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

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

Fundamentals in selecting input and output variables for composting process automatic controllers

El títol del pre-print és Review of fundamentals in selecting input and output variables for composting process automatic controllersThis paper provides a critical analysis of the fundamental principles involved in the selection of input and output variables for automatic controllers of composting processes. Research results and technological advancements make available a number of parameters which may be used by a composting process controller. Parameters based on ventilation have been identified as the most appropriate controller output variables. On-line monitoring of odour generation potential and pathogen destruction has not become feasible, although recent advances indicate potential for electronic noses and biosensors. On-line measurement of reaction rate heavily depends on the suitability of microbial respirometric methods. Water content of the material being composted may be useful in on-line evaluation of reaction rate if relationships between water loss rate and respiration rate are adequately described. Developments in artificial intelligence offer, however, new avenues regarding real-time estimation of reaction rates. In general, the first experiences from application of artificial intelligence on composting process control indicate potential for substantial utility

Potential of the solid-state fermentation of soy fibre residues by native microbial populations for bench-scale alkaline protease production

The production of alkaline proteases by solid-state fermentation (SSF) was evaluated. The effect of three agro-industrial residues was examined: coffee husk, hair waste from the tanning industry and soy fibre residues. Soy fibre presented the highest yield for protease production at the laboratory scale (37 °C, 100 g samples). Consequently, experiments with soy fibre (F) and soy fibre with 10% compost as an inoculum (FC) were performed for 14 days in 4.5 L bench-scale aerobic near-adiabatic reactors. The highest activity occurred under thermophilic conditions and a high respiration activity of 47,331 ± 1391 U/g dry matter for F and 18,750 ± 1596 U/g dry matter for FC, which are much higher values than those reported in other studies. Alkaline proteases showed maximum stability at pH 11 and temperatures of 43.8 °C (F) and 30 °C (FC), which was determined by a full factorial experimental design. Storage assays demonstrated that 90% of enzyme activity was preserved for three months by lyophilising or freezing the samples at −80 °C

Short-time estimation of biogas and methane potentials from municipal solid wastes

Biogas (GB) and methane (BMP) potentials are important parameters for the energy potential of the anaerobic digestion of municipal solid wastes (MSW) and to design full-scale facilities. However, no standard protocol has been defined for this measure. Several samples of mixed MSW and the source-selected organic fraction of municipal solid waste (OFMSW) obtained at different stages of their mechanical-biological treatment were analyzed. GB and BMP values obtained at different times were correlated. Biogas potentials calculated at 3, 4, 5, 6, 7, 14, 21, 50 and 100 days correlated well for the OFMSW samples. In the case of the MSW samples, only GB values obtained at times of 14 or more days correlated well with the ultimate biogas production (considered at 100 days). The biogas potential analyzed at 21 days (as proposed in some standard methods) accounted for 77% of the total biogas potential in OFMSW samples and for 71% in the MSW samples. These results are useful for the correct design and operation of anaerobic digestion plants in terms of retention time estimation and expected biogas and methane production

Anaerobic co-digestion of the organic fraction of municipal solid waste with several pure organic co-substrates

A strategy to improve the operation of working anaerobic digesters treating the organic fraction of municipal solid wastes (OFMSW) to increase the biogas production is studied. It consists of increasing the organic loading rate of the digesters by adding extra organic matter from some problematic organic wastes. Vegetable oil (VO), animal fats (AF), cellulose and protein (protein) were used as pure co-substrates and the co-digestion anaerobic process was analysed in terms of the ultimate methane production, the methane production rate and the hydraulic residence time. The analysis of methane or biogas production led to different conclusions when expressing this parameter on a volatile solids basis or on a reactor volume basis. The need for a combined analysis is highlighted. In addition a new model to predict the biodegradability rate and evaluating the organic matter fraction susceptible to biodegradation was developed and proved to be suitable for assessing anaerobic digestion processes. All four co-substrates used led to some operative improvements. Vegetable oil is the most suitable co-substrate to be anaerobically digested with the OFMSW since all the parameters evaluated were greatly improved compared to the OFMSW digestion

