Semi-quantitative analysis of a specific database on priority and emerging substances in wastewater and sludge

The Water Framework Directive (WFD) has drawn attention to a series of metals and organic compounds because of their demonstrated or potential harmfulness for aquatic environments. The aim of our work was to build and to process a "practical" database focused on the role of wastewater treatment plants for the removal of the 37 priority compounds that have to be reduced or stopped by 2015, and of 34 additional relevant contaminants. About 11,000 concentration values in raw and treated wastewater and sludge, from more than 100 peer-reviewed articles and six French national screening programs, were integrated. A systematic approach showed the global low quality of data for most of the compounds, with missing information about the treatment process, sampling and analysis, leading to 10% of the data available for removal efficiency calculations. A semi-quantitative analysis allowed the identification of 20 priority and 10 additional relevant substances more frequently quantified at significant concentrations in raw wastewater and treated wastewater. Conventional activated sludge was able to remove more than 70% of half of the studied compounds, leaving only 10% of them with less than 50% removal. Physical-chemical treatments appeared to be about 30% less efficient than biological treatments. In addition, very few data are available concerning some compounds and some processes, especially sludge treatment and tertiary wastewater treatment processes. Therefore, complementary on-site measurements and modeling are required to propose adapted solutions for the treatment of priority and emerging substances in wastewater treatment plants.

Organic matter release in low temperature thermal treatment of biological sludge for reduction of excess sludge production

Thermal treatment applied in association with a biological system allows for a significant reduction in excess sludge production (approximately 50%). In general, heat treatment is described as a sludge disintegration technique. This paper offers a thorough study on the impact of heat treatment, at temperatures below 100 degrees C, on the solubilisation of the sludge COD and its biodegradability. Discontinuous heating experiments were performed on activated and digested sludge. At all temperatures tested the released COD for digested sludge was systematically higher than that for activated sludge (15 and 40%, respectively, at 95 degrees C for 40 min of contact time). For the first 30 min, a 1st order kinetic, with respect to the residual COD, was systematically found. In the range of 40-95 degrees C, digested sludge had a lower activation energy than activated sludge (26 kcal/mol compared to 70-160 kcal/mol). COD solubilisation is thus more positively influenced by temperature in the case of activated sludge. This may be due to the significant difference in the ratio of protein/carbohydrate in digested and activated sludge (1-5 and 0.2-0.7, respectively). The increase in the COD/TKN ratio in the solubilised fraction after thermal treatment of activated sludge suggests a preferential solubilisation of proteins over carbohydrates. Respirometric tests performed on the solubilised COD showed that whatever the sludge origin, only 40-50% of released COD is biodegradable at a conventional hydraulic retention time (i.e., 24 h). Hence, heat treatment would act more through organic matter solubilisation rather than by a biodegradability increase.

Specificity and potential applications of the biochemical acidogenic potential method for the anaerobic characterization of wastewater

The biochemical acidogenic potential (BAP) test is an anaerobic characterization method for wastewater. Fermentable organic fractions are obtained through modeling BAP test results. This method was compared to more common fractionation methods such as settling, coagulation, and respirometry, but no direct relationship was found. Biochemical acidogenic potential testing was thus considered to bring new and complementary information. The settleable matter accounted for approximately 50% of the fermentable matter, with a rate comparable to that of aerobic hydrolysis, suggesting a potential assimilable carbon source that could be liberated in sewers or in anaerobic processes. It was also observed that respirometry could underestimate the amount of fermentable substrates while overestimating that of hydrolyzable matter and of heterotrophic biomass involved in anaerobic processes. The BAP fractions are related to the wastewater capacity to produce volatile fatty acids, which are the main substrates of the micro-organisms responsible for enhanced biological phosphorus removal (EBPR). The potential contribution of the BAP fractionation to assist the design, operation, and modeling of the activated-sludge EBPR processes was discussed.

Variations of respiratory activity and glutathione in activated sludges exposed to low ozone doses

Ozonation is one of the most effective treatments for reducing the production of activated sludges in wastewater treatment plants. However, because microorganisms are present in the form of microcolonies, some bacteria may be exposed to sub-lethal ozone doses that could lead to adaptation and resistance to further exposition to oxidative treatment. This represents a major question as it may limit the effect of the treatment, especially when low ozone doses are applied. The critical ozone dosage, defined as the lowest specific transferred ozone concentration leading to a decrease in the maximum oxygen uptake rate was estimated to range between 0.9 and 13.6mg O(3)g(-1) COD(sludges), according to the sludges tested. The lowest ozone dosage leading to the decrease of GSH and GSHt concentrations could be estimated to be lower than 10mg O(3)g(-1) COD(sludges) for GSH, and close to 10mg O(3)g(-1) COD(sludges) for GSHt. After sludge exposure to low ozone doses, no higher amounts of glutathione were synthesized, suggesting that no development of resistance to ozonation occurred after sludge treatment with low ozone doses.

