BOD and sewage water treatment process

2. GENERATION OF WASTE
WATER
Wastewater is generated by
five major sources:
Industrial wastes
Storm water runoff
Groundwater infiltration
Household wastes
Human and animal wastes

3. WHY WATER PLANT IS
DESIGNED?
1. Protect public health.
2. Protect public water supplies.
3. Protect aquatic life.
5. Protect adjacent lands.
4. Preserve the best uses of the waters.

4. Wastewater(sewage
water ) treatment
Step 1-Preliminary treatment
Step 2-Primary treatment
Step 3-Secondary treatment
Step 4-Advanced waste treatment
Step 5-Disinfection
Step 6-Sludge treatment
Wastewater treatment is a series of steps.
In these slides first three steps are discussed in
detail

5. WASTE WATER TREATMENT
Wastewater treatment is a series of steps. Each of the steps can be accomplished using one or more
treatment processes or types of equipment. The major categories of treatment steps are:
1. PRELIMINARY TREATMENT Removes materials that could damage plant equipment or would
occupy treatment capacity without being treated.
2. PRIMARY TREATMENT — Removes settle able and floatable solids (may not be present in all
treatment plants).
3. SECONDARY TREATMENT — Removes BOD and dissolved and colloidal suspended organic
matter by biological action. Organics are converted to stable
solids, carbon dioxide and more organisms.
4. ADVANCED WASTE TREATMENT — Uses physical, chemical, and biological processes to
remove additional BOD, solids and nutrients (not present in all treatment plants).
5. DISINFECTION — Removes microorganisms to eliminate or reduce the possibility of disease
when the flow is discharged.
6. SLUDGE TREATMENT — Stabilizes the solids removed from wastewater during treatment,
inactivates pathogenic organisms, and reduces the volume of the sludge by removing water.

6. PRELIMINARY TREATMENT
Raw influent entering the treatment plant may contain many kinds of materials (trash). The purpose of
preliminary treatment is to protect plant equipment by removing these materials that could cause clogs,
jams, or excessive wear to plant machinery. Preliminary treatment may include many different
processes. Processes include: influent screening, shredding, grit removal, preaeration and chemical addition
SCREENING The purpose of screening is to remove large solids, such as rags, cans, rocks, branches,
leaves, roots, etc., from the flow before the flow moves on to downstream processes
Typically, a treatment plant will remove anywhere from 0.5 to 12 ft3 of screenings for each million gallons
of influent received.

7. SHREDDING As an alternative to screening, shredding can be used Shredding processes include
comminution (comminute means cut up) and barminution devices.
COMMINUTION The comminutor is the most common shredding device used in wastewater treatment. In
this device all the wastewater flow passes through the grinder assembly. Solids pass through the screen
and are chopped or shredded between the two cutters. The comminutor will not remove solids, which are
too large to fit through the slots, and it will not remove floating objects.
GRIT REMOVAL The purpose of grit removal is to remove the heavy inorganic solids that could cause
excessive mechanical wear. Grit is heavier than inorganic solids Processes use gravity
and velocity, aeration, or centrifugal force to separate the solids from the wastewater.
PREAERATION In the preaeration process we aerate wastewater to achieve and maintain an aerobic
state (to freshen septic wastes), strip off hydrogen sulfide (to reduce odors and corrosion), agitate solids
(to release trapped gases and improve solids separation and settling), and to reduce BOD.
CHEMICAL ADDITION Chemical addition is made to the waste stream to improve settling, reduce
odors, neutralize acids or bases, reduce corrosion, reduce BOD, improve solids and grease removal etc.

8. PRIMARY TREATMENT (SEDIMENTATION)
The purpose of primary treatment (primary sedimentation or primary clarification) is to remove settle able
organic and floatable solids. Normally, each primary clarification unit can be expected to remove 90 to 95%
settle able solids, 40 to 60% TSS, and 25 to 35% BOD.
PROCESS DESCRIPTION
1. Primary treatment reduces the organic loading on downstream treatment processes by removing a
large amount of settle able, suspended, and floatable materials.
2. Primary treatment reduces the velocity of the wastewater through a clarifier to approximately 1 to 2
ft./min, so that settling and floatation can take place. Slowing the flow enhances removal
of suspended solids in wastewater.
3. Primary settling tanks remove floated grease and scum, remove the settled sludge solids, and collect
them for pumped transfer to disposal or further treatment.
4. Clarifiers used may be rectangular or circular. In rectangular clarifiers, wastewater flows from one end to
the other, and the settled sludge is moved to a hopper at the one end, either by flights set on parallel
chains or by a single bottom scraper set on a traveling bridge. Floating material (mostly grease and oil) is
collected by a surface skimmer.
5. In circular tanks, the wastewater usually enters at the middle and flows outward. Settled sludge is
pushed to a hopper in the middle of the tank bottom, and a surface skimmer removes floating material.

9. SECONDARY TREATMENT
Secondary treatment refers to those treatment processes that use biological processes to convert
dissolved, suspended, and colloidal organic wastes to more stable solids that can either be removed by
settling or discharged to the environment without causing harm.
The main purpose of secondary treatment is to provide BOD removal beyond what is achievable by
primary treatment.
There are three commonly used approaches,
1. the trickling filter
2. activated sludge process,
3. oxidation ponds or lagoons
TRICKLING FILTERS It was found that if settled wastewater was passed over rock surfaces, slime grew
on the rocks and the water became cleaner. Today we still use this principle.
A trickling filter consists of a rotating distribution arm that sprays and evenly distributes liquid wastewater over
a circular bed of fist-sized rocks, The spaces between the media allow air to circulate easily so that aerobic
conditions can be maintained. The spaces also allow wastewater to trickle down through, around, and over
the media. A layer of biological slime that absorbs and consumes the wastes trickling through the bed covers
the media material.

10. The slime growth on the trickling filter media periodically sloughs off and is settled and removed in a
secondary clarifier that follows the filter.

11. ACTIVATED SLUDGE :The formation of activated sludge is dependent on three steps. The first step is the
transfer of food from wastewater to organism. Second is the conversion of wastes to a usable form. Third is
the flocculation step.
1. Transfer — Organic matter (food) is transferred from the water to the organisms. Soluble material is
absorbed directly through the cell wall. Particulate and colloidal matter is adsorbed to the cell wall, where it is
broken down into simpler soluble forms and absorbed through the cell wall.
2. Conversion — Food matter is converted to cell matter by synthesis and oxidation into end products such
as CO2, H2O, NH3, stable organic waste, and new cells.
3. Flocculation — Flocculation is the gathering of fine particles into larger particles. This process begins in
the aeration tank and is the basic mechanism for removal of suspended matter in the final clarifier. The
concentrated bio-floc that
settles and forms the sludge blanket in the secondary clarifier is known as activated sludge.
OXIDATION PONDS: OXIDATION PONDS also called lagoons or stabilization ponds, are large, shallow
ponds designed to treat wastewater through the interaction of sunlight, bacteria, and algae. Algae grow
using energy from the sun and carbon dioxide and inorganic compounds released by bacteria in water.
During the process of photosynthesis, the algae release oxygen needed by aerobic bacteria.
Mechanical aerators are sometimes installed to supply yet more oxygen, thereby reducing the required
size of the pond. Sludge deposits in the pond must eventually be removed by dredging. Algae
remaining in the pond effluent can be removed by filtration or by a combination of chemical treatment
and settling