Potential of Producing Compost from Source-Separated Municipal Organic Waste (A Case Study in Shiraz, Iran)

sustainability

Article Potential of Producing Compost from Source-Separated Municipal Organic Waste (A Case Study in Shiraz, Iran)

Haniyeh Jalalipour 1,* , Neematollah Jaafarzadeh 2, Gert Morscheck 1, Satyanarayana Narra 1,3 and Michael Nelles 1,3 1 Department of Waste and Resource Management, Rostock University, 18051 Rostock, Germany; [email protected] (G.M.); [email protected] (S.N.); [email protected] (M.N.) 2 Toxicology Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran; [email protected] 3 The German Centre for Biomass Research, 04347 Leipzig, Germany * Correspondence: [email protected]

Received: 30 September 2020; Accepted: 18 November 2020; Published: 20 November 2020

Abstract: Developing countries face serious environmental, social and economic challenges in managing different types of organic waste. Proper treatment strategies should be adopted by solid waste management systems in order to address these concerns. Among all of the treatment options for organic waste, composting is the most approved method as an effective strategy to divert solid waste from landfills. This experimental research aimed to examine the potential of producing compost from source-separated municipal organic waste in Shiraz, Iran. Market waste (fruits and vegetables) and garden waste (plant residues) were used as the raw input materials. They were subjected to the windrow pile composting method in an open site area. The process was monitored against several physical, chemical and biological parameters. In-situ measurements (temperature and moisture content) were carried out on a daily basis. Sampling and lab analyses were conducted over the period of the biological treatment. The final product was of acceptable moisture and nutrient levels, pH, Electrical Conductivity (EC), and Carbon/Nitrogen ratio. All of the analyzed compost samples had lower concentrations of heavy metals than the Iranian and German standards. Overall, the results obtained revealed that composting is a promising method for municipal organic waste treatment. The findings also imply the effectiveness of the source-separation collection method in the production of high-quality compost.

Keywords: municipal organic waste; windrow composting; solid waste management

1. Introduction Worldwide, the development of urban areas, as well as the rapid growth of the population, has resulted in production of waste in an alarming ratio. Organic waste is the main fraction of the Municipal Solid Waste (MSW) produced in urban settlements. It is estimated that 2.6 million tons per day of municipal organic waste are generated globally [1]. Countries face serious environmental, social and economic challenges to manage this waste stream in an environmentally-sound manner [2,3]. The significant proportion of organic materials contributes to a high moisture content, ranging between 55% and 60%, with a thermal value expected to be about 7–8 MJ/kg [4]. This results in the generation of an excessive amount of leachate and landfill gas in dump/landfill sites, leading to serious environmental threats such as the contamination of the soil, and surface and underground water. In order to address these concerns, proper treatment strategies should be adopted by Solid Waste

Sustainability 2020, 12, 9704; doi:10.3390/su12229704 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 9704 2 of 17

Management (SWM) systems. Among all of the management options for organic waste, composting is the most approved method [5,6]. It is an effective strategy to divert solid waste from landfills and improve the heating value of feedstock in case of energy recovery [7]. Previous studies confirmed that composting can reduce the volume of organic materials by more than 30% [8], and can convert them into a hygienic and valuable product. The biological process is carried out through microbial activity. The activity of microbial community is affected by the temperature, pH, particle size, moisture content, aeration, and electrical conductivity of the organic waste [9]. CO2, NH3 and H2O are the by-products of the process, which lead to the recovery of mineral nutrients (nitrogen (N), phosphorus (P) and potassium (K)). The final product, rich in humus, can be utilized for sustainable agricultural purposes. Compost also has the potential to be used for the bioremediation of soil polluted with heavy metals, as it can immobilize them in soil matrix [10]. The quality of the available municipal organic waste significantly depends on the collection method used. The source-separation of the raw organic materials plays a pivoting role in securing the quality of final the product. In fact, the latter is considered to be a major obstacle in developing countries. Iran, as one such country, lacks proper solid waste management systems [11]. The enacted legislation does not effectively address the segregation of different types of solid waste. Only around 7% of the total MSW generated is collected separately as dry recyclables by municipalities. What remains is collected as mixed waste, including more than 70% organic materials [12]. The mixed household waste stream is well-known for containing hazardous and toxic waste (e.g., batteries and paints). The existence of such impurities makes the proper implementation of the composting process questionable. The final product may contain heavy metals and/or foreign materials (e.g., glass). However, recycling municipal organic waste by the composting method, and the utilization of organic residuals to produce soil conditioners has grabbed the local authorities’ attention for many years. The first compost facility in Iran was established in Isfahan in 1969, alongside with a Material Recovery Facility (MRF). Today, it is estimated that 42 MRFs are in operation in the country, and 18% of all of the MSW brought to them is subjected to the composting process [12]. However, these facilities are faced with many operational problems to produce a final product of high quality. Their failure mainly results from the improper raw organic materials of mixed waste origin containing high levels of impurities. In Iran, the MSW segregation system must be adapted stepwise. Launching the source-separation programs for household waste stream is more challenging, since it highly depends on the behavior of the waste generators. Therefore, capacity-building programs to raise public awareness are a pre-requisite for the development of a sustainable SWM system. Furthermore, the fees collected from households for MSW activities are only based on family size, whereas, in case of businesses, the amount of waste generated is determinative. Daily markets, restaurants, and coffee shops, etc., are obliged to pay more for receiving a day-to-day collection service. This could be used as a mechanism for the effective implementation of SWM. For example, in Shiraz, the fifth most populated city of the country, around 30% of the daily markets that generate > 5 kg/d organic waste are given two storage bins with different colors for wet/dry waste segregation. The organic waste collected is subjected to vermicomposting at the small scale. This treatment technique requires high amounts of fresh water. The shortage of water is a problem, especially in the southern parts of the country, including Shiraz. This study aims to adopt an effective waste treatment model to convert source-separated organic fractions into value-added products under the existing SWM system in Iran. The main objective of this work is to practice windrow composting for segregated markets and garden wastes as a viable method in mega cities with established compost facilities. The feasibility of the suggested system is mainly due to its low initial investment, simple technology, and routine monitoring. To this end, this research was carried out to examine the potential of producing compost from source-separated municipal organic waste in Shiraz, Iran. The experiment explores the measurable changes of the organic material—including physical, chemical, and biological properties—throughout the composting process. The research examines the Sustainability 2020, 12, x FOR PEER REVIEW 3 of 17 Sustainability 2020, 12, 9704 3 of 17 The experiment explores the measurable changes of the organic material—including physical, chemical, and biological properties—throughout the composting process. The research examines the qualityquality of offinal final product product attained attained from from the the practice practice of ofthe the state-of-the-art state-of-the-art technology technology for for the the windrow windrow compostingcomposting process process under under arid andand semi-aridsemi-arid weather weather conditions. conditions. It wasIt was set outset toout assess to assess the potential the potentialof compost of compost produced produced for agricultural for agricultural purposes inpurposes the study in area, the andstudy the area, effects and of applyingthe effects organic of applyingamendments organic to amendments the soil. It is hoped to the thatsoil. these It is findingshoped that could these be offindings assistance could to decision-makers be of assistance in to the decision-makersfield of SWM in in Iran. the field of SWM in Iran.

2. Materials2. Materials and and Methods Methods 2.1. Experimental Site 2.1. Experimental Site The study was conducted in an established composting plant, located at the Barmshor landfill The study was conducted in an established composting plant, located at the Barmshor landfill site, 18 km south east of Shiraz, the capital of the Fars province, Iran. The province is located in the site, 18 km south east of Shiraz, the capital of the Fars province, Iran. The province is located in the southwest of the country, and is identified by different agricultural activity. Shiraz has a semi-arid southwest of the country, and is identified by different agricultural activity. Shiraz has a semi-arid climate condition with limited fresh water resources. The total population of Shiraz is around 1.6 million climate condition with limited fresh water resources. The total population of Shiraz is around 1.6 inhabitants, on a surface area covering 217 km2 [13]. million inhabitants, on a surface area covering 217 km2 [13]. The amount of MSW produced per capita in Shiraz is assessed to be 0.645 kg per day. The main The amount of MSW produced per capita in Shiraz is assessed to be 0.645 kg per day. The main ratio of MSW is collected as mixed and low fractions of dry waste, which are segregated at source. ratio of MSW is collected as mixed and low fractions of dry waste, which are segregated at source. The informal sector affects the MSW stream significantly, and it is estimated that half of the dry valuable The informal sector affects the MSW stream significantly, and it is estimated that half of the dry waste is collected by them. Half of the collected mixed waste is subjected to material recovery, and the valuable waste is collected by them. Half of the collected mixed waste is subjected to material remaining half is landfilled directly (Figure1). The disposal method is sanitary landfilling, and the recovery, and the remaining half is landfilled directly (Figure 1). The disposal method is sanitary landfill’s trenches are equipped with a Landfill Gas (LFG) collection system. LFG is used for electricity landfilling, and the landfill’s trenches are equipped with a Landfill Gas (LFG) collection system. LFG production at the biogas power plant. is used for electricity production at the biogas power plant.

