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Article

An Assessment of Two Types of Industrially Produced Municipal Green Waste Compost by Quality Control Indices

by
Daniela V. Borisova
1,
Gergana S. Kostadinova
2,
Georgi S. Petkov
2,
Diyana M. Dermendzhieva
2 and
Georgi G. Beev
3,*
1
Ecoresource Harmanli Ltd., 6450 Harmanli, Bulgaria
2
Department of Applied Ecology and Animal Hygiene, Faculty of Agriculture, Trakia University, 6000 Stara Zagora, Bulgaria
3
Department of Biochemistry, Microbiology and Physics, Faculty of Agriculture, Trakia University, 6000 Stara Zagora, Bulgaria
*
Author to whom correspondence should be addressed.
Appl. Sci. 2022, 12(20), 10668; https://doi.org/10.3390/app122010668
Submission received: 5 September 2022 / Revised: 14 October 2022 / Accepted: 20 October 2022 / Published: 21 October 2022
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Abstract

:
Municipal green waste (MGW) has significantly increased with the development of urban green areas, and its utilization by composting is a good alternative to solve the problem. This paper presents the results from the quality assessment of two industrial composts (from the composting facility of a regional nonhazardous waste landfill) based on their physicochemical properties, hygienic safety (microbiological parameters), fertilizing potential (by fertilizing index, FI) and heavy metal polluting potential (by clean index, CI). Compost 1 (C1) was made from MGW (100%) and Compost 2 (C2) was made from MGW (75%) and discarded green peppers (25%). The evaluation of physicochemical parameters was conducted according to Bulgarian Standards (BDS) methods and microbiological analysis using selective, chromogenic detection systems. It was found that the EC, P, K, Mg, Cu, Cr and Ni were lower for C1 (p < 0.05–0.001). On the other hand, Pb concentration was higher compared to C2 (p < 0.001); the concentrations of Cd, Hg and the E. coli were very low for both composts; presence of Salmonella was not detected. The estimated quality indexes (FI and CI) classified C1 as Class B compost (very-good-quality compost with medium fertilizing potential) and C2 as Class A compost (best-quality compost with high soil fertility potential and low heavy metal content). The C1 and C2 composts meet the requirements of EU and Bulgarian legislation and can be used as soil fertilizers.

1. Introduction

The modern world is characterized with extensive growth of population and ubiquitous urbanization. Along with the benefits it offers to humans, the intensive development of cities is accompanied by a number of negative phenomena, e.g., the rapidly growing amount of municipal solid waste (MSW) [1]. Green waste (GW) is a significant part of MSW. For example, in 2017, the share of GW from total MSW was about 20% in Sweden, 18% in Spain, 33% in Romania, 48% in Germany and 60% in the Czech Republic. For the EU-28, the average level was 35% [2]. Municipal green waste (MGW) commonly consists of wood cuttings from pruning of trees and shrubs, dead and green leaves and grass clippings collected from parks, gardens and households [3,4]. It is a widespread practice to dispose GW in landfills, which is often difficult and expensive due to its low bulk density [5].
Two main practices for environmentally friendly recovery of municipal green waste are currently used—composting and energy production. Although energy production from GW—especially from wood residues—is promising, composting remains the popular option to utilize the organic fraction of municipal solid and green wastes, as the generated products are rich in nutrients and are suitable as soil fertilizers [6,7,8]. Moreover, the composting allows for the potential reuse of GW by incorporating nutrients and organic matter into circulation while maintaining the waste–soil–plant production continuum [9]. In addition, GW management is a key issue due to its high production rate and the variety of physical properties and chemical composition, making composting a promising alternative for GW treatment and valorization [10]. It should be also borne in mind that composts produced from different types of substrates have different characteristics and properties as soil fertilizers, and different potential markets and areas of application [11,12].
Some reports [13,14,15,16] have outlined the positive aspects related to the composting of biodegradable wastes: the reduced amount of waste to be disposed in landfills; reduced emissions of greenhouse gases, as the compost can sequestrate carbon and decrease net CO2 emissions to the atmosphere; the presence of readily available compounds in the organic matter of biowaste that immediately can be used by the composting microbiota. Despite the positive aspects of GW composting and its application, some disadvantages should also be noted: GW compost quality strongly depends on the different compound patterns, which are highly variable and determined by the predominant vegetation in the area, the season and the local collection policy [17,18]; its slow mineralization and the resulting sluggish progress of nutrient release diminish the agricultural value of composts [9].
During the past 20 years, extensive research on composting municipal green waste, either alone [4,17,18,19,20,21,22,23,24] or cocomposting with other organic wastes—food waste [5,8,10,25], MSW [7], woody biomass [26], sewage sludge [9], agricultural plant residues [9,27] and animal manure [28,29], as well as with other additives, such as biomass ash [30], mushroom compost and biochar [31], sugarcane bagasse and grape marc [32], bean dregs and crab shell powder [33], etc.—was performed worldwide. Simultaneously, technologies and facilities/plants for MGW composting/cocomposting were also developed. For example, in thirty-six states of the USA, 3474 GW composting facilities are in operation [34]. Green waste management by composting facilities was reported also from Spain (Madrid [35] and Catalonia [17,29]), Lithuania [19] and Italy [27]. At Sofia University, Bulgaria, a pilot facility for garden waste composting was created to perform research and educational activity [36].
EU waste policy contributes to the circular economy by extracting high-quality resources from waste to the best possible extent. This implies a more efficient utilization of resources, in addition to providing more respect and care for the environment compared to the traditional linear economy [37,38]. EU biowaste—mainly food (about 60%) and garden waste (about 35%)—is a key waste stream with a high potential for contributing to a growing circular economy through delivery of valuable soil-improving material and fertilizers as well as biogas, a source of renewable energy [2]. Moreover, according to the Council Directive [39] on the landfill of waste, EU member states are obliged to reduce the amount of biodegradable municipal waste going to landfills.
In 2017, the EU-28 generated 249 million tons of municipal waste, of which about 86 million tons (34%) was biowaste [2]. About 40% of the collected biowaste was effectively recycled into high-quality compost and digestate. In the last decade, the amount of generated municipal waste in Bulgaria follows a trend towards permanent reduction—from 3572 thousand tons in 2011 to 2838 thousand tons in 2019; for the same period, the amount of landfilled and recycled municipal waste also significantly decreased—70.0% and 81.2%, respectively—while municipal waste submitted for pretreatment increased by 78% [40].
Following the EU waste management policy, the main goal of the new Bulgarian National Waste Management Plan 2021–2028 is the transition from waste management to efficient use of waste as a resource for sustainable development [41]. One of the steps to achieve this goal is MGW composting—either alone or with other biodegradable waste, which in 2017 comprised about 42% of MSW [2]. Until 2021, eighteen facilities for aerobic GW composting were built and put into operation at the regional nonhazardous waste landfills; another thirty-five are under construction [41].
The quality and safety parameters of the compost, monitored by business operators according to Bulgarian and EU legislation, do not provide sufficient information about compost fertility as a soil fertilizer. The recently developed fertilizing index (FI) and clean index (CI) give a good opportunity to compare the quality of composts produced from different raw materials with different composting duration. The FI determines the fertilizing potential, based on total organic C, N, P, K and C:N ratio, whereas CI assesses the potential for contaminating the soil and the food chain, based on heavy metals content [42].
The purpose of this study was to make a quality assessment of industrial green waste composts produced in a composting facility of a regional nonhazardous waste landfill on the basis of: (1) physicochemical properties, heavy metals content and sanitary indicator microorganisms; (2) fertilizing potential through FI and heavy metal polluting potential through CI; (3) Bulgarian and EU legislation regarding soil fertilizers.

