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CONCEPTUAL DESIGN OF A WASTEWATER TREATMENT PLANT FOR THE MUNICIPALITY OF COTORRO, PROVINCE OF

HAVANA, CUBA

Cristina Ortega-Castineiras, Karen Kajder, and Reshma Ramoutar1

As Cuba may be approaching a political and socialtransition, there is increasing interest in the SouthFlorida community regarding planning improvementsfor its decaying infrastructure. Those who have at-tempted to work on this subject have met the complexchallenge of working with the very limited data and of-ten unverifiable information that is published by theCuban government. Our University of Miami seniorenvironmental engineering team took on this plan-ning challenge by tackling a sanitary infrastructure is-sue of great importance to the country, namely, pro-viding wastewater treatment in an underservedcommunity to improve receiving water quality andpublic health.

After performing extensive research on the conditionsof the sanitary infrastructure of Cuba, one major prob-lem was identified in the Municipality of Cotorro, inthe Province of Havana. This municipality’s domesticsewage is currently being discharged with no treatmentinto the upper tributaries of the Almendares River,which shortly after flows into the Ejército Rebelde(Paso Seco) Reservoir. The river, as well as the reser-voir, is hydrologically connected to the Vento Aquifer.This aquifer is very important to Havana because itserves as the source of over 50% of the potable water in

the province. Even though there have not yet beensigns of contamination found in the aquifer water, it isimportant to prevent such pollution because once it isdetected in an aquifer, it is usually too costly and diffi-cult to remediate . The potential contamination of theVento Aquifer, and the unsanitary conditions of theAlmendares River provided the basis for the design so-lutions set out in this paper.

BACKGROUND INFORMATION ON COTORRO

Cotorro is a municipality located to the southeast ofthe city of Havana, the capital of Cuba (Figure 1). It islocated at the center of an important freeway networksystem that serves as its limits. It encompasses an areaof 65 square kilometers. More than half of its total area(about 56%) is devoted to agriculture. Of the remain-ing land, 22% is occupied by water and the rest is notsuitable for agriculture (JCCC 2008).

According to the latest population census, there areapproximately 74,500 people residing in Cotorro. Thepopulation density is about 1,131/km2. There are24,953 housing units, 253 work centers and 108 smallfamily clinic facilities (JCCC 2008).

1. This document reflects the collective guidance and review comments from many individuals both within and outside the University ofMiami Department of Civil, Architectural and Environmental Engineering. The authors would like to specifically thank the following in-dividuals and organizations for their valuable time in providing ideas and review comments: University of Miami—Dr. Helena Solo-Gabri-ele; Camp, Dresser & McKee—Armando Perez, Victor Pujals, Stefan Haecker, Ignacio Lizama, Bruce Chalmers, Layla Llewelyn; Hazenand Sawyer—Peter Robinson, Jason Payge.

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The Municipality of Cotorro is located within the re-gion of the Vento Aquifer, in the surroundings of theupper Almendares River and its intermittenttributaries—Jicotea, Chaelote, Limón and San Fran-cisco (Figure 1). Climate characteristics for this cityare tropical with hot, humid summers, and warm, drywinters. Located in the Caribbean, the city experiencesits warmest temperatures in August with averagesaround 26.5 °C (80°F), and its coldest temperatures inJanuary at 19.8°C (68°F). The average humidity is at81.7%, which is typical for a tropical climate. Coto-rro’s average yearly rainfall is 1567.8mm (JCCC2008).

PROBLEM STATEMENT In the municipality of Cotorro, domestic sewage is notproperly treated. A few houses have septic tanks; otherhomes are connected to pipes that discharge theirwaste directly into streams, and the wastewater fromthose that are connected to the existing sewer collec-tion system ends up in the local streams with no previ-ous treatment. All of these impaired streams and tribu-taries run into the upper section of the AlmendaresRiver, which discharges its waters into the Ejército Re-belde (Paso Seco) Reservoir.

