Two Stage Digestion

Two-stage digestion is carried out in a high-rate digestion tank coupled in series with a second tank (sometimes called a secondary digester), as shown in Figure 5.7. The second tank is neither heated nor mixed. Its main function is to allow gravity concentration of digested solids and decanting of supernatant liquor. Decanting reduces the volume of digested sludge, requiring further processing and disposal. A secondary digester fitted with a floating cover can also provide storage for digested sludge and digester gas. Very little solids reduction and gas production take place in the second tank. In some

Digester Gas

Digester Gas

Primary Secondary

Digester Digester

Figure 5.7 Two-stage high-rate anaerobic digestion.

Primary Secondary

Digester Digester

Figure 5.7 Two-stage high-rate anaerobic digestion.

instances, the second tank is similar in design to the first, with heating and mixing capabilities to provide standby digester capacity.

Many secondary digesters have performed poorly as thickeners, producing dilute sludge and high-strength supernatant. This is because some gas will come out of the solution in small bubbles if there is incomplete digestion in the primary digester or if the sludge transferred from the primary digester is supersaturated with gas. These bubbles attach to the sludge particles and provide a buoyant force that hinders settling. Another reason for poor settling is the fine-sized solids produced during digestion by both mixing and the natural breakdown of particles through biological decomposition (U.S. EPA, 1979). The problem is compounded when the digester is fed with secondary and tertiary solids, especially when those have been flocculated and broken up during digestion. Therefore, two-stage digestion, used frequently in the past, is seldom used in newer facilities.

Was this article helpful?

0 0
Organic Gardeners Composting

Organic Gardeners Composting

Have you always wanted to grow your own vegetables but didn't know what to do? Here are the best tips on how to become a true and envied organic gardner.

Get My Free Ebook


Post a comment