2. Materials and Methods

2.1. Study Area and Description of the Regional Nonhazardous Waste Landfill

The experiment was carried out between June and November 2021 on the regional nonhazardous waste landfill—RNHWL (Complex Permit No 285–H2/2018), located near Harmanli town, Harmanli municipality (27,137 inhabitants) on a surface area of 103 acres (41°54′24.29″ N; 25°53′45.17″ E; 70 m asl) (Figure 1). The climate of the region is Mediterranean continental, characterized with warm summer and mild winter, relatively small annual temperature amplitude, autumn–winter maximum of precipitation and lack of annual snow cover. The annual average temperature and precipitation are 18.4 °C and 550–600 mm, respectively. The RNHWL includes two repositories for nonhazardous waste (R1, capacity—292,254 t and R2, capacity—157,102 t); a facility for waste separation, capacity up to 30,000 t y−1; and a composting facility, capacity up to 2756 t y−1 (Figure 2).
The composting facility occupies an area of 5400 m2 on asphalt pavement and includes 9 areas, as follows: Area No 1 (2000 m2)—for intensive decomposition of the substrates (active phase) formed as windrows (long, narrow piles) where a high temperature is developed (65–70 °C); Area No 2 (400 m2)—for compost maturing (curing phase); Area No 3 (1200 m2)—for storage of the end-products in storage premises; Area No 4 (250 m2)—for storage of wet biowastes; Area No 5 (750 m2)—for storage of dry biowastes; Area 6—a compensatory tank (160 m3) for collection and storage of the infiltrate and rainwater from areas No 1 and No 2; Area No 7 (265 m2)—parking for business vehicles and other mobile equipment; Area No 8 (150 m2)—for storing impurities; Area 9 (225 m2)—for service infrastructure.

2.2. Composting Procedure

Two batches of compost from green biodegradable waste were produced by aerobic treatment under natural conditions: Compost 1 (C1): 100% green waste (GW), total 30.2 t, consisting of branches, shrubs and grass (Waste Code 20 02 01*) collected from parks and gardens of Harmanli municipality; Compost 2 (C2): 75% GW and 25% discarded green peppers (Waste Code 20 01 08*), total 28.6 t (* waste codes are according to Ordinance No 2/2014 [43]). Before initiation of the composting process, separately collected biodegradable wastes were visually inspected in order to discard impurities and other waste (construction, plastic, glass, etc.). Then, the raw materials were mechanically chopped into smaller-size particles (20–40 mm) with a mobile shredder (Figure 3) and were thoroughly mixed. The mixture was formed as windrows (L × H × W—14–15 m × 1.20 m × 3.00 m) (Figure 4).
During composting, two parameters of the substrates were monitored—temperature and moisture content. Temperature was measured in four places at a depth of 40–45 cm inside the windrows at 11.00 a.m., using a portable thermometer (Therma–1, ETI) as follows: 1st week—daily, 2–4 weeks—every 3 days, 2–4 months—weekly. The moisture content of the composted mixtures was simultaneously measured in the same places by means of a portable moisture meter (REOTEMP—Garden and Compost Moisture Meter).
The composting process passed through two phases—phase of decomposition of organic matter and phase of maturation. The first phase started immediately after the formation of compost windrows (Area No 1) and lasted 90 days. During this period, the temperature of both substrates gradually decreased—from 70.4 °C (C1)–70.3 °C (C2) in the beginning to 48.6 °C (C1)–41.4 °C (C2) by the end of the 3rd month—and the moisture content changed from 65.2% (C1)–70.4% (C2) to 55.5% (C1)–58.4% (C2). In order to maintain an optimal temperature–moisture regimen for the life activity of the aerobic microorganisms, the substrates were mixed and moistened periodically with infiltrate and rainwater (collected in the compensating tank in Area No 6), with a special machine pulled/aggregated by a tractor (Figure 5). At the end of this phase, obtained fresh compost was transferred to Area No 2 for maturation. The second phase lasted one month. The temperature and moisture content of the maturing substrates continued to fall and at the end of the period reached values of 32.5 °C/50.5% (C1) and 29.1 °C/53.1% (C2), respectively. The finished composts were sifted through a drum sieve (20 × 20 mm) and the substrates were homogenized (Figure 6). The larger particles, retained on the sieve, were used for subsequent composting. The two composts were transferred to Area No 3 for storage in a covered space at ambient temperature.

