Comparison Of Thermal Drying And Composting

In recent years, thermal drying and composting have become useful technologies for the preparation of class A biosolids. Let us examine three sludge-processing schemes, shown in Figure 9.4, to compare the heat consumption of these two technologies. Let the quantity of raw primary and thickened activated sludge be 800 m3/d with 4% dry solids.

Scheme 1

Scheme 2

Scheme 3

Figure 9.4 Sludge processing schemes.

Scheme 3

Figure 9.4 Sludge processing schemes.

Scheme 1

heat necessary for sludge digestion = (800 m3 /d)(106Mj/ m )

= 84,800 Mj/d heat obtained by sludge digestion = (800m3/d)(235Mj/m3)

= 188,000 Mj/d excess heat from sludge digestion = (800mVd)[(235-106)Mj/m3]

volume of dewatered sludge with 24% solides = (800m3/d)(

volume of thermally dried sludge with 90% solids = (800 m3/d)(

= 35.6mVd moisture to be evaporated for drying = (133 - 35.6)m3/d

= 97.4 m3/d heat required to evaporated moisture = (97.4m3/d)(1000kg/m3 )(3.9MJ/kg)

Utilization of the digester's excess heat (103,200 MJ/d) will reduce the energy consumption for thermal drying.

A heat exchanger following the thermal dryer can recover the heat required for sludge digestion, which is 84,800 MJ/d. Therefore, net heat required for thermal drying of sludge = (379,860 -103,200 + 84,800) Mj/d = 361,460 Mj/d

The process allows the reduction of 188,000 MJ/d of heat.

Scheme 2 The quantity of organics reduced during composting process averages about 25% , but for digested sludge, with the reduced quantity of organics, it can be just 10%. For primary sludge, the reduction is more than 45%, and for activated sludge it is about 30 to 35%. Let us assume that the reduction of organics during composting of the mixture of raw primary and activated sludge is 35%. Then the quantity of reduced organics is (28 kg/m3)(35%) = 9.8 kg/m3, which is (800 m3/d)(9.8 kg/m3) = 7840 kg/d.

heat released from composting = (7840kg/d)(21 MJ/kg)

Reduction of 1 kg of organics results in the removal of 5 kg of moisture. Therefore, moisture removed = (7840kg/d)(5kg/kg)

Scheme 3 The quantity of organics reduced during composting of digested sludge is 10%, which is (28 kg/m3)(40%)(10%) = 1.1 kg/m3, where 40% is the organics reduced in digestion.

organics reduced in composting = (800m3/d)(1.1kg/m3)

=880kg/d heat released during composting = (880kg/d)(21 MJ/kg)

= 18,480 MJ/d excess heat from sludge digestion = 103,200 MJ/d (see Scheme 1) total heat obtained = (18,480 +103,200) MJ/d = 121,680 MJ/d mositure evaporated from sludge = (880kg/d)(5kg/kg)+ 103,200MJ/d

= 30,900 kg/d (30.9m3/d) Table 9.1 compares energy use in the three process schemes.

TABLE 9.1 Comparison of Energy Use (MJ/d)

Scheme 2

Scheme 3

Scheme 1

(Raw Sludge

(Digested Sludge


(Thermal Drying)



Heat required for process


Heat reduced


Heat obtained



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