A BIOZEOLITIC THEORY OF
SEWAGE PURIFICATION EMERY J. THERIAULT U. S. Public Health Service, Washington, D.C.
Previous studies are briefly reviewed and a new theory is proposed to account for the purification of sewage by the biological processes. Practical applications to sewage treatment are pointed out and comparisons are made with the corresponding methods of water purification.
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Liquefaction within the cells may be ascribed to an endoenzyme, but there is no evidence of the existence in the body of the sewage of a liquefying enzyme liberated by the protozoa. As to oxidative action by enzymes, there is no doubt that the bacteria are intimately concerned, although the mechanism of the process is by no means clear. As a rule it is assumed that the oxidation is accomplished by the endoenzymes of the bacterial cells. If so, the rate of oxidation should depend on the rate of diffusion of dissolved oxygen into the cells. It is reasonably certain, however, that the temperature coefficient of the bacterial oxidation process is decidedly higher than for the diffusion of dissolved oxygen. Another view of the matter is to consider that the action of the sewage bacteria is always in the direction of reduction and that the decomposition products liberated by them are oxidized in the solution by dissolved oxygen or other oxidizing agents. I n this sense the bacterial catalyst becomes a reducing agent and not an oxidase. The presence of an oxidizing agent in sewage sludge was tentatively assumed by Theriault and McNamee (17)in experiments on the rate of satisfaction of the oxygen demand of sludge to account for a process which approached completion in 20 hours, in accord with observed rates in certain enzymatic reactions. Bacterial reactions are completed only in 20 days or thereabouts a t ordinary temperatures. The experiments of Spoehr (IS), indicating that sodium ferropyrophosphate acts catalytically in inducing the oxidation of glucose a t ordinary pH values and temperatures, may help to visualize the concept of catalytic action in sewage sludge and in other biological systems. Bacterial precautions, however, were not mentioned by Spoehr and his associates in their earlier communications, and the rate of oxidation deduced from their experiments did not differ materially from that observed with known bacterial cultures acting on the readily decomposable glucose. I n other respects the oxidation curve in Spoehr’s experiments closely simulated the usual oxygen demand curves, including a 24-hour lag. Strictly negative results were obtained by Theriault, Butterfield, and McNamee (16) for the oxidation of glucose by Spoehr’s catalyst when bacteria were excluded from the solutions. Spoehr and Milner (14) claim that certain conditions in the use of the catalyst
VIDENCE to the effect that one important colloid in sewage purification-namely, the activated sludge matrix-is definitely zeolitic in composition and in general behavior has been presented in previous papers by the writer. I n the present paper it is proposed to develop the general theory which is implied in this zeolitic concept of sludge composition and to point out certain applications to the theory or practice of sewage treatment and water purification. First, it will be desirable to summarize previous studies along these lines and to indicate the possible relation of the sludge zeolite to the other colloids of sewage purification, particularly to the so-called sewage colloid. Ardern ( I ) and Wilson (19) have given excellent reviews or critical discussions of colloid theory as applied to sewage treatment. I n general, the sewage colloids have been interpreted either as versatile enzymes or else simply as adsorptive surfaces, although combinations of these hypothetical roles have not been infrequent. With the information a t hand regarding the nature of adsorption by activated sludge, it should be possible to dismiss the unsatisfactory hypotheses of adsorption by surfaces as such. As has already been shown elsewhere, the bacteria embedded in the gelatinous matrix of the biological slimes do not contain any appreciable amount of either aluminum or silica. The aluminosilicate complex or sludge zeolite is therefore wrapped around the bacteria to provide an adsorptive coating of known characteristics. The adsorption of ammonia, amino acids, lignoproteins, and related organic constituents of sewage can be satisfactorily explained in terms of base exchange, without resort to vaguer mechanisms. Enzymatic action has frequently been proposed to account for such diverse phenomena in sewage treatment as the dissolution of the coarser particles of sewage, the oxidation of adsorbed matters in the sludge, and the coagulation of the sewage colloids. The corresponding enzymes may therefore be described as lytic, oxidative, or clotting. All three of these catalytic actions were postulated by Parsons (IO, 11). I n all probability the lytic agents of sewage purification are the protozoa. Their mode of action on the relatively coarse particles of sewage appears to be largely mechanical up to the point where microscopical particles are visibly ingested. 83
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were vitiated by Theriault, Butterfield, and McSamee (16) and they submit new experiments indicative of catalytic action under sterile conditions. It should be of obvious interest to institute other experiments along these lines as further details regarding the proper method of applying the catalyst become available or as further experience is gained in its use. It is of possible significance that all of the ingredients of Spoehr’s catalyst are present in the sludge ash, probably as normal components of the bacteria, although not necessarily in the proper combination for catalytic action. As to the remaining enzyme of sewage purification-i. e., the clotting enzyme of the earlier chemists-it must be considered that the main reliance for the existence of such a catalyst has been the rapidity of action of the clarification process rather than any actual isolation of the active principle. With the demonstration that the adsorption of sewage matters by the sludge zeolite leading to the clarification of a sewage is completed in 30 minutes or thereabouts, and with the knowledge that an equilibrium is reached in conformity with the known behavior of the zeolites, it should be permissible to replace the hypothetical clotting agents of the literature of sanitary chemistry with the aluminosilicate complex or sludge zeolite of the present investigation. This view of the composition of the clotting enzyme is strengthened by the circumstance that the analysis of the sewage colloid isolated b y Wilson (19) from raw sewage by acidification is practically the same as that of the nitrifying sludge given in an earlier paper by the writer (16). As given in Table I, Wilson’s figure for silica (“insoluble matter in aqua regia”) has been compared with actual silica as determined by volatilization. His value for “oxides of iron and aluminum” must be interpreted as “combined oxides” for the reason that “no attempt was made to estimate acid radicals present in the ash” and no mention is otherwise made of phosphorus. Allowing for these differences in analytical procedures and for radical differences in the previous history of the samples, the agreement between the sets of analyses is remarkably close. As shown in a previous paper, the silicasesquioxide ratio for the nitrifying sludge was 3, corresponding to the formula RzOg3SiOz. This formula therefore applies to Wilson’s colloid. The agreement in this respect is probably fortuitous, and the general conclusion regarding the nature of the sewage colloid would not be altered if a different silicasesquioxide ratio should be obtained with other sewages.