Figure 1. Material flow of municipal solid waste in Shiraz. Figure 1. Material flow of municipal solid waste in Shiraz. In the case of MRF, around 4% of the main input is separated manually. Recyclables are used as secondaryIn the case raw of materials MRF, around in the 4% recycling of the main industry. input The is separated fine fraction manually. (<70 mm) Recyclables of about 43%are used is treated as secondaryusing windrow raw materials composting. in the Therecycling coarse industry. fraction (Th>70e fine mm) fraction is sent (<70mm) to the landfill. of about 43% is treated using windrow composting. The coarse fraction (>70 mm) is sent to the landfill. 2.2. Raw Materials 2.2. Raw Materials Different available types of organic waste, fruits and vegetables (market waste), and plant residues (gardenDifferent waste) available were used types as theof organic composting waste, raw fr inputuits and material. vegetables The market (market waste waste), included and inorganic plant residuesmaterials (garden such aswaste) plastic were bags. used They as were the sortedcomposting manually. raw Theinput garden material. waste, The which market was waste mainly includedcomprised inorganic of dry materials leaves and such clippings, as plastic was bags. subjected They to were screening. sorted Woodmanually. particles The weregarden shredded waste, to whichwood was chips. mainly The characteristicscomprised of ofdry the leaves initial rawand materials clippings, are was presented subjected in Table to screening.1. Wood particles were shredded to wood chips. The characteristics of the initial raw materials are presented in Table 1. Sustainability 2020, 12, 9704 4 of 17

Table 1. The characteristics of the initial raw materials used in the composting.

Parameter Fruits and Vegetables Plant Residues Physical properties Bulk density (kg/m3) 628.69 314.2 Moisture Content MC (%) 80 7 Chemical properties Organic matter (%) 54.30 55.00 Total organic Carbon (%) 30.17 30.56 Total Nitrogen (%) 1.4 1.1 C:N Ratio (w/w) 24.55 27.78 pH 7.10 7.7 EC (dS/cm) 2.72 2.98 Total P (%) 0.21 0.24 Total K (%) 0.6 1.4

2.3. Methodology In order to conduct the experiment, source-separated market wastes were received over a period of 8 days in late February, 2019. The proper quantity of fresh materials was received to shape a windrow pile to be aerated using a machinery turner. Since the available market wastes (fruits and vegetables) did not meet the desired C/N ratio, materials rich in C (plants residues) were added as bulking agents to provide the required C/N ratio necessary for eﬀective decomposition [14]. The mixture was chosen with respect to the needed C/N ratio of 25–30 for the beginning of the composting process [15]. The appropriate mixing of carbon and nitrogen beneﬁt the procedure by providing an adequate source of food and energy for the microbial community. Consequently, around 50 tons of fruits and vegetables, and 17 tons of plant residues were combined in a 1:3 ratio in the experimental pile (Table2).

Table 2. Compost pile ingredients.

Raw Input Material Initial Weight Pile Dimension (W, H, L) Fruits & vegetables 50.40 tons 3.5 m, 1.2 m, 20 m Dry leaves 16.80 tons

After blending the raw input materials thoroughly, they were aligned in a long windrow pile using a front-end loader. The dimension of the windrow has high importance in the maintenance of the composting process; height ranges of 1–2 m are crucial for heating in the pile, and the arched surface shape with a slow slope allows a higher absorption of irrigation/ rainfall water. The height and width of the pile is usually set according to the operating range of the machinery turner. The types of the raw materials and the mixture ratio affect the duration of the process and the properties of the final products. The composting method implemented in the Shiraz plant is based on the principles of windrow technology in an open site area. Once the windrow pile has an adequate C/N ratio and bulk density, the aerobic composting process is dependent on providing moisture and oxygen as the necessary requirements for the activity of the microbial community. The in-situ monitoring of the composting process is a must for the production of a secured final product. The turning schedule provides the microbial population with oxygen, and adjusts the moisture content and temperature to an optimum level, as well as speeding up the stabilization of the organic materials [16]. Over the period of the composting process (10 weeks), the temperature and moisture content were monitored on a daily basis. The temperature was measured using a digital thermometer (model TESTO 925, Alton Hampshire, United Kingdom) in at least five points along the windrow pile, and in three depths (30 cm from top, middle, and bottom). The organic wastes used in this research work contained Sustainability 2020, 12, 9704 5 of 17 adequate moisture to start the decomposition process. A Portal Moisture-meter (model REOTEMP, San Diego, CA, United States) was used to determine the moisture content inside the pile. The moisture was kept close to 50% for the first 4–6 weeks. A specialized windrow turner (komptech topturn x53, Hirtenberg, Austria) was used to turn the piles. Turning schedule was adopted based on the temperature and moisture level. The schedule included three turnings in the first and second weeks, and two turnings in third to fifth weeks; from the sixth week onwards, the pile was turned once a week if the microbial activity continued. In order to monitor the physical and chemical operating parameters in different phases of the composting process, samples were taken. The sampling intervals were varied in order to determine the effects of different parameters. Since the moisture content significantly affects the biological activities, it was measured weekly. The EC, pH, total N, carbon and density were measured every other week. This time intervals was chosen because taking more samples (every week) could inhibit the composting process. Phosphorus (P) and potassium (K) have lower decomposition rates; therefore, their evolutions were measured at the beginning and at the end of the process. The parameters that are more crucial to agricultural applications (heavy metals, pathogens, phytotoxicity) were measured only at the end of the process. Every time, at least five points along the windrow piles were chosen for sampling. A section was created by a mini loader in each point in such a way that the materials from the top, middle and bottom part of the pile could be picked and mixed thoroughly. A representative sample of >2 kg was taken using the three times quartering method, placed in a plastic container with a lid, and immediately sent to the lab. The laboratory analysis was carried out on three subsamples at the Shiraz municipal solid waste organization laboratory and the central laboratory of the Isfahan University of Technology (for the heavy metal content). The moisture content was measured following equation 1; the representative sample was placed in a ceramic crucible and held in an oven set at 105 ◦C for at least 24 h, or until no difference in the weight measurement occurred [17]. The moisture was measured three times, and the average was then taken.

(Wet sample weight Dry sample weight) Moisture Content (%) = − 100% (1) (Wet sample weight) ×

The ash content was estimated by calculating the residual mass after heating. In total, 10 g of the prepared samples was placed in a ceramic crucible and subjected to muﬄe furnace heating up to 550 10 C for 6 h under strict time-controlled conditions, sample mass, and equipment speciﬁcations. ± ◦ The total amount of the organic carbon was calculated from the ash content, using Equation (2) [18].

Volatile solid (100) 100 Ash (%) Total organic carbon = = − (2) 1.8 1.8 The EC and pH were evaluated in 1:10 w/v sample: water extract using an EC/pH meter with a glass electrode [19]. The Total Kjeldahl Nitrogen (TKN) was analyzed using the regular Kjeldahl Method using a Gerhardt Vapodest 30S Analyser Unit (Königswinter, Germany). The total phosphorus (P) was measured using the spectrophotometry method (WTW Model photo Lab 6600 uv-vis, Weilheim, Germany). For the heavy metals, an Inductively Coupled Plasma-Spectrometer (Perkin Elmer, 7300 DV, Shelton, CT, United States) was used. The total potassium (K) was measured using ﬂame photometry [17]. The Germination Index (GI) was measured using lab experiment [20]. About 10 mm of 1:10 compost extract was added to ten Garden Cress placed in petri dishes, with three replications. The petri Sustainability 2020, 12, 9704 6 of 17

dishes were placed in the dark for 24–48 h, at room temperature. Water was used as the witness, and the germination index (GI) was calculated using Equation (3).

