decentralised wastewater treatment and reuse components and designing dr. deblina dwivedi senior...
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DECENTRALISED WASTEWATER TREATMENT AND REUSE
Components and Designing
Dr. Deblina DwivediSenior Research Associate-Urban Water
ProgrammeCentre for Science and Environment, New Delhi
3 factors to be considered for designing the DWWT system
Population.
Volume of per capita water consumption.
Volume of wastewater generation.
Thumb rule: 80% of the total water consumption goes out as waste
Step 1: Determine the volume of wastewater generated / day (cum)
Example: Population (P) = 100, Water use = 100 litres / capita / day Volume of water consumed = 100 x 100 = 10000 litres / day
or 10cum/ day Hence average volume of wastewater generated = 10000 x
0.8 = 8000 litres / day or approx 8 cum/ day.
Step 2: Calculate the peak hour wastewater production
Peaking factorHarmon’s Formula: 18 + √P
4+√ PP = Population in thousands
Peak hourly flow = Peaking factor x average flow of wastewater per hr
Example: The average wastewater flow per day = 8 cum The average wastewater flow per hour = 0.333 cum Peaking factor = 4.24 Peak hourly flow = 1.40 cum
Step 3: Calculate the total volume of sludge generated
Thumb rule: Volume of sludge produced per capita per day = 0.1 litres
Example: Population = 100
Volume of sludge produced per day = 100 (P) x 0.1 = 10 litresHence volume of sludge produced per year = 10 x 365 (days) = 3650 litres or 3.6 cum.
Note: at Indian condition the volume of sludge produced in septic tank is 30 litres per capita per year
Note: Sludge volume can be assumed to 0.08lpcd if desludging interval > 2 years.
System components > Modules
Primary Treatment – Pretreatment and Sedimentation inSettler
Secondary anaerobic treatment in Baffled reactor.
Secondary & tertiary aerobic/anaerobic treatment in Planted filter bed.
Tertiary aerobic treatment in Ponds
Design Specifications of settler.
• Rectangular / length to breath ratio: 3 to 1• Depth: between 1.0 to 2.5m• Two chambered: First chamber 2/3 of total length• Three chambered: First chamber ½ of total length• Manholes above each chamber• Watertight, durable and stable tank
Step 4: Calculate the dimensions of settler
Thumb rule > Area required = 0.5 sq m / cum wastewater/day
Volume of wastewater / day = 10 cumThen area required = 10 x 0.5 = 5 sqm.Hence the settler dimensions =L = 3.86 mB = 1.28 m
1.28 m
3.86 m
Step 5: Calculate the depth of settlerThe depth of the settler is based on wastewater retention time.Minimum retention time = 3 hours
• Average wastewater flow per hour = 0.333 cum (8 cum/24 hr)• Hence the volume of the settler = 1 cum ( 0.333 cum x 3hrs)• Final volume of the settler = 4.00 cum (1.00 cum + 3 cumsludge)• The depth of the settler will be = 0.8 or 1 m (4.00 cum/ 5 sq m)
System components > Modules
Primary Treatment – Pretreatment and Sedimentation inSettler
Secondary anaerobic treatment in Baffled reactor.
Secondary & tertiary aerobic/anaerobic treatment in Planted filter bed.
Tertiary aerobic treatment in Ponds
Step 6: Calculate the dimensions of the ABR
Thumb rule >Area required 1 sq m/cum of wastewater per day
E.g. If 10 cum of wastewater is generated per day thenthe size of the baffled reactor will be about 10 sq mDimensions :L = 10 m,B = 1 m,D = 1.5 to 2 m
Hydraulic retention time and Hydraulic load
• HRT = Vol. of the reactor/ Vol. of wastewater applied per day • HL = Vol. of wastewater applied per day / Vol. of the reactor
Measuring HRT
Example:10 cum wastewater flow per day on 15 cum of reactorvolume gives a Hydraulic retention time of 1.5 days I.e. morethan 24 hours ( 15 cum / 10 cum)
Note: 24 hours HRT is better
Step 7: Calculate the area of each chamber
Surface Area of each chamber (sq m) = Peak flow (cum /hr)
Up flow velocity (m / hr)
Example: Peak flow = 1.40 cum/ hrUpflow velocity = 1.5 m /hr
Surface area of each chamber = 0.93 or 1 sq m
Note: The chamber length should be 50-60% of the depth.
If the depth is 1.5 m, length will be 0.75 m
Hence width =1 sq m / 0.75 m = 1.33 m
Up flow velocity must be kept less than 2.0m/hr
0.75 m
1.33
m
Step 8: Calculate the number of chambers
Number of chambers = Total area of the ABR (sq m) / Area of each chamber (sq m)
Example: Total area of chamber = 10 sq mArea of each chamber = 1.33 sq m
No. of chambers = 7.52 or 8
Step 11: Calculate the dimensions of planted gravel bed Horizontal planted filter
Thumb rule >Area required 4 sq m / cum of wwpd or 0.27 sq m / user
E.g. If 10 cum of wastewater is generated per day then the size of the planted filter will be about 40 sq m
Dimensions: L = 20 m, B = 2 m, depth between 0.6 to 1m
Dimensions of gravel bed – by adopting CPCB norms
Design parameters : Expected BOD removal Volume of wastewater
A = Q ( In C in – In C out)
-----------------------
k BOD
A (m2) = Surface area of the bed
Q (cum/d)= Average Wastewater flow
C in = BOD at inlet (mg/l)
C out = BOD at outlet (mg/l)
KBOD = Degradation coefficient which is 0.1 m/d
Example:
Q = 10 cum /d
BOD C in = 80 mg/l
BOD C out = 30 mg /l
A = 10 ( In 80 – In 30) / 0.1
A = 54 sq m
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