Open access peer-reviewed chapter
Abstract
The book chapter discusses the influence of aeration on the decomposition of solid wastes available on the market. The vegetable waste, paper waste, and sawdust as a filler were mixed intensively in a ratio of 75:10:15. The temperature of composting mass inside the reactors was recorded intermittently daily. The weights of total sample and volatile solids were measured for both passive and forced aeration composting tests before and after composting the mixed waste. The temperature rose to a maximum of 52°C with passive aeration and 54°C with forced aeration. The percent decrease in the total sample was higher with forced aeration than with passive aeration. The volatile solids reduced over time at the end of these tests. The degree of volatile solids degradation of the mixed waste over time by the forced aeration was determined for a series of the composting processes. The amount of volatile solids and total sample were determined at 2- to 4-day intervals. The percent reduction in volatile solids and total sample was found to be 4 to 55% and 3 to 68%, respectively. The percent reduction in volatile solids increased with time. The chapter helps to understand the recycling possibility of the mixed waste in the form of compost.
Keywords
- composting
- forced aeration
- organic fertilizer
- passive aeration
- recycling
- volatile solids degradation
1. Introduction
The idea of integrated solid waste management has been developed due to the increase in municipal waste, the decrease in landfill capacity, the increase in waste management costs, communal disagreement to waste management conveniences, and anxieties about the threats related to the solid waste management [1]. The huge amount of organic solid wastes, both putrescible and non-putrescible, is one of the major socio-environmental problems in many developing countries. A number of health and environmental problems may arise due to the solid waste mismanagement. The common problems associated with the improper disposal of solid waste include esthetic nuisance, odor nuisance, water and air pollution, fire hazards, financial losses, disease transmission, etc. [2]. Solid waste management is now becoming an immense task owing to urbanization, explosion of population, fund deficiency, poverty, etc. Landfilling, gasification, incineration, pyrolysis, etc., are some of the waste disposal methods that are effective and, however, have adverse influences on the public health and environment as well. When managed properly, composting is a workable way with numerous benefits, namely the bio-fertilizer production, comparatively low water and air pollution, income generation, low operating costs, etc. [3]. Ecological imbalance and environmental degradation happen constantly because of the improper solid waste planning and management. Composting is an environmentally friendly and sustainable technique to manage the putrescible content of organic solid waste [4].
Composting is a biological conversion of the putrescible organic content of municipal solid wastes to decrease the material weight and volume and produce a humus-like material. Both aerobic and anaerobic practices have taken places in the waste management [5]. Putrescible parts of the organic materials are biodegraded to a stable end product, which can be used as an organic fertilizer [6]. The end product residual after the microbial action in the composting process of organic waste is known as humus or compost [7]. The most common practice in treating the solid waste is aerobic composting due to its easiness and operative treatment and requires the air diffusion through the waste [8]. Huge bacterial action quickens the breakdown of the organic content in the thermophilic phase [9, 10]. A minimum oxygen level is constantly continued to confirm high organic quality [11]. Moisture, pH, temperature, carbon to nitrogen ratio (C: N), etc., are the major factors, which have an effect on the composting process and contribute to the effectiveness of this process [12]. High organic materials and macronutrients in the waste have a high potential in the organic fertilizer production [13]. Organic fertilizers improve soil fertility, reduce soil salinity, require less irrigation water, resist pests and insects, accelerate rapid plant growth, and reduce dependency on expensive chemical fertilizers [14]. The application of compost in agricultural soils significantly increases the water holding capacity and thereby reduces the irrigation water demand of the land as described in several literatures [15, 16, 17]. Compost is often stigmatized when it is made from organic waste. Using compost is a great way to add nutrients to the soil and restore a healthy ecosystem [18]. In developing countries, the high organic part in the municipal solid waste is perfect for the composting process. Composting is well-suited with other forms of recycling [19].
Composting, one of the simplest methods of organic waste stabilization, is the most efficient method of treating organic waste and making a good organic fertilizer. The market solid waste contains numerous nutrients, namely potassium, phosphorus, nitrogen, etc., that contribute to the growth of various plants. In view of this, the study was investigated with the market solid waste for determining the variation of temperature in passive and forced aeration composting tests, for determining the degree of the volatile solids degradation of the mixed waste, and for investigating the recycling possibility of the mixed waste in the form of compost [20].
2. Materials and methods
The study was conducted to determine the influence of aeration, both passive and forced, on the composting of market solid waste. The details of waste materials, reactor and aerator type, measurement of temperature, determination of moisture content and volatile solids, determination of variation of temperature, and determination of rate of volatile solids degradation are described in this section.