(Number o f germinated seed root length) in compost extract GI (%) = × 100 (3) (Number o f germinated seed root length)in distilled water × × 3. Results Sustainability 2020, 12, x FOR PEER REVIEW 6 of 17 3.1. Physical Properties 3.1.1. Temperature 3.1.1.The Temperature oxidation of carbon sources by microbial activities produces abundant energy in form of heat duringThe oxidation the decomposition of carbon sources process by [21,22]. microbial Th activitiese temperature produces fluctuation abundant is energyconsidered in form a proper of heat indicatorduring the for decomposition an efficient composting process [21, 22process]. The temperature[23]. In this fluctuationstudy, the istemperature considered awas proper monitored indicator dailyfor an (Figure efficient 2). composting The temperature process rose [23]. to In the this thermophilic study, the temperature phase (>50 was °C) monitored in the first daily week, (Figure and2). reachedThe temperature to its highest rose tolevel the thermophilicby the seventh phase day. (> This50 ◦C) displays in the first accurate week, initial and reached conditions to its highestin the experimentallevel by the seventhpile. The day. rapid This change displays in the accurate temperat initialure conditions profile during in the the experimental first week pile.also indicates The rapid anchange extreme in reduction the temperature in the organic profile matter during [23]. the firstDuring week the also active indicates decomposing an extreme phase reduction (the first three in the weeks),organic more matter temperature [23]. During fluctuation the active was decomposing observed, which phase consequently (the first three required weeks), more temperatureturnings to maintainfluctuation the was process observed, in the which sanitation consequently phase. The required requirement more turnings for sanitation; to maintain the the elimination process in of the pathogens,sanitation phase.insects, The larvae requirement and weed for seeds sanitation; by reaching the elimination the temperature of pathogens, ≥60 °C insects,for more larvae than and2 weeks weed wasseeds fulfilled by reaching [24,25]. the The temperature composting60 processC for of more different than 2organic weeks wasmaterials fulfilled took [24 place,25]. Theover composting 10 weeks ≥ ◦ [26,27].process The of di lackfferent of rises organic in materialstemperature took from place the over eighth 10 weeks week [26 onwards,27]. The confirmed lack of rises the in temperaturedecreasing microbialfrom the eighthactivities. week The onwards temperature confirmed nearly the reac decreasinghed the microbialambient from activities. week The10, temperaturewhich proves nearly the curingreached phase. the ambient from week 10, which proves the curing phase.

FigureFigure 2. 2.TemperatureTemperature profile proﬁle during during the the composting composting experiment. experiment.

3.1.2.3.1.2. Moisture Moisture MoistureMoisture is anan essential essential factor factor in thein maintenancethe maintenance of any metabolicof any metabolic process. Microbialprocess. metabolismMicrobial metabolismrequires an aqueousrequires mediuman aqueous to gain medium nutrients to gain and nutrients energy from and chemical energy reactions.from chemical The moisture reactions. content The moistureimpacts content the process impacts in terms the process of the oxygen in terms uptake of the rate, oxygen the uptake free air rate, space the and free the air temperature space and the [28 ]. temperatureAccording to [28]. [22], theAccording optimum to moisture [22], the for optimum an eﬀective moisture decomposition for an signiﬁcantly effective decomposition depends on the significantlywaste nature. depends However, ona the moisture waste contentnature. betweenHowever, 40–60% a moisture during content composting between is crucial 40–60% for microbial during compostingactivity [29 ].is crucial for microbial activity [29]. DuringDuring the the initial initial phase phase of of this this experiment, experiment, there there was was no no leachate leachate production. production. It It was was assumed assumed thatthat water water was was only only lost lost through through evaporation. evaporation. This This was was due due to to the the fact fact that that the the composting composting process process waswas carried carried outout underunder optimal optimal conditions, conditions, with with a well-blended a well-blended pile structure pile structure (mixing ratio).(mixing Moreover, ratio). Moreover, the mild weather in winter during the composting process period inhibited the generation of leachate. The active decomposition rate in the initial weeks lead to the reduction of the moisture inside the pile. The moisture content was kept close to 50% throughout the active decomposition process until the tenth week (Figure 3). In addition, 163 mm rainfall during the composting experiment (February to June 2019) provided the pile adequate moisture content. Sustainability 2020, 12, 9704 7 of 17

the mild weather in winter during the composting process period inhibited the generation of leachate. The active decomposition rate in the initial weeks lead to the reduction of the moisture inside the pile. The moisture content was kept close to 50% throughout the active decomposition process until the tenth week (Figure3). In addition, 163 mm rainfall during the composting experiment (February to SustainabilityJune 2019) 2020 provided, 12, x FOR PEER the pile REVIEW adequate moisture content. 7 of 17

Figure 3. Rainfall frequency and moisture profile during the composting experiment. Figure 3. Rainfall frequency and moisture profile during the composting experiment. The proper moisture content, adequate porosity, and accurate aeration sped up the organic matter’sThe proper aerobic moisture degradation. content, The adequate evaporation porosity rate, and is higheraccurate in aeration semi-arid sped areas up and, the asorganic a result, matter’sthe moisture aerobic contentdegradation. decreased The withevaporation a steep sloperate is in higher the curing in semi-arid phase. Atareas the and, end ofas thea result, composting the moistureprocess, content the moisture decreased content with value a steep was slope around in the 36%. curing This phase. is a rather At th remarkablee end of the outcome. composting It gives process,insight the into moisture the processing content andvalue storage was around conditions 36%. of This the final is a product,rather remarkable which is a outcome. significant It mattergives of insightconcern into in the semi- processing arid regions. and storage conditions of the final product, which is a significant matter of concern in semi- arid regions. 3.1.3. Bulk Density and the Weight of the Pile 3.1.3. Bulk Density and the Weight of the Pile During the composting, aerobic microorganisms break down the organic materials into stabilized substancesDuring the through composting, several chemicalaerobic microorganism reactions: mineralization,s break down ammonification, the organic and materials denitrification, into stabilizedetc. The substances byproducts through of biological several process chemical are heat, reactions: carbon dioxide,mineralization, ammonia, ammonification, and humus-containing and denitrification,products of loweretc. The weight byproducts and volume of biological [14,30]. The process mass andare volumeheat, carbon reduction dioxide, during ammonia, the composting and humus-containingof several raw materials products isof a lower key factor weight in theand design volume and [14,30]. operation The ofmass composting and volume facilities. reduction In this duringstudy, the the composting mass and volumeof severa ofl the raw organic materials materials is a key was factor reduced in bytheapproximately design and operation 27% and of 21%, compostingrespectively, facilities. over the In periodthis study, of the the composting mass and process.volume Theof the reduction organic valuesmaterials meet was the reduced results of by [31 ], approximatelywhich reported 27% that and the 21%, mass respectively, loss during over composting the period of sixof the diff compostingerent feedstocks process. ranged The from reduction 11.5% to values31.4% meet of thethe initialresults weight, of [31], and which 18.5% reported to 57.9% that loss the of mass the initial loss during volume. composting of six different feedstocksThe ranged bulk density from 11.5% is an to indicator 31.4% of for the the initial mass weight, of the materialand 18.5% within to 57.9% a specified loss of the volume initial [32 ]. volume.It defines the quantity of the compost that can be placed at a certain site, as well as the machinery size forThe its transportationbulk density is [ 33an]. indicator The waste for within the mass the compostingof the material pile achieveswithin a aspecified higher bulk volume density [32]. by It the definesend ofthe the quantity decomposition of the compost process that [34]. can The be stabilization placed at a ofcertain organic site, materials as well leadsas the to machinery the enhancing size of for theits ashtransportation content in such [33]. a The manner waste that within less volatilethe composting solids remain. pile achieves Figure4 displaysa higher thebulk scatter density diagram by theof end the relationshipof the decomposition between the process bulk density [34]. The and thestabilization volatile solids of organic during thematerials composting leads trial.to the As it enhancingshows, aof significant the ash content negative in correlationsuch a manner was that found. less volatile solids remain. Figure 4 displays the scatter diagram of the relationship between the bulk density and the volatile solids during the composting trial. As it shows, a significant negative correlation was found. Sustainability 2020, 12, 9704 8 of 17 SustainabilitySustainability 2020 2020, ,12 12, ,x x FOR FOR PEER PEER REVIEW REVIEW 88 ofof 1717

FigureFigureFigure 4. 4. The The 4. The relationship relationship relationship between between between the the thebulk bulk bulk density density density and and and volatile volatile volatile solids. solids. solids.

Overall,Overall,Overall, aa aremarkableremarkable remarkable increaseincreaseincrease in inin the thethe bulk bulkbulk density densitydensity of about ofof about 20.74%about 20.74% was20.74% observed waswas observedobserved in the experimental inin thethe experimentalexperimentalpile, while the pile,pile, volatile whilewhile the solidsthe volatilevolatile had a solidssolids 34.75% hadhad reduction. aa 34.75%34.75% These reduction.reduction. data reproduce TheseThese datadata the reproducereproduce results from thethe resultstheresults study fromfromcarried thethe studystudy out by carriedcarried [35], who outout revealedbyby [35],[35], whowho that revealed compostsrevealed thth withatat compostscomposts a moisture withwith content aa moisturemoisture of 35–55% contentcontent generally ofof 35–55%35–55% have a 3 3 generallygenerallybulk density havehave a ofa bulkbulk 500–700 densitydensity kg/ m ofof 500–700.500–700 kg/mkg/m3..

