Characterization of Landfills Solid Waste in Muscat and Estimation of their Energy Recovery

doi:10.21203/rs.3.rs-1948005/v1

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Characterization of Landfills Solid Waste in Muscat and Estimation of their Energy Recovery

https://doi.org/10.21203/rs.3.rs-1948005/v1

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published 09 Sep, 2023

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Solid waste management is one of the most important issues faced by the Sultanate of Oman. Disposal of municipal solid waste (MSW) in engineered landfills without any pre-treatment or separation is the only option available. Engineered landfills themselves are still new and their waste properties have not been well studied. A reliable database of the solid waste compositions, properties, and estimated energy content is important and the first stage in an efficient waste management system. This is an essential requirement, notwithstanding the complexity of the investigations in this area and the associated logistical challenges.

This study investigates the sort of the MSW from landfills in Muscat Governorate. Muscat MSW samples were gathered from Al Amerat and Barka landfills in 2020. MSW compositions were analyzed in terms of the materials ratios (food, plastics, papers… etc.), followed by investigating the ability of recycling that waste. MSW physiochemical properties of both landfills were studied. Therefore, eleven solid waste samples were collected in February from each landfill. Another eleven solid waste samples were collected from Barka landfill in March 2020. All solid waste samples preparation was done manually, and they were converted from solid into the liquid phase for laboratory analysis.

The results demonstrated that about 50% of the weight of disposed waste at both landfills is a recyclable material. MSW biodegradable organic content was high observed. MSW moisture content was observed to be within the range from 21.5 to 43.3%. In addition, MSW volatility and loss of ignition both were on the high sides; between 47.0 and 82.0% and between 56 and 91%; respectively. Total oxides ratio in the MSW were within the range of 12.4 and 44.06%. Silica was the highly influential oxide followed by Calcium Oxide. Furthermore, Muscat MSW found more than 18000 kJ/kg which results in high energy content. Six chemical formulas of the MSW were derived from the waste categories elemental analysis with and without sulfur element.

It has been noticed from this study that almost half of Muscat's municipal solid waste can be recyclable. Thus, the recycling industry should be adopted to utilize solid waste for the production of renewable material sources that can be an alternative to oil resources. Moreover, MSW biodegradable portion is high with a suitable degree of moisture content for the composting and biodegradation process. Waste-to-energy technologies are also feasible for Muscat MSW because of their high energy content associated with high volatility.

Landfills

Solid waste

Energy Recovery

and Solid waste management

Globally, solid waste generation has increased dramatically during past years and it is expected to double in coming decade especially in low- and middle-income countries due to the high rate of consumption and poor management practices (United Nations Human Settlements Programme, 2010). Overall world municipal solid waste in 2006 was 2.0 billion tons per year and expected to reach 2.4 billion tons per year in 2025, assuming per capita waste generation rate is 300 kg per year only (United Nations Human Settlements Programme, 2010).

Safe and viable solid waste management is one of the most critical environmental services that cannot be neglected even though it is intricate and necessitates high operating cost (United Nations Human Settlements Programme, 2010). Improper management of solid waste could cause many human health issues like diarrhea and acute respiratory diseases along with severe environmental damages; in some areas waste could cause flooding as a consequence of drain system clogs by the un-collected wastes (Sharholy et al., 2008).

Zero waste scheme is the recent hope in the developing countries to minimize the adverse environmental effect of solid waste, to move towards a circular economy. This strategy extends the waste management responsibility to users or waste producers like any other services and vigorously advises shifting to waste treatment and recovery technologies (Fudala-Ksiazek et al., 2016).

Efficient solid waste approaches and management techniques in any country necessitates a dependable database of produced waste quantities and composition. The monitoring procedure of these data is highly anticipated for decision-makers in the environmental sector for any waste reduction or treatment that are coherent with the country’s strategic plans. Waste quantities will corroborate the recent facilities’ capacities while the waste composition and characteristics can be adopted for landfills designs, mainly for detecting waste treatment and using technologies such as incineration, composting, and gasification. Thus, frequent solid waste nature studies are significant for the practicable waste management systems (Dahlén and Lagerkvist, 2008).

Solid waste management is one of the extremely important challenges for Oman. Municipal solid waste (MSW) dumping in the engineered landfills without any pre-treatment or separation is the only available alternative in Oman while the modern countries are in the stage of getting rid of solid waste dumping. Engineered landfills themselves are still new and the country has just fully shifted from the open and arbitrary dumpsite to engineered landfills in 2019. The waste composition will provide the initial signal of what section of waste can be recovered and the energy content calculates will assistance identify the proper waste recovery technologies. However, such type of information about Oman solid waste is still insufficient and some do not yet exist. It is worth noticing that MSW is heterogeneous and conveniently changes with time and from one topic to another. Thus, an efficient solid waste management system should rely on a dependable data of Oman solid waste in terms of compositions and physicochemical characteristics.

Literature Review

Solid waste (SW) is described as any solid or semi-solid unwanted materials which end up discarded except radioactive and site mining wastes which are excluded from the definition (Prairie Village, 1999). It is usually categorized in several ways based on the purpose of the categorization. Point source classification is based on classifying the place of waste generation as domestic, commercial, small-scale industries, institutions, market, or public cleansing waste (Hoornweg and Bhada-Tata, 2012). As for waste nature classification, it means indicating whether wastes are organic, inorganic, paper, fiber, plastic, rubber, glass, metal, and so on based on the material properties. Another waste classification is based on the waste heat content which is largely used for incineration purposes (Davis & David, 2008). Municipal solid waste (MSW) is all wastes collected and controlled by municipalities or their sub-contractors from different origins (Igoni, 2008). According to U.S. EPA, MSW includes wastes from residential, commercial, and institutional sources and excludes nonhazardous industrial waste, construction and demolition wastes, and wastewater treatment residuals although they might be dumped in the landfills in some regions (Staley & Barlaz, 2009).

