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

Highlights

• RQ can be an indicator of N₂O emission in forced aerated composting process.

• Emission of N₂O with nitrification was observed with RQ decrease.

• Mass balances demonstrated the RQ decrease was caused by nitrification.

• Conversion ratio of oxidized ammonia and total N to N₂O were ∼2.7%.

Abstract

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 (∼24.8 mmol) was slightly higher than that of CO₂ emission (∼20.0 mmol), 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.

Introduction

The spreading of livestock excrement can contribute to nitrate contamination of groundwater and eutrophication of surface waters. To mitigate these issues, a decentralized differentiable treatment system for livestock excreta that discharges minimum pollutants, produces minimal greenhouse gas (GHG) emissions, and more effectively recovers nutrients has been proposed (Fujiwara, 2012). Producing compost of better quality and less GHG emission is one of the key technologies in this innovative system, through preventing environmental pollution and enhancing resource recycling in agricultural areas. To achieve these outcomes, it is necessary to understand changes of the material during the composting process. In particular, the emission of nitrous oxide (N₂O), which is a GHG and has a reported global warming potential 298 times greater than that of carbon dioxide (CO₂) (IPCC, 2007), has become a problem in agricultural areas. Controls are needed to reduce N₂O emissions produced during the composting process.

Compost stability is an important component of the composting process, because it has implications for compost use as a fertilizer. Stability is related to the types of organic compounds that remain following composting and the microbial activity of composted materials (California Compost Quality Council, 2001). The primary goal of composting is to produce a stabilized end-product (Komilis and Kanellos, 2012). Some of the most common compost stability tests are based on quantification of microbial respiration (Komilis et al., 2011a). Microbial respiration activity indices based on oxygen (O₂) uptake rate (OUR) such as AT4 (static respiration index for 4 days) and specific OUR (SOUR) (Lasaridi et al., 2006), and on CO₂ evolution (Bernal et al., 2009), are commonly used to test compost stability (Gómez et al., 2006). By coupling data of OUR and CO₂ evolution rates, the respiratory quotient (RQ) can be calculated. The RQ is the ratio of the number of moles of CO₂ generated to the number of moles of O₂ consumed. The value of RQ is known to reflect not only microbial respiration but also the biochemical composition of organic material (Gea et al., 2004), and is widely used in the study of soil environments (Dilly, 2003). Previous studies have reported that RQ is a useful index for composting processes (Atkinson et al., 1997), although others reported that RQ did not change significantly throughout those processes (Gea et al., 2004, Gómez et al., 2006). The applicability of RQ as an index of compost stability requires further study.

Stability indices are not only important measures of compost characteristics, but can also be used for process performance monitoring (Gómez et al., 2006). It is important to establish useful indices for both measuring compost stability and monitoring microbial reactions during composting, to identify appropriate conditions that will produce high-quality compost while lowering GHG emissions. To reduce N₂O emissions from the composting process, microbial activity in the nitrogen cycle during nitrogen transformation should be monitored. Previous studies of respiration indices such as RQ in composting processes have only focused on the biochemical composition of organic material (Gea et al., 2004, Komilis and Kanellos, 2012). In an earlier study, we constructed experimental devices for continuous monitoring of emitted gases from a composting reactor, indicating that N₂O emissions occurred simultaneously with an RQ decrease (Tsutsui et al., 2013). However, we could not obtain information about potential mechanisms responsible for this relationship, because we did not conduct either microbial community or mass balance analyses.

In this study, we took a two-step approach to examine RQ usefulness as an indicator, for both compost stability and monitoring of nitrification and N₂O emissions. In the first step, we performed a detailed gas-phase analysis to clarify the relationship between the RQ decrease and N₂O emissions. We used lab-scale forced aerated composting reactors to continuously measure gas components, toward evaluating the suitability of RQ as an indicator. In this experiment, we did not conduct sampling campaigns to avoid the variation of gas components triggered by sample mixing for clarifying the relationship between the RQ decrease and N₂O emissions. In the second step, we investigated mechanisms responsible for the simultaneous occurrence of nitrification and N₂O emissions with the RQ decrease in the forced aerated composting process. This was done by conducting gas phase, sample composition, microbial community, and mass balance analyses. In addition, temporally and spatially complex environment conditions (such as temperature, oxygen concentration, and water content) make us difficult to clear the mechanism of N₂O emission in actual composting process. To achieve the objective of our study, we conducted the forced aerated composting process under isothermal condition and maintained the water content at constant to prevent the effect of large fluctuation in environmental conditions.