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Septic System for La Base
(resumens de los informes en Espaņol (PDF)

International Senior Design:
Santa Cruz, Bolivia

August 2005

S.T.R.8 U.P. Engineering
Lauren Hubbell – Undergraduate, Civil Engineering
Matt Van Slembrouck – Undergraduate, Civil Engineering
Maureen Habarth – Undergraduate, Environmental Engineering


Background

In August, 2005 S.T.R.8 U.P. Engineering traveled to Santa Cruz, Bolivia to gather information needed to design an onsite wastewater treatment design for a local school, La Base. The school is located in UV 118 of District 10, just inside the 6th ring of Santa Cruz. The school serves 1,240 students in grades Kindergarten through 8th in three daily shifts. It is estimated that nearly 1,000 students are turned away each year because of overcapacity.



Entryway sidewalk at La Base. The PVC pipe contains run-off water from school and “grey water” from sink. The water tested at this pipe was contaminated with sewage.

Child playing in sink at La Base.

Toilet rooms at La Base include Astra flush tanks and “squat” toilets. There are currently 8 total toilets.

ONSITE WASTEWATER
TREATMENT DESIGN

Problem Statement

The current wastewater treatment system at La Base School is of the traditional Bolivian design. This system was installed 15 years ago for an estimated 100 students and is currently failing. The system is currently violating Bolivian codes and standards regarding application rates of effluent and disposal of wastewater. S.T.R.8 U.P. believes the following factors are causing the failure:

  • System lifetime of 5-10 years exceeded
  • Current wastewater volume greatly exceeds system capacity

Matt measuring the site layout

Data Gathering

Onsite Testing:
Site Layout
Soil Borings
Topographic Survey

Soil boring results showing mostly silty sand at La Base.

Topographic Survey of site showed level elevations

Drinking water samples showed no growth of E. coli or total coliform bacteria. However, the “grey water” sample (far right) contained extremely high numbers of both E. coli and total coliform colonies. These results are consistent with the results of the P/A test.

Offsite Testing:
Presence/Absence (P/A) Test
E.coli/Coliform Test

Right Bottle: Drinking water samples tested negative for the presence of total coliform bacteria.
Left Bottle: The PVC effluent, or “grey water” sample indicated the presence of total coliform bacteria in the sample. These bacteria species are indicators of fecal contamination. This condition is a health hazard to students and teachers who come in close contact with this water daily.

Basic information was gathered at meetings, including the parent’s concerns.

 

Right: Toilet room survey showed that students used the bathroom 1.76 times per day. This was combined with the flush tank volume to estimate the 24-hour flow to be 7,482 gal.

 At La Base:
Informational meetings with School Director and PTA
Toilet room survey

Traditional Bolivian System

Three-component system that includes and inspection tank, septic tank, and dry well (“poso ciego”).
Advantages:
Familiar to Bolivians.
Can be covered with concrete.
Disadvantages:
Requires 6 poso ciegos for La Base.
High wastewater loading rate.

Recirculating Sand Filter (RSF)

Effluent is pumped from septic tank to a system of perforated pipes in a sand filter. The wastewater passes through the system several times before it is finally dispersed to a drainfield.
Advantages:
Higher application rate can be used in smaller area due to recirculation.
Disadvantages:
Requires several pumps
Unfamiliar to Bolivians.

Modified Bolivian System

Similar design to Traditional Bolivian System. The dry well has a modified trench system that contains perforated pipes for distribution of effluent.
Advantages:
Can be covered with concrete.
Requires only 2 poso ciegos.
Lower construction cost.
Disadvantages:
Requires a pump.
Can only treat 2,000 gpd.

  

System Layout

  1. Inspection Tank
  2. Septic Tank
    10.5m x 2m x 2.5m
    24-hour retention time
    ½ hP pump
  3. Dry wells

Final Site Layout

Modified Dry Well (“Poso Ciego”)

  • 2 dry wells
  • 3.3 m diameter
  • 2.1 m perforated pipes extending out from dry well.

    		•  Recommended Design
    S.T.R.8 U.P.’s final recommendation for La Base is the Modified Bolivian System. The system first moves effluent through an inspection tank used for maintenance purposes, then to a dual chamber septic tank. Solids are removed in the first chamber of the septic tank and cleaner effluent flows over a baffle to the second chamber of the tank. The pump chamber uses a ½ hP pump to dose effluent approximately 6 times a day to the two 3.3m diameter, dry wells located in the courtyard of the school. The dry wells are perforated to allow the effluent to absorb in the soil for final treatment before it reaches the groundwater. The design can handle 2,000 gallons of wastewater each day, so S.T.R.8 U.P. is also recommending La Base take part in a water conservation program to reduce their daily flow. Examples of reducing wastewater include reducing the volume of water per flush and the number of flushes per day. An educational program was developed for La Base about water conservation as part of the final design. The estimated construction cost for the modified system is $6870 USD.
     

    Future Recommendations

    S.T.R.8 U.P. Engineering’s future recommendations for La Base include connecting to municipal sewer lines, starting a basic sanitation and health program, and installing more toilets. The municipal lines do not currently reach La Base, but are expected to within the next 15 years. The team suggests pushing the local government for these lines and connecting as soon as possible because the Modified Bolivian septic system is not a permanent solution.
    The educational program includes water conservation, basic health information, proper hand-washing techniques, and knowledge about septic systems.
    It is estimated that 10 toilets for girls and 10 toilets for boys are needed to meet Bolivian standards at La Base.

     
      Michigan Technological University
    Department of Civil and Environmental Engineering
    1400 Townsend Drive
    Houghton, Michigan, 49931 - 1295, USA
    Department Phone: 1-906-487-2520
    Department Fax: 1-906-487-2943
    Department E-mail: cee@mtu.edu


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