Barbara Scaglia

Assessing amendment and fertilizing properties of digestates from anaerobic digestion through a comparative study with digested sludge and compost

Digestate, with biogas represents the final products of anaerobic digestion (AD). The methane-rich biogas is used to produce electricity and heat, whereas the digestate could be valorized in agriculture. Contrarily to well-recognized biomasses such as digested sludge and compost, the properties of the digestate are not well known and its agricultural use remains unexplored. In this work, a first attempt to study the agronomic properties of digestates was performed by comparing the chemical, spectroscopic, and biological characteristics of digestates with those of compost and digested sludge, used as reference organic matrices. A total of 23 organic matrices were studied, which include eight ingestates and relative digestates, three composts, and four digested sludges. The analytical data obtained was analyzed using principal component analysis to better show in detail similarities or differences between the organic matrices studied. The results showed that digestates differed from ingestates and also from compost, although the starting organic mix influenced the digestate final characteristics. With respect to amendment properties, it seems that biological parameters, more than chemical characteristics, were more important in describing these features. In this way, amendment properties could be ranked as follows: compost≅digestate>digested sludge≫ingestate. As to fertilizer properties, AD allowed getting a final product (digestate) with very good fertilizing properties because of the high nutrient content (N, P, K) in available form. In this way, the digestate appears to be a very good candidate to replace inorganic fertilizers, also contributing, to the short-term soil organic matter turnover.

Odours and volatile organic compounds emitted from municipal solid waste at different stage of decomposition and relationship with biological stability

Odours (OU(E)) and volatile organic compounds (VOC) emission during biological process used to treat MSW were studied under standardized conditions in order to detect potential risk for workers and population. Results obtained indicated that odours and VOCs emitted depend on the biological stability of waste measured by the dynamic respiration index (DRI) and a very good correlation were found between these parameters (OU(E) vs. DRI, r=0.96, p<0.001, n=6; VOC vs. DRI, r=0.97, p<0.001, n=6). GC-MS study of the VOCs indicated the presence of a group of molecules that were degraded during the process. On the other hand, a second group of molecules, i.e. aromatic and halogenated compounds, and furan persisted in the waste sample, although molecule concentrations were always lower than Threshold Limit Value-Time Weighted Average (TLV-TWA).

Two-Stage vs Single-Stage Thermophilic Anaerobic Digestion: Comparison of Energy Production and Biodegradation Efficiencies

Two-stage anaerobic digestion (AD) for integrated biohydrogen and biomethane production from organic materials has been reported to promise higher process efficiency and energy recoveries as compared to traditional one-stage AD. This work presents a comparison between two-stage (reactors R1 and R2) and one-stage (reactor R3) AD systems, fed with identical organic substrates and loading rates, focusing the attention on chemical and microbiological aspects. Contrary to previous experiences, no significant differences in overall energy recovery were found for the two-stage and one-stage AD systems. However, an accumulation in R2 of undegraded intermediate metabolites (volatile fatty acids, ketones, amines, amino acids, and phenols) was observed by GC-MS. These compounds were thought to be both cause and effect of this partial inefficiency of the two-stage system, as confirmed also by the less diverse, and thereby less efficient, population of fermentative bacteria observed (by PCR-DGGE) in R2. The extreme environment of R1 (low pH and high metabolites concentrations) probably acted as selector of metabolic pathways, favoring H(2)-producing bacteria able to degrade such a wide variability of intermediate metabolites while limiting other strains. Therefore, if two-stage AD may potentially lead to higher energy recoveries, further efforts should be directed to ensure process efficiency and stability.

Respiration Index Determination: Dynamic And Static Approaches

Forty-five organic matrices of different types and origin were subjected to the test for the determination of biological stability by means of dynamic respirometric measurement (DRI) and static respirometric measurement (SRI). The results obtained from the comparison between the two indexes indicated that the SRI, compared with the DRI, underestimates the consumption of oxygen by the biomasses and consequently their rate of biological stability by a factor equal to 2. From the analysis of the data it is possible to define the biological stability as DRI<1000 mgO2kg−1VSh−1. The DRI data were correlated with the temperatures of the biomasses showing a linear correlation (R2=0.81, p<0.01) only for stable matrices while for nonstable matrices no significant correlation is found.

Estimating biogas production of biologically treated municipal solid waste

In this work, a respirometric approach, i.e., Dynamic Respiration Index (DRI), was used to predict the anaerobic biogas potential (ABP), studying 46 waste samples coming directly from MBT full-scale plants. A significant linear regression model was obtained by a jackknife approach: ABP=(34.4+/-2.5)+(0.109+/-0.003).DRI. The comparison of the model of this work with those of the previous works using a different respirometric approach (Sapromat-AT(4)), allowed obtaining similar results and carrying out direct comparison of different limits to accept treated waste in landfill, proposed in the literature. The results indicated that on an average, MBT treatment allowed 56% of ABP reduction after 4weeks of treatment, and 79% reduction after 12weeks of treatment. The obtainment of another regression model allowed transforming Sapromat-AT(4) limit in DRI units, and achieving a description of the kinetics of DRI and the corresponding ABP reductions vs. MBT treatment-time.

