Anaerobic Digestion

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<p>Water Research Pergamon Press 1969. Vol. 3, pp. 459-494 Printed in Great Britain.</p> <p>REVIEW PAPER ANAEROBIC DIGESTION II. THE CHARACTERIZATION AND CONTROL OF ANAEROBIC DIGESTIONJ. P. KOTZ~., P. G. THIEL and W. H. J. HATTINGH National Institute for Water Research of the South African Council for Scientific and Industrial Research, P.O. Box 395, Pretoria, Republic of South Africa.(Received 4 February 1969)</p> <p>INTRODUCTION ANAEROmC digestion is a biological process in which organic matter is converted to methane and carbon dioxide in the absence of oxygen (ToERIEN and HATTINGH, 1969). This process is widely used for the purification of wastes containing high concentrations of organic material such as domestic sewage sludge and effluents from various industries. Although well-operating anaerobic digesters will always have a common feature in that organic material is eventually converted to methane and carbon dioxide, digesters can differ vastly one from another with regard to their bacterial populations, the sequence of reactions giving rise to the end-products and the prevailing environmental conditions. The eventual character of any digester is determined by the composition of the substrate, the number of species of microorganisms present in the inoculum and other factors such as temperature, loading rate, hydraulic residence time, sludge retention time, method of mixing of the sludge in the digester and the design of the container of the process. It is necessary to control the process of anaerobic digestion carefully to ensure efficient operation. In contrast to the digestion of domestic sewage sludge, the digestion of many industrial effluents is difficult to control and conditions in which high concentrations of volatile fatty acids accumulate can easily develop, usually leading to a failure of the process. The prevention of such a failure necessitates knowledge of the characteristics of a digester treating a specific waste. By employing certain relevant control parameters the monitoring of these characteristics can serve as indicators of impending failure and motivate steps to prevent failure. An attempt is made in this paper to describe those factors which determine the nature of different anaerobic digesters, the parameters which can serve as indicators of impending failure and the methods which can be employed to avert failure. The characterization and control of anaerobic digesters described in this paper are mainly applicable to the anaerobic treatment of industrial wastes which often prove difficult to control. For a discussion of the factors affecting the disposal of domestic sewage sludge, the reader is referred to the literature survey by POrILAND (1962) and the Water Pollution Control Federation Manual o f Practice on anaerobic digestion (1968). 459</p> <p>460</p> <p>J.P. KOTZ~,P. G. TmELand W. H. J. HATIINGH</p> <p>FACTORS DETERMINING THE NATURE OF ANAEROBIC DIGESTERS The major factors determining the nature of a specific digester are the substrate composition and loading, the temperature of operation, the design of container, the mode of operation and the source of the inoculum of microorganisms required to digest the substrate. In order to evaluate the contribution of the individual factors to the final nature of a digester each of these factors will be discussed in more detail.</p> <p>Composition of substrate General review of compounds occurring in wastes. The composition of the substrateis one of the major factors which determines the characteristics of the ecosystem in an anaerobic digester. The organic and inorganic components in the substrate lead to a selection of those bacteria which are able to metabolize these compounds. The intermediate metabolites formed will lead to a further enrichment of those bacteria which carry the process to the eventual end-products, methane and carbon dioxide. The form in which the compounds are present in the substrate (in solution or in suspension) and the nature of these compounds have a definite effect on the availability of these compounds to the bacteria and consequently the rate at which they are converted to methane and carbon dioxide. A knowledge of the composition of the substrate, to be treated anaerobically, is therefore essential in the interpretation of the behaviour of a digester. Wastes to be treated anaerobically are mainly made up of varying ratios of the following compounds: carbohydrates, polysaccharides, amino acids, proteins, fatty acids, lipids, alcohols and a group of nitrogenous compounds originating from living cells. The carbohydrates occurring in sewage and industrial effluents may consist of a large variety of compounds including, amongst others, hexoses, pentoses, aldoses and ketoses. Industrial effluents can also contain relatively large quantities of polysaccharides such as starch, glycogen, cellulose, hemicellulose, pectins, mannans and xylan. Starch, glycogen and cellulose are all polymers