water waste treatment

PREPARED BY:

SONIA BUTT

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WASTE WATER TREATMENT

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WHAT IS WASTEWATER?

Wastewater is the mixture of liquid and solid materials that residents and businesses flush down toilets and empty down sinks and drains. This material is directed to the plant through a network of pipes that make up the city’s sanitary sewer system to the wastewater treatment plant.

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WHY TREAT WATER? It's a matter of caring for our environment and for our own health. There

are a lot of good reasons why keeping our water clean is an important priority:

Fisheries:- Clean water is critical to plants and animals that live in

water. This is important to the fishing industry, sport fishing enthusiasts, and future generations.

Wildlife Habitats:- Our rivers and ocean waters teem with life that

depends on shoreline, beaches and marshes.

Recreation and Quality of Life:- Water is a great playground  for us all. Visitors are drawn to water activities such as swimming, fishing, boating and picnicking.

Health Concerns:- If it is not properly cleaned, water can carry disease. Since we live, work and play so close to water, harmful bacteria have to be removed to make water safe.

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PARTICLES:- Suspended solids: (physical particles that can clog rivers or channels as they settle under

gravity). Biodegradable organics (e.g. BOD):

which can serve as “food” for microorganisms in the receiving body. Pathogenic bacteria and other disease causing organisms:

These are most relevant where the receiving water is used for drinking, or where people would otherwise be in close contact with it; and

Nutrients, including nitrates and phosphates: These nutrients can lead to high concentrations of unwanted algae, which can themselves become heavy loads of biodegradable organic load. 5

WASTEWATER TREATMENT METHODS:- Physical:-1. Screening (Clarification):- to remove larger entrained objects

2. Sedimentation:- In the process of sedim-

entation, physical phenomena relating

to the settling of solids by gravity are

allowed to operate.

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3. Aeration:- physically adding air, usually to provide oxygen to the

wastewater.

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4. Filtration:- Here wastewater is passed through a filter medium to separate solids. An example would be the use of sand filters to further remove entrained solids from a treated wastewater

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5. Flotation and Skimming:- Permitting greases or oils, for example, to float to the surface and skimming.

6. Degasification:- the removal of dissolved gases from liquids, especially water or aqueous solutions.

7. Equalization:- In certain industrial wastewater treatment processes strong or undesirable wastes are sometimes produced over short periods of time. Since such "slugs" or periodic inputs of such wastes would damage a biological treatment process, these wastes are sometimes held, mixed with other wastewaters, and gradually released, thus eliminating "shocks" to the treatment plant. This is call equalization.

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• CHEMICAL:-1. Chlorination: Chlorine, a strong oxidizing chemical, is used to

kill bacteria and to slow down the rate of decomposition of the wastewater.

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1. Neutralization: Neutralization consists of the addition of acid or base to adjust pH levels back to neutrality. Since lime is a base it is sometimes used in the neutralization of acid wastes.

2. Coagulation: consists of the addition of a chemical that, through a chemical reaction, forms an insoluble end product that serves to remove substances from the wastewater. E.g., Polyvalent metals.

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3. Flocculation:- This is the process whereby smaller particles

(inorganic and organic), water-stable soil aggregates, or flocs aggregate to form larger particles (flocs) in a flowing medium. The formation of flocs is a complicated process that is driven by a combination of mechanisms, physical (e.g., turbulence), chemical (e.g., ionic concentration), and biological (bacterial populations and extracellular polymeric material).

Ferric and aluminum salts when added in water forms insoluble hydroxides, which come out as floppy flocs, whose surface carry positive charge. This positive charge flocs attaches itself with the negative charge particles in water.

Al(pH 6-7) and Iron Hydroxide (pH 5-6) acting as good floc.

Al2(SO4)3 + 6 H2O   →   Al(OH)3 ↓ + 3 H2SO4

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4. Carbon Adsorption: is a widely used method of home water treatment because of its ability to improve water by removing disagreeable tastes and odors, including objectionable chlorine.

Activated carbon effectively removes many chemicals and gases, and in some cases it can be effective against microorganisms. However, generally it will not affect total dissolved solids, hardness, or heavy metals. 

Advantages:-

*. Remove dissolved organics & Cl effectively

*. Long Life (High Capacity) Disadvantages:-

*. Can generate C fines *. Does not remove or treat many

contaminants14

6. Ozonation:- Ozone wastewater treatment is a method that is increasing in popularity.

An ozone generator is used to break down pollutants in the water source. The generators convert oxygen into ozone by using ultraviolet radiation or by an electric discharge field.