Prediction of biogas potentials using quick laboratory analyses: Upgrading previous models for application to heterogeneous organic matrices

This study presents an upgrading of the mathematical models to predict anaerobic biogasification potential (ABP) through quick laboratory analyses that have been presented in an earlier study. The aim is to widen the applicability of the models to heterogeneous organic substrates and to improve their reliability through a deeper statistical approach. Three multiple-step linear regressions were obtained using biomass oxygen demand in 20 h (OD(20)) plus the volatile solids content (VS) of 23 new samples of heterogeneous organic matrices, of 46 samples presented in the earlier work and of the data set comprising all the 69 samples. The two variables chosen were found to be suitable for very heterogeneous materials. To judge the prediction quality, a validation procedure was performed with 12 new samples using model efficiency indexes. The proposed model had good prediction ability for a large variety of organic substrates, and allows the calculation of the ABP value within only 2-days laboratory work instead of the 60-90 days required to obtain ABP by anaerobic test.

Effects of biodrying process on municipal solid waste properties

In this paper, the effect of biodrying process on municipal solid waste (MSW) properties was studied. The results obtained indicated that after 14d, biodrying reduced the water content of waste, allowing the production of biodried waste with a net heating value (NHV) of 16,779±2,074kJ kg(-1) wet weight, i.e. 41% higher than that of untreated waste. The low moisture content of the biodried material reduced, also, the potential impacts of the waste, i.e. potential self-ignition and potential odors production. Low waste impacts suggest to landfill the biodried material obtaining energy via biogas production by waste re-moistening, i.e. bioreactor. Nevertheless, results of this work indicate that biodrying process because of the partial degradation of the organic fraction contained in the waste (losses of 290g kg(-1) VS), reduced of about 28% the total producible biogas.

Microbial community structure and dynamics in two-stage vs single-stage thermophilic anaerobic digestion of mixed swine slurry and market bio-waste.

The microbial community of a thermophilic two-stage process was monitored during two-months operation and compared to a conventional single-stage process. Qualitative and quantitative microbial dynamics were analysed by Denaturing Gradient Gel Electrophoresis (DGGE) and real-time PCR techniques, respectively. The bacterial community was dominated by heat-shock resistant, spore-forming clostridia in the two-stage process, whereas a more diverse and dynamic community (Firmicutes, Bacteroidetes, Synergistes) was observed in the single-stage process. A significant evolution of bacterial community occurred over time in the acidogenic phase of the two-phase process with the selection of few dominant species associated to stable hydrogen production. The archaeal community, dominated by the acetoclastic Methanosarcinales in both methanogen reactors, showed a significant diversity change in the single-stage process after a period of adaptation to the feeding conditions, compared to a constant stability in the methanogenic reactor of the two-stage process. The more diverse and dynamic bacterial and archaeal community of single-stage process compared to the two-stage process accounted for the best degradation activity, and consequently the best performance, in this reactor. The microbiological perspective proved a useful tool for a better understanding and comparison of anaerobic digestion processes.

An index for quantifying the aerobic reactivity of municipal solid wastes and derived waste products.

The organic matter contained in municipal solid waste (MSW) and in the MSW fractions obtained by mechanical separation has strong environmental impact when the waste is used as landfill. This is partly due to the biological activity that occurs under anaerobic conditions. Negative effects on the environment include unpleasant odors, biogas, leachate and biomass self-heating. Measuring the biological reactivity of waste with the help of indicators is an important tool to prevent waste impact. The aim of this study was to develop an index capable of describing the aerobic reactivity of waste, using both biological and chemical indicators. To develop this index, 71 MSW and MSW-product samples, including biologically treated MSW and mechanically separated MSW fractions, were analyzed. Fifty of the 71 samples analyzed represented MSWs and their derived products collected from a number of Italian waste plants and sites. The remaining 21 were MSW samples collected at different times during 8 different full-scale aerobic biological processes in four treatment plants used to reduce the biological reactivity of wastes. Five of these processes used the entire (unsorted) MSW, while the remaining three used the organic fraction of the MSW obtained by mechanical pre-treatment (waste sieving). Respirometric activity (Dynamic Respiration Index, DRI) and eluates characterization (chemical oxygen demand--COD, and 5 days biological oxygen demand--BOD5) were used as indicators of waste strength, as they had previously been reported to be indirect measures of waste impact on landfill. Summarizing all studied indicators, Principal Component Analysis (PCA) was used to develop the Putrescibility Index (Ip). The results revealed Ip index of 204+/-33 (mean+/-standard deviation) and 159+/-14 for the organic fraction of MSW and MSW untreated waste respectively, and of 106+/-16 and 101+/-22 for the corresponding biologically treated waste.

Evolution of organic matter in a full-scale composting plant for the treatment of sewage sludge and biowaste by respiration techniques and pyrolysis-GC/MS.

A full-scale composting plant treating in two parallel lines sewage sludge and the source-selected organic fraction of municipal solid waste (OFMSW or biowaste) has been completely monitored. Chemical routine analysis proved not to be suitable for an adequate plant monitoring in terms of stabilization and characterization of the process and final compost properties. However, the dynamic respiration index demonstrated to be the most feasible tool to determine the progression of the degradation and stabilization of organic matter for both sewage sludge and OFMSW lines. Both lines exhibited an important degree of stabilization of organic matter using rapid and cumulative respiration indices. Pyrolysis-GC/MS was applied to the most important inputs, outputs, and intermediate points of the plant. It proved to be a powerful tool for the qualitative characterization of molecular composition of organic matter present in solid samples. A full characterization of the samples considered is also presented.