A case study of Saudi Arabia

Journal of Cleaner Production 176 (2018) 426e438

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Optimizing the process of food waste compost and valorizing its applications: A case study of Saudi Arabia M. Waqas a, b, A.S. Nizami b, *, A.S. Aburiazaiza a, M.A. Barakat a, c, M.I. Rashid b, d, I.M.I. Ismail b a

Department of Environmental Sciences, Faculty of Meteorology, Environment and Arid Land Agriculture, King Abdulaziz University, Jeddah, Saudi Arabia Center of Excellence in Environmental Studies (CEES), King Abdulaziz University, Jeddah, Saudi Arabia Central Metallurgical R & D Institute, Helwan, 11421, Cairo, Egypt d Department of Environmental Sciences, COMSATS Institute of Information Technology, Vehari, 61100, Pakistan b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 June 2017 Received in revised form 2 December 2017 Accepted 19 December 2017 Available online 20 December 2017

This paper aims to (1) examine the challenges in the compost facilities of the Kingdom of Saudi Arabia (KSA), (2) optimize the composting techniques using indigenous natural zeolite and locally produced biochar from agricultural residues and (3) evaluate the environmental and economic benefits of optimized food waste composting. In KSA food waste is the most abundant stream of municipal solid waste that contribute up to 50% of the total waste. The landfill disposal of this waste results in several environmental and public health issues. Resource recovery through composting is one of the best approaches for treating such nutrient-rich organic waste. There exist several facilities in KSA for the conversion of food waste to compost using conventional methods of compost piles and trenches. However, none of the produced compost is capable of improving the quality and fertility of sandy soils and the growth of the crops due to limited values of organic matter, nutrients and water holding capacity along with high moisture contents, nitrification index, weed seed contents and ammonia emissions. In KSA, vast reservoirs of natural zeolite are available near to Jeddah city. Similarly, in KSA the most cultivated tree is date palm with more than 22 million date trees that would provide sufficient feedstock for biochar production. Therefore, diverting food waste from landfills to optimized composting facilities using natural zeolites and biochar could benefit the KSA economy with a total net savings of about US $70.72 million per year. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Municipal solid waste Food waste Compost Organic fertilizer Composting techniques

1. Introduction In the Kingdom of Saudi Arabia (KSA) and other Gulf countries, food waste is the largest waste stream (up to 50%) of municipal solid waste (MSW) (Anjum et al., 2016). Food waste is mixed with other waste types and disposed to the landfills or dumpsites without efficient material or energy recovery (Ouda et al., 2016). Consequently, the landfills are becoming a devastating source of greenhouse gas (GHG) emissions primarily methane (CH4), contamination of soil and water bodies, leachate, odors, and disease spreading vectors (Fig. 1). Only in KSA, landfills contribute around 76% of the total country's CH4 emissions (Khan and Kaneesamkandi, 2013). The KSA government has launched a new

* Corresponding author. E-mail addresses: [email protected], [email protected] (A.S. Nizami). https://doi.org/10.1016/j.jclepro.2017.12.165 0959-6526/© 2017 Elsevier Ltd. All rights reserved.

policy of Vision 2030 with an objective to seek sustainable solutions for the waste management to minimize the waste-disposal issues and generate economic benefits (Nizami et al., 2016; Waqas et al., 2017a). There are well-established material or energy recovery techniques such as anaerobic digestion (AD), composting, fermentation, gasification and refuse-derived fuels (RDF) to reduce landfill disposal of waste (Kelleher, 2007; Mashat, 2014; Nizami et al., 2017). These methods have less environmental impacts as compared to landfills, but require an additional cost of waste handling and processing (Mu et al., 2017). The production of organic fertilizer from food waste through composting is an ecofriendly and cheap alternative method, which is gaining worldwide attention (Awasthi et al., 2016). For instance, 4 million tons of organic fraction of MSW was treated by 124 compost facilities in the European Union (EU) to produce organic fertilizer during 2006. Similarly, Netherlands, Spain, and France managed 24%, 33% and

M. Waqas et al. / Journal of Cleaner Production 176 (2018) 426e438

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Fig. 1. Current challenges facing by municipalities for waste management in Gulf countries and their potential solutions (Alzaydi et al., 2013; Al-Turki et al., 2013; Ouda et al., 2016; Nizami et al., 2016).