Solid waste disposal is one of the main human concerns around the world that attract public attention and authorities’ discussion. This is possibly due to the significant increment of both solid waste production rate and per capita production value. Economic growth is the main driver of solid waste generation growth which is supposed to continue at the identical high rate and ultimately may lead to high waste generations (Magrinho et al., 2006). In 2012, the international generation rate of municipal solid waste only was around 1.3 billion tons per year and is expected to increase to approximately 2.2 billion tons per year by 2025. That will lead to considerable per capita waste generation rates, rising from 1.2 to 1.42 kg per person per day in less than fifteen years (Hoornweg and Bhada-Tata, 2012).

Many studies demonstrate that improper solid waste management could threaten human health and cause environmental degradation. Open solid waste dumping can directly cause many kinds of pollutions: (1) Land pollution, (2) Surface water pollution, (3) Groundwater pollution, (4) Noise pollution due to the waste operations undertaken near residential and commercial areas, and (5) Air pollution as a consequence of waste burning. Long term implications of open waste dumping will lead to severe environmental degradation and will increase the potential for human infections in that area such as skin diseases, asthma, and diarrhea (Saha, 2013).

The absolute quantity of MSW and per capita generation rate have both increased during past years (Magrinho, 2006). No one can doubt that inappropriate solid waste management will have a direct negative impact on the environment, human health, and economy. Therefore, the waste management system is one of the basic services provided to all citizens in modern cities and a technology-based solution for an effective waste disposal that has lower negative impacts on environment and health is highly required (Beigl et al., 2008).

Efficient solid waste management and waste legislation systems should begin from reliable databases with information on the type of waste and produced amounts per site (Beigl et al., 2008). The old waste management system which focuses on coping with waste through dumping and incineration is no longer a viable and friendly system and most modern countries have shifted to the integrated solid waste management (ISWM) which is based on the 3R approach; reducing waste, reusing and recycling. Nowadays, solid waste management hierarchy is integrated solutions to reduce the produced waste and alleviate the corresponding challenge of un-avoidable waste through reusing recycling, and waste conversion. This system objectives at optimizing the entire waste management, starting from source generation to the final recycled product or disposal. It basically focuses on reducing waste production and maximizing waste recovery at all stages (Memon et al., 2010).

The waste management hierarchy of the ISWM has deemed the umbrella of all new MSW management practices around the world. The hierarchy has numerous waste management operations levels, organized according to the environmental or energy benefits. The hierarchy is an advantageous policy utility for preserving resources and 0reducing associated pollutions for better public health and safety via less direct waste dumping in the environment (Azimi Jibril et al., 2012).

As it can be seen in Fig. 1, the top layer of the pyramid is the most preferred alternative to deal with all kinds of wastes through better processes maintenance and some environmental adjustments to reduce generated waste. The second preferred option is the reuse layer through waste collection and treatment streams such as adopted oils and solvents. Waste might be processed through biological, chemical, or physical techniques to break it down for another useful product that can be applied in other industries for advantageous productions. Landfilling is the last option if not all other preceding ways work and landfills should be well engineered to save the surrounded environment from any adverse effects (Azimi Jibril et al., 2012).

Municipal solid waste management is one of the most crucial difficulties for the Sultanate of Oman. The average municipal waste per capita generation is 1.2 kg per day (Zafar, S., 2020b). The current major disposal way in Oman is landfilling. Oman used to send all collected waste to around 300 open dumpsites across the country until 2013. Old dumpsites were placed near residential areas and lacked the simplest waste management practices and environmental safety and govern measures. However, with the government direction to minimize the impact of improper waste management, Oman government able to completely switch to engineered authorized dumping sites across the country within six years by 2019 (Altouqi, S., 2020).

Solid waste management is among the surpass priorities of the Omani government; it has been decided to privatize the waste management sector, and accordingly the Oman Environmental Services Holding Company (Be’ah) was established, accurately in 2007. The company is charged with establishing solid waste management infrastructure for Oman. Be’ah, has been using the diversion scheme that considers the attributes of the solid waste produced in Oman. Modern solid waste management facilities are under planning, and there are some plan calls for recycling facilities and major national projects whereby MSW will be incinerated to generate energy which will feed into a desalination plant (Prabhu, C., 2019).

Oman started shifting from open and arbitrary waste dumping to engineered landfills in 2011 and completed the successful switch in 2019. Data about the nature and characteristics of Oman engineered landfills waste is limited literature. There is one study done about the ultimate compositions’ evaluation of municipal solid waste, of one set of solid waste samples, in February 2015 from Al Amerat engineered landfill. The study results demonstrated that 24% of Al Amerat landfill solid waste were plastic wraps, 14% of the total waste was cardboard, and approximately 8% was food waste. The energy content analysis attained the following formulas C215H395O120N2S with sulfur and C125H230O70N without sulfur element utilizing modified DeLong equations (Baawain et al., 2017).

Study area

Muscat Governorate is the capital city of Oman with six wilayats, namely Muscat, Mutrah, Al Amerat, Quariyat, Bowsher, and Al Seeb (Al-Awadhi et al, 2018). Muscat Governorate occupies an area of 3800 km² (Hereher et al., 2020). The governorate has witnessed the fastest growth in most sectors over the past decade and becomes one of the most populated city in Oman, with a total population of 1.4 million people in 2020 as per the Omani national center for statistical information.

The solid waste generation rate in Muscat Governorate is the highest throughout the country. The major sources of municipal solid waste are residential waste due to the high population and the large amounts of commercial waste and non-hazard industrial waste; in fact, most commercial and industrial activities are based in Muscat (Hereher et al., 2020). Estimated yearly residential waste generation in Muscat is more than 613 million kg, considering the daily per capita production of 1.2 kg (Haya, 2020). Currently, Be’ah Company is operating two locations of covered landfills Baka and Al Amerat (Fig. 2). The two locations are simply an engineered landfill. There the municipal solid waste is dumped in a well-controlled and efficient procedure to avoid any adverse influence of waste dumping (Be’ah, 2020).

