Municipal solid waste composting with amendments reduces heavy metals hazard, ecological risks and greenhouse gases emission

doi:10.21203/rs.3.rs-3762705/v1

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Municipal solid waste composting with amendments reduces heavy metals hazard, ecological risks and greenhouse gases emission

https://doi.org/10.21203/rs.3.rs-3762705/v1

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Open dumping and landfilling of municipal solid waste (MSW) in India cause environmental hazards and greenhouse gases (GHGs) emissions. Therefore, studies require to assess the potential of aerobic composting of MSW to minimize these risks. This study evaluates the potential of MSW composting with or without some amendments (i.e. G-gypsum, ES-elemental sulphur and RS-rice straw) in minimizing these risks. The composition, physicochemical characteristics including heavy metals (HMs), the potential pollution hazards and GHGs emissions were assessed for the MSW collected from domestic and industrial sites of Haryana, India. Industrial MSW had higher pH (7.31) and HMs concentrations (e.g. Cr, Cd, Cu, Ni, Pb and Zn), than domestic MSW. HMs were found within the critical limits for both sites except Cr and Cu in industrial MSW. Composting lowered the HMs and ecological risk and increased N, P, K and organic carbon. Co-composting with amendments like gypsum, elemental sulphur and rice straw improved C:N ratio, nutrients concentration and accelerated the degradation process. The pollution indices enrichment factor, contamination factor, geoaccumulation index, ecological risk and potential ecological risk index indicated lower ecological risk and HMs concentration in domestic site MSW. On-site GHGs emission significantly differed in space and seasons. Emission of CO₂, CH₄and N₂O from domestic landfill site were 20%, 27% and 64% higher than industrial and higher in monsoon season, followed by summer and winter, respectively. The highest GHGs emission was from center (heap top) (924.58 mg m^-2 h^-1), followed by middle (805.23 mg m^-2 h^-1) and peripheral (717 mg m^-2 h^-1) area of sites. Use of ES and RS amendments in co-composting reduced up to 41% GHGs emission than landfill emissions. MSW co-composting with elemental sulphur and rice straw can be a potentially safer alternative for the management of both domestic and industrial MSW than open landfill dumping.

Municipal solid waste

Composting

Pollution indices

Greenhouse gas emission

Landfill site

Economic and demographic growth of cities brings changes in land use patterns and lifestyle. Technological advancement, coexistent with lifestyle, has led to an increase in the quantity and complexity of urban municipal solid waste (MSW) production and its safer management (Ghosh et al. 2018). In India, about 62 Mt of urban MSW was generated at 450 g /capita/day in 2015 (CPCB 2016). Out of this, about 82% (50 Mt) was collected and only 28% of the total collected was processed and treated, while the remaining 72% (36 Mt) is openly dumped. The present trend of MSW production in 366 major cities indicates that the total MSW generation will increase to ~ 165, 230 and 436 Mt by 2030, 2041 and 2050, respectively (WtR 2014). The collected MSW comprises of about 40–60, 30–50 and 10–30% of compostable, inert and recyclable material, respectively. Landfilling is considered as the most commonly used integral part of any planned MSW management system worldwide (Devi et al. 2016) because it requires less investment and skilled workers. However, GHGs emission and the leaching of pollutants and nutrients remain major problems from insecure MSW landfills in India (Sharma and Siddharth 2019). Therefore, the diversion of waste from landfills is required to reduce the negative environmental impacts such as leachate contamination, GHGs emissions and space limitation.

Global climate change is one of the most serious environmental challenges of the 21st century and the waste sector accounts for approximately 5% of global GHGs emissions (Zhang et al. 2019). Globally, landfills contribute about 40% of methane (CH₄), a significant amount of carbon dioxide (CO₂) (US EPA, 2010) and N₂O emissions. However, the waste composition causes a large variation in GHGs production (i.e. 40 to 250 m³) from 1 ton of waste at different landfill sites (Themelis and Ulloa 2006). Such harmful environmental impacts of MSW disposal in landfills warrant an integration of strategies such as recycling, waste-to-energy combustion, composting and landfills with the gas collection and energy recovery.

Composting is a viable option that can reduce the volume of decomposable organics going to the waste stream (Adhikari et al. 2010). But contradictory estimates have been reported for changes in GHGs emissions during composting. During composting, especially in aerobic decomposition, gases like CO₂ and N₂O, and H₂O are produced in addition to CH₄ emission from anaerobic pockets (Azim et al. 2018). The majority of CH₄ oxidizes to CO₂ and the CO₂ emission is neglected because of its lower global warming potential. Nonetheless, some studies have opposed this concept and reported considerable CH₄ emissions even in well-managed systems. Similarly, contradictory reports are available for N₂O emission during composting as some researchers have suggested its higher emission during the initial stage and negligible after 2–3 days (He et al. 2000), while others suggested higher production with time. Most of these publications provide information on gaseous emission during composting of manures and sewage sludge but only a few deal with the MSW (Colon et al. 2012).

Soil and groundwater pollution due to the disposal or use of MSW may pose considerable risks to ecological and environmental safety. Estimation of pollution indices is an effective tool for assessing the consequent risk related to MSW disposal/application (Mazurek et al. 2017) and can help to protect the ecosystem. Therefore, this research work was carried out to quantify the changes in physical and chemical composition including the HMs and trace elements in MSW collected from domestic (Karnal) and industrial (Panipat) city landfill sites and after composting with amendments like gypsym, elemental sulphur and rice straw. In addition, the risk assessments of HMs pollution were conducted using various pollution indices (e.g. enrichment factor, contamination factor, geoaccumulation index and ecological risk etc). The estimation of the GHGs emissions from both open landfill sites and their subsequent changes in GHGs emissions during composting of segregated MSW were also evaluated. This research work provide an effective aternative for MSW dumping. Municiple solid waste composting especially with amendments like sulphur and rice straw found effective to lower ecological risk and greenhouse gases emissions and also enhance the agronomic value of compost.

