anaerobic digestion—iv. the application of the process in waste purification

Water Research Pergamon Press 1969. Vol. 3, pp. 623--643. Printed in Great Britain

REVIEW PAPER

ANAEROBIC DIGESTION--IV. THE APPLICATION OF THE PROCESS IN WASTE PURIFICATION

G. G. CILLIE, M. R. HENZEN, G. J. STANDER and R. D. BAILLIE

National Institute for Water Research, South African Council for Scientific and Industrial Research, P.O. Box 395, Pretoria, S. Africa

(Received 28 April 1969)

Abstract--Anaerobic digestion is used extensively for stabilisation of sewage sludge and more recently, to a limited extent, for industrial effluents. Present investigations have demonstrated the practical feasibility of the process for specific wastes with high pollution load. Up to 97 per cent purification was obtained.

The cost of anaerobic digestion of strong wastes is considerably less than that of equivalent aerobic processes. The latter are again more economic at C O D concentrations below 4000 mg/1.

I N T R O D U C T I O N

ANAEROBIC digestion, as a unit process in sewage purification, has been practised since the beginning of the twentieth century. IMHOFF (1906) was one of the first to use a plant which separated solids f rom mixed sewage. The settled sludge was then digested in the lower compartment of the well-known Imhoff tank. The process was previously known to occur in marshes and stagnant pools, and hence the gas produced by anaerobic fermentation is commonly known as marsh-gas. Since those early days, anaerobic digestion has gained importance primarily as a means for treating the solid organic matter contained in sewage, but also for handling noxious liquors economically.

The process of anaerobic digestion has been studied extensively over the years (Refer list of literature cited). The stabilization of organic matter in sewage is mainly brought about by bacterial metabolism which converts the complex organic mole- cules into stable end-products. A portion of the organic matter is synthetized into new cell protoplasm while a further portion is oxidized as source of energy. Aerobic processes are similar to anaerobic ones in this regard, but the former can generally be relied upon to produce a higher rate of activity. This is due to the greater difference in energy levels between the raw compounds and the mineralized end-products.

Anaerobic organisms decompose organic carbon into both its fully oxidized and reduced forms (CO2 and CH4 respectively), but the energy level of the latter is relatively high, with the result that very little heat is generated while the rate of reaction is relatively slow. In order to maintain satisfactory rates of digestion i.e. economical practical application, it is necessary to provide opt imum environmental conditions.

In the past, anaerobic digestion has found its main application in the stabilization of sewage sludge, but in recent years has been applied successfully to treatment of industrial wastes carrying high concentrations of dissolved organic matter. This paper

623

624 G.G. CILLIE et aL

considers the application of anaerobic digestion under particular circumstances with specific reference to the economics thereof.

OCCURRENCE OF ANAEROBIC DIGESTION

Anaerobic digestion is characterized by the production of methane (marsh-gas). The presence of methane in nature, therefore, can generally be related to the occurrence of anaerobic digestion.

Methane is the first and the simplest member of the paraffin hydrocarbons. It occurs abundantly in nature as the chief component of natural gas, as a component of firedamp of coal mines, and as a product of the anaerobic bacterial decomposition of vegetable matter under water. Methane formation in nature has interested bacterio- logists and chemists for more than sixty years. Towards the turn of the last century it was known that when moist organic matter was allowed to decompose under re- stricted oxygen conditions, it yielded hydrogen, carbon dioxide, methane and a variety of organic acids in greater or lesser amounts. Little or no quantitative data on the yield of the various end-products were available, and few, if any, studies had been carried out on the anaerobic fermentation of pure compounds. SOHNGEN (1906) showed that the lower volatile fatty acids with an even number of carbon atoms, could be decom- posed by mixed bacterial cultures, with the production of methane and carbon dioxide. Whether hydrogen and fatty acids were necessary intermediates in the process of methane formation was not known. GROSSE and LmaY (1948) showed that the hydro- carbon fraction of the gas from a sewage sludge digester consisted almost entirely of methane with only traces of higher hydrocarbons within the limits detectable by a mass spectrograph. In this regard the former gas was similar to petroleum methane as indicated in TABLE 1.

TABLE 1. COMPOSITION OF PETROLEUM METHANE AND SEWAGE GAS

AFTER PURIFICATION*

Petroleum methane Sewage methane Constituent (Vol. ~) (Vol. ~ )

Methane 98"0 99'2 Ethane 0"6 0.002 Propylene 0" 1 0.002 Nitrogen 0.7 0.2

* From Grosse and Libby, 1948.

Methane fermentation occurs commonly in the bottom sediments of rivers as well as in lakes and water masses where quiescent conditions prevail in the deeper parts. In these cases organic matter settles to the bottom where it is depleted of oxygen and hence undergoes anaerobic decomposition. Anaerobic fermentation thus forms an essential link in the self-purification cycle of nature. It is therefore not surprising that this process had been harnessed by man for sophisticated purification of organic waste.

Anaerobic Digestion--IV 625

SEWAGE SLUDGE DIGESTION

Although extended aeration of whole sewage as a means of decomposing organic solids has found limited application, the use of separate sludge digestion is by far the most prevalent practice. It is also the most economical method of stabilizing a major portion of the organic load carried by sewage (TABLE 2). The solids content of sewage constitutes between 1 and 2 per cent of the total volume (NIWR, 1961) but accounts for 40-80 per cent of the organic load. If, therefore, the primary sedimentation and digestion stages cost less than the latter percentages of the total treatment cost, considerable advantage may be gained.

