- Open Access
Variation in the key indicators during composting of municipal solid organic wastes
© The Author(s) 2019
- Received: 2 March 2018
- Accepted: 23 January 2019
- Published: 11 April 2019
High moisture content in organic wastes leads to a leachate production. Vegetable waste is one kind of municipal solid organic wastes holding very high moisture (88–94%) and thus becoming nuisance to the environment. Composting is a viable technology to treat such organic waste. This research mainly deals with the insight on physic state of composting during composting of vegetable wastes added with the inoculum and bulking agents. This study showed the attainment of thermophilic stage (65–70 °C) during the composting process and completed with the pathogen-free end product. The bulk density increased from 312 to 380 kg m− 3 and was well within the recommended range (less than 1000 kg m− 3). The electrical conductivity of the end product was also within the mentioned range (< 4 dS m− 1). On evaluating the Pearson’s correlation coefficient matrix amongst physical parameters, the moisture content and the free air space exhibited positive relationship.
- Vegetable wastes
- Bulk density
- Free air space
- Statistical correlation
Composting is the degradation of organic wastes in the presence of oxygen. It is one of the best biological treatment that converts waste into a useful compound known as compost. It is an eco-friendly, inexpensive and viable process for organic waste management. It naturally improves the handling characteristics of organic wastes by reducing its volume and weight . Composting process is regulated by the fundamental environmental factors such as temperature, moisture content, pH and aeration and some characteristics (such as C/N ratio, particle size and nutrients) of organic wastes .
Organic wastes such as vegetable wastes possess higher moisture content (88–94%) and slightly acidic pH (5.1) as characterized in this study. Around 50 Mt. of waste is being produced due to the disposal of vegetables and fruits that causes the major environmental pollution at municipal dumpsites . Previous study reported that vegetable wastes decomposes naturally and produces organic acids  and leachate that may cause detrimental effects on water bodies. Moreover, disposal in municipal dumpsites may cause the contamination of soil. The composting process is a proven, safe and economical way to manage these organic wastes. Previous studies also found the composting of vegetable wastes and its application in the soil [5–7]. Varma and Kalamdhad  observed that the moisture content and leachate production have the adverse effects on the properties of the compost with the significant loss of micro-nutrients and heavy metals during composting of vegetable waste. Feasible composting condition may avoid the leachate production, thus achieving the success of compositing process. An application of immature compost may inhibit the seed germination and arrests a growth of plant . The efficiency of the process and the degradation rate of the organic wastes is enhanced by providing the adequate aeration in the composting mixtures . Bulking agents are known to be the best materials to achieve the feasible composting conditions by distributing an optimum aeration required for the complete process.
The primary function of the bulking agents is to absorb an excess moisture content during the composting process, in addition to maintaining the carbon: nitrogen ratio in a feedstock. It not only enhances the microbial activity but also adjusts the porosity of a raw material and thus improves the overall aeration . Several researches have been carried out on the effect of bulking agents during the composting of various types of sludges (sewage and industrial sludge) using distinct composting methods [10, 11]. An experimental study was also performed to evaluate the effect of bulking agents during the composting of municipal wastes and animal manure . Earlier researches have demonstrated the effect of bulking agents on biological/chemical properties and gaseous emissions. However, rarely the literature is lacking on the physical state of composting or variation in the physical parameters such as bulk density (BD), porosity, free air space (FAS) during a rotary drum composting of vegetable wastes added with inoculum and bulking agents. The higher moisture content in the vegetable wastes can have an adverse impact on the rate of microbial growth during the composting process. There is a need to provide a better insight on the variation in physical properties during the decomposition of a vegetable waste added with the inoculum (cow dung) and bulking agents (sawdust and dry leaves). The parameter variation in the temperature, moisture content reduction, and other physical parameters (that controls the entire composting process such as BD, porosity, FAS, and particle density) along with the variation in thermal properties (such as a specific heat capacity) needs to be monitored and evaluated during the composting process.
The physical parameters provide a better insight on the availability of water and oxygen that are vital for the survival of micro-organisms during the composting process. The higher amount of moisture content causes leachate production thus reduces the porosity and oxygen during composting. The favorable moisture content required for the biological decomposition lies between 50 and 70% . According to Haug , FAS should be more than 30% in the end product. The sufficient air flow can be supplied by the bulking agents to improve the structural and functional properties during the composting process. Bulking agents change the biodegradation kinetics and the overall performance of composting process . Mohee and Mudhoo  reported the strong relationship amongst physical parameters that may cause a significant change in the quality of an end product of composting. No such studies carried out during composting of vegetable wastes mixed with the inoculum and bulking agents in high capacity (550 L) rotary drum composter.
The novelty of this research work is to study the physical state of composting or variation in physical parameters during rotary drum composting of vegetable waste mixed with inoculum and bulking agents. Hence, this study aims to determine: (1) how the combined addition of bulking agents and inoculum affects composting during the composting of vegetable wastes; (2) how these additions improvise the chemical and nutritional quality of the final compost; and (3) what are the correlations among various physical parameters.
