Building Services - I
Unit 6.1 - Septic Tank
PRESENTED BY AR. MADHURA RASANE
Septic Tank
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The septic tank is a buried, water-tight container usually made of concrete, fiberglass, or
polyethylene.
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Its job is to hold the wastewater long enough to allow solids to settle down to the bottom
forming sludge, while the oil and grease floats to the top as scum.
Septic Tank
•
The septic tank is designed to retain
the wastewater in the tank
for about 24 hours. During the
retention period, the biological
decomposition of the night soil
occurs by the aid of anaerobic
bacteria.
•
Only a small part of the night soil
thus remains, known as the sludge,
which is then settled in the tank.
•
The treated wastewater that flows
out through the tank is known as
effluent. Thus, the effluent from the
septic tank is disposed of either by
the municipal drainage system or
simply through absorption by
the soak pit.
•
A soak pit is a covered, porous-walled chamber that allows water to slowly soak into the
ground. Soak pits, which can be lined with porous materials, which provide foundational
support to prevent collapse of the underground chamber, may also be used for separate
treatment of greywater.
Soak Pit
o
The water in the septic tank is not pure, it is called grey
water because it still contains organic materials that need
to be filtered out.
o
A Soak Pit is a covered, porous-walled chamber that allows
water to slowly soak into the ground. Pre-settled effluent
from septic tank is discharged to the underground chamber
from where it infiltrates into the surrounding soil.
Working
o
As waste-water percolates through the soil from the soak
pit , small particles are filtered out by the soil matrix and
organics are digested by micro-organism.
o
Soak pit are best suited to soils with good absorptive
properties; clay, hard packed or rocky soils are not
appropriate.
Depends on number of chambers
1.
Single chamber septic tank
o
In a single-chamber tank, everything happens in just
one tank.
o
When the water and solids go into the single tank,
the anaerobic bacteria start to eat the solids.
o
The waste then turns into a sludge that rests on the
bottom of the chamber.
2. Double chamber septic tank
o
A double-chamber tank mostly works in the same way as a single tank does. However, instead of the water
going straight from the original tank into the leech field, the waste water, minus the majority of the solids,
goes into the secondary chamber.
o
In that chamber, the water goes through an additional purification process.
o
A double chamber septic tank has the benefit of being able to hold more waste water before it will need to be
pumped.
o
The water will also be cleaner when it goes into the leech field, meaning it will purify quicker.
Working of Septic Tank
1. Working Chamber:
o The working chamber is the main working space where the anaerobic decomposition as well
as the settling of the sewage particles takes place.
2. Inlet Pipe:
o The inlet pipe is provided in the septic tank to pass the collected wastewater and the night
soil inside the tank.
Working of Septic Tank
3. Baffle Wall:
o The baffle wall is generally provided near the inlet pipe. Baffle walls serve as the breakers for the
incoming sewage.
o Such barriers also prevent the congestion of blockade of the outlet pipe by the overflow of the
effluent.
o The baffle wall’s thickness is kept between 50mm to 100mm.
4. Outlet Pipe:
o The outlet pipe is provided in the septic tank to pass the collected effluent to the drain field for
efficient disposal.
o The outlet pipe is always provided at a level lower than the inlet pipe.
Working of Septic Tank
5. Roofing Slab:
o The roofing slab is the top cover provided to the septic tank. Generally, roofing slab comprises RCC
slabs and may be circular or rectangular.
o The thickness of the top slab must range from 75mm to 100mm based on the size of the tank
o For the circular cover, the minimum diameter must be kept as 500mm; for the rectangular cover,
the minimum size must be kept 600*450mm.
6. Ventilation Pipe:
o The ventilation pipe of the septic tank is also commonly referred to as the Air Vent.
o The main objective of providing the ventilation pipe is to facilitate air circulation inside the tank and
prevent foul odor.
o It is usually made up of cast iron or asbestos. A wire mesh is provided at the top of the ventilation
pipe to check the entry of flies, mosquitoes, and other insects.
https://www.youtube.com/watch?v=bHYuyGJzix8
Design Criteria of Septic Tank
o
The septic tank design mainly depends
upon the number of users expected to
use it.
o
In this regard, the capacity of the
sludge tank mainly depends upon the
number of users and the sludge
removal interval. It is a general practice
to remove the sludge every two years.
o
The liquid capacity of the septic tank is
usually taken as 130 liters to 170 liters
per head.
o
The chamber of the septic tank is
mainly made up of brick walls with
cement mortar.
o
The thickness should not be less than 9
inches, and the foundation floor must
be made of cement concrete of mix
1:2:4.