Understanding the mechanisms of thermal disintegrating treatment in the reduction of sludge production

Among the technologies aimed at reducing sludge production, the combination of thermal treatment at 95 degrees C of sludge and the activated sludge process is a promising route. The feasibility of such a combined process is demonstrated (up to 60% sludge reduction) and the impacts of operating conditions on its efficiency are presented. Major emphasis was put on understanding the complex phenomena occurring within the thermal treatment: release and biodegradability of sludge organic matter, impact on the biological activity (decay, maintenance requirements, etc.). These effects were taken into account for the development of an ASM1-based model. Comparison between the modeling approach and experimental data (continuous and batch) showed that thermal treatment had three major issues partly explaining the reduction of sludge production: (i) a low release of organics; (ii) an immediate and reversible biological inactivation associated with additional maintenance energy requirements; and (iii) a potential inert production.

Oxidative and thermo-oxidative co-treatment with anaerobic digestion of excess municipal sludge

The effect of oxidative and thermo-oxidative co-treatment of excess municipal sludge was investigated. A mixture of primary and waste activated sludge was anaerobically treated using two different configurations: i) two stages and ii) a single stage with recycling. Oxidative or thermo-oxidative co-treatment placed in between the reactor or in the recycle line was studied. A two-stage configuration with no co-treatment served as a control and resulted in 50.1% overall solids removal. Compared to the control, an increase in solids removal of 10.8 and 2.7% was observed when oxidative co-treatment was placed between reactors and in the recycle line respectively. When thermo-oxidative co-treatment was placed between the two stages or in the recycle line an increase in solids removal of 25.1 and 26.9% respectively was observed. The performances of the different configurations were also evaluated with parameters such as COD, TKN, ammonia, and fecal coliform concentration.

Measurement of glutathione in activated sludges

Thermal, electric, mechanical or oxidative stress seem a promising way to reduce the production of excess activated sludge during biological wastewater treatment. However, the adaptation and the resistance of the sludge microbial ecosystem to stress conditions is a major question as it may definitively limit the effect of some treatments. Defence mechanisms developed by aerobic organisms, in particular, in response to oxidative stress involve various antioxidant activities and compounds such as glutathione. An HPLC method was developed for measuring reduced and total glutathione (GSH and GSHt) in perchloric acid sludge extracts. The method was sensitive, highly specific and validated for linearity, precision and recovery. Considering the extraction yield and the oxidation of GSH during extract storage, the measured GSH concentration was estimated to represent 60% of the GSH content from activated sludges. GSHt ranged from 0.32 to 3.34micromolg(-1) volatile solids and the GSH/GSHt ratio ranged from 32% to 91%. Measurements performed on sludges stressed in precise conditions selected to reach a reduction of sludge production showed a decrease of GSH and GSHt concentrations with thermal, mechanical, electric and ozone stress.

Analysis of the structure of very large bacterial aggregates by small-angle multiple light scattering and confocal image analysis

This work aims at developing a more accurate measurement of the physical parameters of fractal dimension and the size distribution of large fractal aggregates by small-angle light scattering. The theory of multiple scattering has been of particular interest in the case of fractal aggregates for which Rayleigh theory is no longer valid. The introduction of multiple scattering theory into the interpretation of scattering by large bacterial aggregates has been used to calculate the fractal dimension and size distribution. The fractal dimension is calculated from the form factor F(q) at large scattering angles. At large angles the fractal dimension can also be computed by considering only the influence of the very local environment on the optical contrast around a subunit. The fractal dimensions of E. coli strains flocculated with two different cationic polymers have been computed by two techniques: static light scattering and confocal image analysis. The fractal dimensions calculated with both techniques at different flocculation times are very similar: between 1.90 and 2.19. The comparison between two completely independent techniques confirms the theoretical approach of multiple scattering of large flocs using the Mie theory. Size distributions have been calculated from light-scattering data taking into account the linear independence of the structure factor S(q) relative to each size class and using the fractal dimension measured from F(q) in the large-angle range or from confocal image analysis. The results are very different from calculations made using hard-sphere particle models. The size distribution is displaced toward the larger sizes when multiple scattering is considered. Using this new approach to the analysis of very large fractal aggregates by static light multiple scattering, the fractal dimension and size distribution can be calculated using two independent parts of the scattering curve.

