Activated Sludge Production Parameters and Nutrient Content of Organic Sludge Components.

An important part of biological treatment system design is quantifying the sludge production and the nutrient removal capacity. Influent wastewater COD fractionation, biomass growth and endogenous respiration directly impacts the composition of the mixed liquor solids in activated sludge systems. The objectives of this project were to determine the model kinetic and stoichiometric parameters associated with activated sludge production and the nutrient content (N and P) of unbiodegradable organic matter components. A complete sludge retention experiment was conducted over 70 days in a pilot-scale membrane bioreactor fed with a real municipal wastewater, and operated with alternating growth and famine periods. Experimental results were simulated and compared using the default values from two well-accepted model parameter sets. The General ASDM parameter set was found to better fit the experimental data than the Metcalf and Eddy parameter set, mainly to characterize endogenous respiration and the heterotrophic biomass concentration. An influent unbiodegradable organic particulate fraction (fXU,Inf) value of 0.16 g COD/g COD was determined by calibration of the accumulated sludge total COD, suspended solids and heterotrophic biomass concentrations. The nutrient content of the accumulated endogenous residue (XE) and influent unbiodegradable organic particulate (XU,Inf) components were calibrated to 0.030 and 0.100 g N/g COD and 0.035 and 0.008 g P/g COD, respectively. These values are in the range of those reported in the literature except for the high P content found in the endogenous residue, possibly due to the presence of coagulants added for P removal in the accumulated sludge. These results were consistent under the wide range of dynamic conditions tested and could improve model prediction of sludge production and composition.

Phosphorus limitation strategy to increase propionic acid flux towards 3-hydroxyvaleric acid monomers in Cupriavidus necator.

Properties of polyhydroxybutyrate-co-hydroxyvalerate (P(3HB-co-3HV)) depend on their 3HV content. 3HV can be produced by Cupriavidus necator from propionic acid. Few studies explored carbon distribution and dynamics of 3HV and 3HB monomers production, and none of them have been done with phosphorus as limiting nutrient. In this study, fed-batch cultures of C. necator with propionic acid, as sole carbon source or mixed with butyric acid, were performed. Phosphorus deficiency allowed sustaining 3HV production rate and decreasing 3HB production rate, leading to an instant production of up to 100% of 3HV. When a residual growth is sustained by a phosphorus feeding, the maximum 3HV percentage produced from propionic acid is limited to 33% (Mole.Mole(-1)). The association of a second carbon source like butyric acid lead to higher conversion of propionic acid into 3HV. This study showed the importance of the limiting nutrient and of the culture strategy to get the appropriate product.

Impact of sustaining a controlled residual growth on polyhydroxybutyrate yield and production kinetics in Cupriavidus necator.

In this study a complementary modeling and experimental approach was used to explore how growth controls the NADPH generation and availability, and the resulting impact on PHB (polyhydroxybutyrate) yields and kinetics. The results show that the anabolic demand allowed the NADPH production through the Entner-Doudoroff (ED) pathway, leading to a high maximal theoretical PHB production yield of 0.89 C mole C mole(-1); whereas without biomass production, NADPH regeneration is only possible via the isocitrate dehydrogenase leading to a theoretical yield of 0.67 C mole C mole(-1). Furthermore, the maximum specific rate of NADPH produced at maximal growth rate (to fulfil biomass requirement) was found to be the maximum set in every conditions, which by consequence determines the maximal PHB production rate. These results imply that sustaining a controlled residual growth improves the PHB specific production rate without altering production yield.

A statistical comparison of protein and carbohydrate characterisation methodology applied on sewage sludge samples.

Biochemical characterization of organic matter is becoming of key importance in wastewater treatment. The main objectives are to predict organic matter properties, such as granulation or flocculation, and hence treatment performance. Although standardized methods do exist for some organic molecules, such as volatile fatty acids or lipids, there are no standard methods to measure proteins and carbohydrates content, both biochemical families being the main components of sewage sludge. Consequently, the aim of the present work is to investigate the efficiency of several colorimetric methods to determine proteins and carbohydrates content as well as their compatibility with the sludge matrices. The different methods have been evaluated based on statistical criteria such as sensitivity, linearity, accuracy, rightness, and specificity using standard molecules such as Bovine Serum Albumin (BSA), glucose, cellulose and a certified reference product. The Lowry and the Dubois methods have been shown to be the best compromise for the considered criteria after having been tested on sewage sludge samples obtained from different locations in a wastewater treatment plant. In average, the measured volatile fatty acids, lipids, proteins and carbohydrates contents represented 80 ± 7% (% volatile solids) of the organic matter. Proteins and carbohydrates represented in average 69 ± 3%. This study underlined that the choice of a relevant methodology is of great importance for organic matter measurement.

