-Liquid
Industrial Wastes-
Boley, A. L., P u b l i c W o r k s , 7 4 , N o . 8 , 20 (1943). Camp, T. R., New England Industrial Waste Conference, Cambridge, Mass. (June 1950). Chicago, Ill., Sanitary District, Chicago J . Commerce (Nov. 2, 1949).
(1942).
Collins, F. L., J . Am. Leather Chemisfs’ Assoc., 46, 176 (1951). Eagl, R. H., I b i d . , 1 4 , 5 7 7 (1919). Esten, P. A,, Ibid., 6 , 4 6 4 (1911). Fales, A. L., H i d e and Leather, 75, KO.17, 48 (1928). Fales, A. L., IXD.ENG.CHEM., 21,216 (1929). Harnly, J. IT., J . Am. Leather Chemists’ Assoc., 3 9 , 24 (1944).
Peck, C. L., J . Am. Leather Chemists’ Assoc., 1 2 , 4 2 2 (1917). Ibid., 13, 417 (1918).
Pennsylvania Tannery Waste Disposal Comm., J . Am. Leather Chemists’ Assoc., 26,70 (1931).
Reuning, H. T., Ibid., 3 8 , 2 9 2 (1943).
(19) Ibid., 46, 169 (1951). (20) Harnly, Wagner, and Swope, Sewage W o r k s J . , 1 2 , 7 7 1 (1940). (21) Hommon, H. B., J . Am. Leather Chemists’ Assoc , 1 2 , 3 0 7 (1917). (22) Hommon, H. B., U. S. Pub. Health Seivice, Public Health Bull. 100 (Sovember 1919). (23) Howal and Cavett, Proc. Am. Soc. Czoil Engis., 53, 1675 (Sep(24)
Nyman, C. L., correspondence (1951). O’Flaherty, F., correspondence (1951). Ohio River Committee, U. S. Pub. Health Service, Appendix XI, Office of Stream Sanitation, Cincinnati, Ohio, Supplement D
tember 1927). Justen, Waite, and Heisig, correspondence (1951).
(25) Kennedy, S. J., Chem. E n g . , 56, 141 (June 1 9 4 9 ) . {26) Leather & Shoes Blue Book, Chicago, Rumpf Pub. Co., p. 385. 1948. ( 2 7 ) Marshall, F. F., J . Am. Leathei Chemists’ A s s o c . , 46, 172 (1951). (28) Massachusetts State Board of Health, Yearly Reports (1895, 1909). ( 2 9 ) Ibid., 3 3 , 18 (1901). (30) Michigan Stream Control Commission, Lansing 1, Mich. (1933, 1934). (31) Mohlman, F. W., IND. ENG.CHEM.,18, 1076 (1926). (32) Murphy, Helper, and Eldridge, Michigan Dept. of Health Bull., Lansing 1, Mich. (May 1 9 2 7 ) .
Ibid., 3 9 , 378 (1944). Ibid., 4 2 , 573 (1947). Ibid., 46, 164 (1951).
Reuning and Cathcart, Public Works, 78, No. 3 (1947). Richards, R. H., J . Am. Leather Chemists’ Assoc., 4 4 , 693 (1949). Schoatzle and Blohm, Maryland State Dept. of Health Bull., Annapolis, Md. (April 1928). Seligsberger, L., correspondence (1951). Stevenson, W. L., J . Am. Leather Chemists’ Assoc., 39; 386 (1940). Stevenson, Tir. L., J . Boston Soc. Civil Engrs., 13, N o . 2 (1926). Sutherland, R., IND. ESG.CHEM.,3 9 , 6 2 8 (1947). Thorstensen, E. B., J . Am. Leather Chemists’ Assoc., 4 6 , 156 (1951).
U. S. Pub. Health Service,I b i d . , 9 , 3 7 6 (1914). Warwick and Beatty, Sewage Works J . , 8 , 1 2 2 (1936). Weldon, R. B., correspondence (1951). West Virginia State Water Commission, Charleston, TT. Va., First Annual Report (1929, 1930). RECEIVED for review September 17, 1951.
ACCEPTEDJanuary 16, 1952.
WASTE DISPOSAL IN BRITAIN B. A. SOUTHGATE, W a t e r Pollution Research Laboratorg, Department o f Scientific and Industrial Research, Langleg R o d , Watf o r d , E e r t s , England
In Great Britain the density of population and industry is relatively high; the rivers are comparatively small and are used as sources of water supply. The climate usually makes disposal by irrigation or lagooning impossible, and manufacturers have a legal right, subject to certain safeguards, to discharge effluents to the public sewers after removal of substances which might be dangerous or might damage sewers or interfere with sewage treatment processes. For many years it has been illegal to discharge polluting liquids to inland waters and recent legislation has empowered River Boards to fix standards for emuents discharged to streams in their districts. hlethods are described which have been developed in Great Britain for treat-
ing various waste waters in conformity with these conditions. Over a period of several years, the Water Pollution Research Laboratory, Watford, England, has carried out experiments on the comparative efficiency of various methods of biological filtration for treatment of sewage to yield a final effluent with a biochemical oxygen demand of not more than 20 p.p.m.; this is usually regarded as the maximum permissible B.O.D. for effluents discharged to the relatively small rivers of Great Britain. The methods investigated have included single filtration, single filtration with recirculation of settled filter effluent, single filtration after a short period of preliminary treatment by the activated-sludge process, and alternating double filtration.
