Assuring a Viable Future for a Historic City

Authors:

  • Fernando Chiriboga, Ana Garcia, Phil Cooke, Pat Davis - Hazen and Sawyer

The waterways of Cartagena, Colombia, one of the most heavily used ports in the Caribbean Sea, were being degraded by releases of untreated wastewater, one of several factors that prompted the establishment of a wastewater master plan.

Hazen and Sawyer collected and evaluated data for the Wastewater Master Plan. Tide gauges were installed at many locations, including the Islas del Rosario, to help determine a suitable location for the ocean outfall.

With the treatment plant and open ocean outfall in service for only a few months, subsequent sampling results along the beaches have shown a marked reduction in bacteriological levels.

Founded in the 1500s by the Spanish as the main port for its famed galleons to carry gold, silver, and jewels to the New World, Cartagena de Indias (Cartagena) today has a population of nearly one million residents and a substantial tourism industry. Tourist income, which has become the mainstay of the local economy, was becoming increasingly threatened by the discharge of all wastewaters to the local estuary (Ciénaga de la Virgen) and bay (Bahía de Cartagena) through outfalls, force mains, and open sewers – without treatment. The Bahia is one of the most heavily used ports in the Caribbean Sea, with more than 28 million tons of cargo handled on an annual basis. Industrial development has been encouraged to support the local economy and now more than 80 industries are situated along the Bahía de Cartagena’s eastern shoreline. Passenger cruise ships have also adopted Cartagena as a regular port of call.

The historical importance of the City, combined with the Colombian government instituting effluent discharge standards in Decree 1594 and an increased need for sustainable development, prompted the local government to pursue the advancement of a master plan to expand the extent and capacity of the city’s sewage collection and address the need for treatment and proper disposal. The Distrito Turístico y Cultural de Cartagena de Indias and Aguas de Barcelona entered into a public/private partnership to upgrade and operate the utility. This entity, referred to as Aguas de Cartagena (ACUACAR), was successful in obtaining more than $110 million in loans from the World Bank and the Inter-American Development Bank for the project.

ACUACAR retained Hazen and Sawyer to evaluate various wastewater treatment and disposal options and prepare an Effluent Disposal Feasibility Study. Due to the limited assimilative capacity of the Bahía and the fact that the southern portion of the 5,500-acre Ciénaga had turned eutrophic, lacking adequate dissolved oxygen to support aquatic life, Hazen and Sawyer staff initiated a comprehensive 12-month field effort which included background water quality and benthic sampling, and conductivity, temperature, and depth profiles of the open ocean. We also utilized tide gauges and an acoustic Doppler current profiler (ADCP) to characterize ocean currents, waves, and tides more than 30 miles off the Colombian coast.

Effluent Disposal Feasibility Study
The study consisted of developing and utilizing computer models to predict hydrodynamics, pollutant transport, initial dilution, and solids deposition. Twelve months of field data collection was conducted to monitor parameters including:
• Water quality
• Current patterns
• Tidal cycles
• Benthic soil characteristics
• Water column stratification

More than 30 sample stations, located along the shore, within the Bahía, and in the vicinity of proposed outfall discharge locations, were monitored for parameters including BOD, TSS, total and fecal coliforms, enterococcoci, total nitrogen, total phosphorus, phytoplankton, zooplankton, dissolved oxygen, temperature, and salinity.

Tidal measurements were used as model input to predict hydrodynamic conditions within the study area. Current measurements were utilized to verify model results and to establish current magnitudes and direction at the proposed outfall locations. A detailed study of the local environment was critical to garner support from the citizens of Cartagena and the villagers of neighboring Punta Canoas, and to justify the project to the World Bank’s independent panel of experts who were charged with assessing the proposed design prior to making a recommendation for funding. Placement of the outfall in an area of naturally high background turbidity and low biodiversity ensured the best possible protection of the environment during both construction and operation. The design of the diffuser to achieve a dilution of 100:1, while accommodating the lowest 10th percentile current speed, coupled with the natural disinfection capability of the open ocean, ensured the best possible protection of the downstream public.

Design and Construction
Upon acceptance of the predesign, Hazen and Sawyer continued with detailed design for various components of the upgraded sewer system, including a new master pump station, a 72-inch conveyance pipeline that extended more than 12 miles, the new Punta Canoas Wastewater Treatment Plant, and a 72-inch diameter ocean outfall extending more than 2.5 miles and located 60-feet deep within the Caribbean Sea.

The escalation of material prices during the design process prompted the implementation of a unique outfall construction approach, wherein the pipe was fabricated out of HDPE in Norway and towed in seven 1,500-feet sections by tug to Cartagena. At the time of construction, the 72-inch I.D. pipe was the largest diameter solid wall HDPE pipe ever fabricated.

Upon reaching the port, the pipe segments were joined together, fitted with concrete ballasts and towed to the outfall site, where a carefully controlled air release from the seaward end enabled connection to the pipe trench in the surf zone. Once positioned and lowered to the ocean floor, the diffuser was installed and the treatment plant and outfall were placed into service in the spring of 2013.

Viable Future
The upgrades to the sewage collection and transmission system now capture more than 95% of all wastewater generated. The plant at Punta Canoas, the first wastewater treatment plant in Cartagena’s storied history, will provide for the City’s sanitary health for the foreseeable future.

For several years before outfall implementation, ACUACAR collected monthly bacteriological samples at the beaches in order to measure the degree of contamination along the shoreline. The data revealed that total and fecal coliforms often exceeded the recreational water quality standards by several orders of magnitude. With the treatment plant and open ocean outfall in service for only a few months, subsequent sampling results along the beaches have shown a marked reduction in bacteriological levels, exhibiting 95th percentile values well within Colombian standards. These water quality improvements have set the stage for recovery of the Bahía and Ciénaga.

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