Sites description

MSW landfill sites of two cities, namely Karnal (29.643883 N, 77.048076 E) and Panipat (29.381814 N, 77.037227 E), of Haryana (India) are open landfills. The Karnal site is spread on an area of ~ 5.3 ha and receives around 225 ton MSW (mainly domestic) daily from Karnal city and nearby small towns. While the second landfill site is spread in about 8.19 ha area and located at Nimbri village near Panipat city (an industrial town). The Panipat city is also known as “City of weavers or Textile city”. This landfill site receives around 420 tons of solid waste daily. Both the landfills are situated in low lying areas with MSW disposed in an uncontrolled mode without any cover material and also devoid of landfill gas (LFG) and leachate collection systems.

Municipal solid waste sampling and physicochemical analysis

Representative MSW samples were collected from different points (i.e. central, middle and peripheral zone) of each landfill site in December 2020 for physicochemical analysis during the GHGs monitoring. Approximately 3.5 tons of MSW was collected from each landfill site for compost preparation. It was segregated into their respective categories including organic matter, plastic and textile. The segregated bio-degradable part of MSW was composted alone and in combination with gypsym, elemental sulphur and rice straw in 1×1×1.2 m³ compost pits for 125 days. The MSW was composted in four treatment combinations. These treatment combinations consisted of 200 kg MSW; 200 kg MSW + 50 kg G; 200 kg MSW + 5 kg ES; 200 kg MSW + 5 kg ES + 25 kg G + 50 kg RS. Samples were drawn for the initial MSW and the final MSW compost (MSWC) for analysis of various physicochemical properties. The electrical conductivity (EC) and pH for all samples were measured (in sample:water ratio of 1:2) using digital pH and EC meter (Page et al. 1985). Organic carbon was estimated using Walkey and Black (1934) method. K⁺ was measured with a flame photometer (Jackson 1973). Cr, Cd, Cu, Ni, Pb and Zn were estimated using inductively coupled plasma-optical emission spectroscopy (ICPOES; Perkin Elmer, USA). Total nitrogen was estimated by the Kjeldahl method using a Kjeltec auto-analyzer, and phosphorus by the vanadomolybdate yellow colour method (Piper 1966) using a UV-VIS spectrophotometer.

Determination of pollution indices

Enrichment factor

Enrichment factor (Ef) was determined to estimate the possible impact of anthropogenic activities (here MSW disposed in open landfills and after its composting with different amendments) on the concentration of HMs in soil. Ef of the given element was calculated relative to the average composition of the crustal abundance (Rudnick & Gao 2003) using Fe as the reference element. Fe/Al/Ca or Mn can be used as reference elements for geochemical normalization. The Ef was calculated using the following Eq. (1):

$$Ef=\frac{(Cn/CFe) sample}{(Cn/CFe) reference}$$

Where, Ef stands for enrichment factor. (C_n/C_Fe) _sample is the content of HM and iron in the analyzed sample whereas (C_n/C_Fe) _reference is the ratio of geochemical background reference of heavy metal and iron (upper continental crust). Ef values were interpreted as proposed by Sakan et al. (2009) that Ef values of < 1, 1–3, 3–5, 5–10, 10–25, 25–50, and > 50 show no enrichment, minor enrichment, moderate enrichment, moderately severe enrichment, severe enrichment, very severe enrichment and extremely severe enrichment, respectively.

Contamination factor

The contamination factor (C_f) index enables the assessment of soil contamination of each metal, which was computed by the ratio between the concentrations of each metal in samples and its background reference value (Maanan et al. 2015;Taylor and McLennan 1995). The C_f was calculated using the following Eq. (2):

C _f $=\frac{{\left[{C}_{n}\right]}_{s}}{{\left[{C}_{n}\right]}_{r}}$ (2)

Here the contamination levels were categorized on a scale of 1 to 6, where C_f values < 1, 1–3, 3–6 and > 6 indicate low, moderate, considerable and very high degrees of possible contamination.

Geoaccumulation index

Geoaccumulation Index (I_geo), an estimate of contamination of HMs by waste, was calculated by the ratio between the present concentration of heavy metal in a sample and its concentration in the background reference value in (UCC) upper continental crust. I_geo was calculated using the following formula of Muller (1979) shown in Eq. (3):

$${ I}_{geo}={log}_{2}\left[\frac{C}{1.5 Bn}\right]$$

Where, C is the identified concentration of a given element in the sample and Bn is the background reference concentration in the upper continental crust. The factor value of 1.5 is a constant and used for reducing the effect of possible disparities in the background values, which may be attributed to the lithogenic variations in the soil. Müller (1979) identified the following soil quality classes based on the I_geo value. I_geo ≤ 0: unpolluted; 0 < I_geo≤1: unpolluted to moderately polluted; 1 < I_geo≤ 2: moderately polluted; 2 < I_geo≤3: moderately to highly polluted; 3 < I_geo≤ 4: highly polluted; 4 < I_geo≤ 5: highly to extremely highly polluted; I_geo > 5: extremely highly polluted.

Ecological risk and potential ecological risk index

Ecological risk (E_i) and potential ecological risk index (PERI) were introduced by Hakanson (1980) and used to calculate using the toxic response coefficient (Tr) of a particular element. The toxic response coefficient for Cr, Cd, Cu, Ni, Pb and Zn are 2, 30, 5, 5, 5, and 1, respectively. These indices i.e. Ei and PERI were calculated by using Eq. 4 and Eq. 5, respectively:

$${ Ei}_{y}={Tr}_{y}\times {Cf}_{y}$$

Where, Tr_y and Cf_y are the toxic response coefficient and contamination factor, respectively for element y.

$$PERI= {\sum }_{y=1}^{m}{Ei}_{y}$$

Ei is a single index of ecological risk factors and m is the number of studied elements or HMs. Based on the ecological risk index soil can be classified into five groups, as E_i < 40: low potential ecological risk; 40–80: reasonable ecological risk; 80–160: considerable ecological risk; 160–320: high ecological risk; and E_i > 320: very high ecological risk. Classes for potential ecological risk according to Håkanson (1980) are PERI < 90: low; 95 < PERI < 190: moderate; 190 < PERI < 380: considerable; and PERI > 380: very high potential ecological risk.