2.1 Waste materials
The vegetable waste, paper waste, and sawdust were selected as the mixed waste and collected at Khulna’s local market. The vegetable waste and paper waste were then cut into small pieces less than 10 mm in size. The vegetable waste, paper waste, and sawdust as a filler were intensively mixed in a ratio of 75:10:15. The different types of waste materials, namely vegetable waste, paper waste, sawdust, and mixed waste, are shown in Figure 1.
Figure 1.
Different types of waste materials (a) vegetable waste, (b) paper waste, (c) sawdust, (d) mixed waste.
2.2 Reactor and aerator type
Twenty thermo-fluxes, each with a capacity of one liter, were used as reactors for the composting processes. The height and diameter of the reactors were 270 and 100 mm, respectively. The reactors are heat insulated to retain the self-heat inside the reactor and to ensure the increase in temperature up to the thermophilic range of 50 to 60°C [21]. The wet mixed waste of 400 to 450 g is put in the reactor. Small pieces of polyurethane sheeting were placed over the mixed waste to protect the whole system from the leakage of self-generated heat of the mixed waste during composting and to keep the system thermodynamically open [22]. Five aerators (Super Pump SP-780) with an air flow of 500 ml/min were used for the purpose of aeration. The types of reactors and aerators used for the composting processes of the mixed waste are shown in Figure 2. Air tubes of 5 mm in diameter and 1000 mm in length were connected with the aerator to the reactors. Four air tubes were used to connect each aerator to four reactors.
Figure 2.
Types of reactors and aerators used for composting process of mixed waste: (a) reactor, (b) aerator.
2.3 Measurement of temperature
Two types of thermometers were used to record the temperature in the room and the temperature generated in the mixed waste within the reactors during the composting processes. The different types of thermometers used for recording the temperature during composting processes are shown in Figure 3. Thermometers were inserted up to midway into the reactors and temperature readings were recorded several times a day.
Figure 3.
Different types of thermometers used for recording temperature during composting process.
2.4 Determination of moisture content and volatile solids
Using a precise digital balance, the weight of the pan (w1) was measured. A small amount of the mixed waste was placed in the pan. The weight of the pan plus wet sample (w2) was measured. The pan with wet sample was then placed in an oven for 24 hours at 105 ± 5°C. The weight of the pan plus dry sample (w3) was measured. A desiccator was used for controlling the moisture of the mixed waste. The measurement of weight of the mixed waste for determining the moisture content and volatile solids is shown in Figure 4. The quantity of moisture content was determined using Eq. (1).
Figure 4.
Measurement of weight of mixed waste for determining moisture content and volatile solids.
The pan with oven-dried sample was placed in a muffle furnace for 5 hours at 550 ± 15°C. The weight of the pan plus fixed sample (w4) was measured. The quantity of volatile solids was determined using Eq. (2).
2.5 Determination of variation of temperature
Three runs of composting tests were conducted according to the previous study [22]. The first and second runs were conducted with 6 reactors (3 reactors for passive aeration and 3 reactors for forced aeration) to determine the temperature variation in the composting processes. The details of the first and second runs for the determination of variation of temperature during the composting processes are discussed below.
2.5.1 Variation of temperature in composting process with passive aeration
The first run was conducted with three reactors for the replication of the composting process with passive aeration. After cutting into small pieces, the vegetable waste (75% of wet weight) and paper waste (10% of wet weight) were intensively mixed with 15% sawdust. The mixed waste of approximately 450 g was placed in three reactors. These reactors were filled with the mixed waste and shaken gently. The reactor openings were sealed with small pieces of polyurethane sheeting. The thermometers were inserted midway into the reactors to record the temperature generated inside the reactors. The test arrangement with temperature measurement for passive aeration composting process is shown in Figure 5. The moisture content, volatile solids, and total sample of the mixed waste were calculated before and after the composting process. The temperature readings were recorded intermittently daily for 30 days.
Figure 5.
Test arrangement with temperature measurement for passive aeration composting process.
2.5.2 Variation of temperature in composting with forced aeration
The second run was conducted with three reactors for the replication of the composting process with forced aeration. After cutting into small pieces, the vegetable waste (75% of wet weight) and paper waste (10% of wet weight) were intensively mixed with 15% sawdust. Air tubes of 5 mm in diameter and 1000 mm in length were inserted midway into each reactor from the air pump before filling the reactors with the mixed waste. These reactors were filled with the mixed waste and shaken gently. The reactor openings were sealed with small pieces of polyurethane sheeting. The thermometers were inserted midway into the reactors to record the temperature generated inside the reactors. The test arrangement with temperature measurement for forced aeration composting process is shown in Figure 6. Air was passed through the mixed waste inside the reactor at the rate of 500 ml/min for 8 hours a day during the day. The moisture content, volatile solids, and total sample of the mixed waste were calculated before and after the composting process. The temperature readings were recorded intermittently daily for 30 days.
