Inuence of Microbial Inoculant on Composting of Biodegradable Domestic Solid Wastes

This study investigated the effect of microbial inoculant (Emuniv) on composting of biodegradable domestic solid wastes for over 30 days. Three different types of treatments were used for the composting of source separated domestic solid waste. The results showed that the use of microbial inoculant in the composting of domestic solid wastes enhanced the biodegradation of composted organic material, which is evident from the considerable changes in temperature, moisture content and volume reduction with time. The compost treatments with Emuniv inoculant recorded higher temperature for the duration of composting phases, compared to treatment without Emuniv inoculant. Whereas, the moisture content and volume reduction of all treatments was showed a decreasing trend during the composting process. The larger reduction was seen in the treatments with Emuniv inoculant. Additionally, according to the results of analysis of variance (ANOVA), there was a statistically signicant difference in temperature, moisture content and volume reduction between treatments. Moreover, the utilisation of microbial inoculant in composting was also can improve the quality of nal compost product as reected by the total nitrogen and available phosphorus contents in the compost product were higher than in the control. activity and thus, the composting rate. To test these hypotheses, from biodegradable produced with applications of microbial and compared to non-microbial inoculant treatment. The results enable the evaluation of the role of microbial inoculant on the composting process.


Introduction
The increasing level of domestic solid waste has been a serious problem over the last few decades due to the increase in population and urbanization, as well as the toxic components of this waste has become a threat to human beings and the environment. The domestic solid waste generally includes degradable (paper, food waste, straw and yard waste), partially degradable (wood, and sludge) and non-degradable materials (leather, plastics, metals, glass, electronics) [1]. Among these, the biodegradable wastes that constitute the major fraction of domestic solid waste load in developing countries, typically characterized by high water content (> 60%) requires greater operating cost and lesser chances of material recovery. Therefore, to establish a sustainable waste management system, it is essential to separate and recycle biodegradable organic material from the domestic waste stream. Of all the recycling methods, composting is recommended due to its environmental and economic bene ts [2].
Composting is one of the best known processes for biological stabilization of solid organic wastes by transforming them into a safer and more stabilized material that can be used as a source of nutrients and soil conditioner in agricultural applications [3,4]. Composting involves the accelerated degradation of organic matter by microorganisms under controlled conditions, in which the organic material undergoes a characteristic thermophilic stage that allows sanitization of the waste by elimination of pathogenic microorganisms [5]. Successful composting generally depends on some factors that in uence the activities of the microorganism. These including the type of composted raw material, the composition of nutrients, temperature, moisture content, alkalinity, bulk density, porosity, particle size, etc. [6].
Temperature as well stimulates the growth and metabolic activity of the microbial community within compost mass. It can directly affect the biodegradation rate of the organic matter during composting [7].
Under optimal conditions, composting process through the different temperature phases required for effective sanitization of the feedstock and e ciency of the process. A mesophilic phase, characterized by the proliferation of the microbiota, a thermophilic phase where a high rate of biodegradation, the growth of thermophilic organisms and the inhibition of non-thermotolerant organisms occur and the nal phase that includes a period of cooling, stabilization and maturation, characterized by the growth of mesophilic organisms and the humi cation of the compost [8,9]. It is interesting to note that the moisture conditions essentially impinge microbial activity, oxygen uptake rate, temperature and the porosity level within composting [10]. An inverse relation exists between moisture content and temperature, exhibiting an increase in temperature as the moisture content drops [11]. During composting, the microorganism's succession is the key for an effective management of the process. The appearances of certain microorganisms in uence the rate of biodegradation and compost maturity, re ected by the quality of the generated compost [12]. Furthermore, microbial inoculants in uence the process of composting by altering the cellulose, hemicellulose and lignocellulose breakdown process, causing alterations to the temperature and nitrogen levels throughout the composting process [13].
Although several studies have addressed the optimization of composting of various organic wastes [7,10,11,[14][15][16][17], information on composting of biodegradable domestic solid wastes using effective microorganism inoculants as an accelerator to speed up the composting process and increased nutrients in the compost has not been well documented. Therefore, it is essential to study the effect of microbial inoculant on the composting process.
The objectives of the current work are to study the effect of microbial inoculant application on the composting process of biodegradable domestic solid wastes and also to evaluate the nutrient at the end of composting. This study is based on the hypothesis that the application of microbial inoculant on biodegradable domestic solid wastes will increase the microbial activity and thus, increase the composting rate. To test these hypotheses, compost from biodegradable domestic solid wastes was produced with applications of microbial inoculant and compared to non-microbial inoculant treatment.
The results enable the evaluation of the role of microbial inoculant on the composting process. An Emuniv inoculant was selected for this investigation. It is a composition in powder form, this is a product of Applied Microbiology Joint Stock Company, Vietnam, which contains a mixture of effective microorganisms that strong decompose organic waste. The composition of Emuniv inoculant is shown in Table 1. Experimental design included three treatments and they were replicated two times. The treatments were: Treatment 1 (E1) was composted only from biodegradable domestic solid wastes (5 kg) and is acted as a control, treatment 2 (E2) was biodegradable domestic solid wastes (5 kg) with Emuniv inoculant (1 g), treatment 3 (E3) was biodegradable domestic solid wastes (5 kg) were chopped at approximately 3 to 5 cm length with Emuniv inoculant (1 g).

