Physico-chemical characterization of compost
The evaluation of quality of compost and its maturity and nutrient level is mandatory to determine the possible use of the compost generated (Mandal et al. 2014). The major parameters used for the evaluation of compost quality are summarized in Tables 3, 4 for both Solan and Mandi study regions, respectively.
The temperature of the compost samples on the 60th day of decomposition process was recorded to be 54 and 59° C for Solan and Mandi region, respectively. Same compost samples showed recorded temperatures of 60 and 64° C at Solan and Mandi when composting was carried out on 20th day. The changes in the temperature could be attributed to the exothermic process exhibited by the microbial decomposition of organic matter (Mandal et al. 2014).
This also signifies that during the decomposition process, temperatures are slightly elevated in comparison to the temperatures observed after complete degradation (Saha et al. 2010). The total organic carbon (TOC) signifies the amount of organic matter present in the municipal solid waste compost material. In this context, it was observed that the range of TOC for both the study regions was between 12 and 18% which is within the specified limits as reported in the literature, wherein it has been mentioned that TOC varied between 4 and 20% in most of the Indian cities (Saha et al. 2010).
Moisture content of the municipal solid waste compost is an important parameter as it helps determining the storage and the ease of transporting of the final compost material (Saha et al. 2010). As such, the range of moisture content of MSW compost for both the study regions varied in between 31 and 45%, whereas the reported literature revealed the range between 3.6 and 45.4% for most of the Indian cities (Mandal et al.2014). However, the reported values were significantly higher than the standards prescribed by Fertility Control Order 1985, wherein the recommended value lies between 15 and 25% (Saha et al. 2010). The pH of the compost samples for the study regions varied in the range from 7.84 to 7.08 from 20th day to 60th day of degradation process for Solan, whereas the same was observed in the range 8.24–7.48 from 20th day to 60th day of degradation process for Mandi. Hence, the pH values of the compost samples for both the study regions were within the alkaline–neutral range, thereby showing the decomposition of organic matter. The past studies revealed that the pH of municipal solid waste compost varied in the range from neutral to alkaline in one of the compost plants of Delhi which was not the sign of matured and nourished compost as a fertilizer. The alkalinity of pH leads to the formation of ammonium gas in the air that tends to increase the harmful pathogenic bacteria in the atmosphere (Mandal et al. 2014).
The electrical conductivity is another important parameter to determine the chemical properties and nutrition level of the compost (Manohar et al. 2016). The value of electrical conductivity was observed to be 6.7 dS/m in the 20th day of composting period, 6.1 dS/m in 40th day of degradation and 5.8 dS/m in the 60th day of composting period for Solan region and was found to be comparatively less from Mandi region which reported 6.0 dS/m in 20th day of degradation, 5.65 dS/m in 40th day of degradation and 5.23 dS/m in the 60th day of degradation period, respectively. The values at both the study locations slightly exceeded the permissible limits as prescribed by FCO India. Comparison with other reported literature for Indian cities reported similar values (Manohar et al. 2016; Mandal et al. 2014).
The concentration of macronutrients such as nitrogen, phosphorous and potassium (NPK) are important parameters to determine the quality of compost as fertilizer (Manohar et al. 2016). The concentration of nitrogen was found 0.64 mg/kg, 0.72 mg/kg and 0.80 mg/kg in the 20th day, 40th day and 60th day of composting period, respectively, for Solan region and 0.69 mg/kg, 0.88 mg/kg and 0.92 mg/kg in the 20th day, 40th day and 60th day of composting period, respectively, for Mandi region and as per FCO standards, India (Saha et al. 2010). The phosphorus content of the compost samples was observed 2.34 mg/kg, 1.98 mg/kg and 0.92 mg/kg in 20th day, 40th day and 60th day of composting period, respectively, for Solan region, whereas 2.86 mg/kg, 2.12 mg/kg and 0.78 mg/kg in 20th day, 40th day and 60th day of composting period, respectively, for Mandi region, respectively. Apart from this, the potassium content was observed 14.1 mg/kg, 9.2 mg/kg and 7.4 mg/kg in 20th day, 40th day and 60th day of composting period, respectively, for Solan region, whereas the concentration was observed slightly more for Mandi region as 16.20 mg/kg, 12.50 mg/kg and 9.20 mg/kg in 20th day, 40th day and 60th day of composting period. Reported concentrations for nitrogen, phosphorus and potassium were 0.702 mg/kg, 2.1 mg/kg and 8.8 mg/kg, respectively, at the end of the maturation stage for Mysore city compost pit (Manohar et al. 2016) and 0.5 mg/kg, 0.6 mg/kg and 5.8 mg/kg were reported for the compost plant of Delhi (Mazumdar 2007) and were quite similar to the values determined for the study locations including Mandi and Solan. It has been further reported in the literature that the compost prepared from rural municipal solid waste generally has a higher proportion of nitrogen and phosphorus content in comparison to prepared content from the urban waste (Saha et al. 2010). Similarly, it has been reported in the literature that compost generated from European countries has higher concentration of carbon, nitrogen and phosphorous (Saha et al. 2010).
