1. Human Health/Environmental Needs. The current effluent quality at the NIWTP is
unable to comply with the newly established National Pollutant Discharge Elimination
System (NPDES), requiring nitrogen removal. Also, the International Outfall
Interceptor and the wastewater collection system allow
excessive amounts of extraneous water to infiltrate the system during
precipitation events.
An additional concern
is the need to maintain and enhance the riparian and aquatic ecosystem existing
downstream of the NIWTP, inhabited by the endangered Gila topminnow (Poeciliopsis occientalis). Additionally,
provisions need to be made to account for the fact that wastewater reclamation
could be a feasible solution to the region’s water resources problems in the
future.
In 1996 the IBWC (both sections) signed Minute 294, which
relates to the solution of sanitation issues along the US-Mexico border. The sanitation project of Nogales complies
with the spirit of environmental cooperation presented in Minute 294.
2. Environmental Assessment.
As part of the planning process, an environmental assessment (EA) was submitted
to the International Boundary and Water Commission (IBWC) and Environmental
Protection Agency (EPA) in December, 1999. The EPA (Region IX) issued a Finding
of No Significant Impact (FNSI) on February 11, 2000. A 30-day public comment
period, ending on March 12, 2000 followed. A comment was received regarding
flooding potential due to increased growth in the City. EPA adequately
addressed the comment.
3. Compliance with Ecology and Cultural Laws
and Regulations. As
part of the environmental review, the Arizona State Museum was consulted to
determine if the project would cause any threats to the ecology or cultural
resources of the area. The Arizona State Museum determined that the project
would not cause any negative impacts in the area.
1. Appropriate Technology. As part of the wastewater facilities plan for
Ambos Nogales, various alternatives were evaluated. Preliminary sizing for
treatment trains utilized maximum month flow
projections for the year 2020. The exceptions, headworks and disinfection
facilities, were based on an estimated peak flow of twice the maximum month
flow.
WASTEWATER TREATMENT
The conceptual design of the treatment
technologies was based on the following concentrations of key constituents in
the influent:
|
Biochemichal Oxygen Demand (BOD5): |
150 mg/L |
|
Total Suspended Solids (TSS): |
200 mg/L |
|
Total Kjeldahl Nitrogen (TKN): |
50 mg/L |
|
Total Phosphorous: |
10 mg/L |
These values are slightly higher than current NIWTP
influent values, since infiltration/inflow improvements are expected to
increase the concentration of these constituents.
Preliminary
Treatment. Three processes were considered for preliminary
treatment at the NIWTP:
·
Trash racks
·
Bar screens
·
Aerated grit chambers
Secondary Treatment. The
upgraded and expanded NIWTP must be capable of achieving stringent ammonia
removal levels, due to toxicity threats to aquatic life or as the first step in
removal of total nitrogen/nitrate, or both. In light of these requirements, the
following alternatives were considered for secondary treatment at the NIWTP.
·
Aerated lagoons.
·
Facultative lagoons.
·
Facultative lagoons with ammonia stripping.
·
Advanced integrated pond systems (AIPS).
·
Lag-Nite Lagon nitrification system
·
Trickling filters.
·
Constructed wetlands
·
Oxidation ditches
·
Activated sludge (Modified Ludzack-Ettinger
process)
Disinfection
facilities. Chlorination and ultraviolet light irradiation
(UV) were considered for disinfection at the NIWTP.
Selected
Alternative. The selected alternative entails the use of
aerated grit chambers, and conversion of the existing complete mix lagoons
(CML) to a nitrification/denitrification reactor based on the Modified
Ludzack-Ettinger process. UV was selected as the disinfection technology at the
NIWTP.
No-Action Alternative.
Under this alternative, no long-term improvements would be
made to the NIWTP. However, the existing treatment train is not capable of meeting
the anticipated ammonia limit of the revised NPDES permit for the plant at any
projected flowrate, and may not be capable of meeting BOD5 discharge
limits at flows higher than presently observed. Failure to meet the proposed
ammonia limit will result in a continued negative impact on population of
aquatic species at points downstream of the NIWTP outfall.
The NIWTP treatment process is presented in the
figure below.
WASTEWATER COLLECTION SYSTEM
Several of the
subcollectors in Nogales, Arizona will be replaced to increase hydraulic
capacity for future needs. In some
cases, a few subcollectors will be replaced in their entirety. Approximately 1,800 m (5,900 ft) of sewer
lines will be replaced throughout the City.
INTERNATIONAL OUTFALL INTERCEPTOR (IOI)
The IOI will be replaced because of its age and
state of deterioration, the need for additional hydraulic capacity to relieve
surcharging in the system, and to minimize the infiltration/inflow flows and
related problems. The proposed IOI will convey the incoming wastewater from
across the U.S.-Mexico International Border to the NIWTP. Also, the IOI will
receive flows from several main subcollectors or interceptors in Nogales,
Arizona. The total length of the IOI
will be 14.4 km (8.9 mi).
O&M Plan. This
plan identifies the requirements of O&M of all
unit processes, including human resources, the frequency of maintenance and
cost estimates. Before operation of the upgraded and expanded NIWTP, and wastewater
conveyance lines, there should be a final O&M plan in place. The O&M
plan would be developed once the final design for the project is completed.
2.
Compliance with applicable design norms and regulations. The requirements for construction of the sewer system, IOI, and
treatment plant have been validated by the Ambos Nogales Binational Technical
Committee.
Financial Feasibility and Project Management
1.
Financial
Feasibility.
The NADB is currently
analyzing the project to determine its financial feasibility. This analysis will help determine the
contributions for each funding source, as well as credit and/or grant
components, and the ultimate rate structure.
The table below
summarizes the project construction costs.
|
Estimated Cost |
|||
|
Concept |
Amount
(US$) |
||
|
|
|
Arizona Wastewater
Collection System |
638,000 |
|
|
|
Expansion and Upgrade of
NIWTP |
26,064,000 |
|
|
|
International Outfall
Interceptor |
19,400,000 |
|
|
|
Total |
$46,102,000 |
|
Estimated
Operation and Maintenance Costs (Annual) |
|||
|
Concept |
Amount
(US$) |
||
|
|
Arizona Wastewater Collection |
4,000 |
|
|
|
NIWTP |
2,279,000 |
|
|
|
IOI |
30,000 |
|
|
|
|
Total |
$2,313,000 |
Based on the analysis developed
by the NADB, the grant and loan amounts for the project will be determined.
Financial
Structure
|
Source |
Amount (US$) |
% |
|
BEIF |
$42,904,000 |
93 |
|
WIFA Loan |
$638,000 |
1.4 |
|
WIFA Loan (non eligible projects) |
$2,560,000 |
5.6 |
|
Total |
$46,102,000 |
100% |
2.
Rate
Model: The rate model prepared by the NADBank was used
to determine the impact that the proposed project would have on the user rates,
considering various grant and loan combinations. The proposed wastewater rate increase
is $6.75 to $9.96 for the base fee over a period of 10 years, and a increase in
the consumption of every 1,000 gallons of $1.17 to $1.71, over the same period.
3.
Project
Management. The project will be managed by the IBWC and
the City of Nogales, the later being the entity responsible for the provision
of water and wastewater services to the community. The
system is expected to operate in a self-sufficient manner, supporting itself
through user fees.