heat and energy requirements in thermophilic anaerobic sludge digestion

Renewable Energy 28 (2003) 2255–2267www.elsevier.com/locate/renene

Heat and energy requirements in thermophilicanaerobic sludge digestion

G.D. Zupancˇic ∗, M. RosNational Institute of Chemistry, Department for Chemistry, Biology and Technology of Water,

Hajdrihova 19, PO Box 660, SI-1000 Ljubljana, Slovenia

Received 6 November 2002; accepted 16 April 2003

Abstract

The heating requirements of the thermophilic anaerobic digestion process were studied.Biogas production was studied in laboratory experiments at retention times from 1 to 10 days.The data gathered in the experiments was then used to perform a heat and energy analysis.The source of heat was a conventional CHP unit system. The results showed that thermophilicdigestion is much faster than mesophilic digestion and therefore produces more biogas in ashorter time or at smaller digester volumes. The major part of the heating requirements con-sisted of sludge heating. The heat losses of the digester were only 2–8% of the sludge heatingrequirements. The heating requirements in thermophilic digestion are about twice those ofmesophilic digestion. Therefore a CHP unit system cannot cover all of the needs for successfuloperation of thermophilic digestion. Heat regeneration was introduced as a solution. Heat isregenerated from the sludge outflow at a temperature of 50–55°C and transferred to the coldinflow sludge at a temperature of 11°C. Enough heat is regenerated in a conventional counterflow heat exchanger to bring the thermophilic process to the same level as the mesophilic one.Considering the smaller digester volumes and the relatively small investment in the regenerat-ive equipment, the construction of thermophilic digestion systems may be a very good alterna-tive to conventional mesophilic sludge digestion systems. 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Anaerobic sludge digestion; Biogas production; Heat requirements; Heat regeneration; Sludgeheating; Thermophilic sludge digestion

∗ Corresponding author. Tel.:+386-1-4760-249; fax:+386-1-4760-300.E-mail address: [email protected] (G.D. Zupancˇic).

0960-1481/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S0960-1481(03)00134-4

2256 G.D. Zupancic, M. Ros / Renewable Energy 28 (2003) 2255–2267

Nomenclature

Agr Digester surface in contact with the ground (m2)Aout Digester surface from sludge to outside air (m2)cps Specific heat of sludge, equal to the specific heat of water (cpw =

4.187 kJ /kgK)kcgrs Heat transfer coefficient through ground walls from inside sludge to

soil (W/m2K)kcgrw Heat transfer coefficient from inside sludge to groundwater (W/m2K)kcout Heat transfer coefficient through walls from inside sludge to outside

air (W/m2K)m�

s Mass flow of inflow sludge (kg/s)Qc Heat loss (W)Qs Heat required to heat the sludge (kW)tgrs Standard calculation temperature of soil, Ref. [5] (0 °C)tgrw Standard calculation temperature of groundwater, Ref. [5] (10 °C)tout Minimum outside air temperature (Table 2) (°C)ts Temperature of sludge in the digester (55 °C)ts0 Minimum sludge temperature before entering the digester on

monthly basis, presented in Table 2 (°C)Vs

� Volume flow of sludge (m3/s)agrs Heat convection coefficient between wall and soil (W/m2K)ain Heat convection coefficient between wall and sludge (W/m2K)aout Heat convection coefficient between wall and outside air (W/m2K)lgrw Heat conductivity of soil between groundwater and digester wall

(W/mK)li Heat conductivity of material in the wall (W/mK)rsl Sludge density (kg/m3)CHP Combined heat and powerHRT Hydraulic retention timePE Population equivalentSTP Standard temperature and pressureVSS Volatile suspended solidsWWTP Waste water treatment plant

1. Introduction

Sludge digestion is the most common process for waste sludge treatment. Theanaerobic mesophilic process (at about 35 °C) is that most widely used. Generally,the anaerobic process is the subject of current research, as a result of the biogasevolved as a by-product of such a process. Degradation of volatile suspended solids