It is the first engineered landfill in Oman, which was commissioned in 2011. It marked the Oman’s first stage in tackling the problem of an efficient waste management. It is also called Al Multaqaa sanitary landfill. Al Amerat Engineered Landfill is designed with five cells and a total capacity of 10 million m³ (Zafar, S., 2020a). The landfill is operated by SUEZ Company, following both local and international operation standards (Be’ah, 2020). On the other hand, Barka Engineered Landfill is the largest landfill in terms of the quantity of waste received in Oman. The Landfill cell was commissioned in 2019 with a capacity of 2.1 million m³ and is operated by SUEZ. The cell is located next to the old dumpsite in Barka, approximately 80 km west of Muscat city (Be’ah, 2020).

Solid waste Sampling

Landfill municipal solid waste (MSW) characterization is a regular activity that takes place every three months during normal site operations. Both Barka and Al Amerat landfills are operated by the SUEZ Company and under the supervision of B’eah Company. Waste sampling in both landfills is comparable and according to the European regulations (Be’ah, 2020).

The sampling procedure is based on the historical data of each landfill. It has been done for fresh waste reaches to the landfill directly from the trucks. It commences with defining the waste source contribution percentage, based on the statistics of the trucks (origin and the tonnage of the trucks) achieving the site on a daily basis. The goal of this step is to have a representative sample of the overall population. After agreeing on how numerous trucks are sent to each location, waste is collected in one place and mixed well to guarantee that waste is homogenous on the waste characterization day. Around 300 kg of waste is adopted for the final waste characterization process; the amount might differ from one cycle to another based on the total waste amount received in the site. Figure 3 illustrates the sampling procedure.

Waste is separated manually (hand sorting) into 17 main categories as per the European regulations defined under the commission decision (2000/532/EC). Then, the sorted waste is weighed to get the fraction of each category out of the total initial amount as it can be seen in figure 4.

The first samples set of MSW from both landfills have been collected in February 2020. Another set of samples was collected from Barka Landfill in March 2020. However, because of Covid-19 pandemic, it was not possible to collect additional samples. It should be also stated that both landfills stop all separation activities to avoid the risk of threatening people's health.

Solid waste samples preparation for laboratory analysis

Most of the municipal collected solid waste samples consist of large pieces that could not be adopted directly for lab analysis. Thus, all kinds of waste have been cut into tiny pieces separately and some of them were additional crushed in order to produce a homogenous sample that can be used for the laboratory analysis. That is necessary because most of the experiments require a small quantity of the samples, reaching to 2 mg for the elemental analysis. Waste was mixed well after every single step to assure having a representative sample. Figure 5 shows row collected samples and the final prepared samples for the laboratory work. It can be seen evidently the solid waste samples as large pieces in figure 5a. All samples then are cutting into smaller pieces as illustrated in figure 5b. Finally, all samples will be crushed into finer samples to assurance homogeneity and workability in the laboratory.

To study the physical and the chemical properties of the solid waste, proximate and ultimate analysis tests have been carried out. Where tests for moisture content (MC), volatile content (VC), loss on ignition, total oxide content (TOC) and elemental composition of the solid waste analysis has been done.

Moisture content (MC) analysis

A well-mixed solid waste sample was operated in the experiment. Dishes were prepared by cleaning and heating them under 103–105°C for one an hour. They were then cooled to ambient temperature prior to the experiment to get rid of any contaminations or moistures. Samples were weighed and evaporated in a weighed dish generated of Platinum in an oven at 103°C to 105°C. The deviation of the weight between the wet and dry sample represents the moisture content in the sample. This experiment has been carried out according to the American Public Health Association (1989).

Volatile Content analysis

Volatile content can be expressed as the lost amount after the sample is put under evaporation (Orhorhoro et al., 2017). The volatile matter content is usually determined by the procedure of ignition of the sample at temperature of 550°C (Abu-Qdais, H. A., 2000).

According to the American Public Health Association (1989), the residue from (MC) measurement is ignited at a temperature of 550 °C using a muffle furnace. The sample is kept in the furnace for one an hour and then cooled to ambient temperature to take the weight in order to calculate the volatile solid matter. The remaining solids are fixed total, dissolved, or suspended solids, while those lost to ignition are volatile total.

Sodium (Na) and Potassium (K) content analysis

Sodium (Na) is a very soluble element. The ratio of sodium to total cations is important in agriculture and human physiology. High sodium ratio can harm the soil permeability (American Public Health Association, 1989). Potassium (K) is also an important element for both plants and human nutrition. It is usually can be found in groundwater (American Public Health Association, 1989).

In this experiment, solid samples were converted into a liquid through a solid digestion procedure using acids that can extract elements to the liquid phase. Which are concentrated Hydrochloric acid (HCL) and Nitric acid (HNO3). Then, the prepared solutions were heated for an efficient digestion procedure. Generally, the extraction procedure was undertaken according to the adjusted APHA for a solid analysis. Amounts of Sodium and Potassium are determined by flame emission photometry at 589 nm and 766.5 nm respectively. Five standard solutions of both Sodium and Potassium were adopted for calibration before concentration measurements.

Total Oxide Analysis

Total oxide is the amount of metal oxides and silica that present in the tested samples. Solid samples containing particulates or organic material generally require pretreatments/a pretreatment before the spectroscopic analysis and conversion to the liquid phase.

The objective of this experiment is to determine silica and total oxide percentage of Aluminum (Al), Calcium (Ca), Magnesium (Mg), Iron (Fe).

Na₂CO₃ is used for digestion purposes after heating the sample under1000 °C in a Muffle furnace. Then, hydrochloric acid (HCl) is used for samples dissolution from solid to the liquid phase. Boiled distilled water and concentrated HCL are used to separate silica from the heated dry samples. The filtration process of silica from aqueous solution is done using the ash-less Whatman filter papers. Filter paper is dried completely in the oven and then burned to measure the silica content. The aqueous solutions are used for oxides concentration measurements using A Perkin-Elmer Model Analyst 300 flame atomic absorption spectrometer.