Requirements of the process

The use of the process in conventional sewage treatment has been discussed extensively in literature by the TECHNICAL PRACTICE COMMITTEE (1966), IMHOFF and FAIR 0956) and BOLITHO (1964). The basic requirements for successful digestion of sewage sludge are as follows:

(a) Maintenance of an acclimatized active seed sludge and intimate mixing thereof with substrate. The sludge density in a digester must not be so high as to impede mixing and not so low as to deplete the inoculum. Successful digestion generally requires a sludge concentration of between 20 and 100 g/l suspended solids. Higher sludge densities in this range will generally support a denser bacterial population and hence greater efficiencies are obtainable.

(b) Mechanical mixing of a digester will generally be desirable. Gas evolution will cause limited convection, and hence increased depth/area ratio will be advantageous.

(c) Continuous or frequent incremental feeding of raw sludge (substrate) must be maintained at a rate which will not exceed the metabolic capacity of the digesting organisms. Metabolic capacity can be measured by means of various control parameters (KOTz~ et aL, 1969).

(d) Temperature must be maintained at or near 35°C throughout the digester, so as to ensure maximum rate for mesophilic digestion.

(e) An adequate supply of nutrients (SPEECE and MCCARTY, 1962) must be available. In this regard a correct C-N-P balance is especially important.

(f) Sufficient reserve alkalinity (> 2000 mg/1 as CaCO3) must be provided to act as buffer for pH control at approximately 7.

Advantages of the process

The main advantages of sludge digestion are summarized as follows:

(a) Proper digestion of sewage sludge reduces both the weight and volume of solids to be disposed of by 30 per cent or more.

(b) Digested sludge is an inoffensive material, and has largely lost the hydrophilic properties of raw sludge. The product can thus be readily dried on drying beds or by more sophisticated means.

(c) Digestion partly destroys pathogenic and parasitic organisms. (d) A valuable by-product, viz. methane gas, is produced. The digested sludge may

also be used as fertilizer.

626 G. G, CILLIE et aL

Cost o f the process

BOLITHO (1964) presented figures on the cost of anaerobic treatment of sewage sludge (TAat,E 2). The cost of anaerobic treatment is less than 50 per cent of aerobic treatment while it handles at least the equivalent organic load. The total cost of sludge treatment could be reduced considerably if cheaper means for sludge disposal were available. It also does not take into account the heat value of the gases produced by the process. In some cases the latter gas may be synthesized to further use as industrial chemicals, which may offset a large portion of the cost of the process (TOWNEND, 1962).

Anaerobic digestion of sewage sludge is the most practical method of dealing with a major constituent of sewage. The process has, in fact, been studied for the treatment o f whole sewage (VAN ECK, 1967). The effluent from anaerobic digestion is generally

TABLE 2. COMPARATIVE COST OF AEROBIC AND ANAEROBIC TREATMENT FACILITIES AT A TYPICAL SEWAGE

WORKS*

Total Capital cost Total annual Treatment capital cost per capita cost per capita

process (Rands) (Rands) (cents)

Aerobic. (Biological filters, humus tanks, sand filters and maturation ponds)

Anaerobic. (Sludge digesters, drying beds and land disposal)

1,550,000 3"45 28'0

740,000 1-65 13.0

Note: Rands and cents refer to South African currency 1 Rand = 1'4 $ U,S.A. = 0-6 £ Sterling.

*From Bolitho, 1964

of poorer quality than that from conventional aerobic purification. The application of the process to the treatment of industrial wastes carrying dissolved organic matter is not as clear cut and requires special consideration.

INDUSTRIAL WASTE TREATMENT

Literature review

Industrial effluents containing high concentrations of organic matter can be dealt with conveniently by means of anaerobic digestion. BUSWELt. (1956) stated that, "'The process finds its greatest economy in wastes running from 1 to 3 per cent digest- ible solids. Less than 1 per cent solids is likely to result in an undesirably large installation and in most cases over 3 per cent solids will usually justify evaporation with the recovery of dry material for either feed or fertilizer".

A survey of recent literature indicates that purification of various industrial wastes by means of anaerobic digestion has been studied, recommended and/or applied by numerous workers. A brief review of recent reports on anaerobic digestion of some representative wastes will serve to illustrate the present scope of the process.


Distillery wastes. SEN and BrIASKARAN (1962) studied anaerobic digestion of molasses distillation wastes and reported a 90 per cent BOD reduction at 37°C when operating at 10 days detention and a BOD loading of 0.2 lb/ft 3/day. Enrichment of the recirculation gas with CO2 resulted in an increased gas production and further BOD reduction.

PAINTER (1960), in pilot-scale studies of distillery waste, found an acclimatization period of several weeks necessary. Thereafter, a BOD loading of 0.25 lb/fta/day at a detention time of 6 days led to a 95 per cent reduction in BOD from a waste containing an initial BOD concentration of 25,000 mg/1. The total N content of the raw waste was approximately 1300 ms/1.

Slaughter-house and meat packing wastes. HEMENS and SHURBEN (1959) reported a 12 month pilot-scale study of digestion of slaughter-house waste waters. At 33°C and a loading rate of 0-11 lb BOD/fta/day, a 95 per cent removal of an original 2000 mg/1 BOD was achieved. The total daily gas production proved insufficient to maintain the operating temperature and had to be supplemented. Effluent from the sludge sedimentation tank was found to be relatively resistant to further aerobic purification. Only approximately 30 per cent further BOD reduction was achieved in a trickling filter at conventional loadings.

