Digital class 7. Technologies for water remediation

Technologies for water remediation

Introduction

It is a pleasure to have you again in this session of the Soil and Water Environmental Engineering course. 

Thanks for continuing! This speaks to your great commitment and interest.

In this session, we will move on to the study of technologies for water treatment where we will analyze the fundamentals and applications of treatments widely used around the world to treat wastewater. This section is designed for you to acquire basic knowledge related to the subject that allows you to understand the foundation, applications, and limitations of these remediation strategies.

The learning sequences discussed above likely allow you, with some ease, to integrate prior knowledge with it. This is desirable since, in this way, you could think about integrating concrete proposals to decontaminate complex environmental systems.

We encourage you to enter this class, tackle it, and finish successfully!

Content developement

Polishing treatments

Water pollution basics

Organic and inorganic compounds are found in wastewater from various industrial facilities. Unlike domestic wastewater, industrial effluents often contain substances that are not removed by conventional treatment, either because they are in high concentrations, or because of their chemical nature. Many of the organic and inorganic compounds that have been identified in industrial wastewater are subject to special regulation due to their toxicity or long-term biological effects.

Among the main pollutants are:

• Organo-halogenated compounds and substances that may affect the aquatic environment.
• Substances and preparations whose carcinogenic or mutagenic properties that may affect reproduction in the aquatic environment.
• Persistent hydrocarbons and persistent and bioaccumulative toxic organic substances.
• Cyanides
• Biocides and phytosanitary products.
• Substances that exert an unfavorable influence on the oxygen balance (computable through aggregated parameters such as Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand COD).

Conventional technologies for water treatment

Treatments for the elimination of suspended matter

Suspended matter can be very diverse, from particles of several centimeters and very dense (usually inorganic), to very stable colloidal suspensions with particle sizes of up to a few nanometers (usually organic). Also their concentration, both in the water to be treated and in the water once treated, plays a fundamental role when choosing the most convenient treatment.

Operations to eliminate this type of water contamination are usually the first to be carried out since the presence of suspended particles is usually not undesirable in many other treatment processes. The elimination of this suspended matter is usually done by mechanical operations. However, in many cases, and to favor this separation, chemical additives are used, in this case, called chemical-physical treatments.

The most common unit operations are described below. 

The use of one or the other is a function of the characteristics of the particles (size, density, shape, etc.) as well as their concentration.

Sieving

It is an operation in which it is a matter of eliminating solids of a larger size than the one usually found by the particles that carry water. The objective is to eliminate them and prevent them from damaging subsequent equipment for the rest of the treatments. It is usually a treatment before any other.

The equipment that is usually used is integrated by bars through which the water is circulated, made of metal bars of 6 or more mm, arranged in parallel, and spaced between 10 and 100. They are cleaned with rakes that are normally mechanically operated.

In other cases, if the type of solids allows it, crushers are used, reducing the size of solids and later separating them by sedimentation or other operations (Jaume, 2013).

Filtration

Filtration is an operation in which water is passed through a porous medium, to retain as much suspended matter as possible. The porous medium traditionally used is a sand bed, of variable height, arranged in different layers of different particle sizes, the upper one being the smallest and between 0.15 and 0.3 mm. It is an operation widely used in the treatment of drinking water, as well as in the treatment of water for reuse, to eliminate suspended matter that has not been eliminated in previous operations (sedimentation). In industrial water, there is more variety in terms of the filtering material used, the use of Diatomaceous Earth being common. It is also common, to improve efficiency, to carry out a previous coagulation-flocculation (Reungoat et al., 2012).

Floatation

It is a physical operation that consists of generating small gas (air) bubbles, which will be associated with the particles present in the water and will be raised to the surface, from where they are dragged and removed from the system. This way of eliminating suspended matter will be suitable in cases where the particles have a density lower or very similar to that of water, as well as in the case of emulsions, that is, a dispersion of drops of an immiscible liquid. As in the case of oils and fats. In this case, the air bubbles help these drops to “float” more quickly, since generally, the density of these liquids is lower than that of water.