Elemental Analysis

The elemental composition of the solid waste sample is one of the important materials studied to define the waste sample heating energy, which gives a direct indication of the amount of recoverable energy (Ionescu et al., 2011). Perkin-Elmer 2400 Series II CHNS/O elemental analyzer is used in this experiment to determine the weight percent of Carbon, Hydrogen, Nitrogen, Sulphur, and Oxygen in the solid waste sample (Tursunov et al., 2015). The analyzer is designed for handling any kind of samples (Perkin-Elmer, 2005). The analysis usually needs a high temperature and O₂-rich atmosphere for combustion (Pradhan, S., 2011). The experiments begin with weighing 2 to 3 mg of each well grinded sample using the Ultra-microbalance attached to the system in Tin capsules. The analyzer running mode is selected first for CHNS analysis where Oxygen is utilized in the combustion process. The analyzer is also heated up to1000°C before starting the analysis for an efficient combustion.

Blanks and calibration standard capsules from Sulfamic acid and cystine are used to ensure that the system is working well before starting the work. Then, weighed samples enter the system for CHNS analysis. Oxygen analysis is done using different calibration standards called benzoic acid and He/H₂ gas mix for the combustion process so that the only amount of O₂ is reported.

Waste Compositions

The composition of MSW has a crucial effect on the waste management system. It could be adopted as raw information to define the ability of MSW recycling and recovering. Moreover, it signifies the environmental emissions associated with waste disposal, or any probable treatments and recycling processes. Population density, life expectancy, income per capita, level of education, and development are the highly important factors affecting the quantity and compositions of the generated MSW (Azam et al., 2020).

Landfills waste composition information in Muscat is collected every three months by the landfill’s operators under normal operations. The latest municipal solid waste composition data of both covered landfills in this study are listed in Figs. 6 and 7.

*WEEE: Waste Electrical and Electronic Equipment

The three main waste categories in Al Amerat landfill are plastic with 24% out of the total waste received in the landfill, followed by food waste with 20% contribution, paper and cardboard waste portion was 16% out of the total MSW.

*WEEE: Waste Electrical and Electronic Equipment

The concentration of different plastic grades waste in Barka landfill makes up the highest percentage, at around 33% out of the total MSW, followed by paper and cardboard of 15% and then food waste of 10%.

From Figs. 6 and 7, it is obvious that the food waste dumped in Al Amerat landfill is double the one in Barka landfill. This mainly could be because of the population lifestyle and concentration of food industry. The area served by Al Amerat landfill has more hotels and restaurants which are responsible for high food waste. The wood portion out of total MSW in Barka is twice that of Al Amerat landfill. Wood recovery could be the key aspect behind this because Muscat municipality wood and green waste are employed in the compost production in Al Amerat.

Overall, Muscat municipal solid waste is characterized by high proportions of recyclable waste in both landfills. However, currently there is no recycling industry and almost all waste is still dumped in the engineered landfills. Among the recycled waste, plastic waste is the highest portion representing about 30% of the total MSW. Most plastic waste is non-biodegradable materials. Non-biodegradable plastic wraps which almost constitute 20% of the total MSW are not appropriate for both composting and combustion procedure and desire to be separated before waste feeds to these processes.

The plastics industry in Oman is supposed to increase sharply shortly which might bring about more plastic waste. Mechanical recycling is the most ecofriendly accessible solution to deal with the huge plastic waste produced and already used widely (Gu et al., 2017). Recycled plastics can be considered as a renewable origin of plastic. Furthermore, in terms of the economic benefits, its price will be less by 20–50% of virgin plastic (Gu et al., 2016). A low cost and high quality of recycling plastic will inspire industry owners to exert it especially in large production and construction (Al-Maaded et al., 2012). Thus, plastic recycling feasibility is worth to be studied further. There is also a possibility to recycle paper and cardboard which represent about 17% of the total waste. This will thus reduce Oman’s dependence on importing of these materials or at least covering the hoped high demand in this field.

The biodegradable waste ratio which primarily consists of food, green, wood, paper, cardboard, and the biodegradable plastic waste is the dominant segment of waste. Thus, biological treatment feasibility could be considered for additional studies.

Landfills hazardous wastes quantity is low because most hospitals waste is directed to incineration facilities somewhat than engineered landfills and there are special industrial waste landfills. These variation in the waste compositions is the evidence of the MSW heterogeneous nature and any solid waste management decisions should be based on a reliable data of each country itself and their long-term plans.

Moisture Content (MC)

Moisture content (MC) is a critical parameter for the degradation of solid waste in landfills. MSW moisture content can be quantified using reported literature moisture contents of the waste categories and their compositions out from the waste separation campaign and can be measured in the laboratory using 103°C ovens for an hour. As it can be seen in Fig. 8, a clear difference in MC for the collected samples in Al Amerat and Barka locations. The main reasons for moisture contents variances are waste content, weather conditions, and the season in which the samples were collected (Hui et al., 2006).

The measured moisture content of MSW samples varied from one sample to another which is normal due to MSW heterogeneous nature. Actual MCs might be higher than this because the analysis was done after days of the fresh waste collection, and the raw sample preparation was done manually because of the unavailability of the shredder machine. However, these results are higher than MCs reported by Baawain in 2017 for Al Amerat landfill waste sample collected in 2015 which ranges from 10.6–14.7% because samples were kept under the sun for many days before the analysis.

MSW moisture contents reported in the literature for Europe normally lower than 30% and it is higher than 60% for East Asia. For example, Hui in 2006 reported values of 53.6–64.1% for mixed waste in Chongqin, whereas Gidarakos in the same year reported 36.72% for the island of Crete in Greece (Hui et al., 2006 and Gidarakos et al., 2006).

Overall, fresh waste moisture content expected to be closed to the reported values. MSW with high MC tends to erode the incineration systems (Hui et al., 2006). While it is required for MSW bioreactor technology (Guérin et al., 2004).

Volatile Content and Loss of Ignition

Volatile content and loss of ignition are both important characteristics to comprehend waste nature. The volatile content (VC) has direct influences on the MSW incineration efficiency. The higher the volatile matter content, the simpler the ignition of MSW will be and more waste fraction can be removed in the incineration process (Azam et al., 2020). In pyrolysis and gasification processes, volatile content impacts production, composition and yield. High volatility leads to more gas pyrolysis and gasification (Zhou et al., 2014).