2.3. Sampling and Sample Preparation

For physicochemical analysis, compost samples were collected 15 days after the maturation stage according to the requirements of BDS EN 12579:2013 [44]. Briefly, 2.4 kg of compost samples were taken with sterilized bulk profile sampler from 12 sites throughout each windrow. Approximately 200 g spot sample was taken at 5 cm from the surface and at 20 cm from the bottom of each of the two windrows. The samples were collected in a mixing vessel, thoroughly mixed, placed in sterile plastic bags, cooled in cooler bag and transported for analysis.

2.4. Analysis of Compost

2.4.1. Physicochemical Analysis

The analyses of the parameters monitored were performed according to BDS complying with international standards: pH—BDS EN 15933:2012 [45] (potentiometric); Electrical conductivity (EC), mS cm−1—CEN/TS 15937:2013 [46] (conductometric); Moisture content, %—BDS EN 15934:2012 [47] (gravimetric); total organic carbon (TOC), %—BDS EN 15936:2012 [48] (infrared detection after dry combustion); total Kjeldahl Nitrogen (TKN), %—BDS EN 16169:2012 [49] (titrimetric); elements: total P (TP), %, total K (TK), % and Mg, %, and Zn, Cu, Pb, Cr, Ni, Cd and Hg, mg kg−1—BDS EN 16170:2016 [50] (optical emission spectrometry with inductively coupled plasma); C:N ratio, by calculation based on the obtained values of total C and total N; C:P ratio, by calculation based on the obtained values of total C and total P. The chemical parameters were determined based on the dry matter (dm) content in the substrate, Wdm = 76.18%, according to BDS EN 15934:2012 [47]. All measurements and analyses were run in triplicate (n = 3).

2.4.2. Microbiological Analysis

Microbiological indicators (Escherichia coli and Salmonella spp.) of the produced composts were determined as recommended in the compost quality standard method PAS 100 (BSI, 2005) [51], BDS ISO 16649-2:2014 [52] and BDS EN ISO 6579-1:2017/A1:2020 [53] with modification of culture media. At the selective enrichment step, RVS (RAPPAPORT-VASSILIADIS-Soya, Merck, Darmstadt, Germany) broth was used—a modification of the original Rappaport medium, where tryptone was replaced with soya peptone to improve Salmonella recovery rates. For the enumeration of Salmonella spp., a 25 g subsample of each compost was taken, placed into a sterile stomacher bag together with 225 mL sterile phosphate buffered saline solution and mixed for 60 s. Further analyses of both parameters were performed by preparation of serial ten-fold dilutions from samples of both composts (C1 and C2) in sterile saline (1:10, 1:100, 1:1000, …, etc.). One mL of sample solutions or appropriate dilutions was transferred on selective, chromogenic culture medium pads RIDA®COUNT E. coli and RIDA®COUNT Salmonella. The test cards were inoculated in triplicate, incubated at 35 °C for 24–48 h and the colonies were counted. The specific microorganisms formed colonies of different colors on the specific test cards. The results are expressed in CFU/g for E. coli and CFU/25 g for Salmonella.

2.5. Assessment of Compost Quality

The quality of produced composts was assessed in three aspects: (a) fertilizing potential and heavy metal polluting potential; (b) as soil fertilizers according to Bulgarian legislation; and (c) as soil fertilizers according to EU legislation.
(a) Determination of compost fertilizing potential and heavy metal polluting potential:
The compost fertilizing potential was determined by the fertilizing index (FI) used for assessment of MSW compost [42] and adapted for other types of compost [54]. FI was determined based on rating scale from one to five, where five corresponds to the best fertilizing potential.
The FI was computed using the formula [42]:
Fertilizing index (FI) = ∑ni=1 (SiWi)/∑ni=1 (Wi)
where ‘Si’ is score value of analytical data for TOC, Total N, Total P, Total K and C:N (in five point scale), and ‘Wi’ is weighing factor of the ‘i’th fertility parameter (in five-point scale); the maximum score of the weighing factor, 5 (TOC), reflects the strongest influence of the factor, and the lowest, 1 (Total K), reflects the weakest, respectively (Table 1).
The compost heavy metal polluting potential was determined by the clean index (CI) [42]. As for FI, the CI determination included a ‘weighing factor’ of each heavy metal, ranking on a scale from one to five, depending on its biological functions in organisms as well as of its phytotoxicity and mammalian toxicity potential, and a ‘score value’ from one to five, according to the obtained value for the specific element (Table 2).
The CI values were calculated using the following formula [42]:
Clean index (CI) = ∑nj=1 (SjWj)/∑nj=1 (Wj)
where ‘Sj’ is score value of analytical data for Zn, Cu, Cd, Pb, Ni and Cr (i.e., 1–6) and ‘Wj’ is weighing factor of the ‘j’th heavy metal (i.e., 1–5) (Table 2).
(b) The compost quality assessment at a national level was based on 3 physicochemical parameters (moisture content, EC and TOC), 7 heavy metals (Cu, Zn, Pb, Cr, Ni, Cd and Hg) and 2 microbiological parameters (E. coli and Salmonella spp.) according to the Ordinance on separate collection of biowaste and treatment of biodegradable waste [55].
(c) The compost quality assessment at the EU level was based on the same 7 heavy metals according to Commission Regulation (EC) No 889/2008 [56].