Coagulation-Flocculation

As has already been mentioned on several occasions, in many cases part of the suspended matter can be made up of very small particles (10-6 – 10-9 m), which form a colloidal suspension. These colloidal suspensions are usually very stable, in many cases due to electrical interactions between the particles.

Therefore, they have an extremely slow sedimentation rate, which would make a classical mechanical treatment unfeasible. One way to improve the efficiency of all suspended matter removal systems is the addition of certain chemical reagents that, firstly, destabilize the colloidal suspension (coagulation) and then favor their flocculation to obtain easily sedimentable particles. It is an operation that is used often, bothin the treatment of urban and potable wastewater as well as in industrial (food industry, paper pulp, textiles, etc.) (Information source 10 et al., 2016).

Treatments for the removal of dissolved matter

As in the case of suspended matter, dissolved matter can have very diverse characteristics and concentrations: from large amounts of dissolved inorganic salts (brines) organic (biodegradable organic matter in the food industry) to extremely small amounts of inorganic (heavy metals) and organic (pesticides) but their elimination is necessary due to their dangerous nature. Some of these treatments are being displaced by more advanced and emerging ones, such as advanced oxidation processes and membrane operations, and especially in the case of industrial waters. For this reason, they deserve further attention and will be described in chapters devoted exclusively to them.

Precipitation

It consists of the elimination of an undesirable dissolved substance, by adding a reagent that forms an insoluble compound with it, thus facilitating its elimination by any of the methods described in the elimination of suspended matter.

Some authors include coagulation-flocculation in this section. However, the term precipitation is used more to describe processes such as the formation of insoluble salts, or the chemical transformation of an ion into another with a greater or lesser oxidation state that causes the formation of an insoluble compound.

A very frequently used reagent in this type of operation is Ca2 +, given the large number of insoluble salts it forms, for example, it is the method used for the elimination of phosphates (nutrient). It also has a certain coagulant capacity, which it is widely used in urban wastewater and industrial wastewater with similar characteristics (Miranda et al., 2015).

Ion exchange

It is an operation in which a material is used, usually called ion exchange resins, which is capable of selectively retaining the ions dissolved in water on its surface, keeps them temporarily attached to the surface, and yields them to a solution with a strong regenerant solution.

The usual application of these systems is, for example, the elimination of salts when they are found in low concentrations, the application for demineralization and softening of water being typical, as well as the retention of certain chemical products and the demineralization of sugar syrups. (Miranda et al., 2015).

The properties that govern the ion exchange process and that at the same time determine its main characteristics are the following:

• Resins act selectively, such that they may prefer one ion over another with relative affinity values ​​of 15 or more.
• The ion exchange reaction is reversible, that is, it can proceed in both directions.
• Electroneutrality is maintained in the reaction.

Adsorption

The adsorption process consists of capturing soluble substances on the surface of a solid. A fundamental parameter, in this case, will be the specific surface of the solid, since the soluble compound to be eliminated has to be concentrated on its surface. The need for higher water quality is making this treatment on the rise. It is considered as a refining treatment, and therefore at the end of the most common treatment systems, especially after a biological treatment (Perrich, 2018).

Disinfection

Disinfection aims to destroy or inactivate microorganisms that can cause diseases since water is one of the main means by which they are transmitted. Disease-causing organisms can be bacteria, viruses, protozoa, and a few others. Disinfection is essential for the protection of public health if the water to be treated is for human consumption. In the case of industrial wastewater, the objective is not only to deactivate pathogens but any other living organism, if the aim is to reuse the water.

To carry out the disinfection, different treatments can be used: Physical treatment (heat, radiation, etc.), acids or bases, etc. but mainly oxidizing agents are used, among which the classic Cl₂ and some of its derivatives are worth noting, or advanced oxidation processes (O₃, heterogeneous photocatalysis) (Howe and Hand, 2012).

