Sewage Treatment

Wastewater TreatmentUtilizes a variety of strategies:

•Physical (mechanical)

•Biological

•ChemicalThe federal government requires the monitoring of sewage discharge and sets limits on pollutant levels:

Clean Water Act (‘72, ‘77, ‘87)

Primary treatmentPollutants

• Large objects: rocks, gravel, twigs, litter, female hygiene products, big food scraps, condoms, etc.

• Smaller: sand, grit, human or animal fecal material

• Even smaller (& water insoluble): fats and grease

Removal methods• Filtration, screening, sieving, nets

• Inflow is slowed in a settling tank. Solids settle to the bottom and are collected (sludge).

• Allowed to float to surface in settling tank and are then skimmed off (sludge).

Secondary treatment

Pollutants

• Dissolved/suspended organics substances (eg fecal matter, soaps, detergents, food particles)

Removal methods• Effluent is pumped into

aeration tanks where aerobic microorganisms break down the organic matter

• There is secondary settling and filtration.

• This sludge is pumped into an enclosed tank with anaerobic decomposers.

Advanced secondary treatment(sometimes called tertiary)Pollutants

• Phosphate

• Nitrates/ammonia

Removal methods• biological removal, chemical binders

• Denitrifying bacteria in anaerobic tank

Final step: Disinfection• Usually done with chlorine which kills pathogens (eg E. coli and other coliform bacteria, Giardia, Cholera, viruses)– What is the negative effect of this?

• Sometimes done with UV radiation or ozone

Sewage treatment summary• Multi-staged approach to cleaning up waste water

• Utilizes mechanical, biological, and chemical strategies

• Products are– Clean(er) water– Sludge – Solid waste (from primary treatment filters)

• A last step includes disinfection with chlorine (for fecal coliform and other harmful bacteria)

The solid waste of the waste

•Sent to already overtaxed landfills

•May contain toxins, heavy metals, POPs, etc that could potentially contaminate groundwater through leaching

•Contributes to the production of methane during decomposition

The sludge waste

• Often used as fertilizer• If not properly disinfected, could contaminate soils with bacteria

• May contain toxins, heavy metals, POPs that could contaminate groundwater, soil, and plants

• This could lead to an accumulation of heavy metals in food chains

• Field runoff could result in surface water contamination, leading to our old friend eutrophication if high in nitrates or phosphates.

OdorsOdors may cause illness or indicate presence of harmful gases

Dust ParticlesParticles of dried sludge carry viruses and harmful bacteria that can be inhaled, infect cuts or enter homes.

ExposureChildren may walk or play in fertilized fields.

Livestock PoisoningCows may die after grazing on sludge-treated fields.

Surface RunoffHarmful chemicals and pathogens may pollute nearby streams, lakes, ponds, and wetlands.

GroundwaterContaminationHarmful chemicals and pathogens may leach into groundwater and shallow wells.

Sludge

Sewage sludge as fertilizer

(1) Raw sewage drains by gravity into the first pool and flows through a long perforated PVC pipe into a bed of limestone gravel.

(3) Wastewater flows through another perforated pipe into a second pool, where the same process is repeated.

(2) Microbes in the limestone gravel break down the sewage into chemicals that can be absorbed by the plant roots, and the gravel absorbs phosphorus.

(4) Treated water flowing from the second pool is nearly free of bacteria and plant nutrients. Treated water can be recycled for irrigation and flushing toilets.

45 centimeterlayer of limestonegravel coated with

decomposing bacteriaFirst concrete pool Second concrete pool

Sewage

Wetland typeplants

Wetland typeplants

Treatedwater

Constructing Wetlands to Treat Sewage

© Brooks/Cole Publishing Company / ITP

Household Wastewater: Septic Tanks

trap greases & large solids & discharge

the remaining wastes over drainage field