I
act of Parliament governing the discharge of waste waters was one passed as long ago as 1876. This act prohibited the discharge of polluting liquids to inland streams but not, except in special circumstances, to tidal waters. As a result there is a great difference between the degree of treatment given in inland districts and on the coast, and large volumes of sewage and industrial u-astesaredischargedto estuaries without treatment or after very incomplete treatment. Many estuaries are seriously polluted, a few of them being almost devoid of dissolved oxygen during hot weather. I n 1951, however, the Rivers (Prevention of Pollution) Act was passed; this contains provisions whereby River Boards (recently set up t o control the condition of rivers in England and Wales) can adopt by-laws specifying the quality of sewage and industrial effluents which may be discharged in their districts, both to fresh water streams and to estuaries. An important factor affecting the disposal of industrial wastes is that a manufacturer has a legal right, subject t o certain safe-
IV ANI‘ country methods used for disposing of industrial wastes
and sewage will be determined partly by geographical and climatic factors and partly by the law relating to pollution and the stringency with which it is enforced. I n Great Britain the density of population and industry is relatively high and the rivers are not only comparatively small but are the source of the greater part of the water distributed for domestic and industrial supply. Consequently, wastes which are to be discharged to inland streams normally have to be treated to yield effluents of high quality. The climate and rainfall are such that treatment of waste water by irrigation or lagooning is not usually possible. Occasionally small quantities of industrial wastes, mainly from factories in rural districts, are disposed of in soakawags or by irrigation, and sewage from a few towns-now mainly from small ones-is still given land treatment. I n general, however, the large sewage farms, which were a t one time extensively used for treatment of sewage, have been superseded by more modern plants. Until recently the most important 524
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j
Liquid hdustrial Wastes guards, to discharge effluents into the sewers of a local authority in whose area his premises are situated. H e may have to apply some pretreatment before discharge t o remove substances which might be dangerous or might damage sewers or interfere with the treatment of a sewage a t a disposal works, and many pretreatment plants of this kind are a t present being designed or built to remove such substances as cyanides, chromates, and toxic metals. There is no doubt t h a t the policy of encouraging the discharge of industrial liquors to domestic sewers greatly simplifies the problem of disposal and leads to a general reduction in the pollution of rivers.
LIQUID INDUSTRIAL WASTES Bearing these considerations in mind it may be of interest to give some notes on methods adopted in Great Britain for disposing of some of the types of waste water dealt with in the present symposium.
Tanneries Many tanneries in Great Britain discharge waste liquors without treatment to tidal waters. Comparatively little has been done to develop methods of purification capable of yielding an effluent of high quality. Among the methods applied are oxidation of sulfide with chlorine, screening, and removal of suspended solids by sedimentation from the mixed wastes. Some tanneries discharge to sewage-disposal works, and where the volume of sewage is large compared with that of the waste water, no special modifications are made in the processes of treatment of the sewage. It is clear, however, that much more information is required on the treatment by biological processes of tannery wastes both alone and mixed with comparatively small quantities of domestic sewage.
Paper and Pulp Pulp is not manufactured from wood on a large scale in Great Britain, and the problem of disposing of sulfite cellulose liquors does not arise. Apart from imported pulp the raw materials mainly used for the manufacture of paper are rags, esparto, and (particularly during World War 11) straw. It is not usual to recover alkali from the boiling of rags, but from the digestion of esparto and straw with caustic soda the liquor is almost always concentrated in multiple-effect vacuum evaporators, the residue being incinerated and used for the recovery of the alkali. The pulp, from which the spent lye has been drained, is then washed with water in breakers and is bleached and made into paper. Separation of the spent lye after digestion is much more difficult when straw is used as the raw material than when esparto is used. With straw the final waste waters may be too alkaline to be treated biologically. Although such liquors have occasionally been treated withacid, there is an obvious need for thedevelopment of more effective methods of separating the alkaline liquor from the pulp in a paper mill, so that a larger proportion of the alkali can be recovered as solid or returned in solution for re-use in the digestion process. This problem is being worked on by the paper industry. It has been shown that provided waste waters from paper mills are not too alkaline they can be treated without difficulty by biological filtration to give an effluent of good quality (7). Paper mill effluents often contain free chlorine and the extent to which this would interfere with biological filtration is not yet known, though it appears, from experimental work of the Water Pollution Research Laboratory, t h a t the interference might be less than was at first thought (8). I n some of these experiments an ammonium salt was added to waste waters from a paper mill to improve the carbon to nitrogen balance before filtration; it is not known whether addition of nutrients would be economic in la1 ge scale practice.
March 1952
Canning Citrus fruits are of course not canned in Great Britain, but there is a large industry concerned with the canning of other fruits and vegetables. As in America, the volume and composition of the waste waters fluctuate widely throughout the year. Some canneries discharge waste waters to domestic sewers; where this is impossible the methods of treatment usually include screening, sedimentation (sometimes with addition of coagulant), and biological filtration. An account of a useful filtration plant with arrangements for recirculating effluent at night was given recently (12).