The use of disinfectants has three purposes: to produce water free of pathogens or living organisms, to avoid the production of undesirable by-products of disinfection, and to maintain the bacteriological quality in the subsequent conduction network.

• Disinfection with chlorine (Cl₂): It is the most widely used oxidant. Several factors influence the process: Nature and concentration of organisms to be destroyed, dissolved or suspended substances in the water, as well as the chlorine concentration and the contact time used. Substances present in water have a great influence on chlorination: In the presence of organic substances, the disinfecting power is lower. The presence of ammonia consumes chlorine (formation of chloramines). Iron and manganese increase the demand for them.

In this sense, it is important to carry out a study of the chlorine demand (breakpoint) to determine the correct chlorine dose for each type of water.

In addition to the dose, the contact time is also important, so that the parameter to be used is the expression:

C·t: Final disinfectant concentration in mg / l (C) and minimum exposure time in minutes (t). 

Normally the expression used is Cn · t = constant, which for chlorine takes values ​​between 0.5 and 1.5. However, one of the main disadvantages of using chlorine as a disinfectant is the possibility of formation, although in very small amounts, of compounds such as trihalomethanes.

Other chlorinated compounds: Sodium hypochlorite, manufactured from Cl₂, is also used as a disinfectant in systems with lower operating flow rates, although the properties are very similar to those of Cl₂. Another compound with possibilities of use is ClO₂, more oxidizing than chlorine, it does not react with ammonium, therefore it does not form chloramines and it seems that the possibility of formation of trihalomethane is much lower than with Cl₂. All these advantages are opening up new possibilities for the use of this compound for disinfection.

Biological treatments

They constitute a series of important treatment processes that have in common the use of microorganisms (among which bacteria stand out) to carry out the elimination of undesirable components from the water, taking advantage of the activity metabolic of the same on those components. The traditional application consists of the elimination of biodegradable organic matter, both soluble and colloidal, as well as the elimination of compounds that contain nutrient elements (N and P). It is one of the most common treatments, not only in the case of urban wastewater but in a large part of industrial water.

In most cases, organic matter constitutes the source of energy and carbon that microorganisms need for their growth. In addition, the presence of nutrients is also necessary, which contain the essential elements for growth, especially the compounds that contain N and P, and finally, in the case of an aerobic system, the presence of dissolved oxygen in the water. This last aspect will be key when choosing the most convenient biological process (Ferrer et al., 2018).

In bacterial metabolism, the electron acceptor element plays a fundamental role in the oxidation processes of organic matter. This aspect also has an important impact on the possibilities of application to water treatment. Considering which is said electron acceptor, we distinguish three cases:

Aerobic biological processes

Activated sludge: Basic process

It consists of putting in contact in an aerobic environment, normally in an aerated basin, the residual water with previously formed biological flocs, which is adsorbed by the organic matter and where it is degraded by the bacteria present. Along with the degradation process, and to separate the flocs from the water, sedimentation must be carried out, where part of the sludge is recirculated, to maintain a high concentration of microorganisms inside the reactor, in addition to a purge equivalent to the increased amount of organisms (Ferrer et al., 2018). A simplified scheme is shown in the following figure:

Within the basic operating parameters, a very important parameter is that of aeration. The solubility of oxygen in water is small (around 8-9 mgO₂ / l depending on pressure and temperature) so it will be necessary to ensure the supply to microorganisms, using surface aerators, capable of supplying 1 kgO₂ / kWh, or diffusers. The minimum recommended operating value of dissolved oxygen concentration is 2 mg / l. Electricity consumption in this operation will be important within the operating costs of the process.

Sludge recirculation.Another key parameter in the process refers to the A / M parameter, sometimes referred to as I, load current. It refers to the relationship between the organic load fed and the number of microorganisms available in the system, with units kg BOD₅ (or COD) / kgSSV · day. It is a fundamental design parameter, having an optimal value between 0.3-0.6 for most operating conditions. It also has a decisive influence on the good subsequent sedimentation. The so-called «cell age» is also an important parameter. It refers to the average time that the sludge (flocs, microorganisms) remains inside the system. This magnitude usually has a value of 5-8 days under conventional operating conditions.

