Trace metal elements vaporization and phosphorus recovery during sewage sludge thermochemical treatment - A review.

Phosphorus (P) plays essential roles in crops growth. Natural mineral sources of phosphate are non-renewable, overexploited and unevenly distributed worldwide, making P a strategic resource for agricultural systems. The search for sustainable ways to secure P supply for fertilizer production has therefore become a critical issue worldwide. Sewage sludge (SS) is an organic waste material considered as a key alternative source of P. Switzerland and the European Union are about to make it mandatory to recover P from SS or its treatment residues. Among the many technical options to achieve this objective, SS thermochemical treatments spiked with Cl-donors appear as a promising approach to recover P from SS and separate it from mineral pollutants such as trace metal elements (TME). The purpose of Cl-donor additives is to fix P within the mineral residues, possibly in bioavailable P species forms, while promoting TME vaporization by chlorination mechanisms. This review paper compares the various thermochemical treatments investigated worldwide over the past two decades. The influence of process conditions and Cl-donor nature is discussed. The presented results show that, except for nickel and chromium, most TME can be significantly vaporized during a high temperature treatment (over 900 °C) with Cl addition. In addition, the fixation rate and solubility of P is increased when a Cl-donor such as MgCl2 is added.

Characterization of municipal biowaste categories for their capacity to be converted into a feedstock aqueous slurry to produce methane by anaerobic digestion.

Biowaste material is a good candidate for the production of energy in urban territories. The presence of undesirable or constituents mixed with the biowaste collected by municipalities makes it difficult to recycle organic matter of sufficient quality for agricultural uses. Methane production is particularly attractive for energy recovery notably because this energy vector can be distributed using the grid already in place for natural gas in many cities. Depending on the origin and biochemical composition of biowaste, methane can be produced using thermochemical (gasification then syngas methanation) or biological processes (anaerobic digestion). The objective of this work was to characterize the ability of biowaste to be used as a feedstock for anaerobic digestion. Based on considerations such as the quantities produced and the availability, four categories of biowaste produced in the city of Lyon were identified as potential key resources: Garden biowaste (GBW), restauration biowaste (RBW), household biowaste (HBW) and supermarkets biowaste (SMBW). Representative samples were taken from the sites of production and analyzed for parameters including biomethane potential (BMP). Each sample was then fractioned by leaching and the distribution of the BMP between the particulate fraction and the readily soluble fraction was assessed. GBW organic matter exhibited high hemicellulose content (over 81% of VS) and a low BMP which was very poorly distributed into its soluble fraction (2 NL·kgTS-1). RBW, HBW and SMBW showed a much higher BMP with a strong distribution in the soluble fraction (100 NL·kgTS-1). Plastic materials were found to account for up to 40% of the mass of SMBW sample. Altogether, GBW was identified as non-favorable for anaerobic digestion and recommended rather for thermochemical conversion. HBW, RBW and SMBW revealed adapted to anaerobic. Pulping was shown to be applicable in order to convert the 3 biowaste materials into a pumpable slurry with high biomethane potential.

Comparative characterization of surface sludge deposits from fourteen French Vertical Flow Constructed Wetlands sewage treatment plants using biological, chemical and thermal indices.

Due to their design and mode of operation, French Vertical Flow Constructed Wetlands (VFCWs) accumulate suspended solids from the inflow wastewater in the form of a sludge layer at the surface of the first filter. In order to maintain the treatment performance over the long term, the characteristics of the sludge deposits and their evolution have to be well described. In this objective, a panel of sludge deposit samples taken from 14 French VFCW sewage treatment plants was investigated. Elemental composition and organic matter content, nature and reactivity were analyzed. Results clearly revealed two categories of sludge deposits, namely the "young-age plants" type (1 year of operation and less) and the "mature plants" type (3 years of operation and more). Sludge deposits from the "mature plants" exhibited same biological, physical and chemical properties. Their organic matter was globally less abundant, more humified and less biodegradable than in the young-age plants type. Their overall contents in trace metals were also higher, although in a limited manner. The effect of additional treatments, particularly FeCl3 injection for phosphorus precipitation, was observable in the "young-age plants" group. Finally, the sludge deposits sampled from one particular plant with specific operating conditions were found to exhibit very different characteristics from those of either groups identified. This observation underlined the influence of local conditions on the typology of the sludge deposits.

Study of aggregation in surface sludge deposits from 14 full-scale French constructed wetlands using particle size distribution and dynamic vapor sorption analyses.

