ENVIRONMENTAL EVALUATION OF TREATMENT / DISCHARGE ALTERNATIVES FOR THE CITY OF LIMA PERU

Transcription

1 I o CONGRESSO BRASILEIRO DE ENGENHARIA SANITÁRIA E AMBIENTAL ENVIRONMENTAL EVALUATION OF TREATMENT / DISCHARGE ALTERNATIVES FOR THE CITY OF LIMA PERU Luciano Meiorin (1) Technical Director, Parsons Engineering Science: Management and design of marine wastewater disposal systems, oceanographic studies, and ocean outfall design projects in: Monterey, California; Colombo, Sri Lanka; Inchon, South Korea; Athens, Greece; St., Thomas, Virgin Islands; Lima, Peru; Los Angeles (Terminal Island Outfall), and San Diego, California. Ruddy Noriega Pissani Project Director SEDAPAL: Project Director for the Promar Project, overseeing the feasibility study for the Wastewater Management and Coastal Pollution Control for the Greater Lima, Peru. Address (1) : Parsons Engineering Science, Inc., Genesee Ave. - La Jolla - California USA. - Tel: Fax: luciano_meiorin@parsons.com ABSTRACT Perched on the edge of the Peruvian desert, Metropolitan Lima overlooks the Bahia de Miraflores a popular summer resort area. Approximately 7.5 million people are concentrated in the urban area, which is projected to reach 1 million within 3 4 years. Current domestic and industrial wastewater generation approaches 19m 3 /s, most of which is discharged near the mouths of three coastal rivers or through shoreline pipes to Miraflores Bay or the Pacific Ocean. Less than two percent is treated in oxidation lagoons and partially reused for crop irrigation. The nearshore waters of Lima show strong evidence of wastewater pollution. Most beaches in the Lima area are unsuitable for swimming with total coliforms exceeding 1 6 NMP/1 ml. Salmonella, Vibrio and several parasites were found in fish and some showed very high metal concentrations. High nutrient levels trigger frequent algal blooms. Contamination from organic materials, trace metals and chemical wastes, coupled with oxygen depletion, seriously reduce water quality which has degraded marine habitats, and reduced marine productivity. In , Parsons Engineering-Science conducted oceanographic surveys to determine the extent of the wastewater contamination and performed hydrodynamic and water-quality modeling to simulate changes under several scenarios. The model took into account Lima s peculiar sea-current pattern where currents alternate north and south every few days and complicate dispersion modeling. The model showed that the use of two ocean outfalls could dramatically reduce nearshore pollution and produce acceptable levels of dissolved oxygen, nutrients, coliform and algae in the surface layer. The outfalls would generate a submerged plume with variable levels of coliform bacteria, organic matter and nutrients at mid-water depths (1-15 m). However, the discharge locations (3 and 8 km offshore) would be removed from the populated areas and allow safe use of the beaches and nearshore waters. KEYWORDS: Lima Peru, Ocean Outfalls, Coastal Pollution, Environmental Baseline, La Chira and Oquendo Outfalls. 2 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4214

2 I o CONGRESSO BRASILEIRO DE ENGENHARIA SANITÁRIA E AMBIENTAL Figure 1. Metropolitan Lima Project Area INTRODUCTION Lima today is an urban conglomerate of 7.5 million people. This metropolitan area is anticipated to grow rapidly towards a population of 1 million within three to four years. The city is located in the arid Pacific coast of Peru between latitudes 11 3 and 12 3 south. Three small rivers, with average flows of a few cm/s and usually dry for a large part of the year, traverse the urban area. The population is mostly concentrated on the sandy coastal plains, which during summer, are heavily used by Peruvians, tourists and swimmers. The metropolitan area and the existing sewerage system are shown in Figure 1. Current wastewater production approaches 19 m 3 /s, and is expected to reach 28 m 3 /s in about 2 years. Less than 2 percent is presently treated in oxidation lagoons constructed in The pond effluent is partially reused for local irrigation of crops. The rest is discharged directly to the ocean or into three rivers, as shown in Figure 2. In September 1996, the Government Of Peru (GOP) contracted with the Association of Nippon Jogesuido Sekkei / Parsons Engineering Science to study and design solutions to improve wastewater treatment and disposal in Metropolitan Lima. A comprehensive Wastewater Treatment Plan has been prepared and approved by GOP for design and implementation. This Plan includes a complex system of multiple treatment plants as listed in Table 1. The plan also included the design and construction of two ocean outfalls, to be located at La Chira and Oquendo near existing discharges F and C, as shown in Figure 2. 2 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4215