14% of their total waste respectively for producing compost during 2005 (Kelleher, 2007). A desert and hot climatic conditions along with sandy soils in Gulf region result in limited or low crops cultivation (Alzaydi et al., 2013). Such natural conditions and massive generation of food waste support the production of compost that could be used to improve the physicochemical characteristics of the soil for agronomic activities (Mu et al., 2017). A limited food waste is utilized throughout the Gulf region for compost production using traditional methods of compost piles and trenches. The chemical characteristics of the produced compost in most of the Gulf countries are not agreed with the international compost standards (Alzaydi et al., 2013). Al-Turki et al. (2013) examined the chemical characteristics and quality of 25 compost producing facilities in KSA and compared the results with the local and international standards. Similarly, Alzaydi et al. (2013) examined the physicochemical characteristics of compost produced in various cities of KSA, including Madinah, Yanbu, Taif, Jeddah, and Makkah. These studies concluded that the compost quality is deteriorated with the presence of high moisture and inorganic contents, unpleasant odors, alkaline pH, and low nutritive values along with long processing time. In addition, the presence of heavy metals and micronutrients in some cases crossed the limits of the EU compost standards (Alzaydi et al., 2013; Al-Turki et al., 2013). Therefore, the locally produced compost is not capable of improving the quality and fertility of indigenous sandy soils, which are suffering from limited organic matter, cation exchange capacity, nutrients, and water holding capacity (Zajonc et al., 2014). It is essential to produce a mature and stable compost in a short period of time. Various amendments and additives are used, including chemicals (hormones), degrading microbes, bulking agents (straw, and leaves), natural minerals (zeolites), and biochar produced from waste biomass (Sun et al., 2016). The use of natural zeolites and biochar to optimize the composting process is getting more attention due to their unique physiochemical characteristics (Awasthi et al., 2016). Zeolites are naturally occurring hydrated aluminosilicate minerals of a porous structure having essential features like cation exchange capacity, sorption, bulking agent and

molecular sieving (Chan et al., 2016). The microporous structure of zeolites makes them capable of absorbing excess moisture and hence provides aerobic conditions to the microbes (Singh and Kalamdhad, 2012). Moreover, due to cation exchange capacity, zeolites could trap excessive ammonium (NH3) and thus inhibit its conversion to free NH3 that controls the odors and provides aeration in the composting process (Awasthi et al., 2016). Similarly, biochar is another amendment that could be used as a bulking agent, carbon source, and sorbent material to improve the compost quality (Jindo et al., 2016). Biochar is produced by burning biomass in oxygen deficient conditions. In compost, it plays a vital role in providing the aerobic conditions to the compost materials (Bass et al., 2016; Zhang et al., 2014). The presence of a wide range of functional groups on biochar surface provides adsorption sites to the dissolved ions (Bass et al., 2016). Moreover, its microporous structure allows absorption of a common solvent such as moisture during the composting process (Wei et al., 2014). This paper aims to examine the challenges in the compost producing facilities of KSA with an ambition to optimize them using indigenous natural zeolite and locally produced biochar from agricultural waste. Moreover, the influence of the produced compost on the properties of the soil along with environmental and economic benefits to KSA are discussed in detail.

2. Food waste to compost Composting is a biological process that is carried out under aerobic conditions in the presence of oxygen (Mu et al., 2017). During the process, microorganisms such as fungi and bacteria break down the complex organic matter into simpler products (Sun et al., 2016). A successful compost technique requires an understanding of the whole process, the involved constituents and the affecting process parameters, including pH, moisture content, aeration rates, temperature and substrate variables such as nutrient content, particle size and C/N ratio (Awasthi et al., 2016). Fig. 2 shows the process of composting, its various stages, and critical parameters that affect the process along with the potential applications of compost.

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Fig. 2. Circular diagram showing the critical parameters and potential application of compost (Waqas et al., 2017a).

