Water Treatment PDF

Water Treatment  Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use.  The end use may be drinking, industrial water supply, irrigation, water recreation or many other uses, including being safely returned to the environment. ...

Water Treatment  Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use.  The end use may be drinking, industrial water supply, irrigation, water recreation or many other uses, including being safely returned to the environment.  Water treatment removes contaminants and undesirable components, or reduces their concentration so that the water becomes fit for its desired end-use.  This treatment is crucial to human health and allows humans to benefit from both drinking and irrigation use. Drinking Water Treatment  Screening To protect the main units of a treatment plant and aid in their efficient operation, it is necessary to use screens to remove any large floating and suspended solids present in the inflow. These materials include leaves, paper, and other debris that could obstruct flow through the plant or damage equipment.  Aeration After screening, the water is aerated (supplied with air) by passing it over a series of steps to take in oxygen from the air. This process helps in expelling soluble gases such as carbon dioxide and hydrogen sulfide (both of which are acidic, so this process makes the water less corrosive) and expels any gaseous organic compounds an undesirable taste to the water. Aeration also removes iron or manganese by oxidation of these substances to their insoluble form. Once in their insoluble forms, these substances can be removed by filtration.  Coagulation and Flocculation After aeration, coagulation occurs to remove the fine particles (less than 1 µm in size) suspended in the water. In this process, a chemical called a coagulant (with a positive electrical charge) is added to the water, which neutralizes the fine particles' negative electrical charge. The coagulant's addition takes place in a rapid mix tank where a high-speed impeller rapidly disperses the coagulant. Since their charges are now neutralized, the fine particles come together, forming soft, fluffy particles called 'flocs.' Two coagulants commonly used in the treatment of water are aluminum sulfate and ferric chloride.  Sedimentation Once large flocs are formed, they need to be settled out, and this takes place in a process called sedimentation (when the particles fall to the floor of a settling tank). The water (after coagulation and flocculation) is kept in the tank for several hours for sedimentation to take place. The material accumulated at the bottom of the tank is called sludge; this is removed for disposal.  Filtration Filtration is the process where solids are separated from a liquid. In water treatment, the solids that are not separated in the sedimentation tank are removed by passing the water through sand and gravel beds.  Chlorination After sedimentation, the water is disinfected to eliminate any remaining pathogenic micro-organisms. The most commonly used disinfectant (the chemical used for disinfection) is chlorine, a liquid (such as sodium hypochlorite, NaOCl), or a gas.  It is relatively cheap and simple to use. When chlorine is added to water, it reacts with any pollutants present, including micro-organisms, over a given period of time, referred to as the contact time. The amount of chlorine left after this is called residual chlorine. This stays in the water through the distribution system, protecting it from any micro-organisms that might enter it until the water reaches the consumers.  Supplementary Treatment Supplementary treatment may be needed for the benefit of the population. One such instance is the fluoridation of water, where fluoride is added to water. It has been stated by the World Health Organization that ‘fluoridation of water supplies, where possible, is the most effective public health measure for the prevention of dental decay. The optimum fluoride level is around 1 mg per litter of water (1 mg l–1). Wastewater Treatment Wastewater treatment and disposal is the act of treating the wastewater (contaminated water) generated either in any residential or industrial area as a by-product. Wastewater treatment involves the processes and mechanisms that are used to treat water that is contaminated by chemical or industrial activities. The main reason why this water is treated is to recycle/reuse the water, discharging it directly to the environment, or a sanitary sewer.  Before the pretreatment process starts fully, there is a need to carry out an evaluation and assessment of the water to be treated.  This will help to determine the methods of wastewater treatment that are needed.  WWTPs employ a series of processes to treat harmful elements in wastewater such as nutrients, toxic compounds and pathogens which can excite the growth of aquatic plants to unsafe level for discharge to the receiving waters and will not negatively impact on human health. Typically, the processes used for wastewater treatment can be grouped together in terms of the treatment levels Levels of Wastewater Treatment Selection of the treatment processes that are to be utilized occurs at the design stage and is based on considerations including. Hydraulic and pollutant loading which are expected to enter the WWTP. The level of treatment required. Preliminary Treatment Preliminary treatment typically serves as a solid’s separation phase prior to any biological or chemical treatment technologies, which may be present downstream. Preliminary treatment, in most cases, includes screening and grit removal to remove objects and insoluble particles which may damage downstream equipment and processes.  