A modified dynamic respiration test to assess compost stability: effect of sample size and air flowrate

In-situ untilization of nitrogen-rich wastewater discharged from a biotrickling filter as a moisture conditioning agent for composting: Effect of nitrogen composition

Although the composting-biotrickling filter coupled system removed ammonia-based odor pollution, other pollutants (nitrogen-rich wastewater) arose. This study intended to determine the effect of in-situ disposal of different kinds of nitrogen-rich wastewater [i.e., multi-nitrogen (NH₄⁺, NO₂^-, and NO₃^-)-rich (STL1), NO₂^--rich (STL2), and NO₃^--rich (STL3)] as a moisture adjustment agent during the composting thermophilic period on nitrogen transformation. Results indicated that nitrogen-rich wastewater addition did not impair the compost maturation, whereas raised the total nitrogen content of fertilizer by 15.8%-46.7% compared to the control group (i.e., tap water group). Moreover, adding STL1 has the potential to reduce CO₂ and NH₃ emissions and avoid incomplete organic nitrogen decomposition. Furthermore, nitrogen flow analysis unveiled that STL1 addition increased nitrogen content by strengthening ammonification, dissimilatory nitrite reduction to ammonium, and high-temperature nitrification pathways. Thus, in-situ disposal of STL1 from biotrickling filters via composting is a suitable technique for coupled systems to achieve zero discharge.

A new liquid-phase method and its comparison to two solid-phase microbial respiration activity methods to assess organic waste stability

Goal of the work was to compare the respiration activities, as measured via oxygen consumption with three different organic waste stability methods so that to propose the optimal one. The novelty of the work is that there exists no comparison of solid-phase with liquid-phase stability assessment techniques in the literature. The respiration activities were assessed using two solid-phase methods and a manometric liquid-phase method (MANLIQ) performed on twenty-seven organic substrates. The methods rely on measuring oxygen consumption (uptake) via pressure drops (liquid-phase test, static solid-phase test) or via direct O₂ measurements on the gaseous phases at the inlet and outlet of the respirometer (solid-phase dynamic test). A positive statistically significant correlation was calculated between the MANLIQ and the static solid-phase indices. The maximum rate MANLIQ index for the raw substrates was 2900 mg O₂ kg^-1 VS h^-1, while most of the processed substrates had cumulative MANLIQ indices below 160 g O₂ kg^-1 VS. The ratio of the liquid indices to the static solid-phase indices ranged from 1.6 to 2.7 and the ratio of the liquid indices to the dynamic solid-phase indices ranged from 0.2 to 0.4. The MANLIQ method failed to result in a good correlation of the processing time with the respiration indices. On the other hand, a correlation was more visible in the two solid-phase tests, despite the large variability of the types and sources of the substrates. Therefore, the solid-phase methods should be preferred over the liquid-phase method to assess stability for various organic substrates.

Portable and Low-Cost Respirometric Microsystem for the Static and Dynamic Respirometry Monitoring of Compost

Composting is considered an option for the disposal of organic waste; however, the development of portable and low-cost systems for its monitoring is of high interest. Therefore, in this study, respirometric microsystems were designed and tested including two integrated oxygen sensors for the measurement of compost samples under static and dynamic conditions with high portability and ease of use. The cost of each sensor was calculated as 2 USD, while the cost of the whole respirometric microsystem was calculated as 6 USD. The electronic system for real-time monitoring was also designed and implemented. The designed systems were tested for over 6 weeks for the determination of compost quality using real samples. The respirometric microsystem was compared to a commercial respirometry system and a standard laboratory test using hierarchical analysis which included costs, portability accuracy, analysis time, and integration of new technologies. The analysis showed a global score of 6.87 for the respirometric microsystem compared to 6.70 for the standard laboratory test and 3.26 for the commercial system.

Seed germination test for toxicity evaluation of compost: Its roles, problems and prospects

Compost is commonly used for the growth of plants and the remediation of environmental pollution. It is important to evaluate the quality of compost and seed germination test is a powerful tool to examine the toxicity of compost, which is the most important aspect of the quality. Now the test is widely adopted, but the main problem is that the test results vary with different methods and seed species, which limits the development and application of it. The standardization of methods and the modelization of seeds can contribute to solving the problem. Additionally, according to the probabilistic theory of seed germination, the error caused by the analysis and judgment methods of the test results can be reduced. Here, we reviewed the roles, problems and prospects of the seed germination test in the studies of compost.