Volatile contents were investigated in this study through the weight loss of residue from moisture contents measurement ignited at a temperature of 550°C. The proportions of the measured MSW volatility as a dry base in these experiments, as it is shown in Fig. 9, varied from 47.0-82.4%. Volatile contents results were closed to the values reported for Lahor in 2020 which were in the range of 75–93% (Azam et al., 2020). Compared with the aforementioned percentages, the volatile contents acquired by Baawain were within the ranges of 28–33% (Baawain et al., 2017), mostly because there are ongoing variations in the MSW properties with the composition changes.

MSW loss on ignition was also investigated by burning residue samples from volatile content under 1000°C for an hour. The loss ignition was measured in the range of 56.2–91.0% which is higher than the study was conducted in 2015 for Al Amerat landfill MSW (Baawain et al., 2017).

High volatile contents and loss on ignition values show that Muscat dried MSW can be a good feed for incineration plants which is extensively used in modern countries. It is also good feed for both pyrolysis and gasification processes where high feed volatility is necessitated.

Total Oxides

MSW samples oxides have been investigated using metal concentration in samples. Aluminum, Calcium, Iron, and Magnesium concentrations were measured using Flame Atomic Absorption Spectrometer which is known for its high precision and capability to detect trace elements amount. Sodium and Potassium concentration were measured using Flame Atomic Absorption Spectrometer. The measured metals concentration with the mount of Silica is shown in Fig. 10.

Metals concentration were then utilized to estimate the percentage of the metal oxides present in each sample using the proportion of the molecular weight between the metal oxide to the metal. Final metal oxides in the samples are presented in Fig. 11.

Figure 11 shows that the total oxides ratio in the MSW was within the range of 12.4–44.06%. Silica is the highly prominent oxide in all MSW samples followed by Calcium Oxide. The major sources of Silica in the MSW are cement, concrete, bricks, stones, ...etc. There were about 6% of the total MSW in both landfills as fine particulates. Calcium oxide is usually produced from the thermal decomposition of limestone or any other materials containing Calcium Carbonate.

The sample collected from Barka landfill in March shows a higher total oxide, almost twice the amount in other samples. This could be due to waste heterogeneous nature or any variation in the sample preparation procedure. In general, Barka MSW samples have higher amounts of total oxides than Al Amerat samples.

Elemental Analysis

The elemental composition of MSW is extremely significant for heating energy calculations that indicate the quantity of recoverable energy in the waste. Perkin-Elmer 2400 Series II CHNS/O Elemental Analyzer was applied in determining the weight percent of Carbon, Hydrogen, Nitrogen, Sulphur, and Oxygen in each waste sample. The experiment result mass percentage of the MSW samples are shown in Fig. 12. It can be concluded that the most dominant compositions in all mixed waste samples are Carbon and Oxygen. Al Amerat February sample has the highest C content (48.7%w) and Barka March sample has the lowest C content (16.0% w) among mixed waste samples. The Sulfur percentage is the lowest in all samples compared to other elements. Nitrogen content in these samples is within the range of 0.7- 3.0%w.

Carbon content in the sub waste categories is high in four waste groups: paper and cardboard, plastic, textile, and wood. Plastic has the highest C content above (51.9% w) in all samples followed by textile with C content above 40.5% weight in all tested samples.

Oxygen is the second main component in most of the waste samples. The highest content of O was observed in wood, textile, paper, and cardboard. Metal and Glass waste have the lowest O content; except glass samples from Al Amerat landfill which could be because the sample was contaminated with another types of wastes.

Nitrogen content in almost all samples is low, mainly lower than 3%. There was one abnormal N content in the wood sample from the Al Amerat landfill with 15% N content.

Sulfur content in all mix and separated waste samples are low. Wood sample from Al Amerat landfill sample is also an exception in S content with the value of 36.2% which could be a consequence of the sample contamination with other wastes.

Overall, the sub waste categories elemental analysis is satisfactory for samples collected from a mixed waste. This indicates the importance of the waste source separations to minimize the amount of waste contaminations and for more easy and efficient recycling processes. The elemental analysis is mainly required to know the waste chemical compositions which will be used further for waste calorific value computations.

Chemical Content

Waste streams energy are available for some regions around the world and the data unavailability has heightened in low and middle-income countries. This initiates the necessity for reliable models and techniques to evaluate the approximate waste energy before any waste utilization or treatment processes adoption. Researchers use the available literature values for energy computation and exert developed models based on the waste physical and chemical properties to estimate the energy content. In this research, Dulong and modified Dulong equations (Kuleape et al., 2014) were used for the proximate municipal solid waste analysis.

The computation approach started with the waste chemical composition calculation for each kind of waste excluding other waste which includes (Other compositable, Non-compositable, Hazard, Weef, and Complex) wastes, and using the dry mass base. The moisture of the sample was converted to Oxygen and Hydrogen using their molar mass ratios. Moles of each element were then computed by dividing the element mass by their molar mass. After that, normalized moles ratio was calculated with Sulfur and without Sulfur content. Four different chemical formulas were developed for each sample set, two for the mixed waste sample and another two using mass percent of each waste streams. All results are presented in Tables 1 to 6. Normal used approach in most of the literature is waste streams compositions. However, an added chemical formula based of direct elemental assessment of mixed waste sample to test the difference and if there is opportunity to adopt such easier way because working with solid waste is intricate and consume a lot of time.