3.2. Chemical Properties 3.2.3.2. ChemicalChemical PropertiesProperties 3.2.1. pH 3.2.1.3.2.1. pHpH The microbial activity in the soil and the availability of different nutrients to the plant’s roots TheThe microbialmicrobial activityactivity inin thethe soilsoil andand thethe availabilityavailability ofof differentdifferent nutrientsnutrients toto thethe plant’splant’s rootsroots are considerably affected by the pH of the growth medium. The optimal pH value for microbial areare considerablyconsiderably affectedaffected byby thethe pHpH ofof thethe grgrowthowth medium.medium. TheThe optimaloptimal pHpH valuevalue forfor microbialmicrobial activity is suggested to be around neutral ranges of 6.5–7.5 [36]. The composting process began with a activityactivity isis suggestedsuggested toto bebe aroundaround neutralneutral rangesranges ofof 6.5–7.56.5–7.5 [36].[36]. TheThe compostingcomposting prprocessocess beganbegan withwith neutral range in the experimental pile (Figure5). A lower pH value may result from the formation of aa neutralneutral rangerange inin thethe experimentalexperimental pilepile (Figure(Figure 5)5).. AA lowerlower pHpH valuevalue maymay resultresult fromfrom thethe formationformation fatty acids during the early stages of the composting process. Subsequently, during the mesophilic ofof fatty fatty acids acids during during the the early early stages stages of of the the composting composting process. process. Subsequently, Subsequently, during during the the mesophilic mesophilic phase,phase, the the mineralization mineralization of ofnitrogenous nitrogenous compound compoundss increases increases the the pH pH value value through through the the formation formation of phase, the mineralization of nitrogenous +compounds increases the pH value through the formation ammonia (NH ) and ammonium (NH )[+37]. In the curing phase, the oxidation process transforms of ammonia (NH3 3) and ammonium (NH4 4 +) [37]. In the curing phase, the oxidation process of ammonia (NH3)+ and ammonium (NH4 ) [37]. In the curing phase, the oxidation process ammonium (NH4 ) to nitrate4+ (NO3−), which3- is defined as the nitrification process [38]. The pH value of transformstransforms ammonium ammonium (NH (NH4+)) to to nitrate nitrate (NO (NO3),-), which which is is defined defined as as the the nitrification nitrification process process [38]. [38]. The The the final product usually reaches a constant level ranging from 7.2 to 8.3, depending on the composting pHpH valuevalue ofof thethe finalfinal productproduct usuallyusually reachesreaches aa constantconstant levellevel rangingranging fromfrom 7.27.2 toto 8.3,8.3, dependingdepending onon feedstocks [39]. thethe compostingcomposting feedstocksfeedstocks [39].[39].

Figure 5. pH values of the composting experiments. FigureFigure 5. 5. pH pH values values of of the the composting composting experiments. experiments. Looking at Figure5, it can be seen that a pH value of 8 was reached in a short period. This resulted Looking at Figure 5, it can be seen that a pH value of 8 was reached in a short period. This fromLooking the considerable at Figure 5, breakdown it can be seen rate, that supported a pH value by the ofhigh 8 was turning reached frequency in a short during period. the This initial resulted from the considerable breakdown rate, supported by the high turning frequency during the resultedweeks from of the the process. considerable The ﬁnal breakdown product indicated rate, supported a reasonable by the range high ofturning pH for frequency ﬁnished compostduring the [35 ]. initialinitial weeksweeks ofof thethe process.process. TheThe finalfinal productproduct indicatedindicated aa reasonablereasonable rangerange ofof pHpH forfor finishedfinished compostcompost [35].[35]. Sustainability 2020, 12, 9704 9 of 17

Sustainability 2020, 12, x FOR PEER REVIEW 9 of 17 3.2.2. Electric Conductivity (EC) 3.2.2. Electric Conductivity (EC) Electrical conductivity (EC) is usually measured in soil and compost in order to estimate the salinityElectrical of the conductivity growth media (EC) [40 is]. usually EC is an measured important in chemical soil and property compost for in the order end to users estimate of compost the salinityproducts of the due growth to the media toxic e ff[40].ects EC that is itan may import haveant on chemical plant growth property [41]. for The the addition end users of compostof compost with productshigh EC due to theto the soil toxic increases effects the that salt it may accumulation have on plant in the growth root zone [41]. and The inhibits addition water of compost absorption with by highroots EC [to40 ].the Crops soil increases can resist the diff salterent accumulation ranges of salinity in the accordingroot zone toand their inhibits species. water The absorption EC of the by final rootsproduct [40]. hasCrops a high can importanceresist different in arid ranges and semi-aridof salinity conditions accordingdue to their to the species. lower organicThe EC matter of the contentfinal productand higher has a salinity high importance level of soils in inarid these and areas. semi-arid conditions due to the lower organic matter contentThe and degradation higher salinity of organic level of materials soils in these by the areas. soluble salts released by the microbial activity during theThe composting degradation process of organic consequently materials increases by the the soluble EC value salts [33 ].released Over the by composting the microbial period, activity the EC duringslowly the increased composting to be aroundprocess 3.18consequently dS/m, and incr thiseases range the remained EC value more [33]. or less Over stable the untilcomposting the end of period,the process, the EC resultingslowly increased in a final to EC be of around 3.36 dS 3.18/m afterdS/m, 10 and weeks this (Figurerange remained6). Compared more to or the less preferred stable untilrange the ofend EC of values the process, for the resulting growth medium, in a final 2–4 EC dS of/m, 3.36 the dS/m final after product 10 weeks has a high(Figure potential 6). Compared to be used to asthe a preferred soil conditioner range [of34 EC]. values for the growth medium, 2–4 dS/m, the final product has a high potential to be used as a soil conditioner [34].

FigureFigure 6. EC 6. EC profile proﬁle of the of the composting composting experiment. experiment.

3.2.3.3.2.3. C/N C/ RatioN Ratio TheThe relative relative proportion proportion of carbon of carbon and andnitrogen nitrogen is one is of one the of major the major parameters parameters in controlling in controlling the nutrientthe nutrient balance balance in the composting in the composting process process[22]. Ca [rbon22]. acts Carbon mainly acts as mainly a source as of a sourceenergy offor energy the microorganisms.for the microorganisms. The population The population growth of growthbacteria ofis bacteriainfluenced is influencedby the nitrogen by the fraction, nitrogen because fraction, theirbecause structure their is structure mainly formed is mainly from formed proteins from [38]. proteins The ideal [38]. range The idealfor the range stimulation for the stimulationof nitrogen of immobilizationnitrogen immobilization or mineralization or mineralization in the composting in the compostingprocess falls processin the initial falls substrate in the initial of about substrate a 25 of to about35 C/N a ratio 25 to [42]. 35 C / N ratio [42]. TheThe decomposition decomposition process process is slowed is slowed down down by bylim limitedited nitrogen nitrogen sources, sources, whereas whereas the the mixtures mixtures withwith a low a low carbon carbon content content lost lost extra extra nitrogen nitrogen as asammonia ammonia during during the the turnovers turnovers [38]. [38 ].In Inorder order to to improveimprove the the decomposition decomposition process, process, it isis essentialessential that that carbon carbon is providedis provided in easily-degradablein easily-degradable forms. forms.Forexample, For example, materials materials of high of high cellulose cellulose content content should should be shredded be shredded prior prior to piling to piling in order in order to become to becomereadily readily available available for microorganism for microorganism consumption. consumption. The microbial The activitymicrobial significantly activity significantly decreases in a decreases<20 C/N in ratio. a <20 For C/N that ratio. reason, For thethat C reason,/N ratio the is mostly C/N ratio used is as mostly stability used index, as stability ranging index, from 15ranging to 20 for froma finished 15 to 20 final for a product. finished final product. FigureFigure 7 7presents presents the the average valuevalue ofof the the C C/N/N ratio ratio during during the the composting composting period. period. In this In figure,this figure,there there is a clear is a trendclear trend of a decreasing of a decreasing C/N ratio, C/Nwith ratio, a with significant a significant reduction reduction of about of 36% about by the36% end by of thethe end process. of the process. It is apparent It is apparent from this from data thatthis thedata organic that the waste organic was waste degraded was edegradedffectively. effectively. This was due Thisto was the idealdue to conditions the ideal conditions in the experimental in the experimental pile created pile by created providing by providing a proper source a proper of carbonsource of and carbonnitrogen. and nitrogen. As a result, As the a result, final product the final appeared product to appeared be stable, to with be stable, an average with Can/N average ratio of C/N about ratio 16.42. of about 16.42. Sustainability 2020, 12, x FOR PEER REVIEW 10 of 17

Sustainability 2020, 12, 9704 10 of 17 Sustainability 2020, 12, x FOR PEER REVIEW 10 of 17

Figure 7. C/N ratio profile during the composting run.

