Nitrous oxide emission mechanisms during intermittently aerated composting of cattle manure

Oxygen dynamics, organic matter degradation and main gas emissions during pig manure composting: Effect of intermittent aeration

To investigate the effect of intermittent aeration on oxygen dynamics, organic matter degradation and main gas emissions, a lab-scale pig manure composting experiment was conducted with intermittent aeration (I_A, 30-min on and 30-min off) and continuous aeration (C_A). Although aeration volume and oxygen supply of I_A was only half of C_A, I_A could obviously enhance the oxygen utilization efficiency by 96.67 % and reduce energy dissipation for aeration by 50.87 %. Based on the comprehensive analysis of total organic matter, total carbon, total nitrogen, cellulose, hemicellulose and lignin contents, there was no significant difference in organic matter degradation between I_A and C_A (p > 0.05). Moreover, a reduction of 21.71 %, 38.93 %, 44.40 % and 62.19 % of CH₄, N₂O and the total GHG emission equivalent as well as NH₃ emissions was realized, respectively, in I_A compared with C_A. Therefore, adopting intermittent aeration was a useful strategy and choice for high-efficiency, high-quality and environment-friendly composting.

Odors Emitted from Biological Waste and Wastewater Treatment Plants: A Mini-Review

In recent decades, a new generation of waste treatment plants based on biological treatments (mainly anaerobic digestion and/or composting) has arisen all over the world. These plants have been progressively substituted for incineration facilities and landfills. Although these plants have evident benefits in terms of their environmental impact and higher recovery of material and energy, the release into atmosphere of malodorous compounds and its mitigation is one of the main challenges that these plants face. In this review, the methodology to determine odors, the main causes of having undesirable gaseous emissions, and the characterization of odors are reviewed. Finally, another important topic of odor abatement technologies is treated, especially those related to biological low-impact processes. In conclusion, odor control is the main challenge for a sustainable implementation of modern waste treatment plants.

Gaseous Emissions from the Composting Process: Controlling Parameters and Strategies of Mitigation

Organic waste generation, collection, and management have become a crucial problem in modern and developing societies. Among the technologies proposed in a circular economy and sustainability framework, composting has reached a strong relevance in terms of clean technology that permits reintroducing organic matter to the systems. However, composting has also negative environmental impacts, some of them of social concern. This is the case of composting atmospheric emissions, especially in the case of greenhouse gases (GHG) and certain families of volatile organic compounds (VOC). They should be taken into account in any environmental assessment of composting as organic waste management technology. This review presents the relationship between composting operation and composting gaseous emissions, in addition to typical emission values for the main organic wastes that are being composted. Some novel mitigation technologies to reduce gaseous emissions from composting are also presented (use of biochar), although it is evident that a unique solution does not exist, given the variability of exhaust gases from composting.

Effects of semi-permeable membrane covering coupled with intermittent aeration on gas emissions during aerobic composting from the solid fraction of dairy manure at industrial scale

In this study, the effects of covering the compost pile with a semi-permeable membrane in combination with intermittent aeration on the gas emissions during aerobic composting from the solid fraction of dairy manure at industrial scale were investigated. A large-scale composting experiment was carried out to compare a membrane-covered (CT) group with a control (CK) group. The results indicated that the CT group could maintain a suitable aerobic and positive micro-pressure environment. The carbon dioxide, methane, nitrous oxide, and ammonia emissions outside the membrane during the aeration interval were reduced by 64.23%, 70.07%, 54.87%, and 11.32%, respectively, compared with that inside the membrane. It was also determined that the methane and nitrous oxide emissions from the CT group were reduced by 99.89% and 60.48% relative to the CK group, confirming that the combined process represented a novel strategy for reducing gas emissions during dairy manure composting.

