Roman latrene

Aerial photo of municipal wastewater treatment plant

Wastewater treatment

Empty Raceway

Barscreen (animation)

Aerial photo of primary treatment tanks

Primary treatment tank (animation)

Aeration tanks, secondary treatment (animation)

Clarifier (animation)

Aerial photo of clarifier

Chlorine tank

Anaerobic digestor

Filter Press for drying sludge

Leach Field

Septic landform

Septic tank

Septic trench

Q2C: What do we want to come out of the plant? And where does it go?
the water from a treatment plant is discharge into natural bodies of water, like the St. Joseph river
we want the water to be safe for humans - no toxins and no pathogenic (disease-causing) microbes
for the environment we want it to be low in nutrients like fixed C and fixed N (especially ammonia
Remember that too much fixed N in lake and streams (like that from farm field runoff) will cause a bloom of autotrophic algae, thus adding lots of fixed carbon to the system, increasing respiration to the point that the lake becomes anaerobic, and the fish are killed

Municipal wastewater treatment plants
what followed was a long description of each stage of a treatment plant - only the highlights will be reported here
primary treatment uses screens to strain out the large particulates (toys, cigarette butts etc)
these materials are collected and landfilled
the wastewater is kept moving rapidly at this point to keep suspended solids from settling
the primary treatment tank is large and wide and the waters slow down
particulates like sand and grit settle to the bottom and are scraped out of the tank mechanically
this primary sludge goes to the anaerobic digesters
soapy detergent and fatty materials float to the top forming a scum that is collected and incinerated or landfilled
the wastewater then moves into the secondary treatment raceway
here microbes take over and degrade the wastes aerobically
these tanks are highly aerated and froth
the microbes turn the complex organics into CO₂
ammonia-like materials are oxidized to nitrate NO₃
the microbes in the aerobic digesters are associated in flocs - clumps of microbes
these represent mixed microbial communities
microcolonies (small clumps) of various bacteria are held onto long filamentous bacteria through a gluey slime (extracellular matrix)
taken together, these flocs are called activated sludge
substantial amounts of biomass are created in these aerobic digesters
clarifiers are funnel-shaped devices that are used to settle out the biomass
the clarifiers allow the flocs to flocculate (settle out)
this material is collected and called secondary sludge
the secondary sludge is directed to the anaerobic digesters
the wastewater is now essentially free of organics and low in ammonia
the water is then treated with chlorine gas to kill any remaining pathogenic microbes
the chlorine gas is allowed to dissipate
the water is discharged to the rive or ocean
the sludges are moved into sealed tanks for anaerobic digestion
here organics are converted through reduction into CH₄ (methane) rather than oxidized to CO₂
the methane is collected and used or sold as fuel. sometimes it is just burned off
the final sludge is collected from the anaerobic digesters and squeeze dried
this material can then be used as compost for agricultural fields

Lagoons
here wastewater is directed into a man-made pond
the pond is not aerated and thus gets stratified
the top layer is aerobic and often has algae
the bottom layer is anaerobic
this simulated the municipal plants
wastes are first oxidized to CO₂
the inevitable biomass produced settles into the anaerobic phase and is further digested
many industrial plants use the lagoon system to treat their wastes

Septic systems
here the anaerobic and aerobic phases are reversed
wastes flow into an underground anaerobic digestor
sludge settles on the bottom and reduction reactions produce methane
the wastewater is then distributed through pipes into a leach field and deposited in the soil
here indigenous soil microbes both anaerobic and aerobic digest the wastes
made a point of showing that the septic system reading had a major mistake
remember that conversion of ammonia to nitrate (nitrification) is an aerobic process
and that saturating soil with water will make the soil anaerobic
thus, heavy rains will inhibit the aerobic process of nitrification