There is great concern for the future of the VentoAquifer, and local and regional authorities have real-ized the paramount importance of the contaminationissue of the Almendares River. To tackle the problem,a sewage collection system was built in the 1980s, withthe goal of collecting and pumping the wastewateroutside of the Vento-Almendares hydrologic system.The San Pedro Pump Station, built in the 1990s tomove the collected wastewater away from the city, hasnever worked properly. As new funding from the Or-ganization of Petroleum Exporting Countries(OPEC) is made available, new decontaminationplans are being implemented. The plans include build-ing additional sewer collection lines that will serve theentire urban population, the restoration of the San Pe-dro pump station, and the construction of a holdingtank and a wastewater treatment plant that will dis-charge treated water via an interbasin transfer into theCojimar watershed, away from the Vento Aquifer.

Currently in Cotorro, the sewer collection system isunder construction, with a schedule to be completedwithin the next five years. The pump station, as well asthe holding tank, has been in place for some time. The

Figure 1. Map of the City of Havana

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piping system connecting the San Pedro pump stationand the holding tank have already been built. The mu-nicipality’s plan also includes the construction of awastewater treatment plant downstream of the hold-ing tank. The aim of this report is to present a concep-tual design for that Wastewater Treatment Plant.

PROJECT LOCATIONThe main constituents of the project are the San PedroPump station and the proposed Cotorro WastewaterTreatment plant (WWTP) (Figure 2). The pump sta-tion is located on the west side of the city, at a lower el-evation than that of the town. Wastewater movesthrough the sewer lines in the city by gravity to thepump station, where the wastewater is then pumpedback over the high ridge of Cotorro to the future Co-torro WWTP. The wastewater will be treated at theplant and then discharged across a hydrologic basin di-vide into the Pitirre reservoir, which is connected tothe Cojimar River. The treated water eventually willmake its way into the ocean on the northern coast ofthe island.

OBJECTIVESThis project aims to develop the conceptual design ofan effective and economically responsible wastewatertreatment plant for the municipality of Cotorro. Theplant will use as many sustainable and energy efficientconcepts as possible, while still keeping constructionand maintenance costs low. The over-arching goal ofthe project is to prevent the contamination of the Ven-

to aquifer, while also minimizing the environmentalimpacts on the surrounding ecosystems.

The design process included taking into account theadvantages and disadvantages of different unit process-es, analyzing their technical details, and determiningthe removal efficiency of each unit operation. In addi-tion, costs of design, construction and maintenance,and durability were other factors used in the selectionof the design process.

THREE-PHASE DESIGN APPROACHDue to anticipated limited funding for the project, thedesign and construction of the wastewater treatmentplant for Cotorro is divided into three phases. Thisphasing allows a minimum level of treatment (i.e., pri-mary) to be provided early on. In the future, secondaryand advanced treatment will be added to the plant astime and finances allow. The three project phases con-sist of the following tasks:

Phase I: Preliminary, and Primary Treatment and Dis-infection

• The construction of preliminary and primarytreatment processes for the proposed WastewaterTreatment Plant, as well as the disinfection units.The sludge treatment processes will also be includ-ed in this phase.

Phase II: Secondary Treatment

• This phase includes the addition of biologicaltreatment to the plant for further biochemical ox-ygen demand (BOD) removal so that the effluentwater meets the national total suspended solids(TSS) and BOD discharge standards.

Phase III: Tertiary Treatment- Nutrient Removal

• The addition of tertiary treatment to the wastewa-ter treatment plant will make sure that the highesteffluent quality standards are met in the finalphase. This additional treatment is recognized asan extremely futuristic plan.

Sewage Characterization, Population Growth and Projected FlowIn order to design a wastewater treatment plant forCotorro, the characteristics of the influent were deter-mined. This information was unavailable through re-search, and therefore educated assumptions using typi-

Figure 2. Map of Cotorro with Project Location

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cal values from the U.S. Environmental ProtectionAgency (EPA), along with information from engineer-ing literature, the United Nations, and the PuertoRico Aqueduct and Sewer Authority were used to cal-culate wastewater generated per capita, and the con-centrations of biochemical oxygen demand (BOD)and total suspended solids (TSS). These parametersare used to measure the relative ecological impacts ofthe waste—the lower the concentrations, the lowerthe impacts when discharged to the environment. Theamount of wastewater generated per person was calcu-lated to be 100 gallons per day (GD), including typicalwater use per capita, infiltration and inflow, and in-dustrial water use. The projected population—with adesign period of 25 years and a growth rate of 0.37%—was calculated to be 82,500 people. Therefore, theprojected average flow rate for the wastewater treat-ment plant was estimated at about 8.25 million gallonsper day (MGD). With this flow rate, the BOD was es-timated at 240 mg/L, and the TSS concentration is300 mg/L. Another parameter that is used to evaluatewastewater discharge is nutrient content, primarily ni-trogen and phosphorus. Presently, nutrient removal isnot mandated in the United States; however, as part ofthe design considerations for the town of Cotorro, var-ious options for nutrient removal processes were ex-amined.