Substitution of chemical dehairing by proteases from solid-state fermentation of hair wastes

The leather industry uses a chemical dehairing process that produces alkaline wastewater and hair as solid waste. A cleaner process is proposed in this work to reduce these environmental impacts. It consists of the valorization of hair waste together with sludge from wastewater treatment by a solid-state fermentation process to obtain proteases that can be used in an enzymatic dehairing process. SSF was undertaken in 4.5 L scale reactors. Neither sterilization of the materials nor inoculation of pure microorganisms was necessary for the development of the process, whereas aeration was provided during the assay to ensure the prevalence of aerobic conditions. Alkaline proteases were produced as a consequence of the degradation of hair. The highest activity of the alkaline protease in crude extracts was determined at 14 days of the process (56,270 ± 2632 activity units/g dry sample), after the thermophilic stage. The final organic matter showed a stability degree similar to that of compost in terms of respiration activity. Potential application of the extracted proteases in dehairing cow hides was successfully proved as an alternative to the chemical dehairing process. The results in the efficiency of the dehairing process were similar to those obtained with the chemical process, thus avoiding the need of chemical reagents and strong chemical conditions. The entire process permits the substitution of the chemical process of dehairing by an environmentally friendly enzymatic process, closing the organic matter cycle

A new control strategy for composting process based on the oxygen uptake rate

Up to now different control strategies to supply the oxygen requirements to the composting process have been studied. All of them seek for the biological activity optimization. In general, temperature and oxygen content are identified as the key parameters to assess the microbial activity. For this reason, the most favorable range of temperature and oxygen content for composting has frequently been studied and used as controllers' set points. On contrast, no previous works have studied the feasibility of oxygen supply according to the biological activity during the process, measured as oxygen uptake rate. In this field, a new automatic composting controller has been developed using the oxygen uptake rate measure as the measured variable. After setting up and to start-up this new technique, two pilot composting trials were undertaken with municipal solid waste. Oxygen, temperature and cyclic controllers were also implemented and tested in the composting of the same waste, in order to compare the results and to determine what the optimum system is. The four systems studied had similar temperature profiles, whereas except for the new controller, the oxygen content was constantly oscillating during most part of the process due to severe airflow changes. Through all the most relevant parameters determined, it can be observed that the new controller offers the most optimum system performance, since with low energy consumption a higher total oxygen uptake is achieved and, in consequence, the most stable end-product is obtained. Hence, the oxygen uptake rate controller is recommended for the airflow regulation in composting systems with automatic control