The non-negligibility of greenhouse gas emission from a combined pre-composting and vermicomposting system with maize stover and cow dung

The acceptance of combined pre-composting and vermicomposting systems is increasing because of the advantage in rapidly stabilizing organic wastes and reducing emission of greenhouse gasses (GHG). However, GHG emission during the pre-composting phase is often neglected when evaluating the system. This study aimed to quantify GHG emission from a combined pre-composting and vermicomposting system and to investigate the effects of earthworms on GHG emission. A combined system using Eisenia fetida was employed to stabilize maize stover and cow dung (mixing ratio 60:40). The inoculating densities were 60 (T₁), 120 (T₂), and 180 (T₃) earthworms per kilogram of substrate. A traditional composting system without earthworms was set as a control (T₀). The results indicated that earthworms increased CO₂ while decreased CH₄ and N₂O emissions compared to the control. Higher emission of CO₂ suggested that the earthworms promoted the degradation of the substrates. Lower emission of CH₄ and N₂O showed the advantage of the combined system because CH₄ and N₂O possess extremely higher global warming potential than that of CO₂. T₂ is recommended for stabilizing maize stover and cow dung when making a tradeoff between stabilization rate and reduction of GHG. The percentages of GHG emission during pre-composting relative to total GHG emission in T₁, T₂, and T₃ were 34%, 35%, and 30%, respectively. GHG emission is non-negligible when using a combined system, especially the emission of GHG during the pre-composting phase cannot be ignored.

Minimization of N2O Emission through Intermittent Aeration in a Sequencing Batch Reactor (SBR): Main Behavior and Mechanism

To explore the main behavior and mechanism of minimizing nitrous oxide (N2O) emission through intermittent aeration during wastewater treatment, two lab-scale sequencing batch reactors operated at intermittently aerated mode (SBR1), and continuously aerated mode (SBR2) were established. Compared with SBR2, the intermittently aerated SBR1 reached not only a higher total nitrogen removal efficiency (averaged 93.5%) but also a lower N2O-emission factor (0.01–0.53% of influent ammonia), in which short-cut nitrification and denitrification were promoted. Moreover, less accumulation and consumption of polyhydroxyalkanoates, a potential endogenous carbon source promoting N2O emission, were observed in SBR1. Batch experiments revealed that nitrifier denitrification was the major pathway generating N2O while heterotrophic denitrification played as a sink of N2O, and SBR1 embraced a larger N2O-mitigating capability. Finally, quantitative polymerase chain reaction results suggested that the abundant complete ammonia oxidizer (comammox) elevated in the intermittently aerated environment played a potential role in avoiding N2O generation during wastewater treatment. This work provides an in-depth insight into the utilization of proper management of intermittent aeration to control N2O emission from wastewater treatment plants.

The influence on carbon, nitrogen recycling, and greenhouse gas emissions under different C/N ratios by black soldier fly

Currently, sustainable utilization, including recycling and valorization, is becoming increasingly popular in waste management. Black soldier fly larvae (BSFL) can convert the carbon (C) and nitrogen (N) from organic waste into biomass and improve properties of the substrate to reduce greenhouse gas and NH₃ emissions. In this study, the recycling of C and N and the emissions of greenhouse gas and NH₃ during BSFL bio-treatment of mixtures of pig manure and corncob were investigated under different C/N ratios. The results indicated that initial C/N ratios of feedstock are a crucial parameter affecting the biomass generation of larvae. The BSFL recycled approximately 4.17-6.61% of C and 17.45-23.73% of N from raw materials under different C/N ratios. Cumulative CO₂, CH₄, NH₃, and N₂O emissions at the different C/N ratios ranging from 15 to 35 were 107.92-151.68, 0.08-0.76, 0.14-1.17, and 0.91-1.18 mg kg^-1, respectively. Compared with conventional composting, BSFL treatment could reduce the total greenhouse gas emissions by over 90%. The study showed that bio-treatment of mixtures of pig manure and corncob with a proper C/N ratio by BSFL could become an avenue to achieve higher nutrient recycling, which is an eco-friendly process.

