Part 4: Wastewater Treatment Flashcards
How much has wastewater consumption increased between 1950 and the present?
85 litres per day to 200 litres per day
What are the relative costs of drinking water and sewage treatment?
Drinking water = £ 0.3 m-3
Sewage treatment = £ 0.7 m-3
What is the main aim of wastewater treatment?
To reduce nutrient concentrations of C, N and P to normal levels.
What is the composition of wastewater (excluding water)?
50% carbohydrates (mostly polymers)
40% protein and urea
10% lipids and their derivatives
How much urea does a person produce per day?
40g per day
What is BOD?
Biochemical oxygen demand - amount of dissolved oxygen needed by aerobes to break down organic material.
How is BOD measured in waste treatment plants.
BOD is measured after 5 days since this represents the first stage of degradation (organics).
What is the second phase of sewage breakdown after organics?
Urea breakdown (nitrification)
When must sewage be diluted?
If BOD becomes too high or too low.
- too high means too much organics
- too low means there are growth inhibiting compounds such as ammonia at harmful levels
What is the BOD of domestic wastewater?
200-300 mg O2 per litre
What is COD?
Chemical Oxygen Demand - BOD + additional compounds that are not microbially degraded.
How much greater than BOD is COD?
Approximately 2x greater.
How is COD measured?
Measured following acidifcation using a strong oxidising agent such as Potassium Dichromate.
What is stage 1 of the primary treatment process?
Mechanical filtering of waste. A mesh is used to retain large objects like plastic and also sand etc.
What is stage 2 of the primary treatment process?
Sewage is allowed to settle into fractions. Oily scum floats to the top, particulates sink to the bottom.
Why must the meat processing and other industries communicate with sewage treatment plants?
Such industries have especially high BOD-demand wastewater, and so have to notify the plant about any unusual flow.
What is secondary treatment of sewage?
Settling of waste in an aeration tank. Organics break down or settle and clear water can be removed.
Why is activated sludge aerated in secondary treatment?
Aerobic degradation is much more efficient than anaerobic. It also produces less acidity (acidity inhibits growth).
What important role do protozoa play in wastewater treatment?
Large numbers of protozoa graze on bacteria, degrading the bacterial population (including pathogens).
Under what conditions might the microbial community in an aeration basin collapse?
Inflow of toxic/inhibitory compounds .e.g acidity, ammonia
How do sewage managers cope with changes to the bacterial community?
Freeze-dried bacteria can be inoculated into the sewage mixture.
How are unusual bacterial assemblages coped with in sewage treatment?
Freeze-dried specialist bacteria are commercially available for halogenated compounds, aromatics etc.
What is tertiary treatment of sewage waste?
Final stage of treatment, may be:
- secondary sedimentation of organics
- phosphorous elimination
- disinfection (using UV light or chemical agents)
For what is anaerobic digestion used in a sewage treatment plant?
Anaerobic digestion reduces organic content in sludge.