2257G.D. Zupancic, M. Ros / Renewable Energy 28 (2003) 2255–2267

(VSS) in the conventional mesophilic anaerobic process is about 40% at retentiontimes between 30 an 40 days [1–3]. In the thermophilic range (about 50–60 °C)sludge degrades at a much higher rate (50% of VSS degradation in 10 days retentiontime [4]). The main problem in thermophilic sludge digestion (compared to meso-philic digestion) is the high heating requirements for sustaining the process. Thetemperature is about 20 °C higher and that requires additional heat resources. In thispaper, an analysis of the heat requirements of thermophilic sludge digestion is perfor-med and a solution that brings thermophilic sludge digestion to the same level asmesophilic sludge digestion (concerning the heat requirements) is presented. Themain attention is given to the heat requirements. The energy requirements are gener-ally equal to those in a mesophilic digestion process and are not the main subjectof this paper.

At the National Institute of Chemistry, Ljubljana, in the Department for Chemistry,Biology and Technology of Water, experiments with thermophilic sludge digestionwere performed. From the results of these experiments, an extensive analysis of heatrequirements was conducted and appropriate solutions to the problem were intro-duced.

The heat requirements of sludge digesters generally consist of three parts [6]; first,that required for raising the temperature of incoming sludge for digestion; second,for compensating heat losses through the boundaries of the digester and third, forcompensating losses that might occur in the piping between the heat source and thedigester. By appropriate construction the heat losses in the piping (the third part)can be minimised to the point where such losses can be neglected.

The energy requirements of anaerobic sludge digestion consist of mixing andpumping. As mentioned above, the energy requirements of the thermophilic processare essentially the same as in the mesophilic process and are described in [4,6].

The main source of energy in all anaerobic digestion processes is the biogas pro-duced in the process. Conventionally, biogas is used in a cogeneration internal com-bustion engine, also called a CHP unit (combined heat and power) for productionof energy (electricity). The waste heat from the CHP unit operation is used as themain source of heat for the digestion process. Standard CHP units carefully utilisewaste heat through a series of three heat exchangers [7], creating a standard heatingcycle of 70/90 °C, using water as the heating medium [8].

To perform a heat analysis, the biogas production rates have to be determined.Biogas is utilised in the CHP unit; therefore energy transformation rates need to bedetermined to establish the heat potential of the biogas produced. Finally the digesterand sludge heat requirements are determined and compared to the heat potential tosee if the heat potential is sufficient for successful thermophilic sludge digestion.

2. Materials and methods

The heat requirements analysis of thermophilic sludge digestion was elaboratedon the basis of experimental results. Thermophilic anaerobic sludge digestion experi-ments were performed in a cylindrically shaped well-mixed anaerobic reactor, with


a gas outlet and a sludge outlet on top. The dosage flask was connected to a sludgeinlet, which was on the bottom of the reactor. The operating volume of the reactorwas 20 l. The reactor was also equipped with a temperature regulation device andheated by an electric heater. The gas outlet was equipped with a water trap and gaswas measured with an OPTIFLOW 420 bubble meter device. The range of the devicewas 0.01 to 55 ml/min. Gas composition (vol. % of CO2) was determined with aspecially designed burette, where CO2 was separated from biogas in NaOH solution.

The main parameter in the experiments was biogas production. For the analysisof further heat requirements the biogas is expressed as specific biogas production inlitres per kg of volatile solids inserted. Other parameters of sludge digestion werealso determined or monitored (volatile solids removal rate, pH and temperature). Theexperiments were performed at retention times (HRT) of 1, 2, 3, 4, 5, 6, 7, 8 and10 days.

The sludge used in the experiments was obtained from a Waste Water TreatmentPlant (WWTP) of 200 000 PE [9,10]. The sludge concentration was between 15 and45 kg/m3. For the laboratory experiments, the sludge was diluted to a concentrationof 10 kg/m3, to avoid clogging of tubes. The sludge volatile solids content was from70 to 85% of total solids.