Acidification of manure reduces gaseous emissions and nutrient losses from subsequent composting process

Manure acidification is recommended to minimize ammonia (NH₃) emission at storage. However, the potential for acidification to mitigate NH₃ emission from storage and the impact of manure acidification (pH range 5-8) on composting have been poorly studied. The effects of manure acidification at storage on the subsequent composting process, nutrient balance, gaseous emissions and product quality were assessed through an analysis of literature data and an experiment under controlled conditions. Results of the data mining showed that mineral acids, acidic salts and organic acids significantly reduced NH₃ emission, however, a weaker effect was observed for organic acids. A subsequent composting experiment showed that using manure acidified to pH5 or pH6 as feedstock delayed organic matter degradation for 7-10 days, although pH6 had no negative effect on compost maturity. Acidification significantly decreased NH₃ emission from both storage and composting, however, excessive acidification (pH5) enhanced N₂O emissions (18.6%) during composting. When manure was acidified to pH6, N₂O (17.6%) and CH₄ (20%) emissions, and total GHG emissions expressed as global warming potential (GWP) (9.6%) were reduced during composting. Acidification of manure before composting conserved more N as NH₄⁺ and NO_x^- in compost product. Compared to the control, the labile, plant-available phosphorus (P) content in the compost product, predominately as water-soluble inorganic P, increased with manure acidification to pH5 and pH6. Acidification of manure to pH6 before composting decreases nutrient losses and gaseous emissions without decreasing the quality of the compost product. The techno-economic advantages of acidification should be further ascertained.

Towards the circular nitrogen economy - A global meta-analysis of composting technologies reveals much potential for mitigating nitrogen losses

Composting is an important technology to treat biowastes and recycle nutrients, but incurs nitrogen (N) losses that lower the value of the final products and cause pollution. Technologies aimed at reducing N losses during composting have inconsistent outcomes. To deepen insight into mitigation options, we conducted a global meta-analysis based on 932 observations from 121 peer-reviewed published studies. Overall, N losses averaged 31.4% total N (TN), 17.2% NH₃-N, and 1.4% N₂O-N, with NH₃-N accounting for 55% of TN losses. The primary drivers affecting N losses were composting method, type of biowaste, and duration of composting. N losses were significantly impacted by the carbon-to-nitrogen (C/N) ratio of the input materials (feedstock of nutrient dense biowastes and C-rich bulking agents), moisture content and pH. Our analysis revealed N-conserving optima with C/N ratios of 25-30, 60-65% moisture content and pH 6.5-7.0. In situ mitigation technologies that control feedstock and processing conditions reduced average N losses by 31.4% (TN), 35.4% (NH₃-N) and 35.8% (N₂O-N). Biochar and magnesium-phosphate salts emerged as the most effective N-conserving strategies, curbing losses of TN by 30.2 and 60.6%, NH₃ by 52.6 and 69.4%, and N₂O by 66.2 and 35.4% respectively. We conclude that existing technologies could preserve ~0.6 Tg of biowaste-N globally, which equates to 16% of the chemical N-fertilizer used in African croplands, or 39% of the annual global increases of 1.58 Tg fertilizer-N. However, the adoption of N-conserving technologies is constrained by a lack of knowledge of best practice, suitable infrastructure, policies and receptive markets. To realize an N-conserving composting industry that supports sustainable practices and the circular nitrogen economy, stakeholders have to act collectively. Benefits will include lowering direct and indirect greenhouse gas emissions associated with agriculture, and facilitating the recarbonization of soils.

The Characteristics of Carbon, Nitrogen and Sulfur Transformation During Cattle Manure Composting-Based on Different Aeration Strategies

This study aimed to investigate the characteristics of gaseous emission (methane-CH₄, carbon dioxide-CO₂, nitrous oxide-N₂O, nitric oxide-NO, hydrogen sulfide-H₂S and sulfur dioxide-SO₂) and the conservation of carbon (C), nitrogen (N), and sulfur (S) during cattle manure composting under different aeration strategies. Three aeration strategies were set as C60, C100, and I60, representing the different combinations of aeration method (continuous-C or intermittent-I) and aeration rate (60 or 100 L·min^-1·m^-3). Results showed that C, N, S mass was reduced by 48.8-53.1%, 29.8-35.9% and 19.6-21.9%, respectively, after the composing process. Among the three strategies, the intermittent aeration treatment I60 obtained the highest N₂O emissions, resulting in the highest N loss and greenhouse gas (GHG) emissions when the GHG emissions from power consumption were not considered. Within two continuous aeration treatments, lower aeration rates in C60 caused lower CO₂, N₂O, NO, and SO₂ emissions but higher CH₄ emissions than those from C100. Meanwhile, C and N losses were also lowest in the C60 treatment. H₂S emission was not detected because of the more alkaline pH of the compost material. Thus, C60 can be recommended for cattle manure composting because of its nutrient conservation and mitigation of major gas and GHG emissions.