2.1. Phases of composting process There are different phases of the composting process such as mesophilic, thermophilic, cooling and maturation (Xiao et al., 2009). Various communities of microorganisms dominate in each phase at different temperature ranges (Hou et al., 2017). In the initial phase, mesophilic microorganisms degrade the organic matter. The primary mesophilic microbes responsible for the degradation of food wastes are Acidovorax families, Acidovorax sp., Alcaligenes sp., Azotobacter sp., Bacillus badius, Chromobacterium sp., Clostridium sp., Comamonas testosterone, Enterobacter sp., Enterococcus sp., Micrococcus luteus, Nitrobacter sp., Nitrosomonas sp., Paenibacillus sp., Pseudomonas sp., Streptomyces sp., and Xanthobacter sp. (Ryckeboer et al., 2003). Heat is generated during the first phase that raises the compost temperature. As the temperature exceeds 40 C, the mesophilic microbes become less active and are replaced by the thermophiles. The thermophiles are the microorganisms that function at temperatures above 40 C (Xiao et al., 2009). During the thermophilic stage, the dominant thermophiles are Actinomyces sp., Bacillus sp., Clostridium thermocellum, Citrobacter freundii, Enterobacter cloacae, Geobacillus sp., Hydrogenobacter sp., Methanothermobacter, Micromonospora sp., Nocardia sp., Pseudomonas stutzeri, Pseudonocardia sp., Streptomyces sp., Symbiobacterium sp., and Thermus thermophilus (Westerman and Bicudo, 2005). The temperature in the compost pile increases rapidly up to 65 C within 24e72 h of pile formation, which is maintained for several weeks. This phase is known as the active phase of the composting process (Ryckeboer et al., 2003). The complex organic molecules such as fats, proteins, and carbohydrates in the form of hemicelluloses and cellulose are broken down during the active phase due to high temperature (Turan and Ergun, 2008). In addition, the pathogens and weed seeds are killed, and the phytotoxic compounds are broken down that are organic compounds and toxic to the plants (Ermolaev et al., 2015). The

common pathogens killed in this phase are Escherichia coli, Staphylococcus aureus, Bacillus subtillus, and Clostridium botulinum (Mashat, 2014). Afterward, the compost pile temperature gradually reduces to around 37 C, and the mesophilic microorganisms recolonize the pile, and compost enters a curing phase (Xiao et al., 2009; Ermolaev et al., 2015). The organic materials continue to decompose during the curing phase and are converted to biologically stable humic substances, mature or finished compost. The chemical breakdown of substrates is prompted through the action of enzymes produced by microorganisms during the composting process (Villar et al., 2016). Fungi and bacteria secrete these enzymes that break down the complex compounds into simpler compounds and finally absorb these components into their cells. Afterward, the sugar, proteins, starch and all other organic matters are oxidized to energy, water and carbon dioxide (CO2) through the catalytic reaction of the enzymes (Wang et al., 2016). The specialized enzymes include cellulase for the breakdown of cellulose, protease for proteins and amylase for starches. Similarly, other produced microbial enzymes that are involved in the degradation are oxygenases that play a critical role in the metabolism of organic compounds, monooxygenases catalyze various oxidative reactions such as ammonification, hydroxylation, biodegradation, biotransformation, and denitrification, laccases capable of catalyzing the oxidation of aminophenols, lignins, aryl diamines, and polyamines, as well as some inorganic ions (Arora et al., 2010; Pandey et al., 2017). Moreover, microbial peroxidases catalyze the oxidation of lignin and other phenolic compounds, whereas lipase plays its role in various reactions such as hydrolysis, interesterification, aminolysis, and esterification (Prasad and Manjunath, 2011). However, the enzyme systems mentioned above are mainly based on the complexity of the original molecules (Awasthi et al., 2016). For instance, the lignins are large polymers that cement cellulose fibers together and therefore are slowly decomposed due to their complex structure (Furhmann, 1999). In thermophilic phase, the soluble sugars from the initial mixture are directly taken up by the bacteria. The other microbes that result in microbial growth, secretion of different enzymes and temperature increase led to the breakdown of complex organic compounds (Chan et al., 2016). 2.2. Types of composting methods There are various composting methods for treating organic wastes (Das et al., 2003). However, the selection of each process depends on the available manpower, and the amount and nature of organic waste to be utilized in composting as well as environmental and economic conditions (Cekmecelioglu et al., 2005). There are mainly two methods of composting such as aerobic and anaerobic composting. In aerobic composting, microorganisms oxidize organic compounds to CO2, nitrate, and nitrite in the presence of oxygen (Hou et al., 2017). The carbon is utilized as a source of energy, whereas nitrogen is recycled from the organic compounds. The thermophilic bacteria are mainly responsible for the breakdown of complex biodegradable organic materials and proteins through the process of oxidation (Hou et al., 2017). On the other hand, organic waste that is highly wet undergoes AD with biogas production. The AD is preferred over other material or energy recovery techniques like incineration, gasification and landfilling (Yin et al., 2016). In both the aerobic and anaerobic processes, CH4 is produced from the organic waste whereas nitrogen is mainly conserved as ammonium. Moreover, the AD results in nutrient-rich digestate that is a source of nutrients to plants and soil (Yin et al., 2016). There are various types of composting based on reactors, aeration rates, and temperature changes. These include windrow composting, aerated static pile (uncovered and covered), in-vessel