Ozone is a very reactive gas that can oxidize bacteria, moulds, organic material and other pollutants found in water.

Benefits:-

1. Kills bacteria effectively.

2. Oxidizes substances such as iron and sulphur

3. no nasty odours or residues produced.

4. Ozone converts back into oxygen quickly, and leaves no trace once it has been used.

Disadvantages:-

1. The treatment requires energy in the form of electricity; this can cost money and cannot work when the power is lost.

2. The treatment cannot remove dissolved minerals and salts.

3. Ozone treatment can sometimes produce by-products such as bromate that can harm human health if they are not controlled.

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• BIOLOGICAL:- It uses microorganisms, mostly bacteria, in the

biochemical decomposition of wastewaters to stable end products.

More microorganisms, or sludges, are formed and a portion of the waste is converted to carbon dioxide, water and other end products.

Generally, biological treatment methods can be divided into aerobic and anaerobic methods, based on availability of dissolved oxygen.

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TREATMENT PLANT:- While the devices used in wastewater treatment are

numerous and will probably combine physical, chemical and biological methods, they may all be generally grouped under six methods:

1. Preliminary Treatment2. Primary Treatment3. Secondary Treatment4. Disinfection5. Sludge Treatment6. Tertiary Treatment

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Detroit's storm water and sewage treatment plant is the largest in the United States serving Detroit and 76 other nearby towns. 20

1. PRELIMINARY TREATMENT:- used to protect pumping equipment and facilitate subsequent

treatment processes. Preliminary devices are designed to remove or cut up the larger suspended and floating solids, to remove the heavy inorganic solids, and to remove excessive amounts of oils or greases.

the following devices are commonly used:

1. Screens -- rack, bar or fine

2. Comminuting devices -- grinders, cutters, shredders

3. Grit chambers

4. Pre-aeration tanks

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1.  Racks and Bar Screens consist of bars usually spaced 3-quarter inches to

6 inches  large screens are sometimes set vertically, at an

angle of 45 to 60 with the vertical.  The incoming wastewater bars or screens

accumulated material is removed.  The racks or screens may be cleaned either

manually or by means of automatically operated rakes.

The solids removed by these units can be disposed of by burial or incineration.

RacksBar

Screen

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2. Comminuting Devices: Grinders, cutters and shredders

These are devices to break or cut up solids to such size that they can be returned to the wastewater without danger of clogging pumps or piping or affecting subsequent treatment devices

 They may be separate devices to grind solids removed by screens or a combination of screen and cutters installed within the wastewater flow channel in such a manner that the objective is accomplished without actually removing these larger solids from the wastewater.

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3. Grit Chambers Grit chambers are long narrow tanks that are

designed to slow down the flow so that solids such as sand, coffee grounds, and eggshells will settle out of the water.

Grit causes excessive wear and tear on pumps and other plant equipment.

Its removal is particularly important in cities with combined sewer systems, which carry a good deal of silt, sand, and gravel that wash off streets or land.

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Pre-Aeration Tanks Pre-aeration of wastewater, that is aeration before primary

treatment is sometimes provided for the following purposes: 1. Greater removal of suspended solids in sedimentation tanks.2. Removal of grease and oil carried in the wastewater.3. To freshen up septic wastewater prior to further treatment.4. BOD reduction. accomplished by introducing air into the wastewater for a

period of 20 to 30 minutes. This may be accomplished by forcing compressed air into the

wastewater at a rate of about 0.10 cu.ft. per gallon of wastewater when 30 minutes of aeration is provided or by mechanical agitation whereby the wastewater is stirred or agitated so that new surfaces are continually brought into contact with the atmosphere for absorption of air.  

 air is usually supplied at the rate of 1.0 to 4.0 cubic feet per minute per linear foot of tank or channel.

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Pre-Chlorination………………………..?????

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2. PRIMARY TREATMENT:- Mechanical treatment. 1st stage of waste water treatment designed to remove gross, suspended and floating

solids from raw sewage. It includes screening to trap solid objects and

sedimentation by gravity to remove suspended solids Can reduce the BOD of the incoming wastewater by

20-30% Total suspended solids by some 50-60%. Biological Treatment = Negligible Settle able solids are separated or removed from the

wastewater by the physical process of sedimentation30

Purpose:- to reduce the velocity of the wastewater Therefore, primary devices may consist of settling

tanks, clarifiers or sedimentation tanks. Settling tanks can be divided into 4 general

groups1. Septic tanks2. Two story tanks -- Imhoff and several proprietary

or patented units3. Plain sedimentation tank with mechanical sludge

removal4. Upward flow clarifiers with mechanical sludge

removal

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• Screening Pumping Aeration Series of

Concrete Tank

Air in Removing Sludge Digester

Removing Scum Chlorine Tank Wastewater

Residuals.