The effect of airflow rates and aeration mode on the respiration activity of four organic wastes: Implications on the composting process

The aim of this study was to assess the effect of the airflow and of the aeration mode on the composting process of non-urban organic wastes that are found in large quantities worldwide, namely: (i) a fresh, non-digested, sewage sludge (FSS), (ii) an anaerobically digested sewage sludge (ADSS), (iii) cow manure (CM) and (iv) pig sludge (PS). This assessment was done using respirometric indices. Two aeration modes were tested, namely: (a) a constant air flowrate set at three different initial fixed airflow rates, and (b) an oxygen uptake rate (OUR)-controlled airflow rate. The four wastes displayed the same behaviour namely a limited biological activity at low aeration, while, beyond a threshold value, the increase of the airflow did not significantly increase the dynamic respiration indices (DRI_{1 max}, DRI_{24 max} and AT₄). The threshold airflow rate varied among wastes and ranged from 42NL air kg^-1DMh^-1 for CM and from 67 to 77NL air kg^-1DMh^-1 for FSS, ADSS and PS. Comparing the two aeration modes tested (constant air flow, OUR controlled air flow), no statistically significant differences were calculated between the respiration activity indices obtained at those two aeration modes. The results can be considered representative for urban and non-urban organic wastes and establish a general procedure to measure the respiration activity without limitations by airflow. This will permit other researchers to provide consistent results during the measurement of the respiration activity. Results indicate that high airflows are not required to establish the maximum respiration activity. This can result in energy savings and the prevention of off-gas treatment problems due to the excessive aeration rate in full scale composting plants.

Effect of air flowrate on the dynamic respiration activity of the raw organic fraction of municipal solid wastes

Scope of this work was to study the effect of the aeration rate on the respiration activity of the fresh organic fraction of MSW and to compare the resulting dynamic respiration indices with those of MSW derived compost. Thus, a categorization of the dynamic respiration activity of MSW throughout a composting facility is provided. A simulated organic fraction of MSW was used as a substrate and four experimental runs were performed to achieve unit airflow rates (UAF) from around 6 to 30Lairkg^-1VSh^-1. Six dynamic respiration activity indices were calculated and compared to the corresponding indices of stable MSW compost from a previous work. Findings indicate that the increase of the UAF results in a corresponding increase of the dynamic stability indices. Dynamic respiration activity indices above 1500 and below 520mgO₂kg^-1VSh^-1 indicate fresh and very stable MSW materials, respectively.

Evaluation of Optimum Moisture Content for Composting of Beef Manure and Bedding Material Mixtures Using Oxygen Uptake Measurement

Moisture content influences physiological characteristics of microbes and physical structure of solid matrices during composting of animal manure. If moisture content is maintained at a proper level, aerobic microorganisms show more active oxygen consumption during composting due to increased microbial activity. In this study, optimum moisture levels for composting of two bedding materials (sawdust, rice hull) and two different mixtures of bedding and beef manure (BS, Beef cattle manure+sawdust; BR, Beef cattle manure+rice hull) were determined based on oxygen uptake rate measured by a pressure sensor method. A broad range of oxygen uptake rates (0.3 to 33.3 mg O₂/g VS d) were monitored as a function of moisture level and composting feedstock type. The maximum oxygen consumption of each material was observed near the saturated condition, which ranged from 75% to 98% of water holding capacity. The optimum moisture content of BS and BR were 70% and 57% on a wet basis, respectively. Although BS’s optimum moisture content was near saturated state, its free air space kept a favorable level (above 30%) for aerobic composting due to the sawdust’s coarse particle size and bulking effect.

The importance of aeration mode and flowrate in the determination of the biological activity and stability of organic wastes by respiration indices

The aim of this study was to assess the effect of different air flowrates and different aeration modes on the respiration activity of three organic substrates of different stability degree: (i) a constant flowrate and (ii) a continuously adjusted air flowrate that optimized the oxygen uptake rate (OUR). Above 20L air kg(-1)DMh(-1), at the constant flow regime, the resulting dynamic respiration index at 24h (DRI24) and the cumulative respiration at four days (AT4) were statistically similar. At the OUR based aeration regime, the DRI24 and AT4 were statistically similar at all initial flowrates tested. Above a minimum threshold, cumulative air flow of around 3000Lairkg(-1) DM during a 5day period, the respiration activity was similar, particularly for the two less active substrates. This study highlights the importance of selecting the aeration to obtain reliable measures of biological activity and stability in organic wastes.

