Different Technologies for Solid Waste Management
Waste treatment management is done using various technologies, among which landfills are the most basic and common technology. The other technologies are compositing and recycling process. All these technologies are widely used for the treatment of wastes, and recently more attention has been given to technologies for converting waste to energy, among which are various thermochemical and biological technologies. These conversion technologies are not more complex than existing techniques, but they have certain advantages like volume reduction, elimination of toxic substances, and recovery of energy (Rajendran et al. 2019).
Landfills are the most common way to dispose of solid waste on land without causing any damage or threat to local health or safety, by using the concepts of engineering to restrict the waste to the smallest possible area, to minimize it to the lowest practical volume, and to cover it with a layer of soil at the end of every day’s operation or at other regular intervals as necessary. Therefore, this process basically consists of spreading the material, which is accompanied by its gradual compaction with the least exposed region to the lowest possible volume and then fully covering with soil. Since the area is exposed is small, the required amount of soil is also less. Covering with soil or inorganic matter will help the waste to be free from flies and rodents, and it also conserves the heat from the decomposition of waste, thus helping to exterminate fly larva and other pathogenic species. Landfilling is most commonly categorized into three types based on site conditions: they are the ramp, area, and trench methods (Rao, Sultana, and Kota 2017). Landfilling involves high management costs, and it has resulted in many idle sites w'ith serious pollution emissions (Sahariah et al. 2015).
Composting is a process in which the wastes are decomposed with the help of microbes into rich organic manure (Suthar, Mutiyar, and Singh 2012). It is traditionally a common process done mainly in rural areas (Narayana 2009). Composting is classified into three types: traditional pit composting, traditional heap composting, and modern large-scale composting. In traditional pit composting, the most common aerobic and anaerobic processes are used, and heap composting is done using aeration techniques. Modern large-scale composting types include in-vessel composting, windrow composting, and effective microorganism-based quick composting (Pergola et al. 2018). The windrow' composting technique is used for the treatment of wastes at large scales. In the aerated turning windrow composting technique, the wastes are distributed through long stretches followed by periodic turning by mechanical aeration activity. Bulking substances are combined with solid wastes in another form of windrow composting called static pile composting. Since there is no mechanical turning involved in the process, the air passages in the layers of waste are formed as gaps and crevices because of the inclusion of bulking substances (Misra, Roy, and Hiraoka 2003). Windrow composting is widely used in Ecuador to recycle urban solid waste (organic/biodegradable) to minimize the waste treatment costs and also for the culturing of vegetables (Gavilanes-Teran et al. 2016). In some Canadian provinces (Saskatchewan, Alberta, and British Columbia), 200 drum composting units were installed for a population of 34.9 million (Zhang et al. 2013). More than 2000 species of microbes (e.g., species of Rhodobacter, Streptomyces, Lactobacillus, etc.) are capable of reducing the lignocellulosic wastes and accelerating the composting cycle (Aminah et al. 2016). Urban development and increasing consumption habits have increased the level of waste generated in the urban areas of the world. It is estimated that 40% of the gross wastes deposited in landfills consists of biodegradable products (Manios 2004). According to one study (Narayana 2009), about 70-80% of waste produced by Asian cities including Singapore, Mumbai, and Tokyo is biodegradable.
Basically, earthw'orm-mediated composting or vermicomposting is the widely followed technique all over the world because of its rapid decomposition rate of waste substances, its substantial enhancement of the finished product, and its eco- friendly sustainability (Suthar, Mutiyar, and Singh 2012). Vermicompost is earthworm’s excreta and it is humus rich. Certain earthworm species (Perionyx excavates, Metaphire posthuma, Ponthoscolex corethrurus, Lampito mauritii, L. terrestris, Lumbricus rubellus, Eudrillus euginae, and Eisenia fetida) have proved to detoxify successfully and balance a broad group of compound domestic and industrial wastes (Bhattacharya and Kim 2016).
Waste treatment does not stop with the reduction of waste volume; it can also be utilized for energy recovery. Due to the rise in the global population and the demand for energy, the world is trying to find more sources for the production of energy. The International Energy Agency reported that the demand for energy supply is growing rapidly and that this is a major concern in developing countries. According to the report, it is predicted that there will be decline in oil and gas demand by the end of 2030 and the waste-to-energy production will rise by another 54%(International Energy Agency 2018). Energy generation from waste is clean and green. It can eliminate the emission of carbon dioxide while burning fossil fuels and methane emissions from landfills; this technique is globally accepted and adopted (Scarlat, Fahl, and Dallemand 2019). Two-thirds of solid wastes is composed of biodegradable substances which can be used for capturing C02-neutral energy, thereby lowering the reliance on fossil fuels (Christensen et al. 2015).