of D-glucose. Starch and cellulose originate from plant and microbial material while glycogen occurs in animal and microbial cells. Hemicellulose, also from plant origin, yields D-glucuronic acid and o-xylose on acid hydrolysis. Pectic acids are components of many plant tissues, and fruit, and appear to be long chains of D-galacturonic acid units. The pectic acids are components of plant materials named pectins which also contain polysaccharides composed of galactose (galactans) or arabinose units (arabans). More detailed information on the chemical structure of these compounds are given by FauToN and SIMMONDS(1958). Mannan and xylan are structural polysaccharides of yeast and plant tissues respectively. Mannan may constitute up to 16 per cent of the dry weight of yeast and is a polymer of o-mannose (FALCONEand NICKERSON, 1956). Xylan is associated with cellulose in wood and wheat and is a polymer of o-xylose (WHISTLER, 1950). TWO classes of compounds originating from wood, which may occur in effluents are the lignins and tannins. Lignin, which usually accompanies cellulose in wood, is believed to consist of p-hydroxy-phenyl-propanes. The phenyl groups often contain methoxy groups (FREUDENBERG, 1954). Tannins occur in the bark of trees and are used in the leather industry. Tannins comprise three groups of substances namely gallotannins and the flavan or stilbene derivatives (NuRSTEN, 1961).</p> <p>Anaerobic Digestion--II. Characterizationand Control of Anaerobic Digestion</p> <p>461</p> <p>Proteins are present in practically all material of biological origin. In abattoir wastes, proteins account for a large proportion of the total organic waste (BoRt~v, 1939). Because proteins are labile and the peptide bonds between the constituent amino acids can be split by the action of extracellular hydrolytic enzymes (proteolyric enzymes), free amino acids and peprides are also present in wastes of organic origin. THmL and DU TOIT (1965) found that the concentration of free amino acids in brewery effluent was of the order of 25-95 mg/l expressed as leucine. Lipids are present in most material of biological origin and in some cases may constitute a major fraction of the organic material in a waste. Domestic sewage can contain as much as 25-2 per cent lipids on a dry weight basis (PoI4LAND, 1962). Lipids are separated into several groups depending on their chemical and physical properties. Simple lipids are esters containing only carbon, hydrogen and oxygen and yield, on hydrolysis, only fatty acids and an alcohol. In the case of the so-called neutral fats and oils, three fatty acid units are joined by ester linkages to the trihydric alcohol, glycerol (FRuTON and SIMMONDS, 1958). The more complex lipids include substances such as the phospholipids (FRUTON and SIMMONDS,1958). The fatty acids present in lipids can include both straight chain and branched chain fatty acids and saturated and unsaturated fatty acids containing from 2-26 carbon atoms, depending on the origin of the particular lipid. Free fatty acids can also occur in waste waters as the end-products of bacterial metabolism or can be formed by enzymatic hydrolysis of lipids. TmEL and r~u TO1T (1965) reported that the concentration of volatile fatty acids in brewery waste could vary from 50 to 350 rag/1 expressed as acetic acid. Another group of compounds which can form an important part of the organic material in a waste is that originating from microbial cells. The importance of this group is borne out by the fact that bacteria make up approximately 25 per cent of the dry weight of faeces (RosEBURY, 1962), and the large amounts of yeast cells which, for instance, may occur in brewery wastes (DmTRICn, 1960). These cells contain, apart from some of the compounds discussed above, the nucleic acids DNA and RNA, which occur in all living cells, poly-fl-hydroxybutyric acid which occurs in microorganisms as a storage product, and the components of bacterial cell walls. The nucleic acids DNA and RNA are polynucleotides in which one of the two acidic groups of the phosphoric acid residue is esterified by one of the sugar hydroxyls of another mononucleoride. The carbohydrate fraction of DNA consists only of deoxyribose and that of RNA only of ribose. Escherichia coli may contain 5-2 and 19.1 per cent DNA and RNA respectively on a dry weight basis, while Pseudomonas aeruginosa on the same basis may contain 4.9 and 4.4 per cent respectively (ZAHN, 1964). Therefore waste waters containing large amounts of microbial cells will also contain considerable quantities of DNA and RNA. Poly-fl-hydroxybutyric acid can account for 50 per cent of the dry weight of microorganisms under growth conditions where nitrogen is deficient (STANIER t al., 1964). e Bacterial cell wails contain rnucopeptides, teichoic acids, polysaccharides, proteins and lipids (STANIER t al., 1964). The mucopeptides are polymers of compounds such as e glucosamine, D- and L-alanine, D-glutamic acid and either L-lysine, DO-, EL-, or meso~-~-diaminopimelic acid, 