Sewage is primarily domestic and sludge is thickened (15 % solids)
MSW is shredded and screened (particle size : 5mm)
Moisture content ( M.C.=60%)
Carbon/Nitrogen ratio (C/N=20)
Table 1. Chemical composition of dewatered MSS sample Parameter Mean Value Standard Error of the Mean pH 7.7 0.15 Moisture Content (%) 54.1 2.24 Organic Matter @ 550 o C (%) 61.5 18.67 Total Organic Carbon (%) 34.2 0.63 Total Kjeldahl Nitrogen (%) 6.1 0.12 C:N Ratio 5.6 0.15
Table 1. Chemical composition of dewatered MSS samples Table 2. Chemical composition of MSW samples Parameter Mean Value Standard Error of the Mean pH 7.7 0.15 Moisture Content (%) 54.1 2.24 Organic Matter @ 550 o C (%) 61.5 18.67 Total Organic Carbon (%) 34.2 0.63 Total Kjeldahl Nitrogen (%) 6.1 0.12 C:N Ratio 5.6 0.15 Parameter Mean Value Standard Error of the Mean pH 5.3 0.23 Moisture Content (%) 61.3 3.95 Organic Matter @ 550 o C (%) 77.93 3.10 Total Organic Carbon (%) 45.2 1.80 Total Kjeldahl Nitrogen (%) 2.47 0.28 C:N Ratio 18.3 3.09
Table 3. Characteristics of mixtures of municipal solid wastes and sewage sludges MSW : MSS ( Ratio) Moisture Content (%) C/N (Ratio) Volatile Solids (%) 1:1 54.06 15.25 71.28 2:1 56.78 16.33 72. 35 4:1 58.12 17.36 74.12
Four identical in-vessel units were used. Each unit was made of a double wall, 364 grade stainless steel, cylindrical shape drum and was supported horizontally by 1100 mm height steel frame. The dimensions of each vessel were 600 mm inside diameter, 764 mm outside diameter, and 1000 mm length with capacity of 200 L. Each drum was electrically- driven by a motor and was insulated by a water jacket which was heated with four 1200 watt heating bars, two from each side. Each cylindrical vessel was fully insulated along its circumference with rockwool insulation to minimize heat loss.
The composting vessels were connected to an air compressor through an air flow meter and regulating valve to control the air flow. The air was supplied into the vessel via an air pocket made of 3 mm opening grill in the bottom of the vessel to ensure proper air distribution throughout the vessel and was vented through an outlet 25 mm diameter pipe. The contents of the reactor were mixed by rotating the drum once a day for 15 minutes (6 rotations/minute) and were mixed manually before sample withdrawal. The temperature of the material inside the vessels was continuously monitored by a thermocouple inserted inside the center of the material and was recorded by an on-line computer system.
To waste biodegradability and to measure the loss of organic matter, expressed as volatile solids during composting , it was necessary to determine process kinetics using data obtained in this study under controlled temperature. The plots shown in the following figures and the correlation coefficient (R 2 ) obtained as shown in Table suggest that the degradation of organic matter during MSW composting at the mesophilic temperature range as a function of time follows a first-order kinetics expressed as:
dC/dt = -kC
where C is the biodegradable volatile solids at any time, t is the time in days, k is the reaction rate constant (day-1)
By integrating this equation and letting C = Co at time = 0 gives:
ln C/ Co = - kt
3.10 Kinetic Analysis for In-Vessel VS Reduction of MSW at 15 o C
3.11 Kinetic Analysis for In-Vessel VS Reduction of MSW at 45 o C
3.12 Kinetic Analysis for Windrow VS Reduction of MSW at 20 o C
Table 4. Kinetic rates (k) for VS reductions during in-vessel composting of MSW
1. Co-composting of mixtures of MSW and MSS at various proportions was more effective than composting of these wastes when treated separately. Reductions of up to 38% of VS were obtained during 30 days of co-composting of MSW and MSS mixtures .
2. In-vessel composting of wastes at controlled temperatures is more effective than windrow piles where temperatures can not be controlled effectively.
3. Optimum operating conditions for temperature is 45 o C and for MSW:MSS mixture is 2:1.
4. A first-order model was suitable to describe the composting process kinetics.