Packinghouse Wastes I n Great Britain the problem of treating packinghouse wastes is not so serious as in America, for much of the meat eaten is slaughtered abroad and imported in carcass form; of the animals slaughtered in the country many are dealt with in relatively small premises from which waste waters are discharged to domestic sewers. There is full agreement with the conclusions reached in America that strict precautions should be taken to recover grease, blood, and other by-products a t the source. Results of operation of the few treatment plants built have confirmed that the waste waters can be treated to yield effluents of good quality. At one establishment the treatment comprises addition of lime and ferrous sulfate, sedimentation, treatment by the Kessener activated-sludge process, and biological filtration, the final result being an effluent of good quality which is discharged to a small stream.
Dairies As in America, great emphasis is now put on the necessity for reducing as far as possible losses of milk and other products to the drains, and a t well-run dairies elaborate precautions are taken to this end. Some dairies are able to discharge theirwaste waters to domestic sewers but many-probably the majority-are in country districts and must discharge direct to a stream. At a few factories the wastes are treated by addition of coagulants, sedimentation, and single-stage biological filtration. At one plant, for example, the wastes are treated with aluminoferric (300 p.p.m.) and soda ash to raise the pH value to 7 and, after sedimentation, are passed through a single percolating filter, 16 feet deep, a t a rate of about 400 U. S. gallons per cubic yard of medium per day. At a second factory the wastes are treated with aluminoferric (170 p.p.m.) and, after sedimentation, are passed through a filter 5 feet deep at a rate of 72 U. S. gallons per cubic yard per day. At these factories the B.O.D. of the crude waste waters was 286p.p.m. and 690 p.p.m., respectively, and that of the final effluents was 14 and 6 p.p.m. ( 1 7 ) . At most dairies, however, coagulants are not added, but the waste waters are treated by sedimentation and alternating double filtration with recirculation of final effluent t o dilute the liquid applied to the primary filter (IO). Many plants of this kind are now operating. I n a recent report from one firm on six such plants the average B.O.D. of the crude waste waters ranged from 200 t o 600 p.p.m., the volume of effluent recirculated per volume of crude liquor from 0.5 to 3, and the average B.O.D. of the final effluent from 3 to 14 p.p.m.
Iron and Steel Much of the spent pickle liquor and washing waters from the steel industry is discharged t o tidal waters without treatment. I n some districts there has been considerable pollution and for this and other reasons (including the present shortage of sulfuric acid) methods of reducing losses are being actively considered. Ferrous sulfate is recovered from pickle liquor a t some works; the liquor is then strengthened by addition of acid and re-used, though, as in America, there would be difficulty in disposing
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525
-.Liquid
hadustrial Wastes
of the recovered copperas if the process were adopted throughout the industry. Recovery of sulfuric acid and iron oxide from copperas is being investigated. At one plant ( 3 ) washing waters are treated by addition of lime and air and, after removal of sludge are re-used in the washing process; this. however, is an unusual case. Washing waters are often discharged after neutral-
LIME SLUDGE I
I
I
VAPOUR
-
CONENSER WATER COOLING
CIRCUIT
I Figure 1.
Radioactive Substances
Flow Sheet of Beet Sugar Factory Showing Re-Use of Water
ization with lime and removal of sludge So far as is ltnoir-n, calcium carbonate is not being used for neutralization on a large scale, though some research on the applicability of this process has been done (13). It may be mentioned that a more advanced stage has been reached in the treatment of'liquors from the pickling of copper. In the larger mills it is common practice to electrolyze pickle liquor continuously for recovery of copper and regeneration of sulfuric acid.
Coke Ovens A large proportion of waste waters from coke ovens is discharged into tidal waters without treatment. All these liquors absorb oxygen from the water; apart from this, their effect in estuaries depends largely on whether they contain cyanidesby far the most toxic constituent likely to be present in waste waters of this kind (9). I n inland districts the waste waters from coke ovens (many of which are operated a t collieries) are not usually treated in a specially built plant but may be spread over colli~rytips or occasionally used for quenching coke. An account of the methods used in the Yorkshire coal fields has recently been given (21). I n Great Britain a widespread problem arises from the discharge of liquors from works making coal gas for domestic supply. Almost every town, down to quite small places, has a gas works and until recently the population served by a gas works was usually about the same as that served by the corresponding sewage M orlrs. Except on the coast the gas liquors were normally discharged to domestic sewers, the proportion by volume not exceeding as a rule about 0.5% There is now a tendency to concentrate manufacture of gas in larger plants and this has increased the difficulty a t some sewagedisposal \\ orks which serve a population smaller than that served by the gas works. There are still some gas works from which liquors are discharged without recovery of ammonia, but it is generally agreed t h a t this is bad practice and recoveiy plants are being installed a t a number of works at which they had not before been provided. The ammonia is often recovered in the 5 26
form of a concentrated solution which is sent to central worlts for preparation of ammonium sulfate. It has been shown quite conclusively that the difficulty in treating gas liquors in admistwe with sewage is much accentuated if the rate of discharge of t,lic. liquor fluctuates, and i t is therefore common practice to install balancing tanks a t a gas works so that the liquor may be diecharged to the sewers a t an even rate. At a very few worke. phenol is recovered, but there is some doubt about the extent t o which this assists the operation of a sewage works since much of the difficulty appears to be due t o the presence of substances other than phenol, including higher tar acids and thiocyanat?. A t one works (15) a plant is being built for complete purification of the liquor. Phenol will be removed by extrac.tioii with benzene and ammonia recovered by distillation; the liquor will then be concentrated to one tenth its volume in multiple-effect evaporators after which polyhydric phenols will be extracted with butyl acetate, and ammonium chloride will be recovered, At some works the tar is removed from the hot gas by electrostatic precipitators; the effect of this is to reduce the concentratioii of polyhydric phenols in the liquor discharged. Air is admitted to the gas stream immediately before the purifiers rather than a t an earlier point in order to reduce the formation of thiocyanate (16). Much work is in progress in Great, Britain a t present with the object of determining more precisely the effects of the various constituents of gas liquor on sewage treatment ( 5 ) .
