1. Executive Summary
This report provides preliminary wastewater collection and treatment facility
design and cost estimates for the Town of Bahía Ballena, Costa Rica. Bahía Ballena is a
small, coastal town along the Pacific Ocean with a population of 1,000, which is estimated
to increase to 3,000 to 4,000 in 10 years. The primary objective was to design a system that
is robust, reliable, low-cost, and low-maintenance.
The design approach was to minimize system complexities and decrease the
likelihood of equipment failure. The collection system relied heavily upon gravity to
convey wastewater from homes to the wastewater treatment facility. The cost to construct
the gravity-fed collection system is approximately $750,000 USD. At the treatment facility
headworks, a lift station is provided to transport the wastewater from the pipes to the
treatment facility. The treatment facility is capable of reducing wastewater effluent
concentrations to 30 mg/L biochemical oxygen demand (BOD) and 30 mg/L total
suspended solids (TSS). The cost of the facility will be approximately $250,000 with
annual O&M costs of $50,000. Ultraviolet (UV) disinfection is included to deactivate
pathogens prior to effluent discharge to protect human health. The well-being of the Bahía
Ballena residents and tourists is of the utmost importance.
The treatment facility was designed to require little operator skill and little
maintenance. The lagoon wastewater treatment system uses dissolved oxygen to increase
biological activity and degrade BOD. After BOD removal, wastewater suspended solids
are removed in settling basins, prior to UV disinfection. Effluent exits through a pipe into
an armored channel and mixes with stream water, south of the treatment facility.
Further analysis of wastewater characteristics is recommended. This information is
critical to ensure that the treatment plant operates at its maximum treatment efficiency and
cost efficiency. Additional soil samples must be collected from the facility location and
analyzed in order to ensure slope stability and construction feasibility. A wastewater
facility operator is recommended to clean the headworks screens and confirm proper
functioning of all aeration equipment on a daily basis.
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Table of Contents
Executive Summary .......................................................................................................................................1
List of Figures...................................................................................................................................................4
List of Tables ....................................................................................................................................................5
1. Introduction ..................................................................................................................................................6
1.1. Problem Statement ............................................................................................................................6
1.2. Location.................................................................................................................................................6
1.3. Project Objectives/Scope................................................................................................................8
1.3.1. Limitations...................................................................................................................................9
1.3.2. Deliverables ................................................................................................................................9
2. Site Selection ............................................................................................................................................ 10
2.1. Site Selection Criteria ................................................................................................................... 10
2.2. Site Rankings ................................................................................................................................... 11
3. Design Sanitary Flow ............................................................................................................................ 14
3.1. Population Projections.................................................................................................................. 14
3.2. Design Flow...................................................................................................................................... 14
3.2.1. Residential and Commercial Flows ................................................................................ 14
3.2.2. Tourism Flows........................................................................................................................ 14
3.2.3. Inflow and Infiltration.......................................................................................................... 14
3.2.4. Total Design Flow................................................................................................................. 15
4. Wastewater Collection System Design........................................................................................... 17
4.1. NR 110 compliance ....................................................................................................................... 17
4.2. Wastewater Collection System Design .................................................................................. 20
4.2.1. Alternative 1: Gravity Flow............................................................................................... 20
4.2.2. Alternative 2: Lift Station................................................................................................... 21
4.2.3. Recommendation ................................................................................................................... 22
5.Wastewater Treatment Facility Design............................................................................................ 24
5.1. Wastewater Strength...................................................................................................................... 24
5.2. Effluent Quality Standards.......................................................................................................... 24
5.3. Wastewater Treatment System Design................................................................................... 24
5.3.1. Headworks................................................................................................................................ 24
5.3.2. Lagoon Alternatives.............................................................................................................. 25
5.3.3. Settling Basins ........................................................................................................................ 29
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List of Figures
Figure 1. Bahía Ballena, Costa Rica site location.............................................................7
Figure 2. Proposed wastewater treatment facility sites. ....................................................7
Figure 3. Start-up average daily loading by month.........................................................15
Figure 4. End of design life average daily loading by month..........................................16
Figure 5. Shallow depth collection system pipe. ............................................................18
Figure 6. Pipes outside roadway limits and requiring non-public land (bold red lines). ..20
Figure 7. 10-inch trunk line (bold red lines)...................................................................21
Figure 8. Location of lift station. ...................................................................................22
Figure 9. Manual bar screen at head of treatment facility. ..............................................24
Figure 10. Completely-mixed aerated lagoon.................................................................26
Figure 11. Triplepoint double bubble technology™ for coarse and fine bubbles. ...........27
Figure 12. Partial-mix aerators on pontoon platforms at the lagoon surface....................28
Figure 13. Profile view of aluminum full round riser with inlet and outlet pipes. ...........30
Figure 14. Top view of full round riser with flashboard track. .......................................31
Figure 15. Installation of UV lamps...............................................................................33
Figure 16. UV treatment banks at wastewater treatment site. .........................................33
Figure 17. Wastewater treatment facility site overview..................................................41
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List of Tables
Table 1. Ranked Site Selection Criteria .........................................................................10
Table 2. Ranking of Each Site per Criterion...................................................................12
Table 3. Business Loading Rates ...................................................................................19
Table 4. High strength wastewater constituent concentrations........................................24
Table 5. BOD Influent and Effluent Concentrations from Start-Up to End of Design....27
Table 6. Two Day Settling Basin Retention Volumes and Corresponding Liquid Depths
for One and Two Tanks.................................................................................................30
Table 7. Flow Range and Required UV Lamps..............................................................32
Table 8. Piping and Manhole Excavation Time (Gordian Group)...................................35
Table 9. Technical Labor Time for Piping (Gordian Group) ..........................................35
Table 10. Technical Labor Time for Manhole Construction (Gordian Group) ................36
Table 11. Collection System Alternative 1 Capital Cost.................................................37
Table 12. Collection System Alternative 2 Capital Cost.................................................37
Table 13. Capital Cost (USD) for Wastewater Treatment Facility (Alternative 1) ..........38
Table 14. Startup O&M Costs per year for Wastewater Treatment Facility (Alternative 1)
......................................................................................................................................39
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1. Introduction
1.1. Problem Statement
Bahía Ballena, Costa Rica is an ecotourism-dependent community that lacks
centralized wastewater treatment for its established population of approximately 1,000.
The need for centralized wastewater treatment in Bahía Ballena was identified by the
Central States Water Environment Association (CSWEA), which established a Global
Water Stewardship program to find solutions for water treatment and environmental issues.
The CSWEA executive director, Mohammed Haque, was the project client. Project scope,
deliverables, and constraints are based on the CSWEA 2016 Student Design Competition
Problem Statement for Bahía Ballena.
Commercial and residential entities currently use plastic or concrete septic tanks
without adequate leach fields. The plastic tanks are susceptible to leakage because they are
not appropriately designed, and are not frequently cleaned or properly maintained.
