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A grit chamber is usually installed before primarysedimentation tanks and it is sensible to say that it

should be placed well before wastewater pumps.

There are basically three different types of gritchambers. One is called the horizontal flow type,another is theaerated grit chamberand the last

one is the vortex type. The design of a horizontalflow type is such that as wastewater flows throughin horizontal direction at a certain velocity

(0.3m/s) particles will start to settle at the channel

before reaching the outlet point. Normally it isconstructed with consideration put in mind to

remove particles that would otherwise be trappedon a 0.21 diameter mesh size. Once the grit settlesto the bottom of the chamber, it can then beremoved out from the system, using conveyor with

buckets or plows.

Aerated grit chamber consists of an aeration tank,

which is designed to create a spiral flow ofwastewater as it moves through the chamber. Withthe velocity created from the movement of water,

a certain particle sized grit will settled at thebottom. A well-designed system should have the

incoming velocity travelling at a precise speed inorder to avoid the grit exiting out of the chamberwith the wastewater as well. A trial and errorobservation can be carried out to determine theright adjustment on the feeding of air to the

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system. Similar like a horizontal flow type, theaccumulated particles which settled at the bottom

can then be removed using conveyor buckets.

A vortex type grit chamber operates on similarprincipal and basically it has a cylindrical tank,

which is designed to create a vortex flow pattern.In order to achieve this, wastewater basically hasto enter the chamber tangentially and as such a

centrifugal force will ensure that the grit is taken

out. Find out more about other forms of physicaltreatment in wastewater plant such as the use

ofDissolved Air Flotation System.

SKIMMING TANK

Different Types of Oil Skimmers Back to Top

For industrial oil skimming, there are six basicdesigns commonly used:

Belt Skimmers:Belt-type oil skimmers use anendless belt of corrosion

resistant steel or syntheticmedium, which is lowered into

the tank or vessel to beskimmed. The belt passesthrough resilient wiper bladeswhere the oil is removed from

both sides of the medium.

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Disk Skimmers:These oil skimmers rotate adisk shaped medium through

the liquid. Oil is wiped off anddischarged into a collectioncontainer in a manner similarto belt oil skimmers. It is

important to consider reach,the portion of the disk thatactually gets immersed, when

looking at a disk oil skimmer.Less disk in the fluid meansless oil removed. Obviously,fluctuating fluids can be a real

problem for disk oil skimmers.

Drum/Barrel Styles:These are similar to the disk

type, but use a rotating drumshaped medium. Compared todisk types, they are usuallymore rugged and have higher

removal capacity. Depending

on the design, these units canalso be rendered ineffective by

fluctuating fluid levels. Also,water pickup with this type ofoil skimmer can be high.

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Mop Skimmers:These oil skimmers use anendless medium shaped like a

rope and having mop-liketendrils that pick up the oil. Asthe medium leaves the liquidand enters the drive unit, it is

pressed and wrung out withpinch rollers. For higherviscosity oils, the medium

tends to mat down and loseeffectiveness. A decantsystem is a must for theseunits, as water pickup can be

very high. Also, replacementoil mops can be veryexpensive, so check prices on

replacements beforepurchasing.

Large Tube Skimmers:Tube oil skimmers use afloating plastic hose thatsnakes out over the surface ofthe liquid and is then drawn

back through the drive unitwhere oil is removed. Thisdesign requires a relatively

large amount of surface areafor proper operation. This oil

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skimmer can skim from veryshallow tanks. As a rule, theremoval capacity is lower than

belt, drum or mop type oilskimmers.

Mini Tube Skimmers:Very similar to the large tube

units, but use either a 3/16"

or a 5/16" tube instead of 1".The pickup rate varies from 1quart/hour to 1.5 gphdepending on the diameter ofthe tube. These units are fairlycompact, and can fit in tight

spots. The better units will

have the motor mountedunderneath, to bring room

required over the lip of thetank down to near zero. The5/16" diameter tube ispreferable as it has a 1 gph

removal capacity and enough

stiffness to not drag on thehousing and prematurely wipe

off oil when being drawn intothe unit.

Floating Suction

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Skimmers:These come in several forms,but all have a floating intake.

They are most suitable forrelatively thick layers of oil(1/4 inch or higher);otherwise, they tend to ingest

large amounts of water. Somemachines will actually emulsifyoil due to churning as it

passes through the suctionpump. This type of oilskimmer requires a coalescingor at least a decanting unit tobe at all effective.

CHEMICAL FLOCCULATION

Location in the Treatment Plant

After the source water has been screened and has passed through

the optional steps of pre-chlorination and aeration, it is ready for

coagulation and flocculation.