2.6. Statistical Analysis

All data were analyzed by statistical software and data analysis tool XLSTAT, Version 2016.02, Addinsoft.

3. Results and Discussion

3.1. Compost Characteristics

3.1.1. Physicochemical Parameters

Among the physicochemical parameters (Table 3), moisture content is a significant factor, as water provides a medium for transporting dissolved nutrients required for microbial activities in the compost [25]. The moisture content of the substrates at the end of the composting process was within the optimal moisture range (45–60%) [57,58], while in the stored compost (two weeks later) it was much lower—21.8% (C1) and 17.9% (C2)—with coefficients of variation determining the indicator as slightly variable (Cv = 7.66–8.10%) (Table 3). The moisture of fresh GW composts was in line with the results of [5] (48.6–56.9%); at the same time, the values were lower than those for compost from plant residue and green biomass (68.6–71.4%) reported by [59] and slightly variable than the compost from mixtures of kitchen and green wastes (mainly raked leaves and grass clippings) (45–70%) [25].
pH is a good indicator for assessing the effectiveness of the different composting stages as well as the quality of the end-product. Normally the fresh compost has a pH value close to neutral [60]. In our study, pH values and the coefficients of variation were almost equal for both composts (C1 and C2) and indicated a slightly alkaline reaction of the resulting substrates (Table 3). The increased pH up to 9 at the end of the composting process is a result of CO2 release, mineralization of the intermediates, good aeration of biomass and ammonia production from protein degradation [25,61]. Previous surveys demonstrated large variations in pH values (lower than 9) of GW compost and that made from GW plus other biodegradable wastes: pH = 8.2–8.9 [35,62], pH = 3.88–8.97 [25], pH = 4.5–6.9 [59], pH = 6.17–7.96 [21,22,32,33,63,64,65]. Some authors [57] considered that the alkaline pH of the substrate was the best for composting, while others stated that the high compost pH was not desirable, because if it exceeded 7.5–8.5, the nitrogen would be converted to NH3 and evaporated as a gas emission [66,67]. Thus, nitrogen transformation reduced the nitrogen reserve of the compost and worsened its properties as a fertilizer. Some researchers reported that the pH of good-quality compost should be below 9 [5,68]; others claimed that pH values from 6.0 to 7.8 were needed for high-quality compost [69]. The pH of the compost may affect the soil quality. The use of compost with pH > 7 as soil fertilizer was not recommended, because it might cause losses of some microelements, i.e., Fe and Mn from soil, that are important for the plants [70]. Thus, the pH values of the produced GW composts could be defined as comparatively high and undesirable.
Electrical conductivity (EC) gives information about the amount of the dissolved salts in aqueous substrate and is an indirect indicator to assess the complex nutrient content in the compost (sodium, chloride, potassium, nitrate, sulfate and ammonium salts); therefore, it is an important parameter for the compost quality assessment. In the present study, the EC values were lower than 2.57 mS cm−1 and varied for both tested composts. The average EC value of C2 was 1.48 times higher than that of C1 (p < 0.05), while the coefficients of variation were close and revealed a slight variability of that parameter. The EC difference between the two composts could be attributed to their different moisture contents—21.8% (C1) vs. 17.9% (C2)—and substrate composition. The higher moisture of C1 (over 1.22 times compared to that of C2) determined better dilution of dissolved salts in C1 (and lower EC, respectively) than in C2. The different substrate composition of the tested composts suggested a different mineral content, but since it has not been studied, could be only hypothesized. Results similar to ours were reported in previous studies about GW compost (0.94–2.6 mS cm−1 [21,28,32,33,62,63]); as well as higher values, (6.73–9.07 mS cm−1 [5]). Lower values were established for GW compost (0.33–0.51 mS cm−1 [35]); and for compost from GW (48%), food waste (39%) and sawdust (13%), which were 0.37–0.51 mS cm−1 [10]. Therefore, it can be assumed that the conductivity of the tested GW composts is usual for this type of compost.
Total Organic Carbon (TOC) concentration is an indicator of organic matter amount of the compost. TOC values varied between both produced composts—8.03% (C1) vs. 13.3% (C2)—but difference was not statistically significant (p > 0.05). Coefficients of variation characterized TOC as a moderately variable indicator (Cv = 13.8–17.6%). TOC concentrations of monitored GW composts were partially comparable to the TOC concentration of compost, produced from leaves and grass (12.35% [65]); and lower concentrations were reported for GW compost and for compost from GW and food waste (18.4–24.76% [5,10,21,22,31]). It is noteworthy that the TOC content of various GW used for composting varied widely (21.60–56.31%), with values significantly exceeding those of the present study, as well as with higher coefficient of variation (Cv = 24.4% [18]). Obviously, the factors and conditions determining the organic carbon content of GW are multiple and different; that is why some types of GW contain more organic carbon and others less. Since the composts evaluated in the current study contained relatively low levels of organic carbon, it could be concluded that the raw materials used for their production, especially municipal green waste, were also poor in this element.

