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Water Research Pergamon Press 1969. Vol. 3, pp. 459-494 Printed in Great Britain.
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)
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
J.P. KOTZ~,P. G. TmELand W. H. J. HATIINGH
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.
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).
Anaerobic Digestion--II. Characterizationand Control of Anaerobic Digestion
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-