Giuliana D'Imporzano

Substituting energy crops with organic wastes and agro-industrial residues for biogas production.

In this study, industrial and agro-industrial by-products and residues (BRs), animal manures (AMs), and various types of organic wastes (OWs) were analyzed to evaluate their suitability as substitutes for energy crops (ECs) in biogas production. A comparison between the costs of the volume of biogas that can be produced from each substrate was presented with respect to the prices of the substrates in the Italian market. Furthermore, four different feeding mixtures were compared with a mixture of EC and swine manure (Mixture A) used in a full-scale plant in Italy. Swine manure is always included as a basic substrate in the feeding mixtures, because many of the Italian biogas plants are connected to farms. When EC were partially substituted with BR (Mixture B), the cost (0.28 euro Nm(-3)) of the volume of biogas of Mixture A dropped to 0.18 euro Nm(-3). Furthermore, when the organic fraction of municipal solid waste (OFMSW) and olive oil sludge (OS) were used as possible solutions (Mixtures C and D), the costs of the volume of biogas were -0.20 and 0.11euroNm(-3), respectively. The negative price signifies that operators earn money for treating the waste. For the fifth mix (Mixture E) of the OFMSW with a high solid substrate, such as glycerin from biodiesel production, the resulting cost of the volume of biogas produced was -0.09 euro Nm(-3). By comparing these figures, it is evident that the biogas plants at farm level are good candidates for treating organic residues of both municipalities and the agro-industrial sector in a cost-effective way, and in providing territorially diffused electric and thermal power. This may represent a potential development for agrarian economy.

Evaluating inhibition conditions in high-solids anaerobic digestion of organic fraction of municipal solid waste

High-solids anaerobic digestion (HSAD) processes, when applied to different types of organic fractions of municipal solid waste (OFMSW), may easily be subjected to inhibition due to organic overloading. In this study, a new approach for predicting these phenomena was proposed based on the estimation of the putrescibility (oxygen consumption in 20 h biodegradation, OD(20)) of the organic mixtures undergoing the HSAD process. Different wastes exhibiting different putrescibility were subjected to lab-scale batch-HSAD. Measuring the organic loading (OL) as volatile solids (VS) was found unsuitable for predicting overload inhibition, because similar VS contents corresponded to both inhibited and successful trials. Instead, the OL calculated as OD(20) was a very good indicator of the inhibiting conditions (inhibition started for OD(20)>17-18 g O(2)kg(-1)). This new method of predicting inhibition in the HSAD process of diverse OFMSW may be useful for developing a correct approach to the technology in very different contexts.

Nanoscale structure of the cell wall protecting cellulose from enzyme attack.

The cell wall structure protects cellulose from enzymatic attack and its successive fermentation. The nature of this protection consists in the very complex macroscopic and microscopic structure of cell wall that limits transport. Explaining this kind of protection is critical in future research to improve cell polymer availability for enzymatic attack. This research shows that the complete description of the cell wall topography at a nanoscale level allows a mechanistic understanding of cellulose protection. For this purpose, we used gas adsorption methods (CO(2) at 273 K and N(2) at 77 K) to detect mesoporosity (pore size of 1.5-30 nm diameter; MeS) and microporosity (pore size of 0.3-1.5 nm diameter; MiS) of the cell wall of five energy crops, i.e., giant cane, rivet wheat straw, miscanthus, proso millet, and sorghum. The presence of both hemicelluloses in the spaces between cellulose fibrils and the unhydrolyzable and highly cross-linked lignocarbohydrate complex (LCC) determines a microporous (80% pores having diameters below 0.8 nm) structure of the cell wall that prevents the cellulase enzymes from coming into direct contact with the cellulose, as their sizes exceed the cell wall pore size. On the other hand, the removal of the hemicelluloses and of the LCC complex determines a reduction of the MiS and an increase of the available surface for enzymatic attack, i.e., pores >5 nm diameter. This was confirmed by the good negative (r = -0.87, P < 0.001, n = 11) and positive (r = 0.78, P < 0.005, n = 11) correlations found for microporosity and mesoporosity (pores of diameters >5 nm), respectively, vs the glucose production, by cellulase enzyme attack in specific enzymatic hydrolysis tests performed on biomass samples.