For the analysis of heat requirements it was assumed that the sludge had a concen-tration of 40 kg/m3 and the volatile solids content was 70%. Biogas production con-sidered in the heat requirements analysis was determined experimentally as men-tioned above; some construction details of the digesters had also to be determined.This information was acquired in the same WWTP as the sludge used for experiments[9,10]. The same digester construction and composition materials were consideredas in actual digesters operating in the mesophilic temperature range (Fig. 1).

The heat requirements analysis was performed for WWTP sizes from 10 000 to500 000 PE. The digester design considered in the analysis was always the same;only the volume was varied appropriately. The sludge quantities considered werestandard recommendations for sludge production in a municipal WWTP (80 kg/m3 × day [11]).

Fig. 1. Digester design [9].


Table 1Heat transfer coefficients for the wall structure in the digestera

From sludge to air From sludge to soil From sludge to groundwater

Thickness li or a Thickness li or a Thickness li or a

Inside ain – 245 – 245 – 245Water insulation 0.005 0.6 0.005 0.6 0.005 0.6Inside mortar 0.007 1.4 0.007 1.4 0.007 1.4Concrete 0.3 2.33 0.6 2.33 0.6 2.33Heat insulation 0.1 0.028 0.1 0.028 0.1 0.028Aluminum plates 0.002 229 – – – –Outside aout, agrs, lgrw – 23.3 – 2.5 2 1.2

kcout = 0.265 kcgrs = 0.235 kcgrw = 0.181

Data used from Refs. [5,9,10].a Thickness is in (m); li is in (W/mK); a is in (W/m2K).

The heat requirements considered were the heat losses of the digester and the heatnecessary for raising the incoming sludge temperature. The heat losses of the digesterwere calculated from Ref. [5] considering losses from the sludge to the outside air,soil and groundwater, using the following equation:

Qc � kcout·Aout·(55�tout) � kcgrs·Agr(55�tgrs) � kcgrw·Agr·(55�tgrw). (1)

The calculated heat transfer coefficients are given in Table 1.The heat losses have to be calculated on a monthly basis, since the outside tem-

perature varies throughout the year. For the calculation, the monthly absolute mini-mum outside temperature has to be considered. For our calculations the 40 yearaverage (1960–2000) absolute minimum monthly temperatures for Ljubljana, Slov-enia, were considered [12], and are presented in Table 2.

For the second heat requirement, the heat needed to raise the sludge temperatureto the operating temperature (in our case 55 °C), the following equation was used:

Qs � mo

s·cps·(ts�ts0) � rsl·Vs

�

·cps·(ts�ts0). (2)

Table 2Monthly absolute minimum outside temperature for Ljubljana, Slovenia, [12] and minimum sludge tem-peratures measured in a WWTP of 200 000 PE (in °C), [9]

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Air �20.3 �18.0 �18.2 �3.6 �1.2 2.7 5.8 4.5 �0.6 �5.4 �14.5 �16.7Sludge 12.21 12.86 12.1 13.88 16.53 17.74 20.14 20.94 17.24 17.22 15.82 11.14


3. Results and discussion

To determine the energy and heat potential of the biogas produced, the relationsor the energy transformation rates from the chemical energy of the fuel to heat andpower have to be determined as already stated above. Usually these transformationrates data are presented with a specific type of CHP unit supplied by the engineproducer. For the analysis in this paper, all of the technical information regardingthe CHP units was acquired from Refs. [8] and [7].

The biogas production was experimentally determined and the results are presentedin Fig. 2. Volatile solids removal rates were poor at low retention times; a conven-tional removal rate of over 40% was reached at retention times above 5 days. Suchremoval rates are much faster than in the mesophilic anaerobic digestion process.

In the CHP unit about 35% of the fuel energy is converted to electrical power,heat losses are about 10% and the portion of heat that can be utilised is 55% [7].This 55% of the heat is available at various temperatures, which also depend on theengine producer. The 70/90 °C standard system is most often used. This means thatthe heating water from the CHP unit is released at 90 °C and the utilisation system(for sludge and digester heating) has to be designed in such a way that the heatingwater is returned at 70 °C.