Measurement of CH₄, CO₂ and N₂O gases

GHGs emission of CH₄, CO₂ and N₂O was monitored using the close chamber technique in December (winter) 2020, April (summer) and August (monsoon)) 2021 simultaneously at both landfill sites (Gupta et al. 2007). Sampling points, in the centre, middle and peripheral regions, were decided based on the height and spread of MSW deposition of landfills. The sampling port-fitted chambers, equipped with a small fan and thermometer, were embedded at the sampling points to a depth of about 0.15 m. Samples were collected in triplicate from each selected point at the interval of 10 minutes for 1 hour in vacuumed borosil headspace vials using 10 ml syringe. Atmospheric temperature and MSW temperature were continuously recorded using a pre-calibrated thermometer.

Gas sampling from the composting pits was done during the decomposition process at 15, 30, 45 and 60 days by using the close chamber technique as mentioned above. The gas samples collected from the landfill site and compost pits were analyzed for CH₄, CO₂ and N₂O using FID of Gas-Chromatograph (Agilent Technology 7890B GC). Analysis of GHGs flux was calibrated against standard concentrations 5, 600 and 1 ppm for CH₄, CO₂ and N₂O, respectively and calculated using Eq. (6) as follows:

F= $\left(\frac{V}{A}\right)p\left(\frac{\varDelta C}{\varDelta t}\right)$ (6)

Where, F is flux (mass per unit area per unit time), V is the chamber volume, A is the chamber area, p is the gas density at ambient temperature, and $\frac{\varDelta C}{\varDelta t}$ is the slope of the regression line of the headspace gases concentration versus the time plot.

Composition of municipal solid waste of domestic (Karnal) and industrial (Panipat) landfill sites

Due to rapid economic and population growth in developing countries, there is tremendous increase in generation of MSW in the last few decades. Source segregation or partial segregation of MSW prior to composting is an effective practice for improving the quality parameters of the finish product. The composition of MSW, collected from two landfill sites, differed with respect to the contents of organic matter, plastic, wood, glass and metals. MSW from domestic waste sites contained higher (42%) organic matter as compared to 34% noticed in industrial waste-dominated MSW landfill sites. In addition to organic matter, the domestic MSW landfill also consisted of higher paper and woods (9.89%) on a weight basis. Whereas, industrial MSW landfill recorded more plastic, textile, glass and metals content as compared to domestic. The major components of MSW of both domestic and industrial cities were organic matter followed by pebbles and stones (inert material). Higher percentage of textile and inert material, in industrial city MSW, envisages mismanagement of textile industries waste and high pace of demolition and construction activities. Figure 1 illustrates the composition of MSW of domestic (Karnal) and industrial (Panipat) landfill sites.

Characterization of municipal solid waste and its compost

MSW samples collected from both the landfill sites and their respective MSWC were analyzed for certain physicochemical properties such as EC, pH, macro-nutrients and HMs as shown in Table 1. Moisture content (MC) in domestic MSW was higher by 43.65% than 35.83% for industrial. Whereas, higher pH (7.91) with lower electrical conductivity (EC; 3.77 dS m^− 1) was recorded in the case of industrial city MSW as compared to respective values of 7.31 and 4.14 dS m^− 1 in MSW from domestic landfill. From this study, we observed relatively lower pH i.e. 5.86 and 6.08 for domestic and industrial MSWC, respectively than the pH values of MSW in respective landfills (Table 1). Lower contents of nitrogen (N), phosphorus (P) and potassium (K) at 0.5, 0.19 and 0.34%, respectively were recorded in industrial MSW landfill as compared to corresponding values of 0.61, 0.36 and 0.48% in the domestic MSW. Similarly, the domestic MSW recorded higher organic carbon (OC) (7.40%) and 12.06 C/N ratio than the industrial (4.01%) with C/N ratio of 8.04. Chromium (Cr), Cadmium (Cd), Copper (Cu), Nickel (Ni), Lead (Pb) and Zinc (Zn) concentrations were recorded significantly higher in MSW from industrial landfill sites as compared to domestic ones. Considerable differences were recorded in different physicochemical parameters of the MSWC of the two sites as compared to the MSW in respective landfills. The variation in concentration of major nutrients and heavy metals in the MSWC of the two sites followed the same trend as observed in respective MSW samples. Likewise, the range of moisture content and EC were higher in the domestic site MSWC and MSW than the corresponding samples of the industrial site. The moisture content of MSWC was in the desired range (15–25%) for biofertilizers (FAI 2007). The moisture content of industrial and domestic site MSWC was 15.79 and 23.25%, respectively. Co-composting of MSW with gypsum and sulfur helped to reduce the pH of MSW from both cities. Contrarily EC was found higher in composts than the MSW. This might be due to the mineralization and dissolution of salts during composting. Reduction in pH also tends to increase the solubility of salts and thus results in higher EC (Wong et al. 2001). Domestic MSWC compost has lower pH (5.86) than industrial (6.08) which helped in more dissolution of mineral salts and resulted in higher EC value (5.21) than that of the industrial (4.43). Major nutrients such as total N, P, K and OC were also higher by 0.71, 0.23, 0.63 and 9.98% in domestic and 0.61, 0.15, 0.54 and 6.28% in industrial MSWC over the respective site MSW samples. Whereas C:N ratio was higher i.e. 13.79 and 10.29% for domestic and industrial site waste samples, respectively than their respective MSWC samples. All the macro-nutrients were recorded significantly higher for domestic MSW as well as MSWC than the industrial city. Higher percentage of organic matter, in domestic MSW (Karnal) than the industrial (Panipat), was the key factor for such differences. Concentration of N and carbon is an important factor for composting. C:N ratio can be balanced by co-composting with straw and other organic matter rich additives. Total N, P, K and total organic carbon were also estimated higher in the MSWC prepared from segregated MSW than that of the unsegregated mix waste. The concentration of major-nutrients (N, P, and K) was augmented due to loss in dry mass of composting substrate (Bernal et al. 2009). Higher and increasing trend of N content has been noticed during composting of MSW by Kausar et al. 2014. As such total nitrogen content in the MSWC significantly correlates with oxidizable organic carbon and TOC (Saha et al. 2010). Similar to our findings of increase in phosphorus content of MSWC, earlier Moturi et al. (2004) also observed higher range of this element after composting of MSW. Domestic MSWC measured relatively lower HMs concentration as compared to industrial MSWC. HMs concentration was significantly higher in industrial city MSW and MSWC as compared to those in domestic. It was mainly because of the presence of many textile industries and indiscriminate dumping of their wastes in industrial city landfill. National Environment Engineering Research Institute (NEERI 1995) also reported higher concentration of metals in the solid wastes generated from industrial or bigger cities as compared to non-industrial cities. Concentrations of all HMs (except Cr and Cu in industrial MSW) for both the sites were found within the range of “Fertilizer (Control) Order 1985” and Indian compost standard of Municipal Solid Waste (Management and Handling) Rule 2001. Use of various types of dyes in textile industries could be the reason for exceeding limits of Cr and Cu in industrial MSW samples, however, MSWC showed significantly lower values. HMs remains unaffected during degradation of organic waste and have toxic impacts on living organisms when exceed a certain limit.

