Performance of in-vessel composting of food waste in the presence of coal ash and uric acid
Highlights
► The amendments of CA and UA could facilitate the composting performance. ► The overall performance is a sum of different events with different mechanisms. ► The added CA and UA might lead to higher pH during the composting. ► The process is correlated with the variations of microbial activity and C/N ratio. ► The presence of CA and UA has significant influence on composting of food waste.
Introduction
With the rapid development of economy and population, a large amount of solid waste has been generated from domestic, industrial and agricultural activities over the last few decades [1], [2]. Food waste, which can be from various sources such as restaurants, markets, residents and many other food-processing facilities, is one of the most general forms of organic solid waste [3]. In some areas of China and Korea, it can account for approximately more than one quarter of the total solid waste generation [4], [5]. Food waste has been demonstrated to be a threat to human health due to the potentially releasing of nuisance odors and leachate. Inappropriate disposal practice has often resulted in various economic, environmental and biological losses across the world.
Composting has been developed as one of the most promising solid waste disposal methods for safe treatment strategies [6], [7], [8]. During the composting process, the microorganisms decompose the substrate through breaking it down from complex to simpler compounds under aerobic conditions [9], [10]. Composting can significantly reduce waste volume within various composting systems, such as non-reactor, enclosed reactor and in-vessel systems [11]. The produced product with humus-like property can be used for stimulating microbial activities and plant growth in various field applications. From the viewpoint of green-recycling in the ecosystem, composting is an environmentally and economically sound manner.
The properties of different solid wastes can be quite variable and this variability could play an important role in the effectiveness of composting. To facilitate the treatment process, pre-separation of organic wastes at source is becoming a more common practice in several European countries [12]. However, it is difficult to achieve such routine classification for many areas due to high cost of operating and maintaining. Massive quantities of food waste often coexist with other agroindustrial and industrial waste. The challenge is especially poignant in many developing countries. Some researches on food waste composting have been reported before, particularly on aspects relating to degradation in simple compost cultures [9], [13], investigations on food waste composting in complex multi-source mixtures are relatively limited in scope. To better understand the composting mechanism within real-world applications, there is a need to investigate the interactions among diverse types of materials for the food waste treatment.
Coal ash (CA) is the major waste produced by the burning of coal in large quantities by electricity-generating power plants [14]. The global amount of CA from coal combustion has been estimated to be 500 million tonnes per year [15]. Some CA can be used in cement products, road bases and structural fill; however, more than 70% of the CA is dumped into landfills as waste [16]. Recently, there have been several reports about the influence of CA on composting. The high porosity and alkali metal oxides in CA might improve the quality of composting substrate. Belyaeva and Haynes [17] have proved the feasibility of co-composting municipal green waste with additional CA to produce manufactured soil material. They also find the amendment of CA could increase the water holding capacity of the substrate. Koivula et al. [18] studied the behavior of the organic carbon during the composting of source-separated catering waste mixed with CA. The results showed that the CA could increase the rate of mineralization of compost and the formation of humic acids. The quality of compost prepared from coal ash and crop residue have also been assessed by Gaind and Gaur [19] and their study showed the compatibility of CA with the microbes. But few efforts have been made to examine the composting of food waste and CA under complicated conditions. As another common solid waste, manure often contains uric acid (UA) as an important bicyclic heterocyclic purine derivative. UA can account for 70% of the total N components in poultry feces and this proportion is influenced by animal diet [20]. Available N supplied from UA could potentially facilitate the microbial activities during the composting. In several previous studies, co-composting of manure and other substrates has also been demonstrated under aerobic conditions. The co-composting of solid swine manure and pine sawdust has been investigated in forced-aeration composting systems [21]. The biological and thermal kinetics of the process has also been analyzed during the composting of dairy manure and tomato plant residues [22]. However, very few studies have been conducted to investigate the composting of manure and food waste, despite the prevalence of composting technique. The potential influences and roles of many manure constituents are still not well understood. The coexistence of food waste with agricultural and industrial wastes may be ubiquitous in many areas. Although the study of multi-component waste composting is of great economic and environmental importance, the complexity of their composition makes related study difficult due to the great variety of raw substances. The study in food waste composting in the presence of other components will be helpful for better understanding of this treatment process.
For the experiment of this study, we will evaluate the performance of food waste composting in the presence of CA and UA. Tests will be conducted in the in-vessel composting system. The patterns of food waste composting are compared among various combinations to investigate the influences of CA and UA. The variations of temperature, pH, organic mass reduction rate, O2 consumption, microbial population and C/N ratio are studied. The results of this study will have important implications for developing appropriate treatment of food waste by composting.
Section snippets
Composted wastes
To investigate the performance of food waste composting in the presence of CA and UA, three experimental treatments were carried out using in-vessel composting reactors. The composite raw materials could offer an advantage to be duplicated from batch to batch allowing ready comparisons among various tests [23]. The composite food waste mixture including potatoes, carrots, steamed rice, meat and leaves was used as the composting raw material. All the raw materials were obtained from a local
Temperature and pH profiles
The profile of temperature is shown in Fig. 2a. In all the reactors, the temperature increased quickly during the first several days. Run B could reach 62.4 °C within 2 days. The temperature variation of Run C also showed a similar pattern at this stage. But the temperature of Run C was significantly higher that of Run B (p < 0.05) during the first 3 days. Although Run A also showed an increasing temperature in the initial stage, the temperature level was lower than that observed in Run B and Run
Conclusion
The influence of CA and UA amendment on the in-vessel composting of composite food waste was investigate in this study. The results showed that the amendment of CA and UA could cause a more significant increase in temperature, particularly in the first half of the process. The alkaline components in CA could buffer the acidity of the composting mixture, while a slightly higher pH level was shown in the series with both CA and UA. When amended with CA and UA, degradation intensity would further
Acknowledgements
This research was supported by the Natural Science and Engineering Research Council of Canada. We are also grateful for anonymous reviewers for their helpful suggestions and advices.
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