The first work carried out in Great Britain on the effects of radioactive substances on sewage treatment has recently bccn published ( 4 ) . The subetances involved are mainly those dis-charged from hospit,als and from certain laboratories, though the use of radioactive substances in industry appears to he beconiing more common. It was shown by Belcher ( 4 ) , who worked with radioactive sodium, phosphorus, cobalt, bromine, and iodine, t,hat none of these substances was removed t o any great cxtmt during primary sedimentation of sewage. When the settled sewage was treated by biological filtrat'ion only a Rmall proportion of the sodium, bromine, or iodine was retained in the filter, but larger amounts of phosphorus and cobalt were adsorhctl. In general, it appears that although some radioactive elements may be retained for a time in a sewage treatment plant it would be xGe, in the interest of public health, to assume that t'hc whole of any radioactive material in sewage would pass through thc works and enter the river to which the effluent n-as discharged. The only way of ensuring that the concentration of radioactive material in a river does not rise to a dangerous level would seem to be to limit in any given area the t'otal amount of such substances supplied for medical or experimental purposes. The amount allowed would no doubt depend on the size of the river available for dilution.
Fermentation .;ilthough sewage sludge is digested a t many works in Grcat Britain very lit,tle progress has been made in the treatment, of industrial waste waters by fermentation. Some csperinicntal work has been done (18) on anaerobic digestion of u.astcs, but as far as is known no full scale plant of this type is in operation.
Re-Use Of Waste Waters In Great Britain, as in America, during recent years much interest has been shown in the disposal of industrial wastes hy re-using them in manufacturing processes. The waste r a t e r s to which this method of disposal has been applied, howevcr, are different in the two countries. I n Great Britain ihc most important examples are in the beet sugar industry and in t h e retting of flax. At beet sugar factories in inland districts, process water-that is, the liquid drained and pressed from the exhausted beet slices-is now often passed through screens
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Liquid Industrlal Wastesand is then returned to the extraction battery (Figure 1). This leads to increased corrosion in the battery circuit, and although various methods, including chlorination (g), have been tried t o reduce it as far as possible, the problem has by no means been solved; nevertheless re-use of the liquor is the most practicable method of disposal yet devised. Fluming and washing water is passed through sedimentation tanks or through lagoons and is also pumped back for re-use. At the end of a sugar campaign there remains a large volume of liquid €or disposal and it has been shown that this can be treated by biological filtration to yield an effluent of good quality suitable for discharge t o an inland stream ( 6 ) . Re-use of liquor from the Getting of flax was developed during World War I1 in order to avoid the extensive treatment which would have been required if the liquor had had t o be treated to an extent sufficient t o allow it t o be discharged to an inland stream ( 1 1 ) . In the process of retting usually employed in Europe the liquor, which A i strongly anaerobic, is run off and replaced by fresh water. I n the new process the liquor is maintained in an aerobic condition by passing bubbles of air through i t during the retting process; its power to cause retting of flax does not then diminish with time and it can be re-used for a large number of successive rets (Figure 2).
the Birmingham ‘lame and Rea District Drainage Board and by the City of Coventry. The present paper contains a short account of an investigation, continued during several years, of the comparative efficiency of treatment of settled sewage by single filtration, by single filtration with the return of an equal volume of settled filter effluent, by single filtration after a short period of pretreatment by the activated-sludge process ( LLbioflocculation’’),and by alternating double filtration. The crude sewage which, after sedimentation, was used in these experiments, contained a considerable proportion of trade wastes, particularly from the pickling and electroplating of nonferrous metals.