Figure 6.
Test arrangement with temperature measurement for forced aeration composting process.
2.6 Determination of rate of volatile solids degradation
The third run was conducted with 20 reactors for composting the mixed waste with forced aeration following the process as described in Section 2.5.2. The test arrangement for determining the rate of volatile solids degradation is shown in Figure 7. The temperature readings were recorded intermittently daily for 50 days. The dry solids, volatile solids, moisture content, and total sample of the mixed waste were calculated at 2- to 4-day intervals.
Figure 7.
Test arrangement for determining rate of volatile solids degradation.
3. Results and discussion
The study was conducted for determining the variation of temperature over time during the composting process of market solid waste with passive and forced aeration conditions and for determining the rate of volatile solids degradation over time for a series of composting processes with forced aeration conditions. The results obtained from this study are discussed below.
3.1 Passive aeration composting
The variation of temperature during composting process with passive aeration is shown in Figure 8. The temperature of composting mass inside reactor-1 increased from 26 to 42°C within the first 7 days. The maximum temperature was 52°C. The temperature slowly dropped to 32°C after 30 days of composting process. The temperature of composting mass inside reactor-2 increased from 26 to 44°C within the first 7 days. The maximum temperature was 49°C. The temperature dropped to 28°C after 30 days of composting process, distant from the ambient temperature of 17°C. A similar pattern was also observed for the composting mass inside reactor-3. The temperature of composting mass increased from 26 to 40°C within the first 7 days. The maximum temperature was 51°C. The temperature dropped to 30°C after 30 days of composting process. Initially, the temperature of composting mass inside the reactors rose rapidly for a few days and then fell slowly in all the reactors. The degradation of the putrescible part of the mixed waste starts within a short period of the passive aeration composting process, which leads to rise in temperature of composting mass inside reactors for a few days. After that the amount of the putrescible part of the mixed waste is reduced significantly, resulting a slight drop in temperature over time. Since no air was blown into the reactors and small pieces of polyurethane sheeting served as insulating materials, there was a slow cooling effect in the composting mass, consequently a significant difference from the ambient temperature of 17°C within 30-day period. The temperature follows a similar pattern for all the reactors as indicated in Figure 8 confirming the replicability of the experiment as described in the literature [23].
Figure 8.
Variation of temperature during composting with passive aeration.
The initial weights of the mixed waste in reactor-1, reactor-2, and reactor-3 were 450, 455, and 435 g, respectively, and the final weights after 30 days of composting process with passive aeration were 370, 380, and 355 g, respectively. The moisture content of the mixed waste was initially 69.4% and increased to 77.9%. The volatile solids of the mixed waste were initially 94.4% and reduced to 90.3%. The percent reductions in dry solids, volatile solids, moisture, and total sample in reactor-1 were found to be 40.60, 43.15, 7.72, and 17.78%, respectively. The percent reductions in dry solids, volatile solids, moisture, and total sample in reactor-2 were found to be 35.06, 37.52, 8.30, and 16.48%, respectively. The percent reductions in dry solids, volatile solids, moisture, and total sample in reactor-3 were found to be 37.87, 40.57, 9.80, and 18.39%, respectively. The average percent reductions in dry solids, volatile solids, moisture, and total sample in these three reactors after 30 days of composting process with passive aeration were found to be 37.84, 40.41, 8.61, and 17.55%, respectively.
3.2 Forced aeration composting
The variation of temperature during composting process with forced aeration is shown in Figure 9. The temperature of composting mass inside reactor-4 increased from 26 to 42°C within the first 7 days. The maximum temperature was 50°C as comparable with the literature [24]. The temperature dropped to 19°C after 30 days of composting process, near to the ambient temperature of 17°C. The temperature of composting mass inside reactor-5 increased from 26 to 39°C within the first 7 days. The maximum temperature was 53°C. The temperature dropped to 20°C after 30 days of composting process. Similarly, the temperature of composting mass inside reactor-6 increased from 26 to 43°C within the first 7 days. The maximum temperature was 54°C. The temperature dropped to 19°C after 30 days of composting process. Initially, the temperature of composting mass inside the reactors rose rapidly for a few days and fell rapidly over 20 days and then slowly decreased in all these three reactors. The degradation of the putrescible part of the mixed waste starts within a short period of the forced aeration composting process, which leads to a rapid rise in temperature of composting mass inside the reactors for a few days. After that, the amount of the putrescible part of the mixed waste is reduced significantly, resulting a sharp drop in temperature within rest of the days. This is due to the air flow into the reactors having ambient temperature and after that become hotter as in the reactor temperature. Finally, it carries more moisture and heat from the reactor, since the moisture carrying capacity of air increases exponentially with the temperature.