Materials And
A certain amount of 100 g of Ca(OH) 2 and 150 g of Ca(H 2 PO 4 ) 2 was added each treatment to reduce the nitrogen loss and improve the NH 3 recovery during composting. The treatments were conducted under semi aerobic conditions for 30 days. During the composting process, the temperature, moisture content and volume reduction of the compost was determined regularly at 3 days interval. Maturity of the compost was estimated by colour and texture changes observed. When the compost was assumed well matured, homogenized compost samples were taken to determine the compost chemical properties. Samples were placed in polyethylene bags and transferred to the laboratory for analysis.

Analysis of chemical and physical parameters of the compost
Chemical and physical parameters of the compost were determined according to the Vietnamese National Standards (TCVN). The temperature was monitored throughout the composting period using digital thermometers. The temperature at different locations of each treatment was measured and the average temperature was reported. Heights, lengths and widths of the compost were measured at speci c

Statistical analysis
Analysis of variance (ANOVA) was used to identify whether there was signi cant difference among treatments. Statistical analyses were performed with Microsoft Excel 2013 and statistical mean differences were considered signi cant at p < 0.05.

Temperature
Temperature is one of the key indicators for active microbial biomass and may be considerable affecting the composting process [6]. Figure 1 shows variation of temperature in each treatment during the composting process. , and cooling phase. The temperature pattern showed that there is a rapid progress from the initial mesophilic phase to the thermophilic phase for three these treatments, which points to a high proportion of readily degradable substances. The change in temperature pattern observed in this study is in accord with other composting studies [14,15,23].
The temperature pro le in the treatments E2 and E3 were similar as temperatures had risen to maximum temperatures of 43.2°C and 45.6°C, respectively on day 12 and remained within the thermophilic range with narrow uctuations until day 18. The treatment E1 followed the same trend of changes like treatments E2 and E3 but only reached a maximum temperature of 42.5°C. In general, treatments E2 and E3 with Emuniv inoculant recorded higher temperature for the duration of mesophilic and thermophilic phases, compared to treatment E1 without Emuniv inoculant. This can be attributed to the fact that the increase in temperature during the composting phases was caused by the heat generated from the respiration and decomposition of the biodegradable material by the population of microorganisms. The increment in temperature is a good indicator that there is microbial activity in the compost pile, as a higher temperature denotes greater microbial activity [23]. Furthermore, the high temperature is the results of the accumulation of microbial metabolic warmth and indicating the occurrence of degradation where organic matter was being transformed [4]. Even though no microbial test was done on the sample, it can be assumed that the compost treatment with Emuniv inoculant (E2 and E3) has higher microbial activity compared to E1 based on the high temperature achieved. It was already reported in the previous investigations that the increase of temperature in the early phases of composting may be the result of mesophilic and thermophilic phases of composting. The temperature rises as a consequence of the rapid breakdown of organic matter by microbial metabolism. The decrease in temperature indicates that the microbial activities started to slow down due to the low amount of organic material available. Hence the microbial activity stopped and reached the cooling phase [16, 24,25].
On the other hand, the highest temperature was recorded for the treatment E3, as re ected by the fact that the particle size of composted organic material also in uence on the microbial activity during the composting process. An appropriate particle size (3-5 cm) can ensure a more available surface area for better microbial activity during composting, resulting in speedy degradation [13,26].
In addition, based on the Anova test (Table 2), it is apparent that the changes in temperature were signi cant gradual change during the composting process in the individual units of E1, E2, and E3. The difference in temperature between treatments E1, E2, and E3 was also statistically signi cant indicating the effect of Emuniv inoculant on the process of composting.

