Senior Design
Friday, April 23, 2010
Chris Crock
Aaron Lammers
Brent Long
Aaron Raak
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Carabuela, Ecuador has a
flawed wastewater
treatment system
 Overloaded septic tank
 Failed leaching field
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Worked with HCJB to
remedy the problem
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Effective Treatment
Culturally Appropriate
Sustainability
Site Appropriate
Low Cost
User Friendliness
Life of Design
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Water Effluent
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E. Coli count < 1000/100 mL
 Biochemical Oxygen Demand (BOD) under 2.0 mg/L
 Helminth eggs < 1 egg/100mL
 (WHO standards set E. coli limit for leafy crops at 1,000/100mL; at this level of
treatment other pathogens are assumed to be treated as well)
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Sludge Effluent
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1000 E. Coli/gram solids
 < 1 Helminth egg/ g solids
 (With alfalfa, requirements need to only meet Class B sludge treatment. The US EPA
determined that sludge which goes through one of six processes of significant reduction
of pathogens may be applied to crops)
Handle the waste of the entire connected population for 20
yrs (1800 residents)
 No electricity
 The system must fit in 0.5 hectares
 No chemical additives
 Shall not need experts outside of the village for construction
General System Description
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Bar Racks
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Grit Chamber
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Settle out discrete organic materials and small particles
Store organics for later treatment
Anaerobic digestion of organic solids
Two tanks and settling chambers
Stabilization Lagoons
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Settle out large particles (sand, grit, etc.)
Two open channels acting as grit chambers
Velocity control weir
Imhoff Tank
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Racks for large solids and objects
Two open channels with inclined bars
Dewatering plate for screenings
One facultative pond for Biochemical Oxygen Demand (BOD) reduction
Two maturation ponds for further BOD reduction and pathogen removal
Sludge Drying Beds
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Treat sludge from Imhoff Tank and Grit Chamber
Four sludge drying beds for treatment cycling
Q = 192 m3/day
BOD = 32 kg/day
TSS = 48 kg/day
FC = 2x107 /100 mL
General System Description
Q = 192 m3/day
BOD = 32 kg/day
TSS = 48 kg/day
FC = 2x107 /100 mL
Bar Racks
Q = 192 m3/day
BOD = 32 kg/day
TSS = 48 kg/day
FC = 2x107 /100 mL
Grit Chamber
Imhoff Tank
Q = 192 m3/day
BOD = 16 kg/day
TSS = 32.6 kg/day
FC 2x107 /100 mL
Solids = 16 m3/month
Sludge Drying Bed
Q = 192 m3/day
BOD = 0.51 kg/day
TSS = 3.2 kg/day
FC = 915 /100 mL
Irrigation
Stabilization Ponds
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Important to remove larger solids
 Bar Racks
▪ Design depends mostly on clear space between bars
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Velocity should be within 0.3—0.6 m/s
Openings between 20—50 mm
Rack for dewatering screenings
Redundant system
 Bar Racks Structural Design
 Analysis of moments in the chamber
 Designed steel and concrete for worst case
loads
 ACI 318M-05 Metric Building Code and
Commentary
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Steel reinforcing requirements
Concrete requirements
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Important to remove particulate
 Grit Chamber
▪ Design largely depends on the velocity
the water (0.3 m/s)
▪ Velocity controlled by Sutro weir
▪ Grit removed is treated in sludge drying
beds
▪ Redundant system
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Structural Design
 Ultimate moment design
 ACI 318M-05 Metric Building Code
and Commentary
 Two open channels and sutro weirs
for redundancy
Outflow
Inflow
V
V0
Stokes Settling Velocity
Stokes Rearranged for Particle Removal
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Environmental Design
 Two tanks in one structure for redundancy
 Sedimentation
▪ Based off design guides and rules of thumb
▪ Overflow rate of 600 gal/ft2 day
▪ Retention Time of 2 hours
 Digestion
▪ Based on case study of Imhoff tank in Honduras
▪ Sludge storage for 0.053 m3 per resident (95.4 m3)
▪ Up to 6 months of sludge storage
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Structural Design
 Analysis of forces and moments in tank
▪ Finite Element Analysis (FEA)
▪ Structural analysis
 Designed steel and concrete to hold for highest loads
 ACI 318M-05 Metric Building Code and Commentary
 Similar to case study tank in Honduras
 Final Design: 9.25m long x 8.6m wide x 7.5 m tall
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Loading Rates
 BOD: 100mg/L
 Helminth Eggs: 1000 Eggs/L
 E-Coli: 2x107 Coliforms/100mL
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Reduced Rates
 BOD: 2.7mg/L
 Helminth Eggs: 0.10 Eggs/L
 E-Coli: 915 Coliforms/100mL
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Pond System
 1 Facultative Ponds
 2 Maturation Ponds
Dimensions
 48 meters x 24 meters
 Depths of 1.5 meters and 0.5
meters
Redundancy
Must hold sludge for
several weeks to dewater
 Must hold sludge for
longer to make it safe for
fertilizer
 Designed to hold 1 year’s
worth of sludge for
Imhoff tank
 Area: 960 m2
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Beds have layers of sand and gravel
Shear gates to control sludge flow
Low walls of earth or concrete
Under drain system of PVC pipe
Townspeople connect roof
drains to sewers
 A large rainfall event could flush the
system
 Model showed 15x increase in flow
during 10-year event
 Will require an overflow weir to prevent
flushing
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Storm inflow: 3100 m3/day
Design inflow: 192 m3/day
Estimated cost of construction =
$31,000
 Probably too much for residents
 We wrote a grant to cover the cost of
construction
 Maintenance costs to be covered by
Carabuela
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 Estimated $14,000/year
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Designed a complete system to treat Carabuela’s
wastewater for irrigation reuse
Removal of BOD, TSS, and Pathogens
 BOD (98.7%)
 TSS (93.4%)
 Pathogen Removal
▪ E. Coli (99.995%)
▪ Helminth Eggs (99.99%)
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Created construction drawing for the system
Wrote an operations and maintenance manual