3.1.2. Macronutrients

Four elements of that group were assayed: total KN; P and K; and Mg. Nitrogen is one of the most important elements for plant growth. The results show that the TKN of C2 was 1.32 times higher compared to that of C1, although not significantly different (p > 0.05). The coefficients of variation were very close and indicated a moderate variation of that parameter (Cv = 17.8–18.9%). Previously reported results for TN content in different GW composts were similar (1.37–1.76% [8,10,24,35]), higher (2.0–3.25% [5,21,31,32,33]) or lower (0.32–1.01% [9,59,63,64,65]) compared to the present data. Summary data from 33 studies indicated that various types of GW used for composting contained nitrogen within a larger range than the monitored composts (0.41–3.40%) and with a considerably higher coefficient of variation– Cv = 48.1% [18]. It is well-acknowledged that the total N content of compost can vary substantially based on feedstocks used, processing conditions, curing and storage [71]. The nitrogen amount largely determines the compost quality [23], as 10 to 21–25% of compost N content can be absorbed by plants in the first year of its soil applications [68,72].
Phosphorus is a constituent of the complex nucleic acid structure of plants, which regulates protein synthesis and therefore is important for plants’ cell division, generation of new tissue and complex energy transformations [73]. The TP contents of the two tested composts were relatively high, but different: C2 contained 1.68 times more TP than C1 (p < 0.05), while the coefficients of variation were comparable (Cv = 7.24–9.75%) and characterized the indicator as slightly to moderately variable. These results are in line with previous data reported for GW compost (0.27–0.75% [5,10,21,31,32,33]) but higher (in some cases, significantly higher) in comparison to the results of other research studies on GW compost, compost from GW and food waste, and GW for composting (0.04–0.38% [9,18,22,24,26]). In addition, the TP concentration of GW used for composting is reported as a parameter of large variability (Cv = 61.4%) [18]. Taking into account that high concentrations of extractable and potentially bioavailable plant phosphorus (30–40%) are typical for the green compost because of insignificant P-sorption capacity of organic matter [28], it can be concluded that the composts produced contained satisfactory levels of that nutrient.
Potassium is an element necessary for proper plant growth. Unlike the total P, Ca and Mg, essentially all of compost TK is plant-available [71]; that is why its amount in the compost is of great importance. TK concentrations varied within a broad range between the two tested composts (0.89–1.80%, p < 0.001) with low coefficients of variation (Cv = 4.44–5.61%), determining slight variability of the parameter. The comparative analysis showed both partial agreement with earlier results (0.57–1.56% [26]), lower (0.22–0.76% [9,21,22,24,31,32,33,64]) or higher (2.92–3.37% [5]) TK concentrations in GW compost. Finally, it can be assumed that the results obtained for TK were within the usual limits for GW compost.
Mg belongs to the group of secondary nutrients (N, P and K are in the first group), but nevertheless, this essential macronutrient is also critical for plant growth and health. Similarly to the concentrations of the other macronutrients (N, P, K), Mg concentrations differed between the two tested composts and were 1.27 times higher (p > 0.01) in C2 than in C1 (0.67% vs. 0.53%, respectively). The coefficients of variation were low and very close defining this parameter as slightly variable. The previously measured Mg levels were within a similar range (0.13–0.76% [9]), higher (0.93–1.19% [21,32]) or lower (0.091% [65]) for GW compost. Therefore, the obtained results can be accepted as positioned in the middle range of analyzed data for Mg in GW compost.

3.1.3. Carbon Ratios (C:N and C:P)

C:N ratio is one of the main parameters (along with temperature, pH and moisture content) contributing to the efficiency of the composting process [74]. The C:N ratio of the tested composts was relatively low (>5.42–<6.80) and slightly to moderately variable, as seen from the close coefficient of variation values (Table 3). Those results are not uncommon for compost from green wastes. The green materials (especially plant wastes) have lower C:N ratio than woody materials and animal wastes [66]. The low C:N ratio of the compost (i.e., insufficient carbon content) is not desirable because the microorganisms in the substrate use the available carbon and convert the excess nitrogen to NH3 or NH4+, the ultimate effect being nitrogen loss from the compost [67]. Many studies [5,8,9,22,25,65] reported higher C:N ratios (8.69–14.9:1) for GW compost and for compost produced from the mixture of different green and biowastes than those in the present study. Higher C:N ratio values, varying within a wide range (13.5–79.0:1) and greater coefficient of variation (Cv = 44.6%), are also reported for GW used for composting [18].
The C:N ratio is widely used for easier evaluation of the compost maturity, as it is a function of TOC and N that changes in organic materials during the composting process [67,75]. Researchers’ opinions for the optimal C:N ratio of compost determining its maturity are not unambiguous. Some consider that the ideal C:N ratio of well-matured compost should be below 10 [42], about 10 [76] or lower than 12 [77]; others offer larger ranges—10–15:1 [66,78,79], 15–20:1 [71], 10–25:1 [69] and 7.8–20.5:1 [12]. It is considered that C:N values lower than 20:1 are sufficient for N supply for plant growth [80]. Finally, it should be emphasized that there is no uniform, standardized limit of this parameter for compost maturity assessment accepted by the scientific community. The main reason is that the C:N ratio varies considerably in raw materials, and as a result often gives misleading information about compost maturity [9]. The results obtained raise the necessity of future research on green composts in relation to their C:N ratio.
Compared to the C:N ratio, research data for the compost C:P ratio in available literature are less numerous. The C:P ratio as an indicator of compost quality should not be underestimated, because the amount of phosphorus can be a limiting factor when that nutrient is deficient in the mixture [81]. C:P ratio in the organic matter determines whether mineralization reactions predominate over immobilization reactions, and in this regard, a critical C:P ratio of 200:1 has been proposed [82]. The results obtained were much lower than this value and were relatively close for both composts—16.7–19.3:1 (Cv = 8.18–10.8%). The C:P ratio of C2 was insignificantly higher compared to that of C1. Due to the lack of data in the available literature on the C:P ratio of GW compost, the comparison with other types of composts revealed that this ratio was significantly higher for compost made from garden waste (pruning remains (40–60%), leaf litter (20–30%), sewage sludge (0–10%) and biomass ash (0–20%)), at 56–155:1 [30]; and for compost prepared from municipal waste, at 45.6–95.0:1 (Cv = 22.0%) [83]. Future studies are needed to gather more information on this parameter as an indicator for GW compost quality assessment.