Impact of soluble organic compounds on permeate flux in an aerobic membrane bioreactor

The relative impact of mixed liquor suspended solids and soluble organic compounds on the permeate flux in an aerobic membrane bioreactor (MBR) were investigated during long-term operation. A statistical correlation analysis performed on data obtained over an approximately 700 day operational period revealed that permeate flux was strongly correlated to soluble organic compounds such as soluble sugars and proteins and was not correlated to total mixed liquor COD. Organic compounds with sizes less than 0.10 microm exhibited the strongest correlation to permeate flux. Specific filtration tests conducted on the MBR showed that the effect of soluble COD was most pronounced in the range of 110-210 mg 1(-1). A critical level of soluble COD was established at 500 mg 1(-1) after which point no correlation was present. The effluent quality remained high throughout the study at below 5 mg 1(-1) total COD, indicating that the membrane was able to retain most organic compounds regardless of mixed liquor soluble COD content. It was concluded that MBR permeate fluxes are enhanced when operating at conditions where bio-degradation is improved and soluble organic compounds are reduced.

Modeling acidogenic and sulfate-reducing processes for the determination of fermentable fractions in wastewater

The biochemical acidogenic potential (BAP) of a wastewater is the maximum concentration of volatile fatty acids (VFAs) that can be measured at the end of an anaerobic fermentation test. A model was constructed to describe the acidogenic reactions occurring during BAP tests and to divide the BAP into organic fractions. The model was calibrated with a set of specific experiments highlighting the role of sulfate-reducing bacteria on acidogenic processes, which description was necessary for correct parameter identification. The model could describe acidogenic fermentation processes, with or without sulfate reduction, at 20 degrees C, for 13 wastewaters of different origin, composition, and settleability using the same optimized parameters. A simplified version of the model, without sulfate reduction, was able to describe VFA production by the adjustment of only three variables: readily fermentable organic matter (Sf), anaerobically hydrolyzable organic matter (Xf), and heterotrophic acidogenic biomass (Xha), which proved to be coherent with the experimental BAP value. The combination of the BAP test and the model developed in this study resulted in a new reliable tool to characterize wastewater under anaerobic conditions. As VFAs are the main substrates for phosphate-accumulating organisms (PAOs), the use of organic fractions VFA, Sf, Xf, and Xha in wastewater treatment plant modeling could improve the predictability and optimization of enhanced biological phosphorus removal (EBPR) processes.

Operating conditions for the determination of the biochemical acidogenic potential of wastewater

The aim of this work was to study the test conditions for the determination of the biochemical acidogenic potential (BAP) of wastewater, which should be useful to predict the performance of enhanced biological phosphorus removal (EBPR). Proposed operating conditions for a simple and reproducible BAP test in 250 ml serum bottles (equipped with black butyl stoppers and magnetic bars) are: use of either frozen or fresh water, no inoculum addition, fermentation carried out in the dark during 15 days, addition of 1 mM bromo-ethane sulfonate (BES) and 2 mM barium chloride (BaCl2), stirring speed strong enough to maintain vortex conditions, no pH control and controlled temperature of 20 degrees C.

Feasibility study of mechanically disintegrated sludge and recycle in the activated-sludge process

The action mechanisms and performances of a combined system associating activated sludge and mechanical treatment (High Pressure Homogenizer) were evaluated for urban wastewater. Discontinuous experiments showed that the energy applied at the first pass was high enough to modify the sludge particulate fraction (high COD release) but without cell lysis. The applied shear forces led to a progressive cell break up (maximal COD release 90% total COD). Continuous experiments showed less than 20% reduction in sludge production (compared to a control run under the same loading conditions Y(TSS) = 0.35 g TSS x g(-1) COD(removed)) through the application of mechanical treatment (stress frequency = 0.2 d(-1)). Recycling of mechanical treated sludge to the aeration tank induced a slight increase in effluent TSS, but the biological performance seemed to be maintained. Significant improvements in sludge settling characteristics were observed.

