Clarifier Design

456 Clarifier Design, Second Editi01t feed tanks to center feed. There are, however, hundreds of peripheral feed tanks, and most correctly designed units do not have this problem. The ducking skimmer d(Cvice offers the advantage of programming the rotating trough. Typically, one and no more than two skimming boards are provided per tank. The trough rotation can be programmed to trip every time the board approaches or can skip some of the cycles to reduce the amount of floatable material removed. This added flexibility is countered by the additional complexity of the system. Some of the models have had binding of the rotating troughs or failure of the motorized device used to rotate the trough. A robust mechanical design and equipment specification is important. BLANKET LEVEL DETECTION. Knowledge of the sludge blanket level in a clarifier has always been essential to good operating practice. The use of a core sam­pler and other means of measuring this are explained in Chapter 7. Automatic measurement of the level of sludge in clarifiers has become increas- . ingly popular among operators and useful in automating control of sludge pumping and wasting. Several means are available to do this. An electronic sensor that is based on light transmittance has been used for years. Additional detail on instrumentation is given in Chapter 10. ALGAE CONTROL. Algae growth is a problem with many clarifiers having weirs and open troughs. Many plants do not have provisions to control this growth, and the operators are left to deal with it as a maintenance chore. This involves occasional hose downs and even putting waders on and walking in the troughs to clean them. This is a tedious task and can also be dangerous. For plants with gaseous chlorine, some designs have provided for a diffuser pipe just upstream of the weir. A periodic release of chlorine solution has served to kill the algae and keep the troughs from excessive growths. In recent years, most plants in the United States have abandoned gaseous chlorine and shifted to hypochlorite or UV light for disinfection. Hypochlorite has a tendency to form a chemical deposit and clog the orifices of a chlorine diffusion line. Thus, chlorine control of algae at clarifiers is declining. Another method used in some new and retrofitted circular tanks is the addition of spring-loaded brushes to the rotating skimmer arms. Figure 8.42 shows such an installation. This has been successful in many applications but does require periodic adjustment and replacement of the brushes. 'II'. 457 Circular Clarifiers FIGURE 8.42 Spring-loaded brushes can be strategically placed to keep the effluent trough clean. Water jet spray systems represent yet another concept to remove algae. In the past several years, an automated arrangement that can be tied into a supervisory con­trol and data acquisition system of a plant has emerged and successfully applied in a few dozen installations. One example is illustrated in Figure 8.43. Supply water is piped to the tank c~nter, where it transfers through a submerged slip ring to a trav­eling pipe attached to the rotating sludge/scum truss structure. Fixed or rotating branch pipes, with nozzles at their ends, emit an intense spray that can be directed at the walls, weirs, baffles, and troughs. This concept has the ability to clean irregular shapes of tanks and weirs, induding the cantilevered double-weir launders. The concept of adding covers to keep the launder areas dark has been practiced for many years. The covers were not used extensively in circular tanks because of the relatively large area involved and the curved shape. Commercially available covers made of fiberglass have become more popular in recent years and have been found effective (Figure 8.44). These can be designed to have hinges at the weir or at the walL Multiple doors can be opened to expose the trough for access and to visibly inspect the quality of effluent. Locating hinges at the wall allows the operators access to the weir and scum baffle, whereas locating them at the weir facilitates observation of the troughs. Coveringof the troughs of circular 458 Clarifier Design, Second Edition fIGURE 8.43 Water spray jet system removing algae from serpentine weiI"s_ tanks can also be done by constructing a concrete deck over the trough and providing multiple access hatches. WALKWAYS AND PLATFORMS. Most circular clarifiers are equipped with a single walkway that extends from the perimeter of the tank to the center area. For some plants, it is advisable to extend this walkway across the full diameter. In some designs, two full diameter walkways, at 90 degrees to each other, are constructed to facilitate operator traffic, guest tours, sampling, and structural support for the floccu­lation baffles, skimming devices, and other facilities. Walkways are typically a minimum of 1 m (3 ft) in width; 1.4 m (4 ft) is preferred by many. A life buoy and a least one hose bib should be provided along the walkway. The center platforms for clarifiers with center drives are sized to give operators a minimum of 0.7 m (2 ft), and preferably 1 m (3 ft), clearance around the mecha­ Copy of MOP p2 Copy of MOP p3 Copy of MOP p4