SILVESTER (1962) and HENZEN (1966) have described the operation of a full-scale plant erected on the premises of Lloyd Maunder Ltd., Devon. This plant, completed in April, 1960, was based upon the pilot-scale data obtained by HEMENS and SnURBEN (1959). Initially, difficulties were experienced with scum build-up in the digester. A layer 42 in. thick had formed alter only 3 months operation. Successful remedial measures taken were:

(a) separation of paunch contents from the effluent (b) the replacement of the comminutor with the automatic screen and (c) the use of a vibrating screen just ahead of the balancing tank.

BOD removal increased to approximately 90 per cent, gas production however, remained low, only 3 ft3/lb BOD. This was insufficient to maintain the desired operating temperature of 90°F and supplementary heating had to be applied. Degasification of the digester Outflow proved essential for efficient sludge separation and return.

FULLEN (1953), SCHROEPFER et al. (1955), AIKINS (1958) and STEFFEN and BEDKER (1961) have all described various facets of anaerobic digestion of meat processing wastes from the Albert Lea Plant at Minnesota, U.S.A. A full-scale plant, erected on data obtained from laboratory and pilot-scale studies, gave a BOD reduction of about 90 per cent, at a detention period of approximately 30 hr. Maximum organic loading attained was 0.156 lb BOD/ft3/day when vacuum degasifying of the displaced mixed liquor was practised. Without degasification the loading had to be reduced to 0.04 lb BOD/ft3/day for successful solids separation. Effluent from this plant proved amenable to aerobic purification giving an overall BOD removal in excess of 90 per cent.

SCHAFFER (1963) examined the use of polyelectrolytes for improving separation in the anaerobic contact type treatment of meat packing plant wastes. BOD and solids removal were improved, but the resultant rapid build up of sludge in the plant caused residual gassing and buoyancy of sludge. Practical application was hampered by the cost of polyelectrolytes.

628 G.G. C-~LLIE et al.

BRAON~Y et aL (1950) described the full-scale treatment of meat packing wastes in two 85 ft dia. digesters followed by trickling filters. During initial operation a heavy scum blanket was formed, which lifted the lids due to sealing of the gas vents and over- flows. Mixers were subsequently installed which eliminated the problem. They found that the digestion of grease was nearly complete. Loading varied from 0.07 to 0.1 lb volatile solids/ft3/day, with an average gas production of 8.0 ft 3 per lb of volatile solids applied.

COWIE (1960) described the operation of an anaerobic digester treating meat wastes in New Zealand. Despite its experimental nature, purification afforded was + 90 per cent (BOD) at a detention time of 1.4 days and temperature of 64°F. Degasi- fication of the mixed liquor was obtained by aeration with diffused air at a rate of 0.1-0-4 fta/gal of effluent. Cowm (1960) also described a treatment plant for meat wastes in South Island, comprising three 80 ft dia. × 30 ft deep, stirred digesters.

RANOS and COOPER (1966) presented data on an anaerobic treatment system for meat wastes at Auckland, New Zealand. The BOD of the raw effluent was reduced from 2000 mg/1 to 30 mg/1 in the final effluent.

The use of anaerobic ponds for treatment of meat wastes has been described by SOLLO (1960), COERVER (1964), PORGES (1963) and STEFFEN (1963). Such pond treatment apparently provides an economic method of purification. It is especially suitable for slaughter-house wastes since the fibrous material contained in paunch contents forms a heavy scum on the surface of the ponds. This scum effectively seals the pond contents from the atmosphere. The process is, however, not entirely free of noxious smells and can generally only be used in places where extensive land is available in uninhabited areas.

Sugar cane waste. GUZMAN (1962) discussed the use of accelerated anaerobic digestion and lagooning to treat sugar cane wastes in Puerto Rico. Anaerobic digestion of cane sugar wastes followed by oxidation pond treatment was found to yield a BOD reduction in excess of 90 per cent in pilot-scale studies (BHASKARAN and CHAKRA- BARTY, 1966). A 60 per cent BOD reduction was achieved at 7 days' detention in an open, unheated digester, whilst closed, heated digesters yielded 70 per cent removal in only 2 days.

Woollen and Textile wastes. BUSWELL et al. (1962) discussed the segregation of strong desizing wastes from a textile industry and its treatment by anaerobic digestion. Removal of 80-90 per cent of the COD content was obtained at a loading of 0.05 lb COD/fta/day.

GRISHINA (1964) treated wool scouring waste waters by anaerobic fermentation at a loading of 0.66 kg BOD/mS]day, (0.04 lb BOD/ft3/day) and a detention period of 23 days, a 93 per cent reduction of BOD was achieved.

TANAKA et al. (1964) ran a successful industrial trial of the treatment of wool scouring waste by thermophilic anaerobic fermentation. Effluent from the process proved amenable to further aerobic purification.

NEVZOROV (1964) and GRISHINA (1964) also described experiments with anaerobic fermentation of wool scouring wastes, and reported efficient purification (up to 99 per cent BOD reduction). Both the mesophilic and thermophilic ranges were investigated and proved successful although the former range was to be preferred due to easier handling.


Tannery wastes. IVANOV (1963) applied anaerobic digestion for pretreatment of tannery wastes followed by aerobic biological purification. By seeding the raw tannery effluent with sewage sludge and reducing the pH with KH2PO 4 some 50 per cent of the BOD was removed in 2 hr.

