Environmental Issues

For a water supply to be considered potable, it must satisfy some specific key quality parameters. These parameters are commonly categorized by their main characteristics, namely the physical, chemical, and biological parameters. To check whether the water supply satisfies the physical parameters, tests are conducted for its electrical conductivity, suspended solids (SS), total dissolved solids (TDS), total solids, and turbidity (cloudiness). For the chemical parameters, the water is tested for the amounts present of ammonia (NH3-N) and Nitrates (NO3-N), and the biochemical oxygen demand (BOD), chemical oxygen demand (COD), dissolved oxygen (DO), and total organic carbon (TOC) are also considered. To satisfy the biological parameters, the water supply must be free of Coliform bacteria, especially the most commonly known strand, Escherichia Coliform, or E. Coli.

The most common unit operations involved in the treatment of source waters with the aim of producing a potable water supply can be broken down into the following steps screening, aeration, coagulation and flocculation, sedimentation and flotation, filtration, and disinfection. The processes involved in this steps, and their respective purposes, are enumerated

First step Screening. The source water is first filtered through a screenmesh, a very basic kind of filtration process. This effectively removes relatively large debris from the water which can not only degrade the water quality, but also damage the other machinery involved in the water treatment. Multiple screens may be utilized for this step for better efficiency, and the water is sifted through the screens multiple times before moving on to the next step. When multiple screens are used, the water passes through these in a specific order, based on the relative sizes of the screen mesh. First, the water enters through the coarse screens, screens with openings which measure about 5cm to 15cm this removes the objects and debris that can significantly damage the treatment equipment. Next, the water enters through fine screens, screens with openings which measure about 5mm to 20mm this removes the small debris, like sand or gravel, which may also cause blockage and damage the machinery. Normally, the screens are placed at an oblique angle to the opening of the system to minimize problems with the water flow and so that they can be easily removed, cleaned, and replaced. The screening process only deals with removing physical impurities.

Second step Aeration. Aeration is required in the water treatment process if, for example, the water has low DO or contains dissolved gases. While many different methods can be used for aeration, cascade aeration, is the simplest and most common method. This process involves simply allowing the water to fall a specific distance into the next container (like a faucet), this method is also used by other industries to cool water. This drop allows oxygen in the air to be dissolved into the water, and thus displace other dissolved gases such as carbon dioxide (CO2), hydrogen sulphide (H2S), or other volatile organic compounds. Carbonic acid, for example, is the solution of CO2 dissolved in water after aeration, this reverts to its gaseous carbon dioxide form, and effectively reduces the risks of having water with acidic pH which can be both unhealthy and corrosive. The process of aeration can also be useful for the eradication of trace metals in the water supply, such as iron and manganese. When the water (solution) has an acidic pH of less that 6.5, which can occur during aeration, trace iron becomes soluble as one of its ionic states (Fe3) can combine with the acidic water to become Fe(OH)3, an insoluble solid. This resulting hydroxide solid can then be filtered out of the water. The removal of manganese is through the same type of chemical process. Thus, the aeration step mostly deals with removing chemical impurities.

Third step Coagulation and Flocculation. Coagulation, which is basically the formation of precipitation, and flocculation, defined as the process wherein colloids come out of suspension, though they have some minute differences, are usually considered to be synonymous and in water treatment, are always considered to form a single process. These effectively remove impurities that are left behind even after filtration and sedimentation, as the particles involved in these processes are very small, usually less that 1m across. These suspended particles can either be partially dissolved, for coagulation, or simply suspended in the water to form a colloid, for flocculation, and can cause the water to have high turbidity or a slight tint. The particles involved in these processes have low settling characteristics, and can be a variety of substances such as clay, metal oxides, microorganisms, proteins, or other minute organic substances their only common characteristic is that they are very small and have negative charges. To induce coagulations and flocculation then, positively-charged multivalent ions or colloids are added to the water. These act as chemical coagulants which are commonly in the form of aluminum or ferric salts, containing Al3 or Fe 3 ions. As the salts dissolve in the water, the positive ions neutralize the suspended negative ions, thus making them coagulate into precipitate, or flocculate into floc or flakes. Coagulation and flocculation deal with removing both chemical and biological impurities.

Fourth step SedimentationFlotation. This process can be done simultaneously with other processes, such as coagulation and flocculation.  Sedimentation occurs when the water is relatively still pulled down by gravity, some solids sink to the bottom of the treatment tank, thus turning into sediment that can be easily removed. While this process is useful for eradicating the solids left behind by filtration, it also effectively removes the precipitate andor flakes formed through coagulation and flocculation.  In the process of sedimentation, the settling of the sediments is most important, so it requires the water to be placed in a sizeable tank wherein the water flow and disturbance are minimal.

Alternatively, flotation, a process which uses small streams of gas bubbles to make the solid impurities float, can be used in conjunction with, or instead of, sedimentation. When the gas bubble stream passes through the minute suspended solids, the impurities are buoyed up to the surface of the water (they float) where the can be easily filtered or skimmed out. Flotation is considered to be more desirable that sedimentation because while sedimentation requires a decent amount of time for the sediments to settle, flotation only needs enough time for the gas bubbles to pass through the entire volume of water. Sedimentation, however, is very simple, only requiring a tank and drains for the sediment and the clean water, while flotation needs a mechanism to form the gas bubbles. Sedimentation and flotation mostly deal with removing small physical impurities.

Fifth step Filtration. Filtration, unlike the initial screening step, involved the water being filtered through a porous substance, such as anthracite or sand. This effectively cleans out both organic and inorganic substances that were left behind by the sedimentationflotation and coagulation and flocculation processes. Some water treatment plants or machines filter the water through special salts with large grains and low solubility for a variety of reasons, one of which is to soften the water. Though it seems unnecessary, most water treatment processes still incorporate this final filtration process to make sure that no unwanted particles are still left in the water. Filtration, when using material such as sand as the filtration medium, deal with mostly physical impurities, but when using salts, it can also remove chemical and biological impurities.

Sixth and Last step Disinfection.  This final process is very important, as it rids the water of all the microorganisms left in the water after all the other processes are finished. Since the purpose of potable water is that people can drink it without getting sick, disinfection should be incorporated in water treatment. However, the term disinfection means to simply deactivate potentially harmful microorganism and not to kill them, as this is sterilization although both terms are commonly interchanged. The most common method of disinfection is by adding chlorine to the water, as chlorine is a strong oxidizing agent that kills microorganisms. Another reason why chlorine is so commonly used is that it is relatively cheap in bulk. However, excessive intake of chlorine can be very harmful, and the presence of chlorine in drinking water can usually be detected because it has a certain smell. Disinfection, of course, deals with removing biological impurities.

Other alternative methods commonly used in water treatment are sterilization using Ultraviolet (UV) light, and processes which make use of reverse osmosis, wherein water flows from relatively higher concentration to lower concentration (the opposite of the normal behavior of flowing from lower to higher concentration).