3.2.4. Nitrogen (N), Potassium (K), Phosphor (P) From an agricultural point of view, the fertilizers should provide the soil with essential nutrient elements to support the crops’ development stages. Nitrogen (N), Potassium (K 2O), and phosphorus (P2O5) are the macro nutrient elements that significantly affect the plant growth. Usually, small parts Figure 7. C/N ratio profile during the composting run. of nutrients are mineralizedFigure by micr 7. C/Nobial ratio activity profile over during the the period composting of the run.composting, resulting in the higher3.2.4. nutrient Nitrogen content (N), Potassium of the final (K), product Phosphor [43]. (P) In this research, the concentrations of the total N, 3.2.4. Nitrogen (N), Potassium (K), Phosphor (P) K2O andFrom P2O5 anwas agricultural measured pointto be 12%, of view, 7%, the and fertilizers 33% higher should in final provide product, the soilrespectively with essential (Figure nutrient 8). What stands out in Figure 8 is that the nutrient content of the compost produced depends elementsFrom an to agricultural support the point crops’ of development view, the fertilizers stages. Nitrogenshould provide (N), Potassium the soil with (K2O), essential and phosphorus nutrient significantly on the type of raw organic waste used [18]. Fruits, vegetables and plant residues have elements(P2O5) areto support the macro the nutrient crops’ developm elementsent that stages. significantly Nitrogen aff ect(N), the Potassium plant growth. (K2O), Usually, and phosphorus small parts higher(Pof2O5 nutrients) areN and the Kmacro are (%), mineralized nutrientwhich can elements by be microbial seen that clearly significantly activity from over these affect the findings. period the plant of One the growth. of composting, the Usually,issues resultingthat small emerge parts in the fromof highernutrients these nutrient findings are mineralized content is that the of by the concentration micr finalobial product activity of [phosphorus43 over]. In the this period research,(P2O5) of in the the composting, final concentrations product resulting doesn’t of the inmeet total the N, the values set up by Iran national compost standard for class ‘A’ compost (1%–3.8%); therefore, higherK2O andnutrient P2O 5contentwas measured of the final to be product 12%, 7%, [43]. and In 33% this higher research, in final the concentrations product, respectively of the (Figuretotal N,8 ). nutrientK2O and adjustment P2O5 was measured using mineral to be fertilizer12%, 7%, is and needed 33% higherprior to in marketing. final product, respectively (Figure 8). What stands out in Figure 8 is that the nutrient content of the compost produced depends significantly on the type of raw organic waste used [18]. Fruits, vegetables and plant residues have higher N and K (%), which can be seen clearly from these findings. One of the issues that emerge from these findings is that the concentration of phosphorus (P2O5) in the final product doesn’t meet the values set up by Iran national compost standard for class ‘A’ compost (1%–3.8%); therefore, nutrient adjustment using mineral fertilizer is needed prior to marketing.

Figure 8. Nutrient content (wt/wt %) at the beginning and end of the composting experiment. Figure 8. Nutrient content (wt/wt %) at the beginning and end of the composting experiment. What stands out in Figure8 is that the nutrient content of the compost produced depends 3.2.5.significantly Heavy Metals on the type of raw organic waste used [18]. Fruits, vegetables and plant residues have higherThe nature N and of K (%),the fresh which organic can be seenwaste clearly and its from collection these findings. method Oneaffects of the issuesheavy thatmetal emerge content from of thesethe compost findings produced is that the concentration[44]. Some authors of phosphorus reported (P 2compostingO5) in the final as product a method doesn’t of heavy meet the metal values immobilizationset up by Iran [45]. national In fact, compost the biological standard forprocess class ‘A’changes compost them (1–3.8%); into unavailable therefore,nutrient forms for adjustment plant uptake.using Nevertheless,Figure mineral 8. fertilizerNutrient the content is mass needed (wt/wtand prior volume%) toat marketing.the reductionbeginning andof endthe ofwaste the composting during the experiment. decomposition process increase the concentration of heavy metals in the final product [46]. The utilization of 3.2.5. Heavy Metals compost3.2.5. Heavy with Metals a high content of heavy metals can affect soil properties, reduce crop productivity, and contaminateTheThe nature nature the foodof ofthe thechain fresh fresh in organic the organic long waste wasteterm and [47]. and its The its co collectionllection limitation method method on heavy affects aff ectsmetal the the heavycontent heavy metal is metal therefore content content ofof ofparticularthe the compost compost concern produced produced in the [44]. application [44 ].Some Some authors of authors the finalreported reported product. composting composting The standard as asa method a limit method of of heavy ofheavy heavy metals metal metal variesimmobilizationimmobilization in different [45]. countries. [45 In]. Infact, fact, Developing the the biological biological countries process process mostly changes changes have them lower them into intolimitation unavailable unavailable levels, forms formswhich for forallow plant plant themuptake.uptake. to useNevertheless, Nevertheless, raw organics the thefrom mass mass mixed andand waste volumevolume streams. reduction reduction of of the the waste waste during during the decompositionthe decomposition process processincreaseThe resultsincrease the concentration obtained the concentration from of the heavy analysis of metals heavy of the in metals thefinished final in productcompostthe final [ 46samples product]. The utilizationwere [46]. compared The ofutilization compost with the withof Iraniancomposta high and contentwith German a high ofheavy standardscontent metals of inheavy canTable ametalsff 3.ect The soil can German properties, affect soil standard properties, reduce has crop one reduce productivity, of the crop strictest productivity, and limitations contaminate and contaminatethe food chain the food in the chain long in term the [long47]. Theterm limitation [47]. The onlimitation heavy metal on heavy content metal is thereforecontent is of therefore particular of concernparticular in theconcern application in the ofapplication the final product. of the final The standardproduct. limitThe standard of heavy metalslimit of varies heavy in metals different variescountries. in different Developing countries. countries Developing mostly countries have lower mostly limitation have lower levels, limitation which allow levels, them which to useallow raw themorganics to use from raw mixedorganics waste from streams. mixed waste streams. The results obtained from the analysis of the finished compost samples were compared with the Iranian and German standards in Table 3. The German standard has one of the strictest limitations Sustainability 2020, 12, 9704 11 of 17

The results obtained from the analysis of the ﬁnished compost samples were compared with the Iranian and German standards in Table3. The German standard has one of the strictest limitations on heavy metal content among developed countries. As can be seen from the table, the compost produced has lower heavy metal concentrations than the values set by the German standard. These results show the importance of waste segregation in order to ensure the quality of the ﬁnal product.

Table 3. Heavy metal content compared with the Iranian and German standards.

Parameter (mg/kg) Value Iran Standard German Standard [48] Class A Class B Pb 27.35 200 150 100 Cd 0.04 10 1.5 1.0 Cr 8.32 150 100 70 Cu 56.89 650 100 70 Ni 15.34 120 50 35 Hg - 5 1.0 0.7 Zn 95.26 1300 400 300 Co 0.4 25 - -

3.2.6. Pathogen Content The metabolic reactions under aerobic composting are mainly controlled by the temperature fluctuation [22]. This has considerable effects on the type, number and species of microorganisms involved in the biological process. Indeed, most pathogens are deactivated and destroyed at high temperatures [38]. Fertilizers can contaminate soil and plant roots in the case where they contain pathogens such as E. coli and Salmonella [49]. Several authors have reported that the thermal death point of 55–60◦C for at least 24 h, alongside moisture of about 50% in the composting pile, is crucial for the removal of most types of pathogens [50,51]. Therefore, maintaining the composting process with favorable moisture and heat has high importance in securing the quality of final products. The Iranian national standard defines limits only for the E. coli and Salmonella content of organic fertilizers. The results, presented in Table4, indicate that the produced compost is pathogen free according to the local standards. As such, the final product could be safely applied for the cultivation of any crops.

Table 4. Pathogen content of the produced compost, compared with the Iranian standards.

Type of Pathogen Content (MPN/g) Iran Standard E. coli <3 1 103 (MPN/g) × Salmonella Absent 3 (MPN/4 g)

3.2.7. Phytotoxicity of the Final Product A phytotoxicity test is used to describe the degree of the maturity of the final product. Maturity is defined as the state of being appropriate for agricultural purposes. The seed growth is influenced significantly by the existence of phytotoxic substances such as ammonia, organic acids, and heavy metals, etc. [52]. The organic acids produced during initial phase are almost degraded in the curing phase under optimum composting conditions. Salt and heavy metals are mineralized or + immobilized [53], and nitrification contributes to the lowering of the NH4 /NO3 [54]. During the curing phase, the microbial activity decreases and the pile temperature drops down to reach near the ambient temperature [55]. The Germination Indexes (GI) of sensitive seeds like Cress and Chinses radish are widely used to determine the toxicities of final products [56]. The compost produced by the experimental pile had an 80% GI in this research. Several studies reported the absence of phytotoxicity by GI 80% [57,58]. ≥ Sustainability 2020, 12, 9704 12 of 17

According to the Iranian standard, the ﬁnal product should obtain a GI > 70% to count as a mature product. These ﬁndings indicate that the experimental pile was subjected to a complete curing phase, and the compost produced can be used for agricultural purposes safely.