Preliminary treatment also includes the control of incoming wastewater using flow splitters, overflow tanks and balance tanks.  Preliminary Treatment Processes Process Description Removal of large objects (rags, plastic, stone) which can damage Screening equipment. Grit Removal Removal of grit which can damage pumping equipment. Pacing of influent wastewater flow into the WWTP to prevent Flow Balancing hydraulic overload; excess influent is stored in a storm tank. Removal of scum, grease and other floating materials from a storm Scum Removal tank. Irritation Control Control of odor emissions, typically to promote public acceptability.  Preliminary treatment by screens or grit chambers is usually followed by primary sedimentation. The main objective of this treatment step is to remove a large fraction (50- 70%) of the total suspended solids in the wastewater. Since suspended solids also contribute to the content of BOD in the wastewater, one should expect 25-40 % of the total BOD to be removed in the process.  Primary Treatment This initial step is designed to remove gross, suspended and floating solids from raw wastewater. It includes screening to trap solid objects and sedimentation by gravity to remove suspended solids. This physical solid/liquid separation is a mechanical process, although chemicals can be used sometimes to accelerate the sedimentation process. This phase of the treatment reduces the BOD of the incoming wastewater by 20–30% and the total suspended solids by nearly 50–60%.  Secondary Treatment Secondary treatment processes are designed to remove organic matter and nutrients in wastewater using biological treatment processes. These are generally divided into two groups: Suspended growth processes and attached growth processes.  Suspended Growth Processes In suspended growth processes, the biomass, which can join to form larger particles known as flocs, can freely move through the wastewater. The methods through which aeration is introduced into the process also aid in providing constant mixing to the wastewater and flocs. These flocs are made up of bacteria and protozoa and do not seek to attach to free surfaces  Secondary Sedimentation Tank As the effluent from biological filters contains biological flocs to a large extent, there is a need of a second clarifier before further distribution of the wastewater. This is normally obtained by a secondary sedimentation tank where solids are settled and removed from the wastewater stream as sludge.  Nutrients such as nitrogen and phosphorus could also possibility be removed either as a part of the solids content or through biological decomposition (for nitrogen removal). As an overview, the main purpose of secondary biological  Tertiary/Advanced Treatment To accomplish nutrient removal sufficient to limit the risk of eutrophication of sensitive water bodies, an additional treatment step after secondary treatment is often necessary. These types of treatment steps, which also go under the term “advanced wastewater treatment” because of their generation of advanced techniques, could be designed in a variety of ways using different techniques.  A typical facility for tertiary nitrogen removal is the use of filtration after secondary treatment. At this stage, the BOD concentration would normally be very low (< 10 mg/l). providing a good basis for nitrification and de-nitrification to occur. Important parameters at this stage are ammonia loading rate, oxygen availability, packing design and temperature.  In nitrification, ammonia (NH4-N) is oxidized to nitrite (NO2-N) before nitrite is oxidized to nitrate (NO3-N), both steps under the presence of oxygen. Each of the steps also depends on the presence of a specific group of autotrophic bacteria, respectively Nitrosomonas and Nitrobacter. To achieve complete removal of nitrogen compounds, denitrification of nitrate needs to occur. This happens as nitrate is reduced to nitric oxide, nitrous oxide and then nitrogen gas which are natural occurring gas in the atmosphere. The reduction steps occurring in de-nitrification are as follows.  