread use and study of enhanced biological phosphorus removal no pure culture, having the above mentioned characteristics, has been isolated yet. All ecophysiological studies on these type of cultures have therefore been performed by enrichment cultures. This paper reviews the research on these type of organisms, and shows that it is possible to understand a complex microbial process on a metabolic level, both stoichiometrically and kinetically, without the availability of a pure culture.

A kinetic study on methanogenesis by attached biomass in a fluidized bed

Abstract The objective of this study is to estimate growth kinetic constants and the concentration of “active” attached biomass in an anaerobic fluidized bed, which decomposes acetic, propionic and butyric acids. The fluidized bed was operated as a methanogenic reactor with synthetic zeolite as support media. The reactor was supplied with synthetic wastewater (1000 mg COD l −1 ), a mixture of the above-mentioned volatile fatty acids (VFA), in the range of hydraulic retention time (HRT) from 0.25 to 2 days. After the effluent has reached a steady state in quality, batch experiments in the bed reactor were conducted using acetic, propionic and butyric acids as substrate in order to investigate the decomposition characteristics of each substrate by the attached biomass. Detached biomass from the support media was also served to batch experiments under the completely mixed condition in order to estimate parameter values of the growth kinetics of the bacteria. The changes of fatty acid concentrations with time were expressed with the Monod growth model. The two kinetics parameters, maximum specific growth rates and saturation constants, and “active” biomass concentrations were obtained by the curve fitting method. The comparison of the measured concentration of volatile suspended solids (VSS) and protein with the estimated “active” biomass concentrations indicated that a large amount of inert organic matter exists in the attached growth reactor.

Kinetics and stoichiometry in the biological phosphorus removal process with short cycle times

Abstract Aerobic or denitrifying biological phosphorus removal in a sequencing batch rector (SBR) with short repetitive anaerobic-aerobic ( A O ) or anaerobic-anoxic (A2) cycling has been studied. These conditions resemble processes with large internal recycle flows. Under these conditions the fatty acid dosage per anaerobic phase is low compared to a conventional SBR or plug flow process. It was shown that at the same sludge retention time (SRT) the average poly-β-hydroxybutyrate (PHB) content of the biomass decreased, with increasing the number of A O or A2 cycles. This had no direct effect on the biological phosphorus removal. In the A2 SBR, however, phosphorus removal was deteriorating due to transfer of nitrate to the anaerobic phase. Most mathematical models for biological phosphorus removal processes show a relation between the biomass growth rate and the PHB content. This research shows that such a unique relation does not exist, since at one SRT different “steady state” PHB levels could be obtained, depending on the cycle number and the use of oxygen or nitrate as an electron acceptor. This indicates the need for an alternative kinetic model.