Hyperthermophilic composting significantly decreases N2O emissions by regulating N2O-related functional genes

This study reported for the first time that hyperthermophilic composting (HTC) could mitigate 90% of the cumulative amount of N₂O emissions compared to traditional composting (TC) in a full-scale experiment. The concentrations of NO₂^--N and NO₃^--N in HTC were significantly lower than those in TC, which may be the main reason for the reduced N₂O emissions. Furthermore, this study found that the decrease in N₂O emissions in HTC compared to TC was mainly due to the inhibition of the abundance of the bacterial amoA and norB genes, which could decrease the nitrification rate and control N₂O formation, respectively. Partial least squares path modeling revealed that a high temperature was the key factor in lowering N₂O emissions in HTC, while physicochemical properties were the dominant factor in enhancing N₂O emissions in TC. These results suggested that HTC is a promising technique for reducing N₂O emissions in manure composting.

Effects of bioleaching pretreatment on nitrous oxide emission related functional genes in sludge composting process

The effect of bioleaching pretreatment on N₂O generation in sludge composting process was firstly investigated in this study. The relationships among physicochemical factors, N₂O and NH₃ emission and related functional genes were analyzed in 60 days composting of bioleaching dewatering sludge (BDS) and filter press dewatering sludge (FDS), respectively. The results showed the cumulative amounts of NH₃ and N₂O emission from the BDS composting system were reduced by 83.52% and 54.76% after bioleaching pretreatment, respectively. The lower moisture and pH, and the higher ORP and the concentrations of NH₄⁺-N, NO₃^--N and NO₂^--N were observed in BDS during the composting compared to FDS. Furthermore, bioleaching pretreatment improved the relative abundance of hao but reduced amoA, nirK and norB in the BDS during the composting. The low pH level and the reduction of nirK and norB in BDS were the main reasons mitigating NH₃ and N₂O emissions, respectively.

Combining biochar, zeolite and wood vinegar for composting of pig manure: The effect on greenhouse gas emission and nitrogen conservation

The effect of enhancing wood vinegar (WV) with a mixture of biochar (B) and zeolite (Z) to compost pig manure (PM) in a 130 L reactor was evaluated to determine the levels of greenhouse gas (GHG) and ammonia emissions. Six treatments were prepared in a 2:1 ratio of PM mixed with wheat straw (WS; dry weight basis): PM + WS (control), PM + WS + 10%B, PM + WS + 10%B + 10%Z, and PM + WS with 0.5%, 1.0% and 2.0%WV combined with 10%B + 10%Z. These were composted for 50 days, and the results indicated that the combined use of B, Z, and WV could shorten the thermophilic phase and improve the maturity of compost compared to the control treatment. In addition, WV mixed with B and Z could reduce ammonia loss by 64.45-74.32% and decrease CO₂, CH₄, and N₂O emissions by 33.90-46.98%, 50.39-61.15%, and 79.51-81.10%, respectively. Furthermore, compared to treatments in which B and B + Z were added, adding WV was more efficient to reduce the nitrogen and carbon loss, and the 10%B + 10%Z + 2%WV treatment presented the lowest loss of carbon (9.16%) and nitrogen (0.75%). Based on the maturity indexes used, nitrogen conservation, and efficiency of GHG emissions reduction, the treatment 10%B + 10%Z + 2%WV is suggested for efficient PM composting.

