Sewage Treatment

Exponential Growth
In reference to the test tube metaphor, species which exhibit this type of growth will reproduce geometrically until they have reached the limit of the resources present
E-folding Residence Time
The time interval in which an exponentially-growing quantity increases by a factor of e
Instantaneous Rate of Change
P=P0ekt  where P0 is initial concentration and P is concentration at time t

The period of time it takes for a substance undergoing decay to decrease by half (opposite of doubling time).

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Doubling Time/ Rule of 70
Doubling time equals 70/growth rate
Sewage Treatment Goals
Removal of suspended sediments, biological oxygen demand, pathogens, and nutrients (nitrogen and phosphorus)
Biochemical Oxygen Demand
A measure of the degree of organic pollution in the water– milligrams of O2 consumed by one liter of sample during 5 days of incubation at 20o Celsius
Primary Sewage Treatment

1. Inflow to course screening and grit removal

2. Primary clarifier

3. (Primary sludge to anaerobic digester)


At least 30% of BOD and SS must be removed at this stage

Secondary Sewage Treatment

1. Sludge from primary clarifier to activated sludge or trickling filter

2. Sludge to secondary clarifier

3. Sludge back to activated sludge/trickling filter, to chlorination, or to anaerobic digester

4. Chlorinated sludge to effluent


85% of BOD and SS must be removed at this stage

Grit Chamber
Remove heavy organic solids. Method: grit (sand, gravel, clay, metal filings) is heavier than the organic solids that should be kept in suspension for subsequent treatment. Centrifugal motion is used to separate the solids from the waste water.
Primary Clarifier
Tank with residence time of a few hours, collects primary sludge for anaerobic digester.
Trickling Filter
Fixed-film or attached growth systems– 70% of organic waste from effluent of primary clarifier is oxidized here. 15-20% is converted to biomass. Characterized by resilient microbial community. Lower sludge production and cheaper to operate once built.
Activated Sludge

A process for treating sewage using air and a biological floc composed of bacteria and protozoans.

Higher SS and BOD removal, cheaper to build.

Anaerobic Digester
Results in 1/3 sludge reduction by weight due to methane and CO2 production, anaerobic catabolism, and dewatering. Methane production can provide energy to heat digester to 27-35oC