Contaminated runoff compromises the livelihood of the people of Bahía Ballena who
depend on the local biodiversity and environmental health. Infrastructure is needed to
collect and centrally treat the Bahía Ballena community’s wastewater. The design
described in this report may be implemented in one phase to include commercial and
residential collection and the wastewater treatment facility.
1.2. Location
Bahía Ballena, Costa Rica is approximately 3 hours south of San Jose. The
community of Bahía Ballena is located approximately 600 m (2,000 ft) east of the Pacific
Ocean (Figure 1). There are three proposed sites for the central wastewater treatment
facility (Figure 2). Note that the dashed lines for Site 3 are areas of possible expansion as
stated by the client.
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Figure 1. Bahía Ballena, Costa Rica site location.
Figure 2. Proposed wastewater treatment facility sites.
Costa Rica
Bahía Ballena
8. Page 8 of 45
1.3. Project Objectives/Scope
The following objectives were outlined to accommodate the wastewater collection
and treatment needs of the community of Bahía Ballena:
1. Choose the best of three proposed wastewater treatment facility sites, with consideration
for:
a. Cost effective collection
b. Ease of attaining property ownership by municipality
c. Elevation and location that minimize need for lift stations
d. Accommodation for future expansion
e. Location of treatment plant effluent discharge
f. Plant layout (including all system processes)
2. Design a wastewater (including graywater) collection system for Phase 1 and 2, with
consideration for:
a. Ability of neighboring community, Uvita, to tie into collection system
b. Ability for Bahía Ballena residents to cost-effectively connect to collection system
c. Location of existing homes and roadways
d. Ability for collected wastewater to gravity drain to centralized treatment facility
with minimal lift stations (using topographic data and land availability)
e. Collection pipe size, pipe bedding and cover, manhole spacing, depth to pipe
crown, and pipe slope
f. Pipe material which accommodates ground shifting
g. Sufficient pressure head to deliver wastewater through a force main to the
centralized treatment system, if applicable
h. Operation and maintenance (O&M) forecast for 10 and 20 year timelines
3. Design a centralized wastewater treatment facility, with consideration for:
a. Reliable system to minimize equipment break-downs
b. Redundancy to account for power outages
c. Redundancy to account for equipment repair on firm capacity processes
d. Ease of expandability to accommodate future population growth
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e. Ability to handle "shock" loads from tourist population fluctuations (30%-50%
increase of average monthly flows for five months of the year)
f. Treatment of wastewater effluent to a maximum level of 30 mg/L biological oxygen
demand (BOD) and 30 mg/L total suspended solids (TSS)
g. Low O&M cost
h. Equipment and parts that are easily replaceable and readily accessible
i. Cost analysis and recommendation of odor control component
j. Feasibility of Supervisory Control and Data Acquisition (SCADA) system
k. Wastewater effluent discharge rate
l. Operation and maintenance forecast for 10 and 20 year timelines
1.3.1. Limitations
The project design followed Wisconsin NR 110 and NR 210 codes. Due to high
electricity costs, an energy-efficient system must be implemented. In addition, low capital
cost will be necessary due to limited funding. A partially self-operating system with a high
level of equipment reliability must be implemented to reduce O&M costs and to limit the
need for highly skilled on-site staff. Finally, the design must accommodate frequent power
outages of up to 6-8 hour duration. Beyond these constraints, the system accounted for
greater loading during tourism months and the rainy season.
1.3.2. Deliverables
A project report for a centralized treatment system with a complete collection
system was compiled according to the Water Environment Federation (WEF) National
Student Design Competition requirements. The report includes the following: a
preliminary design layout for the wastewater treatment plant, detailed plan set for the
collection system, and estimates of capital and O&M costs for the proposed wastewater
treatment facility and collection system.
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2. Site Selection
The CSWEA design competition specified three possible sites (Site 1, Site 2, and
Site 3) for the location of the future Bahía Ballena wastewater treatment facility (Figure 2).
Site 1, which is owned by the municipality, is located to the north of the community center
on the northernmost end of the municipal airport property. Site 2, which is privately owned,
is located to the south of the community center. The land is available for sale. Site 3, which
is owned by a local developer, is located immediately south of a proposed subdivision site.
The client has mentioned that local government may gain ownership of the site by
supplying the proposed subdivision with potable water. Sites 2 and 3 allow for gravity flow
of the wastewater through the majority of the system, unlike Site 1, which is upslope from
the community.
2.1. Site Selection Criteria
Site selection was performed with an alternatives analysis. The criteria for site
selection were ranked 1 to 10, with 10 indicating the greatest importance and 1 indicating
the least importance (Table 1).
Table 1. Ranked Site Selection Criteria
Criteria Importance Ranking
Gravity flow through system 10
Estimated cost of land 9
Availability for expansion 9
Possible treatment technologies 9
Ease of property acquisition 8
Cost to pump from collection system to treatment site 7
Proximity to community (odor concern) 7
Aesthetics (tourism consideration) 6
Tree removal/land clearing required 4
Vehicle Accessibility 4
Location of discharge 3
Waste transportation 3
Gravity flow through the collection system was assigned the greatest ranking
because gravity conveyance helps avoid the expense associated with pumping wastewater
to a higher elevation. Estimated cost of the site land and the availability for expansion of
the treatment system at the site were assigned importance values of 9 in order to avoid high
capital cost of the land, and to provide for the projected expansion of the system to meet
11. Page 11 of 45
the needs of the growing population. The possible range of treatment technologies was also
assigned a value of 9 to avoid limitations due to footprint, slope, or excavation depth (i.e.
lagoons would require greater area than conventional systems).
Land acquisition was given an importance ranking of 8 to ensure acquisition but
not a greater value because the site could also be relocated, if necessary, with adjustments
to the collection system. The cost to pump wastewater from the collection system to the
treatment site was considered to have an importance ranking of 7 because electricity cost
is expensive in Costa Rica (approximately $0.25/kWh). The proximity of the site to the
community was also assigned a value of 7 due to odor and citizen safety concerns. The
wind direction over the community was noted to be highly variable and therefore
impractical to estimate. The aesthetics criterion referred to the degree to which the
treatment facility would be visible to residents and tourists, and was assigned an
importance ranking of 6.
Tree removal or land clearing was considered to have an importance ranking of 4
because it would be an initial cost that would likely be insignificant relative to the rest of
the design (though the process may contribute additional time to construction). Vehicle
accessibility was assigned a value of 4 because equipment and materials may have to be
transported to the site. However, vehicle accessibility is only likely to be an obstacle for
Site 2. The location of discharge and possibility of waste transportation were both assigned
rankings of 3 because each site has an available discharge location and transportation of
solids is projected to occur infrequently.
2.2. Site Rankings
Each site was assigned a preference ranking of 1 through 10 for each of the site
selection criteria (Table 2). A value of 10 indicated greatest suitability whereas a value of
1 indicated least suitability.