Table 1

Chemical composition of Al Amerat MSW samples-Feb. 2020
	Dry mass (kg)	Composition (kg)
		C	H	N	S	O
Mix waste	78.5	37.9	4.6	1.3	0.5	11.1
Food	6.9	1.0	0.3	0.1	0.0	0.6
Paper & Cardboard	17.7	6.3	0.7	0.4	0.1	5.2
Plastics	23.9	12.4	1.9	0.2	0.2	0.0
Glass	4.9	0.0	0.0	0.0	0.0	1.8
Metal	2.8	0.0	0.0	0.0	0.0	0.1
Textile	4.7	1.9	0.1	0.2	0.0	1.9
Wood	1.1	0.4	0.1	0.2	0.4	0.2
Trimming Garden	1.9	0.9	0.1	0.0	0.0	0.8
Sanitary	3.5	1.3	0.1	0.0	0.0	0.6
Fines & Dust	6.0	0.5	0.0	0.0	0.0	1.2
Moisture			3.0			23.7
Total Mass (kg)	68.8	24.7	6.2	1.1	0.7	36.0
Mass (%)		35.9	9.0	1.7	1.0	52.4
Molar Mass (kg/mol)		12.0	1.0	14.0	32.1	16.0
Moles		3.0	9.0	0.1	0.0	3.3
Molar Ratio with S		94	280	4	1	102
Molar Ratio without S		25	75	1	0	27

Table 2

Chemical composition of Al Amerat mixed waste sample- Feb. 2020
	Composition
	C	H	N	S	O
Total Mass (kg)	37.9	7.0	1.3	0.5	30.2
Mass (%)	49.3	9.1	1.6	0.6	39.3
Molar Mass (kg/mol)	12.0	1.0	14.0	32.1	16.0
Moles	4.1	9.0	0.1	0.0	2.5
Molar Ratio with S	215	473	6	1	128
Molar Ratio without S	35	78	1	0	21

Chemical formulas of Al Amerat using different waste streams samples are chemical formula with S: C₉₄ H₂₈₀ O₁₀₂ N₄ S and chemical formula without S: C₂₅ H₇₅ O₂₇ N. While chemical formulas of Al Amerat landfill mixed waste sample using the data in Table 2 analysis are chemical Formula with S: C₂₁₅ H₄₇₃ O₁₂₈ N₆ S and chemical Formula without S: C₃₅ H₇₈ O₂₁ N. From these formulas it is obvious that Carbon and Oxygen are the main chemical compositions in Al Amerat municipal solid waste. Formulas are deviated significantly from each other.

It can be seen that the chemical formulas of Barka Feb. 2020 landfill samples using the data in Table 3 are chemical Formula with S: C₁₂₅ H₃₇₆ O₉₉ N₃ S and chemical formula without S: C₄₀ H₁₂₁ O₃₂ N.

On the other hand, the chemical formulas of Barka Feb. 2020 (Table 4) landfill mixed waste sample analysis are chemical formula with S: C₇₇H₆₀₆ O₁₈₇ N₈ S and chemical formula without S: C₁₀ H₇₉ O₂₅ N.

It showed different chemical formulas of Barka Mar. 2020 landfill samples (Table 5) with S: C₁₉₁ H₄₃₆ O₁₀₉ N₂ S and chemical formula without S: C₈₀H₁₈₃ O₄₆ N.

For the same landfill Barka in Mar. 2020 for mix waste the samples analysis derives the chemical formula with S: C₁₄₂ H₅₈₇ O₃₃₀ N₅ S and chemical formula without S: C₂₇ H₁₁₀ O₆₂ N.

Table 3

Chemical composition of Barka MSW samples-Feb. 2020
	Dry mass (kg)	Composition
		C	H	N	S	O
Mix waste	56.7	14.6	4.9	1.7	0.5	9.1
Food	2.0	0.2	0.2	0.0	0.0	0.2
Paper & Cardboard	20.7	6.5	1.1	0.4	0.1	7.7
Plastics	36.7	21.8	4.1	0.4	0.4	10.0
Glass	3.7	0.0	0.0	0.0	0.0	0.1
Metal	3.6	0.1	0.0	0.0	0.0	0.1
Textile	6.4	3.3	0.6	0.1	0.1	1.9
Wood	3.1	0.6	0.1	0.0	0.0	0.9
Trimming Garden	0.7	0.2	0.0	0.0	0.0	0.2
Sanitary	3.6	0.1	0.5	0.0	0.0	0.4
Fines & Dust	4.9	0.8	0.2	0.1	0.0	1.1
Moisture			1.6			13.0
Total Mass kg	79.4	33.6	8.5	1.0	0.7	35.6
Mass (%)		42.3	10.7	1.2	0.9	44.8
Molar Mass (kg/mol)		12.0	1.0	14.0	32.1	16.0
Moles		3.5	10.6	0.1	0.0	2.8
Molar Ratio with S		125	376	3	1	99
Molar Ratio without S		40	121	1	0	32

Table 4

Chemical composition of Barka landfill mixed waste sample- Feb. 2020
	Composition
	C	H	N	S	O
Total Mass (kg)	14.6	9.7	1.7	0.5	47.6
Mass (%)	19.7	13.1	2.3	0.7	64.2
Molar Mass (kg/mol)	12.0	1.0	14.0	32.1	16.0
Moles	1.6	13.0	0.7	0.0	4.01
Molar Ratio with S	77	606	8	1	187
Molar Ratio without S	10	79	1	0	25

Table 5

Chemical composition of Barka MSW samples-Mar. 2020
	Dry mass (kg)	Composition
		C	H	N	S	O
Mix waste	77.9	12.5	1.9	0.5	0.2	18.9
Food	3.1	0.9	0.1	0.1	0.0	0.5
Paper & Cardboard	14.3	7.2	1.3	0.0	0.1	5.2
Plastics	37.5	24.2	3.3	0.2	0.3	3.7
Glass	3.9	0.1	0.0	0.0	0.0	0.0
Metal	2.9	0.1	0.0	0.0	0.0	0.2
Textile	5.5	3.1	0.4	0.1	0.0	1.2
Wood	2.8	1.1	0.2	0.1	0.0	1.2
Trimming Garden	1.7	0.1	0.0	0.0	0.0	0.1
Sanitary	3.6	0.5	0.1	0.0	0.0	0.4
Fines & Dust	6.5	0.5	0.1	0.1	0.0	1.2
Moisture			2.0			16.1
Total Mass (kg)	76.4	37.9	7.6	0.6	0.5	29.8
Mass (%)		49.6	9.5	0.7	0.7	37.6
Molar Mass (kg/mol)		12.0	1.0	14.0	32.1	16.0
Moles		4.1	9.4	0.1	0.0	2.3
Molar Ratio with S		191	436	2	1	109
Molar Ratio without S		80	183	1	0	46