French vertical flow constructed wetlands (French VFCWs) are widely used for the treatment of wastewaters from small communities. In the system, unsettled wastewater is percolated through two successive stages of filter-cells planted with reeds. This causes the formation of a surface sludge layer. This layer plays positive roles in the treatment performance, but also leads to clogging. The objective of this study was to contribute to the description of the sludge deposits characteristics and their dynamics of evolution, which may control the development of clogging. Representative samples of sludge deposits were taken from 14 French VFCWs full-scale plants and analyzed for particle size, dynamic vapor sorption and other parameters of composition to compare their structure and evaluate the factors of influence. Results showed that ageing of the surface deposits layer over the years of operation in each plant induced the formation and integration of microaggregates within the initial macrostructure of fresh organic matter (OM). The humification process of the OM was found to play a key role in the aggregation process. The injection of FeCl3 operated to precipitate phosphates before filtration was found to accelerate the aggregation process in the early phase (<1 year) of operation of the sludge.

Analytical indicators to characterize Particulate Organic Matter (POM) and its evolution in French Vertical Flow Constructed Wetlands (VFCWs).

The design of French VFCWs leads to the formation of a sludge layer at the surface of the first filters due to the retention of suspended solids from the percolation of unsettled wastewater. This layer plays a major role in the system but still little is known on its characteristics and evolutions. In this study, suspended solids and sludge deposits sampled from two French VFCW plants were analyzed by different methods in the objective to assess the evolution of particulate organic matter (POM) along the treatment chain and within the sludge layer, and identify relevant analytical indicators of these phenomena. The treatment chain included an aerobic trickling filter followed by FeCl3 injection and two successive stages of filters. Thermal analyses showed that OM contents of suspended solids decreased along the treatment chain. POM in inflow suspended solids was predominantly composed of reactive, biodegradable compounds which were partly hydrolyzed and mineralized notably at the trickling filter stage. 3D fluorescence spectra collected from aqueous POM extracts confirmed the evolution of organic matter from low-molecular reactive compounds to more complex and stable structures such as humic-like substances. FTIR confirmed the mineralization of POM's reactive constituents along the treatment chain by the decrease in the intensities of the characteristics bands of aliphatic compounds or proteins, and its humification in the sludge deposits through the relative increase of the bands at 1634cm-1 (vC=O) and 1238cm-1 (δC=O and/or δOH). Isotopic ratios δ2H/1H and δ15N/14N were found to be good indicators of POM evolutions. The higher values of δ2H/1H and δ15N/14N ratios measured in sludge deposits as compared to inflow suspended solids were related to POM humification and to microbial processes of POM hydrolysis and mineralization, respectively.

Characterization of selected municipal solid waste components to estimate their biodegradability.

Biological treatments of Residual Municipal Solid Waste (RMSW) allow to divert biodegradable materials from landfilling and recover valuable alternative resources. The biodegradability of the waste components needs however to be assessed in order to design the bioprocesses properly. The present study investigated complementary approaches to aerobic and anaerobic biotests for a more rapid evaluation. A representative sample of residual MSW was collected from a Mechanical Biological Treatment (MBT) plant and sorted out into 13 fractions according to the French standard procedure MODECOM™. The different fractions were analyzed for organic matter content, leaching behavior, contents in biochemical constituents (determined by Van Soest's acid detergent fiber method), Biochemical Oxygen Demand (BOD) and Bio-Methane Potential (BMP). Experimental data were statistically treated by Principal Components Analysis (PCA). Cumulative oxygen consumption from BOD tests and cumulative methane production from BMP tests were found to be positively correlated in all waste fractions. No correlation was observed between the results from BOD or BMP bioassays and the contents in cellulose-like, hemicelluloses-like or labile organic compounds. No correlation was observed either with the results from leaching tests (Soluble COD). The contents in lignin-like compounds, evaluated as the non-extracted RES fraction in Van Soest's method, was found however to impact negatively the biodegradability assessed by BOD or BMP tests. Since cellulose, hemicelluloses and lignin are the polymers responsible for the structuration of lignocellulosic complexes, it was concluded that the structural organization of the organic matter in the different waste fractions was more determinant on biodegradability than the respective contents in individual biopolymers.

Performance assessment of a vertical flow constructed wetland treating unsettled combined sewer overflow.