Pilot-scale studies by GATES and LIN (1966) on the feasibility of treating tannery waste liquors by anaerobic fermentation were successful by both the contact type anaerobic digestion and anaerobic lagooning. The contact process was preferable but its practical application required further study.

Potato processing wastes. HIND1N and DUNSTAN (1963) showed that mixtures containing up to 50 per cent settled solids from potato chip processing waste in raw sludge could be treated satisfactorily by anaerobic digestion. Greater concentrations of potato wastes resulted in difficulties, mainly due to a nutrient deficiency. A gradual period of acclimatization to the waste was found necessary when the digester used had not previously been digesting carbohydrates.

OLSON et al. (1966) found that 90 per cent BOD reduction was achieved at Grafton, North Dakota, U.S.A. with anaerobic digestion of potato wastes followed by aerobic treatment.

Practical applications. On the whole, it appears that full-scale anaerobic plants have been installed and operated for a limited variety of industrial effluents only, for example: yeast wastes, fermentation wastes and meat packing wastes.

The first practical application of anaerobic digestion for purification of industrial effluents was described by JENSEN (1932). The plant was constructed in 1928 for the treatment of yeast wastes from a molasses fermentation plant at Slagelse in Denmark. It was subsequently enlarged. A similar plant was also commissioned in Sweden. The reduction of organic content amounted to some 80 per cent at a loading of 0.02 lb BOD/ftS/day with 20 days' detention and a temperature of 37°C. The residual organic load was taken care of by lagoon treatment followed by aerobic filtration. The distinctive brown colour of molasses slops, however, persisted throughout the entire purification process.

A full-scale plant for anaerobic digestion of mixed wastes from alcohol distillation and yeast manufacture at Darmstadt, Germany, has been described by HAUV, Y (1958). The fermentation wastes were mixed with an equivalent volume of domestic sewage sludge. The purification obtained has been of the order of 80 per cent. The effluent retained a residual BOD of some 2000 mg/1.

Application of the process at a number of distillery plants in Japan has been described by ONO (1964). Some of these used thermophilic digestion at a loading of 0.4 lb/ft 3/day while the remainder used mesophilic digestion at a load of 0.15 lb/ft3/day. It was claimed that 96 per cent purification was obtained and that the effluent was amenable to further aerobic treatment. The importance of retaining a satisfactory sludge concentration in the digester was stressed.

The most successful anaerobic plants for industrial waste liquors seem to be those dealing with slaughter-hourse and meat packing wastes. In all cases, however, the process was dependent upon maintenance of a properly acclimatized sludge. For instance, in the Albert Lea plant referred to earlier, vacuum degasification was used to obtain satisfactory sedimentation of sludge from the mixed digester liquor. In England the process has been used with some difficulty by SmVESTER (1962) using de-aeration equipment to secure satisfactory sludge settlement.

630 G. G. Cu~m et al.

The collective reports indicate that anaerobic digestion of industrial waste liquors on their own, has found limited practical application. Throughout the world, some 20--30 plants only (excluding anaerobic ponds) have been operated successfully for this purpose. In every instance it was necessary to ensure proper acclimatization of the sludge and satisfactory retention thereof. Major emphasis is accordingly placed upon methods for efficient sedimentation and return of sludge.

South African studies

In South Africa, anaerobic digestion of industrial effluents has been studied extensively over the last decade. In this regard, the discovery by STANDER and SNYDERS (1950) that re-inoculation by means of a well acclimatized sewage sludge would maintain optimum digestion of industrial effluents, constituted a major break-through. The principle has subsequently been used with success for plant-scale purification of effluents arising from glucose-starch manufacture, wine distillation and compressed yeast manufacture. Plant-scale purification of effluents from abattoirs, beer breweries and paper pulp mills by means of this process is also being investigated by the National Institute for Water Research of the South African Council for Scientific and Industrial Research.

It was indicated earlier that the anaerobic stabilization of organic matter requires an acclimatized active seed sludge at optimum concentration. Whereas in conventional sewage sludge digestion the organic solids and aqueous phase of the feed contain the necessary nutrients and organisms for maintaining cell synthesis and metabolic activity, industrial effluents are often poorly balanced in these respects. For successful plant-scale digestion of the latter therefore, it is necessary to recover the active cell material displaced from the digester and to re-introduce it either by inoculation of the raw feed or directly into the digester i.e. the principle of re-inoculation must be adhered to. The anaerobic digestion of neat industrial wastes thus necessitate certain modifications to anaerobic digester design.

It was found that a standard "Dorr-Oliver Clarigester" could be modified to meet the requirements for the treatment of specific industrial effluents. The modification implied that the conventional flow pattern of the clarigester was reversed. Raw feed was thus fed into the lower digester compartment, where it mixed with sludge carrying a dense culture of acclimatized organisms which effected stabilization of organic matter contained in the feed. The incoming feed displaced an equal volume of mixed sludge liquor into the clarifier compartment, situated immediately above the digester, where the sludge was intended to settle. The settled matter was then returned to the digester by means of a scraper system, through the aperture between the two compartments and the remaining clarified supernatant liquid left the clarifier as stabilized effluent.

FIGURE 1 illustrates an experimental plant which incorporated a number of experimental sludge lines, piping and sampling valves which were not all required for practical operation.

Similar plants used for practical treatment of effluents arising from starch manufacture and wine distillation have now been in operation for 10 and 7 years respectively. The average purification obtained with these plants, as well as with an experimental plant used for compressed yeast effluent, is recorded in TABLE 3.


