<!-- wp:paragraph -->
<p>The traditional meaning of Primary treatment implies a sedimentation process to separate the readily settleable and floatable solids from the wastewater.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The treatment unit used to settle raw wastewater is termed the primary sedimentation tank (basin), primary tank (basin), or primary clarifier.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Sedimentation is the oldest and most widely used process in the successful treatment of wastewater.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>After the wastewater passes the screens, comminutors, and grit chambers, the wastewater enters sedimentation tanks.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The suspended solids that are lighter than solids that fell out into the grit chamber will settle in the tank over a few hours.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The settled sludge is then removed using mechanical scrapers or pumped.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Surface skimmer devices are used to remove floatable substances on the tank surface.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Then, the effluent flows to the secondary treatment units or is discharged.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The flow velocity of wastewater in primary sedimentation tanks is reduced by plain sedimentation.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The process usually removes particles with a settling rate of 0.3 to 0.6 mm/s.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>In some cases, chemicals also may be added.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Reduced suspended solids content, equalization of side stream flow, and BOD removal are the benefits of the primary sedimentation process.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The overflow rate of the primary sedimentation tanks ranges from 24.5 to 49 m<sup>3</sup>/(m<sup>2</sup>-d).</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Usually, the detention time in these tanks is 1-3 hours.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Primary tanks (or primary clarifiers) remove 90-95 percent of settleable solids, 50-60 percent of total suspended solids, and 25-35 percent of the BOD<sub>5</sub>.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Type 2 flocculent settlings are the usual settling characteristics in the primary clarifier.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Due to the continuous changes in the shape, size, and specific gravity of flocculated particles, Stoke’s formula for settling velocity cannot be used to determine particle settlement.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Due to these reasons, no mathematical equation can be used to describe flocculent settling satisfactorily. Therefore, laboratory analyses such as settling-column tests are commonly used to generate design information.</p>
<!-- /wp:paragraph -->

<!-- wp:heading -->
<h2 class="wp-block-heading"><strong>RECTANGULAR BASIN DESIGN</strong></h2>
<!-- /wp:heading -->

<!-- wp:paragraph -->
<p>Usually, multiple units with common walls are designed and constructed for independent operations. Basin dimensions are designed based on the surface overflow (settling) rate to calculate the required basin surface area.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The required area of the sedimentation tank is equal to the flow divided by the selected overflow rate.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>An overflow rate of 36 m<sup>3</sup>/(m<sup>2</sup>•d) at average design flow is generally acceptable.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Basin surface dimensions, the length (<em>l</em>) to width (<em>w</em>) ratio <em>(l/w), </em>can be increased or decreased without changing the volume of the basin.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>For a greater <em>l/w </em>ratio, the basin has a proportionally larger effective settling zone and a smaller percentage of inlet and outlet zones.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Increased basin length allows the development of a more stable flow.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The inlet structures in rectangular clarifiers are placed at one end and are designed to disperse the inlet velocity to diffuse the flow equally across the entire cross-section of the basin and to prevent short-circuiting. Typical inlets consist of small pipes with upward ells, perforated baffles, multiple ports discharging against baffles, a single pipe turned back to discharge against the headwall, simple weirs, submerged weirs sloping upward to a horizontal baffle, etc.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The inlet structure should be designed in such a way as not to trap scum or settling solids.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Baffles are installed 0.6 to 0.9 m in front of inlets to assist in diffusing the flow and submerged 0.45-0.60 m with a 5 cm water depth above the baffle to permit floating material to pass.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Scum baffles are placed in front of outlet weirs to hold back floating material from the basin effluent. These extend 15-30 cm below the water surface.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Outlets in a rectangular basin consist of weirs located toward the discharge end of the basin.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Walls of settling tanks should extend at least 150 mm above the surrounding ground surface and shall provide not less than 300 mm freeboard.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Mechanical sludge collection and withdrawal facilities are usually designed to ensure rapid removal of the settled solids.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The minimum slope of the side wall of the sludge hopper is 1.7 vertical to 1 horizontal.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The hopper bottom dimension should not exceed 600 mm.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The sludge withdrawal line is at least 150 mm in diameter and has a static head of 760 mm or greater with a velocity of 0.9 m/s.</p>
<!-- /wp:paragraph -->

<!-- wp:heading -->
<h2 class="wp-block-heading"><strong>CIRCULAR BASIN DESIGN</strong></h2>
<!-- /wp:heading -->

<!-- wp:paragraph -->
<p>Inlets in circular or square basins are typically at the center and the flow is directed upward through a baffle that channels the wastewater (influent) toward the periphery of the tank.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Inlet baffles are 10-20 % of the basin diameter and extend 0.9-1.8 m below the wastewater surface.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>The degree of turbulence in circular basins is higher when compared to rectangular basins. Therefore, circular basins are more efficient as flocculators.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>A typical depth of the sidewall of a circular tank is 3 m.</p>
<!-- /wp:paragraph -->

<!-- wp:image {"id":2135,"sizeSlug":"full","linkDestination":"none"} -->
<figure class="wp-block-image size-full"><img src="https://civilengineeringlectures.com/wp-content/uploads/2024/07/image.png" alt="" class="wp-image-2135"/></figure>
<!-- /wp:image -->

<!-- wp:paragraph -->
<p>As shown in Figure, the floor slope of the tank is typically 300 mm horizontal to 25 mm vertical.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Outlet weirs extend around the periphery of the tank with baffles extending 200-300 mm below the wastewater surface to retain floating material. Overflow weirs shall be located to optimum actual hydraulic detention time &amp; minimize short-circuiting.</p>
<!-- /wp:paragraph -->

<!-- wp:paragraph -->
<p>Peripheral weirs shall be placed at least 30 mm from the well.</p>
<!-- /wp:paragraph -->