d2: sewage treatment

what is sewage?

wastewater from inside or around homes

why is treating sewage important?

removes biologically active portions of our waste

what are the problems with not treating sewage?

disease: promotes the spread of human disease

eutrophication: too many nutrients (algal blooms)

what is eutrophication?

large amounts of organic material are introduced into an ecosystem

ecosystems can drastically change in ways we don’t want to happen because microbes can take advantage of the overflow of nutrients

what happens during primary treatment? what is sludge?

physical removal of large solids

grinding, screening, skimming

sludge = large stuff that settles

removes about 1/3 of the biologically degradable material

what is BOD?

biochemical oxygen demand = measure of remaining material

how much demand on oxygen will this material cause if you release it into the ecosystem

how much BOD do we want to have?

0!

what happens during secondary treatment?

biological step

bacteria aerobically consume most of the remaining BOD

starter cultures of activated sludge are used to start new tanks

what happens after secondary treatment? is the water ready to be released?

if released into a water supply → chlorination only

what treatments are needed to make sewage water potentially safe for drinking?

membrane filtration

ozone treatment

what is tertiary treatment?

additional removal of BOD in man-made lagoons or wetlands

• create a wetland environment

• grow plants that are good at filtering sewage

employed in treatment of liquid wastes and for bioremediation

what is sludge digestion? is is aerobic or anerobic?

sludge is collected in anaerobic sludge digesters

• packed with anaerobic bacteria, usually thermophilic (55℃)

• generate heat from metabolism: kills some microbes

methane gas produced

• often used to heat the system, or even to power the sewage plant

what’s done with the final, treated sludge?

stabilized sludge is disposed of in landfills

what is a septic tank?

used by homes that aren’t part of a larger sewage treatment system

describe what happens in a septic tank

act as primary and secondary sewage treatment facilities

seeded with activated sludge → chew up BOD

relatively clear water will go through pipes into a leaching field

what is a leaching field?

area of land where water leaches out into the soil

why is flushing bactericidal chemicals like bleach down the toilet a potentially bad idea for houses with a septic system?

bleach can kill the activated sludge

wrong kind of microbes can infect the septic tank