Tertiary/Advanced Treatment Processes Process Description Removal of suspended and colloidal solids from secondary treated Sand Filtration wastewater; typically used to achieve lower suspended solids and phosphorus concentrations in the discharged effluent. Disinfection of secondary treated wastewater for the purposes of wastewater Ultraviolet Disinfection reuse. Treatment and disposal of sludge generated by wastewater treatment Sludge Treatment processes. Includes sludge dewatering, conditioning and stabilization. Chemical dosing of wastewater to facilitate phosphorus precipitation; typically, Chemical Precipitation for ferric chloride, aluminum or lime dosing. May lead to excessive sludge Phosphorus Removal production depending on dosage rates. Micro-filtration or ultra-filtration processes combined with a suspended Membrane Filtration growth bioreactor. Methods of Wastewater Treatment There are four different wastewater treatment methods, and each of these methods has a different treatment process. However, every treatment process must start with an assessment and evaluation. The four different industrial water treatment methods are explained below: 1. Physical Methods of Wastewater Treatment Physical water treatment involves the use of physical methods to clean wastewater. This method involves processes like sedimentation, screening, and skimming used for removing the solids in the wastewater. During this process, chemicals are not used at all. Some of the techniques of this method are:  Sedimentation Sedimentation is a wastewater treatment process in which heavy or insoluble particles are collected and separated from the wastewater. The water is separated from the insoluble material when the insoluble material settles down at the bottom of the water.  Aeration In this process, the wastewater is treated by providing oxygen to it through air circulation. Oxygen or air is added to the wastewater in several ways including surface, spray, and diffused aeration. The aeration process begins immediately after the oxygen is in contact with the water. 2-Mechanical Methods of Wastewater Treatment Mechanical filtration for wastewater treatment and disposal is one of the conventional wastewater treatment methods and it can be achieved by either:  Ceramic Membrane Technology The ceramic membrane filtration process is the use of ceramic membranes installed housings to filter wastewater. The filtration process will start when the wastewater begins to move through the membranes. A feed pump is responsible for providing the pressure needed for the water to move through the ceramic membrane. 2-Mechanical Methods of Wastewater Treatment Sand Filter Technology It is more applicable where there are downward-flowing fluids that are driven by either gravity or pressure. One downside of this technology is that it may not provide adequate disinfection. The sand filters consist of a large quantity of special-grade sand in a large tank. 3-Biological Method of Wastewater Treatment This is the use of biological processes to treat wastewater. In this stage, the organic matter like oils, human waste, soap, and food present in the wastewater are broken down. The biological process wastewater treatment method is divided into three categories:  Aerobic Process Here, the organic matter in wastewater is decomposed by bacteria and converted into carbon dioxide. Oxygen and the use of aerobic microorganisms (fungi and bacteria) are needed to decompose the organic matter in the wastewater. The aerobic process takes place in an activated sludge reactor where the wastewater is mixed with a lot of aerobic organisms.  Anaerobic Process In this stage, fermentation is used for decomposing the organic matter in wastewater instead of oxygen. The microorganisms (anaerobic bacteria) used for this process do not require oxygen to break down organic matter. The waste is fermented at a specific temperature by using fermentation.  Composting In this stage, treatment is carried out by mixing the wastewater with sawdust or any other carbon sources. Composting is one of the commonly used methods for treating sludge and transforming it into a useful product that can be used to enrich the soil nutrient. Chemical Methods of Wastewater Treatment There are different chemical treatment processes, but the most common ones are chemical neutralization, adsorption, precipitation, disinfection, and ion exchange. Though the chemical treatment processes listed above differ, one common similarity they have is that the substance is affected by altering extraneous factors.  Neutralization This is the use of acidic or alkaline to control and keep the pH level of the wastewater around 7. If the water lacks sufficient acidity, then an acid will be added to get the required pH level. Also, if the water lacks sufficient alkaline, a base will be added to the water to get the required pH level.  Adsorption This chemical industrial wastewater treatment method is a process whereby adsorbents are used to remove soluble molecules from wastewater. Various organic materials like toxic compounds and detergents can be removed by adsorption. For effective results, the adsorbent must be activated before usage.  Precipitation The chemical precipitation process involves the use of an acid or alkali to remove dissolved inorganic in wastewater. This method is achieved by altering the temperature or by injecting chemicals. The precipitate can then be removed by either floatation or sedimentation.  Disinfection This is a chemical treatment process in which the wastewater is treated by using selected disinfection to inspect or inactivate the pathogens (viruses, microbes, and protozoan) in the wastewater. The primary purpose of this process is to protect the microbial wastewater quality.  Ion Exchange The primary purpose of the chemical treatment process is for softening. This process is known as a reversible reaction because a charged ion is replaced with another similarly charged ion. In other words, polyvalent cations are replaced with sodium.