Anaerobic biological processes

Anaerobic treatment is a biological process widely used in wastewater treatment. When these have a high organic load, it is presented as the only alternative to what would be an expensive aerobic treatment, due to the supply of oxygen. Anaerobic treatment is characterized by the production of so-called «biogas», consisting mainly of methane (60-80%) and carbon dioxide (40-20%) and capable of being used as fuel for the generation of thermal and/or electrical energy. Furthermore, only a small part of the treated COD (5-10%) is used to form new bacteria, compared to 50-70% in an aerobic process. However, the slowness of the anaerobic process forces to work with high residence times, which is why it is necessary to design reactors or digesters with a high concentration of microorganisms (Ferrer et al., 2018).

It is a complex process in which several groups of bacteria intervene, both strict anaerobic and facultative, in which, through a series of stages and in the absence of oxygen, it fundamentally leads to the formation of methane and carbon dioxide. Each stage of the process, described below, is carried out by different groups of bacteria, which must be in perfect balance.

Among the most significant advantages of anaerobic versus aerobic treatment, it is worth highlighting the high efficiency of the systems, even in high load wastewater, low energy consumption, small sludge production, and therefore, low nutrient requirement, as well as its efficacy against important load alterations and the possibility of long periods of the shutdown without significant alteration in the bacterial population. However, the disadvantages include the low effectiveness in the elimination of nutrients and pathogens, the generation of bad odors, and the need for post-treatment, generally aerobic, to achieve the required purification levels, as well as the generally long laying periods. ongoing.

Operating conditions

Both physical and chemical variables influence the habitat of microorganisms. In anaerobic processes, it is important to take into account the influence of environmental factors. Methane-forming bacteria are the most sensitive to these factors, so improper functioning of them can cause an accumulation of intermediates (acids) and completely destabilize the system. Among the most important variables are temperature, pH, and nutrient availability. On the other hand, the mixture is an important factor in the control of the pH and the uniformity of the environmental conditions. A good mix distributes the buffer properties throughout the reactor and avoids the concentration of intermediate metabolites that can be a cause of inhibition for methanogenic bacteria.

The monitoring and control parameters of an anaerobic digester can be located in the solid phase (organic and inorganic materials in suspension); liquid phase (physicochemical parameters and composition) and gaseous (production and composition) These parameters may have different meanings and utility depending on the particular situation of the equipment, which can be found in a start-up period, in a steady-state for continuous systems, or discontinuous systems. Operational parameters include organic loading rate, toxicity, volumetric flow rate, hydraulic retention time, the concentration of volatile solids in the reactor, sludge production, etc.

As you can see, wastewater treatment is complex and requires both theoretical and practical knowledge. 

Since more than one compound is present in contaminated water, which may have different nature and source, particular design configurations and operating conditions are required.

From treatment systems for small volumes such as treatment plants of colossal dimensions, water treatment is necessary to safeguard the water resource and give it subsequent uses and thus avoid the use of water for human consumption in processes where treated water can be used that comply with the parameters established in the standards.

In the following video, we will show you the configuration and operation of water plants on different scales, however, we ask that for each one, you identify the stages of the process, the operations present, and the operating conditions; This has already been reviewed in the theoretical-conceptual elements during the session, so it will serve as an exercise to reinforce the knowledge acquired so far.

As we mentioned, although the dimensions are different, the principle of operation and the components of the complete process for water treatment, follow fundamental principles.

The following video briefly shows the Atotonilco de Tula Water Treatment Plant (WWTP), in the state of Hidalgo, being the largest WWTP in Mexico and one of the largest in the world.

This treatment plant receives and treats the wastewater produced in the Valley of Mexico, so you can imagine its dimensions and magnitude.

Although the basic stages and processes are presented, do not forget that there may be adaptations to treat contaminants with greater specificity. In addition, the development of new technologies allows increasing the efficiency and range of treatability; With this, it contributes to the conservation of the environment and the improvement of the quality of life of all and. all on the planet.

