Municipal wastewater effluent characterization and variability analysis in view of an ozone dose control strategy during tertiary treatment: The status in Belgium.

Ozonation is known for removing trace organic contaminants (TrOCs) from secondary wastewater effluent. However, its implementation and overall efficiency on a broad scale depends on effluent characteristics, which can differ both in time as well as between different treatment plants (nowadays referred to as water resource recovery facilities (WRRFs)). Therefore, water quality was assessed over time at 15 different Belgian sampling locations to increase the understanding of effluent variability in view of online control of the tertiary ozonation step. Conventional and surrogate parameters as well as those specifically related to tertiary ozonation (e.g. instantaneous ozone demand) were assessed. Little differences between the different locations were found for spectral measurements (e.g. UVA254 or fluorescence). The small amount of observed outliers was clearly site or event dependent. A lower variability (for spectral measurements) is advantageous in simplifying the development and application of a generic control framework based on these spectral measurements. In addition, also variations in TrOC concentration levels seemed to be small, as the concentration of most individual compounds resided within one order of magnitude over multiple sampling events at two different WRRFs. The combination of this low variability in TrOC levels in the effluent before ozonation with a control strategy using a TrOC removal efficiency set-point, allows to indicatively assess absolute TrOC levels after ozonation. In contrast, significant variations between different plants (especially smaller sized plants) were observed and could be related to the conventional water quality parameters alkalinity (correlated with the electrical conductivity) and pH which are both known to have an influence on the ozonation process. This confirms that a differential dosing control strategy (i.e. accounting for the matrix reactivity) should be applied instead of one solely based on the (organic) effluent load before ozonation.

Dynamic validation of online applied and surrogate-based models for tertiary ozonation on pilot-scale.

New robust correlation models for ozonation, based on UVA254 and fluorescence surrogate parameters and developed considering kinetic information, have been applied at pilot-scale. This model framework is validated with the aim for operators to control the ozone dose for the removal of trace organic contaminants (TrOCs) in effluents from full-scale municipal wastewater treatment plants. The inflected correlation model between ΔTrOCs and the surrogates predicts the removal of TrOCs (based on statistical evidence) solely using the 2nd order reaction rate constant with ozone (kO3) and in a more adequate manner than similar single correlation models. This allows the use of this new model for current and future TrOCs under investigation which is highly interesting when imposed discharge limits might include more and other TrOCs in future. The use of UVA254 might be preferable at the current timing for online monitoring of TrOC abatement as the model showed a good predictive power (based on statistical evidence and visual confirmation). Reliable online sensors are more widespread (and commercially) available compared to fluorescence sensors which are still under development, with the exception of a few examples. Nevertheless, the data processing of the fluorescence signals, isolating the different intensities associated with moieties reacting similarly to ozone might even increase the predictive power, given the lower degree of interference (i.e. less scattering).

Surrogate-Based Correlation Models in View of Real-Time Control of Ozonation of Secondary Treated Municipal Wastewater-Model Development and Dynamic Validation.

New robust correlation models for real-time monitoring and control of trace organic contaminant (TrOC) removal by ozonation are presented, based on UVA254 and fluorescence surrogates, and developed considering kinetic information. The abatement patterns of TrOCs had inflected shapes, controlled by the reactivity of TrOCs toward ozone and HO• radicals. These novel and generic correlation models will be of importance for WRRF operators to reduce operational costs and minimize byproduct formation. Both UVA254 and fluorescence surrogates could be used to control ΔTrOC, although fluorescence measurements indicated a slightly better reproducibility and an enlarged control range. The generic framework was validated for several WRRFs and correlations for any compound with known kinetic information could be developed solely using the second order reaction rate constant with ozone (kO3). Two distinct reaction phases were defined for which separate linear correlations were obtained. The first was mainly ozone controlled, while the second phase was more related to HO• reactions. Furthermore, parallel factor analysis of the fluorescence spectra enabled monitoring of multiple types of organic matter with different O3 and HO• reactivity. This knowledge is of value for kinetic modeling frameworks and for achieving a better understanding of the occurring changes of organic matter during ozonation.

How well-mixed is well mixed? Hydrodynamic-biokinetic model integration in an aerated tank of a full-scale water resource recovery facility.

Current water resource recovery facility (WRRF) models only consider local concentration variations caused by inadequate mixing to a very limited extent, which often leads to a need for (rigorous) calibration. The main objective of this study is to visualize local impacts of mixing by developing an integrated hydrodynamic-biokinetic model for an aeration compartment of a full-scale WRRF. Such a model is able to predict local variations in concentrations and thus allows judging their importance at a process level. In order to achieve this, full-scale hydrodynamics have been simulated using computational fluid dynamics (CFD) through a detailed description of the gas and liquid phases and validated experimentally. In a second step, full ASM1 biokinetic model was integrated with the CFD model to account for the impact of mixing at the process level. The integrated model was subsequently used to evaluate effects of changing influent and aeration flows on process performance. Regions of poor mixing resulting in non-uniform substrate distributions were observed even in areas commonly assumed to be well-mixed. The concept of concentration distribution plots was introduced to quantify and clearly present spatial variations in local process concentrations. Moreover, the results of the CFD-biokinetic model were concisely compared with a conventional tanks-in-series (TIS) approach. It was found that TIS model needs calibration and a single parameter set does not suffice to describe the system under both dry and wet weather conditions. Finally, it was concluded that local mixing conditions have significant consequences in terms of optimal sensor location, control system design and process evaluation.

