Tiêu đề Module 3_Primary treatment_CEE 381.pdf ✅

5-6 PRIMARY TREATMENT With the screening completed and the grit removed, the wastewater still contains light organic suspended solids, some of which can be removed from the sewage by gravity in a sedimentation tank. These tanks can be round or rectangular. The mass of settled solids is c alled raw sludge. The sludge is iemoved from the sedimentation tank by mechanical scrapers and pumps (Figure 5-14). Floating materials, such as grease and oil, rise to the surface ofthe sedimentation tank, where they are collected by a surface skimming system and removed from the tank for further processing. Primary sedimentation basins (primary tanks) are characterized by Type II flocculant settling. The Stokes equation cannot be used because the flocculating par- ticles are continually changing in size, shape, and, when water is entrapped in the floc, specific gravity. There is no adequate mathematical relationship that can be used to describe Type II settling. Laboratory tests with settling colunms arc used to develop design data (see Chap. 3). Rectangular tanks with common-wall construction are frequently chosen be- cause they are advantageous for sites with space constraints. Typically, these tanks
Discrete seltiing rcgion (type 1) ' Flocculant settling region (rype 2) settling region kype 3) Compression region FIGURE 6.14 Schematic of setfljng regions for activated sludge.
TABLE 6.8 Types of settling phenomena involved in wastewater trealment Type of settling phenomenon Desc.iption Appllcatlon/occurrgnc€ Discrete particle (type 1) Refers to the sedimentation of particles in a suspenslon of low solids concentration. Parlicles settle as individual entities, and lhere is no significant interaction with nejghboring particles Relers to a rather dilute suspension of particles thal coalesce, or flocculate, during the sedimentation operation. By coalescing, the particlGs increase in mass and settle at a iasler rate Refers to suspefisions of intermediale concentration, in which interparticle forces are sufficient to hinde. the setlling of neighboring particles. The particles lend lo remain in fixed positions with respect to each other, and the mass ol particles settles as a unit. A solids-liquid intertace develops at the top of the settling mass Refers to settling in which the particles are ot such concentration that a struclure is formed, and further settling can occur only by compression of the structure. Compression iakes place frorn the weight of the panicbs, which are conelantly being added to the structure by sodimeniation trom the supematant liquid Removes grit and sand padicles from wastewater Removes a portion of the suspended solids in untr€ated wastewator in primary settling facilities, and in uppsr porlions of secondary settling facilities, Also removes chemical floc in settling tanks Occurs in secondary senling facilities used in conjunction with biological treatment facilities Usually occurs in the lower layers of a deep sludge mass, such as in the bottom of deep secondary settling facilities and in sludge- thickening facilities Flocculant {type 2) Hindered, also called zon€ (type 3) Compression (type 4)
Rectangular tanks with common-wall construction are frequently chosen be- cause they are aclvantageous for sites with space constraints. Typically, these tanks