Residus més biodegradables

El tractament de residus sòlids orgànics per obtenir compost -esmena orgànica estable i aplicable al sòl- és un procés d'àmplia aplicació, però complex. Aconseguir que un residu sigui compostat amb èxit, dependrà de que aquest tingui certes característiques químiques i biològiques. Per això, quan aquestes no hi són, existeix la possibilitat de compensar-les mitjançant el cocompostatge. Aquest es basa en l'addició de cosubstrats al residu original, per tal que aquest superi les seves deficiències de partida i pugui ser tractat posteriorment. Seguint aquesta línia, s'han dut a terme una sèrie de proves en el laboratori afegint proteïna, greix i cel·lulosa per separat a fangs de depuradora d'aigües residuals urbanes. Els resultats han estat força positius, tant pel que fa al desenvolupament del procés, com per la conservació del nitrogen en el producte final. Els cosubstrats utilitzats es troben en un nombre important de residus orgànics, especialment aquells que provenen de la indústria agroalimentària. El seu tractament mitjançant compostatge obre una excel·lent via de valorització.El tratamiento de residuos sólidos orgánicos para obtener compuesto - enmienda orgánica estable y aplicable al suelo- es un proceso de amplia aplicación, pero complejo. Conseguir que un residuo sea compostado con éxito, dependerá de que éste tenga ciertas características químicas y biológicas. Por eso, cuando éstas no están, existe la posibilidad de compensarlas mediante el cocompostaje. Éste se basa en la adición de cosustratos en el residuo original, a fin de que éste supere sus deficiencias de partida y pueda ser tratado posteriormente. Siguiendo esta línea, se han llevado a cabo una serie de pruebas en el laboratorio añadiendo proteína, grasa y celulosa por separado a barros de depuradora de aguas residuales urbanas. Los resultados han sido bastante positivos, tanto con respecto al desarrollo del proceso, como por la conservación del nitrógeno en el producto final. Los cosustratos utilizados se encuentran en un número importante de residuos orgánicos, especialmente aquéllos que provienen de la industria agroalimentaria. Su tratamiento mediante compostaje abre una excelente vía de valorización.The treatment of organic solid waste to obtain compost - which can be used as soil conditioner or fertilizer - is a complex process. However, certain physico-chemical and biological characteristics are required in a waste to be successfully composted. When these characteristics are not present, there is the possibility of compensating them through cocomposting. This technique consists in adding a cosubstrate to the original waste to make up for its original deficiencies and facilitate its treatment by composting. Cocomposting was tested with a series of laboratory trials in which protein, fats and cellulose were added to sewage sludge separately. The results were quite positive not only due to enhancement of the composting process evolution but also for the increase in nitrogen conservation in the final product. The cosubstrates used can be found in different organic wastes specially those from the food industry. The treatment of these wastes though composting offers an excellent opportunity for their valorization

Factors affecting air pycnometer performance for its use in the composting process

Air filled porosity (AFP) is a crucial factor in composting to guarantee aerobic conditions inside the composting matrix. Among the different methods proposed to measure AFP in composting processes, air pycnometry is defined as the most adequate. There is a lack of a standard methodology for air pycnometry utilization for AFP determination in heterogeneous samples as those from composting materials. Air pycnometers currently used for this purpose are custom made instruments operating under different conditions (sample volume, initial pressure, etc.). All factors affecting air pycnometry accuracy in the composting process are related to the proper maintenance and handling of the air pycnometer and the composting sample. In this study, AFP measurements have been performed in more than 50 samples of a wide range of composting materials using two different custom made pycnometers, one of them coupled to a composting reactor allowing in situ AFP measurement. While temperature variation during AFP measurement has been discarded as an error source, the determination of the sample volume has been found to be a significant factor affecting the air filled porosity calculation. Regarding the initial pressure to use, a compromise between accuracy and practicality has to be established for each pycnometer design as AFP values obtained with diverse initial pressures (from 200 to 500 kPa gauge pressure) were found to present no statistical differences. An initial pressure in the range of 300-500 kPa (gauge pressure) is recommended. In conclusion, there is a need for a standard methodology for AFP determination or prediction at industrial scale. A complete procedure for air filled porosity determination by air pycnometry is also presented in this work

Biodegradation of animal fats in a co-composting process with wastewater sludge

A composting process was proposed as an effective technology for the biodegradation of fats in a proportion of 40-50%. Anaerobically digested sludge was used as co-substrate for animal fats to balance the C/N ratio of the composting mixture and to provide additional biodegradable organic matter and active biomass. Two different strategies were studied: static pile and dynamic turned pile. Air-filled porosity was initially adjusted to 40% for both experiments. It was observed that non-turned strategy increases the formation of material agglomerates which derived in a non-homogeneous fat distribution, the development of filamentous fungi, and a considerable increase in the amount of leachate generated. Turning the composting material resulted in the best results for composting fat-enriched wastes, preventing the formation of agglomerates. An effective biodegradation up to 92% of the fats was observed under these conditions. Besides, the addition of fats increased significantly the duration of the thermophilic period of the composting process