Biodegradation of the endogenous residue of activated sludge in a membrane bioreactor with continuous or on-off aeration.

The goal of this study was to determine the effect of a long sludge retention time on the biodegradation of the endogenous residue in membrane digestion units receiving a daily feed of sludge and operated under either aerobic or intermittently aerated (22 h off-2 h on) conditions. The mixed liquor for these experiments was generated in a 10.4 day sludge retention time membrane bioreactor fed with a synthetic and completely biodegradable influent with acetate as the sole carbon source. It had uniform characteristics and consisted of only two components, heterotrophic biomass X(H) and endogenous residue X(E). Membrane digestion unit experiments were conducted for 80 days without any sludge wastage except for some sampling. The dynamic behaviour of generation and consumption of filtered organic digestion products was characterized in the membrane digestion unit systems using three pore filter sizes. Results from this investigation indicated that the colloidal matter with size between 0.04 µm and 0.45 µm was shown to contain a recalcitrant fraction possibly composed of polysaccharides bound to proteins which accumulated in the membrane digestion unit under both conditions. Modelling the membrane digestion unit results by considering a first-order decay of the endogenous residue allowed to determine values of the endogenous residue decay rate of 0.0065 and 0.0072 d(-1) under fully aerobic and intermittently aerated conditions, respectively. The effect of temperature on the endogenous decay rate was assessed for the intermittently aerated conditions in batch tests using thickened sludge from tests gave an endogenous decay rate constant of 0.0075 d(-1) at 20 °C and an Arrhenius temperature correction factor of 1.033.

Characterization of the heterotrophic biomass and the endogenous residue of activated sludge.

The activated sludge process generates an endogenous residue (X(E)) as a result of heterotrophic biomass decay (X(H)). A literature review yielded limited information on the differences between X(E) and X(H) in terms of chemical composition and content of extracellular polymeric substances (EPS). The objective of this project was to characterize the chemical composition (x, y, z, a, b and c in C(x)H(y)O(z)N(a)P(b)S(c)) of the endogenous and the active fractions and EPS of activated sludge from well designed experiments. To isolate X(H) and X(E) in this study, activated sludge was generated in a 200L pilot-scale aerobic membrane bioreactor (MBR) fed with a soluble and completely biodegradable synthetic influent of sodium acetate as the sole carbon source. This influent, which contained no influent unbiodegradable organic or inorganic particulate matter, allowed the generation of a sludge composed essentially of two fractions: heterotrophic biomass X(H) and an endogenous residue X(E), the nitrifying biomass being negligible. The endogenous decay rate and the active biomass fraction of the MBR sludge were determined in 21-day aerobic digestion batch tests by monitoring the VSS and OUR responses. Fractions of X(H) and X(E) were respectively 68% and 32% in run 1 (MBR at 5.2 day SRT) and 59% and 41% in run 2 (MBR at 10.4 day SRT). The endogenous residue was isolated by subjecting the MBR sludge to prolonged aerobic batch digestion for 3 weeks, and was characterized in terms of (a) elemental analysis for carbon, nitrogen, phosphorus and sulphur; and (b) content of EPS. The MBR sludge was characterized using the same procedures (a and b). Knowing the proportions of X(H) and X(E) in this sludge, it was possible to characterize X(H) by back calculation. Results from this investigation showed that the endogenous residue had a chemical composition different from that of the active biomass with a lower content of inorganic matter (1:4.2), of nitrogen (1:2.9), of phosphorus (1:5.3) and of sulphur (1:3.2) but a similar content of carbon (1:0.98). Based on these elemental analyses, chemical composition formulae for X(H) and X(E) were determined as CH(1.240)O(0.375)N(0.200)P(0.0172)S(0.0070) and CH(1.248)O(0.492)N(0.068)P(0.0032)S(0.0016), respectively. Data from EPS analyses also confirmed this difference in structure between X(E) and X(H) with an EPS content of 11-17% in X(E)versus 26-40% in X(H).

