2.2 MSW Valorization Technologies
2.2.2 Composting
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].
Compost can sequester carbon and nutrients in the soil, improve plant growth without the need for fertilizers [60]. Life-cycle assessments have shown that composting is en- vironmentally more friendly than waste-to-energy (WtE) method such as incineration or traditional landfilling [61]. Composting is economically more favorable in terms of cost compared to other MSW utilization technologies [62].
Composting systems can be broadly classified into the following:
Passive vs. Active Aeration: Passive aeration relies on natural convection of air coupled with the ”chimney effect” (hot air rises up, replaced by cold air at the bottom). Whereas in active aeration, fans or blowers are used to actively supply air. Although active aeration is expensive, they can handle significantly larger compost sizes [60].
Open vs. In-vessel: Open systems are outdoor open-air facilities, typically open land. In- vessel systems are comprised of drums, silos, concrete-lined trenches, or similar structures where feedstock is fed for composting [63], [60].
Static vs. Managed: Static systems are unturned and unagitated. Whereas managed sys- tems are where the windrows or piles of compost feedstocks are turned and agitated.
On-site vs. Centralized: On-site systems are established by individuals or businesses to compost organic feedstock at the source. Centralized systems are typically community, neighborhood or municipal based composting facilities where composting feedstock is brought in.
2.2.2.1 Types of Composting Systems
Static Systems
These are the most basic composting systems. The piles are static and are passively aer- ated and hence require structural porosity. Static systems are very low cost but requires a long time for composting to take place [60]. Static systems cannot deal with food waste and does not deal with space efficiently [53].
Turned Windrow System
Windrows are long, narrow, and low piles of organic waste [60]. Windrows can be as long as the the available composting space. Windrows need to be turned using heavy equipment such as front-end loaders, tractor-pulled windrow turner or straddle-turners;
depending on the type of equipment used for turning windrows, the windrows can be spaced anywhere between 2 to 20 feet. Special turning machines, such as a straddle-turner, can turn windrows as close as 2 feet from from each other, resulting in proper utilization of compost area [60]. Windrow turning releases heat, water vapor, and gases that were trapped within the pore spaces [53]. Turning frequency had shown indirect correlation to compost preparing time; the time required to produce a compost ready for curing was 60-80 days in well turned windrows, 120-150 days in infrequently turned windrows, and 200-240 days in piles [64].
Turned Mass Bed System
A variation of turned windrow system is the turned mass bed system. The advantage of the turned mass bed system over traditional windrow system is that it can handle a much larger amount of composting feedstock while leaving a smaller operating footprint [53]
and being more environmentally friendly. However, this system has a low level of aeration and requires more frequent turning and a higher level of monitoring. This system is less suitable for composting food waste.
Passively Aerated Windrow System (PAWS)
These are similar to static systems but aeration is enhanced using perforated plastic pipes to allow for air to get inside the piles [60]. Like static systems, PAWS require a long time (1-2 years) for composting but they can handle more composting feedstock than static piles but cannot deal with food waste [53].
Actively Aerated System
In an actively aerated system, the composting pile is aerated by a network of perforated pipes set underneath the composting feedstock using fans or blowers. Aeration can be either positive (air pumped in to the pile) or negative (air pulled out through the piles).
Negative aeration allows for odorous gases from the composting pile to be captured and treated [53]. Actively aerated systems have a higher operating cost than passively aerated systems.
TABLE2.10: Characteristics of composting systems. Adapted from [53]
Static Piles Windrows Turned Mass Bed PAWS Capacity (tons/year) <10,000 <50,000 15,000 - 50,000 <10,000
Food Waste No Yes Yes No
Pretreatment Shredding Shredding Shredding Shredding
/mixing /mixing
Composting time 2-3 years 3-12 months 3-12 months 1-2 years Aeration Passive Passive & MA Passive & MA Passive
RSR1 High Med - High Med - High High
LOC2 Low Low - Med Low - Med Low
Water Requirements Low Low - Med Low - Med Low
Fuel Consumption Low - Med High High Low - Med
RCC3 Low Low - Med Low to Med Low
ROMC4 Low Low - Med Low - Med Low
1Relative Space Requirement
2Level of Odor Control
3Relative Construction Cost
4Relative Operating and Maintenance Cost
2.2.2.2 Composting System Implementation
Individual or Community System of composting requires a backyard or lawn space of individual houses. In the United States (US), multiple homeowners set up individual composting systems and collaborate with each other in providing kitchen waste and yard trimmings, managing the composting system, and in utilizing the compost [60]. Home
owners with adequate space in Bangladesh can set up composting systems using pallets or fencing. The municipal authorities need to educate home owners about individual com- posting and its environmental benefits and encourage them to build individual composting systems.