Effect of aeration interval on oxygen consumption and GHG emission during pig manure composting

To verify the optimal aeration interval for oxygen supply and consumption and investigate the effect of aeration interval on GHG emission, reactor-scale composting was conducted with different aeration intervals (0, 10, 30 and 50 min). Although O₂ was sufficiently supplied during aeration period, it could be consumed to <10 vol% only when the aeration interval was 50 min, indicating that an aeration interval more than 50 min would be inadvisable. Compared to continuous aeration, reductions of the total CH₄ and N₂O emissions as well as the total GHG emission equivalent by 22.26-61.36%, 8.24-49.80% and 12.36-53.20%, respectively, was achieved through intermittent aeration. Specifically, both the total CH₄ and N₂O emissions as well as the total GHG emission equivalent were inversely proportional to the duration of aeration interval (R² > 0.902), suggesting that lengthening the duration of aeration interval to some extent could effectively reduce GHG emission.

Influence of medical stone amendment on gaseous emissions, microbial biomass and abundance of ammonia oxidizing bacteria genes during biosolids composting

This study aimed to evaluate the feasibility of medical stone (MS) on microbial biomass, bacteria genes copy numbers, mitigation of gaseous emissions and its correlation with analyzed parameters during the biosolids composting. Composting of the biosolids by amendment of MS 0%, 2%, 4%, 6% and 10% (on dry weight basis) was performed using a 130-L composting reactor. The results showed that with increasing the dosage of MS, the CH₄, N₂O and NH₃ emission were reduced by 60.5-88.3%, 46.6-82.4% and 38.2-78.5%, respectively. In addition, the 6-10% MS amendment enhanced the organic waste mineralization and prolonged the thermophilic phase. The abundance of ammonia oxidizing bacteria (AOB) and archaea (AOAB) were decreased during the first 28 days, but considerable increment was observed during the maturation phase which indicated that AOB and AOAB were liable for nitrification during the curing phase of composting. A significant correlation was observed among the all analyzed parameters in 6-10% MS blended treatments.

Microbial Abundances Predict Methane and Nitrous Oxide Fluxes from a Windrow Composting System

Manure composting is a significant source of atmospheric methane (CH₄) and nitrous oxide (N₂O) that are two potent greenhouse gases. The CH₄ and N₂O fluxes are mediated by methanogens and methanotrophs, nitrifying and denitrifying bacteria in composting manure, respectively, while these specific bacterial functional groups may interplay in CH₄ and N₂O emissions during manure composting. To test the hypothesis that bacterial functional gene abundances regulate greenhouse gas fluxes in windrow composting systems, CH₄ and N₂O fluxes were simultaneously measured using the chamber method, and molecular techniques were used to quantify the abundances of CH₄-related functional genes (mcrA and pmoA genes) and N₂O-related functional genes (amoA, narG, nirK, nirS, norB, and nosZ genes). The results indicate that changes in interacting physicochemical parameters in the pile shaped the dynamics of bacterial functional gene abundances. The CH₄ and N₂O fluxes were correlated with abundances of specific compositional genes in bacterial community. The stepwise regression statistics selected pile temperature, mcrA and NH₄⁺ together as the best predictors for CH₄ fluxes, and the model integrating nirK, nosZ with pmoA gene abundances can almost fully explain the dynamics of N₂O fluxes over windrow composting. The simulated models were tested against measurements in paddy rice cropping systems, indicating that the models can also be applicable to predicting the response of CH₄ and N₂O fluxes to elevated atmospheric CO₂ concentration and rising temperature. Microbial abundances could be included as indicators in the current carbon and nitrogen biogeochemical models.

Linking N2O emission from biochar-amended composting process to the abundance of denitrify (nirK and nosZ) bacteria community

Manure composting has been recognized as an important anthropogenic source of nitrous oxide (N₂O) contributing to global warming. However, biochar effect on N₂O emissions from manure composting is rarely evaluated, especially by linking it to abundance of denitrifying bacteria community. Results of this study indicated that biochar amendment significantly reduced N₂O emissions from manure composting, primarily due to suppression of the nirK gene abundance of denitrifying bacteria. Pearson’s correlation analysis showed a significant positive correlation between nirK abundance and N₂O fluxes, while a negative correlation between nosZ density and N₂O fluxes. Simultaneously, a linear correlation between nirK gene abundance minus nosZ gene abundance with N₂O fluxes was also observed. In addition, a statistical model for estimating N₂O emissions based on the bacterial denitrifying functional genes was developed and verified to adequately fit the observed emissions. Our results highlighted that biochar amendment would be an alternative strategy for mitigating N₂O emissions during manure composting, and the information of related functional bacterial communities could be helpful for understanding the mechanism of N₂O emissions.

