Pump Applications

Most of the pumping applications associated with the collection and treatment of sewage can be classified according to the general nature of the liquid to be handled. The primary

FIGURE 3 Typical activated sludge plant, with dewatering, incineration, and liquid disposal of ash

classifications are (1) raw sewage, (2) settled sewage, (3) service water, and (4) sludge. There are also, however, a number of specialized applications involving the handling of abrasive materials, such as grit and ash. The types of pumps recommended for sewage applications are indicated in Table 1. Although included in the table, chemical pumps are not discussed in this section. They are covered in Section 9.6.

Raw Sewage Raw sewage pumps are used to lift liquid wastes from one level of the collection system to another or to the treatment plant for processing. Regardless of where the pumps are located, the basic design considerations remain the same.

Even though the sewage is normally screened at larger installations before entering the suction wet well, it still contains a large quantity of problem material, such as grit, rags, stringy trash, and miscellaneous solids small enough to pass through the coarse screens. Screens are often omitted from smaller installations because large objects are not as much of a problem because of the smaller size of the incoming sewers.

Raw sewage pumping installations are usually sized so their firm capacity either is equal to a future maximum flow rate of the incoming sewers or can be expanded to accommodate this level. Firm capacity is defined as total station capacity with one or more of the largest units out of service.

Pneumatic ejectors (Figure 2) are sometimes used where the required capacity is less than that provided by the smallest conventional sewage pump. This type of unit, however, should not be used where more than 50 connections are expected.

Conventional sewage pumps are, by far, the most common pumps used for the handling of raw sewage. A conventional sewage pump is more specifically described as an end-suction, volute-type centrifugal with an overhung impeller of either the nonclog (Figure 4a) or the radial- or mixed-flow type (Figure 5), depending on capacity and head.

Nonclog pumps are all based on an original development by Wood at New Orleans. Actually, no pump has been developed that cannot clog, either in the pump or at its appurtenances. Experience shows that rope, long stringy rags, sticks, cans, rubber and plastic goods, and grease are most conducive to clogging.

Nonclog impellers are used almost exclusively today for pumps smaller than 10 in (25 cm). These pumps differ from clear-water pumps in that they are designed to pass the largest solids possible for the pump size. The conventional nonclog impeller contains two blades, although some manufacturers are now offering a single-blade ("bladeless") impeller. The two-blade impeller has thick vanes with large fillets between the vanes and the shroud at the vane entrance. The bladeless impeller has no vane tips to catch trash. On the other hand, it is inherently out of balance because of its lack of symmetry.

The larger raw sewage pumps are equipped with either mixed-flow or radial-flow impellers, depending on head conditions. Both have two or more vanes, depending on pump size and the size of solids to be handled. The vane tips are sharper than for the non-clog impeller, resulting in a higher operating efficiency. The heavier vanes are not necessary because the vane openings can be larger than on the smaller pumps. Experience indicates that stringy trash will not clog an impeller with vane openings larger than 4 in (102 mm) in diameter.

Conventional solids handling volute type sewage pumps may be of the dry pit type (Figure 4a), wet pit type (Figure 4b), or submersible type (Figure 4c). Dry pit pumps are by far the most popular and widely used type due to their accessibility for observing mechanical operation and ease of maintenance when necessary.

Wet pit solids handling pumps are gaining in popularity especially on return activated sludge services. They are similar in design to a vertical turbine or mixed-flow diffuser pump except the bowl contains a limited number (usually two) of well-rounded diffusers, the impeller is of the solids handling type, and there is no lower suction bell bearing. Therefore, they are not nearly as likely to clog on the debris normally found in raw sewage. Single-stage models available at the time of this publication are limited to operational total heads under 100 ft (30 m).

Submersible solids handling pumps are commonly used in small raw sewage lift stations. They are most widely used in sizes 12 in (305 mm) and smaller with total heads under 100 ft (30 m) and motor ratings under 150 HP (112 kW).

TABLE 1 Types of pumps generally used in sewage applications

Application

Conventional sewage

Diffuser

Torque Clear-water flow volute Ash

Pneumatic Air Positive Screw ejector lift displacement Chemical

Raw Sewage Grit

Primary sludge Less than 2% solids

More than 2% solids Normal primary scum Diluted scum Biological sludge Thickened biological sludge Digested sludge, recirculation Settled sewage Plant effluent Service or nonpotable water Ash sluice Decantate or supernatant liquor Chemical solution

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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