Wastewater COD characterization: biodegradability of physico-chemical fractions

Physico-chemical and biological characterization methods were applied to wastewater samples originating from 7 French WWTPs. The settleable fraction (S), unsettleable-coagulable fraction (US-C) and unsettleable-uncoagulable fraction (US-UC) were separated. Special emphasis was put on the determination of hydrolysis kinetics associated with coagulable- and settleable-fractions in order to model their behaviour in activated sludge processes, as both these fractions are influenced by the SRT. The "soluble" fraction (i.e. US-UC) was composed of readily biodegradable COD (2 to 27%), readily hydrolysable COD (37 to 90%) and inerts (2 to 47%). The "colloidal" fraction (i.e. US-C) was composed of heterotrophic biomass (9 to 24%) and readily hydrolysable COD (6 to 82%), with the rest being inerts or very slowly biodegradable COD (0 to 70%). The "particulate" fraction (i.e. settleable) was composed of biomass (14 +/- 6%), readily hydrolysable COD (21 +/- 14%), slowly hydrolysable COD (about 45-50%) and 15 to 20% which can be considered as inert matter (X(I)). "Readily hydrolysable COD" was correctly modeled by a global first-order reaction. First-order constants (KH) were 9 +/- 2 d(-1) for raw-wastewater, 12 +/- 3 d(-1) for primary settled-wastewater and 16 +/- 5 d(-1) for coagulated-wastewater. "Slowly hydrolysable COD" was correctly modeled by a limited surface reaction. Concerning this fraction, the specific hydrolysis rate (kH) ranged from 0.25 to 1.05 d(-1), and the affinity constant (K(X)) ranged from 0.33 to 0.95 gCOD/gCOD based on settleable solids analysis.

A comparative study between mechanical, thermal and oxidative disintegration techniques of waste activated sludge

The release performances of an organic and mineral activated sludge matrix were studied for a wide range of disintegration treatments like mechanical, thermal, thermal-chemical and oxidative disintegration techniques. The maximal COD release was 35% of total COD after 24 hours contact time at 95 degrees C. A limiting value of 60% COD release was obtained for 500 and 700 bars after 10 passes. Concerning theoxidative disintegration techniques (O3 and H2O2), a limiting value of around 60-65% of TOC release was observed. Therefore, it was hypothesised that thermal and mechanical treatments allow mainly for breaking apart the micro-organisms while the oxidative treatment destroys the sludge flocs and disrupts the micro-organisms. A release effect of the mineral fraction is observed only oxidative disintegration techniques.

Influence of pre-treatments and recycled flows on the COD fractions of a wastewater: a case study

Physical-chemical and biological fractionation methods were used at different parts of the Evry WWTP in order to assess the influence of effluents recycled from the primary settling tanks and sludge treatment processes on the characteristics of the wastewater. The RBCOD doubled between the WWTP influent and the biological reactor inlet, thus doubling the denitrifying potential of the plant. The primary sludge gravity thickener effluent, subject to a great variability, appeared to contribute up to 38% of the RBCOD contained in the biological reactor influent, whereas only 30% seemed to come from the WWTP inlet. No notable increase of RBCOD was observed at the primary settling tanks' outlet for the longer residence time. The significant variations of RBCOD across the plant were not caused by fluctuations of the physical-chemical COD fractions.

Changes in the organic composition of wastewater during biological treatment as studied by NMR and IR spectroscopies

Despite its importance, relatively little is known about the composition and fate of wastewater organic matter (OM) in treatment plants. Monitoring the chemical changes in OM during activated sludge treatment can improve our knowledge of the processes involved in the biological elimination of OM. Direct chemical analyses of treated water OM typically account for about 20% of the OM, and structural information was obtained in this study using Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared (FTIR) spectroscopic techniques. Distinct changes in the OM during wastewater biological treatment were underlined. 13C and 1H NMR showed that aromatic carbons were minor constituents of the samples. Alkyl chains exhibited a more highly branched character in treated water, as compared to long chain aliphatic carbons present in wastewater. Carboxyl signals in the 13C NMR spectrum of wastewater could be due to peptide bonds in proteins, whereas in the treated water spectrum, this signal could be related to the presence of non-proteinaceous nitrogen. Besides the non-degraded compounds, treated water OM could contain recondensation products of simple molecules. Their refractory character probably derives from their complex structures rather than from particular chemical functions, as suggested by the lack of fundamental differences in the chemical structures of wastewater and treated water OM.

