Mixing & Flocculation,Coagulation,and Softening In Waste water technology

 

 

Classification & Applications of different types of screens

 

* The primary treatment incorporates unit operations for removal of floating and suspended

solids from the wastewater. They are also referred as the physical unit operations. The unit

operations used are screening for removing floating papers, rages, cloths, plastics, cans

stoppers, labels, etc.; grit chambers or detritus tanks for removing grit and sand; skimming

tanks for removing oils and grease; and primary settling tank for removal of residual

settleable suspended matter.

Screen is the first unit operation in wastewater treatment plant. This is used to remove larger

particles of floating and suspended matter by coarse screening. This is accomplished by a set

of inclined parallel bars, fixed at certain distance apart in a channel. The screen can be of

circular or rectangular opening. The screen composed of parallel bars or rods is called a rack.

The screens are used to protect pumps, valves, pipelines, and other appurtenances from

damage or clogging by rags and large objects.

Wastewater Screening

Wastewater Screening is the first unit operation in all wastewater treatment plants. Screen is the device used to retain solids found in the influent wastewater to the treatment plant. The main purpose of screening is to remove solid materials that could:

• Cause damage to other process equipment.
• Cause reduction in efficiency of the whole system
• Contaminate waterways

The materials that are removed using screens are called screenings.

 

 

 

 

Classification of Wastewater Screens

Screens are generally classified into three based on the size of their openings in the screening element and mechanism of removal.

• Coarse screens
• Fine screens
• Microscreens

 

Ø  Coarse screens

Coarse screens have a clear openings ranging from 6 to 150 mm (0.25 t0 6 in). Coarse screen consist of parallel bars, rods or wires, wire mesh or a perforated plates with openings generally of circular or rectangular shapes. So it is also call as “bar rack” and used to remove coarse solids such as rags and large objects that may clog or cause damage to other appurtenances. Based on the Wastewater Screening method used to clean them, coarse screens are classified into two:

• Hand cleaned screens
• Mechanically cleaned screens

Fine screens

In Wastewater Screening, Fine screens have clear openings less than 6 mm. They consisted of perforated plates, wire cloth, wedge wire elements that have smaller openings. They are also used to remove the fine solids present in the primary effluent. Fine screens are classified as:

• Static (fixed) wedge wire screen
• Rotary drum screen
• Step type screen

Static wedge wire screens

They have a clear opening of 0.2 to 1.2 mm and designed for a rate of flow of 400 to 1200 L/m² min of screen area. Large floor area should require for installation of these screens and these should be cleaned once or twice daily.

Drum screens

In this type the screening or straining medium is mounted on a cylinder that rotates in the flow channel. The wastewater flows into either end of drum and flows out through the screen outlet with the solids are being collected on this interior or into the top of the unit

 

Microscreens Wastewater Screening

They are rotating drum screens which have a variable low speed (upto 4 r/min), which is continuously backwashed operating in gravity flow conditions. The filtering fabrics used should have a openings ranges from 10 to 35µm and fitted on the periphery of the drum. The influent enters through drum lined with fabric. The solids retained are collected through backwashing and transported for disposal.

 

 

 

 

 

 

 

 

Grit Characteristics, Types of Grit chambers

 

What is Grit?

• Inert material, both organic and inorganic, that is not benefitted by secondary treatment or sludge processing.

 

 

 

Where does grit come from?

The Collection System

 • Materials that are flushed by homeowners.

• Infiltration flow.

How does grit affect my plan

• Disrupts biological processes and reduces effluent quality.

• Increase in energy demand.

 • Reduction of treatment capacity.

 

 

 

 

 

Where grit should be removed?

• Option 1: Head of Plant

- Better protection of process equipment.

- Larger unit required to handle full flows.

- Additional protection or removal may be needed after fixed film treatment processes.

• Option 2: Sludge Stream

- Located prior to thickener or digester.

- Solids concentration should not exceed 2% TS

Option 3: Multiple Locations

- Protects upstream process equipment and basins.

- Removes grit generated within the plant prior to sludge digestion.

-         Cost may be prohibitive.

-  No matter which option is selected, bar screening is required.

Selecting grit removal options

• Range of flow.

• Type of treatment process.

• Location of grit removal.

• Particle size range.

• Equipment and energy requirements.

• Maintenance requirements.

• Allowable headloss.

• Grit testing results.

• Other benefits to treatment

 

Grit chamber are provided to

 (i) protect moving mechanical equipment from abrasion and

abnormal wear.

(ii) Reduce formation of heavy deposits in pipelines.

