Wastewater Treatment Flashcards
On-site waste water treatment
- ~25% of population uses this
- called conventional septic system
- - septic tank
- - leach field
Wastewater collection and treatment system
To remove contaminants (e.g. BOD, nutrients) before effluent release to environment
*Regulation: Clean Water Act
Septic tank
- solids settle, undergo anaerobic digestion (without O2)
* excess liquid drains into leach fields
Leach fields
- organics & inorganics in liquid are degraded in soil
* liquid is lost through various processes e.g. Evaporation, plant uptake
Municipal waste water treatment
1) pretreatment
2) primary treatment process
3) secondary treatment process
Pretreatment
Removes material that may cause operational problems
1) bar screens
2) grit chambers
3) equalization
Bar screens
To remove larger objects, material is sent to a landfill
Grit chambers
To remove inert material e.g. Glass, sand
Equalization
To dampen variation in waste water flow into a WWTP
- collects waste water in large basins
- it’s then pumped to WWTP at a constant rate
Primary treatment
- removes wastes in a settling tank or primary clarifier
- waste water is held in a tank for several hours
- particles settle to the bottom and can be removed by mechanical scrapers
- greases and oils rise to surface and are skimmed from the top
Design parameters for Primary Treatment
- overflow rate (flow rate m^3/ area m^2)
* detention time (volume m^3/ flow rate m^3/hr)
Secondary treatment
- microorganisms degrade the organic material
- BOD & SS removal beyond what’s achieved in primary treatment
- requires healthy microorganisms
Healthy microorganisms w/ in secondary treatment
1) high O2 levels
2) favorable pH range
3) no toxic chemicals
Dispersed or suspended growth
- microorganisms suspended in liquid waste water
* e.g. Activated sludge, oxidation ponds
Activated sludge
- secondary treatment - suspended
- in aeration tank microorganisms consume & oxidize waste, “activated” by O2, waste water flows to settling tank, settled material is returned to aeration tank or removed from system
Fixed or attached growth
- microorganisms grow on a surface
* e.g. Trickling filters, Rotating Biological Contactors RBCs
Trickling Filters
- secondary treatment - attached
* microorganisms grow on the surface of “filter media” (rocks or plastic)
Trickling Filter process
- rotating arm sprays primary effluent over filter media
- biofilm (layer of microorganisms) develops of filter media
- microbes degrade waste as it passes over them
Rotating Biological Contactors (RBCs)
*secondary treatment - attached
- series of discs (~3m diameter)
- half of each disc is submerged
- discs rotate
- biofilm forms on discs
Phosphorus removal
- excess phosphorus can cause eutrophication
- removal accomplished with chemical precipitation
- aluminum hydrate or lime
Nitrogen removal
N can be present in various forms in waste water (NH3, NO3-, NO2-) and can cause problems in receiving water
Biological method in Nitrogen Removal
- nitrification: ammonium (NH4+) to nitrate (NO3-)
* denitrification: nitrate (NO3-) to nitrogen gas (N2)
Chemical method in Nitrogen Removal- Ammonia Stripping
- increase in pH
- this converts ammonium ions (NH4+) to ammonia gas (NH3)
- the water is purged to remove NH3 from water
Final treatment
Disinfection- commonly by the addition of chlorine gas
Sludge treatment
- Final disposal of biosolids/ sludge
* treated sludge or can be disposed in landfills or can be land applied