Table 1

The average value of macro-nutrients (%) and heavy metals (mg kg^− 1) in MSW samples before and after composting of both landfill sites.
Parameters	FCO* std.	MSW samples		MSWC (Final product)
Landfill site	-	Domestic	Industrial	Domestic	Industrial
Moisture cont. (%)	15–25	43.65^A	35.83^B	23.35^A	15.79^B
pH	6.5–7.5	7.31^B	7.91^A	5.86^B	6.08^A
EC (dS/m)	< 4	4.14^B	3.77^A	5.21^A	4.43^B
O.C. (%)	12	7.40^A	4.01^B	9.98^A	6.28^B
N (%)	0.8	0.61^A	0.50^B	0.71^A	0.61^B
P (%)	0.4	0.36^A	0.19^B	0.23^A	0.15^B
K (%)	0.4	0.48^A	0.34^B	0.63^A	0.54^B
C:N ratio	< 20	13.79^A	10.29^B	12.06^A	8.04^B
Cr (mg/kg)	50	39.33^B	52.24^A	33.03^B	45.83^A
Cd (mg/kg)	5	2.11^B	3.46^A	1.42^B	2.07^A
Cu (mg/kg)	300	245.76^B	307.51^A	172.39	242.72
Ni (mg/kg)	50	36.11^B	45.10^A	30.26^B	39.36^A
Pb (mg/kg)	100	74.92^B	96.06^A	52.5^B	75.24^A
Zn (mg/kg)	1000	250.82^B	512.79^A	239^B	314.78^A

* The Fertilizer (Control) Order 1985 (FAI, 2007) quality control (QC) protocol for biofertilizers.

Where A, B show significant difference at P = 0.05 level of significance.

Pollution Indices

Enrichment factor

The composting decreased the Ef of all studied HMs, as Ef of each metal in MSWC was lower than their corresponding Ef values in MSW samples. The Ef for all the HMs (e.g. Cd, Cr, Cu, Pb, and Zn) except Ni was significantly lower in MSWC than in MSW. The industrial site showed significantly higher Ef for Cr, Cu, Ni and Zn than the domestic site as shown in Table 2. Though the Ef for Cd was very severe even in MSWC but decreased by 32% from 36.8 to 25.05 mg kg^− 1. Between sites, the industrial site recorded higher (31 mg kg^− 1) Ef than the domestic (30.39 mg kg^− 1). Whereas Ef for Cr was higher in MSW (1.73 mg kg^− 1) over MSWC (1.3 mg kg^− 1) and in industrial site MSW (1.71 mg kg^− 1) over domestic (1.32 mg kg^− 1). Severe enrichment was calculated for Cu, (11.8 and 14.71 mg kg^− 1), moderately severe for Zn (5.58 and 6.81 mg kg^− 1) and minor enrichment for Ni (2.49 and 2.7 mg kg^− 1) in MSWC and MSW samples, respectively. Pb noticed moderately severe enrichment in MSW (5.44 mg kg^− 1) while moderate enrichment in MSWC (4.54 mg kg^− 1). The Ef for Cu, Ni, Pb and Zn in MSW from the industrial site was expected significantly higher (Table 2) than domestic. The Cu enrichment was anticipated to be severe (> 10) in both industrial and domestic sites. Pb and Zn depicted the possibility of moderately severe (5–10) enrichment but Ni showed minor (1–3) enrichment at both sites.