TREATMENT OF LANDFILL LEACHATE BY COUPLING COAGULATION-FLOCCULATION OR OZONATION TO GRANULAR ACTIVATED CARBON ADSORPTION.

A major concern for landfilling facilities is the treatment of their leachate. To optimize organic matter removal from this leachate, the combination of two or more techniques is preferred in order to meet stringent effluent standards. In our study, coagulation-flocculation and ozonation are compared as pre- treatment steps for stabilized landfill leachate prior to granular activated carbon (GAC) adsorption. The efficiency of the pre treatment techniques is evaluated using COD and UVA254 measurements. For coagulation- flocculation, different chemicals are compared and optimal dosages are determined. After this, iron (III) chloride is selected for subsequent adsorption studies due to its high percentage of COD and UVA254 removal and good sludge settle-ability. Our finding show that ozonation as a single treatment is effective in reducing COD in landfill leachate by 66% compared to coagulation flocculation (33%). Meanwhile, coagulation performs better in UVA254 reduction than ozonation. Subsequent GAC adsorption of ozonated effluent, coagulated effluent and untreated leachate resulted in 77%, 53% and 8% total COD removal respectively (after 6 bed volumes). The effect of the pre-treatment techniques on GAC adsorption properties is evaluated experimentally and mathematically using Thomas and Yoon-Nelson models. Mathematical modelling of the experimental GAC adsorption data shows that ozonation increases the adsorption capacity and break through time with a factor of 2.5 compared to coagulation-flocculation.

A comparative study on the efficiency of ozonation and coagulation-flocculation as pretreatment to activated carbon adsorption of biologically stabilized landfill leachate.

The present work investigates the potential of coagulation-flocculation and ozonation to pretreat biologically stabilized landfill leachate before granular activated carbon (GAC) adsorption. Both iron (III) chloride (FeCl3) and polyaluminium chloride (PACl) are investigated as coagulants. Better organic matter removal is observed when leachate was treated with FeCl3. At a dose of 1mg FeCl3/mg CODo (CODo: initial COD content), the COD and α254 removal was 66% and 88%, respectively. Dosing 1mg PACl/mg CODo resulted in 44% COD and 72% α254 removal. The settle-ability of sludge generated by PACl leveled off at 252mL/g, while a better settle-ability of 154mL/g was obtained for FeCl3 after dosing 1mg coagulant/mg CODo. For ozonation, the percentage of COD and α254 removal increased as the initial COD concentration decreased. Respectively 44% COD and 77% α254 removal was observed at 112mg COD/L compared to 5% COD and 26% α254 removal at 1846mg COD/L. Subsequent activated carbon adsorption of ozonated, coagulated and untreated leachate resulted in 77%, 53% and 8% total COD removal after treatment of 6 bed volumes. Clearly showing the benefit of treating the leachate before GAC adsorption. Mathematical modeling of the experimental GAC adsorption data with Thomas and Yoon-Nelson models show that ozonation increases the adsorption capacity and breakthrough time of GAC by a factor of 2.5 compared to coagulation-flocculation.

Performance and kinetic process analysis of an Anammox reactor in view of application for landfill leachate treatment.

Anammox has shown its promise and low cost for removing nitrogen from high strength wastewater such as landfill leachate. A reactor was inoculated with nitrification-denitrification sludge originating from a landfill leachate treating waste water treatment plant. During the operation, the sludge gradually converted into red Anammox granular sludge with high and stable Anammox activity. At a maximal nitrogen loading rate of 0.6 g N l(-1) d(-1), the reactor presented ammonium and nitrite removal efficiencies of above 90%. In addition, a modified Stover-Kincannon model was applied to simulate and assess the performance of the Anammox reactor. The Stover-Kincannon model was appropriate for the description of the nitrogen removal in the reactor with the high regression coefficient values (R2 = 0.946) and low Theil's inequality coefficient (TIC) values (TIC < 0.3). The model results showed that the maximal N loading rate of the reactor should be 3.69 g N l(-1) d(-).

Performance analysis and optimization of autotrophic nitrogen removal in different reactor configurations: a modelling study.

The autotrophic nitrogen removal process (partial nitritation combined with the Anammox process) is a new and sustainable nitrogen removal technique for nitrogen-rich streams. A modelling study has been performed to define optimal process conditions (temperature, oxygen supply, pH and biomass retention) and to investigate the influence of chemical oxygen demand, nitrogen loading rate and hydraulic retention time on three alternative reactor configurations: a single oxygen-limited partial nitritation reactor, a single Anammox reactor, and a combination of partial nitritation and Anammox in a single reactor. The model applied was compared to experimental data from the literature and gave good agreement for all three reactor configurations. The simulations revealed that a system with separated partial nitritation and Anammox offered a wider range of optimal process conditions than a one-reactor system. The key factors in the successful operation of partial nitritation were found to be control of aeration, ammonium loading rate and temperature. Heterotrophs remained present in all three reactor systems and it was confirmed that interaction between heterotrophs and Anammox and between heterotrophs and ammonium oxidizers was possible.