• Water is a valuable resource for the development of life on the planet. This resource is limited and is under pressure from overuse in human activities, such as agriculture, industrial and human consumption.
• Once the water is used, this resource must receive treatment so that it returns in the best conditions to the environment and does not generate damage to living things.
• As part of a process of reflection, I invite you to imagine a world with limited water for its use and consumption.
• What do you think would be the cost of not having enough water to carry out daily activities? In economic terms, what is the cost of having water outside the minimum parameters necessary to carry out any activity? Do you think that it is worth treating the water to increase its usefulness?
• What contributions, from your knowledge, can you make to improve the treatability of water? Surely you have a new idea that allows you to treat water with certain contaminants that are difficult to remove?
• Keep in mind that any human activity has effects on nature, these can be positive, but some others can be negative, devastating.
• Rethink your actions and assess the impacts they have on the environment.

Conclusion

Water treatment is classified based on the materials removed, i.e., suspended or soluble. There are also biological treatments that are important to remove organic matter and some pollutants; These are classified as aerobic and anaerobic and respond to the conditions in which microorganisms develop their metabolic activity.

In addition to the above, it must be borne in mind that water treatment is essential to ensure the continuity of life on the planet, for this purpose it is important to consider the following aspects to ensure its treatability:

1. It must be approached under a comprehensive approach that includes the analysis of the origins of contamination in the water, the practices associated with the generation of water contamination, consumption patterns, social composition, and economic aspects related to the treatment or lack of it.
2. The infrastructure must be sufficient. Treatment procedures and technologies that are capable of removing toxic compounds must be established. Sufficiency in handling volumes is crucial to guarantee the treatment of all the wastewater generated.
3. Although there is sufficient knowledge related to technologies for water treatment, it is essential to have the training and recurrent updates, so that operational problems can be solved and the operation of the WWTP can be continuously maintained.

Information sources

• Templeton, MR, & Butler, D. (2011). Introduction to wastewater treatment. Bookboon.
• Liu, DH, & Lipták, BG (Eds.). (2020). Wastewater treatment. CRC Press.
• Spellman, FR (2013). Handbook of water and wastewater treatment plant operations. CRC press.
• Cheremisinoff, PN (2019). Handbook of water and wastewater treatment technology. Routledge.
• Grady Jr, CL, Daigger, GT, Love, NG, & Filipe, CD (2011). Biological wastewater treatment. CRC press
• Qasim, SR (2017). Wastewater treatment plants: planning, design, and operation. Routledge.
• Jaume, AT (2013). Purification and regeneration of urban wastewater. University of Alicante.
• Miranda, JPR, Ubaque, CAG, & Pinzón, JP (2015). Selection of technologies for municipal wastewater treatment. Tecnura, 19 (46), 149-164.
• Reungoat, J., Escher, BI, Macova, M., Argaud, FX, Gernjak, W., & Keller, J. (2012). Ozonation and biological activated carbon filtration of wastewater treatment plant effluents. Water Research, 46 (3), 863-872.
• El-Gohary, F., Tawfik, A., & Mahmoud, U. (2010). Comparative study between chemical coagulation/precipitation (C / P) versus coagulation / dissolved air flotation (C / DAF) for pre-treatment of personal care products (PCPs) wastewater. Desalination, 252 (1-3), 106-112.
• Teh, CY, Budiman, PM, Shak, KPY, & Wu, TY (2016). Recent advancement of coagulation-flocculation and its application in wastewater treatment. Industrial & Engineering Chemistry Research, 55 (16), 4363-4389.
• Perrich, JR (2018). Activated carbon adsorption for wastewater treatment. CRC press.
• Howe, KJ, & Hand, DW (2012). Principles of water treatment. Cengage Learning.
• Ferrer Polo, J., Seco Torrecillas, A., & Robles Martínez, Á. (2018). Biological wastewater treatment. Editorial Universitat Politècnica de València.