Transcript Factors Affecting OWTS Performance

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
• Examples:
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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
• Soil system performs multiple tasks in typical
OWTS
– Treat the water to remove contaminants
– Dispose of treated water
• Factors that affect these tasks
Soil
Loading Rates
O& M
Rainfall
Wastewater Characteristics
Users’ Lifestyle
Temperature
Surrounding Development
Soil
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Texture
Structure
Depth
Compaction
Landscape Position
Soil Profiles – Typical Layers
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
• 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 Rates
• Clogging Mat
• Gravitational potential
• Matric Potential
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
• 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)
• 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
Septic Tank
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
• Stabilization Ponds
• Wetlands
• Composting Toilets
• Separation of Waste Streams
• Greywater Sytstems
• Primitive Systems
Facultative Oxidation (Waste Stabilization)
Pond
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 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
• 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
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Outhouses
Privy
Throne
Palace
Comfort station
Castle
Post office
Johnny
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Stool
Doolie
White house
Hut
WC
Ajax
Bog House
Defacatorium
Mr. Turdley Says“Don’t be a fecaphobe”