The calcium and magnesium concentrations in compost are important for growth of microbes (Manohara et al. 2017). Further, these parameters determine the fertility potential of the compost at the maturation stage. The calcium content in the municipal solid waste compost was observed 12.08 mg/l in 20th day of composting, 15.02 mg/l in 40th day of composting and 16.24 mg/l in 60th day of composting period for Solan region, whereas it was observed 15.24 mg/l, 17.01 mg/l and 17.89 mg/l in 20th day, 40th day and 60th day of the composting process, respectively. Apart from this, the magnesium content was observed 9.72 mg/l, 6.84 mg/l and 5.59 mg/l in 20th day, 40th day and 60th day of the composting process, respectively, for Solan region, whereas it was observed slightly more for Mandi region, i.e., 10.02 mg/l in 20th day of composting, 8.18 mg/l in 40th day of composting process and 7.24 mg/l in 60th day of the composting process, respectively. The earlier studies reported the concentration of calcium and magnesium at the maturation stage in the range of 18.02 mg/l and 6.79 mg/l in compost pits of Mysore city as reported by Manohara et al. (2017). The total organic carbon content was observed 18.28, 16.24 and 14.22% in the 20th day, 40th day and 60th day of the composting process, respectively, for Solan region, whereas the same concentration was reported 17.40, 14.35 and 12.46% in the 20th day, 40th day and 60th day of the composting process, respectively, and for Mandi region of Himachal Pradesh that has been found lesser than the minimum value of the FCO standards. In this context, the parameters including magnesium and total organic carbon were observed decreasing up to 60th day of the composting process which has attributed to the liberation of the carbon dioxide and methane gas, whereas calcium goes on increasing with the degradation process that proves beneficial for the compost manufacturing process. The C/N ratio of the municipal solid waste compost is an indication about the maturity level of the compost material which was observed 29.28% in the 20th day of composting, 28.91% in the 40th day of composting and 26.02% in the 60th day of composting period for Solan region, whereas it was found 30.01% in the 20th day of composting, 29.12% in the 40th day of composting and 28.32% in the 60th day of composting period for Mandi region, respectively, hence found more than the minimum criteria of the FCO (fertility control order) limits (Saha et al. 2010). The observations of the earlier studies of compost characterization in 59 cities of India described C/N ratio range in between 7.2 and 36.5% that was quite high (Saha et al. 2010). Higher carbon/nitrogen content relatively more than 30% in the matured compost makes it unsuitable for use as fertilizer for the crop nourishment (Manohara et al. 2017).
Heavy metal analysis
The major parameters used for the heavy metal analysis are summarized in Tables 5, 6 for both Solan and Mandi study regions, respectively.
In general, it was observed from the tables that the concentrations of heavy metal of the compost produced from Solan exceeded that of Mandi but were within the prescribed FCO limits. Further, the evaluation of heavy metals analysis revealed that the compost samples from the Solan were rich in nickel and lead content as compared to other heavy metals due to the excessive amount of batteries and cells being found in the dumpsite as mix wastes and in comparison to Mandi study location. Chromium was found at lower concentrations at the maturation stage of the compost, i.e., at 60th day of the composting period reported as 1.85 mg/kg and 0.23 mg/kg for Solan and Mandi regions, respectively. Similarly, cadmium concentration was observed 1.18 mg/kg at the maturation stage of the compost for Solan region and the same was found below detection levels for Mandi region. The heavy metal concentration of the municipal solid waste compost was found within the permissible limits as prescribed by fertility control order standards (FCO). Further, it was observed that the heavy metal concentration decreased with the increase in composting period and is similar to the pattern as reported earlier (Manohara and Belagali 2016). The reported literature reported comparatively lesser value of cadmium content at the maturation stage of the compost samples in the compost plant of Mysore city, i.e., 0.01 mg/l (Kiran and Srikantaswamy 2014). However, the average content of heavy metals in the compost material prepared in Indian cities was comparatively low than the compost material prepared in USA and European countries (Saha et al. 2010). Higher the concentration of heavy metals signifies greater polluting potential of the compost and lesser value of clean index (CI).