Moisture content
Moisture content is an essential parameter that in uences the changes in physical, chemical, and biological properties of waste materials during the advancement of decomposition of organic matter. Huerta-Pujol et al. [27] reported the moisture content of the waste mix signi cantly in uences biological activity. According to Norbu et al.
[28], the optimum moisture content for an effective decomposition signi cantly depends on the waste nature. However, a moisture content between 50-60% during composting is crucial for microbial activity [4,29].
In the present study, the amount of moisture in each the treatment was showing a decreasing trend with time. Initial moisture content of treatments E1, E2, and E3 were 60.80%, 62.44%, 60.30% and reduced to 58.44%, 54.01%, 52.92% after 30 days of the composting process, respectively (Fig. 2). This phenomenon could be attributed to the changes in temperature and microbial activity. Moisture content affects microbial activity as well as temperature in the composting process, and thus has a central in uence on the biodegradation of organic materials. The high temperatures as mentioned previously contribute to the reduction of moisture content during composting phase due to the evaporation of water from the compost. Treatments E2 and E3 with Emuniv inoculant which enhances microbial activity resulting in increasing the temperature and the rate of degradation of organic matter, and therefore moisture content of these treatments reduced greater than treatments E1 without Emuniv inoculant. Moreover, according to the results of variance analysis of moisture content in Table 3, there was a statistically signi cant difference in moisture content among treatments E1, E2, and E3 and the decrease in moisture content also indicated signi cant differences in each the treatment during the composting time.
These results and interpretation are consistent with the previous observations in the literature. According to Beck-Friis et al. [30] and Raj and Antil [31], moisture content of the composting blend is an important environmental variable as it provides a medium for the transport of dissolved nutrients required for the physiological activities of microorganisms. The main factor of moisture content loss is due to the rising in compost temperature. This is because the heat generated from the microorganisms activity.
Gajalakshmi and Abbasi [29] reported that in addition to normal water loss caused by the difference in moisture content between the substrate and air, the high temperatures during the thermophilic phase contribute to the reduction of moisture content during composting.