For determination of the heat potential of the biogas, the chemical energy of thefuel (the heating capacity of methane) has to be determined. There are two valuesof the heat capacity of methane [13]; the upper heat capacity (Hs = 391 700 kJ /m3) and the lower heat capacity (Hl = 35 700 kJ /m3) at STP. The difference incapacities occurs when the steam in exhaust gases is cooled below 100 °C and con-denses. Generally, exhaust gases are never cooled below 120 °C because of cor-rosion, and therefore for our calculation the lower heat capacity was used. Consider-ing the value of the heat capacity, the methane produced and energy transformationrates (35% for electricity and 55% for heat), the heat potentials as a function ofWWTP size (PE) and HRT are presented in Table 3.

The method of determining the heat requirements was thoroughly described inMaterials and methods. The heat losses of the digester are dependent on the outsidetemperature and on the surface area, since the construction is standard and thereforelittle altered. The digester surface area is then dependent on the digester volume;

Fig. 2. Specific biogas and volatile solids removal.


Table 3Heat and power production potentials produced from biogas in a CHP unit

HRT (days)

WWTP size (in PE) 1 2 3 4 5 6 7 8 9 10

Heat production potential from biogas in a CHP unit (in kW)10 000 3.30 14.6 27.5 31.6 33.9 35.4 36.8 38.3 40.0 41.820 000 6.59 29.1 55.1 63.2 67.8 70.7 73.6 76.5 80.1 83.630 000 9.89 43.6 82.6 94.8 102 106 110 115 120 12540 000 13.2 58.2 110 126 136 141 147 153 160 16750 000 16.5 72.7 138 158 169 177 184 191 200 209100 000 33.0 145 275 316 339 354 368 383 400 418150 000 49.4 218 413 474 508 531 552 574 601 627200 000 65.9 291 551 632 678 707 736 765 801 836250 000 82.4 364 689 790 847 884 919 956 1000 1050300 000 98.9 436 826 948 1020 1060 1100 1150 1200 1260400 000 132 582 1100 1260 1360 1420 1470 1530 1600 1670500 000 165 727 1380 1580 1690 1770 1840 1910 2000 2090Power production potential (electricity) from biogas in a CHP unit (in kW)10 000 2.10 9.26 17.5 20.1 21.6 22.5 23.4 24.4 25.5 26.620 000 4.19 18.5 35.1 40.2 43.1 45.0 46.8 48.7 51.0 53.230 000 6.29 27.8 52.6 60.3 64.7 67.5 70.2 73.0 76.4 79.840 000 8.39 37.0 70.1 80.4 86.3 90.0 93.6 97.4 102 10650 000 10.5 46.3 87.6 101 108 113 117 122 127 133100 000 21.0 92.6 175 201 216 225 234 243 255 266150 000 31.5 139 263 302 323 338 351 365 382 399200 000 41.9 185 351 402 431 450 468 487 510 532250 000 52.4 231 438 503 539 563 585 609 637 665300 000 62.9 278 526 603 647 675 702 730 764 798400 000 83.9 370 701 804 863 900 936 974 1020 1060500 000 105 463 876 1010 1080 1130 1170 1220 1270 1330

the digester volume is, however, proportional to HRT. Naturally, the heat losses ofthe digester are also linearly dependent on the WWTP size, since the sludge flowis proportionally increased in a larger WWTP. Therefore, the greatest heat lossesoccur when the outside temperature is the lowest and the digester volume is thelargest. The heat losses of the digester are presented in Table 4. The effect of outsidetemperature is presented in Fig. 3 on a monthly basis considering the minimum airtemperatures in Table 2. The heat required for sludge heating is dependent only onthe sludge inflow temperature and sludge mass flow. Sludge mass flow is dependenton WWTP size. The digester volume does not affect the heating requirements of thesludge. The sludge heating requirements are presented in Table 5.