2,4-diaminobutyric acid or either D- or L-ornithine (RoGERS, 1965). The teichoic acids are made up of ribitol phosphate, glucose and alanine. The polysaccharides of bacterial cell walls contain amino sugars and/or simple mono-</p> <p>462</p> <p>J . P . KOTZ~, P. G. THIEL and W. H. J. HATHNGH</p> <p>saccharides, while little is known about the composition of the lipids</p> <p>(STANIER t al., e</p> <p>1964).The aforementioned goups of organic substrates will invariably be present in effluents or, in special cases, comprise a significant part of the organic loading. There are, of course, also many other organic substances which may occur in certain effluents. Substances such as gibberellic acids, antibiotics, plant hormones, pesticides and detergents may occur in effluents and have an effect on the anaerobic process but since their concentrations are usually small, they are not discussed in this review. The inorganic composition of a waste will also have a pronounced effect on the eventual nature of a digester. Some inorganic ions between specific concentration levels, are necessary for cell metabolism, while others may prove toxic above certain concentrations. This review is mainly concerned with the organic substrates of waste waters and a detailed account of the effect of inorganic ions on anaerobic digestion is not given. As already indicated, a wide variety of organic substances may occur in waste waters. Heterogeneous microbial populations, such as that of anaerobic digestion, will effect sequential substrate removal when complex substrates are degraded because certain substrates are more easily degraded than others. Sequential substrate removal in aerobic processes has already been studied by GAUDYet al. (1963). In complex substrates certain components might be completely inavailable to the microorganisms resulting in incomplete purification. The composition of a waste fed to a heterogeneous culture determine the eventual combination of species of the system as well as the relative enzyme activities of such a system. The above-mentioned effects of complex substrates on the process of anaerobic digestion indicate the necessity of a thorough knowledge of the composition of the substrate to be used. The available information on the composition of domestic sewage sludge is summarized in the next section as an example of the complexity of such a waste. Detailed information on the composition of different industrial wastes is not given in this article. Composition of domestic sewage sludge. The solids concentration of sewage sludge varies from 8 to 12 per cent (IMHOFFand FAIR, 1956). The volatile matter of the undigested solids may vary from 60 to 80 per cent (RUDOLFSand GEHM, 1942; IMHOFr, 1956), and the inorganic material (ash) of undigested solids from 20 to 40 per cent (POHLAND, 1962). Sewage sludge contains rather large quantities of ether-soluble material. RUOOLFS (1944) analysed sludges from various treatment plants and found that the etherextractable material or grease varied from 5.7 to 44.0 per cent of the dry solids with an average of 13-1 per cent. Various other workers confirmed this range (POHLA~,U~,1962: BUSWELL and NEAVE, 1930; HEUKELEKIAN,1958; RUDOLFSand GEHM, 1942). This ether-soluble fraction of the dry solids was found to be largely destroyed by anaerobic digestion as the percentage grease in digested solids was reduced on average from 25.2 per cent to 6-9 per cent (BuSWELL and NEAVE, 1930). The ether-extractable material consists of many components including free fatty acids, lipids and esters. HEUKELEKIANand Mt~rLr.R (1958) found that the grease from sewage solids conmined 40-60 per cent free fatty acids, 20-40 per cent esterified fatty acids and 15-20 per cent unsaponifiable material and that the fatty acids included palmitic, stearic, and dienoic acids. HUNTER (1962) reported that the settleable solids in sewage con-</p> <p>Anaerobic Digestion--II. Characterizationand Control of Anaerobic Digestion</p> <p>463</p> <p>tained 1.35 per cent free fatty acids, 9.66 per cent glyceride fatty acids, 3.75 per cent unsaponifiable material and very low concentrations of phenols and detergents. About 80-90 per cent of the fatty acids were saturated acids and the rest comprised the following unsaturated acids in order of decreasing concentrations: oleic, linoleic, linolenic and arachidonic acids. Dry fresh sewage solids contain 2.49 per cent alcohol-soluble material and 9.52 per cent material which is soluble in hot and cold water (POHLAND,1962). POHLAND(1962) mentioned that sugars, amino acids, organic acids, starches, tannins and pectic substances were extracted by hot and cold water while the alcohol-soluble fraction contained waxes, resins, alkaloids and choline. HUNTER (1962) analysed the settleable solids in sewage and found that it contained 7.71 per cent alcohol-soluble-etherinsoluble material, which consisted of 16.9 per cent amino acids, 4.4 per cent sugars and 8.3 per cent tannins. Another major group of components of sewage sl...</p>