The reduction of organic load of the effluents varied between 70 per cent and 97 per cent, depending upon the type of effluent treated (TABLE 3). The residual organic content was intractable to anaerobic organisms and the relatively low purification efficiency obtained for yeast wastes did not indicate a decline or intermittent re- gression of the anaerobic process.

Annuk~r Pot t i t ion Resting on Scraper Arms (not s h ~ n )

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The volume of gas produced from "strong" wastes e.g. wine distillery effluent at 22,000 mg/1 COD, constituted up to 14 times the volume of feed. Gas generation was equivalent to between 80 per cent and 90 per cent conversion of the consumed organic carbon (influent COD-effluent COD). It is thus probable that at least 10 per cent of the consumed carbon was synthesized to bacterial protoplasm in the form of sludge.

The synthesis of sludge was continuously balanced by the loss of suspended solids in the clarified effluent and in practice it was seldom necessary to withdraw sludge from the digester compartment. If, however, such withdrawal did become necessary, the sludge could be disposed of by conventional drying.

A detailed description of the purification of glucose-starch effluent has already been published by HEMENS et al. (1962). A brief description of the anaerobic digestion of wine distillery effluent will serve to illustrate the application of the process in general.

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Anaerobic Digestion-IV 633

Digestion of wine distillery efluent. The plant used was a modified Dorr-Oliver Clarigester of 40 ft dia., made of steel and sited above ground level in order to facilitate any modifications resulting from operational experience. Special precautions were taken to ensure even distribution of feed by providing 24 adjustable inlets around the lower perimeter of the digestion compartment. Heating equipment burning digester gas was provided to heat the raw feed, thereby maintaining a temperature of 33°C (FIG. 2).

Since sludge retention in the digester compartment was of primary importance, modifications were progressively effected in an attempt to improve the sedimentation and return of sludge to the digester. The most important alterations were:

(a) Replacement of the standard clarigester “boot”, which acts as a gas seal between the digester and clarifier compartments, with a straight shallow cone (FIG. 1). The latter had a 4 in. clearance at the aperture and was fitted with scrapers to facilitate sludge return.

(b) Provision of a conical partition which fitted below the standard stilling box in the clarifier compartment (FIG. 1). This concentric compartment was intended to segregate actively digesting material, displaced from the digester below, from the quiescent settling zone. It was found that gas evolution within this compartment largely stripped occluded gas from the sludge with concomitant coagulation and improved settleability.

(c) Provision of facilities for external recirculation of sludge. The gravitational return of sludge to the digester compartment was seriously hampered by the upward displacement of digester contents through the same aperture during the feed cycle. By external transfer of digester contents into the central compartment of the clarifier, a nett downward flow through the aperture was effected but this in turn introduced partly digested material into the clarifier. It was found that intermittent transfer of sludge via an intake manifold in the “middle third” of the digester compartment minimised such shortcircuiting of raw feed. Recirculation of sludge in this manner, at a rate of 50 gal/min, for 10 min every 3 hr, was effective for return of thickened sludge to the digester.

The plant finally composed a versatile unit suitably adapted to study a multiplicity of factors influencing the process either individually or in random combination. The major difficulty experienced in carrying out the programme of investigation was to maintain constant conditions while studying the effect of varying any specific factor. This problem was the major disadvantage of full-scale experimentation, but was amply compensated for by the practical applicability of the results obtained.

Plant performance. At an operating temperature of 33°C the optimum feed of “strong” distillery effluent was of the order of 21,000 Imperial gallons per day (gpd) which was equivalent to a detention period (hydraulic residence time) of 6.9 days or an organic load of 0.2 lb COD/ft3/day (3.2 kg COD/mj/day). With a diluted feed (3 strength), up to 28,000 gpd could be digested satisfactorily indicating that organic load was a Iimiting factor.

It was observed repeatedly that gas production declined immediately after feed interruption and it virtually ceased after 224 hr. It was thus concluded that the biological process could be completed within a matter of hours and that the longer hydraulic residence time of up to 7 days was necessitated by hydraulic factors and practical considerations. In this regard it should be pointed out that the loss of sludge

634 G. G. CILL~J et al.

in the clarified effluent at high loading rates was the primary limitation for increased feed rates. This phenomenon, however, provided a convenient and easily observed parameter for incipient overloading of the digester.

The concentration of volatile fatty acids in the mixed sludge liquor is an accepted parameter for gauging digester performance. During the course of the present full- scale experiments an average concentration of less than 100 mg/l, as acetic acid, was maintained and at no time did the concentration rise above 200 mg/l. Laboratory studies indicated that a rise in volatile acid concentration appeared only after advance decline of methane production had occurred and that the former parameter was not sufficiently sensitive to forecast incipient failure of digestion.

The present indications are that, provided sufficient nutrients are available, the primary cause of digester decline would be simple overloading of the metabolic capacity of the micro-organisms under existing conditions. Analyses of wine distillery effluents have indicated that macro-nutrients such as phosphorous, nitrogen and potash, as well as various trace elements, were probably in sufficient supply to support digestion indefinitely. It was found that optimum digestion could be maintained at an ammonia-nitrogen concentration of more than 120 mg/l and an ortbophosphate concentration of more than 50 mg/l as P in the clarified digester liquor.

In 1966 the experimental plant was taken over by the Local Authority on whose site it was constructed. It was thereafter operated by regular Municipal personnel as part of the sewage purification works and performed most satisfactorily. Two other clarigesters for treating starch wastes have similarly been operated successfully by regular sewage works personnel for more than six years. It may therefore be concluded that anaerobic digestion provides an efficient method for practical purification of some concentrated industrial organic wastes.

