Waste Water Treatment

waste water treatment: sewage

• essential societal service (safe drinking and sanitation)

• ~35 billion gal/day treated among 16,000 public WWTPs (not ag or industrial)

  • serves ~75% of US population

• effluent → waterbody or suitably reused; sludge

  • many overflows yearly; ~10 billion gals untreated

• sanitary sewer vs storm sewer; some areas combined

  • drawback: heavy rain events → sewage into waterbody

effluent

waterbody or suitably reused

similarities of stream and WWTPs processes

both “natural” processes: physical and biological purification

streams

settling — microbial activity — sunlight

WWTPs

tanks — aeration — UV light + chlorine

• separates solids (sludge)

• safer, faster, more efficient, human-generated waste

preliminary and primary treatment

raw sewage (influent) solds screened to ¼ in

• heavy solids separated (comminutor/grit chamber)

• sand and grease removed

coagulants and flocculants ($) may be added; primary clarifiers

primary clarifiers

effluent to aeration tank or primary sludge

secondary treatment

biological filtration: microorganisms + aeration ($) to metabolize organic matter + N & P

• aeration tanks

• secondary clarifier

secondary clarifier

in/organic matter settles; activated sludge decanted

• returned (“buffet size”) or disposed of (sludge)

tertiary treatment (not all WWTPs)

chemical: increase water quality for discharge/reuse

• disinfect ($)

• generally, not suitable for drinking w/o additional filtration

disinfect in tertiary treatment

chlorine + sulfur dioxide ($), UV lights ($); ozone

• removes (kills) pathogens (~E. coli)

• high chlorine toxic to aquatic life

• diffuses into TN river

sludge treatment and disposal

• primary and secondary clarifiers moved to biosolids → heated @500F (microbial), dried; kills (most) pathogens…”compost pile”

  • class A biosolid (heavy metals monitored)

• agricultural or other uses (cotton farms — saves them $)

• microplastics,PFAS, etc. persist; can bioaccumulate

septic tanks: rural areas not on city sewer

• primary treatment only; microbial action in tank

  • moderate efficiency (anaerobic)

• sludge builds up over time; pumped every 3-5 years

• set-up and drain field determined by Perc Test

  • soil in drain field filters and treats liquid (contamination)

Perc (Percolation) Test

required for properties w/o municipal sewer line

• determines how quickly soil absorbs water

impacts results of Perc Test

soil type, water table, topography

steps for Perc Test

dig hole / fill with water / measure min per 1” drop (MPI)

<1 on Perc Test

too permeable; (sandy) soil — contaminate groundwater

1 - 60 on Perc Test

acceptable

>60 on Perc Test

too impermeable; (clay) soil drains slow — waste backs up in yard