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

Composting of dewatered wastewater sludge with various ratios of pruning waste used as a bulking agent and monitored by respirometer

The effects of different volumetric ratios of wastewater sludge to bulking agent on the performance of full-scale composting were studied. Volumetric ratios of wastewater sludge to pruning waste, used as a bulking agent, were 1:2 (Pile 1), 1:2.5 (Pile 2) and 1:3 (Pile 3). Experiments were carried out in an uncovered plant using windrow composting with weekly turning. To monitor the evolution of the three composting piles, routine parameters such as temperature and interstitial oxygen level, chemical parameters such as organic matter, moisture and C/N ratio, and biologically related indices such as respiration indices at process temperature (RIprocess) and at 37 °C (RI₃₇) were monitored. Different responses were observed in the three piles; Pile 1 did not accomplish the necessary requirements in terms of sanitation and RIprocess for a typical composting process; Piles 2 and 3 presented a similar behaviour, reaching thermophilic temperatures for a long period and, due to their high biological activity, high RIprocess. The quality of the product obtained in the three piles in terms of stability (RI₃₇ and the Rottegrade self-heating test) and maturity (germination index) were measured, with compost from Pile 3 the most stable. To achieve satisfactory stability and sanitation for application to land, optimisation of the sludge to bulking agent ratio used to process wastewater sludge into compost appears to be crucial

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

Composting of dewatered wastewater sludge with various ratios of pruning waste used as a bulking agent and monitored by respirometer

The effects of different volumetric ratios of wastewater sludge to bulking agent on the performance of full-scale composting were studied. Volumetric ratios of wastewater sludge to pruning waste, used as a bulking agent, were 1:2 (Pile 1), 1:2.5 (Pile 2) and 1:3 (Pile 3). Experiments were carried out in an uncovered plant using windrow composting with weekly turning. To monitor the evolution of the three composting piles, routine parameters such as temperature and interstitial oxygen level, chemical parameters such as organic matter, moisture and C/N ratio, and biologically related indices such as respiration indices at process temperature (RIprocess) and at 37 °C (RI₃₇) were monitored. Different responses were observed in the three piles; Pile 1 did not accomplish the necessary requirements in terms of sanitation and RIprocess for a typical composting process; Piles 2 and 3 presented a similar behaviour, reaching thermophilic temperatures for a long period and, due to their high biological activity, high RIprocess. The quality of the product obtained in the three piles in terms of stability (RI₃₇ and the Rottegrade self-heating test) and maturity (germination index) were measured, with compost from Pile 3 the most stable. To achieve satisfactory stability and sanitation for application to land, optimisation of the sludge to bulking agent ratio used to process wastewater sludge into compost appears to be crucial

Optimization of the hydrolytic-acidogenic anaerobic digestion stage (55ºC) 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 (40-50 g L⁻¹) and volatile solids (25-30 g L⁻¹), named high concentration sludge. The variables considered for this first stage were: hydraulic retention time (1-4 days) and temperature (55 and 65 °C). Maximum volatile fatty acids generation was obtained at 4 days and 3 days hydraulic retention time for 55 °C and 65 °C, respectively. Consequently, 4 days hydraulic retention time and temperature of 55 °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 17-28 g L⁻¹ total solids and 13-21 g L⁻¹ volatile solids. The effect of decreasing the influent sludge pH, when working at the optimal conditions established, was also evaluated

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

Increasing biogas production by thermal (70ºC) sludge pre-treatment prior to thermophilic anaerobic digestion

The objective of this work was to investigate the effect of a low temperature pre-treatment (70 °C) on the efficiency of thermophilic anaerobic digestion of primary and secondary waste sludge. Firstly, effect of sludge pre-treatment time (9, 24, 48 and 72 h) was evaluated by the increase in volatile dissolved solids (VDSs), volatile fatty acids (VFAs) and biogas production in thermophilic batch tests. Secondly, semi-continuous process performance was studied in a lab-scale reactor (5 L) working at 55 °C and 10 days solid retention time. The 70 °C pre-treatment showed an initial solubilization effect (increasing VDS by almost 10 times after 9 h), followed by a progressive generation of VFA (from 0 to nearly 5 g L⁻¹ after 72 h). Biogas production increased up to 30% both in batch tests and in semi-continuous experiments. Our results suggest that a short period (9 h) low temperature pre-treatment should be enough to enhance methane production through thermophilic anaerobic digestion of sludge

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

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 (BOC AE [subíndex] and BOC AN [subíndex]) 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 BOC AN [subíndex]. 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 BOC AE [subíndex] 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

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

Categorizing raw organic material biodegradability via respiration activity measurement : a review

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