4. Discussions

4.1. Scaling-Up the Collection of Source-Separated Organics Shiraz city has two main organic waste streams: business units and households. The solid waste management organization is fully responsible for handling organic waste. The collection service is offered on a day-to-day basis to business units, and on every alternative day to the households. Subjecting the household waste to a source-separated collection system is inapplicable under the existing waste management system. The absence of legal enforcement, the uncertain quality of the raw materials, the small quantity of daily waste generated, and the difficulty in storing it for a long time make this option out-of-scope. Up to 30% of daily markets are covered with a source-separation collection system using five vehicles with different capacities, ranged from 2 to 5 tons. The amount of this organic waste stream accounts for 7 to 10 tons/day of fruits and vegetables. The remaining fractions are collected with the mixed waste stream. Besides this, around 800 tons of garden wastes are collected seasonally (from November to January). In actuality, the existing collection system does not have the capability to cope with the amount of organic waste generated; the system has a shortage of collection vehicles and manpower. Up to 70% of market organics are therefore not covered through the segregated collection system. A plan should be developed that aims to upgrade the source-separation collection system. Consequently, the guaranteed available amount, freedom from impurities, and homogenous mixture of the market organic waste would create a high potential for the raw organic materials to end up in a composting facility. In parallel with the separate collection of the whole amount of the organic waste generated from the markets, attention should also be paid to the quality of the collected segregated organics. This could be guaranteed by making the waste producers an active part of the desired system. Usually, market stores pay fees for each extra bin, which makes them unwilling to cooperate with the segregation. Carrying out incentive programs and awareness campaigns for them are the major tools to obtain the desired collaboration and contribution. Another area that should be focused on is the catering business (restaurants, hotels and canteens). This sector is considered one of the main sources of the provision of rich nutrient-containing organics with a wide range of protein-based products. Overall, the guaranteed availability of such a mixture (fruits, vegetables, food waste, and garden waste) creates considerable potential for the production of compost with high quality.

4.2. The Positive Effect of Using Class Compost in Soil One of the aims of this study was to examine the application of the final product for agricultural purposes. The finished compost with a desired quality has the potential to enhance the soil’s physical, chemical and biological properties. The quality control parameters vary in different countries. They usually define agronomic aspects and the risk-free use of the final products [59]. National and international standards put great emphasis on the cleanliness of municipal solid waste compost regarding heavy metals, while fertility is a matter of marketability. For example, the assessment system developed by [11] evaluates the agronomical and environmental viability of compost produced from MSW, and, accordingly, its final application is distinguished. Following this approach, the compost produced in this research is of high fertility and risk-free. All of the final product samples appeared to be stable and were considered to be class ‘A’, making it suitable for the cultivation of any agricultural crops. The application rate of fertilizer is estimated based on the type of crop, irrigation water (quality, availability), and the growth media properties. Compared to chemical fertilizers, the available form of Sustainability 2020, 12, 9704 13 of 17

nutrients in the compost is lower, which raises the need for a higher rate of application to provide

Sustainabilitythe crop 2020 with, 12 the, x FOR required PEER REVIEW level of nutrients. In the study area, the application rate of 1.5 to 13 30 of tons 17 per hectare is typically recommended [60]. The results obtained from experimental analysis indicated Consideringthat the ﬁnal the product semi-arid contains weather N-K-P conditions, of about it 1.35%,is assumed 1.4%, that and 0.5%,the compost respectively. has a Considering30% moisture the content.semi-arid Consequently, weather conditions, there is it9.45 is assumed Kg, 9.8 thatKg, theand compost 3.5Kg, hasrespectively, a 30% moisture of N-K-P content. in each Consequently, ton of compostthere is produced. 9.45 Kg, 9.8 Kg, and 3.5Kg, respectively, of N-K-P in each ton of compost produced. TheThe application application of ofcompost compost leads leads to tothe the improvement improvement of ofsoil soil fertility fertility by byincreasing increasing the the organic organic matter,matter, cation cation exchange exchange capacity, capacity, and and biological biological activity, activity, asas wellwell asas improving improving the the water water retention retention and andtemperature temperature regulation. regulation. Several Several reports reports have have shown shown that Soilthat Organic Soil Organic Carbon Carbon (SOC) can(SOC) be increasedcan be increasedby between by between 50 and 7050kg and per 70 ton kg ofper (dry ton mass of (dry basis) mass compost basis) compost per hectare per per hectare year [per61, 62year]. Ultimately, [61,62]. Ultimately,carbon sequestration carbon sequestration contributes contributes to prevent to soil prevent erosion soil and erosion greenhouse and greenhouse gass emission. gass The emission. beneﬁt of Thecompost benefit applicationof compost inapplication the study in area the wasstudy evaluated area was by evaluated assuming by a assuming moderate a range moderate of SOC range increase of 1 1 1 SOC(50 increase Kg ha− (50y− Kg.t− hadry-1.y mass)-1. t-1 dry over mass) periods over of periods 10 and 20of 10 years and (Figure 20 years9). (Figure 9). · · ·

Figure 9. Estimated SOC increases under different compost application rates (fresh mass) over 10 and Figure 9. Estimated SOC increases under different compost application rates (fresh mass) over 10 20 years. and 20 years. The figure above illustrates a direct relationship between the amount of SOC and the compost applicationThe figure rates. above The illustrates highest a amount direct relationsh of carbonip sequestration between the thereforeamount of occurs SOC whenand the a highercompost rate application rates. The highest amount of carbon sequestration therefore occurs when a higher rate1 of of compost is applied. Over the periods of 10 and 20 years, SOC will be increased 10.5 t ha− and compost is1 applied. Over the periods of 10 and 20 years, SOC will be increased 10.5 t. ha-1 and· 21 t. 21 t ha− with 30 tons per hectare compost application, which also has a considerable positive effect -1 · haon with the available30 tons per soil hectare water. compost However, application, these results wh mayich notalsobe has attained a considerable due to the positive lower applicationeffect on therate available of compost soil water. on the However, basis of crop these nutrient results demands.may not be attained due to the lower application rate of compostAnother on the interesting basis of crop finding nutrient is obtained demands. by quantifying the amount of organic carbon sequestered Another interesting finding is obtained by quantifying the amount of organic carbon in the soil into the equivalent carbon dioxide using the stoichiometric ratio of CO2 (3.67). Assuming a sequesteredmoderate in range the soil of SOCinto the increase, equivalent this carbon means dioxide that the using application the stoichiometric of each ton ratio of compost of CO2 (3.67). has the Assumingpotential a tomoderate sequester range 128.45 of SOC kg carbon increase, dioxide this means equivalent that the on application a fresh mass of basiseach ton (50 of compost0.7 3.67). × × hasThe the carbon potential sequestrated to sequester in soil stated128.45 as kg carbon carbon dioxide dioxide equivalents equivalent for di ffonerent a fresh compost mass application basis (50×0.7×3.67).rate is shown The in Tablecarbon5, oversequestrated periods of in 5, soil 10, 15stated and 20as years.carbon dioxide equivalents for different compost application rate is shown in Table 5, over periods of 5, 10, 15 and 20 years. Table 5. Sequestered carbon in carbon dioxide equivalents. Table 5. Sequestered carbon in carbon dioxide equivalents. Time (year) Compost Application Rate Time (year) Compost(tons/hectare) application rate (tons/hectare) 5 105 10 1515 20 20 1.51.5 0.96 1.93 0.96 1.93 2.892.89 3.85 3.85 55 3.21 6.423.21 6.42 9.639.63 12.85 12.85 1010 6.42 12.85 6.42 12.85 19.2719.27 25.69 25.69 2020 12.85 25.69 12.85 25.69 38.5438.54 51.38 51.38 3030 19.27 38.54 19.27 38.54 57.8057.80 77.07 77.07

Carbon sequestration is significant at the application rate of 20 and 30 tons of compost per hectare. This would also have the effect of preventing soil erosion, which is in agreement with sustainable development goal target 15.3, i.e., combat desertification and restore degraded land and soil by 2030. Sustainability 2020, 12, 9704 14 of 17

Carbon sequestration is significant at the application rate of 20 and 30 tons of compost per hectare. This would also have the effect of preventing soil erosion, which is in agreement with sustainable development goal target 15.3, i.e., combat desertification and restore degraded land and soil by 2030.

