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THE ACTIVATED SLUDGE PROCESS OF SEWAGE TREATMENT.
1149
Notes, Comments, and Abstracts. THE ACTIVATED
SLUDGE PROCESS OF SEWAGE TREATMENT. Abridged from the Chadwick Lecture delivered at Southampton on Oct. 25th, 1926, BY H. T. CALVERT, D.Sc., F.I.C.
AT an early date imperfect drains were laid with the object of keeping the neighbourhood of dwellings reasonably dry, but the solid waste products of human existence were allowed to accumulate near the dwellings, and it was only when a supply of water became available that the opportunity arose in the more thickly populated districts of swilling the
I
of contact beds and percolating filters the mechanism of the process is somewhat similar, except that the material of the beds and filters is very much larger in size than the soil particles, whilst in the case of the activated sludge process the particles of soil or other filtering material are entirely dispensed with and the gelatinous film or activated sludge is agitated with the sewage and air is either blown in or supplied at the surface of the agitated mixture of sewage and sludge. From what has been said it will have been gathered that activated sludge is produced from the organic matter of sewage and that it is an oxidation product of this organic matter, but the exact function of the oxygen of the air in the production of activated sludge is as yet undetermined. It may be that it fosters the growth of micro-organisms pre-existing in the sewage or even in the air ; or that it kills off another kind of micro-organism ; or again it may be that it destroys certain constituents of the organic matter or sludge which are inimical to its use for the purpose of purifying further quantities of sewage; or the action may be a mechanical one in developing an enormous surface area on the sludge, but experiments have shown that the addition of chloroform, toluol, and other substances which kill micro-organisms, to a fully activated sludge impair or destroy its activity. We are therefore justified in assuming that activated sludge owes its activity, in great part at any rate, to the micro-organisms which it contains. [Dr. Calvert described various methods of applying the activated sludge process to the treatment of sewage, notably those used in this country at Worcester, Manchester, Bury, Birmingham, and Sheffield respectively. He continued :-]
refuse into the drains and thus into the nearest watercourse. This was the first step in rendering the dwellings more cleanly, but the resulting evils of river pollution made it necessary that further steps should be taken. At first there was little or no opposition to this growing pollution, and the general impression seemed to be that the use of rivers was to carry away sewage and trade refuse, but the burden which they were called upon to bear was beyond the power In spite of official inquiries of their purifying ability. matters were allowed to drift from bad to worse, but in 1876, as a result of a Royal Commission, the Rivers Pollution Prevention Act was passed, though it was not until county councils were formed in 1888 that any real progress was Certain Difficulties. made and the necessity for treating sewage before discharge Before the process is applied to any sewage, or indeed into rivers and streams was brought home to local sanitary before any kind of sewage treatment works are constructed, authorities. it is of prime importance that full information should be The Principle of Activation. All artificial biological methods of sewage treatment- obtained as to the volume and nature of the sewage which whether by means of contact beds, percolating filters, or may be expected to reach the works. Included in the question of volume of sewage is the activated sludge-are really developments of the " intermittent downward filtration through land " process of question of storm water, but the indications of existing that if the final settling tanks are of sufficient Frankland, and when once the principles underlying that plants arethe plants are able to deal with three times the process have been grasped the mechanism of these newer capacity weather flow for periods which are generally likely to Oxygen is the purifying dry processes is easily understood. occur in times of storm. This, of course, presupposes that agent and the air furnishes a practically unlimited supply of the first rush of storm water does not wash out of the sewers this gas. If oxygen is brought in contact with organic matter at the ordinary temperature oxidation proceeds only too much solid matter which has been deposited in the during dry weather and become septic, and also slowly, but at higher temperatures the same process takes sewers the flow of storm water is not so prolonged as to give place more rapidly. It is then termed combustion, and this that " is the process which takes place in refuse incinerators. It rise to the phenomenon which has been termed " bulking of the sludge. is obviously impracticable