DECENTRALISED WASTEWATER
TREATMENT AND REUSE
Components and Designing
Dr. Deblina Dwivedi
Senior Research Associate-Urban Water Programme
Centre 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 factor
Harmon’s Formula: 18 + √P
4+√ P
P = 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 litres
Hence 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 in
Settler
Secondary anaerobic treatment in Baffled reactor.
Tertiary aerobic
treatment in Ponds
Secondary & tertiary aerobic/anaerobic
treatment in Planted filter bed.
Types of Settlers
3 chambers
2 chambers
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 cum
Then area required = 10 x 0.5 = 5 sqm.
Hence the settler dimensions =
L = 3.86 m
B = 1.28 m
3.86 m
1.28 m
Step 5: Calculate the depth of settler
The 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 cum
sludge)
• The depth of the settler will be = 0.8 or 1 m (4.00 cum/ 5
sq m)
The final dimension of the settler will be
1.28 m
1.0 m
System components >
Modules
Primary Treatment –
Pretreatment and
Sedimentation in
Settler
Secondary anaerobic treatment in Baffled reactor.
Tertiary aerobic
treatment in Ponds
Secondary & tertiary aerobic/anaerobic
treatment in Planted filter bed.
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 then
the size of the baffled reactor will be about 10 sq m
Dimensions :
L = 10 m,
B = 1 m,
D = 1.5 to 2 m
Cross checking design parameters
Retention
time
Upflow
Velocity
Organic
load
Sludge
storage
volume
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 reactor
volume gives a Hydraulic retention time of 1.5 days I.e.
more
than 24 hours ( 15 cum / 10 cum)
Note: 24 hours HRT is better
Hydraulic load & Hydraulic retention time
80 - 90 % of removal happens in reactor
Step 7: Calculate the area of each chamber
Surface Area of each chamber (sq m) = Peak flow (cum /hr)
Up flow velocity (m / hr)
Up flow velocity must be kept less than 2.0m/hr
Example: Peak flow = 1.40 cum/ hr
Upflow 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
0.75 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 m
Area 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
Polishing Ponds
Thumb rule >Area required 1.2 sq m / cum of wwpd or
0.2 sq m / user
Standard depth= 1-1.5m
E.g.
If 10 cum of wastewater is generated per day
then the size of the planted filter will be about 12 sq
m