Solid-phase fluorescence spectroscopy to characterize organic wastes.

The production of solid organic waste (SOW) such as sewage sludge (SS) or municipal solid waste (MSW) has been continuously increasing in Europe since the beginning of the 1990'. Today, the European Union encourages the stabilization of these wastes using biologic processes such as anaerobic digestion and/or composting to produce bio-energy and organic fertilizers. However, the design and management of such biologic processes require knowledge about the quantity and quality of the organic matter (OM) contained in the SOW. The current methods to characterize SOW are tedious, time-consuming and often insufficiently informative. In this paper, we assess the potential of solid-phase fluorescence (SPF) spectroscopy to quickly provide a relevant characterization of SOW. First, we tested well known model compounds (tryptophan, bovine serum albumin, lignin and humic acid) and biologic matrix (Escherichia coli) in three dimensional solid-phase fluorescence (3D-SPF) spectroscopy. We recorded fluorescence spectra from proteinaceous samples but we could not record the fluorescence emitted by lignin and humic acid powders. For SOW samples, fluorescence spectra were successfully recorded for MSW and most of its sub-components (foods, cardboard) but impossible for SS, sludge compost (SC) and ligno-cellulosic wastes. Based on visual observations and additional assays, we concluded that the presence of highly light-absorptive chemical structures in such dark-colored samples was responsible for this limitation. For such samples, i.e. lignin, humic acid, SS, SC and ligno-cellulosic wastes, we show that laser induced fluorescence (LIF) spectroscopy enables the acquisition of 2D fluorescence spectra.

Biodegradation of the endogenous residue of activated sludge.

This study evaluated the potential biodegradability of the endogenous residue in activated sludge subjected to batch digestion under either non-aerated or alternating aerated and non-aerated conditions. Mixed liquor for the tests was generated in a 200 L pilot-scale aerobic membrane bioreactor (MBR) operated at a 5.2 days SRT. The MBR system was fed a soluble and completely biodegradable synthetic influent composed of sodium acetate as the sole carbon source. This influent, which contained no influent unbiodegradable organic or inorganic materials, allowed to generate sludge composed of essentially two fractions: a heterotrophic biomass X(H) and an endogenous residue X(E), the nitrifying biomass being negligible (less than 2%). The endogenous decay rate and the active biomass fraction of the MBR sludge were determined in 21-day aerobic digestion batch tests by monitoring the VSS and OUR responses. Fractions of X(H) and X(E): 68% and 32% were obtained, respectively, at a 5.2 days SRT. To assess the biodegradability of X(E), two batch digestion units operated at 35 degrees C were run for 90 days using thickened sludge from the MBR system. In the first unit, anaerobic conditions were maintained while in the second unit, alternating aerated and non-aerated conditions were applied. Data for both units showed apparent partial biodegradation of the endogenous residue. Modeling the batch tests indicated endogenous residue decay rates of 0.005 d(-1) and 0.012 d(-1) for the anaerobic unit and the alternating aerated and non-aerated conditions, respectively.

Modified ADM1 disintegration/hydrolysis structures for modeling batch thermophilic anaerobic digestion of thermally pretreated waste activated sludge.

Anaerobic digestion disintegration and hydrolysis have been traditionally modeled according to first-order kinetics assuming that their rates do not depend on disintegration/hydrolytic biomass concentrations. However, the typical sigmoid-shape increase in time of the disintegration/hydrolysis rates cannot be described with first-order models. For complex substrates, first-order kinetics should thus be modified to account for slowly degradable material. In this study, a slightly modified IWA ADM1 model is presented to simulate thermophilic anaerobic digestion of thermally pretreated waste activated sludge. Contois model is first included for disintegration and hydrolysis steps instead of first-order kinetics and Hill function is then used to model ammonia inhibition of aceticlastic methanogens instead of a non-competitive function. One batch experimental data set of anaerobic degradation of a raw waste activated sludge is used to calibrate the proposed model and three additional data sets from similar sludge thermally pretreated at three different temperatures are used to validate the parameters values.