A flow of 900 m3/day of waste sludge (1% SS of which is 70% organic matter) is thickened to an SS-concentration of 60 gSS/L by gravity. The thickened sludge is pumped to a digester. In the digester 50% of the organic matter is degraded to biogas. The digested sludge is dewatered to 25% SS (no chemicals are needed for dewatering).
1a: If 1,075 litre of biogas is produced per kg SS that was degraded, what is the daily biogas production in m³/day? You can assume a desity of the sludg of 1000 kg/m³. This is because sludge mainly consist of water and 1 litre of water is approximately 1 kg of water.
1b: How much dewatered sludge is produced per day in ton/day? Be carefull with the units, this question asks for ton/day!
1c: How many trucks with a capacity of 30 tons are needed per week to transport the dewatered sludge to the central incineration plant?
To find the answers to these questions, we'll follow a step-by-step approach and use the given information and formulas.
1a: To find the daily biogas production in m³/day, we need to calculate the amount of organic matter degraded to biogas and then convert it to volume.
Given:
- Flow of waste sludge = 900 m³/day
- SS concentration before digestion = 60 gSS/L
- Organic matter of SS = 70%
- Degraded organic matter to biogas = 50%
- Biogas production per kg degraded SS = 1,075 litre/kg
Step 1: Calculate the total mass of SS entering the digester per day.
Mass of SS = flow of waste sludge x SS concentration
Mass of SS = 900 m³/day x 60 gSS/L (density of sludge is not needed as it cancels out)
Mass of SS = 54,000 g/day
Step 2: Calculate the mass of organic matter in the SS.
Mass of organic matter = Mass of SS x organic matter content
Mass of organic matter = 54,000 g/day x 70%
Mass of organic matter = 37,800 g/day
Step 3: Calculate the amount of organic matter degraded to biogas.
Mass of organic matter degraded = Mass of organic matter x degradation percentage
Mass of organic matter degraded = 37,800 g/day x 50%
Mass of organic matter degraded = 18,900 g/day
Step 4: Convert the mass of organic matter degraded to biogas to volume.
Volume of biogas = Mass of organic matter degraded / biogas production per kg degraded SS
Volume of biogas = 18,900 g / 1,075 L/kg (as 1 liter of water is approximately 1 kg)
Volume of biogas = 17.58 m³/day
Therefore, the daily biogas production is approximately 17.58 m³/day.
1b: To find the amount of dewatered sludge produced per day in ton/day, we need to calculate the mass of dewatered sludge and then convert it to ton.
Given:
- SS concentration after digestion = 25% (or 250 gSS/L)
- Density of sludge = 1000 kg/m³
Step 1: Calculate the mass of dewatered sludge produced per day.
Mass of dewatered sludge = Flow of waste sludge x SS concentration after digestion
Mass of dewatered sludge = 900 m³/day x 250 gSS/L (density of sludge is not needed as it cancels out)
Mass of dewatered sludge = 225,000 g/day
Step 2: Convert the mass of dewatered sludge to ton.
Mass of dewatered sludge = 225,000 g/day / 1000 g/kg
Mass of dewatered sludge = 225 kg/day
Therefore, the amount of dewatered sludge produced per day is 225 kg/day, which is equal to 0.225 ton/day.
1c: To find the number of trucks needed per week to transport the dewatered sludge to the central incineration plant, we need to convert the daily production of dewatered sludge to ton/day and then divide it by the capacity of each truck.
Given:
- Capacity of each truck = 30 tons
- Time frame = 1 week (7 days)
Step 1: Calculate the number of trucks needed per day.
Number of trucks per day = Dewatered sludge production per day / Capacity of each truck
Number of trucks per day = 0.225 ton/day / 30 tons
Number of trucks per day = 0.0075 trucks/day
Step 2: Calculate the number of trucks needed per week.
Number of trucks per week = Number of trucks per day x Number of days in a week
Number of trucks per week = 0.0075 trucks/day x 7 days
Number of trucks per week = 0.0525 trucks/week
Therefore, approximately 0.0525 trucks (which can be rounded up to 1 truck) with a capacity of 30 tons are needed per week to transport the dewatered sludge to the central incineration plant.