(iii) Reduce the frequency of digester cleaning caused by excessive accumulation of grit and

(iv) To separate inorganic particles from organic and disposed off of these particles just to wash

without passing any further treatment process.

• Grit Chambers are usually located after bar racks and before sedimentation tanks. Similarly, the

installation of screening facilities ahead of the grit chambers make the operation and

maintenance of grit removal easier.

• Two important types of Grit Chambers (i) Horizontal rectangular flow and (ii) Aerated Grit Chamber.

Types of grit chambers

1.   Rectangular Horizontal-flow grit chamber:

·       The unit is designed to maintain a velocity of 0.3 m/s and to provide sufficient time for grit particles to settle at channel while organic particles are kept in suspension.

·       A 25% increase in velocity may result in washout of grit, while 25% reduction result retention of non-target organics.

·       The design of horizontal flow grit chamber be such that, the lightest particles of grit will reach the bed of the channel.

·       Usually grit chambers must be designed to remove particles of diameter of 0.20 mm.

·       The length of channel will be based on the settling velocity and control section, while cross section area will be based on the rate of flow and the number of channels. Allowance should be made for inlet and outlet turbulence.

2.    Horizontal-flow grit chamber

• To maintain a fairly constant velocity of flow, a control

section is used. These control sections are classified as

following:

• Proportional flow weirs.

• Parshall flumes.

• Palmer-Bowlus flumes.

• Sutor weirs.

 

3. Aerated grit chamber

Ø With proper adjustment 100% removal will be obtained, and the grit will be well washed. Wastewater will move through the tank in a spiral path and will make two to three passes across the bottom of the tank.

Ø For grit removal, aerated grit chambers are often provided with grab buckets traveling on monorails and centered over the grit and storage trough. Bucket removal of grit can be further washed by dropping the grit from bucket through the tank contents.

Ø In case of industrial wastewater is discharged having VOCs then either covering of system is required of other type of grit removal will be used.

Ø

 

 

 

 

 

 

                              

 

 

 

 

 

 

 

 

 

 

 

Advantages of Grit Chamber 

The advantages of this chamber are as follows:

✔ To save running mechanical equipment from abrasion and abnormal wear.

✔ To decrease maintenance cost in the frequency of digester cleaning caused by an extreme collection of grit.

✔ To control weighty deposits in pipelines and channels.

✔ It also saves the cost of waste treatment by stopping solid materials in it.

 

Disdvantages of Grit Chamber 

The disadvantages of the grit chamber are as follows:

 

✔ They are more probable to release toxic odors and toxic organic matter.

✔ Aeration system control and maintenance will affect different human resources.

✔ Compared to other girt removal technologies, they need more energy resources.

✔ Initial construction cost is high.

✔ Regular maintenance is needed.

 

 

 

                             

 

 

 

 

 

 

Mixing & Flocculation

 

What is Rapid Mixing?

Rapid Mixing is the process of combining a very quick coagulant, such as FeCl3, in potable water treatment tanks where a large to extremely large volume of dirty water is flowing.

The more common types of facilities used for rapid mixing (or flash mixing) are:

• Static mixers
• Mechanical mixers
• Pumps and conduits
• Baffled chambers

 

1.     Mechanical mixer

• Mechanical mixers consist of a motor driven shaft that turns a propeller(s), impeller or turbine to provide mixing energy.
• This type of mixer is either placed in a small chamber where the coagulant is added to the raw water or installed directly into a pipeline (in-line mixer).
• Most are driven by a variable speed motor to allow for mixing entery control, which means that the mixing energy is independent of plant flow. Chemical addition is made at the point of greatest turbulence to ensure proper mixing. The flash mixing process takes less than 60 seconds.

2.     Static mixers

• Static mixers create turbulence by design to mix the coagulants. These mixers are inexpensive, simple to install, and create significant head loss. Static mixers have no moving parts but provide mixing energy by creating turbulence due to sloping vanes within a pipe section.
• This type of mixer is dependent upon plant flow for mixing energy. When flow is low, coagulation may suffer due to low turbulence.

§  Another type of static mixer is the diffuser mixer, which emits chemicals out of the flow channel.

 

 

 

 

3.     Pumps and conduits

• The coagulant is added just before a low-lift pump which causes turbulence to mix the chemicals.
  

4.     Baffled chambers

Baffled chambers create turbulence by redirecting the flow stream over and under a series of baffles. A negative to this type of system is that the mixing energy is dependent upon plant flow, which means the operator cannot adjust the mixing energy in this system. The chemical is added just prior to the baffled chamber.