and vermi-composting (Cooperband, 2002). In open windrow composting, raw materials are mixed and stacked into a long narrow windrows or piles that are continuously remixed and agitated on a regular basis. The process takes 12e20 weeks for completion (Cooperband, 2002). The height of windrows ranges from approximately 1 m for dense materials such as manures to about 3.5 m for less dense materials like leaves, whereas the width varies from 1.4 to 6 m (Cekmecelioglu et al., 2005). The feedstocks are frequently shredded to assure the optimum porosity and release of the trapped heat, gasses, and vapors (Mason and Milke, 2005). All materials receive an equal exposure to air, light, and temperature through continuous turning, thus provide consistent treatment conditions. Another important composting method is the aerated pile in open and covered forms. This process involves the aeration through a blower, and it takes around 10e13 weeks to produce a mature compost. The composting materials are placed on the top of perforated platform or pipe, whereas the air is passed either through sucked downwards (negative aeration) or blown upwards (positive aeration). The sucked downwards aeration acts as a biofilter and helps in removing the odors. Once the pile is formed, it requires no agitation or turning. The active stage is completed in about 3e5 weeks (Mason and Milke, 2005). In-vessel composting is the production of compost in drums, or channels using high rate controlled aeration system that is designed to provide optimal conditions (Pandey et al., 2016). This system offers favorable conditions for microbes and therefore accelerate the decomposition of organic matter. Continuous agitation accomplishes the required aeration of the material (Latifah et al., 2015). A variety of mechanical turning techniques is used for aerating the substance to accelerate the composting process. Therefore, the in-vessel system provides optimum conditions for microbes to accelerate the decomposition process and results in mature compost in the relatively short period (Makan et al., 2014). This method provides better control than other methods like windrows due to high process efficiency (Pandey et al., 2016). A brief comparison between the different composting methods is presented in Table 1. Vermi-composting is an effective method of composting with the use of worms that disintegrate the complex organic constituents to simpler molecules. For instance, red worms transform the decaying organic matters into worm castings (Cao et al., 2016). Casting contains high nutrient concentration, which is beneficial for plant growth. Since the worms cannot survive at high temperatures, so thermophilic conditions are not achieved in vermicomposting. Wadkar et al. (2013) have experimented that both weed seeds and pathogens were destroyed in vermi-composting even without increasing the temperature to the thermophilic range.

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2.3. Parameters of composting process Compost science requires the understanding of the whole composting process that involves materials and relationships of different environmental and process parameters such as temperature, pH, aeration rates, moisture content, and nature and composition of the feedstock. The nutrient content, particle size, and C/N ratio determine the feedstock composition (Ermolaev et al., 2015). The changes in different parameters that occur with time during the composting process are presented in Table 2. It is well noted that all the composting procedures and parameters are interdependent and correlated to each other (Lin et al., 2016). The optimum moisture contents should range from 35 to 60% for achieving efficient compost. However, the moisture levels from 50e70% have also been reported by many researchers as an optimum moisture (Jindo et al., 2016). The various aeration methods that provide oxygen for microbial activity are forced aeration, natural convection and physical turning of feedstocks at regular intervals (Latifah et al., 2015). Makan et al. (2014) have experimented the composting of MSW. They found significant activities of microorganisms in the reactor when air pressure was increased to about 0.8 bars. The study findings demonstrated that provision of air to the reactor provides favorable conditions to microbes, which degrade organic matters and produce gasses that in turn increase the internal air pressure. A primary role of air is also to eliminate the heat accumulation and remove extra moisture and CO2 (Mohee and Mudhoo, 2005). Makan et al. (2014) have revealed that during the initial stage, a rapid hydrogenation of composting materials dropped the pH from 6.8 to 5 and after 30 h (first temperature cycle) further lowered the pH value. The pH reduction is due to the formation of acids in the initial stage of composting. However, with a time interval, the pH increases from acidic to alkaline range due to the bio-oxidation of the compost materials (Wang et al., 2016). Furthermore, ammonification process and the release of free NH3 during the organic matter degradation lead to high pH (Jindo et al., 2016). Similarly, among other optimizing factors the optimum C/N ratio is from 25:1 to 35:1. Larsen and McCartney (2000) have found the optimum