Individual site rankings were calculated by multiplying each criterion importance
ranking by its corresponding site preference ranking, summing the products, and dividing
the sum by the number of criteria evaluated. For example, the value of 10 assigned to Site
1 for cost of land was multiplied by the cost of land criterion value of 9 for a product of
90. This was done similarly for the rest of the ranking and criterion values, which were
summed and divided by the number of criterion (12) for a weighted score of 38.
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Table 2. Ranking of Each Site per Criterion
Location Site 1 Site 2 Site 3
Cost of land 10 2 8
Difficulty obtaining land 10 2 5
Availability for expansion 4 8 10
Location of discharge 3 7 9
Cost to pump to treatment site 2 8 7
Aesthetics (tourists) 9 5 3
Gravity flow through system 1 8 8
Loss of trees/ Land clearing 7 4 7
Proximity to community 9 3 3
Vehicle Accessibility 7 4 4
Waste transportation 5 7 7
Treatment Technologies 3 7 8
Weighted Average 38 36 45
Sites 1, 2, and 3 were assigned values of 10, 2, and 5, respectively, in consideration
of the difficulty of obtaining land. Site 1 is owned by the community, which eliminates
uncertainty of attainability, Site 2 is privately owned and of unknown attainability, and Site
3 is owned by a developer with whom the community has already begun discussions.
Rankings for the availability for expansion were assigned based on the cleared, flat area
available at the alternative locations. The rankings for the location of discharge were
assigned based on the proximity of the site to a stream or ditch that would travel downslope
without increased flow through the town. Most notably, Site 1 was assigned a suitability
ranking of 3 for the discharge location criterion because the community would be
downslope of any discharge.
The cost to pump to the treatment site was determined by approximating the volume
of wastewater that would have to be pumped to each site, should it be selected. The most
extreme was Site 1, which is at a higher elevation than the majority of the community. Sites
2 and 3 were assumed to require minimal pumping compared to Site 1. Similar rankings
were assigned for the possibility of gravity flow through the system to each site. Site 1 was
assumed to be the least detrimental to the aesthetic appeal of the community. The
oceanfront locations (Sites 2 and 3) were assumed to have greater possible visibility.
Similar rankings were assigned under the proximity to the community criterion. However,
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Site 2 was assumed to have greater tree cover than Site 3, although they would likely have
similar odor contributions.
Land clearing for necessary roadways to the facility or possible future expansion
was assumed to be most difficult and extensive at Site 2 because it is situated in a more
heavily wooded area, relative to the other two sites. Greatest preference for vehicle
accessibility was assigned to Site 1 because it was previously a municipal airport (and
therefore should be reasonably accessible), is located nearer a major thoroughfare than the
other sites, and would eliminate the need to drive through the community when exporting
solid waste fractions to a landfill. Conversely, the site rankings for waste transportation
favored Sites 2 and 3 because of proximity to a major roadway (Costanera Sur). Rankings
for available range of treatment technologies applicable for the site were also greater for
Sites 2 and 3 because there is relatively greater available area.
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3. Design Sanitary Flow
3.1. Population Projections
CSWEA provided estimates that the population of Bahía Ballena, Costa Rica would
increase from the current population of 1,000 to a total of 3,000 to 4,000 in 10 years (2015
to 2025). The facility design life was 50 years, from 2020 to 2070. The population was
predicted to be 2,500 by 2020, and to reach 3,500 by 2025. Population projections beyond
2025 were made under the assumption that the initial rapid population growth rate would
not be sustained, and would be best approximated by Costa Rica’s national average growth
rate of 1.31% (“Population Growth Rate”). This assumption should prevent overestimation
of future treatment facility capacity. The population was approximated to be 4,000 by 2070,
the assumed end of design life.
3.2. Design Flow
3.2.1. Residential and Commercial Flows
Residential and commercial flow predictions were based on the project design
assumption of 200 L/capita/d wastewater generation. At start-up and end of design life the
residential and commercial flows were estimated to be 500 m3
/d (130,000 gpd) and 787
m3
/d (210,000 gpd), respectively.
3.2.2. Tourism Flows
Tourism flow predictions were based on the assumption that tourist season flows
would be 30-50% greater than the average monthly flows. A value of 40% was assumed to
best approximate the increase in flows. Tourist season is December through April. Tourist
flow at start-up and end of design life was estimated to be 200 m3
/d (53,000 gpd) and 315
m3
/d (83,000 gpd), respectively, in addition to commercial and residential flows.
3.2.3. Inflow and Infiltration
Infiltration is the quantity of water that enters the system due to a high water table,
while inflow is the quantity of water that enters during a storm event. Inflow and infiltration
were considered in the design capacity required for the future collection system and
wastewater treatment facility. Costa Rica typically receives 3,900 mm (150 in) of rainfall
per year, varying throughout the wet and dry seasons (CSWEA Problem Statement). The
wet season was assumed to be May through November.
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The infiltration rate through a newly constructed collection system was assumed
to be 0.12 m3
/day/mm-km (500 gpd/in-mile) (Shammas and Wang 2011). Estimates were
calculated for a system that contained 8-inch and 10-inch pipes, and total pipe length of
3.73 km (2.32 miles). The inflow rate through a newly constructed concrete and rebar
manhole was 1.89 m3
/day (500 gpd) per manhole (Shammas and Wang 2011). There were
40 manholes in the collection system. During the dry season, the percent of collection
system piping and manholes below the groundwater table (3 m below ground surface) is
17%. Inflow and infiltration of 178 m3
/d (47,000 gpd) for the wet season and 30 m3
/d
(8,000 gpd) for the dry season was estimated for design start-up.
To account for future breaks in the system, an increase of 1% every year for inflow
and infiltration, starting in 2030, was assumed. End of design life inflow and infiltration
was determined to be 223 m3
/d (59,000 gpd) for the wet season and 38 m3
/d (10,000 gpd)
for the dry season.
3.2.4. Total Design Flow
The total design flow was the sum of residential and commercial loading, tourist
loading, and inflow and infiltration. Average daily loading was evaluated on a monthly
basis at start-up and end of design life (Figure 3 and Figure 4).
Figure 3. Start-up average daily loading by month.
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Figure 4. End of design life average daily loading by month.
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4. Wastewater Collection System Design
4.1. NR 110 compliance
The sanitary sewer collection system was designed to comply with Wisconsin NR
110. The code requires minimum conduit slopes to ensure self-cleansing velocity of 2 ft/s
(0.6 m/s) while flowing full, and a maximum manhole spacing of 122 m (400 ft). Design
of the pipe network focused on minimizing excavation depth due to the high ground water
table. The pipe slopes met the NR 110 minimum required slopes, and matched the ground
surface slope when possible. At manholes where multiple pipes entered, pipe inverts may
not match the invert of the manhole. As required by Wisconsin NR 110, when the distance
between pipe and manhole inverts was greater than 0.6 m (2 ft), a drop pipe was employed
(Appendix C SDD-2; “Duran, Inc.” 2016).