Composting reactor, materials, and process
Characteristics of composing materials used in this study
Moisture Content (%)
89 ± 2.4
82 ± 0.5
10.3 ± 0.2
24.9 ± 3.5
Volatile Solids (%)
34 ± 3.3
88 ± 4.1
65 ± 1.9
72 ± 1.5
EC (dS m− 1)
1.9 ± 0.05
3.4 ± 0.02
0.6 ± 0.03
0.6 ± 0.04
Bulk Density (kg m− 3)
530 ± 5
104 ± 3
375 ± 13
73 ± 4
19 ± 0.3
25.8 ± 1.5
160 ± 16
45.2 ± 1.5
Sampling, monitoring, and analyses
Sampling and monitoring
The samples were collected (300 g per samples) from the center and two extremities (top, middle, and bottom) from each reactor. Three samples were collected on day 0, 2, 4, 6, 10, 14, 18 and 20. Then each sample was divided into two parts. One part was oven-dried, ground to pass through a 212 μm soil sieve and stored in a vacuumed desiccator. The oven-dried samples were used for the determination of physical properties, pH, electrical conductivity (EC), particle density, and the contents of volatile solids (VS), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), total phosphorus (P), and total potassium (K). While the other part, which is not oven-dried, was used to quantify the BD, moisture content, and porosity.
Temperatures in the top, middle, and bottom layers of the composting mixtures in the rotary drum composters were monitored using a digital thermometer with a temperature sensor attached to it. Temperature data was collected daily after every 4-h during the entire composting process, and three reading were averaged per composting mix. Ambient temperature was also recorded using the same temperature sensor.
Physical and nutritional analyses
where ℊ = specific gravity; γw = BD of water; ωwet = wet moisture content; γwet = BD of compost.
where, ASH is the mineral content (%) and MCd.b is the moisture content (%) in dry weight basis.
TKN was determined by using the Kjeldahl distillation method. Stannous chloride method (acid digestion) was used to find the available and TP . Potassium (K) was determined with a flame photometer by digesting 0.2 g air-dried 212 μm sieved sample with 10 mL H2SO4 and HClO4 (5:1) at 300 °C for 2 h .
As noted earlier, the samples gathered from individual composting reactors were treated as triplicates for each sampling time and their mean with standard deviation is reported in this paper, which is calculated using Microsoft Excel, 2010. Parsons’s correlation coefficient was calculated using SPSS 20.0 among various physical parameters.
Composting temperature and moisture content
The temperature 55–60 °C for more than 3 d is essential during the composting process to obtain a pathogen free compost . The addition of the dry leaves (10 kg) substantially increased the period of the thermophilic phase that lasts for more than 5 d during composting process. Similar types of results were obtained during the composting of vegetable wastes . According to Khalil et al. , the conventional composting process requires 90–270 d to produce stable compost. However, in this study the biological activity was observed stable after 18 d, which is indicated by stability in VS reduction on day 20.
The value of compost properties summarized at initial and final experimental periods
Moisture Content (%)
67.9 ± 0.7
57 ± 0.5
Volatile Solids (%)
74 ± 0.7
65.3 ± 0.6
Bulk Density (kg m−3)
312 ± 5.4
380 ± 3.3
Total Kjeldahl Nitrogen (%)
1.54 ± 0.2
2.24 ± 0.3
Available Phosphorus (g kg− 1)
3.41 ± 0.3
4.35 ± 0.5
Specific heat capacity (J kg− 1 K− 1)
29.2 ± 2.1
37.78 ± 1.7
24 ± 0.8
16 ± 0.9
Variation in total organic carbon, porosity and specific heat capacity throughout composting process
Total organic carbon (%)
Specific heat capacity (J kg−1 K− 1)
41.0 ± 0.7
49.2 ± 0.7
32.1 ± 0.3
40.7 ± 0.2
48.7 ± 0.3
36.4 ± 0.4
40.3 ± 0.3
48.5 ± 0.7
35.7 ± 0.1
40.3 ± 0.1
48.3 ± 0.8
37.2 ± 0.0
39.7 ± 0.3
48.3 ± 0.9
39.3 ± 0.4
38.2 ± 0.5
48.2 ± 0.6
41.1 ± 0.3
36.0 ± 0.3
48.2 ± 0.0
42.1 ± 0.4
36.3 ± 0.7
48.1 ± 0.5
43.3 ± 0.5
Composting VS and TOC
Previously reported studies showed that the bulking agents acts differently for the reduction of the organic matter during the composting of various wastes which depends on the type and quality of the bulking agent used during the composting process. The addition of recalcitrant organic materials reduces the degradation of the organic matter and enhances the humification and the quality of the final composts . In the present study, the TOC and VS decreased significantly during composting with the maximum reduction occurring during thermophilic phase (> 45 °C). The 13% decrease from the initial TOC value was observed during entire composting process as tabulated in Table 3. An initial VS was 74% and reduced to 65% after 20 d of composting (Fig. 2b). It was noted that added dry leaves have significantly contributed more organics and VS. Hence, such bulking agents should be reused as Varma et al.  stated that the removal of bulking agents from compost could provide a better reduction in organics and VS.