Assessment of Existing Infrastructure

Currently in Cotorro, there exists a sewer collectionsystem and a pump station. According to local offi-cials, the sewer collection system is going to be re-vamped, and new sewer lines, of twenty-four inch di-ameter, will be installed throughout the entiremunicipality within the next five years. The wastewa-ter from the sewer lines will travel by gravity to the SanPedro pump station, where it will be pressurized totravel two kilometers across the city to an already exist-ing holding tank, where the water will then be dis-charged by gravity to the new wastewater treatmentplant.

The pump station was rebuilt fifteen years ago, and itspurpose was to lift the wastewater to the zone of thePitirre reservoir, which would keep the contaminatedwater away from the Almendares River and VentoAquifer; and instead direct its flow into the Cojimar

watershed. Currently, because the pump station is notworking, the wastewater from Cotorro is bypassingthe lift station and being directly discharged into astream connected to the Ejercito Rebelde reservoir. Atthe entering pool of the pump station, there are manu-ally-cleaned screens, which will be utilized as part ofour treatment design. The station includes two bigpumps and four smaller ones. The combined pumpingcapacity of these six pumps is 64.8 m3/min (24.6MGD). The pumps are thus adequate to send the pro-jected amount of wastewater flow over to the holdingtank and the planned Cotorro wastewater treatmentplant.

Treatment ProcessesBased upon previous assumptions, the influent waterto the treatment plant is characterized with mediumquality. The unit processes within the WWTP are de-signed to efficiently reduce the influent BOD and TSSto the standard levels for Cuba, as well as UnitedStates EPA standards of 30 mg/L. A major consider-ation and goal of the design is to maintain the ecologyof the receiving water bodies.

After studying treatment systems used within otherCaribbean islands such as Puerto Rico, the Virgin Is-lands and Trinidad and Tobago, and the existing Ma-ria Del Carmen WWTP in Havana, we found thatthese plants have several common treatment processes,namely: primary sedimentation, biological filtration,secondary sedimentation, and sludge digestion. Ac-cordingly, we formulated the proposed treatment pro-cesses displayed by Figure 3.

Treatment units are designed for ultimate perfor-mance when all units are working. However, the de-sign will also allow for one unit to be offline for main-tenance purposes, upholding the adequate treatmentand compliance with recommended loadings and re-moval rates.

Phase I: Preliminary and Primary Treatment, and Disinfection Screening: Screening is the first stage of the pretreat-ment process that is necessary to remove large objectssuch as rags, paper, plastics, trash, and metals. Removalof these solids prevents damage to the pumping equip-ment, sludge removal equipment, hang-over weirs,valves, nozzles, channels, or pipelines. Due to the

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unique design of the WWTP, the bar racks will beplaced prior to the pumps at the San Pedro Pump Sta-tion.

Grit removal: Grit removal is the second phase of thepre-treatment process; in the grit chamber all othermaterials in the wastewater that are heavier than or-ganic matter are removed. Common substances re-moved are sand, dust, cinder, bone chips, coffeegrounds, seeds, and eggshells. This extra step is used tofurther protect the equipment against wear, preventclogging, and reduce build up of materials in otherprocessing stages. Different types of grit chamberswere evaluated for potential use in the CotorroWWTP; consequently, based on the analysis of eachtype of grit chamber, we chose an aerated grit chamber,which will deliver proper pretreatment in one easystep.

Metering: Metering systems are built in to the treat-ment plant to measure the flow rate of the incomingwastewater. An effective metering system is the par-shall flume, and this device was selected for our design.

