Municipal solid waste composition
MSW has become a global environmental concern. The contribution of MSW sector to the emission of GHG into the atmosphere has caused negative impacts. The impact of these gases especially methane has led to Climate Change which has further affected other aspects of the ecosystem and public health. For example, coral reefs in the marine habitat have been observed to be undergoing stress and subsequent reduction in population as reported by Chimienti et al.27. Also, Parihar et al.28 has revealed the effect of Climate Change on malaria transmission due to the potential increase in mosquito density as a result of increase in extreme weather condition of temperature and rainfall as one of the health effects of GHGs. The composition of MSW in countries like India showed a 40–60% organic waste content as reported by Mathur et al4. While other findings by Srinivasan et al.16 in Chennai City, India the values is 54% of degradable organic waste. Also, 75% of organic waste was reported by Hosseini et al.29 in Iran. While the value of degradable waste for this study is 52.6%. But in Poland, which is a developed country, the value of 41.1% biodegradable waste was reported by Ciula et al.24. The reason why the value of organic waste from MSW in developing countries is higher than in developed countries could be because, in developing countries, there is no waste segregation to various recycling facilities at the point of waste generation, collection, and transportation as is the case with developed countries. Also, in developed countries, some waste such as cans, plastics, nylons, and cartons are directly transported to recycling facilities. Hence, the waste Collection Coverage (CC) for developed countries is 100%, whereas; the waste CC for developing countries is 40–50% as stated by Froiland-Jesen and Pipatti30. Although, the waste CC for Lagos is quite effective because the waste generated is usually being collected and transported to SWDS by the relevant government agency and PSP. Unfortunately, not all the waste generated is being collected and disposed of due to bad roads and breakdown of the waste trucks. However, some are been dumped in drainage systems or on unauthorized locations.
The waste types deposited in SWDS
The MSW generated and subsequently deposited in Lagos SWDS within the city metropolis comprises rapidly degradable, moderately degradable, slowly degradable, and non-degradable waste. Hence, the difference in the quantity and type of waste deposited in each SWDS is thought to be influenced by the population and economic activities within the location. The reason for the low value of the degradable waste disposed of during the wet season as shown in (Table 2) could be as a result of the degradation process which begins to take place in the waste bins before the arrival of the waste truck since the waste is sometimes not being collected on daily basis. As shown in (Table 4), The IPCC values for West Africa and Nigeria are quite different from the values obtained from this study even though, some waste types are within a close range of values. In addition, the results reported by7,21 have shown that previous Lagos values are also different from the results obtained from this study. This discrepancy could be due to the fact that: during this study sampling and field investigations across the major SWDS was conducted within the last week of the months. This period is fixed by the State LAWMA to compulsorily carryout environmental sanitation when general environmental clean-up exercises are enforced by the State Government.
Previous research results and this study have shown that food waste has the highest value when compared to other waste types. Also, this study has revealed an appreciable value of 7.2% for Grasses/Garden waste while other studies reported zero value, although Oyelola and Babatunde21] indicated the waste category as 4.2%. This difference could be as a result of the sampling period which resulted in an increased volume of grasses, and garden/yard waste being disposed of in SWDS after general environmental clean-up exercise which usually involves cutting of yard grasses and trimming of flowers. The percentage of degradable waste in the total waste stream was 52.6% in this study but Balogun-Adeleye et al7 indicated 83.2%, and Oyelola and Babatunde21 reported 90.78%. However, Ciula et al24 in their findings conducted in Poland kept the value of vegetables, paper, and cardboard waste at 14.5% . This value is small confirming what Mathur et al4 stated that the composition of degradable Municipal Solid Waste in developed countries is low while that of developing countries is about 40–60%.
The degradable organic carbon (DOC)
One factor that determines methane generation in SWDS is the amount of Degradable Organic Carbon (DOC) in the waste stream, which depends on the waste composition as shown in (Figure 2). DOC is one of the main parameters affecting the CH4 emission from SWDS. It is observed to correlate with the quantity of degradable waste deposited, and the physical conditions at the SWDS which leads to the formation of Decomposable Degradable Organic Carbon (DDOC). Meanwhile, the Mass of Decomposable Degradable Organic Carbon (DDOCm) from the waste deposited and accumulated was higher during the dry season than during the wet season because, waste Collection Coverage (CC) was more effective during the period due to the trucks ability to access various waste collection routes. In Poland, Ciula et al.24 used the First Order Decay Model and the DOC value obtained was 0.061 GgC/Ggwaste, while this study has the value at 12.897 GgC/Kgwaste for the dry season and 12.547 GgC/Kgwaste for the wet season. However, the low value of DOC from Poland is evidence that developed countries have less degradable waste in their waste stream when compared to developing countries.