3 I o CONGRESSO BRASILEIRO DE ENGENHARIA SANITÁRIA E AMBIENTAL Figure 2. Untreated Wastewater Discharges in the Lima Area As shown in Table 1, 1.5 percent of the sewage is currently treated. As treatment plants come online, the volume should rise to 43 percent by 2 and reach 1 percent by 224. After completion of the Sewerage Master Plan, an extensive Environmental Impact Assessment (E.I.A.) was prepared covering the following: Environmental effects of two proposed outfalls in the ocean environment, Environmental impacts of the proposed treatment plants in Lima, and Alternative maximum reuse options for treated waters. This paper focuses on the first item: environmental effects of the proposed ocean outfalls. Table 1: Metropolitan Lima Wastewater Treatment Plan ( ) Discharge points (mc/s) South Lima Discharge or Reuse Treatment Level Surco surface no La Chira submarine primary San Bartolo reuse secondary San Juan reuse secondary TOTAL LIMA SUR North Lima Costanero surface no Bocanegra surface no.5.. Comas surface no Callao surface no Rimac River surface no Oquendo surface primary Oquendo submarine primary TOTAL NORTH LIMA LIMA TOTAL (No + So) Without treatment Treated o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4216

4 I o CONGRESSO BRASILEIRO DE ENGENHARIA SANITÁRIA E AMBIENTAL PROJECT APPROACH To evaluate the effects of the outfalls in the existing ocean environment, a three-step approach was utilized: Assessing the present conditions of ocean contamination in the Metropolitan Lima Area (considered as baseline conditions), Reproducing effects of the two outfalls by means of a sophisticated hydrodynamic and water quality 3-dimensional model, and Comparing the baseline conditions and model results, for a full understanding of expected long-term changes in the ocean environment. CONSIDERATIONS ON OCEAN OUTFALLS The basic philosophy of submarine wastewater disposal is the economic disposal of wastewater to avoid adverse effects on the receiving water that would impair beneficial uses. Marine wastewater disposal can be safe, effective and economical. For coastal cities, it can be particularly attractive due to the relatively low level of treatment, maintenance, and energy consumption required, as compared to other methods. A submarine outfall disposal system is an integral part of a waste treatment facility discharging into the marine environment. The design of the treatment facility and outfall are dependent upon the identified beneficial uses of the receiving water, the corresponding water quality criteria to protect those use(s), and the waste-assimilating characteristics of the receiving waters. The ability to assimilate waste depends upon physical oceanographic factors such as wind, wave, swell, littoral currents, variable currents, density gradients, upwelling, etc., as well as conventional physical, chemical, and biological characteristics of the aquatic environment. The function of an ocean outfall is to convey treated wastewater to a point in the ocean where the effect of the waste on the receiving water is minimal even at the point of initial mixing. The discharge point should be sited in an area that can provide rapid and thorough mixing with the receiving water and maintain subsequent transport and dispersion of the wasteseawater mixture, thereby preventing the occurrence of excessive concentrations of waste in the critical areas. The ocean is often the ultimate sink of waste products. Surface drainages typically flow to the ocean. Even land discharges can enter the groundwater and migrate toward the ocean. Most coastal cities have historically discharged into the ocean. Where problems occur, they are usually localized and related to shoreline discharges of inadequatelytreated effluent. Well-designed systems generally function smoothly by treating effluent to an appropriate level and discharging it through relatively long outfalls equipped with diffusers that achieve high levels of dilution and dispersion. A comparison of three hypothetical systems using a bacterial decay rate: T 9 = 4 hours is given below. The tremendous advantage of discharging the wastes offshore through an outfall is obvious, considering that the apparent dilutions achieved differ by several orders of magnitude between Systems A and B or C. 2 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4217