The performance of a vertical flow constructed wetland for combined sewer overflow treatment (CSO CW) has been evaluated. The full-scale site has been monitored for 3 years for major pollutants and for two load events for a range of micropollutants (metals, metalloids and polycyclic aromatic hydrocarbons (PAHs)). Performance were predominantly high (97% for total suspended solids (TSS), 80% for chemical oxygen demand (COD), 72% for NH4-N), even if several loads were extremely voluminous, pushing the filter to its limits. Two different filter materials (a 4:1 mixture of sand and zeolite and natural pozzolana) showed similar treatment performance. Furthermore, environmental factors were correlated with COD removal efficiency. The greatest influencers of COD removal efficiency were the inlet dissolved COD concentrations and the duration and potential evapotranspiration during inter-event periods. Furthermore, sludge was analysed for quality and a sludge depth map was created. The map, and calculating the changes in sludge volume, helped to understand solid accumulation dynamics.

Anaerobic digestion of lignocellulosic biomasses pretreated with Ceriporiopsis subvermispora.

Fungal pretreatment by Ceriporiopsis subvermispora of two forest residues (hazel and acacia branches) and two agricultural lignocellulosic residues (barley straw and sugarcane bagasse) were studied as a pretreatment to improve their subsequent anaerobic digestion for methane production. Biomass samples were grinded to 2 ranges of particle sizes (<4 or 1 mm), autoclaved, inoculated with two strains of C. subvermispora (ATCC 90467 and ATCC 96608) and incubated at 28 °C for 28 days. The effects of fungal pretreatment were assessed by analyzing the samples before and after incubations for dry solids mass, biochemical composition, bio-methane production (BMP) and availability of cellulose to hydrolysis. The production of ligninolytic enzymes MnP and/or laccase was observed with both strains during incubation on most of the samples tested. It almost doubled the hazel branches BMP per unit mass of dry solids but did not improve however the BMP of the agricultural residues and acacia branches. These observations were explained by the fact that although both strains were able to degrade 20-25% of lignin in <1 mm and <4 mm hazel branches samples, none of them was successful however to significantly degrade lignin in the other samples, except for sugarcane bagasse.

Pilot-scale study of vertical flow constructed wetland combined with trickling filter and ferric chloride coagulation: influence of irregular operational conditions.

The aim of this study was to characterize the efficiency of an intensified process of vertical flow constructed wetland having the following particularities: (i) biological pretreatment by trickling filter, (ii) FeCl3 injection for dissolved phosphorus removal and (iii) succession of different levels of redox conditions along the process line. A pilot-scale set-up designed to simulate a real-scale plant was constructed and operated using real wastewater. The influences of FeCl3 injection and water saturation level within the vertical flow constructed wetland stage on treatment performances were studied. Three different water saturation levels were compared by monitoring: suspended solids (SS), total phosphorus (TP), dissolved chemical oxygen demand (COD), ammonium, nitrate, phosphate, iron, and manganese. The results confirmed the good overall efficiency of the process and the contribution of the trickling filter pretreatment to COD removal and nitrification. The effects of water saturation level and FeCl3 injection on phosphorus removal were evaluated by analysis of the correlations between the variables. Under unsaturated conditions, good nitrification and no denitrification were observed. Under partly saturated conditions, both nitrification and denitrification were obtained, along with a good retention of SSs. Finally, under saturated conditions, the performance was decreased for almost all parameters.

Physical-chemical characterization of sludge and granular materials from a vertical flow constructed wetland for municipal wastewater treatment.

The use of vertical flow constructed wetlands (VFCWs) is well developed in France and other countries for the treatment of wastewaters from small communities. The patented Azoé® process has been developed by a French company, SCIRPE, in order to improve denitrification and phosphorus removal as compared to classical VFCWs. It includes a biological trickling filter pretreatment followed by two stages of partially flooded VFCW. The performances of partially flooded VFCW are well demonstrated for the removal of organic matter and nitrogen. The system is now being considered for phosphorus removal as well. In this article, sludge and granular materials sampled from the filters of a municipal plant where the Azoé® system has been operated for 8 years were analyzed in order to provide data that may contribute to a better understanding of the dynamics of phosphorus retention. Elemental analyses showed that phosphorus was predominantly captured in the sludge layer accumulated at the surface of the first stage. The progressive mineralization of the sludge over time was also clearly highlighted. The phosphate phases were mainly associated with iron and calcium. The transport of phosphorus via the migration of fine particles through the porous medium in the first stage was also observed.

Study of the VOC emissions from a municipal solid waste storage pilot-scale cell: comparison with biogases from municipal waste landfill site.