Waste decomposition impact on methane generation
Sandoval-Cobo et al.31 stated that fresh waste disposed of produces more biodegradability and methane generation potential than excavated waste. However, methane generation is assumed to start immediately after the deposition of the waste, but in reality, this is not the case. A time lag of 2 to 6 months is considered after which the normal first-order decay process starts32. This explains why the value of methane generated during the study period is low. The degradable organic carbon is consumed slowly by the bacteria throughout a period at negative exponential rates, reflected in the process by which methane is generated and eventually emitted to the atmosphere33. However, the result for the DDOC decomposed in the disposal site was observed to have a quantity higher during the wet season likewise to methane generated in SWDS as stated by Dasgupta et al19. 34also revealed the effect of season on the decay rate (K) value that results to degradation.
Methane generation based on seasonal variation
The FOD Model adopted in this study, as explained by Oonk35 considers CH4 generation from organic matter, which is assumed to progress in a complex order of reaction steps. First, the enzymes break apart the solid organic macro molecules into smaller one that is further processed by a micro-organism that leads to CH4 generation25. The CH4 emission process involved stages in which waste generators disposed of the waste into approved waste collection bins in designated locations within the city metropolis. After that, the waste collection trucks collect the waste into trucks and disposed of SWDS where they are decomposed after scavengers have picked the valuable materials for recovery and recycling. This follows the steps involved in MSW management such as waste generation, storage, collection, processing, and disposal as stated by IPPC and Maiyaki et al2,36. According to Kaza et al.37, the amount, complexities, and characteristics of generated waste depend on urbanization, population growth, economic prosperity, and improved standard of living. Other socio-economic factors identified to influence the MSW generation are: culture, tradition, attitudes, family life, and education according to Oluwalana et al9. However, this study has provided information that confirms that seasonal variation is also a factor that can affect the quantity of CH4 generation based on the degradability of the organic waste38.This study has the total value of methane generated as 0.465 Gg. Similar results generated in Ontario33 using Mass Balance models showed methane emissions of 0.74KgCO2Eq/KgMSW and 0.39KgCO2Eq/KgMSW. In Chennai City, India, the results of methane emission using a close flux chamber were in the range of 6.1 Gg/yr–9.1 Gg/yr, and 8.8 Gg/yr–11.3 Gg/yr, according to Srinivasan et al16. In Poland,24 used the First Order Decay Model and the Methane Generation Potential was 0.018 GgCH4/Ggwaste however, the estimated CH4 emission from the Landfill was 0.393 Gg/yr. But, the CH4 generated for this study during the dry season was 0.134 Gg while the value generated during the wet season was 0.331 Gg which is a 42.2% increase higher than the value obtained for the dry season. This variation could be explained as follows; A higher value than the 56 percent of the DOC as stated in the guidelines by IPCC is been converted to CH4 and CO2 during the periods when the temperature is < 20 °C39. Then, when the temperature becomes > 20 °C it leads to low precipitation, resulting in unintentional and uncontrollable fire outbreaks in SWDS, a frequent incidence during the dry season. Jain et al.40 in the research conducted in the United States, investigated waste composition kinetics for closed landfills and assumed that a fraction of the in-place biodegradable waste may not be decomposing, potentially due to a lack of adequate moisture content during the dry season, the results showed that the decay rate is significantly correlated with annual precipitation, It does not correlate with per capita income or lifestyle as stated by Akintayo and Olanisakin11. Therefore, methane generated is equal to methane emitted because there is no methane recovery for energy purposes. Also, the SWDS are unmanaged with a shallow depth of < 5 m. Hence, the oxidation is zero. This poor waste management practice requires an increased public awareness about the effect, so that they would be improved perception and disposition towards sustainable MSW Management approach as stated by Adekola et al41. Thereby considering the relocation of all the dumpsites located in the mainland (Fig. 1) to outskirt of the city metropolis. Finally, Food waste, Grasses/Garden and Wood waste quantities in SWDS depends on seasonal variation while Paper/Textile waste does not.