to evaporate sewage and to This phenomenon, about which little definite information oxidise the remaining organic matter in a refuse incinerator, and hence in sewage purification we have to aim at oxidising is available, is due to a condition which activated sludge the organic matter by introducing oxygen in the concentrated assumes under certain unknown conditions. At times it is found that the sludge, in a comparatively short space of or active form in which it occurs in various chemicals (such time, say a few hours, swells up and occupies a much larger as permanganates) or by having present along with the volume of the tank space than normally. If the normal oxygen some other substance which hurries up the process of oxidation. The former method proves more expensive amount of sludge is, say, 20 per cent. (measured after than the latter, which is the essential feature of land settlement for one hour) it is sometimes found that the treatment and artificial biological methods. The other sludge, whilst containing no more dry solid matter, will substance which hurries up the process of oxidation is a occupy a volume twice as great or even greater. Under it is often difficult to maintain the biological one, either bacteria and other micro-organisms or such circumstances requisite amount of sludge in the aerating or agitating tank even larger living organisms, but the actual mechanism of the change is not yet thoroughly understood. It would and to prevent it escaping with the effluent from the final lead us too far from our subject to discrss the various settling tanks. Included in the question of the nature of the sewage we theories which have been formulated to explain the biological oxidation of sewage, but it may serve a useful purpose have not only the variations in strength due to varying to cite the analogous process of manufacturing vinegar from water consumption and to variations due to the class of wine. Wine (for this purpose a weak solution of alcohol) the population tributary to the sewers, but also the question is allowed to trickle over shavings which have been inoculated of any trade refuse which may be admitted to the sewers. with a specific bacterium (Mycoderma aceti), and in doing Here our knowledge is not by any means definite, but the indications are that with domestic sewage the application so the alcohol in the wine is oxidised to acetic acid ; the wine is converted into vinegar. Vinegar is an oxidation of the process presents little difficulty. With trade refuse, matter is otherwise, and difficulty has been product of wine, just as carbon dioxide and nitrates are however, thewith such classes of refuse as spent gas liquor, oxidation products of the organic matter in sewage. The oil and tar, and refuse from the manufacture analogy serves to suggest that the future may teach us to I ofbrewery waste, The dyestuffs. process is able to deal with certain recognise, to cultivate, and to use specific organisms for proportions of these classes of refuse in the sewage, but it the better oxidation of the organic matter of sewage. Apart from all theories we are on safe ground if we say would appear that when once the sludge has acclimatised that the presence of organisms enables the oxygen of the itself to those proportions any very material variation either by depriving air to oxidise the organic matter in sewage very rapidly, up or down is likely to cause difficulty, either " and it remains to consider what are the best methods of the sludge of its activity or by giving rise to bulking." These difficulties can be counteracted to some extent by bringing the three reacting substances-the organic matter of the sewage, the oxygen of the air, and the organisms- the preliminary treatment to which the sewage is subjected into as intimate contact as possible. In land filtration each before it enters the activated sludge tanks. Such preliminary particle of the soil becomes coated with a gelatinous film treatment should always include screening and the removal of derived from the organic matter of the sewage. This film grit in detritus tanks. The final settling tanks, following the aeration, agitation, forms a nidus for organisms and at the same time such an extensive surface that it attracts more organic matter out or activated sludge tanks, serve the purpose of separating of the sewage and a partially purified liquid passes on to the activated sludge from the purified sewage and concenthe next particle of soil, where the process is carried a stage trating the former for return into the process tanks or for further. Following the sewage comes a supply of air, and disposal of the surplus, for more is always produced than is we thus have the three reacting substances together. required in the process tanks. The form of construction Oxidation takes place and the gelatinous film on the soil which has been almost always adopted for these tanks is that particle is ready for another dose of sewage. In the case of the Dortmund or hopper-bottomed tank.