5 I o CONGRESSO BRASILEIRO DE ENGENHARIA SANITÁRIA E AMBIENTAL Treatment Est. Pollutant Discharge Dilution/Decay of Initial System Level Removal Point Concentration C after 12 hrs A Secondary 9% Shoreline C /1 B Adv. Primary 75% Avg. Outfall C /28, C Primary 5% Long Outfall C /3, Treatment beyond the primary level is not warranted in some cases. Secondary treatment usually involves biological processes to reduce Biochemical Oxygen Demand (BOD) which can cause an oxygen demand in the receiving waters. In the ocean, however, dissolved oxygen depletion is rarely a problem due to the high dilutions, vast quantities of stored dissolved oxygen, and large surface area available for re-oxygenation. Indeed, ocean disposal can function as a secondary treatment process in which the treatment is carried out in the ocean, rather than on land. Secondary treatment also has high-energy costs and generates large quantities of sludge with attendant disposed costs. Secondary treatment is, therefore, not indicated for discharges into open coastal areas, but may be necessary for discharge into enclosed estuaries and embayments with poor flushing. In the U.S., the Clean Water Act still requires secondary treatment for all ocean discharges. Some progressive states, such as California, allow a lower level of treatment, provided that specific requirements are met. Recently, in 1993, the National Research Council investigated the issue and published its findings in the report: Managing Wastewater in Coastal Urban Areas. The findings clearly support the assertion that lessthan-secondary treatment followed by a long, deep ocean outfall is adequate to protect the marine environment. The U.S. Environmental Protection Agency has recently approved the request of the City of San Diego to continue to discharge its advanced primary effluent through its new, very deep, ocean outfall. ESTABLISHING AN ENVIRONMENTAL BASELINE Two extensive oceanographic surveys were carried out in November 1996 and March 1997 (covering the spring and summer seasons in the Southern Hemisphere) The two campaigns covered a potential impact area of about 5 Km 2 (5 Km of shoreline and 1 km offshore). Fifty-eight stations were sampled at surface, mid-water and bottom depths. Nearly 8, seawater samples were collected and analyzed. The survey results characterize the existing or baseline water quality as follows: 1) Surface water temperature varies from 16.6 C 22.5 C. The vertical temperature profile shows an intrusion of warmer waters at the -1 to -3 m depth. Surface salinity ranges between ) Suspended solids in the middle of Miraflores Bay show values up to 34 mg/l, (compared to a typical seawater value of 2 mg/l). Water transparency is below standard in all coastal areas, with the lowest transparency of.3 m (Secchi disk) close to the major discharge points of La Chira and Comas. This condition seriously affects primary phytoplankton productivity with adverse consequences to marine life in general. 3) BOD concentrations range between ml/l, which are higher than normal seawater levels, and are indicative of external inputs of organic matter from 2 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4218