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.

Biological compost stability influences odor molecules production measured by electronic nose during food-waste high-rate composting

Composting is a technique that is used to convert organic waste into agriculturally useful products. Composting is an aerobic, solid-state biological process, which typically can be divided into two phases, a high-rate composting phase and a curing phase. High-rate composting plays an important role during the composting process, owing to the high microbial activity occurring during this phase. It requires an accurate plant design to prevent the formation of anaerobic conditions and odors. The formation of anaerobic conditions mainly depends on the rate of O(2) consumption needed to degrade the substrate, i.e., the biological stability of the substrate. In this study, we investigated the relationship between the biological activity, measured by the dynamic respiration index (DRI) and the odor molecules production, measured by an electronic nose (EN) during two food-waste high-rate composting processes. Although the O(2) concentration in the biomass free air space (FAS) was kept optimal (O(2)>140 ml l(-1), v/v) during composting, strong anaerobic conditions developed. This was indicated by the high levels of sulfur compounds, methane, and hydrogen in the outlet air stream. Both the high level of O(2) consumption, needed to degrade the high-degradable water-soluble organic matter and the low water O(2) solubility, caused by high temperature reached in this stage (up to 60 degrees C), led to the anaerobic conditions observed in the biofilm-particle level. The application of the partial least square (PLS) analysis demonstrated a good regression between the DRI and the odor molecules produced that was detected by the EN (R(2)=0.991; R(2)(CV)=0.990), signifying the usefulness of the DRI as a parameter to estimate the potential production of odor molecules of the biomass.

Biogas from dedicated energy crops in Northern Italy: electric energy generation costs

Agricultural anaerobic digestion facilities are increasing in many EU member States and biomass supply is sometimes an issue. Dedicated energy crops (DEC) (mainly Maize, Triticale and Sorghum) are often used to integrate other substrates, such as agricultural residues, manure and organic waste. However, DEC production includes onerous agricultural operations (soil preparation, harvest, transport and storage) and may result in high unit costs (UC) of electric energy (EE, € kWhe−1), compared to other renewable sources. In this work, seven different types of DEC (4 different combinations of crop successions) were cultivated in 30 different parcels, distributed along the Po Valley (northern Italy), using different varieties of seeds for each crop type. All agricultural operations were accounted for their costs (988–3346 € ha−1). Biomass production was measured and reported as average of different parcels for each type of crop (31.2–187 Mg ha−1). Biomass dry matter content and biogas potential were measured on representative samples and the EE obtainable was calculated (7.9–35.3 MWhe ha−1), by assuming conservative factors (CH4 contents in biogas and electric generation yields). The costs of ensiled biomass sensibly varied (13.8–40 € Mg−1) among crop solutions, as well as the same UC of EE (0.068–0.150 € kWhe−1). These costs were considered together with typical plant management and investment costs (plant size: 0.5–1 MWe): total UC of EE generation through anaerobic digestion (considering 100% DEC) varied in a relatively wide range (0.143–0.279 € kWhe−1). When the biomass mix is ‘blended’ with low‐cost residues or organic waste, this range could be lowered to 0.096–187 € kWhe−1. Only this strategy and strong efforts in reducing technological investment/management costs can candidate biogas‐based EE as a really competitive renewable alternative to traditional sources, in the next future.

Sanitation ability of anaerobic digestion performed at different temperature on sewage sludge.