After flash mixing, coagulation occurs.  During coagulation, the coagulant chemicals neutralize the electrical charges of the fine particles in the water, allowing the particles to come closer together and form large clumps.  You may already be familiar with the process of coagulation from cooking.  You can see coagulation occurring when preparing gelatin (jello) or when cooking an egg white.

 

Jar Test Procedure

Sometimes it's hard to find the correct coagulant dose needed for your water. It would take too much time and cost too much money to just experiment with the system. The solution is to perform a jar test procedure. During this procedure you can determine:

• the optimum coagulant dose needed to get the lowest turbidty result
• optimum pH
• how much acid to add during the enhanced coagulation process
• if alkalinity addition is necessary.

Jar test is simply a device used to determine this optimum coagulant dose required. The jar test, device consists of a number of stirrers (4 to 6) provided with paddles. The paddles can be rotated with varying speed with the help of a motor and regulator. Samples will be taken in jars or beakers and varying doses of coagulant will be added simultaneously to all the jars. The paddles will be rotated at 100 rpm for 1 minute followed by a slow mix step at 40 rpm for 20 to 30 minutes, corresponding to the flash mixing and slow mixing in the flocculator of the treatment plant.

              

 

Factors That Impact the Process

Factors that can impact the coagulation and flocculation process include:

Water temperature	Cold water negatively impacts the process
Alkalinity	Low alkalinity negatively impacts the process
Turbidity	Low turbidity negatively impacts the process
pH	Alum is impacted the most by pH
Mixing energy	Too low = poor floc formation Too high = floc shear
Coagulant dose	Too low = insufficient coagulation Too high = colored water
Flocculent aid	Addition of polymer can aid flocculation and settling

 

COAGULATION & FLOCULATION

Ø  The coagulation-flocculation processes facilitate the removal of SS and colloidal particles. It’s used in the first stage of solids-liquids separation: settling, flotation or filtration.

Ø  Coagulation is the destabilization of colloidal particles brought about by the addition of a chemical reagent called as coagulant.

Ø  Flocculation is the agglomeration of destabilized particles into microfloc and after into bulky floccules which can be settled called floc. The addition of another reagent called flocculant or a flocculant aid may promote the formation of the floc.

 

 

 

 

COAGULATION

·         Coagulant chemicals with charges opposite those of the suspended solids are added to the water to neutralize the negative charges on non-settlable solids (such as clay and color-producing organic substances).

·         Once the charge is neutralized, the small suspended particles are capable of sticking together.

·         These slightly larger particles are called microflocs, and are not visible to the naked eye. Water surrounding the newly formed microflocs should be clear. If not, coagulation and some of the particles charge have not been neutralized. More coagulant chemicals may need to be added.

·         A high-energy, rapid-mix to properly disperse coagulant and promote particle collisions is needed to achieve good coagulation. Over-mixing does not affect coagulation, but insufficient mixing will leave this step incomplete. Contact time in the rapid-mix chamber is typically 1 to 3 minutes.

·         The factors, which can promote the coagulation-flocculation, are the velocity gradient, the time, and the pH. The time and the velocity gradient are important to increase the probability of the particles to come together. Moreover the pH is a prominent factor in the removal of colloids.
PROCESS :

The coagulants

Trivalent cations : the neutralization of the negative surface of the colloid is accomplished by the addition of cations in the case of inorganic coagulants. The trivalent ions are ten times more effective than the divalent ion. Trivalent iron and aluminum salts keep on the be be widely used in all the water coagulation treatments.

The influence of the pH : Inorganic coagulant because of their hydrolysis change the physical-chemical characteristics of water to be treated (pH, conductivity, …) :

M ³⁺ + 3 H₂O <=> M(OH)₃ + 3 H⁺

The pH necessary for coagulation may be adjusted by addition of an acid or a base.

 

 

Cation	Optimum pH for Coagulation-Flocculation
Al3+	6.0 – 7.4
Fe3+	> 5

 

Sludge production : The formation of metallic hydroxide causes the production of a substantial amount of sludge. This sludge should be removed in the final solids-liquids separation process.

Organic coagulants may also be used. The advantage of those cationic polyelectrolytes is because they directly neutralize the negative colloids. Consequently to this direct action the amount of sludge is considerably reduced.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

COAGULANT SELECTION AND CHEMISTRY

Inorganic Coagulants Inorganic coagulants such as aluminum and iron salts are the most commonly used. When added to water, these highly charged ions to neutralize the suspended particles. The inorganic hydroxides that are formed produce short polymer chains which enhance microfloc formation. Inorganic coagulants usually offer the lowest price per pound, are widely available, and, when properly applied, are effective in removing most suspended solids. They are also capable of removing a portion of the organic precursors which may combine with chlorine to form disinfection by-products. Inorganic coagulants produce large volumes of floc which can also entrap bacteria as they settle.