3.1.4. Heavy Metals (Zn, Cu, Pb, Cr, Ni, Cd, Hg)

Heavy metal content is another important parameter of compost quality, as it can influence soil fertility, plant health and growth, and ground water pollution. The heavy metal amount in the produced composts was different with respect to the individual elements and ranged from <0.3 mg kg−1 (C1, C2) for Hg to 174.3 mg kg−1 (C1)–188.2 mg kg−1 (C2) for Zn (Table 3). A trend of higher content of Cu, Zn, Cr and Ni in C2 compared to C1 was observed: 1.26, 1.08, 1.12 and 1.11 times (p < 0.01–0.001), respectively. The recorded values of Cd and Hg were below the limit of quantification of the method used, which implies the presence of negligibly low concentrations, without significant differences between the two composts. The only exception was the Pb concentration, which was 1.30 times higher in C1 than in C2 (p < 0.001). Because of the same composting method (aerobic composting) applied for both composts, the observed differences in heavy metal concentrations among them suggested different amounts of heavy metals in the initial raw materials (municipal GW and discarded green peppers). It is likely that municipal GW contained lower concentrations of Cu, Zn, Cr and Ni, and higher amounts of Pb than discarded green peppers, while the Cd and Hg levels of all substrates were very low. As the content of heavy metals of the raw materials used to produce both composts has not been analyzed, this hypothesis was not proven. Coefficients of variation for the individual elements of both composts indicated that all were very slightly variable (Cv = 0.54–3.40%). Depending on their amount, the elements in the monitored composts were arranged in the following descending order: Zn > Cu > Pb/Cr > Ni > Cd > Hg.
The comparative analysis between the obtained results and data of previous studies revealed both similarities and discrepancies. Similar Cu, Pb, Cr, Ni and Cd content and lower Zn content were reported in GW compost from Germany, France and Belgium [84] along with similar Ni, Cr, Pb and Cd content and lower Zn and Cu content in GW used for composting [18]. Other studies demonstrated diverse data on heavy metals levels in GW compost [21,22,31,33], which is completely understandable given the multiple different factors affecting their content in the produced compost in each studied case.
Referring to the heavy metals ranking by amount, a similar trend was reported for GW compost (Zn > Cu > Pb > Cr > Ni > Cd [85] and Pb > Cr > Cd > Hg [22]), while the results in this study partly coincided with the ranking for GW used for composting (Zn > Ni > Cu > Cr > Pb > Cd) [18]. Full compliance with the arrangement of the monitored elements was observed for compost from municipal waste [55,83]. Anyway, this brief analysis illustrates the great variety of the heavy metal contents in GW compost, which is logical given the great diversity in their concentrations in the different types of green wastes used for compost production over the world. It was noteworthy that regardless of the compost type, the amount of Zn was the highest, and that of Cd and Hg was the lowest.

3.1.5. Microbiological Parameters

The mature compost should not contain pathogens; that is why two key microbial parameters of the tested composts were monitored: Escherichia coli, from the group of sanitary indicator bacteria; and Salmonella spp., from the group of pathogenic microorganisms. The results showed that in both composts, the E. coli counts were lower than 1.0 CFU g−1, while Salmonella was not detected in 25 g of analyzed samples (Table 3). The insignificant number of E. coli and the absence of Salmonella in the end-products indicates that composting occurred at optimum temperature and humidity values. During the active phase of composting, the compost temperature between 49 and 60 °C (as in this study) can kill pathogens [66]. Another study noted that 2–10% of bacterial pathogens survived the composting process [23]. Therefore, a potential risk for human health may exist if pathogens from the compost, used as a soil fertilizer are transferred through the food chain to the consumers [86,87].

3.2. Assessment of Compost Quality

The quality assessment of the produced composts was made in three aspects: (a) fertilizing potential and heavy metal polluting potential, and appropriateness as soil fertilizers according to (b) the Bulgarian legislation and (c) the EU legislation.
The evaluation through the fertilizing index (FI) and the clean index (CI) [42,54] used the equality-based parameters for determining compost’s agronomic value in terms of fertilizing potential and heavy metal polluting potential. The results showed that the FI value of C2 was higher than that of C1, namely 4.33 vs. 3.40 (Table 3). According to the suggested classification [42], these values classified C1 as Class B compost (FI = 3.1–3.5) and C2 as Class A compost (FI > 3.5). The C2 compost prepared from the mixture of GW (75%) and discarded green peppers (25%) was a better soil fertilizer than the C1 compost prepared from GW only (100%). The fertilizing index reported for GW compost (FI = 4.47) and for compost made from green and food wastes (50–60%:50–40%) (FI = 4.27–5.0) was close to the FI value of C2 and higher than that of C1 [5].
Regarding the CI, the values were comparatively high and very similar for the two composts: CI = 4.26 (C1) and CI = 4.13 (C2). Based on the scores, the tested composts were classified as Class A (CI > 4.0), the highest class in the above-mentioned classification [42], corresponding to minimum heavy metal levels as well as minimum pollution threat to the environment [54].
FI and CI values of the two composts were comparable to—and in some cases were even higher than—those of composts produced by municipal solid wastes: FI = 1.8–4.2; CI = 0.5–5.0 [42]; FI = 4.47–4.6; CI = 2.33–2.38 [54]; FI = 3.5–3.6; CI = 4.0–5.0 [88]. Therefore, the composts produced from green biodegradable wastes were equally valuable soil fertilizers as composts produced from solid municipal organic waste.
The final classification of both composts on the basis of the two indexes (FI and CI) classified C2 as Class A compost (FI > 3.5, CI > 4.0) and C1 as Class B compost (FI = 3.1–3.5, CI > 4.0). Class A determined C2 as a GW compost of best quality with high soil fertility potential and low heavy metal content, which could be used for high-value crops, such as in organic farming. Class B characterized C1 as a GW compost with very good quality with medium fertilizing potential and low heavy metal content [42].
Regardless of the established high fertilizing potential and low heavy metal polluting potential of the produced GW composts, they can be used as soil fertilizers only if they meet the requirements of Bulgarian and EU legislation in terms of environmental and human health protection. The national-level compost quality assessment, carried out through three physicochemical parameters (moisture content, pH and EC), seven heavy metals (Zn, Cu, Pb, Cr, Ni, Cd and Hg) and two microbiological parameters (E. coli and Salmonella spp.) showed that all monitored parameters for both composts (C1 and C2) were below the limits of the relevant standard [55] (Table 3). Only the moisture content was above the permissible standard limit (10%)—2.18 times for C1 and 1.79 times for C2. Unlike the other evaluated parameters, excess compost moisture could be relatively easily removed and brought to the standard limit, for example, by better aeration of stored compost batches.
On the EU level, the assessment based on the seven above-mentioned heavy metals revealed that the concentrations of all elements in the produced composts were under the permissible limits according to Commission Regulation (EC) No 889/2008 [56] (Table 3). The only exception was the Cd content, which exceeded the permissible limit of the standard 1.28 times. With this regard, it should be borne in mind that the method used to determine the Cd content has a limit of quantification below 0.9 mg kg−1, i.e., the exact amount of Cd in both composts under this limit is unknown, making the evaluation of the tested composts illegitimate, as it is not known whether Cd levels are higher or lower than EU standard (0.7 mg kg−1).
Hence, the present study revealed that industrial composts produced from MGW alone and its cocomposting with discarded green peppers in a composting facility of a RNHWL were of good quality and suitable for use as soil fertilizers. The FI and CI as well as the suggested compost quality classification system are a good option to compare the quality of composts made from different raw materials.

