1. Domestic Waste Water Treatment

2. Composition of domestic sewage
Human feces and urine
g of feces and liters of urine/person/day
“graywater”
Water from sinks, bathtubs, yard sprinklers
Assessment of amount of organic matter in sewage
Total organic carbon (TOC)
Biochemical oxygen demand (BOD)
Chemical oxygen demand (COD)

3. Objective of wastewater treatment
To reduce BOD
BOD is the amount of dissolved oxygen consumed by microorganisms during the biochemical oxidation of organic and inorganic matter to carbon dioxide
5-day BOD test
Purpose of BOD test
Determine amount of oxygen required to oxidize the organic matter in the wastewater
Determine size of treatment system needed
Assess the efficiency of the treatment process
Determine compliance with wastewater discharge permits

4. 5-day BOD test P is decimal volumetric fraction
BOD (mg/l) = __________
D0 – D5 P
P is decimal volumetric fraction
of wastewater used in test
D is dissolved oxygen
concentration at Time=0
and Time = 5 days

5. Modern Wastewater Treatment
Primary treatment
Separation of large debris following sedimentation
Gravel, sand, twigs. leaves
Grit Chamber
Primary clarifiers
Bar Screen
Primary
Settling
Tank
Anaerobic sludge
digestor
To land
application
sludge

6. Primary Clarifiers Separate liquids from solids
Skimmer removes grease at the surface and sends it to anaerobic digester

7. Secondary treatment: Aerobic
Activated sludge process
Trickling filters
Rotating discs
Rotating drums
Oxidation Ponds

8. Activated Sludge Process
Remaining suspended solids are decomposed and number of pathogens are reduced
Final
settling tank
or clarifier
Aeration tank or
Trickling filter
Primary
settling
tank
sludge
To anaerobic
sludge digester
Sludge
digester
Gravity thickener plant
1% 6% solids content
Land application

9. Activated Sludge Process (aerobic microbial metabolism)
Mixed Liquor Suspended Solids (MLSS)
Air is pumped through the wastewater
Sludge is removed from the bottom and sent to the anaerobic sludge digestor
Some of the sludge is used to inoculate the fresh, incoming wastewater entering the aeration tank
Q = flow rate of sewage in millions of
gallons per day (MGD)
MLSS is in mg/l
V is volume of aeration tank (gallons)
Q x BOD
Food/Microbes
Ratio
= _____________
MLSS x V

10. Trickling Filters Not a true filtering or sieving process
Material only provides surface on which bacteria to grow
Can use plastic media
lighter - can get deeper beds (up to 12 m)
reduced space requirement
larger surface area for growth
greater void ratios (better air flow)
less prone to plugging by accumulating slime

11. Trickling Filter Porous media Microbial biofilm Organic matter
CO2 + H2O

12. Rotating Biological Contactors
Called RBCs
Consists of series of closely spaced discs mounted on a horizontal shaft and rotated while ~40% of each disc is submerged in wastewater
Discs: light-weight plastic
Slime is 1-3 mm in thickness on disc

13. Rotating Biological Contactors

14. Aerobic ponds Shallow ponds (<1 m deep) Light penetrates to bottom
Active algal photosynthesis
Organic matter con- verted to CO2, NO3-, HSO4-, HPO42-, etc.

15. Secondary treatment: Anaerobic
Anaerobic digestion
Anaerobic Up flow filters (AUF)
Anaerobic filters

16. Anaerobic Sludge Digester
CH4
Energy to run plant
Maintain temp at 37C
30-day retention time
Kills pathogens
Produces methane
used to run facility
from settling
tanks
sludge
Gravity thickener plant
1% 6% solids content

17. Tertiary treatment
Involves a series of steps to further reduce organic concentration, turbidity, N, P, metals, and pathogens
Settling tank
Sand or mixed
media filter
Disinfection
tank
Discharge to
environment
filters out
protozoans
& pathogenic
bacteria
Sludge
digestor