 

 

 

The flocculant

Inorganic polymers (activated silicia) and natural polymers (starches, alginate) were the first to be used. But the use of synthetic flocculants often results in a minimum amount of sludge. Combined with modern separation techniques can allow to produce very dense sludge that can be directly treated in a dewatering unit.

Flocculation

 

Flocculation follows coagulation in the conventional water treatment process. During flocculation, a process of gentle mixing causes smaller particles called microfloc to collide and form into larger macrofloc. The speed at which flocculation occurs is directly related to the rate of particle collisions. More mixing energy results in more collisions per second, allowing flocculation to occure more rapidly.

 

Flocculation typically lasts for about thirty to forty-five minutes.  As the floc increases in size, it becomes more fragile. This means the mixing energy should be reduced in levels as the floc progresses towards the sedimentation basin

 

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The first stage has the highest mixing energy of the three chambers. This is to ensure a high number of particle collisions to encourage flocculation. The mixing energy is lowered in stage 2 since the floc has grown and becomes more fragile as it increases in size. You are shootin for the look of snowflakes floating in clear water. In the last chamber, the mixing energy is furthered reduced while the floc continues to grow. Total detention time in the flocculation process should be between 30 to 60 minutes. The flow velocity should range beween 0.5 and 1.5 ft/min in the flocculation basin.

 

 

 

 

 

Retention (or detention) time is the amount of time that water spends in a process. It is calculated by dividing the liquid volume (in gallons) of a basin by the plant flow rate (gallons per minute). Actual detention time in a basin will be less than the calculated detention time because of “dead areas” and short circuiting, which could be due to inadequate baffling.

 Retention time = basin volume (gallons)/ gpm flow

 

 

What are coagulant aids examples?

Coagulant aids can be used to improve the coagulation process whether they are organic or inorganic by producing bigger flocs and reducing the amount of coagulants used [4]. Common coagulant aids used are Bentonite ,Calcium carbonate , Sodium silicate , Anionic polymer , Nonionic polymer.

A coagulant aid is a chemical or material, which is not a coagulant, used to assist or modify coagulation. Coagulant aids add density to slow-settling flocs and add toughness to the flocs so that they do not break up during the mixing and settling processes.

A coagulant aid improves the effectiveness of a coagulant by:

• Forming larger or heavier particles
• Speeding reactions
• Permitting reduced coagulant dosage
• Coagulant aids are also known as flocculants.

Coagulant aids are chemicals which are added to water during coagulation to:

• 1)      Improve coagulation;
•  
• 2)      Build a stronger, more settleable floc;
•  
• 3)      Overcome slow floc formation in cold water;
•  
• 4)      Reduce the amount of coagulant required;
•  
• 5)      Reduce the amount of sludge produced.

Coagulant aids may be:

• Nonionic, cationic or anionic polymers
• Sodium aluminate
• Activated silica
• Clay
• Acids
• Alkalis

actors which affect how well a coagulant aid works include:

• Mixing conditions
• pH
• Alkalinity
• Water temperature
• Turbidity

 

TYPES OF COAGULANT AIDS

Activated Silica

1)      Increase the coagulation rate;

2)      Reduce the amount of coagulant needed;

3)      Widen the pH range for effective coagulation;

4)      Chief advantage-strengthens floc

5)      Usually added after coagulant, never directly with alum.

Weighting Agents

1)      Provide additional particles that can enhance floc formation.

2)      Used to treat water that is high in color;

3)      Used to treat water that is low in turbidity;

4)      Used to treat water that is low in mineral content

5)      Different types include: Bentonite Clay, Powdered Limestone, and Powdered Silicia

 

Polyelectrolytes

Extremely large molecules which produce highly charged ions when dissolved in water.� These are also called polymers.

 

Types of Polyelectrolytes

Cationic (+)

1)      Have a positive (+) charge.

2)      Allows reduced coagulant dose;

3)      Floc settles better;

4)      Less sensitivity to pH;

5)      Improved flocculation of organisms such as bacteria and algae.

 

Anionic (-)

1)      Have a negative (-) charge

2)      Used primarily as a coagulant aid

3)      Increases floc size;

4)      Improve settling;

5)      Produce a stronger floc;

6)      Not materially affected by pH, alkalinity, hardness, or turbidity

 

Nonionic

1)      Balanced or neutral charge;

2)      Used as a primary coagulant or coagulant aid. 

 

Factors which affect how well a coagulant aid works

1)      Mixing conditions

2)      pH

3)      Alkalinity

4)      Water temperature

5)      Turbidity