Volume reduction
The volume reduction behaviour of organic solid waste over 30 days of composting of treatments E1, E2, and E3 is shown in Fig. 3. It was observed that the volume reduction of all treatments was showed a sharp reduction during the composting process. The largest reduction of 72.23% was seen in the treatment E3 with Emuniv inoculant. The lowest reduction of 58.09 % was seen in the treatment E1 without Emuniv inoculant, followed by the treatment E2, with a reduction of 67.68 %. This fact can be explained by the high degradability of the wastes used in this study. The reduction in the volume of compost due to the degradation of organic matter during composting may be due not only to weight loss but also the existence of the smaller particles as a result of the deterioration of the structure of organic compounds. Findings in this study also show clearly that composting of organic waste with effective microbial inoculant led to an increase in decomposition of organic waste, and consequently to a greater volume reduction. These results are also con rmed by a variance analysis (Table 4). There are signi cant differences in volume reduction between treatments E1, E2, and E3 as well as the decrease in volume reduction of each the treatment with increasing time of composting. It may therefore be concluded that microbial inoculant considerably affected the volume reduction of compost materials.
The volume reduction is a result of the organic matter degradation by microbial activity during the composting process has also been pointed out by several investigators [10,32]. Additionally, previous studies investigating the change of physical properties of organic wastes during the composting phase have demonstrated that high levels of biodegradable organic matter increased the capacity of the microorganisms to oxidise the material, and consequently reduced the volume [17,33].

The quality of compost product
By the 30th day of composting, except for the control sample, the composting materials showed temperature stability, ne texture, much darker brown or black colours and homogeneity of materials. The composting process was, therefore, halted and samples collected for compost analysis. The compost product quality are listed in Table 5. It is apparent that organic carbon content in treatments E2 and E3 were signi cantly lower than that of treatment E1, while the total nitrogen and available phosphorus contents of these treatments were higher than in the control (E1). Treatment E3 recorded the lowest mean OC of 21.57%, and the highest mean TN and TP of 1.46 and 0.08%, respectively. The low OC after composting might be as a result of microbial decomposition of organic substrates, which was evident of temperature increases during composting as microorganism consume carbon for energy [4]. Furthermore, the low OC was served as an indicator of compost maturity and stability [34]. The high level of the total nitrogen in nal compost product could be attributed to the Emuniv inoculant effect as a result of substantial degradation of organic carbon compounds to nitrogen component, and the contribution of nitrogen-xing bacteria [35,36]. The high level of the available phosphorus is probably due to mobilization and mineralization and mobilization of phosphorus due to microbial activities of Emuniv inoculant.
These results are similar to those obtained by several other previous investigations. According to Teshome and Amza [37] and Tibu et al. [15], the OC values of the matured composts for all the treatments were found to be in between 19-33%. Benito et al. [38] found that the total nitrogen rate in the compost product ranged from 0.99 to 2.01%. Similar trends were observed by Dessalegn et al. [39] who indicated the total nitrogen content of the matured compost was between 1.05 and 1.13 %. Therefore, it can be concluded that composting of organic solid wastes using microbial inoculants (Emuniv) was not only can enhance composting rate, but also can improve the quality of nal compost greatly.

Conclusions
The results indicated that microbial inoculant Emuniv had a signi cant impact on the composting of biodegradable domestic solid wastes and the quality of nal compost product. The temperature in the treatments E2 and E3 with Emuniv inoculant increased to maximum temperatures of 43.2°C and 45.6°C, respectively, while the treatment E1 without Emuniv inoculant only reached a maximum temperature of 42.5°C. Initial moisture content of treatments E1, E2, and E3 were 60.80%, 62.44%, 60.30% and decreased to 58.44%, 54.01%, 52.92% after 30 days of the composting process, respectively. The volume reduction of all treatments was showed a considerable reduction during the composting process. The largest reduction of 72.23% was observed in the treatment E3. The lowest reduction of 58.09% was observed in the treatment E1, followed by the treatment E2, with a reduction of 67.68%. Moreover, the results of ANOVA demonstrated that the relationship between the chemical and physical parameters such as temperature, moisture content and volume reduction of treatments was statistically signi cant difference. In addition, composting of organic solid wastes using microbial inoculants improved the quality of nal compost greatly, which is evident from the considerable higher in the total nitrogen and available phosphorus contents of treatments E2 and E3, compared to treatment E1. Overall, the results obtained revealed that composting with microbial inoculant is a promising method for domestic organic waste treatment.

Declarations
Availability of data and material All data generated or analyzed during this study are available upon request.

Figure 2
Variation of moisture content during the composting period