Toda agua servida o residual debe ser tratada, tanto para proteger la salud pública como para preservar el medio ambiente. Antes de tratar cualquier agua servida se debe conocer su composición. Esto es lo que se llama caracterización del agua. Permite conocer qué elementos químicos y biológicos están presentes y da la información necesaria para que los ingenieros expertos en tratamiento de aguas puedan diseñar una planta apropiada al agua servida que se está produciendo.
Una Estación depuradora de aguas residuales tiene la función de eliminar toda contaminación química y bacteriológica del agua que pueda ser nociva para los seres humanos, la flora y la fauna, de manera que se pueda devolver el agua al medio ambiente en condiciones adecuadas. El proceso, además, debe ser optimizado de manera que la planta no produzca olores ofensivos hacia la comunidad en la cual está inserta. Una planta de aguas servidas bien operada debe eliminar al menos un 90 % de la materia orgánica y de los microorganismos patógenos presentes en ella. Las aguas residuales también provienen de laboratorios químicos (universitarios), algunos de los contaminantes son: plomo y cadmio.[4].
La etapa primaria elimina el 60 % de los sólidos suspendidos y un 35 % de la DBO. La etapa secundaria, en cambio, elimina el 30 % de los sólidos suspendidos y un 55 % de la DBO.
Stages of wastewater treatment
The wastewater treatment process can be divided into four stages: pretreatment, primary, secondary and tertiary. Some authors call the preliminary and primary stages united as the primary stage.
The preliminary stage must fulfill two functions:
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- Measure and regulate the flow rate "Flow (fluid)") of water that reaches the plant.
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- Extract large floating solids and sand (sometimes also grease).
Normally, plants are designed to treat a constant volume of water, so they must be adapted to the fact that the flow of wastewater produced by a community is not constant. There are times, generally during the day, when the volume of water produced is greater, so regulation systems must be installed at the entrance to the plant so that the flow that circulates through the treatment system is uniform.
Likewise, at the entrance to the plant it is necessary to filter the water that arrives, separating solids and fats from it so that the process can be carried out normally. The devices responsible for this function consist of "grids", "sieves", "crushers" (sometimes), "degreasers" and sand traps. At this stage, pre-aeration can also be carried out, whose functions are:
• - a) eliminate the volatile compounds present in the waste water, which are characterized by being smelly, and.
• - b) increase the oxygen content of the water, which helps reduce the production of bad odors in the subsequent stages of the treatment process.
Its objective is to eliminate suspended solids "Suspension (chemistry)") through a simple sedimentation process by gravity or assisted by coagulants and flocculants. Thus, to complete this process, chemical compounds (iron, aluminum salts and flocculating polyelectrolytes) can be added in order to precipitate phosphorus, very fine suspended solids or those in a colloid state.
The structures in charge of this function are the "primary sedimentation ponds" or "primary clarifiers." They are typically designed to remove particles that have "sedimentation rates" of 0.3 to 0.7 mm/s (millimeters per second). Also, the "retention period" is usually short, 1 to 2 h (hours). With these parameters, the depth of the pond fluctuates between 2 and 5 m (meters).
In this stage, around 60 to 70% of the suspended solids are removed by precipitation. In most plants there are several primary settlers and their shape can be circular, square to rectangular.
Its objective is to eliminate organic matter in solution and in a colloidal state through an oxidation process of a biological nature followed by sedimentation. This biological process is a "controlled natural process" in which the microorganisms present in the wastewater participate, and which develop in a reactor or aeration tank, plus those that develop, to a lesser extent, in the secondary settler. These microorganisms, mainly bacteria, feed on solids in suspension and colloidal state, producing carbon dioxide and water in their degradation, creating a bacterial biomass "Biomass (ecology)" that precipitates in the "secondary decanter." Thus, the water remains clean in exchange for producing sludge for which we must find a way to eliminate it.
In the secondary settler, there is a calm flow of water, so that the biomass, that is, the bacterial flocs produced in the reactor, settle. The sediment that is produced and which, as mentioned, is mainly made up of bacteria, is called active sludge.
The microorganisms in the aerated reactor can be suspended in water ("suspended growth processes" or activated sludge), adhered to a suspension medium ("adhered growth processes") or distributed in a mixed system ("mixed growth processes").
Structures used for secondary treatment include "intermittent sand filters", "trickling filters", "rotating biological contactors", "fluidized beds", "active sludge ponds", "stabilization or oxidation lagoons" and "sludge digestion systems".
Its objective is to suppress some specific contaminants present in wastewater such as phosphates that come from the use of domestic and industrial detergents and whose discharge into water courses favors eutrophication, that is, an uncontrolled and accelerated development of aquatic vegetation that depletes oxygen and kills the existing fauna in the area. Not all plants have this stage since it will depend on the composition of the wastewater and the destination that will be given to it.
Main steps of wastewater treatment
Treated wastewater usually contains pathogenic microorganisms that survive the previous treatment steps. The quantities of microorganisms range from 10,000 to 100,000 total coliforms and 1,000 to 10,000 fecal coliforms per 100 mL (milliliters) of water, as well as some viruses and parasite eggs. For this reason it is necessary to proceed to disinfect the water. This disinfection is especially important if these waters are going to be discharged to recreational waters, waters where shellfish are grown, or waters that could be used as a source of water for human consumption.
Wastewater disinfection methods are mainly chlorination and ozonation"), but bromination and ultraviolet radiation have also been used. The most used is chlorination because it is cheap, easily available and very effective. However, as chlorine is toxic to aquatic life, water treated with this element must be subjected to dechlorination") before disposing of it to natural water courses.
From a public health point of view, waste water containing less than 1000 total coliforms per 100 mL and with a BOD of less than 50 mg/L (milligrams per liter) is acceptable.
The structure used to carry out chlorination is the "contact chamber." It consists of a series of interconnected channels through which the treated wastewater flows so that it is in contact with the chlorine for at least 20 minutes, the time necessary to kill pathogenic microorganisms.
The sediments generated in the primary and secondary stages are called sludge. These sludge contain a large amount of water (99%), pathogenic microorganisms and organic and inorganic contaminants. Various methods have been developed for sludge treatment and include: 'anaerobic digestion', 'aerobic digestion', 'composting', 'chemical conditioning' and 'physical treatment'. The purpose of sludge treatment is to destroy pathogenic microbes and reduce the percentage of moisture.
"Anaerobic digestion" is carried out in a closed pond called a digester and does not require the presence of oxygen as it is carried out by bacteria that develop in its absence. For optimal growth of these microorganisms, a temperature of 35 °C (degrees Celsius) is required. Anaerobic bacteria degrade the organic matter present in the wastewater, in a first phase, to propionic acid, acetic acid and other intermediate compounds, to later give as the final product methane (60-70%), carbon dioxide (30%) and traces of ammonia, nitrogen, sulfur dioxide and hydrogen. Methane and carbon dioxide are odorless; On the other hand, propionic acid has a rancid cheese smell and acetic acid has a vinegar smell.
“Aerobic digestion” takes place in an open pond and requires the presence of oxygen and therefore the injection of air or oxygen. In this case, the digestion of organic matter is carried out by aerobic bacteria, which carry out their activity at room temperature. The final product of this digestion is carbon dioxide and water. No methane is produced. This process, done well, does not produce odors.