Nitrous Oxide Emissions from Open-Lot Cattle Feedyards: A Review

Nitrous oxide (NO) emissions from concentrated animal feeding operations, including cattle feedyards, have become an important research topic. However, there are limitations to current measurement techniques, uncertainty in the magnitude of feedyard NO fluxes, and a lack of effective mitigation methods. The objective of this review was to assess NO emission from cattle feedyards, including comparison of measured and modeled emission rates, discussion of measurement methods, and evaluation of mitigation options. Published annual per capita flux rates for beef cattle feedyards and open-lot dairies were highly variable and ranged from 0.002 to 4.3 kg NO animal yr. On an area basis, published emission rates ranged from 0 to 41 mg NO m h. From these studies and Intergovernmental Panel on Climate Change emission factors, calculated daily per capita NO fluxes averaged 18 ± 10 g NO animal d (range, 0.04-67 g NO animal d). This variation was due to inconsistency in measurement techniques as well as irregularity in NO production and emission attributable to management, animal diet, and environmental conditions. Based on this review, it is clear that the magnitude and dynamics of NO emissions from open-lot cattle systems are not well understood. Further research is required to quantify feedyard NO fluxes and develop cost-effective mitigation methods.

Influence of zeolite and lime as additives on greenhouse gas emissions and maturity evolution during sewage sludge composting

This study aimed to evaluate the role of different amount of zeolite with low dosage of lime amendment on the greenhouse gas (GHGs) emission and maturity during the dewatered fresh sewage sludge (DFSS) composting. The evolution of CO2, CH4, NH3 and N2O and maturity indexes were monitored in five composting mixtures prepared from DFSS mixed with wheat straw, while 10%, 15% and 30% zeolite+1% lime were supplemented (dry weight basis of DFSS) into the composting mass and compared with treatment only 1% lime amended and control without any amendment. The results showed that addition of higher dosage of zeolite+1% lime drastically reduce the GHGs emissions and NH3 loss. Comparison of GHGs emissions and compost quality showed that zeolite amended treatments were superior than control and 1% lime amended treatments. Therefore, DFSS composting with 30% zeolite+1% lime as consortium of additives were found to emit very less amount of GHGs and gave the highest maturity than other treatments.

Nitrous Oxide Production at a Fully Covered Wastewater Treatment Plant: Results of a Long-Term Online Monitoring Campaign

The nitrous oxide emissions of the Viikinmäki wastewater treatment plant were measured in a 12 month online monitoring campaign. The measurements, which were conducted with a continuous gas analyzer, covered all of the unit operations of the advanced wastewater-treatment process. The relation between the nitrous oxide emissions and certain process parameters, such as the wastewater temperature, influent biological oxygen demand, and ammonium nitrogen load, was investigated by applying online data obtained from the process-control system at 1 min intervals. Although seasonal variations in the measured nitrous oxide emissions were remarkable, the measurement data indicated no clear relationship between these emissions and seasonal changes in the wastewater temperature. The diurnal variations of the nitrous oxide emissions did, however, strongly correlate with the alternation of the influent biological oxygen demand and ammonium nitrogen load to the aerated zones of the activated sludge process. Overall, the annual nitrous oxide emissions of 168 g/PE/year and the emission factor of 1.9% of the influent nitrogen load are in the high range of values reported in the literature but in very good agreement with the results of other long-term online monitoring campaigns implemented at full-scale wastewater-treatment plants.