Design Criteria of Septic Tank
o
It must be noted that both the inside as well as the outside faces of the wall and the top
layer of the floor must be plastered with a minimum thickness of 12mm i.e. one and a halfinch thick.
o
The mix of the cement mortar for the plastering work should be 1:3. Some water-resistant
admixtures may be added to the mix.
o
The floor of the septic tank must be designed to have a slope of 1:10 to 1:20 towards the
inlet. This is done to ensure efficient collection and removal of the sludge.
Types of Septic Tanks
A. Depends on material used for construction
Types of Septic Tanks
Discharge from septic tank
Drain field (Trench) :
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A solid pipe leads from the septic tank to a distribution box where the waste water is channeled into one
or more perforated pipes set in trenches of gravel. Here the water slowly infiltrates (seeps) into the
underlying soil.
•
Dissolved wastes and bacteria in the water are trapped or adsorbed to soil particles or decomposed by
microorganisms.
•
This process removes disease-causing organisms, organic matter, and most nutrients (except nitrogen
and some salts).
•
The purified wastewater then either moves to the ground water or evaporates from the soil. Trench
systems are the most common type of system used in new home construction.
Discharge from septic tank
Sand mound system
o
Another alternative is the Sand Mound System: These systems are used in areas where the site is not
suitable for traditional septic systems.
o
The liquid is then pumped from the tank to perforated plastic pipes buried in a mound of sand built on the
original soil surface.
o
In these systems, the waste water flows from the septic tank to a storage tank
o
Vegetation growing on the mound helps to evaporate some of the liquid. This is particularly important in
areas with shallow water tables.
Discharge from septic tank
Leach pit / Seepage pit (Dry well)
o An alternative to the common drain field is the Seepage Pit (Dry Well).
o In this type, liquid flows to a pre-cast tank with sidewall holes, surrounded by gravel. (Older
versions usually consist of a pit with open-jointed brick or stone walls.)
o Liquid seeps through the holes or joints to the surrounding soil.
Size of Septic Tank
•
•
Generally, the residential septic tank is typically 4.5 feet wide x 8.0 feet long x 6 feet
high. Your septic tank may be a different size.
Tanks are generally buried 4 inches to 4 feet deep, depending on local site conditions,
shape, slope, and other factors.
Importance of Septic Tank
o It is necessary to collect the wastewater and night soil from the connected drains
and toilets in the households that are generally not connected to the municipal sewer
systems.
o Septic tank efficiently treats the sewage before it is discharged to the environment
thereby preventing the environmental degradation and pollution.
o Septic tank allows the wastewater to be replenished by natural means and the
treated water can also be re-used for purposes such as industrial works, irrigation,
groundwater recharge etc. Septic tanks are vital for safe disposal of the night soil,
particularly from the restrooms
Size of Septic Tank
Dimensions of the Septic Tank :
• The septic tank’s design must be done so that the tank’s width is not less than 750mm.
• The length is usually taken as two to four times the width and the depth is taken as 1000 to
1300mm plus 300mm to 450mm is taken as the freeboard.
A 5-person household's daily water consumption
05 people - 5 liters per day for cooking
85 liters per person for bathing & toileting.
That means for 5 five people it is (85* 5 =425)
•
•
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30 liters is for washing clothes and utensils.
10 liters is for the cleaning solution.
The rest is the 5 liters.
So you can take 475 liters on average for 5 persons.
(As a rule, we count three days as the period of detention. It should be able to hold the household wastewater
for at least three days so that the septic tank can perform properly. )
475 x 3 (No. of Days) = 1425 liters of wastewater in 3 days
A home should have a minimum capacity of 2000 liters. As a result, septic tanks should have a
minimum depth of 1.8m. Approximately 30 liters of sludge settle down per person per year.
Here, we remove it every two years. A home with five members would require a tank of this size.
Design Criteria of Septic Tank
Detention Period: It varies between 12 to 36 Hours, Commonly adopted as 24 Hours.
Length to Width Ratio: For Rectangular tank, length is 2 to 4 times the width.
Depth: Range between 1.2 to 1.8 m.
Space for Digestion: 0.0425 m3/cap.