Fertility index and clean index
The fertility index (FI) and clean index (CI) of the compost are quality-based parameters for determining the gradation of compost which determines its market value. To summarize, there are total seven classes of compost quality classification including A, B, C, D, RU-1, RU-2 and RU-3 based on the determination FI and CI. The four classes A–D depicted good quality compost and have good market value and can be used for organic farming and high value crops. The remaining classes are having restricted usage and cannot be applied for organic farming. The classification of MSW compost for their marketability and use in different areas is shown in Table 7.
The criteria for ‘weighing factor’ to heavy metal parameters and ‘score value’ to compost for the evaluation of fertility index and clean index for both the study locations are presented in Tables S1–S12 of supplementary material. Further, a compost quality evaluator software (Saha et al. 2010) based on the MS excel blueprint has also been utilized to check the results obtained. The FI determined after 20th, 40th and 60th day of sampling of compost at the study location was 3.5. It was observed that the FI of compost generated at the Solan study location was similar for all of the sampling period. This signifies that primary degradation of the waste occurs by 20th day
The FI value for compost generated from Solan (3.5) can be categorized as Class D as outlined by FCO standard and description mentioned in Table 7 for samples tested after full maturation (60th day). Similarly, the FI values for compost generated at Mandi site were determined to be 3.5 and 3.6 for samples tested after 20th day and for both 40th and 60th day. Hence, the compost can be categorized as Class A after full maturation period. Similarly, the CI value of the compost samples was determined to be 4.0 and 5.0 for Solan and Mandi study locations, respectively, for all the sampling periods. The compost generated at the study locations meets the specified criteria as per FCO standards since the determined heavy metal concentrations are less than the FCO standards. Different FI and CI values are summarized in Table 8 and the classification of compost category is shown in Table 9.
The results obtained from our study locations were compared with the compost generated at Okhla compost plant in Delhi, wherein the determined FI and CI values were 4.54 and 2.60 (Mandal et al. 2014), respectively. This revealed that the compost generated at the Okhla compost plant in Delhi was of poor quality due to high concentration of heavy metals and hence can be classified for Restricted Usage (RU 3).
To summarize, the mature compost generated at Solan study location is of medium quality having medium fertilizing and pollution potential and hence may be used for producing livestock for non-food crops. For, the compost generated from Mandi site, it can be classified as class A having very high fertilizing potential and low heavy metal content and hence suitable for growth of livestock of food crops.
SEM–EDS analysis of compost material
The SEM micrographs show the physical changes occurring during various stages of microbial degradation of municipal solid waste compost of both the study regions (Solan and Mandi) for 20th day, 40th day and 60th day of degradation process and are shown in Figs. 2(a, b), 3, 4, 5, 6, 7(a, b), whereas XRD analysis of compost samples is shown in Fig. 2(c), 3, 4, 5, 6, 7(c).
From the images obtained by SEM–EDS analysis for both the study regions, it is observed that in the time frame of the composting process from 1 to 20 days, the morphology showed a larger-sized solid material with smaller voids created on the surface. With the increase in time (20–40 days) and increased degradation process, the particle size started to reduce and finally after the complete degradation process (60th day), the particles were converted into smaller-sized solid particles with increase the voids on the surface indicating the formation of mature compost. This was observed for both the study locations
The electron dispersive spectroscopy (EDS) of the compost sample in Solan showed the presence of eight natural elements, namely oxygen, silicon, carbon, aluminum, sodium, calcium, potassium and magnesium (Table 10).
The concentration of oxygen was determined to be relatively higher (60.35%) in comparison to other elements. The carbon fraction was determined to be 16.19% for samples collected on the 60th day of degradation process of compost when analyzed through EDS whereas the organic carbon content was found to be 14.22% by the Walkley method. In this context, it is revealed that out of total carbon percentage in the municipal solid waste compost samples, the maximum amount is observed as organic carbon fraction. A similar electron dispersive spectroscopy analysis was carried out on the compost samples from Mandi region and was reported to have the eight elements including oxygen, carbon, silicon, aluminum, potassium, calcium, sodium and iron (Table 11).