The ground surface elevation, location of houses, and lateral house connection
locations and elevations were considered during collection system design. The system
followed roadways to avoid intercepting or navigating backyard obstacles, and to avoid
obtaining easements. Erratic placement of homes and businesses within community lots
would cause any proposed sewer system layout to greatly deviate from straight paths, to
avoid buildings and private septic tanks. In addition, a system layout through backyards
would require many extra manholes in order to comply with NR 110, which specifies that
there be a manhole at each direction change of the pipe. Optimal placement of manholes,
to minimize the number used, reduced the project cost. Additionally, the roadway option
ensures that all buildings will be able to conveniently connect to the system.
The pipe network was created using Bentley SewerCAD (V8i 2015). Sewer main
invert depths were a minimum of 1.5 m (5 ft) from the ground surface in the west and
northwest residential areas, and a minimum of 0.45 m (1.5 ft) from the ground surface in
the southwest business area. An excavation depth of 1.5 m (5 ft) was assumed to be
sufficient for house connections, at elevations lower than the roadway, to gravity flow to
the sewer. The shallow depth of 0.45 m (1.5 ft) was selected in the southwest section
because area business connections are at the same relative elevation as the road. Pipe at
0.45 m (1.5 ft) depth was located alongside the road to avoid damage from heavy traffic
loads (Figure 5).
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Figure 5. Shallow depth collection system pipe.
Pipe inverts deeper than 1.2 m (4.0 ft) were designed to be under the roadway, as
needed. The location and depth of the water main was considered during the collection
system design. The water main is located along the roadway centerline at a depth of
approximately 1.0 m (CSWEA Supporting Information, Water Plan Set). If a placement
conflict occurs, for either pipe or manhole, the structures can be shifted to either side of the
roadway. The entire collection system plan view can be referenced in Appendix C (Sheet
C-1).
The loading rate assigned to each manhole depended on the business, institutional,
and residential contributions to each pipe length that was immediately upstream. Each
home was assumed have three or four occupants that collectively generate an average of
750 L/d (200 gpd) wastewater. Loading rates for institutional and commercial buildings
were estimated from Table 4-1 in Crites et al (1998, refer to Table 3), and used to estimate
collection system flows and treatment facility capacity.
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Table 3. Business Loading Rates
Pipe size was designed to be predominantly 8-inch, with 10-inch trunk line to
provide a greater flow of 0.033 m3
/s (1.15 ft3
/s) compared to 0.022 m3
/s (0.77 ft3
/s) for 8-
inch pipe. High density polyethylene (HDPE) pipe was chosen to accommodate ground
shifting during earthquakes and to prevent excessive infiltration as the system ages.
Throughout the system, 8-inch pipe was sufficient to convey design life flows and meet
minimum diameter requirements from NR 110. Slopes were 0.4% and 0.28% for 8-inch
and 10-inch pipe, respectively, to ensure self-cleansing velocity at full flow. Where the
ground slope was greater than the required minimum slopes, the pipe slope was equal to
that of the ground so that pipe elevations were lower than building elevations.
Crushed stone or gravel pipe bedding is recommended. Crushed stone or gravel is
readily available in the Bahía Ballena area and will allow water to infiltrate through the
trench, rather than remain around the pipe. The recommended bedding depth to be used is
0.1 m (4 inch) with aggregate fully covering the pipe. Native soil can be used to fill the
remaining trench depth. For pipe below the roadway, the top 0.3 m (1 ft) of cover should
consist of gravel, or traffic bond, as the base course of the roadway. Both the aggregate and
native soil should be compacted in lifts of 20.3 cm (8 inch) or less. Two 3.8 cm (1.5 inch)
lifts of asphalt will be used to construct the roadway surface.
The design of the pipe network and placement of trunk lines was done with
consideration for system expansion. Population growth is predicted to occur in the northern
and northwestern areas of Bahía Ballena, and south of Site 2 (toward the ocean). Therefore,
the trunk line diameter was increased from 8-inch to 10-inch on the southeast side of town,
near the treatment facility, to accommodate the addition of flows from the town of Uvita,
if necessary.
Business Type
Number in
Community
Guest Employee -
Hotel 190 38 4
Restaurant 23 34 6
Church 10 - 2
Shopping Center 11 11 14
Loading Rate
(L/day)
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4.2. Wastewater Collection System Design
The two alternatives described subsequently will convey more than Bahía Ballena
design flows, and prevent excessive inflow and infiltration throughout the design life of
the wastewater treatment facility.
4.2.1. Alternative 1: Gravity Flow
Alternative 1 is a pipe network that conveys flow only by gravity until it reaches
the wastewater treatment facility where the wastewater is lifted by a pump into the
headworks. The pipe network generally follows the roadway, with the exception of
approximately 390 m (1,280 ft) of pipe on private property (Figure 6).
Figure 6. Pipes outside roadway limits and requiring non-public land (bold red lines).
A trunk line conveys flow where the majority of the pipes converge toward the
treatment facility (Figure 7).
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Figure 7. 10-inch trunk line (bold red lines).
The sewer interceptor reaches the wastewater treatment facility at a depth of 3.7 m
(12 ft) below ground surface, which requires a lift station to pump collected wastewater to
the headworks (Appendix C SDD-1). A Godwin Dri-Prime CD80D Pump is recommended
to lift the wastewater because it requires minimal O&M. The pump will be at the ground
surface and will use a suction hose within the wet well. The CD80D Pump is able to deliver
flow from 250 m3
/d (66,000 gpd) to 1,900 m3
/d (500,000 gpd) at a range of 4.57 m (15.0
ft) to 30.5 m (100 ft) of head, which provides operational flexibility. The pump is equipped
with a Yanmar 20 hp diesel engine with 113 L (30 gallon) fuel capacity. After fueling, the
pump can run for 33 continuous hours. In addition, the CD80D Pump is equipped with Dri-
Prime so the facility operator does not need to prime the pump at the beginning of
operation, nor when the pump runs dry and loses prime. Floats will be used to regulate
pump on and off controls, which can also be adjusted at the surface with a control panel
mounted to the pump. A metal housing unit covers the pump and engine to dampen noise
and protect from weather damage. Pump O&M is limited to checking fluid levels and
refueling.
4.2.2. Alternative 2: Lift Station
Alternative 2 is a pipe network that operates similarly to Alternative 1 with the
exception of an additional lift station near the center of town (Figure 8).
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Figure 8. Location of lift station.
An extra lift station could reduce excavation by 18,200 m3
(23,800 CY) because
the system would not be gravity flow dependent. Maximum required excavation depth
would be 3.34 m (11.0 ft) and pipe trenches would only need to be dewatered for 270 m
(890 ft) of pipe (in comparison to 620 m (2,000 ft) for Alternative 1).
The sewer interceptor that leads to the wastewater treatment facility would arrive
at a depth of 2.7 m (8.9 ft); wastewater would need to be pumped to the headworks through
a second lift station. The same pump, Godwin Dri-Prime CD80D, could be used for both
lift stations. However, an additional lift station would result in an increase in fuel
consumption, O&M, and project cost.