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In theory and at the chemical level, coagulation and flocculation

is a three step process, consisting of flash mixing, coagulation,and flocculation. However, in practice in the treatment plant,

there are only two steps in the coagulation/flocculation process -

the water first flows into the flash mix chamber, and then enters

the flocculation basin.

In this lesson, we will primarily be concerned with the theory

behind coagulation/flocculation. In later lessons, we willconsider the practice in more detail.

Purpose

The primary purpose of the coagulation/flocculation process isthe removal of turbidity from the water. Turbidity is a cloudy

appearance of water caused by small particles suspended

therein. Water with little or no turbidity will be clear.

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Turbidity is not only an aesthetic problem in water. Water with

a high turbidity can be very difficult or impossible to properly

disinfect. As a result, the maximum allowable level of turbidity

in water is 0.5 NTU, while the recommended level is about 0.1

NTU. (NTU, or TU, stands for nephelometric turbidity units, a

measurement of the turbidity of water.)

In addition to removing turbidity from the water, coagulation

and flocculation is beneficial in other ways. The process

removes many bacteria which are suspended in the water and

can be used to remove color from the water.

Turbidity and color are much more common in surface water

than in groundwater. As surface water flows over the ground to

streams, through streams, and then through rivers, the waterpicks up a large quantity of particles. As a result, while aeration

is more commonly required for groundwater, treatment

involving coagulation and flocculation is typical of surface

water.

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Three Steps

As I mentioned above, the chemistry of coagulation/flocculation

consists of three processes - flash mix, coagulation, and

flocculation. Each of these processes is briefly explained

below.

In the flash mixer, coagulant chemicals are added to the water

and the water is mixed quickly and violently. The purpose of

this step is to evenly distribute the chemicals through the water.

Flash mixing typically lasts a minute or less. If the water is

mixed for less than thirty seconds, then the chemicals will not be

properly mixed into the water. However, if the water is mixed

for more than sixty seconds, then the mixer blades will shear thenewly forming floc back into small particles.

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

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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.The final step is flocculation. During flocculation, a process of

gentle mixing brings the fine particles formed by coagulation

into contact with each other. Flocculation typically lasts for

about thirty to forty-five minutes. The flocculation basin often

has a number of compartments with decreasing mixing speeds as

the water advances through the basin. This compartmentalized

chamber allows increasingly large floc to form without beingbroken apart by the mixing blades.

Common Types of ISFs:-

Gravity Discharge ISFs:-

One variety of buried ISFs, the gravity discharge ISF, is usually

located on a hillside with the long axis perpendicular to theslope to minimize the excavation required. Because the effluent

leaving the sand filter flows out by gravity, the bottom of the

sand filter must be several feet higher than the drain field area.

To achieve that difference in elevations, a sand filter may be

constructed partially above ground.

Pumped Discharge ISFs:-

Another type of buried sand filter, the pumped discharge sand

filter, is usually sited on level ground, but its location in relation

to the drainfield is not critical since a pump located within the

sand filter bed allows effluent to be pumped to a drainfield at

any location or elevation. Discharge piping goes overnot

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throughthe sand filter liner, so the integrity of the liner is

protected.

Bottomless ISFs:-

A third type of buried sand filter has no impermeable liner and

does not discharge to a drainfield, but rather directly to the soil

below the sand.Pumps and controls should be checked every 3months, and the septic tank or aerobic unit should be checked

for sludge and scum buildup and pumped as needed. Control

panels with programmable timers and hour and event meters

simplify troubleshooting and diagnosis.

What are the advantages and disadvantagesof using ISFs?

Advantages

ISFs produce a high quality effluent that can be used for drip

irrigation or can be surface discharged after disinfection.

Drainfields can be small and shallow.

ISFs have low energy requirements.

ISFs are easily accessible for monitoring and do not require

skilled personnel to operate.

No chemicals are required.

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If sand is not feasible, other suitable media could be

substituted that may be found locally.

Construction costs for ISFs are moderately low, and the labor

is mostly manual.

The treatment capacity can be expanded through modular

design.

ISFs can be installed to blend into the surrounding landscape.

The soil cover prevents odors.

Disadvantages

The land area required may be a limiting factor.

Regular (but minimal) maintenance is required.

Odor problems could result from open filter configurationsand may require buffer zones from inhabited areas.

If appropriate filter media are not available locally, costs

could be higher.

Clogging of the filter media is possible.

ISFs could be sensitive to extremely cold temperatures.

ISFs may require an NPDES Permit when the effluent is

surface discharged.