Preliminary Treatment

WASTEWATER TREATMENT PROCESSES

Preliminary Treatment

Primary Treatment

Secondary (Biological) Treatment

Tertiary Treatment

Disinfection

Solids Handling

PRELIMINARY TREATMENT

Used to protect equipment and downstream processes.

Preliminary treatment can consist of:

Screening

Shredding

Grit removal

SCREENING

Screening removes such items as metal, wood, plastic objects, string, cloth, and paper products.

These items could cause pipes to plug, damage or plug pumps, or jam sludge collector mechanisms.

BAR SCREENS AND RACKS

Screening devices usually consist of parallel bars set at an angle in the wastewater channel.

When the spacing between the bars is between 3 to 4 inches it is generally considered to be a rack.

When the spacing between the bars is about 3/8 inch to 2 inches it is considered to be a bar screen.

BAR SCREENS

Bar screens are used to screen the influent flow on a continuous basis.

Bar screens may be manually or mechanically cleaned.

Racks are usually found in bypass channels where flows are diverted when bar screens are being serviced or repaired.

BAR SCREENS - CLEANING

Screenings should be removed from bar screens before they visibly hinder the wastewater flow.

Screenings are usually dewatered and disposed of by landfilling.

When wastewater is allowed to back up in the influent line, organic waste will settle out and can cause septic conditions which produce hydrogen sulfide

BAR SCREENS - CLEANING

The sudden rush of wastewater which can occur when a screen is cleaned infrequently can:

carry grit into the clarifiers;

carry additional solids over the clarifier weirs; and

reduce the efficiency of the clarifiers and secondary treatment units.

SHREDDING

Can can be used in addition to or in place of bar screens.

Most common shredding devices are comminutors and barminutors.

Comminutors and barminutors act both as a cutter and a screen. Their purpose is to shred the solids and leave them in the wastewater.

SHREDDING

Advantages of shredding:

No screenings to dispose of; and

No flies or odors.

Disadvantages of shredding:

Plastics, woods and plastics are rejected and have to be removed manually; and

Shreddings can reform.

GRIT REMOVAL

Grit is the heavier inorganic matter found in wastewater which will not decompose or break down.

Will generally have a settling velocity greater than that of organic material in the wastewater.

Grit can consist of sand, cinders, rocks, coffee grounds, cigarette butts, etc.

GRIT REMOVAL

Grit removal protects equipment by;

Reducing clogging in pipes;

Protecting moving mechanical equipment and pumps from abrasion; and

Preventing accumulations of materials in aeration tanks and sludge digesters

GRIT REMOVAL

Grit may be removed by settling in:

Hand-cleaned channels;

Mechanically cleaned channels; and

Aerated channels.

Grit may be removed by centrifugal force with:

Cyclone degritters.

GRIT CHANNELS

Channels allow the wastewater velocity to be reduced to around 1.0 foot per second.

The objective is to keep the lighter organic material in suspension and flowing to the next treatment unit while allowing the grit to settle.

GRIT CHANNELS

Velocity can be controlled by:

Multiple channels;

Proportional weirs; and

Shape of the grit channels.

GRIT CHANNELS

Grit may be removed from channels manually or mechanically.

The advantages of hand-cleaned channels are:

Cheaper to build and operate

No moving parts.

GRIT CHANNELS

The disadvantages of hand-cleaned channels are:

Must be manually cleaned (Its hard work!)

Safety concerns (back strain, slipping, fumes, etc.).

AERATED GRIT CHAMBERS

Are generally rectangular chambers that are wider, deeper, and shorter than grit channels.

Air is injected along the wall of the tank. The mixture of air and water has a lower specific gravity than water so the grit settles out better.

The rolling action of the water in the tank moves the grit along the bottom to a grit hopper.

CYCLONE DEGRITTER

Cyclone degritters use centrifugal force in a cone-shaped unit to separate grit from the wastewater.

Wastewater enters the cyclone near the top and to the side. This causes a spinning action and creates a centrifugal force that causes heavier particles to be forced outward and downward, where they are passed out of the cyclone.

GRIT- WASHING & DISPOSAL

Grit may be washed before final disposal to remove the organic matter that settled out with the grit.

Grit is usually disposed of by landfilling.

GRIT REMOVAL EFFICIENCIES

Inflow and infiltration (I/I) may cause reduced grit removal efficiency due to:

High flow rates through the chambers; and

Excessive amounts of grit washed into the plant.

ADDITIONAL PRELIMINARY CONSIDERATIONS

Flow Equalization reduces flow fluctuation into the plant

Odor Control to prevent odor at and around the plant

Septage Management to control the discharge of wastes from septic

tanks, grease traps, and other sources

Flow Measurement to measure the influent flow and provide for flow

proportional sampling

Questions?