Rajeev Goel

Enzyme activities under anaerobic and aerobic conditions in activated sludge sequencing batch reactor

Abstract An experimental study was performed to observe the effect of electron acceptor conditions on the enzyme activities in an anaerobic–aerobic activated sludge sequencing batch reactor (SBR). Four types of extra-cellular enzymes, i.e. alkaline phosphatase, acid phosphatase, α -glucosidase and protease were selected for the study. In addition to these enzymes, dehydrogenase, an intracellular enzyme, was also assayed. A lab scale anaerobic–aerobic SBR was operated and used for the experimental work. It was observed that the specific enzyme activities did not change significantly during anaerobic and aerobic phase of SBR. The enzymatic behavior of sludge observed under anaerobic and aerobic conditions was explained by considering three important factors: (1) enzyme synthesis under aerobic and anaerobic conditions, (2) location of enzymes, and (3) stability of enzymes in activated sludge. Based on the experimental results, it was established that, even though synthesis of the enzyme is to some extent affected depending on the anaerobic and aerobic incubation, it is the stable and the floc bound nature of these extra-cellular enzymes which results in no significant difference in enzyme activity under anaerobic and aerobic phase of a steady state operated single sludge anaerobic–aerobic system.

CFD simulation of mixing in anaerobic digesters

A three-dimensional CFD model incorporating the rheological properties of sludge was developed and applied to quantify mixing in a full-scale anaerobic digester. The results of the model were found to be in good agreement with experimental tracer response curve. In order to predict the dynamics of mixing, a new parameter, UI (uniformity index) was defined. The visual patterns of tracer mixing in simulation were well reflected in the dynamic variation in the value of UI. The developed model and methods were applied to determine the required time for complete mixing in a full-scale digester at different solid concentrations. This information on mixing time is considered to be useful in optimizing the feeding cycles for better digester performance.

Xenobiotic removal efficiencies in wastewater treatment plants: Residence time distributions as a guiding principle for sampling strategies

The effect of mixing regimes and residence time distribution (RTD) on solute transport in wastewater treatment plants (WWTPs) is well understood in environmental engineering. Nevertheless, it is frequently neglected in sampling design and data analysis for the investigation of polar xenobiotic removal efficiencies in WWTPs. Most studies on the latter use 24-h composite samples in influent and effluent. The effluent sampling period is often shifted by the mean hydraulic retention time assuming that this allows a total coverage of the influent load. However, this assumption disregards mixing regime characteristics as well as flow and concentration variability in evaluating xenobiotic removal performances and may consequently lead to biased estimates or even negative elimination efficiencies. The present study aims at developing a modeling approach to estimate xenobiotic removal efficiencies from monitoring data taking the hydraulic RTD in WWTPs into consideration. For this purpose, completely mixed tanks-in-series were applied to address hydraulic mixing regimes in a Luxembourg WWTP. Hydraulic calibration for this WWTP was performed using wastewater conductivity as a tracer. The RTD mixing approach was coupled with first-order biodegradation kinetics for xenobiotics covering three classes of biodegradability during aerobic treatment. Model simulations showed that a daily influent load is distributed over more than one day in the effluent. A 24-h sampling period with an optimal time offset between influent and effluent covers less than the half of the influent load in a dry weather scenario. According to RTD calculations, an optimized sampling strategy covering four consecutive measuring days in the influent would be necessary to estimate the full-scale elimination efficiencies with sufficient accuracy. Daily variations of influent flow and concentrations can substantially affect the reliability of these sampling results. Commonly reported negative removal efficiencies for xenobiotics might therefore be a consequence of biased sampling schemes. In this regard, the present study aims at contributing to bridge the gap between environmental chemistry and engineering practices.