4.2.3. Recommendation
Alternative 1, the gravity flow collection system, is the recommended collection
system design alternative. This recommendation is based on lower capital, operation, and
maintenance costs, as outlined in the Cost Analysis section. Alternative 1 reduces and
focuses O&M by locating the only lift station at the wastewater treatment facility, thereby
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reducing operator travel to check pumps. The gravity flow system will provide greater
reliability and require less dependence on pumps.
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5.Wastewater Treatment Facility Design
5.1. Wastewater Strength
The wastewater generated by Bahía Ballena was predicted to be “high strength”
wastewater and of comparable composition to the high strength constituent concentration
information provided by Tchobanoglous (2003, refer to Table 4). The assumption was
based on the prediction that Costa Rican water use would generally result in little dilution
relative to most wastewater generated in the United States.
Table 4. High strength wastewater constituent concentrations.
Contaminant Concentration Unit
BOD5, 20 ºC 350 mg/L
Total Suspended Solids 400 mg/L
Nitrogen (total as N) 70 mg/L
Phosphorus (total as P) 12 mg/L
Fecal Coliform 105
-108
No./100 mL
5.2. Effluent Quality Standards
Effluent quality standards were provided by CSWEA. The specified maximum
BOD and TSS effluent concentrations were each 30 mg/L. To achieve sufficient treatment,
BOD and TSS will each have to be reduced by approximately 90%. Disinfection of the
effluent prior to discharge will reduce the number of pathogens that endanger human
health.
5.3. Wastewater Treatment System Design
5.3.1. Headworks
Headworks are vital to the success of the wastewater treatment facility. The role of
the headworks is to protect downstream processes from potential harm by large or abrasive
objects from the sewer system. If objects were to pass directly into the treatment system,
there is a high probability that much of the aeration equipment would be damaged or
clogged, and BOD removal efficiency would be drastically decreased.
From the lift station, wastewater enters a box culvert, which transitions the
wastewater into uniform channel flow (Appendix C Sheets H-1 and H-2). Channeled
wastewater then flows through a manually-cleaned bar screen. The bar screen will remove
any large objects (Figure 9, Appendix C Sheet SDD-5).
25. Page 25 of 45
Figure 9. Manually cleaned bar screen example.
A successive fine screen will further remove contaminants, such as plastic bags,
small stones, and rags. Rather than capture objects which could damage equipment, the
fine screen primarily collects items which could disrupt BOD removal through clogging or
increased solids settling in the aerated lagoons. Screens should be checked daily and
cleaned if there is debris accumulation. After screening is complete, the water flows to
aerated lagoons.
5.3.2. Lagoon Alternatives
Two alternatives, complete-mix and partial-mix lagoons, were proposed to provide
low-cost, low-maintenance BOD removal.
5.3.2.1. Alternative 1: Completely-Mixed Aerated Lagoons
Completely-mixed aerated lagoons were chosen to be the primary BOD treatment
technology, based on the assets and needs of Bahía Ballena. One benefit of the Bahía
Ballena area is that the terrain is relatively flat and will not limit lagoon siting. A
completely-mixed aerated lagoon system has few moving parts, as compared to a
conventional system. Therefore, the system has fewer items which might break-down and
cause decreased treatment efficiency. Though the area does not have skilled operators,
local workers do have a working knowledge of pump systems; adjusting to the aerators in
the lagoons will not be difficult. The system mimics natural cycles and only quickens the
biological processes without the use of chemicals. One design constraint was that
26. Page 26 of 45
construction and maintenance equipment should be sourced in the Bahía Ballena region.
Figure 10 illustrates basic complete mixing.
Figure 10. Completely-mixed aerated lagoon.
Each lagoon is square, which encourages complete mixing (Middlebrooks 1983).
The bottom base is a 10 m (33 ft) square and the side wall slopes are 2:1
(horizontal:vertical) with design water depth of 3 m (10 ft). The inlet elevation to the pond
will be 0.3 m (1.3 ft) greater than the outlet elevation to ensure gravity flow through the
ponds. In the event that wastewater operations must be stopped, the elevation difference
between inlet and outlet will provide storage for wastewater and prevent back-up into
upstream processes. The lagoon system will have a start-up hydraulic retention time (HRT)
of 3 days and utilize two aeration ponds. The end of design life HRT will be reduced to 2
days but three lagoons will be used.
Oxygen transfer efficiency, overall pump energy efficiency, ease of operation, and
mixing efficiency were the design criteria used for pump selection. Based on these
conditions, the Triplepoint M.A.R.S. Aerator (8D) is recommended. The M.A.R.S. aerator
uses Double Bubble Technology™, which provides both coarse and fine bubbles (Figure
11). Coarse bubbles provide adequate mixing and fine bubbles ensure proper oxygen
transfer for BOD treatment (“MARS” 2016). The Triplepoint system has been optimized
to supply only the required amount of air to provide mixing and aeration. The energy
requirement for completely-mixed aeration basins is 15 kW/1000 m3
tank volume (US EPA
1986). Four units are required for each lagoon because the 8D model is a 3 kW unit and
each lagoon has a volume of 850 m3
(225,000 gallons). However, five units per lagoon are
recommended to be installed during construction. Though aerators rarely break down or
clog, five aerators are recommended to be installed and in the event that one is out of order.
27. Page 27 of 45
Although a dead zone may be present when one of the five aerators are out of order, the
wastewater treatment facility will meet the mixing requirements in the meantime.
Figure 11. Triplepoint double bubble technology™ for coarse and fine bubbles.
At start-up, only two of the three constructed lagoons should be used for complete
mixing. The BOD load is such that only two lagoons are necessary to meet the 30 mg/L
BOD permit at start-up. Any subsequent mixing will increase the cost to power the aerators
with minimal increase in BOD removal. The potential impact of employing two, then three
basins was analyzed (Table 5).
Table 5. BOD Influent and Effluent Concentrations from Start-Up to End of Design
Flow (m3
/d)
BOD Conc.
(mg/L)**
Final BOD
Effluent***
500 55.6 28
600 61.7 30
700* 19.6 12
800 21.6 13
900 23.2 14
1000 24.6 15
1100 25.9 16
1200 27.1 16
*Denotes addition of third aeration basin.
**BOD concentration after aerated lagoons.
***BOD concentration after settling basins.
At start-up, the third lagoon can serve as an additional settling basin. By utilizing a
third settling basin, overall lower TSS concentrations may be achieved. However, longer
HRT could cause unwanted algal growth, which would increase effluent TSS. If algal
growth does pose a problem, a bypass valve will allow the water to be transferred directly
from the second completely-mixed lagoon to the settling basin.
28. Page 28 of 45
The third lagoon should go online as the effluent BOD level approaches the permit
limit. The community will save money on energy and maintenance costs by waiting to add
additional aeration equipment. After BOD treatment, the water flows to the settling basins
for TSS removal.