I experienced
1150 The volume of sludge which is returned from these tanks into the process tanks is regulated according to the amount which it is desired to maintain in the process ; 15 or 20 per cent. (measured after settlement for one hour) may be regarded as an average amount. It is difficult to give figures relating to the cost of the process, the power consumption, and the size of plant which may be regarded as the smallest economical unit, since local circumstances vary so enormouslv. The capital constructional costs of a plant to deal with a daily dry-weather flow of a million gallons of average domestic sewage, and with three times this volume in wet weather, would be under average conditions, approximately £30,000, and the annual working costs from £1200 to £1400. The power required has been found to vary from 15 to 30 h.p. per million gallons, but the price of electrical energy varies at the several plants from d. to 4d. per Board of Trade unit. Skilled supervision is necessary where the process is installed, and it is not usual, though it might be advantageous, to provide such supervision at small works where the daily dry-weather flow is less than, say, 500,000 gallons. A Modified Process. The methods of application of the activated sludge process aim at what is termed the complete purification of the sewage-that is, the production of an effluent which is nonputrescible and which is not likely to give rise to a nuisance when discharged even into the smallest watercourse. We will now turn our attention for a moment to a modification of the process which has been worked out at Birmingham and which promises a wider applicability. At Birmingham the basic idea is to stop the purification process at an early stage, separate the partially purified liquid from the partially deactivated sludge, and reactivate the sludge separately for re-use in the process. The partially purified liquid is then passed on to existing percolating filters at a rate of two or three times the ordinary rate of feed. The short period of agitation enables the plant to be constructed correspondingly smaller, but the reactivation or reaeration of the sludge in the concentrated form is a more difficult matter than in the dilute form, so that the size of plant necessary for this part of the process is not reduced strictly in proportion to the concentration of the sludge. A large-scale plant to deal with 21; million gallons per day has been operated on these lines for some time at Birmingham, and the size of the plant is now being trebled. Local conditions make the process peculiarly applicable. The settlement tanks are situated some four or five miles higher up the valley than the percolating filters, and in traversing the carrier between the two portions of the works the tank liquor becomes septic and is liable to give rise to nuisance from smell when it is sprayed on to the percolating filters. The partial treatment, or bio-flocculation, as it has been termed, for it consists essentially in a flocculation of the colloidal organic matter in the sewage, deprives the sewage of its septic character, so that no nuisance from smell is caused when the sewage is sprayed on to the filters, and owing to the high rate of feed of the filters the normal extension of filter area to meet the needs of increasing population is avoided. It is also probable that with this partially purified liquor the question of fly nuisance on the percolating filters will be lessened. The process is a peculiarly inviting one, for there are few large towns in a position to afford absolutely new sewage disposal works, and most of the activated sludge plants installed in this country have been adjuncts to existing works which required extension. In most cases they deal with a portion of the whole flow of sewage and not with the whole flow and its daily variations. The process in use at Birmingham affords an opportunity of extending existing works, especially where percolating filters exist, without separating the works into two parts and without constructing further expensive percolating filters. The Nature of Activated Sludge. When seen in the aeration tanks activated sludge has a brown flocculent particulate appearance, and the experienced eye can judge its activity, knowing that a greyness and ultimately a blackness comes over it when it loses its activity. Under the microscope great differences are seen in activated sludge produced at different places. From its mode of formation activated sludge contains the whole of the mineral matter present in the settleable and colloidal solids of the sewage, and such portions of the organic matter as have not been oxidised to gaseous or soluble forms. The particles of sludge are heterogeneous masses which have a tremendous surface per unit of weight, and they teem with life. Many forms of bacteria and other organisms have been recognised in the sludge, but up to the present our knowledge of these forms of life and their functions is very limited. It would appear that the phenomenon of bulking," to which reference has been made, is associated with the preponderance of protozoal forms. Activated sludge has a higher percentage nitrogen content than ordinary sewage sludge, and percentages of "
5
or
7, measured
on
the
absolutely dried sludge,
are
not
uncommon.
