Lecture 6: Wastewater treatment 2 Flashcards
What are the different types of secondary treatment?
- trickling filters
- Rotating biological contractors
- Activated sludge treatment
- anaerobic digestion (also a type of sludge stabilisation - no more biological activity going on)
Leading to waste stabilisation ponds, soil aquifer treatment, and artificial & constructed wetlands
What is a trickling filter?
Type of aerobic attached-growth process (dev 1900s)
Watertight basins filled with rocks or highly permeable medium
wastewater trickled throguh medium and air diffuses up
microbes attach to medium to form a biofilm and degrade orgnaic matter in incoming water to reduce BOD
Second most common type of WWT
What are rotating biological containers (RBC)
type of aerobic attached-growth process
Circular disks (PVC) that are submered in the wastewater and rotated slowly
Biological growth forms a slime layer on disks, rotatin maintains biomass in aerobic condition
What is activated sludge wastewater
a type of aerobic suspended growth processes (suspended biofilm)
the most common form of wastewater treatment
What is the process of using ‘activated sludge wastewater’ as a secondary treatment?
Oxidation in the aeration tank; biosynthesis = increased activated sludge (feeback inoculum) and biodegeneration of CO2, H2O, NO3, SO4, PO4
What is a settling tank used for?
Removal of solids from treated wastewater following the aerobic suspended growth processes
How is sludge processed?
Sludge treatment and disposal is costly
sludge from primary and secondary processes contains ~1-5% solids
Treatment involves thickening and dewatering by centrifugion and filtration
How is sludge processed?
Sludge treatment and disposal is costly
sludge from primary and secondary processes contains ~1-5% solids
Treatment involves thickening and dewatering by centrifugion and filtration
results in solid content 20-40%
dewatering is followed by stabilisation and final disposal of sludge
What is sludge stabilisation
Purpose: breakdown organic fraction of the sludge to reduce its mass, odour and increase its safety
Process
1. anaerobic digestion
2. composting
3. potential heat stabilisation or lime stabilised for fetiliser use
4. incineration ( not environmentally friendly)
5. land fill
What happens during anaerobic digestion of sludge stabilisation?
1, series of microbiological processes that convert organic compounds to CH4 and CO2 and reduce volatile solids by 35-60%
- carried out by absence of O2 by bacteria and archaea
- uses CO2 as electron acceptor
what are the two types of anaerobic digestion in sludge stabilisation?
two-stage process: uses one tank for heating and mixing and another for thickening and storage
one-stage process: single tank, sludge digestion and settling odour at the same time
CH4 produced can be used to generate electricity
how does anaerobic digestion occur
Acidogenic bacteria carry out initial stage (prod of organic acids). Methanogenic bacteria convert acids or CO2 and H2 to methane (CH4). T
How can sludge by stabilised aerobically via composting?
Aerated static pile: sludge is mixed with eg woodchips and is aeratef for 20-30 days. Then cured for anther 30 days, dried and screened
Windrow: pile is turned and mixed during composting (30-60 days)
What is the first stage of tertiary treatment?
Disinfection
chlorination and UV treatment most common for secondary effluent disinfection (ozone also used)
What is UV disinfection?
wastewater is irradiated with UV light
Microbial inactivation by DNA damage
UV treatment is an effective (and expensive) method