The emission of volatile organic compounds (VOCs) from municipal solid waste stored in a pilot-scale cell containing 6.4 tonnes of waste (storage facility which is left open during the first period (40 days) and then closed with recirculation of leachates during a second period (100 days)) was followed by dynamic sampling on activated carbon and analysed by GC-MS after solvent extraction. This was done in order to know the VOC emissions before the installation of a methanogenesis process for the entire waste mass. The results, expressed in reference to toluene, were exploited during the whole study on all the analyzable VOCs: alcohols, ketones and esters, alkanes, benzenic and cyclic compounds, chlorinated compounds, terpene, and organic sulphides. The results of this study on the pilot-scale cell are then compared with those concerning three biogases from a municipal waste landfill: biogas (1) coming from waste cells being filled or recently closed, biogas (2) from all the waste storage cells on site, and biogas (3) which is a residual gas from old storage cells without aspiration of the gas. The analysis of the results obtained revealed: (i) a high emission of VOCs, principally alcohols, ketones and esters during the acidogenesis; (ii) a decrease in the alkane content and an increase in the terpene content were observed in the VOCs emitted during the production of methane; (iii) the production of heavier alkanes and an increase in the average number of carbon atoms per molecule of alkane with the progression of the stabilisation/maturation process were also observed. Previous studies have concentrated almost on the analysis of biogases from landfills. Our research aimed at gaining a more complete understanding of the decomposition/degradation of municipal solid waste by measuring the VOCs emitted from the very start of the landfill process i.e. during the acidogenesis and acetogenesis phases.

Effect of leachate injection modes on municipal solid waste degradation in anaerobic bioreactor.

Three pilots simulated landfill bioreactors were used to investigate the effect of leachate injection modes on anaerobic digestion and biogas production from municipal solid waste. The technical modes used to increase waste moisture consisted of an initial saturation of the waste by flushing with leachate followed by a quick drainage, or weekly leachate injections with two different rates. The results confirmed that increasing moisture content is a key parameter to boost the biological reactions. Weekly leachate injection with high flow rate led to better results than the initial saturation of the waste in terms of biogas production kinetics. Water percolation was found to be an important factor to accelerate the degradation of solid waste. However, a modelling of the collected data by Gompertz model clearly showed that the intrinsic biogas potential determined on the initial solid waste was not reached with any of the progressive leachate injection modes.

Assessment of the effectiveness of an industrial unit of mechanical-biological treatment of municipal solid waste.

An assessment of the French municipal solid waste (MSW) mechanical-biological treatment (MBT) unit of Mende was performed in terms of mass reduction, biogas emissions reduction and biostability of the biologically treated waste. The MBT unit consists of mechanical sorting operations, an aerobic rotating bioreactor, forced-aeration process in open-air tunnels (stabilization), ripening platforms and a sanitary landfill site for waste disposal in separated cells. On the overall plant, results showed a dry matter reduction of 18.9% and an oxidative organic matter reduction of 39.0%. A 46.2% biogas production decrease could also be observed. Concerning the biotreatment steps, high reductions were observed: 88.1% decrease of biogas potential and 57.7% decrease of oxidative organic matter content. Nevertheless, the usually considered stabilization indices (biogas potential, respirometric index) remained higher than recommended by the German or Austrian regulation for landfilling. Mass balance performed on each step of the treatment line showed that several stages needed improvement (especially mechanical sorting operations) as several waste fractions containing potentially biodegradable matter were landfilled with very few or no biological treatment.

Characterization of screenings from three municipal wastewater treatment plants in the Region Rhône-Alpes.

The objective of this study was to analyze the composition of the screenings sampled from three municipal wastewater treatment plants (wwtp) located in the Region Rhône-Alpes, France. The plants were equipped with multi screening stages with gap sizes ranging from 60 to 3 mm. Waste production flows from each plant were monitored over at least 48 hours in each sampling campaign in order to calculate average production rates. Waste samples of at least 7 kg were collected from each screening stage in each plant at different seasons to evaluate the influence of different parameters on the composition of the waste. An overall 30 samples were thereby collected between May 2007 and February 2008, dried at 80 degrees C for a week, and subsequently hand sorted into 10 fractions of waste materials. Results showed that the average production varied between 0.53 and 3.49 kg (wet mass) per capita per year. The highest production rates were observed during or immediately after rainy weather conditions. The dry matter content ranged between 14.4 and 29.2% of wet mass, and the volatile matter content was between 70.0 and 90.5% of dry mass. The predominant materials in the screenings were found to be sanitary textiles which accounted for 65.2% to 73.6% of dry weight and fines (<20 mm) which accounted for 15.2% to 18.2% of dry weight. These proportions were relatively similar in each plant and each sampling campaign.

Assessment of the multi-compound non-equilibrium dissolution behaviour of a coal tar containing PAHs and phenols into water.

