Sergio Ponsá

In search of a reliable technique for the determination of the biological stability of the organic matter in the mechanical-biological treated waste

The biological stability determines the extent to which readily biodegradable organic matter has decomposed. In this work, a massive estimation of indices suitable for the measurement of biological stability of the organic matter content in solid waste samples has been carried out. Samples from different stages in a mechanical-biological treatment (MBT) plant treating municipal solid wastes (MSW) were selected as examples of different stages of organic matter stability in waste biological treatment. Aerobic indices based on respiration techniques properly reflected the process of organic matter biodegradation. Static and dynamic respirometry showed similar values in terms of aerobic biological activity (expressed as oxygen uptake rate, OUR), whereas cumulative oxygen consumption was a reliable method to express the biological stability of organic matter in solid samples. Methods based on OUR and cumulative oxygen consumption were positively correlated. Anaerobic methods based on biogas production (BP) tests also reflected well the degree of biological stability, although significant differences were found in solid and liquid BP assays. A significant correlation was found between cumulative oxygen consumption and ultimate biogas production. The results obtained in this study can be a basis for the quantitative measurement of the efficiency in the stabilization of organic matter in waste treatment plants, including MBT plants, anaerobic digestion of MSW and composting plants.

Comparison of aerobic and anaerobic stability indices through a MSW biological treatment process

A complex mechanical-biological waste treatment plant designed for the processing of mixed municipal solid wastes (MSW) and source-selected organic fraction of municipal solid wastes (OFMSW) has been studied by using stability indices related to aerobic (respiration index, RI) and anaerobic conditions (biochemical methane potential, BMP). Several selected stages of the plant have been characterized: waste inputs, mechanically treated wastes, anaerobically digested materials and composted wastes, according to the treatment sequence used in the plant. Results obtained showed that the main stages responsible for waste stabilization were the two first stages: mechanical separation and anaerobic digestion with a diminution of both RI and BMP around 40% and 60%, respectively, whereas the third stage, composting of digested materials, produced lesser biological degradation (20-30%). The results related to waste stabilization were similar in both lines (MSW and OFMSW), although the indices obtained for MSW were significantly lower than those obtained for OFMSW, which demonstrated a high biodegradability of OFMSW. The methodology proposed can be used for the characterization of organic wastes and the determination of the efficiency of operation units used in mechanical-biological waste treatment plants.

Different indices to express biodegradability in organic solid wastes.

Respiration indices are suggested in literature as the most suitable stability determination and are proposed as a biodegradability measure in this work. An improved dynamic respiration index methodology is described in this work. This methodology was applied to 58 samples of different types of waste including municipal solid wastes and wastewater sludge, both raw materials and samples collected in a mechanical-biological treatment plant at different stages of biodegradation. The information obtained allowed to establish a qualitative classification of wastes in three categories: highly biodegradable, moderately biodegradable, and wastes of low biodegradability. Results were analyzed in terms of long and short-term indices and index expression: dynamic respiration indices expressed as average oxygen uptake rate (mg O(2) g(-1) dry matter [DM] h(-1)) at 1 and 24 h of maximum activity (DRI(1h), DRI(24h)); and cumulative oxygen consumption in 24 h of maximum activity and 4 d (AT(24h), AT(4)). The statistical comparison of indices and wastes is also presented. Raw sludge presented the highest biodegradability followed by the organic fraction of municipal solid waste and anaerobically digested sludge. All indices correlated well but different correlations were found for the different wastes analyzed. The information in the dynamic respiration profile allows for the calculation of different indices that provide complementary information. The combined analysis of DRI(24h) and AT(4) is presented here as the best tool for biodegradable organic matter content characterization and process requirements estimation.

Optimization of the hydrolytic-acidogenic anaerobic digestion stage (55 °C) of sewage sludge : Influence of pH and solid content

In conventional single-stage anaerobic digestion processes, hydrolysis is regarded as the rate-limiting step in the degradation of complex organic compounds, such as sewage sludge. Two-stage systems have been proposed to enhance this process. However, so far it is not clear which are the best conditions for a two-stage anaerobic digestion process of sewage sludge, in terms of temperature and hydraulic retention time of each stage. The aim of this work was to determine the optimal conditions for the hydrolytic-acidogenic stage treating real sludge with a high concentration of total solids (40-50gL(-1)) and volatile solids (25-30gL(-1)), named high concentration sludge. The variables considered for this first stage were: hydraulic retention time (1-4 days) and temperature (55 and 65 degrees C). Maximum volatile fatty acids generation was obtained at 4 days and 3 days hydraulic retention time for 55 degrees C and 65 degrees C, respectively. Consequently, 4 days hydraulic retention time and temperature of 55 degrees C were set as the working conditions for the hydrolytic-acidogenic stage treating high concentration sludge. The results obtained when operating with high concentration sludge were compared with a low concentration sludge consisting of 17-28gL(-1) total solids and 13-21gL(-1) volatile solids. The effect of decreasing the influent sludge pH, when working at the optimal conditions established, was also evaluated.

