Dissolved Oxygen Sag Model for Discharge to Streams

Question 1

What decreases and increases O2 in stream?

Answer 1

• O2 in air increases
• BOD decreases O2

Question 2

Overall Oxygen Deficit (D) over time (Streeter-Phelps Model)

Answer 2

rate of oxygen consumption (basic microbial biodegradation) minus rate of reaeration from atmosphere

Question 3

Is there more oxygen deficit in winter or summer?

Answer 3

More oxygen dissolves in cold water, so more oxygen deficiency in summer

Question 4

Extrapolations to basic Streeter-Phelps Model

Answer 4

• photosynthesis, respiration
• sludge deposits
• nitrogenous biodegradation (need to remove NOD by nitrification)
• particulate BOD (wood dust)

Question 5

Solutions to DO Deficit Problems

Answer 5

1. Conventional WWTP
   - suspended growth
   - attached growth
2. Wetlands
   - natural
   - engineered
3. In stream engineering

Question 6

Why use wetlands for wastewater treatment?

Answer 6

• natural/green process
• relatively simple engineering
• relatively cheap (low installation cost little manitenenace)
• possible agricultural benefits
• ecological benefits (eg biodiversity)
• most possible for small systems (farms, small factories etc)