FLAX-RETAINING BEAUS WOODEN GRIDS
Choice of Factory Sites Many of the difficulties in treating industrial wastes in Great Britain arise from the fact that the question of treatment and disposal was not considered when factories were built; indeed a few factories have been moved during recent years because of difficulties in effluent disposal. The wisdom of considering this before deciding on the site of a new factory is, however, now well recognized and the Water Pollution Research Laboratory has been asked on several occasions to survey rivers and coastal waters and t o predict the probable effect of discharging industrial wastes t o them. A manufacturer considering the erection of a new factory is a t present in some difficulty since River Boards have not published standards of quality to which his effluent must conform. I n the act previously mentioned, however, provision is made for the formulation by River Boards of bylaws containing standards of quality for effluents. It is believed that each of the standards adopted will assess some general property of an effluent-for example, its oxygen-absorbing capacity, its toxicity, and its content of suspended matter. If this system is adopted it would be of great assistance to a manufacturer embarking on the building of a new plant for he would know to what specification his effluent would have to conform a n d could arrange his treatment processes and the location of his factory accordingly.
BIOLOGICAL FILTRATION OF SEWAGE A large proportion of the sewage of Great Britain is treated by biological filtration. The rivers into which sewage effluents are discharged are comparatively small and the maximum biochemical oxygen demand permitted for .an effluent does not usually exceed 20 p.p.m. and is sometimes less than this. I t is not usually possible to reduce the B.O.D. of an effluent by chlorination since, with the small dilution available, there would be a serious risk of discharging chlorine in concentrations that would he toxic in a river. Moreover the sewage from most towns in Great Britain contains liquor from the manufacture of coal gas; effluent from the treatment of this sewage usually contains thiocyanate with which chlorine would form the toxic cyanogen chloride ( 1 ) . The development of processes of biological filtration of high efficiency, and capable of yielding a final effluent with a B.O.D. not exceeding 20 p.p.m. and preferably well below this figure, is therefore of great importance. During the past few years several alternative methods have been compared by the Water Pollution Research Laboratory in large scale plants put a t the disposal of the department by March 1952
STORAGE TANK
A
PLAN
COM~ESSED AIR WATERTIGHT
SECT/ON A-A
Figure 2. Diagram of Retting Tanks and Storage Tank for Retting of Flax, with Aeration of Liquor
-
Sewage from a large part of the Birmingham area receives primary treatment at a separate works where spent gas liquor, which is conveyed to the works through a separate sewer, is added in regulated amount. The settled sewage flows through a sewer, 41/*miles long, to another works at Minworth, where it passes through a second set of sedimentation tanks and is then treated by biological filtration. For the experimental work four circular filters, earh about 6 feet deep and 1100 square *yards in area, were used. Each filter was provided with an upward-flow humus tank with a peripheral weir; the capacity of each tank was 76,000 U. S. gallons, and the rate of upward flow a t the surface was 2 feet per hour when a filter was treating liquid at a rate of 120 U. S. gallons per day per cubic yard of medium (1,160,000U. S. gallons per acreper day). The pipes and valves of these four filters were so designed that settled effluent could be added to the settled sewage immediately before it was applied to a filter, or settled sewage could be passed through two filters in series.
Recirculation Recirculation of effluent has been employed in Great Britain for many years in the treatment of industrial wastes: for example it was used in 1904 in the biological filtration of gas liquors and
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Liquid Industrial Wastes by the Water Pollution Research Laboratory in an investigation, between 1935 and 1938, of the filtration of waste waters from dairies and cheese factories. It has also been used from time to time in the treatment of domestic sewage and interest in the process for this purpose was greatly stimulated about
fq ,-I
.e
ai i Figure 3. Diagram of Arrangement of Plant for Treatment of Settled Sewage by Alternating Double Filtration
1936 by work in America with semiscale plant. In treating sewage at a comparatively low rate to yield an effluent of the high quality generally necessary in Great Britain, the limiting factors are usually the excessive growth and accumulation of biological film on the filtering medium. There seems to be no doubt that one of the chief benefits of recirculating effluent is to restrict this accumulation. The rate of treatment of the sewage can then be increased, possibly by 100 t o 300'%, without causing so great a deterioration in the quality of the final effluent as to make the process impracticable. Though operating costs are increased by the additional cost of pumping, and it will be necessary to install larger pipes, humus tanks, and other equipment, the total cost, in many cases, is less than the capital cost of providing the additional filtering capacity which would have been necessary without recirculation of effluent, and the process is therefore an economic one.
$tias 150 p.p.m. Excess sludge discharged from the bioflovculation plant 'ii as equivalent to the removal of 60 p.p.m. suspended solids from the settled sewage. Alternating Double Filtration The process of alternating double filtration was developed from observations made by the staff of the Birmingham Tame and Rea District Drainage Board some years ago and was first used on a large scale for the treatment of milk wastes (10). In this process, as applied to the treatment of sewage (14,ZS), the settled sewage (Figure 3) passes first through a filter, A , then through a humus tank, A , and then passes through a secoiid filter and humus tank B. At intervals, usually of 1 week or 1 day, the order is reversed and the liquid passes first through the filter and humus tank, B, and then through the filter and tank, A. This process again is designed t o limit the accumulation of film and sludge in the filters since the material accumulated in a filter when it is in the primary position is dispersed TI hen the direction of flow is reversed and the filter is treated nith partially purified liquid. Growth of Film. Some results, reported by Tomlinson (19) of determination of the comparative rates of accumulation of film ill filters operated by alternating double filtration and by the ordindr 3 process of single filtration are given in Table I. Sets of cylindrical vessels iTith perforated bottoms and containing medium similar to that in the filter were placed on the surface of a filter treating settled sewage a t a rate of 700,000 to 900,000 U. S. gallons per acre per day and on the two filters of an alternating double filtration plant treating similar sewage a t a rate of 1,900,000 U. S. gallons per acre per day in the two filters together. The order of these two filters in series was changed each week and, immediately before the change, the \\eight of film and sludgc associated n i t h the medium in one vessel from each filter wab determined. This experiment was made during a period ni the minter when the quantity of film in the filters was on the whole increasing. Average weights are given for each month between October and January, and the weights a t the end of each week are given for February and March. In the single filter the amount of film increased rapidly and smoothly from October to January and thereafter remained reasonably steady.
