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<p>Natural waters in streams, rivers, lakes, and reservoirs have a natural waste assimilative capacity to remove solids and organic matter. Even toxic chemicals in the wastewater can be removed in natural systems up to some extent. However, it is a long process. Therefore, Wastewater treatment facilities are designed to speed up the natural purification process that occurs in natural waters. If not, the contaminants in wastewater might interfere with the natural process in the receiving waters.</p>
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<p>Varying quantities of suspended and floating solids, organic matter, and fragments of debris are carried in wastewater. Therefore, conventional wastewater treatment systems are designed as combinations of physical and biological processes to remove all impurities.</p>
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<p>The alternative methods for municipal wastewater treatment are classified into three major categories:</p>
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<p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; (1) primary (physical process) treatment,</p>
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<p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; (2) secondary (biological process) treatment, and</p>
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<p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; (3) tertiary (combination of physical, chemical, and biological processes) or advanced treatment.</p>
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<p>The ultimate goal of wastewater treatment processes is to produce clean effluents and to protect public health, natural resources, and the ambient environment.</p>
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<h2 class="wp-block-heading"><strong>Preliminary Treatment Systems&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</strong></h2>
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<p>Preliminary systems are designed to remove the larger suspended and floating materials. It also removes heavy inorganic solids and excessive amounts of oil and grease.</p>
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<p>Another purpose of preliminary treatment is to protect pumping equipment and the subsequent treatment units.</p>
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<p>Preliminary systems consist of flow measurement devices and regulators (flow equalization), racks and screens, comminuting devices (grinders, cutters, and shredders), flow equalization, grit chambers, pre-aeration tanks, and (possibly) chlorination.</p>
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<p>However, the quality of wastewater is not substantially improved by preliminary treatments.</p>
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<p>The object of primary treatment is to reduce the flow velocity of the wastewater sufficiently to permit suspended solids to settle, i.e. to remove settleable materials. Floating materials are also removed by skimming. Hence, a primary treatment device may be called a settling tank.</p>
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<p>Settling tanks can be divided into four groups: plain sedimentation with mechanical sludge removal, two-storey tanks (Imhoff tank), upflow clarifiers with mechanical sludge removal, and septic tanks.</p>
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<p>When chemicals are applied, other auxiliary units such as chemical feeders, mixing devices, flocculators, and sludge management (treatment and disposal of) are required if there is no further treatment.</p>
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<p>The physical process of sedimentation in settling tanks removes approximately 50-70 percent of total suspended solids from the wastewater.</p>
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<p>The BOD<sub>5</sub> removal efficiency of the primary system is 25-35 percent. When some coagulants are applied in settling tanks, more colloidal as well as settleable solids, or a total of 80-90 percent of Total Settlable Solids (TSS), can be removed.</p>
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<p>Approximately 10 percent of the phosphorus corresponding insoluble is normally removed by primary settling.</p>
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<h2 class="wp-block-heading"><strong>Secondary Treatment Systems</strong></h2>
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<p>After primary treatment, the wastewater still contains organic matter in suspended, colloidal, and dissolved states. This matter should be removed before discharging to receiving waters. Otherwise, these interfere with subsequent downstream users of stream water.</p>
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<p>Secondary treatment is used to remove the soluble and colloidal organic matter which remains after primary treatment.</p>
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<p>Although the removal of those materials can be affected by physicochemical means providing further removal of suspended solids, secondary treatment is commonly referred to as the biological process.</p>
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<p>Biological treatment consists of the application of a controlled natural process in which a very large number of microorganisms consume soluble and colloidal organic matter from the wastewater in a relatively small container over a reasonable time.</p>
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<p>Secondary treatment devices may be divided into two groups: attached and suspended growth processes.</p>
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<p>The attached (film) growth processes are trickling filters, rotating biologic contactors (RBC), and intermittent sand filters.</p>
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<p>The suspended growth processes include activated sludge and its modifications, such as contact stabilization (aeration) tanks, sequencing batch reactors, aerobic and anaerobic digestors, anaerobic filters, stabilization ponds, and aerated lagoons.</p>
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<p>Secondary treatment processes may remove more than 85 percent of BOD<sub>5</sub> and TSS. However, they are not effective for the removal of nutrients (N and P), heavy metals, non-biodegradable organic matter, bacteria, viruses, and other microorganisms.</p>
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<p>Disinfection is also needed to reduce the densities of microorganisms. Additionally, secondary clarifiers are required to remove solids from the secondary processes.</p>
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<p>Sludge generated from the primary and secondary clarifiers needs to undergo treatment and proper disposal.</p>
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<h2 class="wp-block-heading"><strong>Advanced Treatment Systems</strong></h2>
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<p>Advanced wastewater treatment is defined as the methods and processes that remove more contaminants from wastewater than conventional treatment.</p>
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<p>The term advanced treatment may be applied to any system that follows the secondary, or that modifies or replaces a step in the conventional process. The term tertiary treatment is often used as a synonym; however, the two are not synonymous.</p>
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<p>A tertiary system is the third treatment step that is used after primary and secondary treatment processes.</p>
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<p>Advanced processes include chemical coagulation of wastewater, wedge-wire screens, granular media filters, diatomaceous earth filters, micro screening, and ultrafiltration and nanofiltration, which are used to remove colloidal and fine-size suspended solids.</p>
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<p>Techniques such as biological assimilation, nitrification (conversion of ammonia to nitrogen &amp; nitrate), denitrification, ion exchange, breakpoint chlorination, and air stripping are used for nitrogen control. Soluble phosphorus is removed by chemical precipitation and biological (bacteria and algae) uptake for normal cell growth in a control system.</p>
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<p>Filtration is required after chemical and biological processes. Physical processes such as reverse osmosis and ultrafiltration are used for phosphorus reduction, but these are primarily for overall dissolved inorganic solids reduction. The use of lagoons, aerated lagoons, and natural and constructed wetlands is used for nutrient (N and P) removal.</p>
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<p>Removal of some species of groups of toxic compounds and refractory organics can be achieved by activated carbon adsorption, air stripping, activated sludge, powder-activated carbon processes, and chemical oxidation.</p>
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<p>Conventional coagulation-sedimentation-filtration &amp; biological treatment (trickling filter, RBC, and activated sludge) processes are also used to remove the priority pollutants &amp; organic compounds.&nbsp;</p>
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