6 I o CONGRESSO BRASILEIRO DE ENGENHARIA SANITÁRIA E AMBIENTAL sewage. Surface dissolved oxygen ranges between ml/l. Low oxygen levels produce hypoxia in the water column and anoxia in the bottom sediments. Close to the outlets of the rivers, ph values were recorded as high as 1.27, which is inhospitable to marine life. 4) Surface concentration of phosphate ranges between µg-at/l, while nitrate reaches 6.55 µg-at/l. Sulfur ranges between µg-at/l. High nutrient levels spur algal proliferation and algae blooms may occur in spite of poor transparency. Organic matter in sediments ranges between %. 5) Organic and chemical contamination, coupled with oxygen depletion, subject all marine biota to high stress conditions. This can seriously impoverish marine life and reduce the self-recovering capability of the ocean environment. 6) Coliform bacteria analyses show concentrations above standards in almost all coastal areas and in Miraflores Bay. Near the discharges, total coliform concentrations have exceeded 1 6 MPN/1 ml and up to 2.5 x 1 4 MPN/1 ml have been found in the middle of Miraflores Bay. 7) Copper, lead and mercury are present in high levels throughout the entire northern area, and are especially concentrated in sediments. Copper was measured up to 22 mg/kg. Hydrocarbons reached 26.2 mg/kg of dry weight. THE MODEL General The study employed a three-dimensional hydrodynamic and water quality model. This model represents one of the outstanding features of the EIA by linking together 11 physical-chemical and biological constituents (BOD 5, DO, Total N, NH 3, NO 2, NO 3, Total P, PO 4, algae, coliforms and other tracers) in a series of equation systems. The interaction of these constituents is shown in Figure 3. Four scenarios posing existing conditions, construction of one or two outfalls, and varying treatment levels before wastewater discharge were studied. Model runs were conducted on each scenario simulating two seasons (summer and winter), three depths, and 11 separate constituents, leading to 264 different cases evaluated. How the model works RMA11 is a finite element water-quality model for simulation of three-dimensional estuaries, lakes and rivers, and was especially adapted for the conditions in Lima. A four-stage approach was used: 1) Grid generation; 2) Hydrodynamic simulation; 3) Water quality simulation; and 4) Display of results. Each task was carried out with a different model that produced linkable inputs and outputs. 2 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4219

7 I o CONGRESSO BRASILEIRO DE ENGENHARIA SANITÁRIA E AMBIENTAL Figure 3. RMA11 water quality model: nutrient interactions. ATMOSPHERE ORG-N setting demand BOD BED benthic uptake setting BED NH3 NO2 oxidation DISSOLVED OXIGEN setting BED ORG-P NO3 oxidation growth respiration benthic uptake BED DIS-P respiration growth ALGAE growth respiration Stage 1: Grid generation (Model RMA1) Grid generation started with the specification of the study region geometry (extent of modeling domain and land boundaries) and the bathymetry (water depths). The model covered the whole study area and generated a finite element grid. A higher grid resolution was used in the vicinity of the various discharges (shoreline or outfall) in order to obtain more detail on the steep concentration gradients in these areas. Higher resolution was also used in areas of complex rapidly-changing bathymetry to increase the hydrodynamic resolution of the model. Stage 2: Hydrodynamic Simulation (Model RMA2) The RMA2 Model solves the depth-averaged form of the shallow-water flow equations. The water motion was driven by the applied boundary conditions. These were specified water-surface elevations or flows, or some combination of the two, at the open boundaries. These boundary conditions could be specified as time-varying, leading to a dynamic simulation. In the Lima project, the flow across the northern and southern boundaries was determined by extrapolation from the measured ocean currents. The model was calibrated using approximately six months of current data, recorded at several current-metering stations. Stage 3: Water Quality Simulation (Model RMA11) The water quality RMA11 Model is significantly more complex than the hydrodynamic model. This model simulates the transport, mixing and chemical interactions of a number of constituents. In the Lima 2 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 422