Influence assessment of a lab-scale ripening process on the quality of mechanically-biologically treated MSW for possible recovery

In this study, the influence of an additional ripening process on the quality of mechanically-biologically treated MSW was evaluated in the prospective of recovering the end material, rather than landfilling. The biostabilised waste (BSW) coming from one of the MBT plants of Rome was therefore subjected to a ripening process in slightly aerated lab test cells. An in-depth investigation on the biological reactivity was performed by means of different types of tests (aerobic and anaerobic biological tests, as well as FT-IR spectroscopy method). A physical-chemical characterisation of waste samples progressively taken during the ripening phase was carried out, as well. In addition, the ripened BSW quality was assessed by comparing the characteristics of a compost sampled at the composting plant of Rome which treat source segregated organic wastes. Results showed that the additional ripening process allowed to obtain a better quality of the biostabilised waste, by achieving a much higher biological stability compared to BSW as-received and similar to that of the tested compost. An important finding was the lower heavy metals (Co, Cr, Cu, Ni, Pb and Zn) release in water phase at the end of the ripening compared to the as-received BSW, showing that metals were mainly bound to solid organic matter. As a result, the ripened waste, though not usable in agriculture as found for the compost sample, proved anyhow to be potentially suitable for land reclamation purposes, such as in landfills as cover material or mixed with degraded and contaminated soil for organic matter and nutrients supply and for metals recovery, respectively. In conclusion the study highlights the need to extend and optimise the biological treatment in the MBT facilities and opens the possibility to recover the output waste instead of landfilling.

Relationship between respiratory quotient, nitrification, and nitrous oxide emissions in a forced aerated composting process

We assessed the relationship between respiratory quotient (RQ) and nitrification and nitrous oxide (N2O) emission in forced aerated composting using lab-scale reactors. Relatively high RQ values from degradation of readily degradable organics initially occurred. RQ then stabilized at slightly lower values, then decreased. Continuous emission of N2O was observed during the RQ decrease. Correlation between nitrification and N2O emission shows that the latter was triggered by nitrification. Mass balances demonstrated that the O2 consumption of nitrification (∼24.8mmol) was slightly higher than that of CO2 emission (∼20.0mmol), indicating that the RQ decrease was caused by the occurrence of nitrification. Results indicate that RQ is a useful index, which not only reflects the bioavailability of organics but also predicts the occurrence of nitrification and N2O emission in forced aerated composting.

Static, dynamic and inoculum augmented respiration based test assessment for determining in-vessel compost stability

The purpose of this work was to evaluate compost (and related industry) stability tests given recent large-scale changes to feedstock, processing techniques and compost market requirements. Five stability tests (ORG0020, DR4, Dewar self-heating, oxygen update rate (OUR) and static respiration) were evaluated on composts from ten in-vessel composting sites. Spearman rank correlation coefficients were strong for the ORG0020, OUR and DR4 (both CO2 and O2 measurement), however, OUR results required data extrapolation for highly active compost samples. By comparison the Dewar self-heating and static respiration tests had weaker correlations, in part the result of under reporting highly active, low pH samples. The findings suggest that despite differences in pre-incubation period both dynamic respiration tests (ORG0020 and DR4) are best suited to deal with the wide range of compost stabilities found.

Static respiration indices to investigate compost stability: effect of sample weight and temperature and comparison with dynamic respiration indices

Goal of this work was to study the effect of sample weight and temperature on the microbial respiration indices of a static microbial respiration test used to quantify compost stability. The static respiration tests (SRT) were performed at two different temperatures (20°C and 35°C) using five different sample weights (19, 38, 56, 75 and 94 dry g). Results showed that at 35°C, as sample weight increased, the magnitude of the respiration indices reduced. In addition, the 35°C temperature resulted in higher static respiration activity indices compared to that at the 20°C for two sample weights. The static respiration tests led to 2-2.5 times lower oxygen-related indices compared to those calculated in dynamic respiration tests (DRT); the 7-day CO(2) cumulative generations were, however, almost similar for both types of tests. Respiratory quotients (RQs) were constantly between 0.8 to 1.2 during the SRT.