Water treatment system particle removal

This chapter is written with three objectives in mind. First, the importance of the size and concentration of the particles to be treated in determining the eflFectiveness of some solid-liquid separation processes is evaluated. Second, past theories are used to examine how particle sizes and concentrations are altered by these treatments. Third, interrelationships among the individual unit processes that comprise a complete treatment system are investigated to provide a base for an integral treatment plant design. These aims are undertaken using a typical water treatment system as employed in practice to remove turbidity from surface water supplies. Before addressing these objectives, it is useful to review some mathematical expressions of particle size distributions, and to identify some important properties of these functions. 

Research projects in sanitary engineering include seeking processes and equipment for improved purification efficiency. One example is the development of large, portable water-treatment systems that are suitable for providing clean water to survivors of natural disasters and the bivouac medical units that treat them. Another example is a nanofiltration system that desalinates ocean water for use on naval ships, especially during times of conflict, and extended private offshore operations such as oil drilling. A related nanofiltration system is necessary for oil-spill cleanup. A third example is the specialized absorbent removal of microcontaminants that may be present in small yet detrimental amounts. These may include elements such as arsenic and lead, industrial solvents, and radioactive particles. 

For wet ESPs, consideration must be given to handling wastewaters. For simple systems with innocuous dusts, water with particles collected by the ESP may be discharged from the ESP system to a solids-removing clarifier (either dedicated to the ESP or part of the plant wastewater treatment system) and then to final disposal. More complicated systems may require skimming and sludge removal, clarification in dedicated equipment, pH adjustment, and/or treatment to remove dissolved solids. Spray water from an ESP preconditioner may be treated separately from the water used to wash the ESP collecting pipes so that the cleaner of the two treated water streams may be returned to the ESP. Recirculation of treated water to the ESP may approach 100 percent (AWMA, 1992). 

The practice of corrosion inhibition requires that the inhibitive species should have easy access to the metal surface. Surfaces should therefore be clean and not contaminated by oil, grease, corrosion products, water hardness scales, etc. Furthermore, care should be taken to avoid the presence of deposited solid particles, e.g. stones, swarf, building materials, etc. This ideal state of affairs is often difficult to achieve but there are many cases where less than adequate consideration has been given to the preparation of systems to receive inhibitive treatment. Acid treatments, notably with 3-5% citric acid, with or without associated detergent washes, are often recommended and adopted for cleaning systems prior to inhibition. However, it is not always appreciated that these treatments will not remove particulate material particularly when, as is often the case, the material is insoluble in acids. 

According to the vendor, costs for sediment removal, dewatering, water treatment, and segregation by particle size are approximately 250/m or less. For removal only, costs are 100/m or less. These estimated costs do not include mobilization and demobilization and/or supplying temporary enclosures (used in pretreatment) if none are available at the site (personal communication Roger Carr, Eriksson Sediment Systems, Inc., May, 1997). 

Ozone is applied in three-phase systems where a selective ozone reaction, oxidation of residual compounds and/or enhancement of biodegradability is required. It can be used to treat drinking water and waste water, as well as gaseous or solid wastes. Especially in drinking water treatment full-scale applications are common, e. g. for particle removal and disinfection, while in waste water treatment sludge ozonation and the use of catalyst in AOP have been applied occasionally. Current research areas for three-phase ozonation include soil treatment and oxidative regeneration of adsorbers. Ozonation in water-solvent systems is seldom studied on the lab-scale and seems favorable only in special cases. In general, potential still exists for new developments and improvements in ozone applications for gas/watcr/solvent and gas/waler/solid systems. 

Ozone applications in gas/water/solid systems cover a wide range of media such as sludges, soils, adsorbents and catalysts. Disinfection, which can be regarded as a three-phase system, is a well-described and established application (see Section A 3.2.1 and 3.3.2). The preozonation for particle removal is discussed frequently, especially in the treatment of surface water, where different organic (e. g. bacteria, viruses, algae, suspended organic matter) and inorganic (e. g. silica, aluminum and iron oxides, clay) particles can be present (see Section A 3.2.4). 

Figure 14.23. Variables that typically determine the efficiency of coagulation and filtration in natural waters and in water and waste treatment systems, (a) How the variables determine the coagulation efficiency, (b) Marked increase in filtration rate can be achieved by counterbalancing a reduction in contact opportunities by chemically improving the contact efficiency, with similar efficiency in particle removal.

Coagulation is one of the most common unit processes involved in a wide variety of water and wastewater treatment systems. It is particularly popular when applied to colour and turbidity removal. The efficiency of removal depends on many different factors such as the physicochemical composition of the water to be treated, on the type and structure of the particles responsible for colour and turbidity generation, on pH, temperature, etc. 

The treatment of wastes and water supplies primarily involves the removal of particulates and, therefore, is accomplished by solid-liquid separation processes. However, present design procedures for such treatment systems do not utilize or even recognize the importance of the physical properties of particulates in solid-liquid separation. In fact, until very recently, eflForts at measuring particle size in wastewaters and raw water supplies have been very limited. Among early eflForts, the investigations at Rutgers University (3,4,5,6) are especially notable. Very recent renewed interests in particle size determinations have used Coulter, Hiac, and Zeiss Videomat particle counters (7,8,9). 

Activated carbon, in powdered (PAC) or granular (GAC) form, has many applications in drinking water treatment. It can be used for removing taste and odor (T O) compoimds, synthetic organic chemicals (SOCs), and dissolved natural ot] nic matter (DOM) from water. PAC typically has a diameter less than 0.15 mm, and can be applied at various locations in a treatment system (Fig. 1). GAC, with diameters ranging from 0.5 to 2.5 mm, is employed in fixed-bed adsorbers such as granular media filters or post filters. Despite difference in particle size, the adsorption properties of PAC and GAC are fundamentally the same because the characteristics of activated carbon (pore size distribution, internal surface area and smface chemistry) controlling the equilibrium aspects of adsorption are independent of particle size. However, particle size impacts adsorption kinetics. 

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