The Thermophilic Anaerobic Digestion Process

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<p>I%ter Research rot. 1I. pp. 129 to 1-t3. Pergamon Press t977. Printed in Great B~tain.</p> <p>REVIEW PAPER THE THERMOPHILIC ANAEROBIC DIGESTION PROCESSH. O. BUHR Department of Chemical Engineering. University of Cape Tov,n. Rondebosch. CP 7700, South Africa and J. F. ANDREWS Department of Civil Engineering, University of Houston, Houston, TX 77004. U.S.A.</p> <p>(Received 30 March 1976)Abstract---The paper presents a review of the thermophilic anaerobic digestion process, which has been successfully employed in Los Angeles, U.S.A., and Moscow, U.S.S.R. The chief advantages of operating at higher, thermophilic temperatures are lower detention times, improved dewaterability of the sludge, and increased destruction of pathogenic organisms. It is anticipated that operation of the thermophilic process wilt require closer control than is conventionally practised. A dynamic process model is proposed, and the expected response of a thermophilic digester to changes in feed rate, concentration and temperature is illustrated.</p> <p>NOMENCLATURE</p> <p>PH:o</p> <p>a,b,c,d A,B, AI, A,_t"i</p> <p>(COz)o(CO,)T, Cr D E~. E2 Fo Ft G H</p> <p>HCO~ HS ! kd kr K.</p> <p>stoichiometric coefficients constants molality of ith species, mol kg- t concentration of dissolved CO_,, mol 1-t concentration of dissolved CO2 at equilibrium, tool I-l concentration of total carbonic species, (HCO~ + CO2o), mol 1-* gas volume conversion factor, 1 m o l - t activation energies for synthesis and degradation, respectively, J m o l - t liquid feed rate to digester, 1 d - t effluent withdrawal rate, 1 d - t net growth rate per unit mass of organisms, d - t hydrogen ion concentration, tool 1-t concentration of bicarbonate ion, mol 1- t un-ionized substrate (volatile acids) concentration, mol 1-t ionic strength, X; ci ~ , tool kg-x biological decay coefficient, d - t rate constant for death due to toxicity, d-t dissociation constant for acetic acid, toolI- ~</p> <p>Pr Q Qcs.,, Qco2 R Rs R~ SSrt ' T</p> <p>TxVV6</p> <p>X Ycm:x</p> <p>Yco,.:xY,~:x</p> <p>KH Kt Kt.a K, Kt NHI Nr p (prefix) Pco;w.R. 11/2--~,</p> <p>dissociation constant for ammonia, mol I- l Henry's Law constant for CO2/water, mol 1- t atm- 1 inhibition coefficient, mol Ioverall mass transfer coefficient for COz, d -x saturation coefficient, mol 1-t first dissociation coefficient for carbonic acid system, mol 1-t concentration of ammonium ion, mol 1- t concentration of total nitrogen species, mol 1- t negative log partial pressure of CO, in gas phase, atm</p> <p>Y,v:sZ</p> <p>7</p> <p>7H" t.t t~ 0 ~Subscript 0129</p> <p>vapor pressure of water at operating temperature, atm total pressure in gas phase, atm rate of dry gas flow from digester, 1 d - t production rate of methane and carbon dioxide, respectively, in digester off-gas, ld-t gas constant, J tool-t K - t rate of biolo~cal COs production per unit digester volume, mol 1- * d - t rate of CO, transfer to gas phase per unit digester volume, mol 1- t d - t ionized substrate (volatile acids) concentration, tool 1-x total substrate (volatile acids) concentration, tool 1-' time, d absolute temperature, K concentration of conservative toxic material, mol 1-1 digester liquid volume, I volume of gas space in digester, 1 organism concentration, mol 1-* yield of methane per mole of organisms formed carbon dioxide yield rate of nitrogen consumption per mole of organisms formed yield of organisms per mole of substrate consumed net concentration of (cations-anions) other than C O l , H , HCO~, N H I , O H - and S-, tool l - t dielectric constant activity coefficient activity coefficient for hydrogen ion specific growth rate, d maximum specific growth rate, d-1 temperature, :C ionic valence input conditions</p> <p>130INTRODUCTION</p> <p>H.O. Burro and J. F. ANDREWS sidered for a large treatment plant in Vienna in 1971 Won der Emde &amp; Muller. 1972). However. according to the U.S.S.R. experience, pasteurization would appear to be unnecessaD ' when thermophilic anaerobic digestion is practised. Possible disadvantages of the thermophilic process, on the other hand. may include the following: (a) High energy requirement for heating. (b) Poor supernatant quality. (c) Poor process stability. The use of the process in Moscow, U.S.S.R., as well as simple heat balances indicate that the energy requirements lbr heating are not excessive and can easily be supplied by the produced gas. An exception would be the digestion of sludge with a low solids concentration. There are mixed reports in the literature concerning the quality of the supernatant produced in thermophilic anaerobic digestion (Fischer &amp; Greene, 1945: Garber, 1954). However, it appears likely that the thermophilic supernatant will contain larger quantities of dissolved materials. There are also mixed reports (Fischer &amp; Greene, 1945; Heukelekian &amp; Kaplovsky, 1948; Garber 1954) concerning process stability, especially with respect to perturbations in temperature. However, Garber (1954) experienced no difficulty with process stability thus indicating, at least for large installations, that the process can be operated in a stable manner. Nevertheless, it is expected that poor process stability could be the most significant disadvantage of the process since this is also one of the major problems encountered with the mesophilic process. It is well known that as biological processes approach environmental extremes (pH. salinity, temperature, etc.), fewer species are able to survive and the process becomes less resistant, or more unstable, with respect to change, it would therefore be expected that the thermophilic anaerobic digestion process might not be as stable as mesophilic digestion and. accordingly, adequate process control would be an important requirement for the successful application of thermophilic digestion as a treatment operation. The purpose of this paper is twofold. First, to review the results presented by a number of investigators, in order to identify the operating characteristics and the advantages that may accrue from the application of the thermophilic anaerobic digestion process and, second, to present a dynamic model for the process as a firs't step toward a study of the operational stability and controllability of the process.PREVIOUS W O R K</p> <p>There has been an interest in thermophilic anaerobic digestion since at least i930 when Rudotfs &amp; Heukelekian (1930) conducted bench scale tests on the batch process. Since that time. several other investigators have studied the process in the laboratory and there have been at least four plant scale investigations. Operation of anaerobic digesters in the thermophilic range of temperatures (50-60-C) offers several potential advantages over conventional mesophilic {30--38"C) operation. Included among these are: Ca) increased reaction rates with respect to the destruction of organic solids. (b) increased efficiency with respect to the fraction of organic solids destroyed. (c) improved solids-liquid separation. (d) increased destruction of pathogenic organisms. Increased reaction rates would permit the use of lower detention times, thereby decreasing capital costs, and increased organic solids destruction would decrease the mass of solids for ultimate disposal while simultaneously yielding larger quantities of methane gas for in-plant energy requirements. There are, of course, interactions between these two items, in that the fraction of organic solids destroyed is a function of residence time for both thermophilic and mesophilic digestion. As an example of increased reaction rates-, conversion from mesophilic to thermophilic digestion in Moscow, U.S.S.R. (Popova &amp; Bolotina, 1964) permitted the detention time to be decreased from 18 to 9 days with a reduction in total gas output of only 3 to 4 . Improved solids-liquid separation is of importance if digested sludge is to be dewatered prior to further processing or ultimate disposal. In 1953 Garber (1954) studied the vacuum filtration of thermophilic (50~C) anaerobic digested sludge and reported greatly improved vacuum filter yields for thermophilic as compared to mesophilic (29 ) sludge, together with a lower coagulant demand. Improved solids-liquid separation would also be of value in land application of digested sludge, by decreasing the quantity of liquid sludge for disposal and thereby lowering the cost of transport to the disposal site. The increased destruction of pathogenic organisms at thermophilic temperatures is of special significance in light of the current trend toward land disposal of digested sewage sludge. In 1962, Popova &amp; Bolotina (1964) discussed the use of thermophilic (51C) anaerobic digestion in Moscow, U.S.S.R., and stated, "The most essential advantage of this process is the sanitary quality of the thermophilic sludge. According to the sanitary officials of the health department, viable eggs of helminths are absent from such a sludge." The public health aspects of the disposal of digested sludge on land are of considerable concern throughout the world and in this connection it should be noted that pasteurization of digested sludge prior to land disposal is being used at the Niersverband in West Germany (Kugel, 1972a, b) and was being con-</p> <p>Bench scale studies</p> <p>In 1930, Rudolfs &amp; Heukelekian (1930) studied the batch digestion of primary municipal sludge at thermophilic temperatures. Using thermophilic digestion, the yield of gas per gram of volatile matter added was higher and a greater percentage of the volatile matter was destroyed. The composition of the gas was</p> <p>The thermophilic anaerobic digestion process not materially affected by digestion at the higher temperatures. They concluded that the digestion of primaD" municipal sludge at temperatures of 4%55-C was feasible and that the time required for digestion of seeded solids in this temperature range was materially shorter than for the mesophitic temperature range, provided that the seed sludge had been produced under thermophilic