The Basics of Centralized Sewer Systems

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The Basics of Centralized Sewer Systems
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• Types of Systems--Sanitary Sewer System and a
  separate Stormwater Sewer System--Combined Sewer
  System What are the advantages and disadvantages
  of each?
• The Basic SystemLaterals ? Submains ? Trunk
  Sewers ? Treatment Plant Designed to flow
  half-full, at a slope of 0.5-2.0 (2 ft.ps),
  Who is responsible for these different
  pieces?Manhole covers Changes in direction,
  Confluence of major lines, and at specified
  intervals on straight linesGravity Mains vs.
  Force Mains Uses, Advantages, Disadvantages?
• Tallahassees Sewer SystemEstimated 675 miles in
  length (100 of FM), 15,000 manholes, 85 pumping
  stationsLake Bradford Road Wastewater Treatment
  Plant (1934) Capacity to treat 4 ½ million
  gpdThomas P. Smith Wastewater Treatment Plant
  (1966-1992) Capacity to treat 27.5 million gpd
  (Peak of 55 mil.)

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Wastewater Treatment
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• Major Elements to be RemovedSuspended solids,
  Biological oxygen demand (BOD), nitrogen,
  phosphorous, and colliform bacteria
• Major Stages of TreatmentPrimary Treatment
  (Reduce Solids 50-60, BOD 30-50) Bar
  screens, Comminutor, Grit chamber (Settling tank)
  Secondary Treatment (Solids 85-95, BOD 80-95,
  Col. 90-95) Trickling filter, Activated
  sludge, Water treatment ponds Advanced
  Treatment (Solids 100, BOD 99, Col.
  99) Land Application, Coagulation-sedimentation
  , Adsorption, Electrodialysis
• Outputs of TreatmentEffluent Sprayed on fields,
  Treated to make potableSludge (Treated
  Biosolids) Old Way Burned, Landfilled, Dumped
  in ocean (illegal now) New Way Fertilizer,
  Dewater and treat (then reuse) the water,
  Digestion in heated sludge tanks, Process and
  reuse the methane gas

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Wastewater Package Plants

• Package Plants Intermediate sized biological
  wastewater treatment systems. They are designed
  and engineered specifically for areas that are
  temporarily or permanently outside the reach of
  municipal waste disposal systems.
• Pre-built at the factory and shipped to the
  project site as a self-contained unit requiring
  only minimal field assembly.
• Typically PPs handle between 3,000-500,000 gpd,
  usually designed for between 10 and 500 dwelling
  units.
• Advantages1) Service areas not serviced by
  municipal sewer relatively cheaply2) Make land
  available for development where water and roads
  exist 3) Plants are easy to operate, many are
  manned for 2-3 hours per day4) Superior to
  septic tank systems in treatment and cost
• Disadvantages1) Promotes leapfrog development2)
  Often poorly operated and maintained3) Allows
  development in undesirable areas (from view of
  planners)
• For more info go to http//www.epa.gov/owm/mtb/pa
  ckage_plant.pdf

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Example Package Plant
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Wastewater Septic Tanks

• Over 100 years old rapidly spread in usage
  (Created in 1860s, Came to the US in 1884)
• Septic Tank System
• The Septic Tank A large container to hold solid
  wastes
• 1) tank stores
• 2) tank distributes
• 3) tank provides a place for bacterial
  digestion
• Tanks designed to hold wastewater for at least
  24 hours before flowing into the drainfield.
• Piping A perforated 4-inch pipe in a specially
  dug trench
• Drainfield A trench dug to capture and treat
  effluent as of flows out of the septic tank.
  Trench has gravel in it to help with percolation
• Basically a system that provides Primary and
  Secondary Treatment.
• 1) Solids settle, Greases float
• 2) Effluent is cleaned by settling/floating,
  bacterial digestion, percolation and
  infiltration into the ground

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Wastewater Septic Tank Issues

• Septic Tanks are a mixed blessing because while
  they brought the convenience of indoor plumbing
  to rural areas, they also brought problems
  associated with their use in dense urban areas.
• A centralized sewer system can be designed on the
  concept of the average user. What needs to be
  determined is the projected number of average
  users and then the size of the system can be
  determined.
• -A septic tank cannot be designed for the
  average user. A septic tank must be designed
  for the worst condition or the 99th percentile
  of users.
• To have a 99 chance of performing
  satisfactorily, a septic tank system must be
  designed for those families in the top 1 with
  respect to water use.
• Research in the 1970s and 1980s found that most
  systems fail in twenty years due to poor
  construction or a lack of maintenance. Septic
  systems have a distressing perversity with
  regard to predictability of performance.
• In terms of their economy, septic tanks are
  cheaper only in the short run. Studies have shown
  that centralized sewer systems are less expensive
  in the long run (over 60 years). Septic tanks
  become economical at very low densities
  (approximately 3 acre lots).

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Wastewater Septic Tank Successes

• Why are Septic Tanks so important
  historically?--Provided the convenience and
  safety of interior plumbing to rural
  areas--Helped to foster suburbanization/rural
  growth--Brought wastewater treatment to
  undeveloped areas
• In recent years, septic tanks have become more
  and more safe because1.      Better septic tank
  technology (better materials, better
  builders)2.      Better understanding of septic
  tank problems3.      Better regulations
  (permitting and building), Better
  enforcement4. Specialized firms for septic tank
  work
• Septic Tank Administration
• A permit is required for septic tanks. Location
  of drainfield is the KEY.
• Inspected every 3-5 years (3 is best)
• Clean out annually, especially when you have a
  garbage disposal
• When failure occurs--Must hook up to sewer line
  if available ()--Must dig new drainfield (if
  there is room) ()--If no room for new
  drainfield, must clean out entire old drainfield
  ()

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Wastewater Planning Considerations

• Important Legislation Federal Water Pollution
  Control Act (1972) Clean Water Act (1977) Water
  Quality Act (1987) Resource Conservation and
  Recovery Act (1976)
• Determining Sewer System Size and Type--Building
  a system of appropriate capacity--Cost
  factors--Balancing cost and treatment levels
• Linkage of Sewer System Planning and
  Comprehensive Planning--Federal monies and
  federal requirements
• Siting Sewer Facilities--Ideally at the lowest
  point in the area--Away from residential/commerci
  al areas--Good transportation access --Soil
  considerations
• Importance of Maintenance--EPA study showed that
  while most needs are for new facilities,
  retrofitting and repairing older facilities are
  very expensive, so maintenance is essential.