Sedimentation: For Indian Conditions, surface area will be 0.92 m2 for every 10 lpm peak flow rate.
Cleaning Period for Sludge: 6 months to 3 Years, generally taken as 12 Months.
Free Board: 0.3 to 0.6 m.
https://www.youtube.com/watch?v=t37zUtDtXNo
Design Criteria of Septic Tank
Design a septic tank for a small residential colony having a population of 50 persons. The rate of
water supply is 150 liters per head per day. Design also the soak well, if the effluent from the
septic tank is to be discharged in it?
Dimensions of Septic Tank Components
•
•
•
•
•
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Length, Width and Depth of Septic Tank
Width = 750mm(min)
Length = 2 to 4 times width
Depth = 1000 to 1300mm. (min below water level) + 300 to 450mm free board
Maximum depth = 1800mm + 450 mm free board
Capacity = 1 cubic meter (10 cubic feet) minimum
Building Services - I
Unit 6.2 – Waste Water and Sewage
Treatment Plant
PRESENTED BY AR. MADHURA RASANE
Waste Water Treatment and Sewage Treatment Plant
o Wastewater
treatment,
also
called sewage treatment, the removal
of impurities from wastewater, or
sewage, before it reaches aquifers or
natural bodies of water such
as rivers, lakes, estuaries, and oceans.
o Sewage Treatment plants are used to
treat wastewater by removing organic
pollutants and pollutants that are made
from human waste.
o While Water Treatment plants are used
to clean and purify water, wastewater
treatment plants are used to clean and
purify wastewater.
o In
general,
the
wastewater
discharged from domestic premises
like residences, institutions, and
commercial establishments is termed
as sewage waste.
o It includes waste liquid from toilets,
baths, showers, kitchens, and sinks
draining into sewers.
o Sewage treatment refers to the
process of removing contaminants,
micro-organisms and other type of
pollutants from wastewater, primarily
from household sewage.
o Sewage treatment includes physical,
chemical, and biological methods to
remove contaminants and produce
environmentally
safe
treated
wastewater to make it usable again.
OBJECTIVES OF SEWAGE TREATMENT
o Sewage treatment plant is simple and affordable treatment plant to maintain in apartments.
Installation of STP in apartments/flats will upgrade the quality of life.
o STP treats the water that goes down drains before releasing it back into the environment.
o Usage of STP in apartments yields clean reusable water that can be used for other purposes
like washing, toilet, watering plants and outdoor floor cleaning etc., so that you can save
water.
o Using STP the amount of waste from the apartments that is usually released into the
environment is reduced thus improving environment’s health.
OBJECTIVES OF SEWAGE TREATMENT
Sewage from your apartment can be treated by implanting Sewage Treatment System or
Sewage Treatment Plant (STP) which involves following stages:
o Preliminary Treatment
o Primary Treatment
o Secondary Treatment
o Tertiary Treatment
Stages in Sewage Treatment Plant
o Pre-treatment: Pre-treatment removes materials that can be easily collected from the raw
wastewater before they damage or clog the pumps and skimmers of primary treatment
clarifiers.
o The influent sewage water is strained to remove all large objects carried in the sewage
stream. This is most commonly done with an automated mechanically raked bar screen in
modern plants serving large populations, whilst in smaller or less modern plants a
manually cleaned screen may be used.
o
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
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Conventional sewage treatment may involve three stages, called
Primary,
Secondary
Tertiary treatment.
OBJECTIVES OF SEWAGE TREATMENT
o Primary treatment:
o
o
Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids
can settle to the bottom while oil, grease and lighter solids float on the surface.
The settled and floating materials are removed and the remaining liquid may be discharged or subjected
to secondary treatment.
o Secondary treatment:
o
o
o
Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed
habitat.
Secondary treatment may require a separation process to remove the micro-organisms from the treated
water prior to discharge or tertiary treatment.
Secondary treatment removes dissolved and suspended biological matter.
o Tertiary treatment:
o
Tertiary treatment is effected by sand filters, mechanical filtration or by passing the effluent through a
constructed wetland such as a reed bed or grass plot.
o
Treated water is sometimes disinfected chemically or physically (for example by lagoons and micro
filtration) prior to discharge into a stream, river, bay, lagoon or wetland, or it can be used for the
irrigation of a golf course, green way or park.
o
If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.
