8.11 - Sewage treatment

Question 1

Water Treatment Process

Answer 1

1. Primary Treatment:
   - Physical removal of large debris (TP, leaves, plastic, sediment) with a screen or grate
2. Secondary Treatment:
   - Biological breakdown of organic matter (feces) by bacteria; aerobic process that requires O2
3. Tertiary Treatment:
   - Ecological or chemical treatments to reduce pollutants left after primary & secondary (N, P, bacteria)
   Also 3. Disinfectant:
   - UV light, ozone, or chlorine is used to kill bacteria or other pathogens, such as e. Coli (considered part of 3)

• Effluent: liquid waste (sewage) discharged into a surface body of water, typically from a wastewater treatment plant

Question 2

Primary and Secondary Treatment

Answer 2

Primary:
- Screens or grates filter our large solids
- grit chamber allows sediment to settle out and be removed

Secondary:
- O2 is bubblers into aeration tank filled with bacteria that break down organic matter into CO2 and nutrients like N and P
- Secondary treatment removed 70% of P and 50% of N
- Does not removed POPS such as medications or presticides

• Sludge: inorganic, solid waste that collects at the bottom of tanks in primary and secondary treatment
• Water is spun/pumped off to concentrate it further
  Dry, remaining physical waste is collected to be put in landfill, burned, or turned into fertilizer pellets
• After primary & secondary treatment, some plants go directly to disinfectant (UV, ozone, chlorine) & discharge into surface water, while some will use tertiary treatment to remove more nutrients before discharge

Question 3

Tertiary Treatment

Answer 3

• uses chemical filters to remove more of the nitrates & phosphates from secondary treatment discharge
• Critical step because effluent that is discharged into surface waters with elevated nitrate/phosphate levels leads to eutrophication
• Expensive and not always used

Question 4

Sewage Treatment Issues

Answer 4

• Combined sewage and stormwater runoff systems can cause wastewater treatment plants to flood during heavy rains, releasing raw sewage into surface waters
• Beneficial b/c it treats stormwater runoff normally, but causes overflow during heavy rains
• Raw sewage release contaminates surface waters with:
  - E. coli
  - Ammonia
  - Nitrates
  - Phosphates
  - Endocrine disruptors (medications)
• Even treated wastewater effluent released into surface water often has elevated N/P levels and endocrine disruptors (medications passed through the body)