Table 2

Enrichment factor and contamination factor for heavy metals in domestic and industrial city MSW as well as MSW compost samples
Pollution indices	Sample	Cd (mg/kg)	Cr (mg/kg)	Cu (mg/kg)	Ni (mg/kg)	Pb (mg/kg)	Zn (mg/kg)
Enrichment factor (EF)	MSW	36.8^A	1.73^A	14.71^A	2.7	5.44^A	6.81^A
	MSWC	25.05^B	1.3^B	11.8^B	2.49	4.54^B	5.58^B
	SE(d)	1.4	0.12	0.69	0.13	0.12	0.14
	LSD at 5%	3.61	0.31	1.79	NS	0.31	0.35
	Domestic	30.39	1.32^B	12.31^B	2.37^B	5.08	5.9^B
	Industrial	31.46	1.71^A	14.19^A	2.83^A	5.12	6.5^A
	SE(d)	2.07	0.17	0.61	0.11	0.2	0.12
	LSD at 5%	NS	0.37	1.35	0.25	NS	0.27
Contamination Factor (C_f)	MSW	28.44^A	1.31^A	11.07^A	2.03^A	4.27^A	5.38^A
	MSWC	17.82^B	0.88^B	8.3^B	1.74^B	3.19^B	3.9^B
	SE(d)	1.09	0.09	0.4	0.09	0.1	0.09
	LSD at 5%	2.81	0.23	1.04	0.24	0.27	0.24
	Domestic	18.03^B	1.01	8.36^B	1.66^B	3.19^B	3.45^B
	Industrial	28.23^A	1.19	11.01^A	2.11^A	4.28^A	5.83^A
	SE(d)	1.52	0.12	0.35	0.08	0.11	0.09
	LSD at 5%	3.38	NS	0.78	0.17	0.25	0.2

NS: Not Significant. Figures in a column followed by different superscripts differ significantly at P = 0.05 level of significance. SE(d) means standard error of difference. LSD means least significant difference.

Contamination factor

Cf values of each of the studied HMs in MSWC were significantly lower than the analogous MSW samples (Table 2), but higher (except Cr) in MSW at the industrial site as compared to domestic. Even though the Cf for Cd and Cu was at a very high degree (> 6) in MSWC (17.82, 8.3 mg kg^− 1) but remained significantly lower than 28.44 and 11.07 mg kg^− 1 for respective site MSW. Domestic site MSW observed significantly lower Cf for Cd and Cu than in industrial. However, Cr and Ni of both sites represented a moderate degree of contamination (1–3) in MSW and MSWC samples as well as in industrial and domestic sites. Pb and Zn were found in the category of a considerable degree of contamination (3–6) in both MSW (4.27, 5.38 mg kg^− 1) and MSWC (3.19, 3.9 mg kg^− 1) samples as well as in industrial and domestic waste samples. Overall, composting of MSW significantly reduced the contamination risk than open dumping landfills but still posed a considerable risk of contamination.

Geoaccumulation Index

I _geo was significantly higher for all studied HMs, except Cr and, in MSW than MSWC samples as well as higher in industrial sites than domestic shown in Table 3. Cd recorded high to extremely high I_geo (4–5) in MSW and industrial sites as compared to lower I_geo (highly polluted) in MSWC and in the domestic site. Cr in MSW and MSWC as well as in industrial and domestic sites did not pose any risk of I_geo. Whereas Cu represented moderate to high I_geo pollution (2–3) in MSW, MSWC (2.87 and 2.39 mg kg^− 1) and industrial and domestic sites (2.77 and 2.5 mg kg ^− 1, respectively). Ni showed low to moderate I_geo (0–1) for both MSW and MSWC and also for industrial and domestic sites. Pb and Zn illustrated moderate (1–2) I_geo for MSW (1.49 and 1.75 mg kg ^− 1) and MSWC (1.05 and 1.3 mg kg^− 1, respectively) as well as for industrial and domestic sites.

Table 3

Range of Geoaccumulation Index (I_geo) and Ecological risk (E_i) in MSW and MSWC of two cities
Pollution indices	Sample	Cd(mg/kg)	Cr(mg/kg)	Cu(mg/kg)	Ni(mg/kg)	Pb(mg/kg)	Zn(mg/kg)
Geoaccumulation Index (I_geo)	MSW	4.2^A	-0.22^A	2.87^A	0.46^A	1.49^A	1.75^A
	MSWC	3.41^B	-0.97^B	2.39^B	0.19^B	1.05^B	1.33^B
	SE(d)	0.04	0.14	0.05	0.05	0.05	0.03
	LSD at 5%	0.1	0.35	0.14	0.13	0.13	0.08
	Domestic	3.59^B	-0.63	2.5	0.21^B	1.13	1.25^B
	Industrial	4.02^A	-0.56	2.77	0.44^A	1.41	1.83^A
	SE(d)	0.16	0.13	0.14	0.09	0.12	0.09
	LSD at 5%	0.35	NS	NS	0.21	NS	0.21
Ecological risk (E_i)	MSW	853.25^A	2.62^A	55.33^A	10.15^A	21.37^A	5.38^A
	MSWC	534.46^B	1.77^B	41.51^B	8.7^B	15.97^B	3.9^B
	SE(d)	32.74	0.18	2.01	0.46	0.52	0.09
	LSD at 5%	84.17	0.45	5.18	1.19	1.34	0.24
	Domestic	540.82^B	2.01	41.81^B	8.3^B	15.93^B	3.45^B
	Industrial	846.89^A	2.37	55.03^A	10.55^A	21.41^A	5.83^A
	SE(d)	45.52	0.23	1.75	0.38	0.56	0.09
	LSD at 5%	101.43	NS	3.89	0.84	1.25	0.2

Ecological risk

Significantly higher ecological risk (E_i) was found for all HMs in MSW than MSWC and likewise, the industrial site recorded higher E_i than domestic (refer to Table 3). Cd posed very high E_i (> 320) in both MSW (853.25 mg kg ^− 1) and MSWC (534.46 mg kg ^− 1). E_i for Cr, Ni, Pb and Zn in MSW was 2.62, 10.15, 21.37 and 5.38 mg kg^− 1, respectively than corresponding values of 1.77, 8.7, 15.97 and 3.9 mg kg ^− 1, respectively in MSWC. In addition, Cu posed a reasonable level of E_i both in MSW and MSWC (55.33 and 41.51 mg kg ^− 1, respectively) as well as in industrial (55.03 mg kg ^− 1) and domestic (41.81 mg kg ^− 1) waste samples.