5. Conclusions This project was undertaken in order to adopt a feasible method for the effective treatment of municipal organic waste in mega cities in Iran. Therefore, segregated municipal organic wastes were subjected to aerobic pile composting technology in an established compost plant in Shiraz, Iran. A windrow pile was prepared from source-separated market and garden waste. The raw organic materials were blended with 1:3 ratios in order to ensure the required initial C/N ratio for efficient microbial activity. In-situ measurements were carried out on daily basis; the temperature and moisture were monitored and, accordingly, the pile was subjected to aeration. Sampling was carried out periodically; representative samples were taken every alternative week in order to monitor and ensure an accurate degradation process. Lab analyses were performed in equipped laboratories for several key parameters; moisture content, bulk density, pH, EC, Nitrogen, C/N, and nutrient contents. The final product was subjected to a strict evaluation. The finished compost was examined for pathogens, heavy metals and phytotoxicity, and the obtained results were compared with the values set by the national and German standards. The final product of the experimental pile was of acceptable moisture, nutrients, pH, EC, and C/N ratio. All of the analyzed samples appeared to be pathogen-free. Concerning the heavy metals, all the compost samples analyzed had lower concentrations than those values set by the Iranian and German standards. The findings indicate that the state-of-art technology for the composting process was performed successfully under ideal conditions. The characteristics of the final product demonstrate a complete degradation of the organic waste within a relatively short period of time. Indeed, after the successful implementation of this pilot project, the number of daily markets subjected to the source-separation program were increased. This was mainly due to the fact that, compared to vermicomposting (on average 3 months), this method could turn the organic materials, in 10 weeks, into a value-added product. Furthermore, it requires less labor work, and the process is less vulnerable to semi-arid weather conditions. The findings of this study suggest that a wet/dry source-separation program can be effectively adopted for business units in Iran. Indeed, in the initial phase, among all of the municipal organic waste streams, the segregation and separate collection of market and garden waste are more viable under the existing SWM system. These findings are particularly relevant to decision makers in mega cities with established compost facilities.

Author Contributions: Conceptualization, H.J., S.N. and G.M.; methodology, H.J., S.N. and G.M.; investigation, H.J.; data curation, H.J.; writing—original draft preparation, H.J.; writing—review and editing, H.J., G.M. and S.N.; supervision, S.N., G.M. N.J. and M.N. All authors have read and agreed to the published version of the manuscript. Funding: The research work was funded by the Solid Waste Management Organization (SWMO) of Shiraz municipality by providing all of the requirements needed to perform the study in terms of raw input materials, working areas, machineries, equipment, staff and laboratory analysis. The research was also funded by Deutsche Forschungsgemeinschaft and Universität Rostock within the funding programme “Open Access Publishing”. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Sustainability 2020, 12, 9704 15 of 17

References

1. International Solid Waste Association (ISWA). Global Assessment of Municipal Organic Waste Production and Recycling. Available online: https://www.iswa.org/uploads/media/Report_1_Global_Assessment_of_ Municipal_Organic_Waste_Compressed_v2.pdf (accessed on 25 June 2020). 2. Awasthi, M.K.; Pandey, A.K.; Khan, J.; Bundela, P.S.; Wong, J.W.C.; Selvam, A. Evaluation of thermophilic fungal consortium for organic municipal solid waste composting. Bioresour. Technol. 2014, 168, 214–221. [CrossRef][PubMed] 3. Sukholthaman, P.; Sharp, A. A system dynamics model to evaluate effects of source separation of municipal solid waste management: A case of Bangkok, Thailand. Waste Manag. 2016, 52, 50–61. [CrossRef][PubMed] 4. Pohlmann, A. Waste Situation in Refugee Hosting Communities in Northern Jordan, with the Example of the City of Irbid. Master’s Thesis, Rostock University, Rostock, Germany, September 2017. 5. Qian, X.; Shen, G.; Wang, Z.; Guo, C.; Liu, Y.; Lei, Z.; Zhang, Z. Co-composting of livestock manure with rice straw, Characterization and establishment of maturity evaluation system. Waste Manag. 2014, 34, 530–535. [CrossRef][PubMed] 6. Onwosi, C.; Igbokwe, V.; Odimba, J.; Eke, I.; Nwankwoala, M.; Iroh, I.; Ezeogu, L. Composting technology in waste stabilization: On the methods, challenges and future prospects. J. Environ. Manag. 2017, 190, 140–157. [CrossRef] 7. Storino, F.; Arizmendiarrieta, J.S.; Irigoyen, I.; Muro, J.; Aparicio-Tejo, P.M. Meat waste as feedstock for home composting: Effects on the process and quality of compost. Waste Manag. 2016, 56, 53–62. [CrossRef] 8. El-Sayed, K. Some physical and chemical properties of compost. Int. J. Waste Resour. 2015, 5, 1–5. 9. Li, Z.; Lu, H.; Ren, L.; He, L. Experimental and modeling approaches for food waste composting: A review. Chemosphere 2013, 93, 1247–1257. [CrossRef] 10. Kulikowska, D.; Gusiatin, Z.M.; Bułkowska, K.; Kierklo, K. Humic substances from sewage sludge compost as washing agents effectively remove Cu and Cd from soil. Chemosphere 2015, 136, 42–49. [CrossRef] 11. Saha, J.K.; Panwar, N.; Singh, M.V. An assessment of municipal solid waste compost quality produced in different cities of India in the perspective of developing quality control indices. Waste Manag. 2010, 30, 192–201. [CrossRef] 12. Rupani, P.F.; Delarestaghi, R.M.; Abbaspour, M.; Rupani, M.M.; EL-Mesery, H.S.; Shao, W. Current status and future perspectives of solid waste management in Iran: A critical overview of Iranian metropolitan cities. Environ. Sci. Pollut. Res. 2019, 26, 32777–32789. [CrossRef] 13. Shiraz Municipality, Annual Yearbook of Shiraz City (In Farsi). Available online: https://moba.shiraz.ir (accessed on 2 June 2020). 14. Abbassi, B.; Abubaker, S.; Al-Manaseer, E.; Nassour, A.; Dababneh, B.; Shqairat, W.; Al-Jaar, M. Optimization of operating parameters of windrow composting of animal manures. J. Solid Waste Technol. Manag. 2015, 41, 60–67. [CrossRef] 15. Saad, N.F.M.; Ma’min, N.N.; Zain, S.M.; Basri, N.E.A.; Zaini, N.S.M. Composting of mixed yard and food wastes with effective microbes. J. Teknol. 2013, 65, 89–95. 16. Rich, N.; Bharti, A. Assessment of different types of in-vessel composters and its effect on stabilization of MSW compost. Int. Res. J. Eng. Technol. 2015, 2, 37–42. 17. APHA. Standard Methods for the Determination of Water and Wastewater, 23th ed.; American Public Health Association: Washington, DC, USA, 2005. 18. Hemidat, S. Feasibility Assessment of Waste Management and Treatment in Jordan. Doctoral Dissertation, Rostock University, Rostock, Germany, October 2019. 19. Pan, I.; Sen, S.K. Microbial and physico-chemical analysis of composting process of wheat straw. IJBT 2013, 12, 120–128. 20. Warman, P.R. Evaluation of seed germination and growth tests for assessing compost maturity. Compost Sci. Util. 1999, 7, 3–37. [CrossRef] 21. Epstein, E. The Science of Composting; Technomic Publishing Company: Lancaster, PA, USA, 1997; pp. 19–32. 22. Norbu, T.; Visvanathan, C.; Basnayake, B. Pretreatment of municipal solid waste prior to landfilling. Waste Manag. 2005, 25, 997–1003. [CrossRef] Sustainability 2020, 12, 9704 16 of 17