Table 6

Chemical composition of Barka mixed waste sample- Mar. 2020
	Composition
	C	H	N	S	O
Total Mass (kg)	12.5	4.3	0.5	0.2	38.5
Mass (%)	22.2	7.7	1.0	0.4	68.7
Molar Mass (kg/mol)	12.0	1.0	14.0	32.1	16.0
Moles	1.9	7.6	0.1	0.0	4.3
Molar Ratio with S	142	587	5	1	330
Molar Ratio without S	27	110	1	0	62

From the chemical formulas analysis for the two landfill, it can be argued that there is no clear association between the waste collection area and the waste chemical formulas. The Table 7 shows that, the MSW formula of Al Amerat is closed to Barka March sample formula. The variation in the chemical formulas is significant between the mixed waste sample chemical formulas and the formulas established by using the landfill waste stream elemental mass percentages. Thus, the mixed waste samples are not typical of the overall waste chemical formula and not adequate to evaluate the solid waste chemical formula. Waste streams elemental analysis are required for accurate results regardless of their complexity.

Table 7

Chemical formulas of solid waste samples
Location	Sample	Formulas with S	Formulas without S
Al Amerat (Feb. 2020)	MSW	H₂₈₀ O₁₀₂ N₄ S	C₂₅ H₇₅ O₂₇ N
Al Amerat (Feb. 2020)	Mixed waste	C₂₁₅ H₄₇₃ O₁₂₈ N₆	C₃₅ H₇₈ O₂₁ N
Barka (Feb. 2020)	MSW	C₁₂₅ H₃₇₆ O₉₉ N₃	C₄₀ H₁₂₁ O₃₂ N
Barka (Feb. 2020)	Mixed waste	C₇₇H₆₀₆ O₁₈₇ N₈ S	C₁₀ H₇₉ O₂₅ N
Barka (Mar. 2020)	MSW	C₁₉₁ H₄₃₆ O₁₀₉ N₂	C₈₀H₁₈₃ O₄₆ N
Barka (Mar. 2020)	Mixed waste	C₁₄₂ H₅₈₇ O₃₃₀ N₅	C₂₇ H₁₁₀ O₆₂ N

Energy Content

Waste energy computation of MSW is important to evaluate the viability of any energy recovery initiative (Azam et al., 2020). To evaluate the overall MSW energy content, each waste stream energy was quantified separately. Table 8 shows the waste calculated energy. Waste categories composition of the latest separation of each landfill was used for the waste stream energy calculation.

Table 8

Energy content of MSW samples using waste composition from Be’ah and waste streams energy
	Wet Mass (kg)			Energy (kJ/kg)^*	Total Energy (kJ)
	Al Amerat Feb. 2020	Barka Feb. 2020	Barka Mar.2020	Energy (kJ/kg)^*	Al Amerat Feb. 2020	Barka Feb. 2020	Barka Mar. 2020
Mix waste	100.0	100.0	100.0	10500	106,328	31,205	48,783
Food	22.9	6.7	10.5	4650	309,953	361,768	250,826
Paper & Cardboard	18.8	21.9	15.2	16525	793,530	1,219,362	1,247,969
Plastics	24.3	37.4	38.3	32600	743	561	603
Glass	5.0	3.7	4.0	150	1,992	2,618	2,109
Metal	2.8	3.7	3.0	700	91,939	124,780	107,115
Textile	5.3	7.2	6.1	17450	25,479	71,617	64,353
Wood	1.4	3.9	3.5	18600	31,507	11,441	26,842
Trimming Garden	4.8	1.8	4.1	6500	41,096	42,354	41,295
Sanitary	8.2	8.5	8.3	5000	45,732	36,964	49,219
Fines & Dust	6.5	5.3	7.0	7000	106,328	31,205	48,783
Total					1,448,298	1,902,671	1,839,113
*(Peavy te al., 1985)

The highest energy content was observed in four waste categories: plastics, wood, textile, paper, and cardboard which represent almost more than 50% of the total MSW in all collected samples. Other wastes consist of hazard waste, non-combustible and other combustible, weef and complex waste; energy calculations were avoided because most of them were non-combustible.

The total waste energy content for the wet base is within the range from 1.4 to 1.9 Mn kJ which is in the high range of waste energy content. As for the dry base waste, computations were done using measured waste moisture in Fig. 8. Unit energy was also assessed after normalizing the new total waste without the other waste stream. The outcome of these computations are listed in Table 9.

Table 9

Unit energy content of MSW samples
	Total Energy (kJ)
	Al Amerat Feb. 2020	Barka Feb. 2020	Barka Mar. 2020
Total Energy (kJ)	1448298.2	1902670.5	1839112.7
Total Mass (kg)	100.0	100.0	100.0
Unit Energy (kJ/kg)	14483.0	19026.7	18391.1
Moisture (%)	21.5	43.3	22.1
Energy on Dry basis (kJ/kg)	18449.7	33556.8	23608.6

The energy unit of the MSW samples on the dry base is within the range from 18450 to 33560 kJ/kg. The dry energy content of Muscat MSW estimated by Baawain et al., 2017 was within the range from 15243 to 23779 kJ/kg which are close to the estimated values in this study. MSW typical unit energy as indicated in Peavy, 1985 for wet mix waste is 10500 kJ/kg which is associated with about 14000 kJ/kg for dry base calculations.

MSW incineration as a waste disposal option is debated throughout history because of its high costs and associated air pollution. However, the low and limited economic values of MSW separation and recycling in the past positioned the incineration procedure as the suitable choice for waste disposal for a long time (McKay, 2002). The incineration procedure reduces MSW volume to 80% of its original volume, the cost of the facility can be offset by energy sale, and an efficient flue gas treatment system could control pollutants in air discharge to a satisfactory level (McKay, 2002).

