Wastewater Treatment Operations

Ideal velocity for sanitary sewer collection systems

2 ft/sec

Typical distance between manholes

300-500 ft

Purpose of preliminary treatment processes

Remove course material from wastewater

Purpose of primary treatment processes

Remove settleable and floatable materials

Purpose of secondary treatment processes

Remove suspended and dissolved solids

Sulfur dioxide (SO2) is used for...

Used to neutralize chlorine & detoxify the effluent

Ideal flow-through velocity of a grit channel

0.7-1.4 ft/sec

1 ft/sec is best

Types of flow measuring devices in an open channel

Parshall Flume & V-notch weir

Types of flow measuring devices in a closed channel

Venturi meter & magnetic flowmeter

Advantages of measuring flow with a Parshall flume

Smooth constriction

no protruding edges

waste will not collect behind the metering device

Ideal detention time in a primary clarifier

1.5-2 hrs

Humus

Layer of organisms (aka biomass) on trickling filter media that eventually gets thick enough to slough off and must be removed by settling in the secondary clarifier

Typical BOD & suspended solids removal rate of an activated sludge plant

90-99% removal

Mixed liquor

Activated sludge + raw wastewater + return sludge in an aeration tank

Types of bacteria present in a digester

Acid formers & gas formers

Typical organic matter removal rate of a digester

50% removal

If temperature is kept at 95F, what is the typical digestion time in a digester?

30 days

Byproducts of digestion

Organic acids

carbon dioxide gas

methane

Typical gas ratio in a digester

70% methane, 30% CO2

Types of dewatering methods

sludge drying beds

lagoons

centrifuges

vacuum filters

filter presses

Shallow pond

Aerobic

3-6 ft deep

Oxygen throughout their depth

Deep pond

Anaerobic

8-12 ft deep

Treatment depends on fermentation of sludge at the bottom

Odorous

Mainly for industrial waste

Facultative pond

Most common type of pond

Top layer is aerobic

Bottom layer is anaerobic

Algae supplies oxygen to supernatant

Short-circuiting

When flowing water entering a tank or basin flows along a nearly direct pathway from inlet to outlet, resulting in shorter contact or settling times

Bar Racks

Spacing between bars is 3-4 in

Usually found in bypass channels

Manually cleaned

Bar Screens

Spacing between bars is 3/8-2 in

The more debris that collects on bar screens leads to...

Greater head loss through bar screens

Organic wastes settled out

Dissolved oxygen is depleted

Septic conditions

Shear pin

A straight pin that will break when a certain load or stress is exceeded

Typical limit of allowable head loss at a bar screen

3 in of head loss

Benefits of a communitor

Reduction in screenings disposal

Reduction in flies

Reduction in odors

Most grit channels are designed to remove...

0.2 mm size sand & and all other heavier materials

Typical settling rate of sand (0.2 mm)

0.075 ft/sec

Settling time (sec) =

Depth (ft) / Settling Rate (ft/sec)

Grit channel length (ft) =

[Depth of channel (ft) x Flow velocity (ft/sec)] / Settling Rate (ft/sec)

If inspection of grit reveals a high amount of organic material...

Velocity in the grit channel may be too low

If inspection of grit reveals mainly large or heavy sand...

Velocity in the channel may be too high

Aerated grit chambers work on the basis of...

Air and water have a lower specific gravity than water, so grit settles out better

What is a slurry?

A watery mixture or suspension of insoluble matter

How does a cyclone grit separator work?

Water and suspended particles are carried upward in the secondary vortex while heavy particles move outward and down in the primary vortex.

Typical grit quantities per volume wastewater

1-4 cubic ft / 1 MG

When disposing of grit by burial, how much soil should cover the grit?

6 in

What are the benefits of pre-aerating wastewater before grit removal?

Improve grit removal efficiency

Freshen wastewater

Remove gases

Add oxygen

Promote flotation of grease

Aid coagulation

Primary sludges are usually ______ than secondary sludges

Denser

Types of sludge handling and disposal

Sludge digestion

Vacuum filtration

Filter presses

Incineration

Land disposal

Lagoons

Burial

Surface Loading (GPD/sq ft) =

Flow (gal/day) / Surface Area (sq ft)

Detention time (hr) =

[Tank Volume (cu ft) x 7.48 gal/cu ft x 24 hr/day] / Flow (gal/day)

Recommended analyses for a primary clarifier

Settleable solids

Suspended solids

BOD

DO (effluent only)

pH

Temperature (influent only)

Ideal velocity in a primary clarifier

1.0-2.0 ft/min

Factors that influence settling characteristics in a primary clarifier

Temperature

Short circuits

Detention time

Weir overflow rate

Surface loading rate

Solids loading

What is the importance of knowing the weir overflow rate in a clarifier?

To prevent short circuits or high velocity near the weir, which might pull settling solids into the effluent

Weir Overflow Rate (GPD/ft) =

Flow (GPD) / Length of Weir (ft)

Recommended weir overflow rate for a primary clarifier

10,000-20,000 GPD/linear ft of weir

Recommended surface loading rate in a clarifier

300-1200 GPD/sq ft

Solids loading (lbs/day/sq ft) =

[Flow (MGD) x MLSS (mg/L) x 8.34 lbs/gal] / Surface Area (sq ft)

Typical solids loading in a secondary clarifier

12-30 lbs/day/sq ft

Rationale for secondary clarifiers

Biological treatment processes convert additional solids to the settleable form and also produce floc containing microorganisms

The sludge volume in the secondary tank will be _____ from the activated sludge process than from the trickling filter process

Greater

Typical detention time for a secondary clarifier

2-3 hrs

Typical weir overflow rate for a secondary clarifier

5000-15000 GPD/lineal ft

Typical return sludge rates at an activated sludge plant

10-50% of plant inflow

Recommended analyses for a secondary clarifier

Suspended solids

Turbidity

DO

pH

BOD

Ammonia Nitrogen

Sludge levels

What is a sedimentation-digestion unit?

