Senior Design Friday, April 23, 2010

Aaron Raak

Aaron Lammers

Brent Long

Chris Crock

Introduction

Carabuela, Ecuador has a flawed wastewater treatment system Overloaded septic

tank Failed leaching field

Worked with HCJB to remedy the problem

Design Norms/Criteria

Effective Treatment Culturally Appropriate Sustainability Site Appropriate Low Cost User Friendliness Life of Design

Performance Requirements Water Effluent

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) Sludge Effluent

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)

Functional Requirements

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

General System Description Bar Racks

Racks for large solids and objects Two open channels with inclined bars Dewatering plate for screenings

Grit Chamber Settle out large particles (sand, grit, etc.) Two open channels acting as grit chambers Velocity control weir

Imhoff Tank 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 One facultative pond for Biochemical Oxygen Demand (BOD) reduction Two maturation ponds for further BOD reduction and pathogen removal

Sludge Drying Beds Treat sludge from Imhoff Tank and Grit Chamber Four sludge drying beds for treatment cycling

General System Description

Bar Racks Grit

Chamber Imhoff Tank

Sludge Drying Bed

Stabilization Ponds

Q = 192 m3/dayBOD = 32 kg/dayTSS = 48 kg/dayFC = 2x107 /100 mL

Q = 192 m3/dayBOD = 16 kg/dayTSS = 32.6 kg/dayFC 2x107 /100 mL

Q = 192 m3/dayBOD = 32 kg/dayTSS = 48 kg/dayFC = 2x107 /100 mL

Q = 192 m3/dayBOD = 32 kg/dayTSS = 48 kg/dayFC = 2x107 /100 mL

Solids = 16 m3/month

Irrigation

Q = 192 m3/dayBOD = 0.51 kg/dayTSS = 3.2 kg/dayFC = 915 /100 mL

Environmental Design - Bar Rack

Important to remove larger solids Bar Racks▪ Design depends mostly on clear space between

bars▪ Velocity should be within 0.3—0.6 m/s▪ Openings between 20—50 mm▪ Rack for dewatering screenings▪ Redundant system

Structural Design - Bar Rack

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 Steel reinforcing requirements Concrete requirements

Environmental Design – Grit Chamber

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

Structural Design – Grit Chamber

Structural Design Ultimate moment design ACI 318M-05 Metric Building

Code and Commentary Two open channels and sutro

weirs for redundancy

How an Imhoff tank works

Sedi

men

tatio

n

Anae

robi

c

Diges

tion

V

V0

Inflow Outflow

Stokes Settling Velocity

Stokes Rearranged for Particle Removal

Environmental Design – Imhoff Tank

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

Structural Design – Imhoff Tank

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

Concrete Structures Walkthrough

Environmental Design – Lagoons

Loading Rates BOD: 100mg/L Helminth Eggs: 1000 Eggs/L E-Coli: 2x107 Coliforms/100mL

Reduced Rates BOD: 2.7mg/L Helminth Eggs: 0.10 Eggs/L E-Coli: 915 Coliforms/100mL

Structural Design – Lagoons

Pond System 1 Facultative Ponds 2 Maturation Ponds

Dimensions 48 meters x 24 meters Depths of 1.5 meters and

0.5 meters Redundancy

Environmental Design – Sludge Treatment

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

Structural Design – Sludge Treatment

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

Storm Water

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

Storm Water

Storm inflow: 3100 m3/day Design inflow: 192 m3/day

Project Funding

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 Estimated $14,000/year

Conclusion

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%)

Created construction drawing for the system Wrote an operations and maintenance manual

Questions??