Table 2 Details of variations in compost parameters on weekly basis (Wadkar et al., 2013). Weeks

pH

Moisture content (%)

Volume reduction (%)

Temperature (oC)

1 2 3 4 5 6

8.2 7.0 6.0 5.6 6.0 6.8

37.36 28.27 22.92 15.41 12.36 10.36

0 15.80 26.77 38.19 49.43 47

31 45 58 37 31 32

Table 1 Comparison between different composting methods. Composting method

Advantages

Limitations

Windrow system

- Time: 12e20 weeks - Cost: medium

Static pile

- Time: 10e13 weeks - Cost: low

In-vessel

-

-

Channels (enclosed)

Rapid composting process Better nuisance control Multiple turnovers of site footprint Rapid composting process Better nuisance control Multiple turnovers of site footprint

Slow process Low nuisance control Require large area Slow process Low nuisance control Require large area High capital and operating cost

- High cost

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biodegradation rates with C/N ratio of 29:1. In general, the values of C/N ratio varying from 25 to 38 promote a satisfactory composting process (Jindo et al., 2016). C and N are the critical elements required for microbial decomposition. C is utilized as an energy source as well as the basic building block making up to 50% of microbial biomass. Similarly, N is an essential part of the amino acids, proteins, enzymes, and DNA required for cell growth and function. The C:N ratios below 30:1 allow rapid decomposition and rapid microbial growth, but cause undesirable odors due to the loss of N in the form of NH3. In addition, C:N ratios above 30:1 do not provide sufficient N for microbial growth and result in lower degradation rates. A key parameter affecting the composting process is temperature. It is reported that any variation in temperature directly affects the microbial activities (Ermolaev et al., 2015). The microbial activities at high temperature result in higher organic matter degradation (Xiao et al., 2009). Aerobic respiration of microbes during their activities leads to the release of water. However, aeration and increase in temperature during the thermophilic stage make the feedstocks dry (Makan et al., 2014). Furthermore, the temperature rise during the thermophilic stage kill pathogens present in raw materials. Therefore, sanitization is achieved in the composting process (Hou et al., 2017). The efficiency of microbes also depends upon the particle size of the materials. Particle size provides the surface area available to microbes for degradation. Lower the particle size; the larger is the surface area available for microorganisms to degrade the materials (Zhao et al., 2017). All the parameters mentioned above are correlated with the microbial activities that produce certain types of enzymes (Wang et al., 2016). In addition, there are guidelines for the concentration of heavy metals and pathogens in the final compost. Table 3 shows the acceptable concentration of metals and pathogens on a dry weight basis in the final compost according to California Code of Regulations and Compost Quality in America (CQSG, 2000). 3. Optimization of food waste compost in the Kingdom of Saudi Arabia (KSA) The practices of making compost from food waste are started in KSA and other Gulf countries since long due to the presence of high nutrients and organic contents in food waste (Table 4) along with

Table 3 Acceptable metals concentration and pathogens in compost in g/kg (CQSG, 2000). Metals

Concentration on dry weight basis CCR

EU

Mercury (Hg) Nickel (Ni) Cadmium (Cd) Chromium (Cr) Arsenic (As) Copper (Cu) Selenium (Se) Lead (Pb) Zinc (Zn)

0.01 0.42 0.03 1.20 0.04 1.50 0.03 0.30 2.80

0.0007e0.01 0.02e0.20 0.0007e0.01 0.07e0.20 NR 0.07e0.60 NR 0.07e1.0 0.21e4.00

Pathogens

Level

Salmonella