conditions. In further studies, Heukelekian 1t930~ found that gasification was essentially complete in 14 days at a temperature of 50:C. Eleven and twelve days were required, respectively, for temperatures of 55 and 60:C. Temperatures above 60:C resulted in a retardation of gasification. The gas yield/g of volatile matter added, volatile matter reduction and decomposition of nitrogenous substances was greater and the decomposition of fats slightly less for 14 days digestion at 50-C than for 35 days digestion at 22:C. Fair &amp; Moore 11932, 1937) studied the batch digestion of both primary and waste activated sludge, and concluded that the digestion time could be substantially shortened by thermophilic digestion. They reported an optimum temmperature of 50~C for both primary and waste activated sludge. In 1948, Heukelekian &amp; Kaplovsky used batch digesters to study the effect of changes of temperature on thermophilic digestion. Their experimental procedure was to make pulse changes (durations of 2-5 days) from the normal operating temperature of 50~C down to either 40 or 20'C. Similar experiments were also performed on seed sludge developed at 40"C: however, in addition, upward pulse changes from 40 to 5 0 C were made on these digesters. These workers observed that rates of gasification were greatly affected by the drop in temperature for both the 40 and 50~C digesters, with gasification completely stopping at 20:C. However, no lasting effect on the subsequent digestion was noted when the digesters were returned to their original operating temperatures. The results obtained from the experiments in which the 40:(2 digesters were raised to temperatures of 50C were somewhat inconclusive. The digestion continued at this higher temperature; in most cases, however, the time required for digestion was somewhat longer than if no change had been made to the higher temperature. Golueke (1958) studied the effects of temperature on the digestion of primary sludge using bench scale apparatus but with daily feeding and mixing. All of his studies were conducted at a detention time of 30 days and an organic loading of 1,4 kg volatile matter m-3d -t (0.091bft-3d-~). As would be expected at this long detention time and low solids loading, there was no appreciable difference in solids destruction for temperatures ranging from 35 to 5YC. However, very little solids destruction was observed at 65~C. Gas production rates, gas composition, and general sludge appearance were also similar at temperatures ranging from 35 to 60-C. Two significant differences were that the sludge produced at 50 and 60C had substantially</p> <p>131</p> <p>better dewatering characteristics, as measured by the amount of coagulant required, and sludges produced at the higher temperatures had higher volatile acid concentrations. There was an especially sharp increase from approximately 500 mgl-~ of volatile acids at 50:C to 21./00mg 1-~ at 55~C. Golueke also observed that when a population was not well established in a particular digester, it became very sensitive to any abrupt drop in temperature, which occasionally happened because of a temporary failure of equipment. Thus, a drop of 5~C lasting 16--18 h in the 55:C digester resulted in a decline in destruction of volatile matter from 49.9 to 38";, over a 4-day material balance period: similar reductions were experienced at 60 and 55~C. In 1961, Malina (1961) studied the effects of temperature on the digestion of waste activated sludge in bench scale apparatus using daily feeding and continuous gas recirculation for mixing. At a temperature of 52.5-C he was able to obtain 420 volatile matter destruction at a detention time of 6 days and an organic loading of 4.8kg volatile matter m-3d -* (0.3 lb ft- 3d- 1). Corresponding reductions in volatile matter under the same conditions but at temperatures of 42.5 and 32.5:C were 41 and 39~-o, respectively. He also observed higher volatile acids concentrations at the higher temperatures and somewhat less gas production at 52.5C than at 32.5~C, with gas production at 42.5-C being lower than at either of the other two temperatures. Mixed results have been reported for the thermophitic anaerobic digestion of industrial wastes. In tests on a desulphated waste from the production of alcohol from sugar cane molasses, Stander &amp; Elsworth (1950} reported appreciably higher percentages of organic carbon converted to CH~ and CO2 for thermophilic (55~C) than for mesophilic (33C) digestion (57.8 vs 48.30). At high sulphate concentrations (5800 mg 1- t SO.d, however, thermophilic digestion was inhibited to a marked extent, giving an average conversion of 37.40 as against 46.8.:0 for mesophilic digestion. Anaerobic digestion of a yeast waste containing 1800 mg 1- t SO.~ {Stander, 1950) also showed a lower percentage conversion for thermophilic than for mesophilic digestion (138.8 vs 53.7"Q. Basu &amp; Leclerc 11975) studied the anaerobic digestion of a beet sugar molasses distillery waste containing 200 mg 1- ~ of zinc and 29 mg 1- t of copper and found...</p>