Evaluating and comparing the results for various heat requirements, it is evidentthat the heat losses of the digester are a very small proportion of the total heatrequirements and the majority of the heat requirements are sludge heating require-ments. A comparison of digester heat losses and sludge heating requirements isshown in Fig. 4. The percentage of the digester heat losses (compared to sludge


Table 4Heat losses of the digester as a function of HRT and WWTP size (in kW) for minimum temperature(in January)

HRT (days)

WWTP 1 2 3 4 5 6 7 8 9 10size (inPE)

10 000 0.76 1.21 1.59 1.93 2.23 2.52 2.80 3.06 3.31 3.5520 000 1.21 1.93 2.52 3.06 3.55 4.00 4.44 4.85 5.25 5.6330 000 1.59 2.52 3.31 4.00 4.65 5.25 5.82 6.36 6.88 7.3840 000 1.93 3.06 4.00 4.85 5.63 6.36 7.04 7.70 8.33 8.9450 000 2.23 3.55 4.65 5.63 6.53 7.38 8.17 8.94 9.67 10.4100 000 3.55 5.63 7.38 8.94 10.4 11.7 13.0 14.2 15.3 16.5150 000 4.65 7.38 9.67 11.7 13.6 15.3 17.0 18.6 20.1 21.6200 000 5.63 8.94 11.7 14.2 16.5 18.6 20.6 22.5 24.4 26.1250 000 6.53 10.4 13.6 16.5 19.1 21.6 23.9 26.1 28.3 30.3300 000 7.38 11.7 15.3 18.6 21.6 24.4 27.0 29.5 31.9 34.2400 000 8.94 14.2 18.6 22.5 26.1 29.5 32.7 35.7 38.7 41.5500 000 10.4 16.5 21.6 26.1 30.3 34.2 37.9 41.5 44.9 48.1

Fig. 3. Digester heat losses as a function of monthly minimum temperatures (minimum air temperaturefrom Table 2).

heating requirements) decreases with the size of the WWTP (Fig. 4). In a largerWWTP the heat losses of the digester have even less effect on the total heat require-ments of the sludge digestion process.

From the results it is evident that the digester heat losses affect the heat require-ments very little. At a certain size of digester the heat losses are a small proportionof heat requirements, from only 2–8.5%. In a practical sense this means that for acertain size of WWTP it is reasonable to have a larger digester. With a larger digesterhigher VSS removal rates can be achieved and the specific biogas production is alsohigher (comparing 3 days HRT and 10 days HRT, biogas production is over 40%


Table 5Sludge heating requirements (in kW) as a function of WWTP size and sludge temperature (minimumsludge temperature from Table 2)

WWTP size Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec(PE)

10 000 41.60 40.97 41.71 39.98 37.40 36.23 33.89 33.11 36.71 36.73 38.09 42.6420 000 83.20 81.94 83.42 79.96 74.80 72.45 67.78 66.23 73.42 73.46 76.18 85.2830 000 124.8 122.9 125.1 119.9 112.2 108.7 101.7 99.3 110.1 110.2 114.3 127.940 000 166.4 163.9 166.8 159.9 149.6 144.9 135.6 132.5 146.8 146.9 152.4 170.650 000 208.0 204.9 208.5 199.9 187.0 181.1 169.5 165.6 183.6 183.7 190.5 213.2100 000 416.0 409.7 417.1 399.8 374.0 362.3 338.9 331.1 367.1 367.3 380.9 426.4150 000 624.0 614.5 625.6 599.7 561.0 543.4 508.4 496.7 550.7 551.0 571.4 639.6200 000 832.0 819.4 834.2 799.6 748.0 724.5 677.8 662.3 734.2 734.6 761.8 852.8250 000 1040 1024 1043 999.4 935.0 905.6 847.3 827.9 917.8 918.3 952.3 1066300 000 1248 1229 1251 1199 1122 1087 1017 993.4 1101 1102 1143 1279400 000 1664 1639 1668 1599 1496 1449 1356 1325 1468 1469 1524 1706500 000 2080 2048 2085 1999 1870 1811 1695 1656 1836 1837 1905 2132

Fig. 4. Comparison of heat losses of the digester (HRT 10 days, January temperature) and sludge heatingrequirements (December temperature) for 500 000 PE.

higher). However, the heat requirements are not much higher (comparing 3 daysHRT and 10 days HRT, less than 4.5%). These findings are also applicable to diges-tion processes that are performed in two or more stages. Because sludge heating isthe major heat requirement, adding more stages (or more digesters) to the digestionprocess increases the overall heating requirements relatively little. Only the heatlosses of the additional digesters have to be covered (since the sludge is alreadyheated) and these are only a few percent of the overall heat requirements.

