Power Efficiency Guide

Incineration reduces sludge volume dramatically. Prior to incineration, sludge must be dewatered and thermally dried, with drying being the most energy-intensive step. Therefore, when considering the use of incineration, one must look for methods and techniques to reduce the amount of energy required and to provide some, if not all, of the required energy from another sludge process.

Dewatering can be accomplished by any number of mechanical processes, such as belt filter presses, pressure filter presses, and centrifuges. The less moisture in the dewatered sludge, the less the total consumption of energy (see Figure 9.2). Therefore, it is most energy efficient to remove as much moisture as possible prior to drying. The lower moisture content will reduce the energy requirements in the succeeding steps of thermal drying and incineration.

Let us examine the technology of treatment of a mesophilically digested primary and secondary sludge mixture using the thermal drying process. In

Moisture of Dewatered Sludge, %

Moisture of Dewatered Sludge, %

Moisture of Original Sludge, %

Figure 9.2 Moisture content versus energy consumption for dewatering and drying of sludge.

Moisture of Original Sludge, %

Figure 9.2 Moisture content versus energy consumption for dewatering and drying of sludge.

dryers with opposing jet streams of air, approximately 3.4 to 3.9 MJ of heat and 0.02 to 0.06 kWh of electricity are used for each 1.0 kg of evaporated moisture. Consider a wastewater treatment plant with a capacity of 100,000 m3/ d (26 mgd). The quantity of mesophilically digested primary-secondary sludge mixture with a solids content of 4% (96% moisture) is about 800 m3/d. Assume that this 4% solid sludge is dewatered mechanically to a solids content of 18 to 24%, and compare the amount of energy required to dry the sludge with 24% solids versus one with 18% solids to a sludge with 60% solids.

The volume of dewatered sludge is calculated using the formula where

V2 = volume of dewatered sludge, m3

V1 = volume of sludge prior to dewatering, m3 (800 m3)

C1 = dry solids concentration of sludge prior to dewatering, % (4%)

C2 = dry solids concentration of dewatered sludge, %

volume of dewatered sludge with 24% solids = (800 m3

Thermal drying of dewatered sludge requires evaporation of 133 - 53 = 80 m3/d of moisture. The energy required to evaporate the moisture is as follows:

heat = (80 m3/d )(1000 kg/m3)(3.9 Mj/kg) = 312,000 MJ/d electricity = ( 80m3/d )(1000kg/m3)(0.03kWh/kg ) = 2400kWh/d

Therefore, it is necessary to evaporate 178 - 53 = 125 m3/d of moisture for thermally drying the 18% solid sludge. The energy required to evaporate this moisture is as follows:

heat = (125 m3/d)(1000 kg/m3)(3.9 Mj/kg) = 487,500 MJ/d electricity = (125 m3/d)(1000 kg/m3)(0.03 kWh/kg) = 3750 kWh/d

This is almost 1.6 times more heat and electricity than that required to dry a sludge that has 24% solids. Utilization of the methane gas generated by the mesophilic digestion process will reduce the required quantity of heat for thermal drying by (129 MJ/m3)(800 m3/d) = 103,200 MJ/d.

The heat value of municipal wastewater sludge (Qb) in incineration normally ranges from 23.4 to 26.9 Mj/kg. This heat is obtained from the organics that are typically 65 to 72% of the sludge solids. The heat value is higher for raw sludge from primary clarifiers than for raw activated sludge from secondary clarifiers and digested sludge. When 1.0 m3 of a 1 : 1 primary/secondary sludge mixture that contains 40 kg of dry solids (or 28 kg of organics) with a Qb value of 25.5 MJ/kg is incinerated, approximately (28 kg)(25.5 MJ/kg) = 714 MJ of heat is obtained. Similarly, when 1.0 m3 of mesophilically digested sludge with a Qb value of 23.5 is incinerated, (16.8 kg)(23.5 MJ/kg) = 395 MJ of heat is obtained. For thermophylically digested sludge, the heat obtained is (14.0 kg)(23.5 MJ/kg) = 329 MJ.

The total energy from anaerobically digested sludge includes the energy from the methane gas produced from digestion plus the energy obtained from the combustion of the remaining sludge organics. For the mesophilically digested sludge, the total energy obtained is 129 + 395 = 524 MJ, and for thermophylically digested sludge it is 64 + 329 = 393 MJ. It follows from these calculations that it is reasonable to incinerate the raw sludge because of its higher heat value (714 MJ versus 524 or 393 MJ).

Combining the thermal drying process with the incineration of sludge may significantly reduce the energy expenditures for thermal drying. As pointed out before, a significant quantity of heat may be obtained by the incineration of sludge. However, most of the heat is spent for moisture evaporation and

Pretreatment

Primary Clarifiers

Aeration Tanks

Secondary Clarifiers

Pretreatment

Primary Clarifiers

Aeration Tanks

Secondary Clarifiers

Thermal Incineration Drying

Figure 9.3 Schematic of autothermic incineration of sludge.

Thermal Incineration Drying

Figure 9.3 Schematic of autothermic incineration of sludge.

heating of the blast air, and there are some system loses. Therefore, incineration may cover only a part of the heat that is necessary for the thermal drying of sludge.

Incineration of a mechanically dewatered primary-secondary sludge mixture in an autothermic process (conducting the process of thermal drying and incineration without additional consumption of fuel) may be achieved when the moisture of the dewatered sludge mixture is 64 to 66%. An increase in the moisture content of the dewatered sludge requires using the appropriate quantity of energy for evaporation of moisture. An illustration of autothermic incineration of sludge is shown in Figure 9.3. It becomes reasonable to use incineration when toxic substances in the sludge prevent its use as a fertilizer or in a municipal landfill.

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