Primary Treatment: Settling, or clarification, is per-formed in rectangular or circular tanks where thewastewater is held quiescent to permit particulate sol-ids to settle out of suspension. To prevent short-cir-cuiting and hydraulic disturbances in the tank, flowenters behind a baffle to dissipate inlet velocity. Over-flow weirs, placed near the effluent channel, are ar-ranged to provide uniform effluent flow. Floating ma-terials are prevented from discharge with the liquidoverflow by placing a baffle in front of the weir. A me-chanical skimmer collects and deposits the scum in apit outside the tank. Settled sludge is slowly moved to-ward a hopper in the tank bottom by a collector arm(Hammer and Hammer 2007). In order to determinethe type of sedimentation tank to be used, advantages

and disadvantages were evaluated for the circular andrectangular tank alternatives. We selected circular clar-ifier tanks because they have lower capital and mainte-nance costs and they can achieve greater weir lengths.In these clarifiers, raw wastewater enters through portsin the top of a central vertical pipe into a center wallbaffle then flows radially to a peripheral effluent weir.The center well directs flow downward to reduceshort-circuiting across the top.

Disinfection: The process of disinfection includes theinactivation of pathogenic organisms, such as entericbacteria, viruses, and protozoa. The purpose for disin-fecting wastewater effluent is to protect public health.There are 3 main methods of disinfection used forwastewater treatment: chlorination, ozone, and UV.Based on its cost-effectiveness, advantages over theother technologies, and its traditional use, chlorina-tion was selected as the disinfection process for theCotorro WWTP. Additionally, the potable water inHavana is disinfected through chlorination and there-fore the chemicals are presumed to be readily available.Moreover, the gas chlorination system was chosen forthe design based on its clear economic and practicaladvantages over other available technologies such ascalcium and sodium hypochlorite. Admittedly, the ini-tial capital investment for a chlorine gas system isslightly higher than for hypochlorite treatment; how-ever, the savings in chemical cost far outweigh theequipment costs (Hydro Instruments).

Sludge Treatment: Sludge, a mostly wet mixture ofsuspended solids and wastewater, is generated at boththe primary settling tank and the trickling filters. Forthe Cotorro wastewater treatment plant to be sustain-able, the sludge will be treated so that it can be used asa soil conditioner or fertilizer in the agricultural regionof the municipality. To achieve this goal, calculationswere performed to determine the amount of sludge be-

Figure 3. Wastewater Treatment Process

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ing generated based on the incoming flow rate project-ed at the wastewater treatment plant, and the assumedwastewater characteristics regarding the total suspend-ed solids. The calculated volume of sludge is about14,100 lb/d. To treat this amount of sludge, the fol-lowing processes were selected based on their effective-ness for a tropical region, and their low cost: prelimi-nary blending, thickening by gravity, stabilization inan anaerobic digester, and final dewatering on a paveddrying bed. Each of these treatment units were sizedfor the daily generation of sludge, and the final prod-uct will be safe and dry enough to use for agriculturalpurposes. The preliminary treatment helps to makethe sludge from the two sources into a homogenousmixture; the gravity settling helps to thicken thesludge and remove some of the water; stabilizing thesludge helps to remove odors and destroy any patho-gens; and laying out the sludge in a thin layer to dry inthe sludge turns the “waste” product into a resource.From the anaerobic digester, energy can also be gener-ated from the methane gas being produced.

Phase II: Secondary Treatment

Secondary Biological Treatment: Secondary biologicaltreatment is designed to remove dissolved organicmatter from wastewater, which in turn reduces BOD.Sewage microorganisms are cultivated and added tothe wastewater. In turn, these microorganisms absorborganic matter from the sewage as their food supply.There are three main alternative categories used toachieve secondary treatment: fixed film, suspendedfilm and lagoon systems. A fixed film alternative wasdetermined to be the most suitable choice for CotorroWWTP, because it is a relatively inexpensive alterna-tive that works well in tropical climates. After per-forming a thorough investigation of the biological pro-cesses available, it was concluded that the tricking filterwith a rock media best fits the desired WWTP designbecause it is very efficient, simple, and economical. Itworks very well in tropical climates and consequentlyit is commonly used in islands of the Caribbean. Thesuspended film processes (activated sludge and oxida-tion ditch) entail a higher energy use and more com-plex operation.