When the overall heat requirements (Table 4 and Table 5 added) are comparedwith the heat potential from a CHP unit (Table 3), it is evident that the heat producedfrom a CHP unit is not sufficient for successful thermophilic sludge digestion. Asolution to this problem can be found in the regeneration of heat. The basic idea isthe following. The sludge outflow temperature is 55 °C. By using a conventional


counter flow heat exchanger some of that heat in the sludge outflow can be trans-ferred to the sludge inflow, which has a minimum temperature of 11.14 °C (Table2). The inflow sludge can then be preheated to a temperature where the heat producedin the CHP unit is sufficient to sustain the process.

The first part of Table 6 shows the amount of heat that is necessary for successfuloperation. The second part of Table 6 shows the temperature to which the sludgehas to be preheated so that the heat from the CHP unit can successfully cover allof the heat requirements. When regeneration of heat is applied, the heat exchangerbetween the outflow and inflow sludge has to preheat the sludge to the temperaturerequired for successful operation. There are many commercially available heatexchangers that can successfully fulfill such a task. We examined the case of theVARITHERM 20 CDS-10 heat exchanger (produced by Ipros-GEA Ecoflex [14]).The heat exchanger was designed for a WWTP of size 50 000 PE. The producer’s

Table 6Amount of heat required for the thermophilic sludge digestion process and the temperature of preheatedsludge necessary for successful operation

WWTP size HRT (days)(in PE)

1 2 3 4 5 6 7 8 9 10

Amount of heat required to sustain the thermophilic digestion process (in kW)10 000 �40.10 �29.30 �16.70 �13.00 �11.00 �9.800 �8.700 �7.400 �5.900 �4.40020 000 �79.90 �58.10 �32.70 �25.20 �21.10 �18.50 �16.20 �13.60 �10.40 �7.30030 000 �119.6 �86.80 �48.60 �37.20 �30.90 �27.10 �23.40 �19.50 �14.70 �9.80040 000 �159.3 �115.4 �64.40 �49.10 �40.70 �35.50 �30.50 �25.20 �18.70 �12.2050 000 �199.0 �144.0 �80.10 �60.90 �50.30 �43.70 �37.50 �30.80 �22s.70 �14.50100 000 �397.0 �286.6 �158.4 �119.5 �97.90 �84.50 �71.60 �58.00 �41.40 �24.70150 000 �594.8 �428.8 �236.1 �177.5 �144.9 �124.5 �105.0 �84.30 �59.10 �33.90200 000 �792.5 �570.8 �313.7 �235.3 �191.6 �164.1 �137.9 �110.2 �76.40 �42.60250 000 �990.2 �712.7 �391.0 �292.8 �238.0 �203.4 �170.5 �135.7 �93.30 �50.90300 000 �1188 �854.6 �468.3 �350.2 �284.3 �242.6 �202.9 �161.0 �110.0 �58.90400 000 �1583 �1138 �622.5 �464.7 �376.4 �320.5 �267.3 �211.0 �142.8 �74.40500 000 �1978 �1421 �776.4 �578.9 �468.2 �397.9 �331.2 �260.6 �175.0 �89.30Necessary temperature of preheated sludge in °C10 000 52.22 41.16 28.24 24.43 22.39 21.17 20.01 18.76 17.19 15.6120 000 52.07 40.91 27.89 24.03 21.92 20.64 19.42 18.11 16.49 14.8630 000 51.99 40.78 27.74 23.83 21.70 20.39 19.14 17.80 16.15 14.5040 000 51.94 40.70 27.63 23.71 21.55 20.22 18.95 17.60 15.94 14.2750 000 51.90 40.65 27.56 23.62 21.45 20.10 18.82 17.46 15.79 14.11100 000 51.81 40.50 27.36 23.38 21.17 19.79 18.48 17.08 15.38 13.67150 000 51.76 40.42 27.27 23.26 21.04 19.64 18.31 16.90 15.18 13.46200 000 51.73 40.38 27.20 23.19 20.95 19.54 18.20 16.78 15.05 13.32250 000 51.71 40.34 27.16 23.14 20.89 19.47 18.13 16.70 14.96 13.23300 000 51.70 40.32 27.13 23.10 20.85 19.42 18.07 16.64 14.90 13.15400 000 51.67 40.28 27.08 23.04 20.78 19.35 17.99 16.54 14.80 13.05500 000 51.66 40.26 27.05 23.00 20.73 19.29 17.93 16.48 14.72 12.97


