## Heating

Maintaining a constant temperature [typically, 35°C (95°F) for mesophilic organisms and 55°C (130°F) for thermophilic organisms] is important for the efficient operation of an anaerobic digester. Heating is required to raise the feed sludge to the operating temperature of the digester and to compensate for heat loss through the walls, floor, and roof of the digester.

Heating Requirements The amount of heat required to raise the temperature of the sludge is given as

 where Qi = heat required, J/d (Btu/d) Wf = feed sludge weight, kg/d (lb/d) Cp = specific heat of sludge (assumed to be same as water), 4200 J/kg • °C (1 Btu/lb-°F) T2 = design operating temperature of digester, °C (°F) Ti = temperature of feed sludge, °C (°F)

The amount of heat required to make up for the heat loss from the digester is given by

where

U = heat transfer coefficient, J/m2 ■ s ■ °C (Btu/ft2-hr-°F) A = surface area of digester through which heat losses occur, m2 (ft2) T2 = temperature of sludge in digester, °C (°F) T3 = temperature outside the digester, °C (°F)

Various values of heat transfer coefficients for different wall, floor, and roof construction are given in Table 5.5. Coefficients, hence heat losses, can be

TABLE 5.5 Heat Transfer Coefficients for Anaerobic Digester Components

U.S. Customary SI Units Units

Digester Component W/m2-°C Btu/ft2-°F-hr

Plain concrete walls (above ground)

U.S. Customary SI Units Units

Digester Component W/m2-°C Btu/ft2-°F-hr

Plain concrete walls (above ground)

 300 mm (12 in.) thick, not insulated 4.7-5.1 0.83-0.90 300 mm (12 in.) thick with air space plus 1.8-2.4 0.32-0.42 brick facing 300 mm (12 in.) thick with insulation 0.6-0.8 0.11-0.14 Plain concrete wall (below ground) In dry earth 0.57-0.68 0.10-0.12 In moist earth 1.1-1.4 0.19-0.25 Plain concrete floors 300 mm (12 in.) thick in dry earth 0.85 0.15 300 mm (12 in.) thick in moist earth 0.7 0.12 Floating covers With 35-mm (1.5-in.) wooden deck, 1.8-2.0 0.32-0.35 built-up roofing, and no insulation With 25-mm (1-in.) insulating board 0.9-1.0 0.16-0.18 installed under roofing Fixed concrete covers 100 mm (4 in.) thick, covered with built-up 4.0-5.0 0.70-0.88 roofing, and not insulated 100 mm (4 in.) thick, covered, and insulated 1.2-1.6 0.21-0.28 with 25-mm (1-in.) insulating board 225 mm (9 in.) thick, not insulated 3.0-3.6 0.53-0.63 Fixed steel covers, 6 mm (0.25 in.) thick 4.0-5.4 0.70-0.95

Source: Adapted from U.S. EPA, 1979, and Metcalf & Eddy, 2003.

Source: Adapted from U.S. EPA, 1979, and Metcalf & Eddy, 2003.

reduced by insulating the cover and the exposed walls of the digester. Common insulating materials are glass wool, insulation board, urethane foam, and dead air space. A facing is placed over the insulation for protection and to improve aesthetics. Common facing materials are brick, metal siding, and stucco.

In computing heat losses, various surfaces should be considered separately (roof, walls exposed to air, walls below ground, and floor) to develop the total heat loss. Walls below ground level are exposed to different temperature regimes depending on groundwater level and frost penetration. An average temperature is generally assumed for the entire wall below grade. If the groundwater level is not known, it is assumed that the digester floor is in saturated earth.

Heating Equipment External heat exchangers are the most commonly used heating method. Other heating methods, such as steam injection directly into the tank, direct flame heating by passing hot combustion gases through the sludge, and heat exchanger coils placed inside the tank, have been used in the past but have been discontinued because of the better heat transfer efficiency of the external heat exchangers. Another advantage of external heat exchang ers is that recirculating digester sludge can be blended with raw sludge feed before heating in order to seed the raw sludge with anaerobic organisms. Cold sludge should never be added directly to the digester because the thermal shock will be detrimental to the anaerobic bacteria.

Three types of external heat exchangers are commonly used: water bath, jacketed pipe, and spiral. In a water bath heat exchanger, boiler tubes and sludge piping are located in a common water-filled container (see Figure 5.13). Hot water is pumped in and out of the bath to increase heat transfer. In a jacketed pipe heat exchanger (also known as a tube-in-tube heat exchanger), hot water is pumped countercurrent to the sludge flow through a concentric pipe surrounding the sludge pipe. The spiral heat exchanger (see Figure 5.13) is also a countercurrent flow design; however, the sludge and the