A trickling filter is an aerobic sewage treatment meth-od in which the sewage is dispersed via a revolving

sprinkler suspended over a bed of porous material. Thebase material for the selected trickling filter is crushedstone, with a depth of 6 feet above the under drain sys-tem. The influent sewage slowly moves through theporous bed and the effluent is collected at the bottom.The crushed stone eventually becomes coated with amicrobial community that absorbs and breaks downdissolved organic nutrients in the sewage; thus consid-erably reducing the BOD. This process is also aeratedby the movement of air through the spaces betweenthe stones.

Secondary Settling Tank: Settling tanks that followbiological filters are similar to primary clarifiers. Forsecondary settling tanks, the same choice of circular vs.rectangular tank exists, and due to reasons discussed inthe previous sections, circular clarification tanks werechosen. The purpose of gravity settling following atrickling filter is to collect biological growth, or hu-mus, flushed from the filter media. These sloughed sol-ids are generally well-oxidized particles that settle read-ily (Hammer and Hammer 2007). These biosolids arereturned to the primary sedimentation tank wherethey re-settle and then are pumped to the sludge treat-ment units. This recycling system also serves the pur-pose of facilitating the control of an equalized flowthrough the plant, and the proper recycling of treatedwater through the trickling filters.

Phase III: Tertiary Treatment

The idea for the addition of tertiary treatment to theCotorro WWTP is acknowledged as a very futuristicplan. Tertiary treatment technologies are expensive,and usually not required by the regulatory agencies.However, laws are beginning to become more strin-gent, and people are beginning to realize that the waterdischarged today is the water they will be drinking to-morrow. It is important then to provide an insightinto the current available technology that would bestfit into the design for Cotorro, and would providetreatment necessary for compliance of more stringentregulations. The main priorities for the implementa-tion of tertiary treatment in Cotorro are to achieve ni-trogen and phosphorous removal and additional sus-pended solids removal. Due to its ability to achievesuccessful removal of both suspended solids and dis-solved nutrients, the recommendation is that a high-

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rate, up-flow biological fixed-film system be used.These systems employ a proprietary dense granularsupport media that acts as a biological contactor aswell as a filter. Both the influent wastewater and pro-cess air flow from the bottom of the unit in an upwarddirection. This allows the water with the highest pol-lutant load to contact the media where the dissolvedoxygen level is the greatest.

Figure 4 displays the layout of the treatment unitswithin the Cotorro WWTP. The proposed treatmenttrain, including up to Phase II, encompasses two aerat-ed grit chambers, a Parshall flume, four circular prima-ry clarifiers, four trickling filters, four circular second-ary clarifiers and disinfection by gas chlorination withtwo contact basins. In addition, a set of units are in-cluded for the treatment and recycling of the biosolidsgenerated at the plant.

Summary of BOD, TSS and Nutrient Removal

Each of the treatment units selected for the CotorroWastewater treatment plant works to reduce the con-centrations of BOD, TSS, and nutrients. From our se-lected processes, the wastewater effluent will meet thequality characteristics of a Class “B” water body (“riv-ers, reservoirs and hydrogeologic zones which are uti-lized for irrigation, where products may be consumedraw, or for recreational purposes”) as defined by theCuban government. The final characteristics of thewastewater after secondary treatment are: BOD = 18

mg/L, TSS = 13 mg/L, Total Phosphorus = 4 mg/L,and Total Nitrogen = 22 mg/L. For the effluent tomeet the characteristics of a Class “A” discharge (“riv-ers, reservoirs and hydrogeologic zones which are uti-lized as a source of potable water for public use and/orindustrial use for the processing of food”), the tertiarytreatment, which will further remove nutrients andsuspended solids in the wastewater, needs to be imple-mented. A Class “A” effluent will not have any harm-ful effects on the local ecosystem or environment, andwill also prevent public health issues within the com-munities of people living along the Cojimar River.