THE ECONOMICS OF ANAEROBIC DIGESTION

It is generally accepted that concentrated organic wastes can best be treated by anaerobic digestion, whereas more dilute wastes are more economically dealt with aerobically. There is, however, considerable controversy regarding the threshold organic concentration which determines preference between the two processes. In order to clarify the relative economic merits of the two alternatives, individual evaluation is necessary.

Aerobic treatment

The cost of conventional aerobic treatment is largely dependent upon the size of the plant used and the quality of effluent required.

Size of plant. TOWNEND (1962) showed that the capital cost of sewage works per capita was almost halved for a population increase from 250 to 5000. It was reduced by a further 20 per cent for a population of 1,000,OOO. Operating costs were reduced even more markedly with increasing size of works. Similarly MEIRING and MALAN (1965) have postulated costs in relation to size of population served. They concluded that unit costs increased sharply for populations of less than 30,000 [average dry weather flow (dwf) = 1 mgd] and that packaged plants would probably be more economical for populations of less than 20,000.


In order to arrive at a basis for comparison, Figure 3 has been prepared to illustrate the variation in unit cost of sewage purification relative to size of plant used. In preparing this figure, respresentative cost data for Germany (NEUHAUS, 1963), Britain (TOWNEND, 1962), the U.S.A. (THOMAN and TRAIN, 1963) and South Africa (BOLITHO, 1963) have been extrapolated and adapted to South African conditions.

2. Change of stole for sludge volume

3. Cost of sludge treatment in Rand per IOOOg. sludge

‘. -- _ e Treatment of settled Sewage = 40% of Total

-----_,-_

---_ Anaerobic Treatment of Sludge = 20% of Total Cost --_-_--- -------__

Sludpe Volume in Thousands g.p.d. I I I I

0 5 IO I5 20 = 2 m.g d. 25

SEWAGE FLOW IN HUNDRED - THOUSANDS g.p,d.

FIG. 3. Decrease of unit costs with plant size.

FIGURE 3 indicates that unit costs decrease fairly regularly with increased flow rates above 1 mgd average dwf. Below 1 mgd the slope of the curve steepens progressively. On the other hand, the regular inverse relation between unit costs and flow rates above 1 mgd does not hold indefinitely. It would appear that an optimum size plant is reached at a flow rate of about 20 mgd and that very little reduction in unit costs is possible with still larger plants.

For the present economic evaluation, the unit costs for a plant of 2 mgd average dwf has been used, since this capacity falls within the linear range of cost/size relation- ship. Unit cost for any given size of plant within the range l-20 mgd. may then be derived from the equation:

Total treatment cost = 14.5-G cents/1000 g (where x = flow in mgd)

This equation was extrapolated from the curve of FIG. 3. Aerobic treatment may be taken to constitute 40 per cent of this total cost and

sludge digestion and disposal will constitute approximately 20 per cent. This has been

B

636 G. G. CILLIE et al.

indicated in FIG. 3. Treatment costs are largely determined by the quality of sewage and the standard of purification required. The above cost estimates assume that a normal domestic sewage is purified to comply with the General Standard (SOUTH AFRICAN GOVERNMENT GAZETTE, 1962). Salient limiting values of this standard are as follows :

D.O.

pH (COD P.V.

S.S. NH,-N

The dissolved oxygen content should be at least 75 per cent of saturation values. Shall be between 5.5 and 9.5. COD shall not exceed 75 mg/l after chloride correction. Oxygen absorbed from N/80 Acid potassium permanganate in 4 hr at 27°C shall not exceed 10 mg/l. Not more than 25 mg/l of suspended solids are permitted. A maximum free and saline ammonia content of 10 mg/l is permitted.

Heavy metals etc. These substances should not exceed the toxic threshold for the aquatic fauna and flora of the receiving stream.

From the preceding data, it is estimated that the average unit cost of aerobic stabilization of normal domestic sewage to comply with the General Standard amounts to 5.7 cents per 1000 galls i.e. 1.1 cents per lb COD or 1.65 cents per lb BOD. The latter figure was confirmed by actual cost calculations for activated sludge treatment as follows :

Oxygenation capacity utilized = 2 x BOD load (at sea level). Power consumption per lb oxygenation = 0.25 kWh (at sea level).

Therefore 1 lb BOD requires 0.5 kWh at max. efficiency. Frictional losses and additional energy requirements for pumping and control facilities increase this figure to O-7 kWh per lb BOD removed. With power cost of lc per kWh the energy consumption costs 0.7 cents.

Loan charges for the aerobic stabilization section of a plant (MALAN, 1968) of 2*5 mgd capacity based on redemption of civil construction over 30 years and mechanical and electrical equipment over 15 years amounts to R18,600.00 per annum i.e. 0.68 cents per lb BOD.

Operational and supervisory costs vary with individual circumstances, generally from 30 per cent to 60 per cent of the capital charges. On the average it will amount to 0.34 cents per lb BOD.

Total cost of aerobic stabilization therefore amounts to O-7 -I- 0.68 + 0.34 = 1.72 cents/lb BOD.

Quality ofe@ent. The cost of biological treatment increases exponentially with the degree of purification required. TOWNEND (1962) has drawn attention to the determin- ing influence of the standard of treatment on the cost of purification. BOLITHO (1963), in comparing the performance of biological filters at various large works in Johannes- burg, pointed out that, whereas a final effluent having a P.V. of 16 mg/l (10 mg/l BOD) could be produced at relatively low cost from a settled sewage with a P.V. of 50-80 mg/l(240-320 mg/l BOD), a final effluent of 10 mg/l P.V. (5 mg/l BOD) required twice the biofilter capacity.