Table I. Weight of Film Associated with Medium in Filters Operated by Single Filtration and by Alternating Double Filtration (Rate of treatment of settled sewage: single filtration, 700,000-900,000 U S. gal./acre/day; alternating double filtration, 1,900,000 U. S. gal./acre/day)
October November December January February March
Biof loecnlation The process known in Great Britain as bioflocculation consists in treating settled sewage by the activated-sludge process with only a short period of aeration; the liquid is then usually passed through percolating filters, the object of the preliminary treatment being to increase the rate a t which the filters can be operated, I n one of the bioflocculation plants at Minworth settled sewage, in admixture with activated sludge, is aerated by diffused air for approximately 1 hour, the sludge being reconditioned by aeration in a separate tank for about 10 hours. During one period of experiment the settled sewage had an average B.O.D. of 190 p.p.m. and the corresponding figure for the efffuent from the bioflocculation tank, which was applied to the filters,
528
(1
b
20 120 360 540 580 560 540
100 140 220 270 310a 370 6 300a
520 . ~ .
3 m b
530 540 580 560
260a 280 b 240"
~~~
270 6
End of week of cycle during which filter received settled primary effluent. End of week of cycle during which filter received settled sexage.
I n the alternating double filters, the amount also increased but though these filters were more heavily loaded, the maximum weight of film never reached so high a value. During the week in which a filter of the alternating double filtration plant occupied the primary position the weight of film increased markedly, but there was a decrease during the week in which it occupied the secondary position; the net effect was to limit the total accumulation of film.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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-Liquid Table 11. Average Concentration of Suspended Solids in Effluents from Treatment of Settled Sewage by Single Filtration and by Alternating Double Filtration June1940- June1941- June 1942May1941 May1942 May1943 Av. concn., p.p.m. Single filtration Alternating double filtration Primary filter Secondary filter Maximum av. concn. in any 3 months,
Range of av. concn. as proportion of mean, % Single filtration Alternating double filtration Primary filter Secondary filter
64
72
72
40 73
37 66
64 53
125
64
65
38 33
16 41
45 41
Industrial Wustes-
rate at which sewage can be treated by the process. It is thought that the loading of the primary filter should not exceed a value a t which the B.O.D. of the settled primary effluent to be applied to the secondary filter exceeds about 50 p.p.m. Comparative Experiments. Comparative average results of treatment of Birmingham sewage over about 4 years by single filtration with and without recirculation of effluent and by alternating double filtration are given in Table 111. During this period the weight of B.O.D. applied, per cubic yard of medium per day, was considerably higher in the alternating double filtration plant than in the single filter, the ratio being at first about 2:1 and a$ the end of the'experiments about 4:l. The quality of the final effluent was about the same from the two processes; from neither did the B.O.D. of the settled final effluent greatly exceed 10 p.p.m, No more difficulty from ponding was encountered with the alternating double filtration plant than with the single filtration plant operated at the much lower rate. As compared with alternating double filtration, filtration with recirculation of settled effluent, in a ratio 1:1, was comparatively inefficient. Some more recent results are given in Table IV which refers to experiments in which alternating double filtration was compared with filtration with recirculation of effluent and with filtration of settled sewage previously treated in a bioflocculation plant. Of these three processes alternating double filtration was the most efficient. The limit to the loading permissible in treating settled sewage by alternating double filtration in Great Britain is set by the necessity of producing a final effluent of the high standard normally required and a primary effluent of sufficiently good quality to remove biological growths from the secondary filter. Experiments have been made, however, in an attempt to increase
With a percolating filter treating settled sewage at a low rate by the ordinary process, there is a marked seasonal fluctuation in the concentration of humus in the filter effluent. In Great Britain the most intense sloughing of biological film usually occurs during the spring but the exact time differs a t different works and is possibly correlated with the types of predominant metazoa in different filters. In the process of alternating double filtration the variation in rate of discharge of humus is much less marked (Table 11), no doubt because the periodic treatment of each filter with partially purified effluent causes sloughing during the whole of the year. The mechanism wherebv film is dispersed when treated with partially purified liquid from a primary percolating filter is at' present very little understood. It has been shown (19), Table 111. Comparative Average Results of Treatment of Settled Sewage from Birmingham (England), by Three Processes however, that one effect of the Mar. 1941- Mar. 1942- Mar. 1943- July 1944treatment is to cause marked Feb. 1942 Feb. 1943 May 1944 May 1946 changes in the condition of the Rate of Treatment, Million Gal./Acre/Day fungi, which are an important 1. Single filtration 0.91 0.79 .2. .. 