Section 5.3.2.2 Alternative 2: Partially-Mixed Aerated Lagoons
Alternatively, partial-mix aerated lagoons may be used for primary BOD treatment.
The lagoon operates similarly to the completely-mixed system by providing oxygen to
enhance biological processes. However, the partial-mix system would only aerate the
uppermost 2 m (6 ft) of a lagoon. Partial-mix oxygen addition would come from surface
aerators, which are mechanically driven blades that mix the water surface and incorporate
oxygen.
There are a couple of advantages to partial-mix systems over completely-mixed
systems. Many surface aerators are mounted on pontoon platforms, which can be tied to
the shoreline with cable (Figure 12). These are easily maintained because they can be
quickly pulled over to shore for inspection and repair. The second advantage is that the
aerators only mix the top 2 m (6 ft) of water, while the bottom section of the lagoon serves
as a settling basin (US EPA 1986). This will decrease the necessary size of the settling
basins after partial-mix treatment. When properly maintained, surface aerators should only
need attention two or three times a year for greasing.
Figure 12. Partial-mix aerators on pontoon platforms at the lagoon surface.
29. Page 29 of 45
Even though partially mixing a lagoon has benefits, there are substantial
disadvantages to this system. First, the surface aerators are not as efficient at providing
oxygen as diffuser heads, so savings gained on maintenance would only be replaced by
additional energy costs. Secondly, a partially mixed aerated lagoon would need to be
between five and six times as large as a completely-mixed lagoon due to lower treatment
efficiency. Greater lagoon area requirements would require increased land purchasing,
excavation, and construction costs. A facility with greater footprint would decrease the
available land for development of tourist locations. The treatment plant location is very
close to beachfront property, so it is essential to minimize the wastewater facility footprint
and maintain beachfront property values.
Section 5.3.2.3. Recommendation
Alternative 1, completely mixed aeration lagoons, is the recommended BOD
treatment alternative. The primary driver behind the selection was keeping beachfront
property available for development. Completely mixed systems have a shorter HRT and
more efficient oxygen transfer, both of which decrease the necessary size of the lagoons.
5.3.3. Settling Basins
In order to further improve effluent quality, two settling basins were designed to
allow TSS, including microbial biomass from the aerated lagoons, to settle prior to
disinfection and discharge. The basins were designed for a range of flows from 500 m3
/d
to 1,100 m3
/d, which corresponds to average start-up and end of design life flows,
respectively. The designed hydraulic detention time was 2 days; sufficient time to allow
for settling but insufficient for problematic algal growth (Tchobanoglous 2003). Each basin
was designed to be 10 m (33 ft) wide and 38 m (125 ft) in length, support a range of depths
from 2 m to 3.25 m (6 ft to 11 ft), and have 2:1 slopes. As the facility reaches flowrates of
600 m3
/d and greater, the flow should be divided between the basins (Table 6). The
minimum operational depth for each tank was designed to be 2 m (6 ft) to prevent release
of odors due to the warm climate (Tchobanoglous 2003). Though settling basins may have
greater depth, the design liquid depth was limited to approximately 3 m (10 ft) to avoid
deep excavation. Lined (geosynthetic 60 mil) earthen settling basins were designed to avoid
the construction cost associated with conventional tanks (Appendix C, Sheets S-1 and S-
2).
30. Page 30 of 45
Table 6. Two Day Settling Basin Retention Volumes and Corresponding Liquid Depths for One and Two Tanks
Combined effluent from the settling basins will flow from the outlets to the
disinfection process.
5.3.4. Settling Basin Outlets
The outlet structures were designed to be adjustable across a 2 m range to
accommodate the range of liquid depths in the settling basins. Each basin will be equipped
with an aluminum, 2 m tall, 1.07 m (42 inch) diameter full round riser with 0.31 m (12
inch) inlet and outlet pipes (Figure 13 and Figure 14).
Figure 13. Profile view of aluminum full round riser with inlet and outlet pipes.
Average Flowrate
(m3
/d)
Wastewater
Volume (m3
)
Liquid Depth (m)
for 1 Tank
Liquid Depth (m)
for 2 tanks
500 1000 2.84 1.86
600 1200 3.16 2.09
700 1400 3.44 2.30
800 1600 3.70 2.49
900 1800 3.94 2.67
1000 2000 4.17 2.84
1100 2200 4.38 3.00
Image: Pomona Pipe, Inc.
31. Page 31 of 45
Figure 14. Top view of full round riser with flashboard track.
The risers will have center tracks to stack 1-1/2 inch by 3-1/2 inch boards
(commonly known as 2x4’s) to the appropriate height to allow overflow from the settling
basin to flow to the disinfection process at a controlled rate. The riser outlet is cost-
effective, may be built on-site, and is manually adjustable.
5.3.5. Disinfection
No requirements for disinfection were stated by the client but disinfection is
strongly recommended to protect the health of Bahía Ballena’s residents. UV disinfection
was selected, over other conventional disinfection methods, because it presents the least
challenge to operators with little training, while still providing adequate pathogen
inactivation. Though the cost to treat with UV is moderately high, relative to chlorination
or ozonation, UV does not depend on chemical storage, on-site generation, nor does it leave
a residual in the treated water, which would require additional chemicals for residual
removal. Chemical storage, ozone generation, and residual removal are processes that
require skills for safe chemical handling and greater operational knowledge than the UV
disinfection process.
Effluent from the settling basins was designed to flow through 73 m of 8-inch pipe
and then 10 m of 0.435 m wide rectangular, open channel to three banks of low-pressure,
low-intensity UV lamps. The wastewater will be introduced in the bottom section of the
Image: Pomona Pipe, Inc.
32. Page 32 of 45
channel and exit through an effluent pipe. Short-circuiting will be minimized by locating
the influent inlet near the bottom of the channel. Wastewater will flow through the channel
parallel to the lamps and will have a contact time of 20 seconds (Tchobanoglous 2003).
This contact time ensures that coliforms can be decreased to approximately 200 fecal
coliforms/100 mL (Darby 1995). Each UV bank will consist of sixteen 1 m UV lamps.
Only sixteen lamps are required for start-up flows. However, to ensure public safety and
redundancy, two sets of lamps should be installed during initial construction. Space for a
third and fourth UV bank will also be built at start-up. Additional UV banks could be
installed as the flowrate increases (Table 7).
Table 7. Flow Range and Required UV Lamps
Flow
(L/min)
UV Lamps
Required
300 15
350* 16
400 20
450 23
500 25
550 28
600 30
650 33
700 35
750** 38
800 40
* Denotes Startup Flow
** Denotes EDL Flow
Installation and maintenance is relatively easy and the UV banks require relatively
little space (Figure 15 and Figure 16).
33. Page 33 of 45
Figure 15. Installation of UV lamps.
Figure 16. UV treatment banks at wastewater treatment site.