A small amount of ammonia is used in full-scale plants to partially sanitize sewage sludge, thereby allowing successive biological processes to enable the high biological stability of the organic matter. Nevertheless, ammonia and methane are both produced during the anaerobic digestion (AD) of sludge. This paper describes the evaluation of a lab-scale study on the ability of anaerobic process to sanitize sewage sludge and produce biogas, thus avoiding the addition of ammonia to sanitize sludge. According to both previous work and a state of the art full-scale plant, ammonia was added to a mixture of sewage sludge at a rate so that the pH values after stirring were 8.5, 9 and 9.5. This procedure determined an ammonia addition lower than that generally indicated in the literature. The same sludge was also subjected to an AD process for 60 days under psychrophilic, mesophilic and thermophilic conditions. The levels of fecal coliform, Salmonella spp. helmints ova, pH, total N, ammonia fractions and biogas production were measured at different times during each process. The results obtained suggested that sludge sanitation can be achieved using an AD process; however, the addition of a small amount of ammonia was not effective in sludge sanitation because the buffer ability of the sludge reduced the pH and thus caused ammonia toxicity. Mesophilic and thermophilic AD sanitized better than psychrophilic AD did, but the total free ammonia concentration under the thermophilic condition inhibited biogas production. The mesophilic condition, however, allowed for both sludge sanitation and significant biogas production.

Anaerobic digestion of straw and corn stover: The effect of biological process optimization and pre-treatment on total bio-methane yield and energy performance

Anaerobic digestion (AD) is a useful method for producing renewable energy/biofuel. Today, biogas production uses a large amount of energy crops (EC), with the effect of increasing AD costs and creating conflict between food/feed vs. energy use. A partial solution to this might be the substitution of EC with agricultural wastes, e.g. straw. Straw and corn stover are widely available in the world and approximately 1600millionMgyear-1 of these substrates are available. Straw can be useful used for biogas production but its characteristics limit its performance so that sometimes the energetic balance can be negative. In this review, the limits for the conversion of this substrate into biogas were investigated and solutions/proposals for getting higher straw biogas production performance are reported. In addition, energetic balances for untreated and pre-treated substrates are reported, giving indicative evaluations of the sustainability of straw and corn stover use for biogas production.

Solid and liquid fractionation of digestate: Mass balance, chemical characterization, and agronomic and environmental value

Solid-liquid (S/L) separation of digestate (D) represents a simple technology able to produce two fractions having different composition. The aim of this work was to study the effect of S/L separation on dry matter (DM), nitrogen (TKN), phosphorus (P2O5) and heavy metals (HM) repartition into these two fractions and to characterize them. Therefore, thirteen full-scale digestion plants were studied and D, LF and SF were collected during three seasons of the year. Results obtained indicated that unexpectedly, on a mass balance, the liquid fraction still contains the majority of DM, i.e. 67% of the total of D. LF also contained 87% and 71% of TKN and P2O5 respectively. HM contents were in line with typical NP-organic fertilizers. Chemical characterization suggested that the LF can be used as a substitute for mineral N fertilizers because of its high N content, while SF can be proposed as an NP-organic fertilizer.

Mixotrophic cultivation of Chlorella for local protein production using agro-food by-products.

A local strain of Chlorella vulgaris was cultivated by using cheese whey (CW), white wine lees (WL) and glycerol (Gly), coming from local agro-industrial activities, as C sources (2.2gCL-1) to support algae production under mixotrophic conditions in Lombardy. In continuous mode, Chlorella increased biomass production compared with autotrophic conditions by 1.5-2 times, with the best results obtained for the CW substrate, i.e. 0.52gL-1d-1 of algal biomass vs. 0.24gL-1d-1 of algal biomass for autotrophic conditions, and protein content for both conditions adopted close to 500gkg-1 DM. Mixotrophic conditions gave a much higher energy recovery efficiency (EF) than autotrophic conditions, i.e. organic carbon energy efficiency (EFoc) of 32% and total energy efficiency (Eft) of 8%, respectively, suggesting the potential for the culture of algae as a sustainable practice to recover efficiently waste-C and a means of local protein production.