6. constituent of the film; 2. Single filtration with recirculation of effluent (1 :1) .0...9. 3 2.4 2.8 3. Alternating double filtration 2.3 2.8 2.8 3.3 chlamydospores are produced and the protoplasm becomes B.O.D. Applied, Lb./Cu. Yd./Day concentrated in these and in 1. Single filtration 0.12 0.11 .... 2. Single filtration with recirculation of effluent (1 :1) .0.09 ., . 0.32 0.39 0 39 parts of the hyphae, which are 3. Alternating double filtration 0.23 0.36 0.40 0.49 separated by sections of hyphae B.O.D., P.P.M. devoid of protoplasm. These Settled sewage 116 153 163 174 empty sections are vulnerable Final effluent 1. Single filtration 11 12 10 t o bacterial attack and 2. Single filtration with recirculation of effluent (1 :1) . ,, 14 15 16 after a time a r e readily 3. Alternating double filtration 10 11 11 11 broken. I t is not at present known whether the action is due to a general deficiency in Table IV. Comparative Average Results of Treatment of Settled Sewage from nutrient substances in the priBirmingham (England), by Three Processes mary effluent or whether the Sept. 1946AprilAug. 1947Mar.Feb. 1947 Aug. 1947 Feb. 1948 Aug. 1948 primary treatment removes specific organic compounds Rate of Treatment, Million Gal./Acre/Day necessary for the growth of 1. Single filtration after bioflocculation 1.8 1.7 1.7 1.7 2. Single filtration with recirculation of effluent (1 : 1) 1.7 1.7 1.7 1.7 the fungi. It is clear, how3. Alternating double filtration 1.7 1.7 1.8 1.7 ever, that the process will be B.O.D. Applied, Lb./Cu. Yd./Day successful only where the de1 Single filtration af r bioflocculation 0.18 0.12 0.15 0.17 gree of treatment given in a 2: * Single filtration wi% recirculation of effluent (1 :1) 0.24 0.22 0.25 0.24 3. Alternating double filtration 0.24 0.22 0.25 0.24 primary filter is sufficient to reduce the concentration of B.O.D. of Liquids Applied to Filters, P.P.M. nutrient material in the settled Effluent from bioflocculation plant (1) 115 83 103 114 161 144 168 161 Settled sewage (2 and 3) sewage to a point a t which B.O.D. of Final Effluent, P.P.M. the primary effluent will cause 1. Slngle filtration after bioflocculation 26 21 17 21 d i s i n t e g r a t i o e rather than 2. Single filtration with recirculation of effluent (1 :1) 18 14 15 19 growth of the film on the sec3. Alternating double filtration 14 10 11 14 ondary filter. This limits the
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March 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
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-Liquid
Industrial Wastes-
Table V. Treatment of S e t t l e d Sewage by A l t e r n a t i n g Double F i l t r a t i o n in Circular Filters w i t h Distributors Revolving at Different R a t e s Loading: 1,700,000 to 2,200.000-gal./aore/d~~g;0 24 to 0.36 Ib. B.O.D./ cu. yd./day Rate of rotation of four-armed distributor: filters 4 and B, 1 rev. in 1 to 4 min.; filters C and D, 1 rev. in 8 niin. Dee. 1948Feb. 1950
Settle sewage Settle2 rimary effluent A and3B
B.O.D., P.P.M. 240
MarchJ u l y 1980
Aug.-Oct. 1950
191
lZ8
28
C and D
4s
!5
42
33
21
22 25
20 23
11
C and D
Settled secondary effluent A and B
Xitrite plus Xitrate (as pi), P.P.hI. Settled final effluent A and B 8 3 C and D 12 10
15
10
15
the efficiency of the process by changes in the method of operation. There would be a considerable saving if the humus tank, which follon-s the primary filter, could be omitted and experiments have been made over a long period both at Birmingham and a t Coventry to observe the effect of pumping primary effluent to the secondary filter without sedimentation. In both places omission of the primary humus tank caused a sinall but quite significant deterioration in the quality of the final effluent and in the condition of the two filters. Without the intermediate humus tank there was a noticeably greater tendency toward ponding, and it seems probable that omission of the intermediate stage of sedimentation would not usually be justifiable in large ~ c n l epractice. Another large scale experiment was made to ascertain whether it would be satisfactory t o discharge part of the settled primary effluent direct to a river, using only a portion of it to treat the secondary filter. The object of this was, of course, to reduce the cost of pumping to the secondary filter. In this experiment the method failed since the mixture of primary effluent and secondary effluent to be discharged often had a B.O.D. greater than 20 p.p.m. It is possible, however, that some modification of the method might be used. A variation would consist in having three filters, two primary and one secondary, each in turn becoming the secondary filter, to which only part of the primary effluent would be pumped. This system