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Successful OWTS Treatment. Siting Site Evaluation System Location Design System Sizing System Selection and Design Installation Operation/Maintenance. Failure. A condition that threatens public health by failure to adequately treat sewage or creating potential for direct contact

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successful owts treatment
Successful OWTS Treatment
  • Siting
    • Site Evaluation
    • System Location
  • Design
    • System Sizing
    • System Selection and Design
  • Installation
  • Operation/Maintenance
  • A condition that threatens public health by failure to adequately treat sewage or creating potential for direct contact
  • Examples:
    • Pooling on ground surface
    • Back up into structure
    • Leaking tanks, pump chamber or collection system
    • Impacted water quality (surface or ground)
factors affecting owts performance
Factors Affecting OWTS Performance
  • Soil system performs multiple tasks in typical OWTS
    • Treat the water to remove contaminants
    • Dispose of treated water
  • Factors that affect these tasks

Soil Wastewater Characteristics

Loading Rates Users’ Lifestyle

O& M Temperature

Rainfall Surrounding Development

  • Texture
  • Structure
  • Depth
  • Compaction
  • Landscape Position
soil horizons soil properties according to depth
Soil Horizons: Soil Properties According to Depth
  • Distinct soil horizons or layers; form from weathering processes
  • Layers have distinct chemical compositions; determines:
    • amounts and state of organic matter
    • amounts of nutrient elements
  • Each layer supports varying amounts and types of microbial communities
    • Surface layers of soils (O layers) are organic
    • Dominated by organic matter (e.g. leaves, twigs, etc.) (= O1 layer)
    • Dominated by unrecognizable organic matter in next lower layer
      • some decomposition has occurred (O2 layer)
    • Sub-surface soil layers (A layers): various combinations of organic and mineral materials which experience increasing amounts of leaching (= eluvial layers)
    • Lower layers (B layers): experience leaching and horizontal movement of materials (= illuvial layers)
    • Lowest soil layers (C layer) experience least weathering; in contact with bedrock
loading rates
Loading Rates
  • Hydraulic overloading is one of leading causes of OWTS failure
  • Design flow typically based on number of people in house (assumption generally 2 per bedroom)
    • 60 gpd/person (common assumption in design)
  • Loading rates determined by type of soil
loading rates1
Loading Rates
  • Clogging Mat
  • Gravitational potential
  • Matric Potential
users lifestyle
Users’ Lifestyle
  • Low-flow devices (toilet, faucets, shower heads, etc.)
  • Rainfall collection
  • Use of garbage disposals
  • Excessive use of FOG
  • Laundry habits
  • Time in Residence
wastewater characteristics
Wastewater Characteristics
  • BOD5 (above 230mg/L will reduce life of system and level of treatment)
  • Biologically active chemicals (bleach, antibiotics, etc.)
  • FOG
  • Other chemicals (Cleaners, solvents, degreasers, etc.)
operation and maintenance o m
Operation and Maintenance (O&M)
  • Pump Tanks every 3 to 5 years (plan on every 4)
    • Actual time period should depend on active monitoring of system
  • Conduct at least biannual monitoring of tank levels, baffles, and drainfield
    • This is the minimum, more frequent monitoring recommended (required for advanced systems)
    • Pump tank when sludge layer thickness exceeds 25% of working liquid capacity of the tank, or if scum layer is within three inches of bottom of outlet baffle
temperature and rainfall
Temperature and Rainfall
  • Temperature
    • Affects flow and mixing in septic tank
    • Soil treatment relies on biological activity
    • Cold slows down biological processes
      • 50% loss in activity for 10ºC drop in temp
      • Activity effectively stops at 2 ºC
  • Rainfall
    • Additional hydraulic load on soil
    • Reduction in vertical separation
    • Benefit-dilute nitrates, however also increase transport rate
alternative waste treatment
Alternative Waste Treatment
  • Stabilization Ponds
  • Wetlands
  • Composting Toilets
  • Separation of Waste Streams
  • Greywater Sytstems
  • Primitive Systems
stabilization ponds or lagoons
Stabilization Ponds or Lagoons
  • Aerobic and Facultative Ponds:
  • Biologically Rx by complementary activity of algae and bacteria.
  • Used for raw sewage as well as primary‑ or secondary‑Rx’d. effluent.
  • Bacteria and other heterotrophs convert organic matter to carbon dioxide, inorganic nutrients, water and microbial biomass.
  • Algae use CO2 and inorganic nutrients, primarily N and P, in photosynthesis to produce oxygen and algal biomass.
stabilization ponds or lagoons1
Stabilization Ponds or Lagoons
  • Many different pond designs have been used to treat sewage:
  • facultative ponds: upper, aerobic zone and a lower anaerobic zone.
  • Aerobic heterotrophics and algae proliferate in the upper zone.
  • Biomass from upper zone settles into the anaerobic, bottom zone.
  • Bottom solids digested by anaerobic bacteria.
constructed wetlands and enteric microbe reductions
Constructed Wetlands and Enteric Microbe Reductions
  • Surface flow (SF) wetlands reduce enteric microbes by ~90%
  • Subsurface flow (SSF) wetlands reduce enteric microbes by ~99%
  • Greater reduction in SSF may be due to greater biological activity in wetland bed media (porous gravel) and longer retention times
  • Multiple wetlands in series incrementally increase microbial reductions, with 90-99% reduction per wetland cell.
primitive systems




Comfort station


Post office




White house




Bog House


Primitive Systems