Solid wastes have high energy content, and their latent energy can be used for waste management and conversion technologies for heat, oil or gas, into electricity (Das et al. 2019). The two main waste-to-energy technologies are thermochemical conversion and biochemical conversion. In thermochemical conversion, the energy is produced from waste material by thermal decomposition. The main types of this technology are incineration, gasification, pyrolysis, and carbonization.
Incineration is technology in which wastes are combusted and converted into heat energy, flue gas, and ash (Pan et al. 2015). The process occurs at a temperature of 800-1000°C with a supply of sufficient oxygen in which thermal breakdown and deposition of wastes are done. The heat energy recovered from the combustion process can be utilized for heat exchangers or power plants. The flue gases from the process should be cleaned and pollutant-free gaseous should be released into atmosphere. The ash from the process is due to the inorganic waste and they are released as solid pollutants with the flue gases. Incineration will decrease the solid waste volume up to 80-85% of the original waste, and it results in a significant reduction in the volume for disposal (Pham et al. 2015). In fact, incineration can be used for the treatment of medical wastes and other hazardous wastes where high temperature is required to destroy the microbes and toxins (Rodriguez-Garcia et al. 2014). Potential pollution correlated with waste incineration is a big problem for incinerators. The various emissions from incineration are hydrogen fluoride, hydrochloric acid, ammonia, methane, furan, polychlorinated dibenzodioxine, fluorine, chlorine, sulfur, SO, NOx, CO,. CO, and N (Tabasova et al. 2012). It is cost intensive and requires skilled labor and specialized handling equipment (Abramov et al. 2018). Emissions can be controlled by altering the composition of fuel, the moisture content particle size, and design of the incinerator (Karademir, Bakoglu, and Ayberk 2003).
Pyrolysis is a combustion technique in which organic compounds are broken down in the absence of oxygen at high temperature to produce various biofuels such as bio-oil, biochar, and syngas. The syngas is composed of CO and H2 at 85%, and the remainder consists of C02 and CH4. The pyrolysis is done at temperatures of 400-1200°C, and the bio-oil which is produced has a high calorific value of 17 MJ/ kg, which can be added to petroleum diesel. The product yield of pyrolysis is mainly dependent on their process parameters such as temperature and residence time for the production of biochar (Tripathi, Sahu, and Ganesan 2016). The various advantages of the pyrolysis are as follows: fuel gas production, ability to add the bio-oil to diesel to make a blend mixture, less pollution emission compared to incineration, and the compact nature of the pyrolysis. The major disadvantages of this technique are the formation of hazardous gases such as polyacrylonitriles and hydrogen cyanides, greater oil viscosity, high moisture content, and lower total energy output; this technique also requires pre-treatment of the solid wastes for production of bio-oil, which incurs more cost, and furthermore, treatment is required during oil storage due to its instability (Czajczynska et al. 2017).
Gasification is another thermal process in which wastes are treated like incineration. The main difference between incineration and gasification lies in their products. In incineration, high heat energy is produced, whereas in gasification the product is combustible gases. Thermal decomposition of wastes is done under partial oxidization in the temperature range of 800-900°C, which converts solid wastes into combustible and non-combustible gases. The gas produced has low calorific value and it can used for direct burning or as a fuel for gas turbines and gas engines. The gas or syngas produced is a mixture of CO,, CO, and H20; the remaining by-product is called vitreous slag. The produced gas can be used for the production of alcohols (such as methanol). Solid waste gasification is done on in a series of sequential endothermic and exothermic phases in relation to the reactant and final products (Rollinson 2018). The advantage of gasification technology is the production of syngas; the vitreous slag by-product can be used in road construction as a filling substance, and chemicals such as sulfuric acid can be produced from the sulfur, which is in impure form. The limitations of this technology are the high cost, high capital investment, high coal requirement, and production of hazardous organic polyhalogenated compounds (Das et al. 2019). The gasification technique uses a variety of reactor beds such as rotary kilns, moving grate furnaces, vertical shafts, entrained beds, fluidized beds, all determined based on their applications (Pham et al. 2015).