Perspectives on modelling micropollutants in wastewater treatment plants

Models for predicting the fate of micropollutants (MPs) in wastewater treatment plants (WWTPs) have been developed to provide engineers and decision-makers with tools that they can use to improve their understanding of, and evaluate how to optimize, the removal of MPs and determine their impact on the receiving waters. This paper provides an overview of such models, and discusses the impact of regulation, engineering practice and research on model development. A review of the current status of MP models reveals that a single model cannot represent the wide range of MPs that are present in wastewaters today, and that it is important to start considering classes of MPs based on their chemical structure or ecotoxicological effect, rather than the individual molecules. This paper identifies potential future research areas that comprise (i) considering transformation products in MP removal analysis, (ii) addressing advancements in WWTP treatment technologies, (iii) making use of common approaches to data acquisition for model calibration and (iv) integrating ecotoxicological effects of MPs in receiving waters.

Novel anaerobic digestion process with sludge ozonation for economically feasible power production from biogas

A novel process scheme was developed to achieve economically feasible energy recovery from anaerobic digestion. The new process scheme employs a hybrid configuration of mesophilic and thermophilic anaerobic digestion with sludge ozonation: the ozonated sludge is first degraded in a thermophilic digester and then further degraded in a mesophilic digester. In small-scale pilot experiments of the new process scheme, degradation of VSS improved by 3.5% over the control (mesophilic-only configuration) with 20% less ozone consumption. Moreover, biogas conversion also improved by 7.1% over the control. Selective enrichment of inorganic compounds during centrifugation produced a dewatered sludge cake with very low water content (59.4%). This low water content in the sludge cake improved its auto-thermal combustion potential during incineration and added to the overall energy savings. We conducted a case study to evaluate power generation from biogas for a municipal wastewater treatment plant with an average dry weather flow of 43,000 m3/d. Electricity production cost was 5.2 ¢/kWh for the advanced process with power generation, which is lower than the current market price of 7.2 ¢/kWh. The new anaerobic digestion scheme with power generation may reduce greenhouse gas emissions by about 1,000 t-CO(2)/year compared with the conventional process without power generation.

Evaluation of state variable interface between the Activated Sludge Models and Anaerobic Digestion Model no 1

For plant wide modelling of wastewater treatment, it is necessary to develop a suitable state variables interface for integrating state of the art models of ASM and ADM1. ADM1 currently describes such an interface, however, its suitability needs to be experimentally evaluated. In this study, we characterised activated sludge under aerobic and anaerobic conditions to obtain representative state variables for both models. ASM state variables of X(S), X(H) and X(I) (as obtained from aerobic tests) and ADM1 state variables of X(C) and X(I) (as obtained from anaerobic tests) were then correlated to assess the suitability of current interface. Based on the seven datasets of this study and seven datasets from literatures, it was found that in general ASM state variables were well correlated to the state variables of ADM1. The ADM1 state variable of X(C) could be correlated to the sum of state variables of X(S) and X(H), while X(I) in both the models showed direct correspondence. It was also observed that the degradation kinetics of X(C) under anaerobic condition could be better described by individual degradation kinetics of X(S) and X(H). Therefore, to establish a one to one correspondence between ASM and ADM1 state variables and better description of degradation kinetics in ADM1, replacing the composite variable of X(C) by the state variables of X(S) and X(H) is recommended.

Modelling clogging and biofilm detachment in sponge carrier media

Sponge carrier media provide a large surface area for biofilm support; however, little information is known about how to model their dual nature as a moving bed and as porous media. To investigate the interaction of mass transfer and detachment with bio-clogging, a novel biofilm model framework was built based on individual-based modelling, and hydrodynamics were modelled using the lattice Boltzmann method. The combined model structure enabled the simulation of oxygen and biomass distribution inside the porous network as well as inside the biofilm. In order to apply the model to moving bed biofilm reactors (MBBR), biofilm detachment due to abrasion (carrier collisions) was modelled to be dependent on intracarrier distance. In the initial growth stage, biofilm grew homogeneously on the internal skeleton after which a more discontinuous growth developed which significantly increased permeability. Low detachment rates caused clogging in the outer pores which limited growth of biofilm to the surface region of the sponge. High detachment rates on the surface enabled deeper oxygen penetration with higher internal biomass activity. The degree of clogging was also sensitive to the presence of extracellular polymeric substances because of its large spatial occupancy.