BD, porosity, FAS and PD
Excessive moisture present in the organic wastes causes leachate formation and reduction in the porosity and FAS. Physical parameters such as porosity and FAS are evaluated for optimizing the composting process. FAS should be present more than 30% for proper aeration during composting process . Bulking agents are carbonaceous material that provides optimal FAS. Moreover, it regulates the moisture content during the composting process. An interesting observation was that despite of 22% increment in BD, the variation in the porosity was not significant during the entire composting process as shown in the Fig. 3a and Table 2. Bulking agents also increased the pore space . Furthermore, volume reduction after composting revealed that enough pore space was available for microbial activity to degrade the organic matter during the composting process.
Percentage of free air space occupied in the different compost mixtures at the end of treatment
Particle density in the different compost mixtures at the end of treatment
End compost mix
Particle density (kg m− 3)
Woodchips, cattle manure and mixed vegetables
Five mixtures of garbage, sludge cake, paper and vermiculite
Wheat straw and chicken manure
Vegetable wastes, cow dung, sawdust and dry leaves
Composting specific heat capacity
The thermal properties such as heat storage capacity, or specific heat, of the bulk material usually influences the amount of material required to maintain the appropriate rise in temperatures . The specific heat capacity is also the function of moisture content. As anticipated, the increase in the ash content was observed with the decrease in the VS contents (Table 3). An increase in ash content directly influences the increase in specific heat capacity during the composting process. Moreover, Rahman  revealed a linear relationship between moisture content and thermal specific heat. For current the study, the 16% increment in specific heat capacity was observed during the thermophilic phase.
Composting pH, EC and nutritional properties
pH is an important parameter in governing the decomposition process . Biological activities are mostly influenced by the change in pH, and a neutral pH is recommended to be optimal for the composting process . The pH showed an increasing pattern during the composting process as illustrated in Fig. 3b. During the initial rise of temperatures, the pH increased quickly from 6.8 to 7.1 in the thermophilic phase. Subsequently, increase in the pH was noted during 20-d composting process. The pH for end compost was recorded as 7.6. Others revealed that the range of pH between 7 and 8 supports the microbial activity for the decomposition of organic matter during composting process . In the present study, the pH was close to the desired optimal range for composting process and in the later stage, pHs were relatively constant as the composting process ended indicating the stabilization of process and lower microbial activities. Increase in the pH during the composting process occurs mainly due to activity of proteolytic bacteria and higher buffering capacity of bulking agents avoids further pH fluctuation .
The mineral cation concentration increases with the decomposition of organic materials and is not reduced by binding to stable organic complex . The changes in EC showed a typical trend during the composting process as shown in Fig. 3b. During initial rise of temperatures, the EC increased quickly, or from the initial 2.1 increased to 3.5 dS m− 1 during composting. Zhang and Sun  reported that addition of bulking agents usually increases the EC values during the composting process as it increases the production of inorganic compounds and the release of ions. The EC value less than 4 is recommended for the compost application in soil.
The bulking agents could enhance organic matter decomposition and hence increase the rate at which ions are released. It may be the reason for an increase in EC during composting added with inoculum and bulking agents.
Nutritional properties such as nitrogen, phosphorus, and potassium were measured during 20-d composting process. It was observed that all nutritional parameters were observed to be increasing during the composting process. The TK concentration was observed to be increased from 14.3 to 25.2 g kg− 1 during the composting process as illustrated in Fig. 3c. The addition of bulking agents absorbed relatively large amount of moisture and intact structural integrity and porosity could be responsible for preventing the loss of potassium during the composting process. Phosphorus concentration was also found gradually increasing from 3.4 to 4.4 g kg− 1 during the composting process (Table 2). In the present study, TKN also followed an increasing trend; from the initial value of 1.5 to 2.2% - an increase by 1.5-fold during the composting process (Fig. 3c). An increase was due to the addition of bulking agents that absorbed the higher amount of both aqueous NH4-N and gaseous NH3 during composting process .
Pearson’s correlation between physical properties during composting process
Vegetable wastes, cow dung, sawdust and dry leaves
The results of the current study signified the scope of additives aided composting process. The moisture content was observed to be 57% at the end of the composting process, and was within the recommended range. The maximum temperature rise was observed up to 68.9 °C during composting process. BD followed the increasing trend during the composting process. The physical parameters such as moisture content, BD, porosity and FAS found to be in the relationship with each other. FAS and moisture content exhibited a positive correlation with an extremely high correlation coefficient (r = 0.98).
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