A type of package treatment plant which combines a small clarifier over a sludge digester

Chemical added to wastewater to raise the pH in a digester

Lime or anhydrous ammonia

In a sedimentation-digestion unit, how often should you remove supernatant?

Twice a day for 15-30 min

In a sedimentation-digestion unit, how often should you remove sludge?

Start wasting when solids start to appear in the supernatant

Stop wasting when sludge starts to run thin

What type of dewatering process is typical for a sedimentation-digestion unit?

Sand drying beds

Ideal temperature range for a digester

80-95 degrees F

Recommended analyses for a sedimentation-digestion unit

Inflow

Suspended solids

Settleable solids

BOD (optional)

Temperature

pH

Quantity of sludge withdrawn

Chlorine residual (effluent)

What is an Imhoff tank?

A conical tank that combines sedimentation and digestion in the same unit

Typical detention time in a septic tank

12-24 hrs

What are colloids?

Small, finely divided solids that remain dispersed in a liquid for a long time due to their size and electrical charge

Describe nitrification

An aerobic process

Bacteria oxidize ammonia and organic nitrogen in wastewater into nitrate

First stage BOD is called carbonaceous BOD

Second stage BOD is called nitrogenous BOD

Describe denitrification

Anoxic biological reduction of nitrate/nitrate nitrogen to nitrogen gas

Nitrogen bubbles attach to the floc and float

Common cause of rising sludge in secondary clarifiers

What is ponding?

A condition occurring on trickling filters

When hollow spaces become plugged

Water is unable to pass through the filter media

Result of excessive slime growth, trash, or media breakdown

Typical suspended solids and BOD removal efficiency of secondary treatment

90% or more

List the components of a trickling filter

Media

Underdrain system

Distribution system

Typical size of rock media in a trickling filter

2-4 in wide

Uniform in size

3-8 ft deep

Void space is 35% of filter media

Typical size of synthetic media in a trickling filter

15-30 ft deep

Void space is 95% of filter volume

Advantages of recirculation in a trickling filter

Increased efficiency

Dilutes strength of wastewater

Prevents excessive buildup of slime growths

Helps seed lower portions of filter with active organisms

Helps keep biofilm healthy during low flow

Addition of DO

Controls flies

Advantage of synthetic media

Considerably more surface area per unit of dead space for slime to accumulate

What effect does warm temperature have on a trickling filter?

Organisms are more active

Higher loadings

Greater efficiency is possible

What is hydraulic loading?

The total volume of liquid (including recirculation) applied to a filter

Expressed as GPD/sq ft

What is organic loading?

The weight of BOD applied per day per 1000 cubic feet of filter media

Expressed as lbs BOD/day/1000 cu ft

Typical hydraulic loading of a standard-rate trickling filter

25-100 GPD/sq ft

Typical organic loading of a standard-rate trickling filter

5-25 lbs BOD/day/1000 cu ft

What is a roughing filter?

A high-rate trickling filter that receives a very high organic loading (100-300 lbs/day/1000 cu ft)

What should you check before starting a trickling filter?

Replace packing grease of filter bearings

Correct oil is used

Distributor arms are level

No foreign objects in fixed-spray nozzles

No debris in underdrain system

Valves operate smoothly

Why should you record the speed of rotation of a trickling filter distributor arm during initial start-up?

The distributor arm speed is proportional to the influent flow rate

Provides a reference for daily inspection and operation of distributor arm

Typical time for slime to build up on trickling filter media

Several days for slime growth to start

Several weeks for fully developed slime

How can you speed up the development of slime growth on a trickling filter?

Recirculate wastewater prior to treating the main wastewater flow stream

Add waste activated sludge

Recommended daily observations in a trickling filter plant

Indications of ponding

Filter flies

Odors

Plugged orifices

Roughness/vibration of distributor arms

Leakage past the distributor turntable seal

Splash beyond filter media

Cleanup of slimes not on media

Where should recirculation take place at a trickling filter plant and why?

Primary clarifier

Adds oxygen to incoming waste-stream

Prevents odors

Recommended analyses for a trickling filter

DO (final effluent & filter underflow)

Settleable solids

pH

Temperature

BOD

Suspended solids

Chlorine residual (final effluent)

Coliform bacteria (final effluent)

Clarity (final effluent)

Advantage of performing COD analyses over BOD analyses

4 hrs to obtain results vs 5 days

Nutrient requirements of a trickling filter

For each incoming 100 lbs of BOD, there should be 5 lbs of ammonia nitrogen and 1 lb of orthophosphate

Advantages of a trickling filter

Less operating attention

Shock loads are less frequent because recovery is faster

Operational challenges of using a trickling filter

Ponding

Odors

Flies

Freezing

Major operational consideration/strategy at a trickling filter plant

Use the lowest possible recirculation rate that will produce good results

Operational criteria for a trickling filter plant

Hydraulic loading rate

Organic loading rate

Recommended DO concentration of trickling filter effluent

3-6 mg/L (rock)

4-8 mg/L (synthetic)