Wastewater –
Its Journey to Treatment and
Return to the Environment
ESI
Where does it all go!
Where does the
water from the
washer go?
When you flush the
toilet where does
the contents go?
By gravity flow, the waste is on its way
to your local wastewater treatment plant!
Why treat wastewater?
• Causes a demand for dissolved oxygen
(lower DO levels of streams)
• Adds nutrients (nitrate and phosphate)
to cause excessive growth
• Increases suspended solids or sediments
in streams (turbidity increase)
Sewagesheds
- an area where
sewage is collected
and delivered to a
WWPT
Parkway
WWTP
Bowie WWTP
Western
Branch
WWTP
Blue Plains
WWTP
Marlboro
Meadows
WWTP
Piscataway
WWTP
Mattawoman
WWTP
approximate
boundaries
A
Where do you locate the
WWTP in the watershed?
B
C
D
Consider the four possible sites.
The volume of wastewater
Capacity
WWTP
30
Average Flow
25
20
15
10
5
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million gallon/day
35
PGC 1992 Comprehensive Ten Year Water and Sewerage Plan
Levels of Treatment
Primary
– removal by physical separation of grit and large
objects (material to landfill for disposal)
Secondary
Mostly dead
microbes
– aerobic microbiological process (sludge)
organic matter + O2  CO2 + NH3 + H2O
aquatic nutrient
NH3  NO3- lowers suspended solids content (into sludge)
Aeration
and rapid
mixing
Settling
collects sludge
on bottom
From primary process
To tertiary process
air
diffuser
Secondary process
Levels of Treatment continued
Tertiary (advanced)
– anaerobic microbiological process with a
different microbe where O2 is toxic (more
sludge)
NO3-  N2 (escapes to atmosphere)
– PO4-3 if not removed in sludge in secondary
process
PO4-3 + Al+3  AlPO4 (s) (into sludge)
- aeration to strip N2 and re-oxygenate (add DO)
add methanol as food source
Slow mixing
to keep suspended
and O2 out
Settling
collects sludge
on bottom
Effluent
From secondary process
Tertiary process
When the treatment is done…
• Effluent back to stream after
– a final carbon filtration and
– chlorination/dechlorination
• Sludge – very nutrient rich
– applied directly to land as fertilizer
– incinerated (good fuel after drying)
– composted (Compro® from WSSC)
Note – Leafgro® is composted
leaves and grass from MES