The waste-to-Energy project approved in 2019 at Barka planned to generate between 120 to 160 MW of electricity on yearly basis (Prabhu, 2019). From Table 9, the average unit energy on a dry basis of Muscat mixed waste without any separation was calculated 25205 kJ per kg of MSW. Thus, the planned unit will be necessary to process 479904 kg of MSW per day which is almost a quarter of the residential solid waste produced in Muscat.

One of the major limitations of MSW combustion reported through history is the greenhouse gas (GHG) production which is interrelated with the MSW characteristics and the procedure technology. For example, plastic waste which has high LHV also generates an increment in CO₂ emission (Yang et al., 2012). Another main GHG is CH₄ and N₂O (Hwang et al., 2017). However, mixed MSW incineration contribution to global warming was observed to be less compared to landfilling. This is because generated energy is largely used for electricity production which is usually produced from non-renewable petroleum resources (Hupponen et al., 2015). Thus, adopting incineration in the integrated management system necessitated to investigation all these perspectives in detail based on the Muscat MSW compositions, characteristics, and the region's long-term plans which might affect the compositions.

High Heating Value Content

High heating value (HHV) calculation based on the material elemental analysis has been approved for coal by Dulong (1785–1838) before the availability of a calorimetric bomb which has been then employed for more exact measurement. The formula has been appraised and modified for coal since that time. The first instance of using the equation for MSW HHV estimation was in 1980 and has been reviewed and modified several times for a large number of samples which corroborates its validity (Maksimuk et al., 2020).

Dulong (Kuleape et al., 2014) is still adopted widely and numerous experiments verified that the error ratio between the equation and bomb calorimeter is for coal is 1.5% (Maksimuk et al., 2020). Waste high heating values (HHV) were quantified using Dulong equation with Nitrogen and modified Dulong equation without Nitrogen (Kuleape et al., 2014)

\(HHV\left(\frac{KJ}{Kg}\right)=337C+1428(H- \frac{O}{8}\) ) +93S + 23N [4.1]

\(HHV\left(\frac{KJ}{Kg}\right)=337C+1428(H- \frac{O}{8}\) ) + 9S [4.2]

Where, C: Carbon (%), H: Hydrogen (%), O: Oxygen (%), S: Sulfur (%) and N: Nitrogen (%)

Table 10

High heating values of MSW using Dulong equations
		Percent by Mass					Energy kJ/kg with N	Energy kJ/kg without N
		C	H	N	S	O	Energy kJ/kg with N	Energy kJ/kg without N
Al Amerat Feb. 2020	Waste streams	35.9	9.5	0.7	0.7	37.6	19062.9	18988.2
Al Amerat Feb. 2020	Mix Waste Sample	49.3	9.1	1.6	0.6	39.3	22758.6	22669.5
Barka Feb. 2020	Waste streams	42.3	10.7	1.2	0.9	44.8	21699.5	21595.0
Barka Feb. 2020	Mix Waste Sample	19.7	13.1	2.3	0.7	64.2	14057.8	13947.3
Barka Mar. 2020	Waste streams	49.6	9.5	0.7	0.7	37.6	23668.5	23593.8
Barka Mar. 2020	Mix Waste Sample	22.2	7.7	1.0	0.4	68.7	6313.1	6255.7

Table 10 shows MSW calculated high heating values using the findings from both Dulong and modified Dulong equations (Kuleape et al., 2014). All studied MSW samples have high HHV. It is evident that the range of the MSW HHV is within the range from 6300 to 23700 kJ/kg which is within the estimated range by Baawain et al., 2017 (22000 kJ/kg). Overall, compared with different studies conducted in this field; this level of waste energy is high and waste to energy technology is a good alternative to be study further utilizing the direct heating values (Tian et al., 2001 and Bosmans et al., 2013).

In order to improve the solid waste management technique in Muscat, the study analyzes the compositions of municipal solid waste samples from Al Amerat and Barka landfills to estimate the potential of waste recycling.

MSW samples were prepared manually for laboratory analysis due to the unavailability of proceeding required apparatus for solid waste studies. Moisture content was quantified using a 103–105°C Oven. Furthermore, volatility was measured using a 550°C Muffle furnace and l000°C Muffle was adopted for loss of ignition measurements. Solid samples were converted into liquid phase for metal oxides analysis using adjusted APHA.

Energy content was estimated using the energy content of the waste stream as indicated in literature besides Muscat's new MSW compositions and moisture content. Dulong equations (Kuleape et al., 2014) were implemented to estimate the high heating values of the samples.

Muscat MSW was found with the proportion of recyclable materials which causes up to 50% of the total received waste. It has been observed that Plastic is responsible for about 28% and paper and cardboard for approximately 17% of the recyclable waste. Also, it is clear that both metal and glass make up 5% of the total MSW. MSW has a high fraction of biodegradable portion and moisture content within the range from 21.5 to 43.3%. Total oxides percentage in the MSW was found medium but on the other hand, silica is the most dominant followed by Calcium Oxide. All MSW investigated samples were found with high energy content (more than 18000 kJ/kg). In this study, the Chemical formulas that have been obtained from mixed waste samples deviate significantly compared to the ones obtained from waste stream elemental analysis. Thus, mixed waste chemical formals are not representative and cannot replace the long procedure required to obtain waste chemical formulas from elemental analysis of all waste streams.

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Authors Contributions Talal Etri was involved with the conceptualization, methodology, data analysis, and writing of the first draft of the manuscript. Omaima Al Samsami assisted in the conceptualization, methodology, data analysis, provision of resources, review, and editing of the paper. Mahad Said Baawain assisted in the conceptualization, data analysis, and provision of resources.

Funding Not applicable

Competing Interests The authors declare no competing interests

Availability of data and materials Not applicable

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Characterization of Landfills Solid Waste in Muscat and Estimation of their Energy Recovery

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Abstract

Figures

Introduction

Research Methodology

Materials And Methods

Study area

Results And Discussion

Waste Compositions

Moisture Content (MC)

Volatile Content and Loss of Ignition

Total Oxides

Elemental Analysis

Chemical Content

Energy Content

High Heating Value Content

Conclusion

Declarations

References

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