For the present comparison of costs it is assumed that the degree of purification

must comply with the General Standard, referred to earlier, so as to eliminate the variation in cost due to quality of final effluent. The effect of organic concentration in

the feed, however, requires further consideration. Increased concentration of soluble organic material in the feed does not require a

corresponding increase in treatment facilities while the oxygen required for stabilization will of course increase according to the applied load. It may thence be derived that energy consumed for oxygenation will cost 0.7 cents per lb BOD removed (refer above cost estimate) while loan charges and operational costs will depend upon the volumetric loading only.

300 i.e. cost per lb BOD removed = O-7 + s cent

(where S = concentration of soluble BOD in the feed, assumed to be 300 mg/l for settled sewage).

This relation is an approximation and assumes that volmetric loading remains constant at all concentrations of feed. Volumetric loading must in practice be reduced with increasing concentration to avoid process decline. Stabilized effluent is generally recirculated to dilute raw feed to an organic strength compatible with that of domestic settled sewage (300 mg/l BOD). Hence the loan and operational costs for aerobic treatment will also increase with organic strength. The additional pumping required for recirculation of effluent may further involve considerable expense.

Plant scale studies with biological filtration of industrial wastes, mostly derived from fruit and vegetable processing (CILLIE and DE VILLIERS, 1966), have indicated that the unit cost of aerobic stabilization reduces somewhat with increasing organic strength as measured by COD concentration. This is depicted in FIG. 4.

There was, however, a distinct limit to the concentration of organic material which could be handled with the available recirculation facilities (4:l). Organic concentration in excess of 5000 mg/l COD was found to inactivate the biological filters. This was especially significant with highly concentrated wastes e.g. spent wine (> 20,000 mg/l COD) which could not be handled by the biological filters even after extreme dilution (1 :lOO) with domestic sewage.

The unit cost for aerobic treatment as depicted in FIG. 4 provides for a degree of purification equivalent to the General Standard, referred to earlier. In the case of highly concentrated wastes it was found that approximately 1.5 per cent of the initial soluble COD was biologically intractable. There are indications that a similar per- centage of the organic content of a large variety of wastes, including domestic sewage, remains unchanged during either aerobic or anaerobic stabilization. This intractable residue amounts to excessively high COD concentrations in the effluent resulting from waste liquors with an initial soluble COD of >5000 mg/l. In these cases further reduction of COD can only be achieved at considerable expense e.g. by flocculation or adsorption on activated carbon. The cost curves of FIG. 4 do not include such supplementary treatment and relate to efficient biological purification only, i.e. BOD values reduced to ~10 mg/l.

Anaerobic treatment The cost of anaerobic digestion reflected in FIG. 3 and TABLE 2 includes the cost

of drying and/or disposal of digested sludge. With anaerobic digestion of industrial


wastes there is very little accumulation of sludge (HEMENS et al., 1962) with the result that the disposal thereof need not be added to the cost.

Sewage sludge generally requires a lengthy period of digestion, of the order of 30 days, in order to decompose cellulose and fibrous materials. The soluble organic material contained in industrial wastes, on the other hand, are generally amenable to rapid decomposition. In this regard STANDER et al. (1967) reported that the reaction ceased after 1 to 4 hours of interruption of feed to a plant scale digester. The cost of anaerobic digestion of the latter wastes should therefore be considerably lower than that of sewage sludge.

plus subsequent Acrobi

500 1,000 5.000 10,000

FEED CONCENTRATION IN rng. C.O.D. PER LITRE

FIG. 4. Variation of unit lost with organic strength.

In FIG. 3 the cost of sewage sludge treatment has been related to volume of raw sewage from which it is derived. The volume of sludge treated is, however, generally about 1 per cent of the original sewage volume. Unit costs decrease with increasing size of plant and thus a 2 mgd aerobic plant has been accepted for the present comparison. In view of the reduced volume of wastes which are dealt with anaerobically it would be unrealistic to base a cost comparison on similar volumetric capacities. The following estimates of cost of anaerobic treatment of industrial wastes has therefore been based on a plant with organic load capacity (10,000 lb COD/day) equivalent to that contained in 2 mg settled sewage.

Capital costs. Digester capacity of some 150,000 ft3 providing for an organic load rate of 0.067 lb COD/ft3/day, would normally be required for a 2 mgd sewage treatment plant. The digestion of industrial wastes described earlier in this paper, however, affords a higher organic load rate (O-2 lb COD/ft3/day) and hence capital costs for a digester of 50,000 ft3 plus appurtenances have been estimated below. The inherent


economy of large components is also manifested in the case of digesters up to an optimum capacity of about 100,000 ft3 (75 ft dia. x 25 ft depth). The present comparison is therefore less favourable to anaerobic digestion than might actually be obtained with larger plants.

Recent South African tender prices (1968) for digester equipment with a capacity of 50,000 ft3 amounted to the following:

Construction of concrete digester 55 ft dia. x 21 ft SWD* R20,000.00 Mechanical equipment e.g. heating (steam injection), gas collection,

mixing 15,OOO.OO Sludge clarifier and thickener, 50 ft dia. x 13 ft SWD (concrete,

scraper bottom tank) 7500.00 Mechanical equipment for clarifier 7000.00 Appurtenances e.g. pump house, feed sumps, sludge pumps, feed

pumps, control gear, gas meters, float valves, interconnecting pipe- work, electrical installation 11,500,oo

Total capital cost R61 ,ooo.oo

Annual charges for interest and redemption of this capital cost would amount to R5,800.00.