HydraulicsIn order to achieve the goals of cost-effectiveness whileproviding the best treatment for the wastewater in Co-torro, the plant was designed to operate entirely bygravity. If the wastewater can flow without the use ofpumps or other energy intensive units, the cost andmaintenance of the plant can be lowered. By analyzingthe topography of the Cotorro region, specifically inthe north-east corner where the treatment plantwould be located, it was determined that there is ap-proximately 25 feet of elevation difference from wherethe wastewater enters the plant to the effluent pointafter the chlorine contact basin. There is an additional25 ft elevation difference between this point and theeffluent point at the reservoir where the treated waste-water would be discharged. Using the Hazen-Williamstables for head loss within pipes and typical head lossvalues for each of the treatment units (Metcalf andEddy 1991), the hydraulic head loss through the plantwas calculated. It was verified that the proper head willbe available for the Cotorro WWTP to operate ongravity, saving costs and also taking advantage of thelocal environment and topography.

CONCLUSIONAs the local and regional authorities have realized theimportance of the contamination of the AlmendaresRiver, and how the elevated contamination inputs ofthe wastewater discharge in Cotorro contributes tothe impairment of this watershed, a plan has been putin place to revert the current situation. The conceptualdesign of the Cotorro WWTP described in this reportis a very essential part of addressing current pollutionproblems.

Figure 4. Cotorro Wastewater Treatment Plant Layout

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This conceptual WWTP design proposed for Cotorroin the short term consists of up to secondary treat-ment. With the proposed design, Cotorro’s WWTPeffluent characteristics would comply in the shortterm with Class “B” standards, and it has thepotential—with the addition of the tertiarytreatment—to comply with Class “A” standards.

Therefore, a futuristic plan for the addition of tertiarytreatment is also recommended. All the proposedtreatment units were designed to achieve acceptable ef-fluent characteristics in compliance with the nationalCuban standards, using the least expensive, most tradi-tional, and energy efficient technologies available.

WORKS CITED

Chin, David A. Water Resources Engineering. UpperSaddle River: Prentice Hall , 2000.

Clearinghouse, National Small Flows. “EnvironmentalTechnology Initiatives.” Ozone Disinfection. 22February 2009. http://www.nesc.wvu.edu/pdf/WW/publications/eti/Ozone_Dis_tech.pdf.

Corps-of-Engineers. “Ch 10: Trickling Filter Plants.”11 May 1984. 28 February 2009. http://140.194.76.129/publications//eng-manuals/em1110–3–172/c-10.pdf.

Hammer, Mark J. and Mark J. Hammer, Jr. Water andWastewater Technology. Upper Saddle River:Prentice-Hall, Inc., 2007.

Hydro Instruments. Hydro Instruments. http://www.hydroinstruments.com/page.aspx?page_id=38.

JCCC, Los Joven Club de Computación y Electróni-ca. Cotorro. August 2008. November 2008 http://www.cha.jovenclub.cu/municipios/cotorro/in-dex.php?op-tion=com_content&task=view&id=18&Itemid=55.

Lima Cazorla L, Olivares Rieumont S, Columbie I, dela Rosa Maderos D, Gil Castillo R. “Niveles dePlomo, Zinc, Cadmio y Cobre en el Río Alm-endares, Ciudad Habana, Cuba.” Revista Interna-cional de Contaminacion Ambiental (2005): 115–124.

Metcalf and Eddy, Inc. Wastewater Engineering:Treatment, Disposal, and Reuse. New York: Mc-Graw-Hill, Inc., 1991.

NC, Oficina [Cubana] Nacional de Normalización.Vertimiento de Aguas Residuales a las Aguas Te-rrestres y al Alcantarillado. Especificaciones. Ciu-dad de la Habana, 1999.

“Process Design Manual for Sludge Treatment andDisposal.” September 1979. Environmental Pro-tection Agency. January 2009. http://www.epa.gov/nrmrl/pubs/625179011/625179011.htm.

Puerto Rico Aqueduct and Sewer Authority. “Rulesand Regulations for the Design Standards.” Man-ual. 1983.

Qasim Syed. Wastewater Treatment Plant. CRC Press,1998.

Rodríguez, A. J., Castano Z., Marsan R., Lugo D., Ro-evros N., Chou L., Rojas N, and Heydrich M.”Contaminantes Inorgánicos Tóxicos en lasAguas del Río Almendares, Cuba.” Contami-nación Química y Carga Microbiana en las Aguasdel Río Almendares (2004).

WEF and ASCE. Design of Municipal WastewaterTreatment Plants. 1998.

Williams, Gardner S. and Allen Hazen. “HydraulicTables.” Hazen and Sawyer, 1965.