Improving composting as a post-treatment of anaerobic digestate

This work investigated the influences of practical parameters upon composting of digestate. The yardsticks for evaluation were digestate stabilization, nitrogenous emissions mitigation and self-heating potential. The results suggest choosing an "active" bulking agent like dry wood chips (WC) which served as free-water and nitrogen sink through composting. At an optimal volumetric WC:digestate mixing ratio of 4:1, nearly 90% of the initial NH4(+)/NH3 were fixed, which reduced significantly nitrogenous emissions. This mixing ratio also improved the stabilization and self-heating potential. Using small particle size WC increased narrowly O2 consumption and reduced NH3 emission. Storing used WC prior to recycling reduced 40% N2O emission compared to directly recycled WC. Recycling compost helped to decrease NH3 emission, but quadrupled N2O emission. The optimal aeration rate (15Lh(-1)kg OM0) which was lower compared to composting of organic waste, was enough to ensure the O2 supply and ameliorate the self-heating potential through composting of digestate.

Complete nitrification-denitrification of swine manure in a full-scale, non-conventional composting system

A full-scale composting plant (track type, aerated by screws), treating liquid swine manure (94.8% on mass basis) with straw (<0.8%) and sawdust (4.4%), was monitored. The main objectives were testing the performance of the process and assessing its environmental sustainability. Particular attention was dedicated to verify the possibility that this process could determine significant mass reduction, along with Nitrogen reduction, mainly by denitrification. Emissions were evaluated by measuring NH3, N2O and CH4 (by static chamber), H2S and odor emissions (by dynamic olfactometry). Quality and quantity of inputs and outputs and process parameters (redox, oxygen, and temperature) were monitored. The process produced a mature, highly humified (Humification Index=0.27), solid product with 92.8% mass reduction (mainly evaporation), and nitrogen reduction (85.8% referred to input TN). The process was revealed to be environmentally sustainable: emissions of odors and H2S resulted negligible; emissions of N-N2O represented 0.18% of TN input, while emissions of N-NH3 represented 0.87% of input TN. Microbiological analyses determined the presence of 10(7) CFU/g of bacteria related to N cycle and real time PCR demonstrated the presence in the final product of 4.77 ⋅ 10(7) couples of genes of Bacterial amoA/gTS and 2.46 ⋅ 10(7) couples NosZ/gTS, indicating nitrification and complete denitrification. These results exhibit that nitrification and complete denitrification can efficiently occur in a composting process effectively transforming N2O into N2 as consequence of the optimized alternation of aerated and anoxic phases in the feedstock.

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.

Greenhouse gas emissions from home composting in practice

In Sweden, 16% of all biologically treated food waste is home composted. Emissions of the greenhouse gases CH4 and N2O and emissions of NH3 from home composts were measured and factors affecting these emissions were examined. Gas and substrate in the compost bins were sampled and the composting conditions assessed 13 times during a 1-year period in 18 home composts managed by the home owners. The influence of process parameters and management factors was evaluated by regression analysis. The mean CH4 and N2O concentration was 28.1 and 5.46 ppm (v/v), respectively, above the ambient level and the CH4:CO2 and N2O:CO2 ratio was 0.38% and 0.15%, respectively (median values 0.04% and 0.07%, respectively). The home composts emitted less CH4 than large-scale composts, but similar amounts of N2O. Overall NH3 concentrations were low. Increasing the temperature, moisture content, mixing frequency and amount of added waste all increased CH4 emissions.

Correlating denitrifying catabolic genes with N2O and N2 emissions from swine slurry composting

This work evaluated N dynamics that occurs over time within swine slurry composting piles. Real-time quantitative PCR (qPCR) analyzes were conducted to estimate concentrations of bacteria community harboring specific catabolic nitrifying-ammonium monooxygenase (amoA), and denitrifying nitrate- (narG), nitrite- (nirS and nirG), nitric oxide- (norB) and nitrous oxide reductases (nosZ) genes. NH3-N, N2O-N, N2-N emissions represented 15.4 ± 1.9%, 5.4 ± 0.9%, and 79.1 ± 2.0% of the total nitrogen losses, respectively. Among the genes tested, temporal distribution of narG, nirS, and nosZ concentration correlated significantly (p<0.05) with the estimated N2 emissions. Denitrifying catabolic gene ratio (cnorB+qnorB)/nosZ ≥ 100 was indicative of N2O emission potential from the compost pile. Considering our current empirical limitations to accurately measure N2 emissions from swine slurry composting at field scale the use of these catabolic genes could represent a promising monitoring tool to aid minimize our uncertainties on biological N mass balances in these systems.