High Rate Digestion

In the 1950s, several improvements to low-rate digestion were developed, resulting in a high-rate anaerobic digestion system. Heating, auxiliary mixing, thickening the raw sludge, and uniform feeding, the essential elements of a high-rate digestion system, act together to create a uniform environment. As a result, the tank volume is reduced and the stability and efficiency of the process are improved. Figure 5.6 shows the basic layout of this system.

Heating Heating is essential because the rate of microbial growth, and therefore the rate of digestion and gas production, increase with temperature. Most commonly, high-rate digestion operates in the mesophilic range 30 to 38°C (86 to 100°F). The contents of the digester are heated and maintained

Digester Gas

Digester Gas

Raw Sludge

Heat Exchanger

Heat Exchanger

Recycle

Figure 5.6 Single-stage high-rate anaerobic digestion.

Figure 5.6 Single-stage high-rate anaerobic digestion.

consistently within 0.5°C (1°F) of design temperature because anaerobic bacteria, especially methanogens, are easily inhibited by even small changes in temperature. Heating by external heat exchangers is the most common method of heating because of their flexibility and ease of maintenance. Other heating methods include internal heat exchangers and steam injection. Heating methods are discussed further in Section 5.5.4.

Auxiliary Mixing Auxiliary mixing of digester contents is beneficial for (WEF, 1998):

• Reducing thermal stratification

• Dispersing the raw sludge for better contact with the active biomass

• Reducing scum buildup

• Diluting any inhibitory substances or any adverse pH and temperature feed characteristics

• Increasing the effective volume of the reactor

• Allowing reaction product gases to separate more easily

• Keeping in suspension more inorganic material, which has a tendency to settle

The three types of mixing typically used are mechanical draft tube mixing, pumping, and gas recirculation. Methods of mixing are discussed further in Section 5.5.3.

Thickening In the early 1950s, (Torpey, 1954), thickening raw sludge before feeding it to the digester was found to be beneficial. Thickening reduces the biomass volume and hence the digester volume. It also reduces the heating requirements because of the smaller volume of raw sludge entering the digester. Thickening also reduces the volume of supernatant to be withdrawn from the digester (in a two-stage operation). Since thickener supernatant is of far better quality than digester supernatant, it has a less adverse impact when recycled to the treatment plant influent. If thickening is used to enhance solids loading and performance of an existing digester, the limits imposed by pumping, heating, and mixing systems should be evaluated carefully. Increasing the solids concentration affects the viscosity, which in turn affects the pumping and mixing. Thickening to more than 7% concentration can cause digester mixing problems. Excessive thickening can also increase the chemical concentrations to levels that can inhibit bacterial activity.

Uniform Feeding Sludge is fed into a high-rate digester continuously or at regular intervals to help maintain steady-state conditions in the digester. Since the methanogens are sensitive to changes in volatile solids concentrations, uniform feeding can alleviate shock loadings. Multiple feed points in the digester can also alleviate or reduce shock loadings. Shock loading a digester affects its temperature and dilutes the alkalinity necessary for the buffering against pH in the digester.

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