4. Conclusions

The composts produced from 100% MGW (C1) and those from 75% MGW and 25% discarded green peppers (C2) in a composting facility of a regional nonhazardous waste landfill (RNHWL) differed in terms of analyzed physicochemical parameters and the content of heavy metals. C2 demonstrated higher values of EC, P, K, Mg, Cu, Cr and Ni, and lower values of Pb compared to C1 (p < 0.05–0.001). In both composts, the Cd and Hg values and the E. coli counts were very low. The presence of Salmonella was detected in neither of the two composts.
The fertilizing index (FI) values determined C2 as a better organic soil fertilizer (FI = 4.33) compared to C1 (FI = 3.40), while the clean index (CI) values (CI > 4.0) characterized both composts as clean with low potential for heavy metal pollution. The complex assessment of the produced composts by FI and CI values categorized C1 as Class B compost with very good quality with medium fertilizing potential and low heavy metal content, and C2 as Class A compost of the best quality with high soil fertility potential and low heavy metal content. Based on this assessment, both composts were comparable to—and in some cases even better than—composts produced from other organic wastes.
The produced composts meet the requirements of Bulgarian legislation (BGL) regarding soil fertilizers and environmental protection for pH, EC, heavy metals (Zn, Cu, Pb, Cr, Ni, Cd and Hg) values, E. coli and Salmonella spp. counts and those of EU legislation for the levels of the same heavy metals, and deviate from the standard only as far as moisture content (BGL) and Cd content (EU) are concerned.
The municipal GW composting in the industrial composting facility of a RNHWL was an effective method for environmentally friendly management of municipal GW, resulting in the production of good-quality compost for soil application.