Sewage
Treatment
Plant
OBJECTIVES OF WASTEWATER TREATMENT
o The purpose of waste water treatment is to remove contaminant from water so that the
treated water can meet the acceptable quality standard. The quality standard usually
depends whether the water will be reused or discharged into river.
o Before disposing of sewage into river or land, sewage has to be treated to make it safe and
to make it harmless. Methods of waste water treatment depends on composition of
waste water and required quality for treated water.
o Treatment process are broadly classified as physical, chemical and biological treatments.
Physical treatment methods utilize physical separation of pollutant such as by filtration etc.
o Chemical treatment methods utilize chemical characteristics of pollutant for purification.
For e.g. Coagulation etc.
o Biological treatment methods utilize biological characteristics of pollutants such as
bacteria, viruses by purification.
Waste Water Treatment Plant
Decentralized Wastewater Treatment (DWWT)
o
There are several kinds of wastewater treatment systems which are in use. They range from
conventional low-cost options and decentralized small systems to large, expensive, centralized systems.
o
Decentralized systems are small, individual or cluster type wastewater facilities to provide wastewater
treatment services to residents. In the decentralized wastewater treatment systems, wastewater can be
treated onsite through aerobic and anaerobic techniques. The anaerobic modules comprise of settlers,
baffle reactors and anaerobic filters.
o
The aerobic modules have horizontal planted gravel filters and polishing ponds. This approach is based
on different natural treatment techniques, put together in different combinations according to need. It
is used for recycling both “grey” and “black” domestic wastewater.
This systems include:
o Primary treatment, which includes pre-treatment and sedimentation in settlement tank or septic tank;
o Secondary anaerobic treatment in baffled reactors;
o Tertiary aerobic/anaerobic treatment in reed bed system; and polishing in Ponds
Stage 01
Pre-treatment is used for the screening and separation of the floating oil and grease in a grease trap.
Fig. 1 Separation of floating oil & grease in a grease trap
Primary treatment takes place in a settler or a septic tank in which the liquid part is separated from the
solid matter through sedimentation process. The settled sludge is stabilized by anaerobic digestion.
Stage 02
o Secondary treatment of the wastewater takes place in the Baffled Reactors which consist of a
series of chambers, in which the wastewater flows up-stream.
o
On the bottom of each chamber activated sludge is retained. During inflow into the chamber
wastewater is intensively mixed up with the sludge whereby it is inoculated with wastewater
organisms, which decompose the contained pollutants.
Stage 02
o Tertiary treatment takes place in the planted Gravel Filter through root zone system. .
o The main removal mechanisms are biological conversion, physical filtration and chemical
adsorption. The treated water at this stage meets the requirement for recycling and
reuse of water for horticulture/irrigation purposes.
Polishing Ponds
Here, both aerobic degradation and pathogen removal takes place. It is simple in construction,
reliable in performance if properly designed, high pathogen removal, can be used to create an
almost natural environment, fish farming is possible in large and low-loaded polishing ponds.
Fig. 4 Polishing pond for post treatment
https://www.youtube.com/watch?v=fSuLK8JLPkE
Building Services - I
Unit 6.3 – Biogas Plant
PRESENTED BY AR. MADHURA RASANE
Bio-Gas plant and it’s functioning
•
Biogas plant is an airtight container that facilitates fermentation of material under anaerobic
condition. Other names given to this device are ‘Biogas Digester’, ‘Biogas Reactor’, ‘Methane
•
‘Generator’ and ‘Methane Reactor’. Recycling and treatment of organic wastes (biodegradable
material) through anaerobic digestion (fermentation) technology not only provides biogas as
a clean and convenient fuel but also an excellent and enriched bio-manure.
•
The BGP also acts as mini bio-fertilizer factory; hence some people refer it as ‘Biogas fertilizer
plant’ or ‘Bio-manure plant’. Anaerobic digestion of organic matter produces a mixture of
methane (CH4) and carbon dioxide (CO2) gas that can be used as a fuel for cooking, lighting,
mechanical power and the generation of electricity, or a replacement for other fuels.
Bio-Gas plant and it’s functioning
Classification of biogas plants
• Classification of biogas plants depends upon the plants design and mode of working. One
common way to classify them is
Bio-Gas plant and it’s functioning
Batch type biogas plant
o Batch type biogas plants are appropriate where daily supplies of raw waste materials are difficult
to be obtained. Batch type plant is charged at 50-60 day intervals.
o Once charged, it starts supplying the gas after 8-10 days and continuous to do so for about 40-50
days till the process of digestion is completed. Afterwards it is emptied and recharged.
o Gas production in batch type is uneven. Several digesters occupy more space. This type of plants
requires large volume of digester, therefore, initial cost becomes high. Such plants are installed
in European countries. Do not suit the conditions in Indian rural areas.