Herein, an experimental study coupled with a model in order to assess the non-equilibrium and multi-compound dissolution behaviour of a coal tar containing PAHs and phenols into water, is presented. For this aim, two experimental studies has been carried out: (1) coal tar-water partition equilibrium and (2) dissolution dynamics of coal tar under controlled hydrodynamic conditions in percolation columns packed with glass beads. The dissolution amount of the three target constituents (i.e. phenol, naphthalene and phenanthrene) was monitored by UV detection. The dissolution behaviour was modelled using a predictive fraction approach. The partition coefficients have been estimated from experimental data and the obtained results show that the partition coefficient of each constituent between the aqueous phase and the tar depends on the activities of the constituent in both phases and cannot be estimated only from the solubility of the pure compound in water. The non-equilibrium dissolution model was established, applied for the experimental conditions and validated for three target compounds adjusting the effective interfacial area between tar and water. This parameter is specific of the experimental set-up. The global behaviour of coal tar has been modelled taking into account four categories of compounds according to their water solubility and volatilities. The mass transfer parameters have been estimated using available correlations. The results of this paper indicate that a model based on component fractions can be used to assess the non-equilibrium dissolution behaviour of a coal tar.

Effect of microbial activity on the mobility of chromium in soils.

The effect of microbial activity on the chemical state of chromium, in a contaminated soil located in the Rhjne-Alpes region (France), has been investigated. This soil contained 4,700 mg kg(-1) Cr, with about 40% present in the soluble hexavalent form. Indigenous microbial activity was found to significantly reduce Cr(VI) to the less mobile form (III) when the soil was incubated at 30 degrees C in an aqueous medium containing glucose and nutrients. A Cr(VI)-reducing strain of Streptomyces thermocarboxydus was isolated from the contaminated soil. The strain was found to metabolize Cr(VI) in a similar manner as an exogenous inoculum of Pseudomonas fluorescens LB300, and to precipitate chromium as a Cr oxyhydroxide with a gammaCrOOH-like local structure. The Cr(VI)-reducing activity of S. thermocarboxydus was induced, or significantly accelerated, by the aggregation of bacterial cells or their adhesion to suspended solid particles, and was stimulated in pure culture by glycerol and chromate.

Kinetics of acetate, propionate and butyrate removal in the treatment of a semi-synthetic landfill leachate on anaerobic filter.

The kinetics of acetate, propionate, and butyrate removal was studied in conditions of leachate treatment in a plug flow anaerobic fixed-film reactor made of a sequence of seven perfectly mixed compartments. An original experimental procedure was followed under sequential feeding conditions so as to maintain the Bacteriol biomass in a quasi-steady state all along the study. With an appropriate computer program based on the least squares method, the apparent kinetic parameters of VFA removal were calculated within concentration ranges below the levels of salt inhibition. The models proposed are based on simple theoretical considerations. For acetate and n-butyrate removal, the best fits were given by the Michaelis-Menten equation with respectively: V(m) (spec) = 0.49 +/- 0.06 g CH(3) COOH g(-1) biomass h(-1)and 0.18 +/- 0.02 g n-CH(3)CH(2)CH(2)COOH g(-1) biomass h(-1) and: K(s) = 21.2 +/- 0.9 g CH(3)COOH L(-1) liquid phase and 8.2 +/- 0.9 g n-CH(3)CH(2)CH(2)COOH L(-1) liquid phase, Iso-butyrate was produced during n-butyrate catabolism and the apparent removal rate of (n + iso)-butyrate considered as a whole was also described by the Michaelis-Menten equation with V(m) (spec) = 0.14 +/- 0.02 g(n + iso)-butyrate g(-1) biomass h(-1) and K(s) = 9.0 +/- 1.2 g (n + iso) butyrate L(-1) liquid phase. On the other hand in the case of propionate, the best fit was obtained with a first-order equation with K(spec) = (0.88 +/- 0.05) 10(-2) L liquid phase g(-1) biomass h(-1). These constants were subsequently used to predict the removal of mixtures of the three major VFAs under study, at various feed concentrations. Three sets of concentrations were tested, and the experimental data were compared to the simulations. This study, together with other experimental observations previously reported, tends to show that under sequential feeding conditions the classical assumption of butyrate beta-oxidation should be rejected. Butyrate seems to be anaerobically decarboxylated, but propionate thus formed inside the biofilm is degraded as soon as its formation proceeds. It was therefore considered that butyrate degradation produces, through propionate intermediate, 1 mole acetate per mole butyrate removed. When propionate or butyrate concentrations were high, the same phenomenon was noted, to a much lower extent, for the degradation of acetate formed inside the biofilm.