guarantee was a minimum preheated sludge temperature of 32 °C (if the inflowsludge is 11.14 °C, and the outflow sludge 50–55 °C).

Basically, the heat exchanger takes the heat from the sludge outflow (50–55 °C)and transfers it to the sludge inflow (11.14 °C). Therefore the inflow sludge is heatedto at least 32 °C. With this sludge inflow temperature, in almost all significant casesthe heat requirements are covered. Furthermore, there is excess heat that can beutilised elsewhere in the premises of a WWTP. Only in the cases of HRT of 1 and2 days is such a solution insufficient. This is because of poor biogas production. Thewhole anaerobic process is still in the acidogenic phase (methane-forming bacteria donot have enough time to evolve, and therefore there is little biogas available). How-ever, in these two cases VSS removal is very poor, and therefore these two casesare not even considered for use in single stage thermophilic sludge digestion.

If thermophilic anaerobic sludge digestion is compared to the mesophilic process,it is obvious that the thermophilic one is much faster. In less than 10 days’ HRT,the same levels of VSS removal can be achieved, while in the mesophilic processit takes 30–40 days. From the heating perspective, thermophilic sludge digestion ismuch more demanding, but with proper use of regenerative heat exchangers, theheating demand is brought down to the same level. For mesophilic sludge digestion,sludge has to be heated from about 11 to 35 °C, or a difference of 24 °C. In thermo-philic sludge digestion with the use of heat regeneration, the sludge must be heatedfrom about 32 to 55 °C. This is a difference of 23 °C, which is about the same asin mesophilic sludge digestion, only consideration must be given to the fact that inthermophilic digestion, though this difference is the same, it is at a higher tempera-ture. Therefore, heating must occur at a higher temperature. However, a standardCHP unit operates with a standard heating cycle of 70/90 °C with water as theheating medium [8]. With proper design of the heating equipment, there should beno problem in utilising the CHP heating cycle for thermophilic conditions.

The gain in thermophilic sludge digestion is faster digestion to the same levels ofVSS removal, or even better levels of VSS removal at retention times much shorterthan in mesophilic digestion. The consequence of this is smaller digesters (a thermo-philic digester is only 30% of the size of a mesophilic one for the same effectiveness).That also means smaller costs of construction, since the construction materials arethe same in mesophilic and thermophilic digesters. The only additional cost is thecost of construction of a regenerative heat exchanger. The heat exchanger for aWWTP of 50 000 PE costs only 1950 [14]. This additional cost is usually less thanthe amount saved on account of the smaller digester. Considering the smaller costsof the digester and all the other benefits of thermophilic digestion, the improvementis substantial.

4. Conclusions

A heat analysis of thermophilic anaerobic sludge digestion was performed. Sludgewas anaerobically digested in thermophilic conditions in the Laboratory for WaterChemistry Biology and Technology at the National Institute of Chemistry Ljubljana,


Slovenia. In these experiments biogas production was studied for retention times of1 to 10 days. The resulting biogas production was the basis of the heat requirementsanalysis. In the analysis, the heat and energy potentials from the biogas producedwere first determined using the conversion rates of a CHP unit. Second, the heatrequirements were determined, which basically consist of the heat losses of the diges-ter and inflow sludge heating. Then the heat production and requirements were com-pared. The results of this analysis showed:

� Thermophilic sludge digestion is much faster than mesophilic. This means thatthermophilic digesters would be only up to 30% of the size of mesophilic diges-ters.