Running costs. The operational expenditure for anaerobic digestion of industrial wastes depends upon whether or not the plant is run in conjunction with a sewage works or other waste treatment facility. In the former event, labour, supervision, laboratory services and maintenance would be partially shared by the sewage works organization and thereby reduce nett costs. On the assumption that such integration of manpower would generally be possible, the following estimates of annual running costs, based on actual experience, were made :

Equipment maintenance and renewals RI ,200.OO Labour 1 ,*.OO Supervision 1 ,ooo.OO Laboratory services 2400.00 Power Consumption (20 H.P. average) 1,800.OO

Total Running Costs R7,000.00

Actual data for power consumption were not available and the above cost figure may therefore require slight modification according to the type of equipment used and the method of operation.

Fuel costs for heating of digesters are not provided for, since sufficient gas should be available from the digestion process.

Subsequent treatment. The effluent from anaerobic digestion requires further aerobic stabilization to reduce its oxygen demand to acceptable values. Experience has shown

* SWD = side wall depth. The bottom cone of this type of digester is shallow and does not con- tribute to active digestion space.


that about 2.5 per cent of the initial COD of industrial wastes remained in the effluent after anaerobic digestion. Extended aeration of the anaerobic effluent reduced the residual COD by 40 per cent i.e. 1 per cent of the initial load. The ultimate residual COD (about 1-5 per cent of the original COD) remained biologically intractable.

The unit cost for anaerobic purification of industrial wastes should therefore include an item for subsequent aerobic treatment of 1 per cent of the applied COD. This has been reflected in the cost curves of FIG. 4.

Overall costs. From the preceding discussion the unit cost of anaerobic treatment of industrial wastes can now be derived.

At an organic load rate of 0.2 lb COD/ft3/day a digester of 50,000 ft3 capacity will handle 10,000 x 365 lb COD p.a. = 3.65 million lb COD.

The annual cost for anaerobic purification of this load would then be R5,800.00+ R7,000.00 = R12,800.00 i.e. cost per lb COD = 0.35 cents.

The permissible load rate per ft3 of digester capacity decreases with decreasing organic concentration of the feed (TABLE 3). This is primarily caused by the larger volumetric loading associated with an equivalent organic load and the increased sludge clarification and recirculation necessitated thereby. The unit cost of anaerobic treatment of industrial wastes will consequently vary from about O-25 cents per lb COD for strong wastes (50,000 g/l COD) to l-5 cents per lb COD for dilute wastes (3000 g/l COD).

This has been reflected in the cost curves of FIG. 4.

Comparison of cost

The collective data indicate that anaerobic digestion becomes economical at effluent concentrations of more than 4000 mg/l COD. Below this concentration aerobic methods are preferable. The angle of intersection for the two curves of FIG. 4 is, however, comparatively small and the point of intersection is not sharply defined. There would thus be a range of concentrations where either aerobic or anaerobic methods may be applied with advantage. The actual process to be adopted will then be determined by local circumstances.

The economic advantage of anaerobic digestion is primarily manifested at higher concentrations, i.e. above 20,000 mg/l COD. At this concentration anaerobic methods cost about & of equivalent aerobic treatment.

The upper economic limit of COD strength for anaerobic digestion has not been established. It is probable that with very high concentration (more than 50,000 mg/l) evaporation and solids recovery may well compete with anaerobic digestion. As a rough guide it may be stated that below COD concentrations of 4000 mg/l aerobic methods should receive priority between 4000 and 50,000 mg/l COD anaerobic digestion would be preferable and above this value evaporation with possible solids recovery should receive serious consideration.

CONCLUSIONS

Anaerobic digestion, being one of the oldest natural self-purification processes, may be harnessed to purify industrial waste liquors satisfactorily. Anaerobic digestion is especially suitable for purification of concentrated organic waste. Methane gas produced during anaerobic decomposition of organic matter provides a useful source of heat which in some cases may be used profitably.


Practical application of the process, as described in this paper, ensures reliable purification of industrial wastes. The process is easily controlled and, by using well- established control parameters, it can be operated indefinitely. Sludge disposal will seldom cause difficulties since very little accumulation thereof has been experienced throughout extensive operation of full-scale experimental plant.

The cost of anaerobic digestion of concentrated organic waste is considerably less than that of equivalent aerobic processes. Aerobic processes are again more economic at lower concentrations (less than 4000 mg/l COD).

The use of a modified clarigester unit as described in this paper provides for easy control and operation of the process by normal sewage works personnel. It has, however, also been demonstrated that a digester with separate clarifier and suitable sludge return facilities may be used equally well.

Acknowledgements-The full-scale experimental clarigester was supplied and installed free of charge by Messrs E. L. BATEMAN of the Dorr-Oliver Company. Their continued assistance throughout the experimental operation of the plant is gratefully acknowledged.

The Municipality of Paarl and various members of staff, particularly Mr. D. T. V1s.s~~ (sewage works superintendent) and Mr. H. A. DE VILLIERS (biochemist), rendered valuable services.

Mr. H. BLERSCH of Ninham Shand & Partners (consulting engineers) provided data for cost estimates and also advised on the actual costs of recently constructed sewage plants.

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