23. Xiao, Y.; Zeng, G.M.; Yang, Z.H.; Shi, W.J.; Huang, C.; Fan, C.Z.; Xu, Z.Y. Continuous thermophilic composting (CTC) for rapid biodegradation and maturation of organic municipal solid waste. Bioresour. Technol. 2009, 100, 4807–4813. [CrossRef] 24. Insam, H.; De Bertoldi, M. Microbiology of the composting process. Compost Sci. Technol. 2007, 8, 25–48. 25. Zhang, L.; Sun, X. Influence of bulking agents on physical, chemical, and microbiological properties during the two-stage composting of green waste. Waste Manag. 2016, 48, 115–126. [CrossRef] 26. Sánchez-Monedero, M.A.; Fernández-Hernández, A.; Higashikawa, F.S.; Cayuela, M.L. Relationships between emitted volatile organic compounds and their concentration in the pile during municipal solid waste composting. Waste Manag. 2018, 79, 179–187. [CrossRef] 27. Francou, C.; Linères, M.; Derenne, S.; Le Villio-Poitrenaud, M.; Houot, S. Influence of green waste, biowaste and paper–cardboard initial ratios on organic matter transformations during composting. Bioresour. Technol. 2008, 99, 8926–8934. [CrossRef][PubMed] 28. Petric, I.; Helic, A.; Avdic, E.A. Evolution of process parameters and determination of kinetics for co-composting of organic fraction of municipal solid waste with poultry manure. Bioresour. Technol. 2012, 117, 107–116. [CrossRef][PubMed] 29. Bernal, M.P.; Alburquerque, J.A.; Moral, R. Composting of animal manures and chemical criteria for compost maturity assessment: A review. Bioresour. Technol. 2009, 100, 5444–5453. [CrossRef][PubMed] 30. Yue, B.; Chen, T.; Gao, D.; Zheng, G.; Liu, B.; Lee, D. Pile settlement and volume reduction measurement during forced-aeration static composting. Patent of PR China. Bioresour. Technol. 2008, 99, 7450–7457. [CrossRef][PubMed] 31. Stentiford, E.; de Bertoldi, M. Composting: Process. J. Solid Waste Technol. 2010, 1, 513–532. 32. Mohee, R.; Mudhoo, A.; Unmar, G.D. Windrow co-composting of shredded office paper and broiler litter. Int. J. Environ. Waste Manag. 2008, 2, 3–23. [CrossRef] 33. Agnew, J.M.; Leonard, J.J. The physical properties of compost. Compost Sci. Util. 2003, 11, 238–264. [CrossRef] 34. Hemidat, S.; Jaar, M.; Nassour, A.; Nelles, M. Monitoring of composting process parameters: A case study in Jordan. Waste Biomass Valorization 2018, 9, 2257–2274. [CrossRef] 35. Sullivan, D.M.; Miller, R.O. Compost quality attributes, measurements, and variability. In Compost Utilization in Horticultural Cropping Systems; Stoffella, P.J., Kahn, B.A., Eds.; CRC Press: Boca Raton, FL, USA, 2001; pp. 95–120. 36. Wong, J.W.C.; Fung, S.O.; Selvam, A. Coal fly ash and lime addition enhances the rate and efficiency of decomposition of food waste during composting. Bioresour. Technol. 2009, 100, 3324–3331. [CrossRef] 37. Oviedo-Ocaña, E.R.; Dominguez, I.; Komilis, D.; Sánchez, A. Co-composting of green waste mixed with unprocessed and processed food waste: Influence on the composting process and product quality. Waste Biomass Valorization 2019, 10, 63–74. [CrossRef] 38. Gajalakshmi, S.; Abbasi, S.A. Solid waste management by composting: State of the art. Critical Reviews in Environ. Sci. Technol. 2008, 38, 311–400. [CrossRef] 39. Nakasaki, K.; Yaguchi, H.; Sasaki, Y.; Kubota, H. Effects of pH control on composting of garbage. Waste Manag. Res. 1993, 11, 117–125. [CrossRef] 40. VanderGheynst, J.S.; Pettygrove, S.; Dooley, T.M.; Arnold, K.A. Estimating electrical conductivity of compost extracts at different extraction ratios. Compost Sci. Util. 2004, 12, 202–207. [CrossRef] 41. Chan, M.T.; Selvam, A.; Wong, J.W. Reducing nitrogen loss and salinity during ‘struvite’ food waste composting by zeolite amendment. Bioresour. Technol. 2016, 200, 838–844. [CrossRef] 42. Tchobanoglous, G.; Theisen, H.; Vigil, S. Biological and chemical conversion technologies. In Integrated Solid Waste Management: Engineering Principles and Management Issues, 2nd ed.; Tchobanoglous, G., Ed.; McGraw-Hill Higher Education: New York, NY, USA, 1993; pp. 671–716. 43. Golueke, C.G. Bacteriology of composting. Biocycle 1992, 33, 55–57. 44. Zhang, H.; He, P.J.; Shao, L.M. Implication of heavy metals distribution for a municipal solid waste management system—A case study in Shanghai. Sci. Total Environ. 2008, 402, 257–267. [CrossRef] 45. Singh, J.; Kalamdhad, A.S. Reduction of Heavy Metals during Composting. J. Environ. Prot. 2012, 2, 36–43. 46. Smith, S.R. A critical review of the bioavailability and impacts of heavy metals in municipal solid waste composts compared to sewage sludge. Environ. Int. 2009, 35, 142–156. [CrossRef] 47. Wong, J.W.C.; Selvam, A. Speciation of heavy metals during co-composting of sewage sludge with lime. Chemosphere 2006, 63, 980–986. [CrossRef] Sustainability 2020, 12, 9704 17 of 17

48. Ordinance on the Recovery of Biowaste on Agricultural, Forestry and Horticultural Uses (Biowaste Ordinance—BioAbfV). Available online: https://www.gesetze-im-internet.de/bioabfv/BioAbfV.pdf (accessed on 15 April 2020). 49. Strauch, D. Occurrence of Microorganisms Pathogenic for Man and Animals in Source Separated Biowaste and Compost—Importance, Control, Limits, Epidemiology. In The Science of Composting; Bertoldi, M.D., Sequi, P., Lemmes, B., Pap, T., Eds.; Springer: Dordrecht, The Netherlands; Stuttgart, Germany, 1996; pp. 224–232. 50. Polprasert, C. Organic Waste Recycling Technology and Management, 3rd ed.; IWA Publishing: Bangkok, Thailand, 2007; pp. 88–134. 51. Hassen, A.; Belguith, K.; Jedidi, N.; Cherif, A.; Cherif, M.; Boudabous, A. Microbial characterization during composting of municipal solid waste. Bioresour. Technol. 2001, 80, 217–225. [CrossRef] 52. Barral, M.T.; Moldes, A.B.; Cendón, Y.; Díaz-Fierros, F. Assessment of municipal solid waste compost quality using standardized methods before preparation of plant growth media. Waste Manag. Res. 2007, 25, 99–108. 53. Kim, E.Y.; Hong, Y.K.; Lee, C.H.; Oh, T.K.; Kim, S.C. Effect of organic compost manufactured with vegetable waste on nutrient supply and phytotoxicity. Appl. Biol. Chem. 2018, 61, 509–521. [CrossRef] 54. Morisaki, N.; Phae, C.G.; Nakasaki, K.; Shoda, M.; Kubota, H. Nitrogen transformation during thermophilic composting. J. Ferment. Bioeng. 1989, 67, 57–61. [CrossRef] 55. Cesaro, A.; Conte, A.; Belgiorno, V.; Siciliano, A.; Guida, M. The evolution of compost stability and maturity during the full-scale treatment of the organic fraction of municipal solid waste. J. Environ. Manag. 2019, 232, 264–270. [CrossRef] 56. Barral, M.T.; Paradelo, R. A review on the use of phytotoxicity as a compost quality indicator. Dyn. Soil Dyn. Plant 2011, 5, 36–44. 57. Emino, E.R.; Warman, P.R. Biological assay for compost quality. Compost Sci. Util. 2004, 12, 342–348. [CrossRef] 58. Riffaldi, R.; Levi-Minzi, R.; Pera, A.; De Bertoldi, M. Evaluation of compost maturity by means of chemical and microbial analyses. Waste Manag. Res. 1986, 4, 387–396. [CrossRef] 59. Van Fan, Y.; Lee, C.T.; Klemeš, J.J.; Bong, C.P.C.; Ho, W.S. Economic assessment system towards sustainable composting quality in the developing countries. Clean Technol. Environ. Policy 2016, 18, 2479–2491. [CrossRef] 60. Tadayon, M.S.; KHoghar, Z.; Moafpourian, G.H.; Ziaeian, A.H.; Keshavarz, H.; Nabavi, F.; Hemati, A.; Haghighatnia, H.; Rastegar, H.; Mirzavan, J.; et al. Fertilizer Recommendations for Agricultural and Horticultural Crops in Fars Province (in Farsi); Sana Publication: Tehran, Iran, 2005; pp. 1–46. 61. Powlson, D.S.; Bhogal, A.; Chambers, B.J.; Coleman, K.; Macdonald, A.J.; Goulding, K.W.T.; Whitmore, A.P. The potential to increase soil carbon stocks through reduced tillage or organic material additions in England and Wales: A case study. Agric. Ecosyst. Environ. 2012, 146, 23–33. [CrossRef] 62. International Solid Waste Association (ISWA). Quantifying the Benefits of Applying Quality Compost to Soil. Available online: https://www.iswa.org/uploads/media/Report_4_Quantifying_the_Benefits_to_Soil_ of_Applying_Quality_Compost.pdf (accessed on 25 September 2020).

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