� The major part of the heat requirements in thermophilic sludge digestion is inflowsludge heating. The heat losses of the sludge digester make up only 2 to 8% ofthe heat requirements. This means the digester size influences the heat require-ments very little. If there are several stages of thermophilic sludge digestion, everystage brings only 2 to 8% more heat demand, but generally much better sludgedigestion.

� The heat produced from biogas in a CHP unit does not satisfy all of the heatrequirements. However, by introducing heat regeneration the problems are com-pletely resolved.

� Heat regeneration with a conventional counter flow heat exchanger betweensludge outflow (50–55 °C) and sludge inflow (about 11 °C) preheats the inflowsludge to a temperature of 32 °C, which is more than sufficient for successfulthermophilic sludge digestion. Even more, excess heat can be utilised elsewherein the WWTP premises. In such a way, from the heat requirements point of view,thermophilic sludge digestion is equal to mesophilic sludge digestion.

� The costs of the regenerative heat exchanger are usually less than the cost saveddue to the smaller digester.

Considering all the benefits of thermophilic sludge digestion and the solutionsregarding the heat requirements, thermophilic sludge digestion is a very good alterna-tive to conventional mesophilic sludge digestion.

References

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Control Federation: Task Force on Sludge Stabilization, 1986.[3] Owen WF, Parkin GF. Fundamentals of anaerobic digestion of wastewater sludges. Journal of

Environmental Engineering 1986;112:867–920.[4] G.D. Zupancic, Two stage anaerobic-aerobic mineralization-stabilization of excess activated sludge.

Doctoral thesis, School of Environmental Sciences, Nova Gorica Polytechnic: Nova Gorica, 2002.[5] Sprenger E, Recknagel H. Heating and Airconditioning. Vrnjacka Banja: Interklima, 2002 In Serbian:

Grejanje i Klimatizacija.[6] Metcalf and Eddy. In: Tchobanoglous G, editor. Wastewater Engineering: Treatment, Disposal, and

Reuse. Boston, MA: Irwin/McGraw-Hill; 1991.


[7] Jenbacher AG, Jenbacher Energy—interactive CD ROM, 1999.[8] V. Vasic, An analysis of energy flows in the trigeneration plant (in Slovene: Analiza energijskih

tokov v trigeneracijskih postrojih). Masters thesis, Faculty for Mechanical Engineering. 2000, Uni-versity of Maribor: Maribor.

[9] WWTP Domzale-Kamnik, Annual WWTP Working Report for 2001 (in Slovene: Porocilo za delov CCN v letu 2001). Wastewater treatment plant Domzale-Kamnik: Domzale, 2002.

[10] WWTP Domzale-Kamnik, Annual WWTP Working Report for 2000 (in Slovene: Porocilo za delov CCN za leto 2000). Wastewater treatment plant Domzale-Kamnik: Domzale, 2001.

[11] Imhoff KR, Imhoff K. Taschenbuch der Stadtentwaesserung, 28th edn. Muenchen, Wien: R. Olden-bourg, 1993.

[12] HMZS, Average monthly temperatures for Ljubljana and surroundings (in Slovene: Poprecnemesecne temperature za Ljubljano). Hidrometeroloski zavod republike Slovenije: Ljubljana, 2001.

[13] Kraut B. Mechanical Engineering Handbook. Zagreb: Tehnicka knjiga, 1987 In Slovene: Strojni-ski prirocnik.

[14] Tender for sludge heat exchanger VARITHERM 20 CDS-0 (in Slovene: PONUDBA za toplotnimenjalnik za tekoce blato VARITHERM 20 CDS-10), Ipros GEA Ecoflex, 2002.

heat and energy requirements in thermophilic anaerobic sludge digestion

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