Continuous type biogas plant
o In continuous type biogas plant, the supply of the gas is continuous and the digester is fed with
biomass regularly. Plant operates continuously and is stopped only for maintenance or for
sludge removal. The gas produced is stored in the plant or in a separate gas holder.
o The period during which the biomass remains in the digester is known as ‘the retention
period’. The thin dry layer often formed at the top of the slurry is known as scum. This type of
plant are very popular in India and China.
Other features:
• Retention period is less
• Less problems as compared to batch type.
• Small digestion chambers are required
KVIC type biogas plant
o
o
o
o
o
o
This mainly consists of a digester or pit for
fermentation and a floating drum for the
collection of gas. Digester is 3.5-6.5 m in depth
and 1.2 to 1.6 m in diameter.
There is a partition wall in the center, which
divides the digester vertically and submerges in
the slurry when it is full. The digester is
connected to the inlet and outlet by two pipes.
Through the inlet, the dung is mixed with water
(4:5) and loaded into the digester. The
fermented material will flow out through outlet
pipe.
The outlet is generally connected to a compost
pit. The gas generation takes place slowly and
in two stages.
In the first stage, the complex, organic
substances contained in the waste are acted
upon by a certain kind of bacteria, called acid
formers and broken up into small-chain simple
acids.
In the second stage, these acids are acted upon
by another kind of bacteria, called methane
formers and produce methane and carbon
dioxide.
Types of biogas plants: Biogas plants basically are two types.
• Floating dome type
Eg. KVIC-type (KVIC- Khadi Village Industries Commission)
• Fixed dome type
Eg. Deenabandu model
Gas holder
o
The gas holder is a drum constructed of mild
steel sheets. This is cylindrical in shape with
concave. The top is supported radically with
angular iron.
o
The holder fits into the digester like a stopper. It
sinks into the slurry due to its own weight and
rests upon the ring constructed for this purpose.
o
When gas is generated the holder rises and
floats freely on the surface of slurry. A central
guide pipe is provided to prevent the holder
from tilting.
o
The holder also acts as a seal for the gas. The
gas pressure varies between 7 and 9 cm of
water column. Under shallow water table
conditions, the adopted diameter of digester is
more and depth is reduced.
o
The cost of drum is about 40% of total cost of
plant. It requires periodical maintenance. The
unit cost of KVIC model with a capacity of 2
m3/day costs approximately Rs.14, 000
Advantages of KVIC Plant
o
o
o
o
High gas yield
No problem of gas leakage.
Works under constant pressure naturally.
No problem of mixing of biogas with external air, thus
no danger of explosion.
Janata type biogas plant:
o
The design of this plant is of Chinese origin but it
has been introduced under the name “Janata biogas
plant” by Gobar Gas Research Station, Ajitmal in
view of its reduced cost. This is a plant where no
steel is used, there is no moving part in it and
maintenance cost is low.
o
Good quality of bricks and cement should be used
to avoid the afterward structural problems like
cracking of the dome and leakage of gas. This model
have a higher capacity when compared with KVIC
model, hence it can be used as a community biogas
plant.
o
This design has longer life than KVIC models.
Substrates other than cattle dung such as municipal
waste and plant residues can also be used in Janata
type plants.
o
The plant consists of an underground well sort of
digester made of bricks and cement having a dome
shaped roof which remains below the ground level.
Janata type biogas plant:
o
At almost middle of the digester, there are two
rectangular openings facing each other and coming
up to a little above the ground level, act as an inlet
and outlet of the plant. Dome shaped roof is fitted
with a pipe at its top which is the gas outlet of the
plant.
o
The principle of gas production is same as that of
KVIC model. The biogas is collected in the restricted
space of the fixed dome, hence the pressure of gas is
much higher, which is around 90 cm of water column.
Advantages of Fixed Dome Type Plant
o
o
o
o
Cost of plant is less compared to floating drum type plant.
Loss of heat is negligible since these are constructed
underground.
No corrosion problems as in fixed drum type.
It is maintenance free.