However, the oxygen fraction (54.53%) for the samples analyzed for 60th day sampling period was less than observed for the samples from Solan study area. Interestingly, a higher proportion of carbon content was reported for compost samples from Mandi being determined as 28.71% on the 60th day of degradation process of compost, whereas the organic carbon content was found 12.46% by Walkley method because out of total carbon fraction, the percentage of organic carbon was observed more in the sample. The percentage of organic carbon in the compost sample of Mandi region was found slightly less than the organic carbon in the compost sample of Solan region. This was due to the reason that the organic matter has been observed more in the municipal solid waste of Solan region than Mandi region. The sufficient amount of oxygen and carbon promotes the aerobic composting of the waste carried out in both of the study locations.
XRD analysis of compost material
XRD analysis of compost material for three decomposition stages in the Solan region and Mandi region of Himachal Pradesh is shown in Figs. 2(c), 3, 4, 5, 6, 7(c) (presented in above section). X-Ray diffraction is the useful tool to get the structural information of any compound (Kiran and Srikantaswamy 2014). In particular, each signal in XRD represents the plane of a crystal. The spectra of composting piles of both study regions revealed acute peaks for composting samples collected on 20th day. Subsequent collected samples on 40th and 60th day at both the study locations showed the reduced number of these peaks due to increase in degradation process leading to formation of mature compost. The intense peaks appearing on the spectra of XRD analysis of compost material revealed the presence of some major compounds including calcium carbonate, calcium thallium nitride, silicon oxide, niobium sulfide and zinc sulfide and minerals including quartz, calcite and dolomite. The presence of minerals including quartz, calcite, and dolomite was due to the disposal of mix waste in the dumping site. The presence of inert materials, sand, grit, egg shells, and green waste was attributed minerals in the municipal solid waste compost samples. From the overall data obtained from the X-ray diffraction spectra, it is clear that particle size of solid waste decreases during the degradation process of the compost material.
In this context, the results of scanning electron microscopy of compost samples from both the regions (Mandi and Solan) revealed that with the starting phase of degradation process up to the 60th day of degradation process of compost, the large-sized particles were converted into smaller particles showing maturation of the compost, whereas XRD spectra also indicated the sharp peaks of minerals at the starting degradation phase of compost and up to the 60th day of degradation process; very few peaks were shown in the spectra revealed breakdown of particles and indicate matured compost. The results obtained from XRD are having good collaboration with the SEM analysis of compost samples.
Discussion on compost generation in state of Himachal Pradesh
The MSW generated in Himachal Pradesh has a high proportion of biodegradables which can be easily generated into compost and can be used as a natural fertilizer. However, the existing scenario of waste management is not very satisfactory in Himachal Pradesh (Sharma et al. 2018). Previous literature studies (Saha et al. 2010) have reported the practice of composting in regions of Kullu, Solan, Shimla and Manali in Himachal Pradesh. Though the type of feedstock use was different, some composting practices were in use at the time of reported literature by Saha et al. 2010. At the time of the reported study of Saha et al. 2010, the municipalities of respective regions had received and utilized funds for the construction of composting yards but the efforts to make compost were not very successful. The reasons for discontinuation of composting at the study locations reported by Saha et al. 2010 are manifold with major reasons being failure to implement complete and proper segregation process at Kullu and Manali study location leading to generation of poor quality compost (Personal Communication with an official of Municipal Corporation Shimla). The reason for discontinuation of composting process in Shimla is due to non-availability of land to meet the increased MSW generated and hence the more preferred treatment of MSW is to burn them in Refuse Derived Fuel (RDF) plant as it leads to immediate reduction of waste (Personal Communication with an official of Municipal Corporation Shimla). Other proposed reasons for discontinuation of the composting process in Himachal Pradesh are due to poor segregation, uneconomic viability of compost generated, lack of funding and lack of manpower (Personal Communication with an official of Municipal Corporation Shimla).
At the time of carrying out the present study, two compost pits were under working condition in Solan and Mandi locations but these plants have also presently stopped their composting process due to their non-economic viability. This is an alarming trend in the state that the once existing composting system is gradually losing its way. The reasons for closure of the composting units in Solan were its geographical issue (dumpsite being located adjacent to National Highway), breakdown in communication of functioning of the plant under public–private partnership (PPP), non-economic feasibility of compost generated and lack of funds for smooth operation of the plant. For the Mandi site, the major reason for closure of the plant was the lack of availability for the operation of the compost plant. As observed from our study, the compost generated in this study location is of very high quality. Apart from this, currently the RDF plant has been installed in the dumpsite of Shimla for the processing of biodegradable waste which is under working condition.