Author Contributions

Conceptualization, G.S.P., G.S.K. and D.V.B.; methodology, G.S.P., G.G.B. and D.M.D.; software, G.G.B. and D.M.D.; validation, G.G.B., G.S.K. and D.V.B.; formal analysis, G.S.P., G.G.B. and G.S.K.; investigation, D.V.B. and D.M.D.; resources, D.V.B. and G.G.B.; data curation, G.S.P., D.M.D. and D.V.B.; writing—original draft preparation, G.S.P. and G.G.B.; writing—review and editing, D.V.B., G.S.K., G.S.P., D.M.D. and G.G.B.; visualization, G.S.P., G.G.B. and D.V.B.; supervision, G.S.P. and G.G.B.; project administration, G.S.K. and D.V.B.; funding acquisition, D.V.B. and G.G.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Bulgarian Ministry of Education and Science under the National Research Programme “Healthy Foods for a Strong Bio-Economy and Quality of Life” approved by DCM #577/17.08.2018.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to Harmanli City Council and the management team of Ecoresource Harmanli Ltd.—Regional nonhazardous waste landfill and the National Program for Research “Young Scientists and Postdoctoral Students” for the help during the performance of this experiment.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 12 10668 g001 550
Figure 1. Map of Bulgaria with location of Harmanli Municipality (in red color).
Figure 1. Map of Bulgaria with location of Harmanli Municipality (in red color).
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Figure 2. General view of the regional nonhazardous waste landfill site, town of Harmanli.
Figure 2. General view of the regional nonhazardous waste landfill site, town of Harmanli.
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Figure 3. Mobile shredder, R 640TN, NRGRI.
Figure 3. Mobile shredder, R 640TN, NRGRI.
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Figure 4. Compost windrows in Area No 1.
Figure 4. Compost windrows in Area No 1.
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Figure 5. Compost turner (AFN 3000, KRAUSE).
Figure 5. Compost turner (AFN 3000, KRAUSE).
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Figure 6. Autonomous drum sieve, Ecogreen Screener L1000, Zago.
Figure 6. Autonomous drum sieve, Ecogreen Screener L1000, Zago.
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Table
Table 1. Criteria for fertilizing index (FI) scoring (Source: [42,54]). Adapted with permission from [J.K. Saha, N. Panwar, M.V. Singh], 2022, Elsevier.
Table 1. Criteria for fertilizing index (FI) scoring (Source: [42,54]). Adapted with permission from [J.K. Saha, N. Panwar, M.V. Singh], 2022, Elsevier.
NoParametersScore Value, SiWeighing Factor, Wi
54321
1TOC, %>2015.1–20.012.1–15.09.10–12.0<9.15
2Total N, %>1.251.01–1.250.81–1.000.51–0.80<0.513
3Total P, %>0.600.41–0.600.21–0.400.11–0.20<0.113
4Total K, %>1.000.76–1.000.51–0.750.26–0.50<0.261
5C:N ratio<10.110.1–15.015.1–20.020.1–25.0>253
Table
Table 2. Criteria for clean index (CI) scoring (Source: [42]). Adapted with permission from [J.K. Saha, N. Panwar, M.V. Singh], 2022, Elsevier.
Table 2. Criteria for clean index (CI) scoring (Source: [42]). Adapted with permission from [J.K. Saha, N. Panwar, M.V. Singh], 2022, Elsevier.
NoHeavy Metal,
mg kg−1 dm		Score Value, SjWeighing Factor, Wj
543210
1Zn<150151–300301–500501–700701–900>9001
2Cu<5151–100101–200201–400401–600>6002
3Cd<0.30.3–0.60.7–1.01.1–2.02.0–4.0>4.05
4Pb<5151–100101–150151–250251–400>4003
5Ni<2121–4041–8081–120121–160>1601
6Cr<5151–100101–150151–250251–350>3503
Table
Table 3. Mean ± SD values of the monitored parameters of the tested composts.
Table 3. Mean ± SD values of the monitored parameters of the tested composts.
NoParametersCompost 1
n = 3		Compost 2
n = 3		MAC *
Bulgarian Norms		MAC **
EU Norms
IPhysicochemical parameters
1Moisture, %
Cv, %		21.8 ± 1.67
7.66		17.9 ± 1.45
8.10		10%-
2pH (H2O)
Cv, %		9.27 ± 0.18
1.94		9.17 ± 0.18
1.96		--
3EC, mS cm−1
Cv, %		1.74 ± 0.05 c
2.87		2.57 ± 0.08 c
3.11		≤3 -
4TOC, % dm
Cv, %		8.03 ± 1.41
17.6		13.3 ± 1.83
13.8		15% -
IIMacronutrients
5TKN, % dm
Cv, %		1.48 ± 0.28
18.9		1.96 ± 0.35
17.8		--
6P, % dm
Cv, %		0.41 ± 0.04 a
9.75		0.69 ± 0.05 a
7.24		--
7K, % dm
Cv, %		0.89 ± 0.05 c
5.61		1.80 ± 0.08 c
4.44		--
8Mg, % dm
Cv, %		0.53 ± 0.02 b
3.77		0.67 ± 0.02 b
2.98		--
IIICarbon ratios
9C:N ratio
Cv, %		5.42 ± 0.44
8.11		6.80 ± 0.71
10.4		--
10C:P ratio
Cv, %		16.7 ± 1.82
10.8		19.3 ± 1.58
8.18		--
IVHeavy metals
13Zn, mg kg−1 dm
Cv, %		174.3 ± 5.2
2.98		188.2 ± 6.4
3.40		>400200
14Cu, mg kg−1 dm
Cv, %		41.2 ± 0.5 c
1.21		52.1 ± 0.6 c
1.15		>10070
15Pb, mg kg−1 dm
Cv, %		29.1 ± 0.2 c
0.68		22.4 ± 0.5 c
2.23		13045
16Cr, mg kg−1 dm
Cv, %		23.3 ± 0.2 b
0.85		26.2 ± 0.3 b
1.14		6070
17Ni, mg kg−1 dm
Cv, %		9.12 ± 0.05 c
0.54		10.2 ± 0.07 c
0.68		4025
18Cd, mg kg−1 dm<0.9 ***<0.9 ***1.30.7
19Hg, mg kg−1 dm<0.3 ***<0.3 ***0.450.4
VMicrobiological parameters
20Escherichia coli, CFU g−1<1.0<1.0<100-
21Salmonella, CFU 25 g−1Not detectedNot detectedNot allowed-
VIIndexes
11Fertility index, FI ****3.404.33--
12Clean index, CI ****4.264.13--
Notes: * MAC—maximum allowable concentration according to [55]; ** MAC according to [56]; *** limit of quantification of the method; **** fertility index and clean index are calculated on a scale from 1 to 5, with 5 indicating the most beneficial effect to plant growth (FI) (Table 1) and the lowest heavy metal polluting potential (CI), respectively (Table 2); differences between Compost 1 and Compost 2 are significant at p < 0.05—a; p < 0.01—b; p < 0.001—c.
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Borisova, D.V.; Kostadinova, G.S.; Petkov, G.S.; Dermendzhieva, D.M.; Beev, G.G. An Assessment of Two Types of Industrially Produced Municipal Green Waste Compost by Quality Control Indices. Appl. Sci. 2022, 12, 10668. https://doi.org/10.3390/app122010668

AMA Style

Borisova DV, Kostadinova GS, Petkov GS, Dermendzhieva DM, Beev GG. An Assessment of Two Types of Industrially Produced Municipal Green Waste Compost by Quality Control Indices. Applied Sciences. 2022; 12(20):10668. https://doi.org/10.3390/app122010668

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Borisova, Daniela V., Gergana S. Kostadinova, Georgi S. Petkov, Diyana M. Dermendzhieva, and Georgi G. Beev. 2022. "An Assessment of Two Types of Industrially Produced Municipal Green Waste Compost by Quality Control Indices" Applied Sciences 12, no. 20: 10668. https://doi.org/10.3390/app122010668

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