Treatment and energy efficiency depend largely on the TSS concentration. As the
concentration increases, greater fractions of the UV radiation is absorbed by the suspended
solids, rather than destroying pathogens. One possible shortcoming of low-intensity UV
treatment is the photoreactivation of bacteria and viruses, which occurs when sunlight
repairs the previously disrupted DNA of the cells. Revegetating the stream bank to shade
the treatment plant discharge and decrease the likelihood of photoreactivation is
encouraged.
34. Page 34 of 45
5.3.6. Outfall
After passing through the UV disinfection, the water enters the outfall pipe which
leads south, from the disinfection banks to the stream (Appendix C Sheets W-1 and O-2).
The water will flow out of the pipe and into an armored, riprap channel. Standard riprap
outlet protection dictates that the channel width be 1 m (3 ft) with the length being 10 m
(33 ft) (Appendix B, “Minnesota TR-3”). The depth of riprap must be 0.3 m and have an
average stone size between 5 cm and 7.5 cm (2 to 3 inches) in diameter. Protecting the
channel will avoid stream erosion and prevent TSS from disrupting downstream wildlife.
35. Page 35 of 45
6. Cost Analysis
6.1. Wastewater Collection System
A cost analysis was prepared for both wastewater collection system Alternative 1
and 2. Costs included excavation and site work, technical labor, piping, manholes,
dewatering, house connections, road repair, pumps, and removal of excess soil.
Excavator capacity was assumed to have a maximum digging depth of 5.5 m (18
ft) with a 46 cm (18 inch) bucket of 6.12 (10-2
) m3
(0.08 CY) capacity. Piping and manhole
excavation was calculated based on Table 8, which indicates the time required per cubic
meter of soil removed. Trenches with depths greater than 1.83 m (6 ft) would require
tapering above the 1.83 m (6 ft) depth at a 1V:3H slope to accommodate the sandy soils.
Alternative 1 would require approximately 27,000 m3
(35,300 CY) of excavation and
Alternative 2 would require approximately 8,830 m3
(11,500 CY) of excavation.
Table 8. Piping and Manhole Excavation Time (Gordian Group)
The total technical labor for piping was calculated based on the values in Table 9,
which indicates the time required per linear meter of piping. The total technical labor for
manhole construction was calculated based on the values in Table 10, which indicated the
time require per manhole based on depth.
Table 9. Technical Labor Time for Piping (Gordian Group)
Depth Hours / m
3
(CY)
0-6' 0.084 (0.110)
6-10' 0.107 (0.140)
10-14' 0.115 (0.150)
14-20' 0.138 (0.180)
Criteria Hours / LM (LF)
8"piping 0.063 (0.207)
10"piping 0.065 (0.213)
36. Page 36 of 45
Table 10. Technical Labor Time for Manhole Construction (Gordian Group)
The 8-inch pipe unit price, which includes excavation and stone, was assumed to
be applicable for depths less than 3 m (10 ft). For pipes greater than 8 inches, the unit price
was assumed to increase by 20%. At depths greater than 3 m (10 ft), the unit price was
assumed to increase by 100%.
The dewatering unit price was assumed to be 20% greater to account for fuel
expenses. The dewatering pump would be operated with equal time relative to technical
labor.
The average piping distance from a house or business to the centerline of the road
was 24 m (80 ft). Cost estimates for house and business connections to the collection
system only considered the existing 188 houses and 25 businesses.
Asphalt density of 2,323 kg/m3
(145 pcf), thickness of 76.2 mm (3 inches), average
width of 6.71 m (22 ft), and approximate road length to be replaced of 1,580 m (5,190 ft)
was assumed in order to approximate the unit price of road repair.
The CD80D Pump was chosen for the collection system and had a unit price of
$24,000 USD. Alternative 1 would require one pump at the entrance to the headworks of
the wastewater treatment system and Alternative 2 would require two pumps (one at the
headworks and one at the lift station).
Excess soil will be generated from pipe installation. For 8-inch pipe, the excess soil
was estimated to be 0.05 m3
per m (0.06 CY per LF) of piping. For the 10-inch pipe, the
excess soil was estimated to be 0.06 m3
per m (0.07 CY per LF) of piping. Hauling of
excess soil was estimated to be $5 USD per loose cubic meter (Gordian Group). Volume
was assumed to increase by 20% from cubic meter to loose cubic meter. The unit price
includes equipment, labor, and gas expenses for a 6.12 m3
(8 CY) truck traveling a 9.67
km (6 mile) travel cycle at 40 km/h (25 mph). Alternative 1 would have approximately 180
m3
(236 CY) of excess soil removed and Alternative 2 would have approximately 138 m3
(180 CY) of excess soil removed.
37. Page 37 of 45
The capital cost for collection system Alternative 1 is $761,000 USD (Table 11)
and the capital cost for Alternative 2 is $767,000 USD (Table 12).
Table 11. Collection System Alternative 1 Capital Cost
Table 12. Collection System Alternative 2 Capital Cost
Due to the durable materials of the piping and manholes, minimal repairs would
occur during the 10 and 20 year O&M forecast. The major operating cost was assumed be
the gas expenses for pump operation. The CD80D Pump requires an estimated 53 L/d (22
gal/d) with a tank capacity of 113 L (30 gallons).
6.2. Wastewater Treatment Facility
Cost analysis was limited to wastewater treatment facility Alternative 1. Aeration
and land requirements were much greater for Alternative 2, which are dominant system
expense components; therefore, a cost analysis was not calculated for Alternative 2.
Purchase of a Supervisory Control And Data Acquisition (SCADA) system is not advised.
SCADA systems are expensive and require a trained and skilled operator. As the system
Item Unit Measurement Quantity Unit Price (USD) Total (USD) Unit Price Reference
Excavation and Site Work Excavator Hours 1600 75$ 121,000$ Client
Technical Labor Hour 3100 4$ 12,000$ Client
SS Main: 8" Pipe - depth <3 m Meter 2800 33$ 92,000$ Client
SS Main: 10" Pipe - depth <3 m Meter 400 40$ 16,000$ 20% increase from 8"
SS Main: 10" Pipe - depth >3 m Meter 700 79$ 55,000$ 100% increase from 8"
Concrete Manhole Manhole 40 1,113$ 45,000$ Client
Dewatering - Trench Pump Day 200 41$ 8,000$ Client
SS Service: 4" Pipe - depth <3 m Meter 4900 33$ 162,000$ Client
House Connection Home 200 425$ 85,000$ Client
Road - Asphalt Tons 2100 65$ 137,000$ Client
Pump Unit 1 24,000$ 24,000$ Godwin
Hauling Excess Soil LCM 900 5$ 4,000$ RSMeans
Estimate 761,000$
Item Unit Measurement Quantity Unit Price (USD) Total (USD) Unit Price Reference
Excavation and Site Work Excavator Hours 1500 75$ 113,000$ Client
Technical Labor Hour 3000 4$ 12,000$ Client
SS Main: 8" Pipe - depth <3 m Meter 2700 33$ 89,000$ Client
SS Main: 10" Pipe - depth <3 m Meter 700 40$ 28,000$ 20% increase from 8"
SS Main: 10" Pipe - depth >3 m Meter 300 79$ 24,000$ 100% increase from 8"
Concrete Manhole Manhole 40 1,113$ 45,000$ Client
Dewatering - Trench Pump Day 130 41$ 5,000$ Client
SS Service: 4" Pipe - depth <3 m Meter 5200 33$ 172,000$ Client
House Connection Home 213 425$ 90,000$ Client
Road - Asphalt Tons 2100 65$ 137,000$ Client
Pump Unit 2 24,000$ 48,000$ Godwin
Hauling Excess Soil LCY 800 5$ 4,000$ RSMeans
Estimate $767,000
38. Page 38 of 45
grows and treatment efficiency becomes increasingly important, SCADA could be
considered as a possible improvement. At start-up, the SCADA capital cost would be too
great and would have minimal payoff through increased treatment efficiency. The only
process that SCADA would regulate would be airflow to the aeration basins; the UV lamps
would already have a SCADA-like system to operate efficiently.