A benchmark simulation to verify an inhibition model on decay stage for nitrification

Activated Sludge Models (ASMs) are widely used for biological wastewater treatment plant design, optimisation and operation. In commonly used ASMs, the nitrification process is modelled as a one-step process. However, in some process configurations, it is desirable to model the concentration of nitrite nitrogen through a two-step nitrification process. In this study, the benchmark datasets published by the Water Environment Research Foundation (WERF) were used to develop a two-step nitrification model considering the kinetics of Ammonium Oxidising Bacteria (AOB) and Nitrite Oxidising Bacteria (NOB). The WERF datasets were collected from a chemostat reactor fed about 1,000 mg-NH3-N/L synthetic influent with at different sludge retention times of 20, 10 and 5-d, whereas the pH in the reactor varied in the range of 5.8 and 8.8. Supplemental laboratory batch experiments were conducted to assess the toxicity of nitrite-N on nitrifying bacteria. These tests suggested that 500 mg-N/L of nitrite at pH 7.3 was toxic to NOB and resulted in continuous decrease in bulk oxygen uptake rate. To model this phenomenon, a poisoning model was used instead of the traditional Haldane-type inhibition model. The poisoning model for NOB and AOB with different threshold poisonings for unionised NO2-N and NH3-N concentrations could successfully reproduce the three WERF datasets.

A modified anaerobic digestion process with chemical sludge pre-treatment and its modelling.

Activated Sludge Models (ASMs) assume an unbiodegradable organic particulate fraction in the activated sludge, which is derived from the decay of active microorganisms in the sludge and/or introduced from wastewater. In this study, a seasonal change of such activated sludge constituents in a municipal wastewater treatment plant was monitored for 1.5 years. The chemical oxygen demand ratio of the unbiodegradable particulates to the sludge showed a sinusoidal pattern ranging from 40 to 65% along with the change of water temperature in the plant that affected the decay rate. The biogas production in a laboratory-scale anaerobic digestion (AD) process was also affected by the unbiodegradable fraction in the activated sludge fed. Based on the results a chemical pre-treatment using H2O2 was conducted on the digestate to convert the unbiodegradable fraction to a biodegradable one. Once the pre-treated digestate was returned to the digester, the methane conversion increased up to 80% which was about 2.4 times as much as that of the conventional AD process, whilst 96% of volatile solids in the activated sludge was digested. From the experiment, the additional route of the organic conversion processes for the inert fraction at the pre-treatment stage was modelled on the ASM platform with reasonable simulation accuracy.

High nitrite concentration accelerates nitrite oxidising organism's death

High nitrite is a known operation parameter to inhibit the biological oxidation of nitrite to nitrate. The phenomenon is traditionally expressed using a Monod-type equation with non-competitive inhibition, in which the reaction associated with the biomass growth is reduced when high nitrite is present. On the other hand, very high nitrite is also known to slay nitrifiers. To clarify the difference between the growth inhibition and the poisoning, cell counting for living microorganisms in the nitrite oxidiser-enriched activated sludge was conducted in batch conditions under various nitrite concentrations together with measurements of biomass chemical oxygen demand (COD) concentration and oxygen uptake rate. The experiments demonstrated that these measureable parameters were all decayed when nitrite concentration exceeded 100-500 mgN/L at pH 7.0 in the system, indicating that nitrite poisoning took place. Biomass growth was recognised in lower range of nitrite which was expressed with growth inhibition only. Based on the response, a kinetic model for the biological nitrite oxidation was developed with a modification of IWA ASM1. The model was further utilised to calculate a possibility to wash out nitrite oxidiser in the aeration tank where a part of the return activated sludge was exposed to high nitrite liquor in a side-stream partial nitritation reactor.