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analysis used by Dienert are not given in his paper, it may be that his figure for silica is not comparable with the more definite values given in Table I. It should be possible, nevertheless, to institute comparisons between the analyses given by Dienert. The ratios deduced from his analyses are of the same order of magnitude as those derived from the data of Wagenhals, Theriault, and Hommon (18) when the “HClinsoluble matter” is used instead of the actual silica content. It is not necessary to assume, however, that the sludge zeolite is of unvarying composition. Mellor (9) gives a range of A1203.2Si02 to A1203.10Si02 for the members of the zeolite family of minerals. As will presently be shown, other evidence exists that the composition of the sludge zeolite may vary between wide limits. The essential feature of Dienert’s analyses is that the aluminosilicate complex tends to persist unchanged in its transformation from the septic to the fully activated condition.
Biozeolitic Theory of Sewage Purification A great deal of work still remains to be done on the subject, but from the data a t hand the following mechanism can be proposed for the purification of sewage by activated sludges or biological slimes:
1. The inorganic colloid of sewage is a zeolite, derived from feces, and accordingly fully saturated with organic matters, largely nitrogenous. 2. This sewage colloid is negatively charged and may be agglomerated by plain subsidence in tanks to give negatively charged sludge particles. The agglomeration of the sewage colloid may be accelerated by properly modulated agitation, not necessarily by aeration. The agglomeration of the sewage colloid is akin to the growth of precipitates in water purification from the initial stage of a turbid suspension to the condition of a visible floc. 3. Starting with a se tic sludge or an agglomerated sewage colloid, the activation o f the sludge is accomplished when the adsorbed organic matters are dialyzed to the bacterial cells, leaving the nondiffusible aluminosilicate complex behind for further adsorption from the liquid. There is a tendency for the accumulation in the sludge zeolite of products unassimilable by the bacteria, leading to a gradual decrease in the efficiency of the sludge unless it is continually replaced by fresh material. 4. The clarification of a sewage is accomplished by the adsorption of organic matters on the sludge zeolite, without accompanying oxidation or bacterial intervention. The time required for reaching an equilibrium with the organic matters of sewage is about 30 minutes, after which further adsorption will depend on the rate of bacterial action. Clarification in a different sense is also accomplished by the plankton but only as a relatively slow process not circumscribed by any 30-minute limitition. 5. The cations transferred to the bacteria from OF WILSON’S COLLOID WITH ACTIVATED SLUDGE the sludge zeolite are decomposed and liberated to TABLEI. COMPARISON ~ 1 2 0 8+ the solution by diffusion. These decomposition FezOa + products are probably released as highly reactive Material P epzos r cent inpzos Ash Mgo radicals, capable of combining directly with dissolved Activated sludge L5.77 (24.96) (1.08) (8.39) 34.43 7.58 2.54 90.32 oxygen or other oxidizingagents, if present. StartWilson’s colloid 44.24 ... *. . . 34.25 8.04 2.45 88.98 ing with fresh material, the first stage of the bacterial oxidation process proceeds at much the same rate as in river waters. Under sustained aerobic conditions, the decomposition products of the first TABLE 11. EFFECTOF ACTIVATION ON INORQANIC CONSTITUENTS OF SLUDGE stage of bacterial activity appear in the solution N _. Total a8 Organic largely as bicarbonates and water, whereas the Results by Dienert (6) Si02 Ah08 Fez08 PzOa CaO MgO so4 6 “4 N Ash ammonia is retained by the sludge zeolite to the Milligrams per grain of dry 8 l U d Q e extent defined by the equilibrium existing between Before activation 262 48 23 13.6 136 7.1 6.9 15.1 0.65 9 . 6 666 the ammonia in the solution and in the zeolite. After activation 261 44 28 1s.9 129 9.2 14.0 0 . 6 5 11.9 554 With the development of nitrifying zotiglea, both stages of the bacterial oxidation-may proceed simultaneouslv without anv apparent interference. The decomposition produ&s of the nit