Potential Ecological Risk

The potential ecological risk index (PERI) was significantly higher in MSW than in MSWC. The industrial site showed a higher PERI as compared to the domestic. However, the overall PERI was very high (> 380) for both the MSW and MSWC. But it significantly decreased from 948.09 mg kg ^− 1 for MSW to 606.31 mg kg ^− 1 for MSWC. PERI was 942.088 mg kg ^− 1 and 612.32 mg kg ^− 1 for industrial and domestic sites, respectively. All the pollution indices showed higher values and risk for industrial MSW than domestic. This might be due to the improper dumping of unsorted industrial waste in landfill site. Dumping of wastes from textile factories and oil refineries seems to be the major sources of higher concentration of HMs in industrial MSW (Table 1). Meena et al. (2019) reported that continuous accumulations of HMs in soils can lead to killing of soil-microbes and reduce the quality of agricultural soil and also enter in the food chain. In present study, the lower values of pollution indices and reduced concentration of HMs after composting of MSW indicated the potential of reducing the environmental and health hazards by composting the MSW and thus can prove effective in the minimization of environmental pollution.

Seasonal variation of greenhouse gases emissions from landfill areas

Emissions of CO₂, CH₄ and N₂O were significantly higher at domestic landfill sites than at industrial as shown in Table 4. GHGs emissions also varied significantly among different seasons. The highest emission of CO₂, CH₄ and N₂O was recorded in the monsoon season followed by summer and winter. High moisture content and high temperature favored the higher emission of methane (~ 50.49%) during monsoon as compared to the summer (30.74%) and winter (18.77%) seasons in both the landfill sites. During the monsoon and summer seasons, high temperature promotes the microbial metabolic activity and tends to increase the GHGs emission inside the landfill sites (Henneberger et al. 2015). Emission of N₂O during monsoon was estimated at 53% followed by 30.23% in summer and 16.28% in winter season (Table 4). Presence of N₂O and large portion of CO₂ specify that air was present inside the landfill and there was some aerobic degradation. Aerobic degradation was observed, due to open dumping of MSW without soil cover, in both landfill sites. It might have led to oxidation of considerable fraction of CH₄ resulting in lower percentage of CH₄. Out of total GHGs emission during all the seasons, only 38% of CH₄ was estimated while CO₂ and N₂O accounted for 61 and 1%, respectively. The sampling location i.e. upper, middle and periphery also significantly influenced the GHGs emission. The highest CO₂, CH₄ and N₂O emission were recorded at central (582.54, 341.85, and 0.19 mg m^− 2 h^− 1, respectively) followed by middle (513.76, 291.33 and 0.14 mg m^− 2 h^− 1, respectively) and peripheral (462.77, 254.13 and 0.10 mg m^− 2 h^− 1, respectively) locations of sampling. Wide variations in greenhouse gases emissions between the sampling locations may be due to the heterogeneous nature of landfill and uneven height and compaction across the landfill areas. Out of the total GHGs emissions, 37.36, 38.53 and 44.19% of CO₂, CH₄ and N₂O emission, respectively was recorded from the central location. The central or upper part of both the landfill sites contained earlier dumped wastes, which significantly affects the emission of GHGs. Whereas lower emission of CO₂, CH₄ and N₂O was observed at peripheral location of both the landfill sites because of lesser moisture waste being filled in the region. On average higher emissions of CO₂, CH₄ and N₂O were recorded from domestic landfill sites (49.54, 29.68 and 0.016 ton ha^− 1yr^− 1, respectively) than industrial (41.16, 23.37 and 0.010 ton ha^− 1yr^− 1) as shown in Table 5. Higher emission was recorded during every sampling time at domestic landfill site as compared to industrial. Domestic MSW had high percentage of organic matter and moisture content as shown in. The organic portion of the waste could be responsible for this high GHGs emission, because of the uncontrolled anaerobic and aerobic degradation. Gurijala and Suflita (1993) reported greater CH₄ emission at pH 6.8–7.4 and at higher moisture contents in landfill areas. Waste characteristics, disposal method, climatic conditions and age of disposal sites are some important factors, which influenced the wide range of variation in GHGs emission between the domestic and industrial landfill sites. Rinne et al. (2005) stated that the production of N₂O may be related with the age of the deposited waste in landfill areas. Soil C: N ratio, total organic carbon, and activity of nitrifiers and denitrifiers (Long et al. 2018) and environmental factors like temperature and oxygen content inside the landfill site determine N₂O flux. Here combined effect of available nitrogen and high moisture content resulted in the higher emission of N₂O at domestic landfill site.

Table 4

Greenhouse gases emission from two landfill sites
		CO₂ (mg m^− 2 h^− 1)	CH₄ (mg m^− 2 h^− 1)	N₂O (mg m^− 2 h^− 1)
Site	Industrial	469.87^B	266.75^B	0.11^B
	Domestic	565.51^A	338.79^A	0.18^A
	SD	9.11	1.12	0.01
	LSD	39.18	4.84	0.02
Season	Winter	338.24^C	166.53^C	0.07^C
	Summer	469.43^B	272.77^B	0.13^B
	Monsoon	751.40^A	448.02^A	0.23^A
	SD	9.83	1.33	0.01
	LSD	28.09	3.79	0.02
Location	Peripheral	462.77^C	254.13^C	0.10^C
	Middle	513.76^B	291.33^B	0.14^B
	Central	582.54^A	341.85^A	0.19^A
	SD	7.93	2.54	0
	LSD	19.79	6.34	0.01

Figures in a column followed by the different superscripts differ significantly at P = 0.05 level of significance. SE(d) means standard error of difference. LSD means least significant difference.