Community systems are scaled-up version of individual systems, designed to handle ap- proximately 230-380 cubic meters of organic waste [60]. Community system composting is typically done in multiple backyard systems or in small-scale in-vessel system operated by the community. The benefits of community system composting are: improved local soil, more local jobs, less truck traffic hauling garbage [60]. Various communities in the US have implemented community composting systems and have benefited from it [65].
Municipal Systemsare set up by city and municipal authorities to compost sewage sludge and yard trimmings. Municipal composting systems can handle between 1530-76,500 cubic meters of organic waste annually. City corporations across Bangladesh can collect yard trimmings and dead leaves from government properties, parks, roads and compost at municipal composters alongside sewage sludge and food waste from the municipality.
These systems can be open-air turned windrow systems; however, if there are residential activity nearby, fabric-covered forced-aeration system is recommended to minimize odor [60].
Industrial and Agro-Farm Systemsare composting systems that can be set up in various industrial facilities to deal with the organic waste generated from its manufacturing pro- cess or by agro-farms to deal with their organic waste.
2.2.2.3 Composting in Bangladesh
A NGO called Waste Concern had been carrying out composting since 1995 at their 3 tons capacity composting plant in Mirpur neighborhood of Dhaka city [66–68]. The Waste Concern site at Mirpur is a Passively Aerated Turned Windrow composting sys- tem [66, 67]. The feedstock for composting at this plant was collected by Waste Concern
from adjacent neighborhoods around the composting site using three-wheeledrickshaw vans [68]; by the end of 2001, Waste Concern was collection 3 tons of MSW from 1430 households in Mirpur [68]. Furthermore, Waste Concern had provided individuals and families in low-income areas of Dhaka with concrete or steel barrels for on-site in-vessel composting [67]. The individuals and families use their daily generated waste for in- vessel composting, which is then bought by Waste Concern at 2 Taka/kg (US $0.024/kg).
In Chittagong, only 0.16% of all MSW is sent to Composting and Garbage Treatment Plant (CGTP) located near Anandabazar landfill site [30]. CGTP is capable of producing only 25 tons of compost annually; it was reported that CGTP sells compost for 20 Taka/kg (US $0.24/kg) [30]. In contrast, Waste Concern produced 600 kg of compost in Dhaka and sold it at a price of 2.5 Taka/kg (US $0.03/kg) [66]. Other sources have claimed that Waste Concern sells compost to households or individual customers at 10 Taka/kg (US $0.12/kg), and high volume sales to a fertilizer trading company at 2 Taka/kg (US
$0.024/kg) [67].
It is estimated that if all the compostable waste is utilized in Bangladesh, 911,816 tons of compost can be produced annually and it would create 24,981 jobs [69]. Composting reduces carbon footprint, and it is possible to reduce the release of 2,279,541 tons of carbon dioxide by utilizing all compostable waste in Bangladesh [69]. The payback period for 3 tons/day, 10 tons/day, and 20 tons/day capacity composting plants are 2 years, 1.7 years, and 1.5 years, respectively [66]. Waste Concern’s 3 tons compost plant in Dhaka within 23 months; and it is not recommended to build a compost plant in urban areas where land is expensive [69]. Additionally, it would make more sense to have a compost plant in rural areas as the demand for compost is high in rural areas [69].
2.2.2.4 Negative Impacts of Composting
Although composting is a good alternative to landfilling, there are some negative impacts associated with composting which needs to be addressed and prevented. Some of the negative impacts of composting are discussed below:
Odor
As the organic components are composted, significant amount of odor is generated due to the release of hydrogen sulfides, volatile organic sulfides, ammonia, pyridine, alcohols, esters, ketones, and aldehydes [70]. Typically odor at composting sites are tackled by suf- ficient aeration and by the addition of bulking agents such as cornstalks, rice straws, wheat straws, wood chips, and sawdust [70]. Negative active aeration is an efficient method for tackling release of odor [53, 70].
Bioaerosol
Airborne microorganisms, called bioaerosol, are released when composting feedstock is handled or turned during composting. Bioaerosols can cause respiratory diseases, and other chronic lung diseases in composting facility workers [70]. It is recommended that the operating cab of heavy equipment being used at composting sites be equipped with pressurization systems and high efficiency particulate air (HEPA) filters [71].
Heavy Metals
Heavy metals in compost can reduce its quality. Sources of metal in compost are compost feedstocks. Source separation of the metal contaminants have been shown to be the most efficient in improving compost quality [72].