2.2 MSW Valorization Technologies
2.2.1 Landfilling and Anaerobic Digestion
As the seen in Table 2.7, the landfill or dump site conditions in Bangladesh are not fair or up to global standards. The worst practice is conducted by BCC who are dumping waste directly in Kirtonkhola river. Bangladesh is a densely populated country with a scarcity of land; this creates significant hindrances is acquiring land for landfilling or waste dumping as seen in the case of DNCC who failed to find another landfill site to replace Amin Bazar landfill site. It may be possible that BCC is unable to find enough land for the purpose of dumping waste. The high volume of waste generated in Bangladesh is mostly dumped untreated and the majority of the waste contains organic components as shown in Table 2.2. There are technologies available to utilize MSW to generate energy or value- added products. Bangladesh cannot afford to merely dump the generated amount of MSW without extracting any value out of it. The following chapter will discuss the pertinent technologies available to valorize MSW.
In developed countries, a landfill is covered with an impervious layer to prevent rainwater from seeping into the landfill after it reaches a desired maximum height [47]. A disparate form of retiring a landfill requires recirculating the aqueous effluent to speed up the bio- logical degradation of MSW [47]. Developed countries, like the United States of America (USA), require the authorities to monitor, collect and treat landfill gas and liquid runoffs for as long as 30 years after a landfill is closed [47].
Biological pretreatment of MSW, such as anaerobic digestion, is an attractive alternative method to landfilling due to various negative ramifications as discussed above. In anaer- obic digestion, biogas and other value-added products can be produced by utilizing the organic fraction of MSW [50, 51]. However, anaerobic digestion is inefficient in captur- ing carbon in the product [52].
2.2.1.1 Anaerobic Digestion Reactions
Anaerobic digestion reactions are carried out in reactors or chambers called digesters, in the absence of oxygen. The reactions taking place during anaerobic digestion are hydroly- sis or depolymerization, acidogenesis, acetogenesis, and methanogenesis [49, 50, 53–56], out of which hydrolysis or depolymerization is the rate-determining step [53, 55]. Ta- ble 2.8 provides detail of the reactions. These reactions are influenced by temperature, pH, and moisture content and hence these parameters are monitored [53]. The primary product of anaerobic digestion are biogas and digestate. Biogas contains methane and carbon dioxide; digestate is a semi-solid leftover material which can be used as fertilizer [53]. The energy potential of pure methane is 37 MJ/m3; biogas with 60-70% methane concentration has calorific value between 22-26 MJ/m3[53].
TABLE2.8:AnaerobicDigestionReactions.Adaptedfrom[53] ReactionTypePurposeComment hydrolysisordepolymerizationcomplexorganicmoleculesarebrokendownintoapHof5favorsthisstep smallermoleculesbyparticularorganismsinthedigester acidogenesishydrolyzedmaterialsarebrokendownintoalcoholsfermentativemicroorganismsareused andacidsinthisstep acetogenesisalcoholsandacidsarefermentedtoformadifferenttypeoforganismwhich short-chainvolatilefallyacidsandhydrogengasisacidtolerantisusedinthisstep methanogenesisanaerobicorganismscalledmethogensconvertmethogensneedapHrangebetween6.5-7.2 thebyproductstobiogas
Properties such as moisture content, temperature, retention time, carbon to nitrogen (C:N) ratio, ammonia concentration, and sulfide concentration affect the type of product pro- duced in an anaerobic digester. Anaerobic digesters are classified into wet and high-solids;
in wet digesters, the moisture content is above 80%, whereas in high-solids digesters the moisture content is between 60-80% [53]. Digesters are also divided into mesophilic and thermophilic depending on the temperature they operate at; mesophilic digesters oper- ate between 30-38◦C, thermophilic digesters operate between 50-60 ◦C. In mesophilic digesters, microorganisms are more robust; however, in thermophlic digesters, a higher yield of biogas is achieved quicker [53]. Pre-aeration is reported to generate biological heat in which the temperature of the organic fraction of MSW can reach as high as 60◦C, hence no external heating is required for thermophilic digestion [55]. Aeration followed by wet thermophilic aerobic digestion was found to be optimum for producing biogas with higher methane concentration requiring a low retention time of 12 days [55]. Some typical anaerobic digester conditions are given in Table 2.9.
TABLE2.9: Typical anaerobic digester parameters. Adapted from [53]
Parameter Typical Range
Moisture Content 60 - 80% for high-solids digester 80%f orwetdigesters
pH 6.0 - 7.0
Temperature 30 - 38◦C (mesophilic digesters) 50 - 60◦C (thermophilic digesters) Retention time 14 to 40 days (dependent on technology)
C:N ratio 30:1
Ammonia 200 mg/L
Sulphide 50 mg/L
Wet digesters are continuous processes and high-solids digesters are batch processes with some exception. Batch digesters have a solid retention time between 14-30 days, con- tinuous digesters have a solid retention time between 3-55 days [53]. Retention time is of paramount importance in determining the yield of biogas. Short retention times do not produce enough biogas, whereas longer retention time will curtail biogas recovery efficiency [53]. Although batch digesters are the simplest type of digesters, they are no- torious for having high fluctuations in methane yield [49, 56]. Continuous digesters can
be further classified into one-stage, two-stage, or multi-stage continuously fed systems.
In one-stage continuously fed digesters, all the biochemical reactions take place inside one reactor; in two-stage or multi-stage continuously fed systems, reactions take place separately in different reactors [49, 56].
It is important to control the C:N ratio of the feedstock as excess nitrogen in the feedstock can lead to ammonia accumulation in the digester, which inhibits the digestion process [53]. Excess sulfide, like excess ammonia, can also inhibit the digestion process. Sources of sulfide in MSW are proteins; samples of the feedstock are collected intermittently and the introduction of excess sulfide is controlled [53].
2.2.1.2 Anaerobic Digestion of MSW in Bangladesh
In Bangladesh, especially in the rural areas, there are a considerable amount of small scale anaerobic digestion processes for producing biogas [57]. Organizations, such as Bangladesh Biogas Development Foundation, are working in the rural areas to help peo- ple develop individual anaerobic digesters to produce biogas primarily for cooking [58].
Households in the villages use kitchen waste, and livestock manure as feedstock for bio- gas generation [58, 59].