Wastewater

Treatment

Municipal WW Management Systems

Sources of

WastewaterProcessing at

the SourceWastewater

Collection

Transmission

and PumpingTreatment Reuse/Disposal

Sources & Types of WW

Domestic Wastewater

from residences, commercial & institutional

flows ~70-90% of water supplied

Industrial Wastewater

highly dependent on industry

Infliltration/Inflow

enters through leaks, foundation drains, etc.

Stormwater

for combined sewers - largely in older cities

Typical WW Characteristics

Parameter Conc.BOD 250 mg/L

TSS 250 mg/L

COD 500 mg/L

Ammonia 30 mg/L

TOC 100 mg/L

Chloride + 50 mg/L

1972: Federal Water Pollution Control Act

PL 92-500 subsequently amended and now

called the Clean Water Act

established water quality goals fishable &

swimmable and timetable

established National Pollution Discharge

Elimination System (NPDES)

construction grants for WW treatment

required secondary treatment (30/30)

30 mg/L BOD5

30 mg/L TSS

Conventional WW Treatment

Biological ProcessPreliminary

Treatment

Secondary

Sedimentation

Sludge

Disinfection

Primary

Sedimentation

Sludge

Preliminary Treatment

To remove materials that will interfere with

subsequent treatment

Coarse Screening (bar racks)

Medium Screening

Comminution

Flow measuring

Pumping

Grit removal

Pre-aeration

Bar Racks

Metal bars spaced a few cm apart across

water flow

mechanical or manually cleaned

size of unit set by approach velocity

0.6-1.0 m/s for mechanically cleaned

0.3-0.7 m/s for manually cleaned

see Fig 10.4 and example 10.2 on pg. 311

Grit Removal

Grit chambers intended to remove sand,

cinders, gravel that may enter system by

cracks in pipes, inflow etc.

Grit can cause excess wear in pipes and

pumps

small sedimentation tanks; designed with

the help of Stokes Law

no organics removal

Grit Chamber

Primary Sedimentation

Purpose: to remove suspended solids

(smaller than grit, and less harmful)

Typical efficiency

67% TSS removal

33% BOD removal

Design parameters

overflow rate

weir loading rate

detention time

Primary effluent is

largely composed of

soluble and colloidal

organics which can be

converted to settleable

microbial solids and

CO2 by biological

treatment

Primary Sedimentation Primary Treatment

Removes ~50% of suspended solids

Parameter Design Range Typical

ValueOverflow

Rate

35-45 m/d

800-1200 gal/ft2/d

40 m/d

1000 gal/ft2/d

Detention

Time

1.5-2.5 h 2 h

Weir loading

rate

125-500 m2/d

10,000-40,000 gal/ft/d

275 m2/d

20,000 gal/ft/d

Primary Sed. Tank

Primary Clarifier: Center Feed

Secondary Treatment

Generally includes some biological process

plus secondary clarification

Required under PL92-500

Converts soluble and colloidal organic

materials to biomass and CO2

Biological Treatment

Suspended Growth

Activated Sludge

Conventional, Extended Aeration, Contact

Stabilization

Aerated lagoons

Aerobic digestion

Attached Growth

Trickling Filters

Rotating Biological Contactors

Suspended Growth Systems

Sludge

Secondary

Sedimentation

Return Activated Sludge

Waste

Activated Sludge

Aeration Tank

Air

Attached Growth: Trickling Filters

Rocks or

Plastic

Media

Underdrain

Rotary Distributor

With rocks, depth is limited to 2-3 m because of oxygen needs

Air

Rotating Biological Contactors

Drum diameters are typically 10-12 ft.

Rotation speed: ~1.5 rpm

May be in several stages

No flow recycle

Requires piloting

Secondary Clarifier

Sludge Disposal

Thickening

gravity, flotation

Digestion

aerobic, anaerobic

Mechanical Dewatering

Vacuum filtration, centrifugation, pressure filtr.

Disposal

land application, burial, incineration

Anaerobic Digestion

Sludge held without aeration for 10-90 days

Process can be accelerated by heating to 35-40oC

These are called High Rate Digestors (10-20 days)

Advantages

low solids production

useable methane gas produced

Disadvantages

high capital costs

susceptibility to shocks and overloads

Sludge Dewatering

Sludge drying beds

historically the most common

sand bed, 15-30 days, evaporation & seepage

Vacuum Filtration

cylindrical rotating drum covered with fabric

submerged with applied vacuum

Continuous belt filter presses (follows)

Plate pressure filters

vertical plates mounted on a frame

Belt Filter Press (Komline-Sanderson)