was in fact used in Great Britain during the war with some success for treating a mixture of domestic 867%age and waste waters from a factory handling trinitrotoluene and other explosives ( 2 2 ) . A feature of the sewerage systems of Great Britain is that niost of them are operated on the combined system, so that with increased rainfall the rate of flow increases and the strength of the sewage normally decreases. It is probable that if a works is altered to operate by alternating double filtration this system will be used only when the rate of flow of sen age does not greatly exceed the dry-weather flow; when the sewage contains much storm water it will be treated by single filtration. The object of this would, of course be to avoid excessive pumping and the provision of very large pipes and distributors. Frequency of Dosing. There are many works in Great Britain in which sewage is treated in rectangular filters with distributors which travel from one end to the other, driven as a rule by paddle wheels turned by the flow of sewage. With this system the period between the application of successive doses of sewage to any particular part of the bed is considerably longer than in a circular filter with a rotary distributor. Some experiments have been made a t Minworth to ascertain the performance of filters working as alternating double filters with a comparatively long interval between successive applications of liquid. The four circular
530
filters, previously dc ibed, were used but whereas in one pair the four-arm distributor revolved freely, driven by reaction jets, each of the other pair was fitted with a four-armed distributor driven by a geared electric motor. I n the first pair t,he distributor made one revolution in 1 to 4 minutes and in the other pair in 8 minutes. Up to the present there has heen no very great difference between the performance of the two pairs (Table V); longer intervals betTTeen successive dosiriga are to be tried. Applications in Full-scale Plants. It seems probable that the process of alternating double filtration will be quite extensively adopted in Great Brit’ain in the next few years, the designed loading being a t first about two to two and one half t’iines the value usually adopted with Ringle filtration. If the process proves to be satisfactory at this loading it will often be cheaper than single filtration, taking into account both capital costs and costs of operation. I t is already in m e a t Inan? dairies f o r treating milk wastes and appears to be cheaper than the other method commonly used-that i p , trestment a-ith a coagulant (usually aluminum sulfate) and passage through a single percolating filter. The large sewage-disposal works of the Birmingham Tame and Rea District Drainage Board at, Minworth, which contain about 42 acres of percolating filters. mainly of the rectangular type, are a t present being altered to operate by alternating double filtration (go), and plans w e in hand for the adoption of the process a t a number of other smaller works. A t one of these works experiments are in progress with pilot plant to ascertain the possibility of final trcatmcnt of the effluent by mechanical filtration through sand in backwashed rapid gravity filters.
Acknowledgment This paper is published by permission of the Departniciit’ of Scientific and Indust,rial Research, Great Britain
Lil.ernture Cited (1) 411en, L. A, Rlezard, K., and Wheatland, A. B., J . Ifvg.,46, IS4 (1948j . (2) Allen, L. B., Cairns, A., E d e n , G. E., TTheatland, ,I B., \T.oriiiwell, F., a n d Nurse, T. J., J . SOC.Chem. Ind., 67 70 (1948). ( 3 ) Anon., I n d . Finishing, 2, No. 18, 335 (1949). (4) Belcher, E. H., paper presented a t Annual Conference, Iiistitute of Sewage Purification (1951). (5) Blackburn, W.If., Tomlinson, T. G., and Summers, T. I I . , Gas Research Board Paper, GRB 63 (1951). (6) Brandon, T.W., I n t e r n . Sugar J . , 49, 98, 124 (1947). ( 7 ) Department of Scientific and Industrial Research, Lotidon, H.hZ. Stationery Office, Ann. Rept. Water Pollutioii Ilesearch Board, 1949, p. 36 (1950). (8) Ibid., Ann. R e p t . , 1950, p. 26 (1951). (9) Ibid., Tech. Paper 5 (1935). (10) Ibid.,Tech. P a p e r 8 (1941). (11) Ibid.,Tech. Paper 10 (1948). (12) Dickinson, D., J . Proc. Inst. Sewage Purif., 1949, Part 1 , 5 4 . (13) Eden, G. E , and Truesdale, G. A, J . Iron Steel Inst. ( L o d o n ) , 164, 281 (1950). (141 Mills, E V., J Proc. I n s t . Sewage Purif., 1945, P a r t 2 , 35 (15) Nicklin, T., Gas W o r l d , 128, 300 (1948). (16) Riniiilei, G. J . , I b f d . , 124, 332 (1946). (17) Southgate, B. A., Dairy l n d s . , 13, 233 (19481. (18) Southgate, B. 8.. ’“l?reatnient and disposal of industrial waste waters,” p 28:3 London. 1-I.M. Stationery Office, 1948. (19) Tomlinson, T. G., Ibid., 1941, 39. (20) Vokes, F. C., J . Roy. Sanit. Inst., 68, 69 (1948). (21) West Riding of Yorkshire Rivers Board, Rept. Trade Refuse (coal trade) in West Riding, Wakefield (1948). (22) Wilkinson, R., I n d . Chemist, 27, 9, 59 (1951). ( 2 3 ) Wishart, J. M., and Wilkinson, R., J . Proc. Inst. Sewage Purif., 1941, 15. RECEIVED for review September 19, 1951.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 44, No. 3
