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

Types of Settlers

2 chambers

3 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 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)

1.28 m1.0 m3.

86 m

The final dimension of the settler will be

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

Cross checking design parameters

UpflowVelocity

Retentiontime

Organicload

Sludgestoragevolume

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

80 - 90 % of removal happens in reactor

Hydraulic load & Hydraulic retention time

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

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