21112005

M.K. PANDEY/P. Jenssen

Centralised –Decentralised transportation system

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Collection system 70 - 90 %

Treatment 10 - 30 %

(Otis 1996, Mork et al. 2000)

The cost of conventional gravity system is up to 4 times higher than the cost of treatment and disposal

Wastewater treatment plant

Centralised system

Wastewater treatment plant

Sewer lines

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House hold human waste and wastewater

＋

Urine

Feaces＋

＋Flush

Black water

} Excreta

{Greywater

Wastewater

Or Sewage

Anal cleansing＋

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＋

Urine

Faeces＋

＋Flush

Black water

} Excreta

{Greywater

Wastewater

Or Sewage

Anal cleansing＋

Important constituents

•Organic matter•Neutrients- Nitrogen, Phosphorus, Potassium•Pathognes

House hold human waste and wastewater

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Wastewater transportation

Wastewater transported to treatment plant as quickly as possible

Self cleansing velocity should be maintained at low flow

Velocity should not be higher than the maximum allowable velocity – to prevent wear and tear of the pipes

Formation of Hydrogen sulphide, airlock should be prevented

Should not be close to W/S lines

Proper selection of type and shape of sewer

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Conventional gravity sewer

River

WW Treatment Plant

Pumping system

G.L

•Pollution due to combined sewer overflow•Large dia sewer•Interference to other infrastructure•Contamination of water distribution system•High chances of system failure

Over flowstructure

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Combined sewer

• Storm and sanitary sewage (wastewater) collected in one sewer• Suitable at places where rainfall is evenly distributed throughout

the year• Overflow structure required to divert the flow more than the

design flow• Large dia sewer required• Large volume of wastewater to be treated• Plumbing work reduced in houses

• Separate sewer – Storm sewage and sanitary sewage conveyed in separate sewer Chances of cloggingProne to formation of H2S

Partially separate systemRainwater from houses and yards discharged into sanitatry sewers

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Types of conventional sewerage system

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Collection system 70 - 90 %

Treatment 10 - 30 %

(Otis 1996, Mork et al. 2000)

The cost of conventional gravity system is up to 4 times higher than the cost of treatment and disposal

Wastewater treatment plant

Investment Cost

Wastewater treatment plant

Sewer lines

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Collection in a septic tank and transport the effluent wastewater to nearby treatment system

Natural Treatment Septic tank (S.T)

Compost or transport to faecal sludge treatment facilities.

Decentralized system

•Soak pit•Constructed wetland•Infiltration system•Pond system•Sand filter

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Collection and treatment of blackwater and Greywater separately

Natural Treatment

Compost or Transport to faecal sludge treatment facilities

Settling tank and greese tap

Low flush or pour flush

Septic tank (S.T)

•Soak pit•Constructed wetland•Infiltration system•Pond system•Sand filter

Decentralized system

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Collection and treatment of urine, faeces and greywater separately

Natural Treatment

Low flush or pour flush

Faeces

UrineSettling tank and greese tap

Decentralized system

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Decentralized system- STEG

Septic tank effluent gravity (STEG)

Can be laid in variable grade - because no solid to settle

Uniform slope with no high points to prevent airlock

H2S formation

Air release valve in high points

Clean out ports at junction

100 mm

50 mm 50 mm to 200 mm

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Decentralized system- STEP

Septic tank effluent pump (STEP) and pressure sewer

with grinder pumps

Sewer are under pressure – pressure generated by high

head turbine pump

Advantage in high groundwater and rocky soil and rolling

terrain - can follow the terrain

If grinder pumps used- septic tank not required

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Decentralized system- Vaccum sewer

Vacuum sewer

Vacuum applied to transport sewage

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Hydraulics of wastewater collection system

Velocity and headloss are two governing parameter

Hazen williams equation (1)

Where

V= Velocity of flow, m/s

C = Hazen –williams coefficient, 150 may be used PVC pipe

R = Hydraulic Radius, (wetted area/wetted perimeter), m

(e.g for pipe flowing full

D = inside dia of sewer, m

S = Slope of energy gradeline, m/m,

hf = head loss due to friction, m

L = Length of pipeline

54.063.0849.0 SRCV

4/DR

L

hS f

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Hydraulics of wastewater collection system

Manning`s equation (2)

Where

V= Velocity of flow, m/s

n = Manning’s coefficient, 0,013 to 0.009 may be used for PVC pipe

R = Hydraulic Radius, (flowing full)

D = inside dia of sewer, m

S = Slope of energy grade line, m/m,

hf = head loss due to friction, m

L = Length of pipeline

Sewer line (gravity sewers) are designed as a open channel or

flowing just full

4/DR

L

hS f

n

SRV

5.03

2

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Hydraulics of wastewater collection system

The velocity should be less than 1.5 m/s to avoid excessive frictional loss.

No minimum velocity required for STEG system – (but usually kept at 1m/s)

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Information's required for design and layout of STEG collection system

Site characteristics

–Topography of the area

–Depth of soil

–Depth of water table

–Depth of freezing zone

Equivalent dwelling unit (EDU)

–Residence with given number of residents e.g if 1 EDU is

defined as residence with 4 person then 8 person residence is

2EDU

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Information's required for design and layout of collection system

Peak flow rates

–collection system based on peak flow rates

–1.3 to 1.9 lit/min/EDU (USA)

–0.8 to 1.2 lit/min/EDU (Norway)

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STEPS for the design of Sewer collection system

Prepare a longitudinal profile

Select a pipe size

Calculate the velocity using Hazen Williams equations

Calculate the pipe cross sectional area and determine the actual

capacity

Check for the surcharged condition

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