A back-up generator is included in the cost estimate for operation of the aeration
equipment. The generator will maintain treatment efficiency in the event of a power outage.
As previously stated in Section 1.3.1., the O&M fuel cost was based on a weekly eight hour
power outage.
Wastewater treatment plant capital costs will primarily come from lagoon
excavation and liner, aeration pumps, and UV treatment equipment. Table 13 and 14 below
detail specific capital and O&M costs associated with Alternative 1. Present worth was
determined using an assumed discount rate of 4% per year.
Table 13. Capital Cost (USD) for Wastewater Treatment Facility (Alternative 1)
Process Labor Materials Total Cost
Headworks $ 1,000 $ 25,000 $ 26,000
Complete Mix Lagoons $ 14,000 $ 112,000 $ 126,000
Sedimentation Basins $ 10,000 $ 16,000 $ 26,000
UV Disinfection $ 8,000 $ 40,000 $ 48,000
Piping Between Processes N/A* $ 7,000 $ 7,000
Back-up Diesel Generator N/A $ 10,000** $ 10,000
TOTAL $ 33,000 $ 210,000 $ 243,000
*Quoted price includes excavation
**Based on Automatic Isuzu 21 kW Generator (Central Maine Diesel)
39. Page 39 of 45
Table 14. Startup O&M Costs per year for Wastewater Treatment Facility (Alternative 1)
Process Electricity/Fuel Labor Total Cost
Headworks N/A $ 2,000 $ 2,000
Complete Mix Lagoons* $ 24,000 $ 6,000 $ 30,000
Sedimentation Basins N/A $ 1,000 $ 1,000
UV Disinfection $ 2,000 $ 2,000 $ 4,000
Outfall Riprap N/A N/A N/A
Back-up Diesel Generator $ 2,300 N/A $ 2,300
TOTAL $ 28,300 $ 11,000 $ 39,300
Present Worth (10-year O&M) $ 370,000
Present Worth (20-year O&M) $ 660,000
*Price for Complete Mix Lagoons is for two aerated lagoons from start-up to Year 5, three lagoons
will be needed after Year 5 with a price increase to a total of $36,000 for electricity/fuel.
40. Page 40 of 45
7. Conclusion and Considerations
This report provides preliminary wastewater collection and treatment facility
design and cost estimates for the Town of Bahía Ballena, Costa Rica. The designed system
is robust, reliable, low-cost, and low-maintenance. The collection system relied heavily
upon gravity to convey wastewater from homes to the wastewater treatment facility. At the
treatment facility headworks, a lift station is provided to transport the wastewater from the
pipes to the treatment facility. The system is designed to reduce wastewater effluent
concentrations to 30 mg/L each BOD and TSS. After treatment in completely-mixed
aerated lagoons, suspended solids are removed in settling basins. UV disinfection is used
to inactivate pathogens. Effluent exits through a pipe into an armored channel and mixes
with stream water, south of the treatment facility.
7.1. Collection System
Flowrates should be monitored in order to determine whether or not there is
excessive inflow and infiltration. Excessive inflow and infiltration may indicate that there
are broken pipes or leaking manholes. Additionally, the exact locations of community
expansion should be considered in order to appropriately size collection system piping.
7.2. Wastewater Treatment Facility
The wastewater treatment facility would require additional analysis for berm and
lagoon design to account for construction and operational considerations. Further analysis
of slope stability, anchor trenches, nailings for the HDPE liner along the top of the berm,
and protection against failure modes of sand (piping, seepage, liquefaction, boiling,
heaving, etc.) should be completed. Construction should take place during in the dry season
to ensure optimal conditions. All lagoons must be filled throughout the entire design life to
maintain slope stability (i.e. extra lagoons must be filled with water when not in use).
Fencing should surround the wastewater treatment facility for resident safety and
to prevent animals from entering the treatment system. Additionally, shallow-rooted trees
should be planted around the fence to provide a more aesthetically pleasing view.
41. Page 41 of 45
7.3. Cost Analysis
The cost analysis for the collection system does not include a specific hauling
location for excavated soil. A location must also be selected for settling basin solids
disposal and included in facility O&M cost estimates.
7.4. Recommendation
The recommended collection system design alternative utilizes gravity conveyance
(Collection System Alternative 1). Alternative 1 is more cost effective with regards to both
capital cost and O&M, because there is one less pump and lift station to maintain. The
completely-mixed aerated lagoon system (Wastewater Treatment System Alternative 1)
was deemed most fit for Bahia Ballena. Additional aeration treatment is possible for greater
quantities of water and a smaller footprint than other low-cost options (Figure 17). The
designed treatment and gravity collection systems provide a centralized location which will
minimally impact economic development along valuable, coastal properties.
Figure 17. Wastewater treatment facility site overview.
40 m
42. Page 42 of 45
Acknowledgments
Costa Rica Senior Design Team 1A would like to express our very great appreciation to
Dr. Gretchen Bohnhoff, Dr. Michael Penn, and Dr. Philip Parker for guidance and
enthusiastic encouragement.
Braden O’Leary (Triplepoint Environmental, LLC) and Rich (Mulcahy Shaw Water) were
very helpful with questions relating to complete aeration and UV disinfection, respectively.
Brad Etzel (Lincon Contractors Supply, Inc) assisted in the recommendation of the pump
for our lift station and provided us with a quote.
We would also like to extend our thanks to Mr. Mohammed Haque for his time and
guidance, and providing us the opportunity to compete in the Global Stewardship Design
Competition as part of our senior design project.
Additional thanks go to Mr. Michael Holland for providing data and coordinating the
design competition.
43. Page 43 of 45
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45. Page 45 of 45
Appendices
CD800 Dri-Prime Pump Data Sheet ……………………………………….....Appendix A
Wastewater Treatment Facility: Sample Calculations...………………....…....Appendix B
Design Plan Sets……………………………………………………………….Appendix C