Table 5

Total (overall) emission of GHGs from both landfill sites as well as from MSWC
Landfill emission	CO₂ (ton ha^− 1yr^− 1)	CH₄ (ton ha^− 1yr^− 1)	N₂O (ton ha^− 1yr^− 1)	* kg CO₂eq ton^− 1 ha^− 1yr^− 1
Industrial	41.16	23.37	0.010	628.39
Domestic	49.54	29.68	0.016	796.31
MSWC emission
Industrial	17.65	9.59	0.004	258.59
Domestic	20.39	11.26	0.006	303.678
* GWP over 100 years for CH₄ & N₂O were taken from IPCC 4th Assessment Report (AR4), 2007

Gases emission during compost formation

Composting is an eco-friendly method for organic waste management and the final product can be used as an amendment and soil conditioner in agriculture sector. During composting of MSW, emission of all three studied gases (i.e. CO₂, CH₄ and N₂O) was higher for domestic than industrial sites. The concentration of CO₂ was significantly higher in the case of domestic (i.e. 312.55, 226.75, 202.92 and 188.73 mg m^− 2 h^− 1 ) at 15, 30, 45 and 60 days of composting, respectively than for industrial MSWC (291.47, 200.9, 162.3 and 151.05 mg m^− 2 h^− 1, respectively). A similar trend was observed for CH₄ emission with values being 184.23, 125.32, 106.62 and 97.8 mg m^− 2 h^− 1 at 15, 30, 45 and 60 days of composting, respectively for domestic and 146.27, 115.55, 92.8 and 83.22 mg m^− 2 h^− 1 for industrial MSW compost. N₂O emission also showed significantly higher values in domestic MSW at all sampling stages than industrial MSWC. N₂O emissions at 15, 30, 45 and 60 days of composting for domestic MSW were 0.05, 0.05, 0.08 and 0.08 mg m^− 2 h^− 1, respectively in comparison to 0.03, 0.04 0.06 and 0.07 mg m^− 2 h^− 1, respectively for industrial MSWC. Decreasing trends were observed for CO₂ and CH₄ emissions from 15 to 30, 45 and 60 days of composting as shown in Fig. 2A and Fig. 2B, but the opposite trend was observed for N₂O as shown in Fig. 2C. Presence of sulphur and rice straw hastened the degradation process and higher temperature led to higher oxygen demand. This leads to anaerobic spots within the compost piles and favored the higher emissions of CH₄ and CO₂. MSW (control) showed least emission during initial sampling possibly due to late entry in thermophilic phase. However, N₂O emission showed opposite trend, lesser emission observed at 15 days sampling stage as compared to other later sampling stages. It was mainly due to the fact that higher temperature reduces the N₂O emissions (Amlinger et al. 2008). At the time of 30, 45 and 60 days of sampling, an overall reduction was noticed in CO₂ and CH₄ emission. Least emission of CO₂ and CH₄ was recorded during 60 days of sampling which indicated the stability of end product or maturation of compost. During later sampling stages, higher emissions of CO₂ and CH₄ were observed for MSW treatment followed by MSW + G and further by MSW + ES, whereas least being recorded in MSW + G + ES + RS. Conversely during 30 days sampling, control (MSW) entered into the thermophilic phase and resultant high temperature and lower C:N ratio led to higher emissions in MSW treatment. However, presence of sulphur and rice straw in MSW + G + ES + RS seemed to have facilitated the least emission of CO₂ and CH₄. Because sulphur reduces the pH of compost and had a negative impact on methanogens activity and rice straw increased the C:N ratio as well as enhanced the aeration within the compost piles. N₂O showed an increment during 30, 45 and 60 days of sampling stages as the compost was entering to the maturation/cooling phase. Earlier Amlinger et al. (2008) also observed higher concentration of CH₄ along with NH₃ during initial stage of composting and higher concentration of N₂O during cooling phase. Many factors such as temperature, particle size, C:N ratio, pH, moisture along with some additives such as sulphur, straw and phosphorus play an important role in the biochemistry of compost degradation process. Sommer and Moller (2000) investigated the effect of straw addition in pig manure and observed that there was less emission of CH₄ and N₂O in straw amended low density piles as compared to high density piles (without straw). Whitehead (1995) recorded that higher C: N ratio reduced the mineralization and FYM decomposition, which leads to less emission. Hence, the addition of organic and inorganic additives for composting of MSW need to be thoroughly evaluated and promoted to minimize the global warming impact and enhance the agronomic value of compost (Awasthi et al. 2017). Overall higher emission of CO₂, CH₄ and N₂O was recorded in domestic MSWC formation (20.39, 11.26 and 0.006 ton ha^− 1yr^− 1, respectively) than in industrial MSWC (17.65, 9.59 and .004 ton ha^− 1yr^− 1, respectively).

Entire emission of GHGs was incorporated in terms of CO₂ equivalent, as CH₄ and N₂O have a GWP 21 and 298 times that of CO₂ over 100 year (IPCC 2007). Significantly lower GWP was recorded during composting of MSW (258.59 and 303.678 kg CO₂eq ton^− 1 ha^− 1yr^− 1in industrial and domestic MSW, respectively) than that of landfill sites (628.39 and 796.31 kg CO₂eq ton^− 1 ha^− 1yr^− 1, ¹in industrial and domestic MSW, respectively). In present study, approximately 43, 41 and 46% reduction was recorded in CO₂, CH₄ and N₂O, respectively, for industrial MSWC than the emission from industrial landfill site. While 41, 38 and 36% reduction in CO₂, CH₄ and N₂O respectively, was observed for domestic MSWC than the emission from domestic landfill site. The total GHGs (CO₂, CH₄ and N₂O) emission, due to composting of MSW, was reduced by 42.27 and 39.95% in industrial and domestic, respectively as compared to the emission recorded at respective city MSW landfill sites. Industrial MSW compost showed considerable higher reduction in GHGs emissions than the domestic compost. This might be due to fact that domestic waste had higher percentage of moisture content and organic material than industrial. High water content in MSW reduces the air space and facilitates the gases emission. Babel and Vilaysouk (2015) also found similar results in offsetting of GHGs emission during composting. They have reported approximately 41% lower CH₄ and N₂O emission from composting than landfill. Hubbe et al. (2010) also reported that composting is an effective method of decreasing emission of GHGs and subsequently minimization of global warming.