Figure 9.
Variation of temperature during composting with forced aeration.
The initial weights of the mixed waste in reactor-4, reactor-5, and reactor-6 were 450, 455 and 465 g, respectively, and the final weights after 30 days of composting process with forced aeration were 235, 265, and 270 g, respectively. The moisture content of the mixed waste was initially 69.4% and reduced to 66.1%. The volatile solids of the mixed waste were initially 94.4% and reduced to 88.2%. The percent reductions in dry solids, volatile solids, moisture, and total sample in reactor-4 were found to be 42.12, 45.15, 50.27, and 47.78%, respectively. The percent reductions in dry solids, volatile solids, moisture, and total sample in reactor-5 were found to be 36.21, 39.12, 44.21, and 41.76%, respectively. The percent reductions in dry solids, volatile solids, moisture, and total sample in reactor-6 were found to be 37.60, 41.70, 43.85, and 41.94%, respectively. The average percent reductions in dry solids, volatile solids, moisture, and total sample in these three reactors after 30 days of composting process with forced aeration were found to be 38.64, 41.99, 46.11, and 43.83%, respectively.
3.3 Volatile solids degradation
Composting is a self-heating biological transformation of organic waste, which produces a stable end product, for example, organic fertilizer. Thermophilic phase is a very dynamic phase where high bacterial action leads to enhance the organic matter degradation [9]. The maximum temperature of composting mass in both passive and forced aeration tests was within the range of composting process (below 60°C). The percent reduction in total sample was higher with forced aeration than with passive aeration. The composting process with forced aeration was selected for determining the rate of volatile solids degradation over time. The temperature increased from 30 to 52°C. The moisture content of the mixed waste was initially 68.9% and reduced to 41.7%. The volatile solids of the mixed waste were initially 92.4% and reduced to 87.4%. The volatile solids reduced over time at the end of the composting. The percent reductions in dry solids, volatile solids, moisture, and total sample in reactor-1 after 3 days of the composting were found to be 6.99, 7.23, 2.43, and 3.85%, respectively. The percent reductions in dry solids, volatile solids, moisture, and total sample over time are shown in Figure 10. The percent reductions in dry solids, volatile solids, moisture, and total sample increased over time. The temperature of composting mass inside the reactors rose significantly after a few days under the forced aeration composting condition. The temperature was higher in composting mass inside the reactors than the ambient, resulting in a rapid reduction in volatile solids and moisture content of the mixed waste. The rate of volatile solid reduction increased with temperature and was usually double in every 10°C temperature increase. The moisture reduced in composting mass as the moisture carrying capacity of exhaust air increased exponentially with the temperature. For this combined reduction effect, that is volatile solids degradation and dryness, the amount of dry solids and total mass also reduced significantly over time. The percent reduction rates in dry solids, volatile solids, moisture, and total sample were not uniform in the biochemical reaction of the mixed waste. The gradient or slope of the trend line of the reduction in volatile solids with time curve was used to approximate the percent rate of reduction in biochemical reaction of the mixed waste during the forced aeration composting process. The rate of reduction or degradation of volatile solids was found to be 0.92% per day.
Figure 10.
Reductions of different parameters during forced aeration composting (a) dry solids, (b) volatile solids, (c) moisture, and (d) total sample.
4. Conclusions
The first run with passive aeration and the second run with forced aeration were conducted to determine the variation of temperature over time during the composting processes. The third run with forced aeration was conducted to determine the rate of volatile solids degradation over time for a series of composting processes. For the composting process with passive aeration, the temperature of composting mass inside the reactors rose rapidly for a few days from 26 to 52°C and then fell slowly to 28°C, distant from the ambient temperature of 17°C. For the composting process with forced aeration, the temperature of composting mass inside the reactors rose rapidly for a few days from 26 to 54°C and fell rapidly to 26°C for a few days and then dropped slowly to 19°C, near to the ambient temperature of 17°C. During the composting process, the biochemical reaction in the mixed waste produced heat, rising the temperature inside the reactors. The percent reductions in volatile solids were found to range from 4 to 55%. The percent reductions in total sample were found to range from 3 to 68%. The percent reductions in dry solids, volatile solids, moisture, and total sample increased with time. The reduction or degradation rate of volatile solids was found to be 0.92% per day. The mixture of vegetable waste and paper waste was well degraded during composting processes in both aeration conditions. There is a possibility of recycling the putrescible part of organic solid waste in the form of compost for its further beneficial usages.
Acknowledgments
The authors would like to express their sincere gratitude to the staff of the Environmental Engineering Laboratory of the Department of Civil Engineering, Khulna University of Engineering and Technology, Khulna, Bangladesh, for assisting the research work.
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