Hydrothermal carbonization is another thermal solid waste conversion technique in which the waste is converted into carbonaceous residues (hydrochar) under a water-based setup with a temperature range from 180°C to 350°C. The advantages are the low energy required to initiate the setup, the exothermic nature, and the ease of operation with high-moisture-content waste products. The disadvantages are cost and the increase in water toxicity (Mihajlovic et al. 2018).
Biochemical conversion is a technique in which wastes are converted with the assistance of microbes and enzymes. The two types of biochemical conversion are anaerobic digestion and fermentation. Solid wastes which have high moisture content and putrescible fractions are preferred for these processes due to their microbial activity nature.
Anaerobic digestion is a sequential biological process where various microbes in the absence of oxygen break down biodegradable materials. The feedstocks mostly chosen for anaerobic digestion are wet organic wastes such as agricultural wastes, animal slurries, and food wastes. The final product of the digester is biogas, which contains CH4 60% and CO, 40%, which can be combusted for heat energy and generation of electrical energy or upgradation of the biogas to automotive fuels and natural gas. The digestate which is collected as a by-product has high levels of rich nutrients and it can be used a soil fertilizer. Anaerobic digestion is eco-friendly with various benefits like renewable energy production, the prospect of recycling nutrients, and reduction in waste volume (Xu et al. 2018). Anaerobic digestion plants have received attention in many countries in recent years. In Germany, there are about 9000 plants for biogas production and in 2013 Europe had about 14,572 biogas plants (Torrijos 2016). It was found that the methane content was rich if the biogas plant was provided with a mixture of food waste and wastewater sludge, and sequentially an increase in food waste up to 35% not only increased the methane yield but also improved the mechanism of methane formation (Koch, Helmreich, and Drewes 2015). The major drawbacks of anaerobic digestion is its longer duration: the digestion process can take 20-40 days for the microbial reaction. Hydrolysis, acidogenesis, acetogenesis, and methanogen- esis are the four major biological processes happening during anaerobic digestion (Zhen et al. 2017).
Fermentation techniques are used to generate bioethanol or biohydrogen from wet organic wastes includes sewage sludge, agricultural wastes, and food wastes. Certain wastes need hydrolysis or saccharification for the conversion of carbohydrates into sugars before the start of fermentation (Rajendran et al. 2019).
In biohydrogen production, dark hydrogen fermentation is done in which the sugars are converted into hydrogen and vaporous fatty acids with the help of hydrogengenerating bacteria (like Enterobacter and Clostridium). Since hydrogen is found to have a high energy density per unit mass, it can be used as a substitute for liquid fuels; in fact, it is has energy content 2.8 times greater the hydrocarbon fuels, which makes it a versatile energy source (Chandrasekhar, Lee, and Lee 2015). The combustion of hydrogen fuel produces water as the final product, and thus it is considered as an option for reducing greenhouse gas emissions. Production of hydrogen by the dark hydrogen fermentation technique is quite in its early stages, and the research has been limited to the pilot scale so far. The estimated hydrogen yield is 4 mol/mol of glucose by dark fermentation (Lin et al. 2016). Process parameters like pH levels, evaporative fatty acids, and accumulated hydrogen of the bacterial strain are the main causes for the low hydrogen yield (Ren et al. 2016).
As for the production of bioethanol, conversion of sugars into bioethanol is done in the fermentation process. Compared to biohydrogen production, the production of ethanol is very old and well developed. In this, microbes like Candida shehatae, Pachysolen tannophilus, Zymomonas mobilis, Escherichia coli, and Saccharomyces cerevisiae are used for the conversion of glucose to ethanol. The industrial-scale production of ethanol is done by process such as sugar fermentation, distillation, dehydration, and denaturation (if needed). Pretreatment of wastes and enzymatic hydrolysis are done for cellulosic wastes before bioethanol production in order to improve the glucose concentration level. Ethanol as fuel can be used in automotive fuel and it has the potential to replace petrol in the global market. However, the rate for bioethanol production is high compared to hydrocarbon fuels, and research is ongoing to find low-cost enzymes and a suitable selection of microorganisms which have the ability to produce inhibitory compounds (Pham et al. 2015).