Saving freshwater from salts

Ion-specific standards are needed to protect biodiversity Many human activities—like agriculture and resource extraction—are increasing the total concentration of dissolved inorganic salts (i.e., salinity) in freshwaters. Increasing salinity can have adverse effects on human health (1); increase the costs of water treatment for human consumption; and damage infrastructure [e.g., amounting to

Determining C/N ratios for typical organic wastes using biodegradable fractions

700 million per year in the Border Rivers catchment, Australia (2)]. It can also reduce freshwater biodiversity (3); alter ecosystem functions (4); and affect economic well-being by altering ecosystem goods and services (e.g., fisheries collapse). Yet water-quality legislation and regulations that target salinity typically focus on drinking water and irrigation water, which does not automatically protect biodiversity. For example, specific electrical conductivities (a proxy for salinity) of 2 mS/cm can be acceptable for drinking and irrigation but could extirpate many freshwater insect species (3). We argue that salinity standards for specific ions and ion mixtures, not just for total salinity, should be developed and legally enforced to protect freshwater life and ecosystem services. We identify barriers to setting such standards and recommend management guidelines.

From Wastes to High Value Added Products: Novel Aspects of SSF in the Production of Enzymes

It is well established that an optimal aerobic and anaerobic microbial metabolism is achieved with a C/N ratio between 20 and 30. Most studies are currently based on chemically-measured carbon and nitrogen contents. However, some organic wastes can be composed of recalcitrant carbon fractions that are not bioavailable. To know the biodegradable C/N ratio, two different methods to determine the aerobic and anaerobic biodegradable organic carbon (BOCAE and BOCAN) are proposed and used to analyze a wide variety of different organic samples. In general, raw wastes and digested products have more amount of BOCAE. On the contrast, the samples collected after an aerobic treatment have higher content of BOCAN. In any case, all the BOC fractions are lower than the total organic carbon (TOC). Therefore, the C/N ratios based on BOC are always lower than the total C/N ratio based on the TOC measure. The knowledge of the real bioavailable C/N ratio is crucial for the biological treatments of organic materials. To reduce the test time necessary for BOC determination, the values of BOC for all the samples obtained at different times were compared and correlated with the final BOC. A method that allows for the determination of BOCAE in 4 d is proposed. In relation to the anaerobic assay, the biogas potential calculated after 21 and 50 d was positively correlated with the final potential defined after 100 d of assay.

The effect of storage and mechanical pretreatment on the biological stability of municipal solid wastes

Solid-state fermentation (SSF), a process that occurs in the absence or near absence of water, has been used for the production of various high value added products such as enzymes and other organic components. This paper reviews the recent studies reported on the use of SSF for the production of enzymes: lipases, proteases, cellulases, hemicellulases, ligninases, glucoamylases, pectinases, and inulinases. The microorganisms used for fermentation are mostly fungi, and substrates are waste materials from the agriculture and food industry. This shows the advantages of SSF from an economical and environmental viewpoint. The paper provides an update on several issues, viz. wastes, microorganisms, and issues of scaling up and controlling the process of fermentation in solid state.

Categorizing Raw Organic Material Biodegradability Via Respiration Activity Measurement: A Review

Modern mechanical-biological waste treatment plants for the stabilization of both the source-separated organic fraction of municipal solid wastes (OFMSW) and the mixed stream of municipal solid wastes (MSW) include a mechanical pretreatment step to separate recyclable materials such as plastics, glass or metals, before biological treatment of the resulting organic material. In this work, the role of storage and mechanical pretreatment steps in the stabilization of organic matter has been studied by means of respiration techniques. Results have shown that a progressive stabilization of organic matter occurs during the pretreatment of the source-separated OFMSW, which is approximately 30% measured by the dynamic respiration index. In the case of mixed MSW, the stabilization occurring during the reception and storage of MSW is compensated by the effect of concentration of organic matter that the pretreatment step provokes on this material. Both results are crucial for the operation of the succeeding biological process. Finally, respiration indices have been shown to be suitable for the monitoring of the pretreatment steps in mechanical-biological waste treatment plants, with a strong positive correlation between the dynamic respiration index and the cumulative respiration index across all samples tested.

Modelling the aerobic degradation of organic wastes based on slowly and rapidly degradable fractions

A massive characterization in terms of respiration activity for the most common types of organic solid wastes is presented in this compilation. Respiration activity for a solid waste is a crucial parameter to understand the behaviour of the waste in the environment and for waste management aspects such as the definition of a suitable biological treatment and the determination of the potential rate of microbial self-heating if organic wastes are to be used as solid recovered fuels. The respiration data compiled in this work are the result of five years of research focused on the determination of the biological activity of organic wastes. A compilation of respiration data found in the literature is also presented. The main groups of organic wastes analyzed are: municipal solid wastes (including mixed wastes and source-selected organic fraction), wastewater sludge (including digested and nondigested sludge from primary and secondary operations in municipal and industrial wastewater treatment plants), different types of manure (of different origin), other particular wastes (animal by-products, hair waste, fats, etc.) and some mixtures of different wastes. Results suggest that respiration activity can be used to classify the biodegradability of organic wastes into three main categories: i) highly biodegradable wastes (respiration activity higher than 5 mg O2 g Organic Matter−1 h−1), which includes source-selected organic fraction of municipal solid waste, nondigested municipal wastewater sludge and animal by-products; ii) moderately biodegradable wastes (respiration activity within 2 to 5 mg O2 g Organic Matter−1 h−1), including mixed municipal solid waste, digested municipal wastewater sludge and several types of manure; iii) wastes of low biodegradability (respiration activity lower than 2 mg O2 g Organic Matter−1 h−1), which includes few organic wastes such as some particular wastes from the food industry.