Wood, Plastic, Rocks, dead animals, etc.

FORCE OF GRAVITY

H2S, etc

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Thickening

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3. SECONDARY TREATMENT

the dissolved organic matter that escapes primary treatment

This is achieved by microbes consuming the organic matter as food, and converting it to carbon dioxide, water, and energy for their own growth and reproduction.

biological process is then followed by additional settling tanks (“secondary sedimentation", see photo) to remove more of the suspended solids.

About 85% of the suspended solids and BOD can be removed

depends primarily upon aerobic organisms

Devices Used in Sec. Treatment:-

1. Trickling filters with secondary settling tanks2. Activated sludge and modifications with final

settling tanks3. Intermittent sand filters4. Stabilization ponds

a. Activated sludge:- Pri. Effluent Aeration tank (O2) sec. Settling tank

Ter. Treatment

Microbes

Aeration tank

Sludge Digester

Natural Water System

Pros:1. Low construction cost2. Occupies small area3. Relatively low odor4. Removes a high percent of BOD

Cons:1. High operating cost (air pumps)2. High energy expenses for oxygen demand

b. Trickling filter

the wastewater is dispensed upon a bed of media, such as rocks, stones, plastics, or salts

use a type of rockwool

Slow rates of flow to allow microbial growth on the surface of media

biofilm that falls off the media flows through the bed of material and will be transported to the secondary settling tank to remove excess microorganisms

Settled Sec. effluent then enter either into a digester or re-enter into the trickling system for:

1. further treatment,

2. preventing the microorganism from drying out,

3. diluting or supplementing primary effluent.

Pros:1. Low construction cost2. Cheap oxygen delivery3. Non-electric systems available

Cons:1. Temperature Dependent2. Vulnerable to congestion, which can lead to

flooding and system failure, low oxygen supply, and restricted water flow

3. Can occupy a larger area than activated sludge4. High Maintenance on some older types

c. Oxidation ponds: Process:- large & shallow depth range= 1-2.5m Composed of microbes, feed on organic matter Algae= key feature, much like aeration tank in the

activated sludge steady flow of O2 to bac. Slow process requiring large areas of land typically used in areas with small pop.

Pros:1. Small energy input2. Degrades nitrogen and phosphorus

Cons:1. Occupies a large area2. Possible odors3. Slow process4. Long retention times5. Climate dependent

d. Rotating Biological Contactors

Use of light weight plastic discs (3-3.5m in diameter)

Mounted on a horizontal shaft, half immersed in water

Rotated at an adjustable speed

Microbes stick & grow onto the plastic

Rotation exposed microbes to O2.

4. CHLORINATION:- Disinfection, final step before discharge of the effluent Application:1. Disinfection or destruction of pathogenic organisms2. Prevention of wastewater decomposition --

(a) odor control, and (b) protection of plant structures Aid in plant operation -- (a) sedimentation, (b) trickling filters, (c) activated sludge bulking Reduction or delay of biochemical oxygen demand

(BOD)

5. SLUDGE TREATMENT:- 2 objectives:

1. the removal of part or all of the water in the sludge to reduce its volume,

2. the decomposition of the putrescible organic solids to mineral solids or to relatively stable organic solids.

Fulfilled by following 2 or more methods:

1. Thickening2. Digestion with or without heat3. Drying on sand bed -- open or covered4. Conditioning with chemicals5. Elutriation6. Vacuum filtration7. Heat drying8. Incineration9. Wet oxidation10.Centrifuging

Then, dewatered before disposal Dewatered sludge contains 70% water, BUT Behaves as a liq. & can be handled as a solid

material Sludge-drying beds provide the simplest method

of dewatering, it includes: Rotary drum vaccum filter Centrifuge Belt filter press Requires less space sludge conditioning, in which chemicals are added

to the liquid sludge to coagulate solids and improve drainability.

6: TERTIARY AND ADVANCED WASTEWATER TREATMENTCan remove 99% of all the impurities

from sewageVery expensive, requiring high level of

technical know-how and well trained treatment plant operators

A steady energy supplyChemicals & specific equipment may

not be readily availablethe modification of a conventional

secondary treatment plant to remove additional phosphorus and nitrogen.

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QUESTIONS…???

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