In present study, operating and management cost was not included in the calculation of GHGs emission during composting process because the factors involved, like transportation, salaries and prices with respect to sales, are site specific.

The MSW composition was different in domestic and industrial landfills. Domestic MSW had higher organic matter (42%), major nutrients and C:N ratio, had lower pH and higher EC than industrial. Contents of heavy metals viz., Cr, Cd, Cu, Ni, Pb and Zn were significantly higher in industrial MSW. Composting significantly increased N, P, K and organic carbon with simultaneous reduction in contents of HMs than the respective raw MSWs. Co-composting with gypsum, sulphur and rice straw increased C:N ratio, nutrients and accelerated the degradation process. The studied pollution indices (Ef, Cf, I_geo, EI and PERI) indicated lower ecological risk in domestic MSW than industrial. Cr and Cu contents were higher than critical limits in industrial MSW. The higher PERI and EI, in MSW of both cities, emphasize the associated higher ecological risk. MSW composting was effective in reducing ecological risk and other pollution indices for each studied HM as compared to raw MSW. Both domestic and industrial landfill sites measured significant seasonal and spatial variation in emission of GHGs. Emission of CO₂, CH₄ and N₂O from domestic landfill site was 20.4, 27 and 63.6% higher than industrial landfill site. Approximately 32% higher emission of CO₂, CH₄ and N₂O was observed in monsoon season, followed by summer and winter. The order of overall GHGs emission remained 924.58, 805.23 and 717 mg m^− 2 h^− 1 from central (upper), middle and peripheral part, respectively of the landfill sites. As far as phases of composting are concerned, initial phase for control showed least emission while other treatments with additives showed considerably higher emission of gases. However, the trend reversed with passage of time. Overall least emission was recorded in MSW + G + ES + RS followed by MSW + ES, MSW + G and MSW. Co-composting of MSW with elemental sulphur and rice straw reduced GHGs emission up to 41% as compared to landfill emission. MSW composting especially with amendments like sulphur and rice straw found effective to lower ecological risk and GHG emissions. Therefore, sulphur and rice straw co-composted MSWC, which is a rich source of plant nutrients and an effective soil ameliorant, can be a potential alternative to open dumping and landfilling of both domestic and industrial MSW.

Acknowledgements

We are thankful to the Directors of ICAR–CSSRI Karnal and Institute of Environmental Studies, Kurukshetra University, Kurukshetra, Department of Science & Technology, India and DST INSPIRE for providing the necessary logistics/facilities/funding support for this research.

Statements and Declaration

Ethical approval Not applicable

Consent to Participate Not applicable

Consent to Publication

The authors declare that they are agree with the content and give explicit consent to submit and that obtained consent from the responsible authorities at the institute where the work has been carried out, before the work is submitted.

Authors' contributions

Conceptualization R. K. Yadav, Gajender Yadav and Smita Chaudhry; Methodology, Manisha Devi and Gajender Yadav; Validation, Manisha Devi, Gajender Yadav; Formal Analysis, Manisha Devi, Gajender Yadav, R. K. Yadav; Investigation, Manisha Devi, Gajender Yadav, R. K. Yadav; Resources, R. K. Yadav and Gajender Yadav; Data Analysis, Manisha Devi, Gajender Yadav; Writing—Original Draft Preparation, Manisha Devi, Gajender Yadav; Writing—Review and Editing, R. K. Yadav, and Smita Chaudhry; Visualization, R. K. Yadav; Supervision, R. K. Yadav; Project Administration, R. K. Yadav and Gajender Yadav; Funding Acquisition, Manisha Devi and Gajender Yadav. All authors read and approved the final manuscript.

Funding

This work was supported by Department of Science and Technology, India under DST-INSPIRE Fellowship (IF 180548) and DST Young Scientist awards.

Competing Interest

The authors declare no competing interest.

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Municipal solid waste composting with amendments reduces heavy metals hazard, ecological risks and greenhouse gases emission

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Abstract

Figures

Introduction

Research methodology

Sites description

Municipal solid waste sampling and physicochemical analysis

Determination of pollution indices

Enrichment factor

Contamination factor

Geoaccumulation index

Ecological risk and potential ecological risk index

Measurement of CH₄, CO₂ and N₂O gases

F= \(\left(\frac{V}{A}\right)p\left(\frac{\varDelta C}{\varDelta t}\right)\) (6)

Results

Composition of municipal solid waste of domestic (Karnal) and industrial (Panipat) landfill sites

Characterization of municipal solid waste and its compost

Pollution Indices

Enrichment factor

Contamination factor

Geoaccumulation Index

Ecological risk

Potential Ecological Risk

Seasonal variation of greenhouse gases emissions from landfill areas

Gases emission during compost formation

Conclusion

Declarations

References

Supplementary Files

Status:

Version 1

Municipal solid waste composting with amendments reduces heavy metals hazard, ecological risks and greenhouse gases emission

Status:

Version 1

Abstract

Figures

Introduction

Research methodology

Sites description

Municipal solid waste sampling and physicochemical analysis

Determination of pollution indices

Enrichment factor

Contamination factor

Geoaccumulation index

Ecological risk and potential ecological risk index

Measurement of CH4, CO2 and N2O gases

F= \(\left(\frac{V}{A}\right)p\left(\frac{\varDelta C}{\varDelta t}\right)\) (6)

Results

Composition of municipal solid waste of domestic (Karnal) and industrial (Panipat) landfill sites

Characterization of municipal solid waste and its compost

Pollution Indices

Enrichment factor

Contamination factor

Geoaccumulation Index

Ecological risk

Potential Ecological Risk

Seasonal variation of greenhouse gases emissions from landfill areas

Gases emission during compost formation

Conclusion

Declarations

References

Supplementary Files

Status:

Version 1

Measurement of CH₄, CO₂ and N₂O gases