8 I o CONGRESSO BRASILEIRO DE ENGENHARIA SANITÁRIA E AMBIENTAL project, attention was focused on algae-related constituents. These are DO (dissolved oxygen), BOD, various nutrients (nitrogen and phosphorus in different forms), and algae. In addition, a conservative tracer (for purposes of tracking other wastewater constituents) and coliform bacteria were also included. The major nutrient-related constituents interactions modeled are shown in Figure 3. The model reads input data from several files. The hydrodynamic result file generated by RMA2 provides the advective currents that drive the transport processes. In the Lima project, an initial 186-day run was performed, and the resultant output restart file was used as the input for another 186-day simulation period. This process allowed the modeling of discharges with background conditions that had reached a steady-state. Model outputs The model produced 264 representations which were viewed as consecutive steps rolling dynamically on the computer screen. The wastefield is identified with contrasting colors at specific concentration contours. On screen, the wastefields evolve and spread from the release point, under the influence of the currents. In total, 168 screens representing average yearly concentrations of 7 parameters (DO, NH 3, NO 3, PO 4, algae, coliform bacteria and other tracers) were plotted. Time series, representing the various scenarios, seasons and water depths, were also plotted. These series show the expected temporal changes at 2 selected stations considered most representative of the 58 stations sampled for the baseline study. WATER QUALITY SIMULATION RESULTS The RM11 Model output matches, with appreciable accuracy, the field results and ensures a reliable representation of future scenarios. The two proposed outfalls, even with only a preliminary level of wastewater treatment, clearly represent a viable solution. Based on the simulation results, surface contamination will be essentially eliminated and acceptable ocean water-quality conditions should be rapidly reached when the two outfalls are constructed and put into operation. The outfall design was based on density trapping the wastefield as low as possible in the water column to impede shoreward migration. At deeper layers, the impact of discharged sewage will be detectable, with a large wastefield moved by currents. This wastefield, however, will not affect the population, as it will be confined below the 1 15 m depths, 3 km from the shore. This residual negative effect will dissipate with time as the wastefield disperses and decays. Comparisons of the offshore area potentially impacted by pollutants for the different scenarios are shown in Figure 4. In Scenario 1, (existing conditions), the top ocean layer is heavily affected by the coastal discharges. Most of the beaches are polluted and water quality standards will be exceeded over 27, ha. Although the pollution is density-limited to the surface layer, the resultant reduced light transmittance will extend to the lower depths. 2 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4221

9 I o CONGRESSO BRASILEIRO DE ENGENHARIA SANITÁRIA E AMBIENTAL Scenario 2 offers primary treatment and discharge through two outfalls. Modeling indicates dramatic improvements over existing conditions with minimal standards exceedance. Higher concentrations of coliforms will be limited to the bottom depths and decay with time in the ocean. In the simulation, a conservative limit of 1, MPN/1 ml was used over the current 5, MPN/1 ml standard. Scenario 3 considers primary treatment for all the sewage except for one outfall at the southern end of Lima. One-third of the treated wastewater would be discharged through that outfall. Comparisons between the results of Scenarios 1, 2 and 3 indicate that the greatest improvement is obtained with an outfall rather than increased treatment. The Scenario 4 modeling confirms the above conclusion. This scenario is similar to Scenario 2 with two outfalls, but treatment is reduced to only screening and degritting. Scenario 4 appears to achieve good results, similar to those of Scenario 2. Anticipated frequency of compliance with water-quality standards among the four scenarios is shown in Figure 5. The two scenarios with two outfalls show a dramatic improvement over the existing conditions in Scenario 1. Scenario 3 with only one outfall in the south, shows continuing (albeit reduced) problems in the north. The projected compliance with water-quality standards in Scenarios 2 and 4 indicates that these two alternatives would be most suitable for the safe use of beaches and nearshore waters for recreational activities and other public uses. Figure 4. Comparison of Potential Offshore Impact Area by Pollutant type and Water Depth Area Exceeding Standard a (ha). Scenario Scenario Scenario Scenario Surface Water Layer (-8m) Nutrients DO DO Nutrients DO DO Mid- Water Layer (8-2m) Bottom Water Layer (>2m) a Water Quality Standards: Nutrients (NH 4 ) >.6 mg/l <1:1 Effluent Dilution Factor Dissolved Oxygen < 4. mg/l Coliform Bacteria <1, MPN/1 ml 2 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4222

10 I o CONGRESSO BRASILEIRO DE ENGENHARIA SANITÁRIA E AMBIENTAL Figure 5. Comparison of Pollutant Effects on Recreational Resource Areas Predicted Percent Exceedance of Water Quality Standard a (ha). Scenario Rio Rimac+ Rio Chillon Miraflores W Miraflores E La Chira Scenario Scenario Rio Rimac+ Rio Chillon Scenario Rio Rimac+ Rio Chillon Miraflores W Miraflores E La Chira Rio Rimac+ Rio Chillon Miraflores W Miraflores E La Chira a Exceedance of Water Quality Standards for = <1:1 Dilution Factor 2 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4223