Application of COD fractionation by a new combined technique: comparison of various wastewaters and sources of variability

A new respirometric method for COD fractionation was applied to various wastewaters collected on French treatment plants. Great variations were observed especially in the readily biodegradable fraction (RBCOD) ranging from 1 to 16% of total COD. Variability of the results among the origin can be explained by the reactions occurring in the sewers. In one of the towns studied, the wastewater samples collected directly in the sewer show COD characteristics very different from the sewage which reaches the treatment plant. By analysing the same wastewater after aerobic and anaerobic storage, it was proved that RBCOD respectively decreased or increased significantly. Seasonal fluctuations were also quantified, showing that RBCOD fraction decreased from 75 mg/L to -15 mg/L during the summer whereas the variation of soluble fraction was less important.

Transfer of organic matter between wastewater and activated sludge flocs

The organic matter of wastewater was fractionated into settleable (i.e., particulate) and non-settleable (i.e., colloidal + soluble) fractions by settling followed by 0.22 micron filtration. Particulate, colloidal and soluble proportions were found to be relatively constant (45, 31 and 24% of the total COD, respectively). Transfer of soluble fraction always occurred from the wastewater to the activated sludge flocs, whereas bidirectional transfer occurred for the colloidal fraction. The transfer of soluble and colloidal matter reached a steady state after 40 min-mixing and 20 min-mixing, respectively. Desorption of a part of the colloidal organic matter pre-sorbed on the activated sludge flocs was evidenced. The biosorption capacity of activated sludge was around 40-100 mgCODg-1TSS. The biosorbable fraction of wastewater represented on average 45% of the non-settleable fraction.

Hydrolysis of wastewater colloidal organic matter by extracellular enzymes extracted from activated sludge flocs

Enzymatic activities associated with the exopolymeric substances (EPSs) extracted from activated sludges were tested for their ability to hydrolyse the organic colloidal fraction of wastewater. Bacteria extracted with EPS and concentrated by wastewater microfiltration were inhibited with NaN3 or KCN. The protein hydrolysis mainly resulted from the enzymatic activity of EPS, whereas the glycolytic activity was mainly present in the organic colloidal fraction of the wastewater.

Structural Interpretations of Static Light Scattering Patterns of Fractal Aggregates

A method based on static light scattering by fractal aggregates is introduced to extract structural information. In this study, we determine the scattered intensity by a fractal aggregate calculating the Structure and the Form factors noted, respectively, S(q) and F(q). We use the approximation of the mean field Mie scattering by fractal aggregates (R. Botet, P. Rannou, and M. Cabane, appl. opt. 36, 8791, 1997). This approximation is validated by a comparison of the scattering and extinction cross sections values calculated using, on the one hand, Mie theory with a mean optical index n) and, on the other hand, the mean field approximation. Scattering and extinction cross sections values differ by about 5%. We show that the mean environment of primary scatterers characterized by the optical index n(s) must be taken into account to interpret accurately the scattering pattern from fractal aggregates. Numerical simulations were done to evaluate the influence of the fractal dimension values (D(f)>2) and of the radius of gyration or the number of primary particles within the aggregates (N=50 to 250) on the scatterers' mean optical contrast (n(s)/n). This last parameter plays a major role in determining the Form factor F(q) which corresponds to the primary particles' scattering. In associating the mean optical index (n) to structural characteristics, this work provides a theoretical framework to be used to provide additional structural information from the scattering pattern of a fractal aggregate (cf. Part II). Copyright 2000 Academic Press.

Structural Interpretations of Static Light Scattering Patterns of Fractal Aggregates

Information on the size and structure of aggregates is critical in predicting the formation kinetics, settling velocities, and reactivity of particle aggregates. For some systems, however, accessing this information may be very difficult. Light scattering measurements are among the most useful techniques for accessing such information. In the case of large primary particles forming aggregates, the common Rayleigh approximation is not valid. Instead, Mie scattering must be used and multiple scattering must be accounted for. Moreover, size polydispersity and structure of aggregates are combined in the scattered intensity measurements. This work presents an experimental validation of a new theoretical approach for extracting information on both aggregate structure and size when multiple scattering cannot be neglected. The chemically controlled aggregation of 0.8-µm latex particles demonstrates the following: (1) Polydispersity effects prevent the interpretation of data to obtain structural information from the Structure factor S(q). (2) The calculated optical contrast decreasing during the aggregation can be correlated with the structural changes in the growing aggregates independently of size polydispersity. We have shown that a strict correlation can be obtained between the fractal dimension D(f) and the scatterers' mean optical contrast calculated at large scattering angles. (3) The changes in the Form factor (F(q)) due to multiple scattering when particles are close together yield a predicted structure that is in agreement with expected fractal dimension values and therefore S(q) can be described in term of both structure and size polydispersity. Copyright 2000 Academic Press.