Experiments have demonstrated that activated sludge a higher manurial value than ordinary sewage sludge and that its nitrogen is as available for plant life as the nitrogen of sulphate of ammonia. For horticultural work and the dressing of golf greens and in market gardens the activated sludge produced at Milwaukee and Chicago has been dried and put up in 100 lb. bags and has found a ready sale. The problem of using activated sludge as a fertiliser has not yet been tackled seriously in this country, and at the various works where it is produced it has been dealt with much on the lines in which ordinary sewage sludge has been dealt with-by shallow ploughing into the land, by lagooning, or by drying on special drying beds and then spreading on the land. At most works it is mixed with the ordinary sewage sludge produced at another part of the works, but in no case has a preliminary dewatering in presses or filters with subsequent drying by heat in mechanical driers proved successful. At Milwaukee and other places in America very large plants have been constructed to deal with the sludge on these lines, but whether the success which is reported from these plants is due to some difference in the American sludges or to a want of vision on our part is not clear. There is still a chasm of ignorance concerning the activated sludge process of sewage treatment. Much more research will have to be done before we are in a position to define the future possibilities of the process, but there is every reason to expect that these possibilities will be large. possesses
OPHTHALMOLOGY IN EGYPT. MODERN Egyptian ophthalmology, which owes its inception to the late Sir Ernest Cassel, who endowed the ophthalmic hospitals, and to Mr. A. F. MacCaIlan, who during his twenty years’ residence in the country organised their work, continues to flourish. This year’s report of the Ophthalmological Society of Egypt records an increase of three permanent and three travelling hospitals during the years 1924 and 1925, the number of new patients treated in the latter year having increased to 236,903. The main cause of blindness, as in all Eastern countries, is trachoma, but it is encouraging to read that whereas in 1919 the percentage of blindness in one or both eyes among the patients was 15-3, this percentage has been gradually reduced until in 1925 it was 10’4. There is an entertaining paper in the current Bulletin of the Society by Dr. Max Meyerhoff entitled New Light on the Early Period of Arabic Medical and Ophthalmological Science. It contains photographs of anatomical sketches from an Arabic manuscript of the year 1003 A.D., now preserved in a private library at Cairo. One of the diagrams shows the external ocular muscles, and another the decussation of the optic nerves at the chiasma. DENTAL CARE OF MOTHERS AND YOUNG CHILDREN. AT the quarterly meeting of the National Baby Week Council, held at Carnegie House on Nov. 17th, a discussion on Dental Care of Expectant and Nursing Mothers and Children under Five was opened by Dr. Helen P. Dent, of Leicester. She said that anaemia, digestive troubles, and debility followed an unhealthy mouth, and it was quite possible that pyorrhoea might have a share in the production of puerperal sepsis. There was much ignorance and prejudice amongst mothers in regard to the need and the safety of dental treatment during pregnancy. The provision of dental treatment for mothers should always be part of the local authority’s maternity and child welfare scheme, and it should be available at least for all those attending the welfare centres, whether they were expectant or nursing at the time or not.-Dr. Harry Campbell said that caries was absolutely preventable, and pyorrhcea largely so; if teeth were kept perfectly clean they would never decay. The food given to young children was often not of a kind to develop mastication ; the greater part of the right sort of food belonged to the vegetable kingdom, and a considerable proportion of farinaceous food which compelled grinding should be provided.-Dr. Vincent Denne emphasised the importance of the hereditary factor in connection with teeth. Many women suffered from malnutrition owing to, diseased teeth. Toxins from the teeth were absorbed, and’ the infant was not only badly nourished but poisoned. A number of women were unable to suckle their infants ;y they could not elaborate the ingredients of breast-milk because they were damaged by bacterial infection. There was no reason why fillings should not be carried out at any time during pregnancy and extractions until quite an advanced stage, if proper precautions were taken. A CHEQUE for 100 guineas has been sent to the New Health Society by a firm of cigarette manufacturers as an expression of regret for having inadvertently included in an advertisement a photograph of Sir W. Arbuthnot Lane.
Notes, Comments, and Abstracts. THE ACTIVATED
SLUDGE PROCESS OF SEWAGE TREATMENT. Abridged from the Chadwick Lecture delivered at Southampton on Oct. 25th, 1926, BY H. T. CALVERT, D.Sc., F.I.C.
AT an early date imperfect drains were laid with the object of keeping the neighbourhood of dwellings reasonably dry, but the solid waste products of human existence were allowed to accumulate near the dwellings, and it was only when a supply of water became available that the opportunity arose in the more thickly populated districts of swilling the
I
of contact beds and percolating filters the mechanism of the process is somewhat similar, except that the material of the beds and filters is very much larger in size than the soil particles, whilst in the case of the activated sludge process the particles of soil or other filtering material are entirely dispensed with and the gelatinous film or activated sludge is agitated with the sewage and air is either blown in or supplied at the surface of the agitated mixture of sewage and sludge. From what has been said it will have been gathered that activated sludge is produced from the organic matter of sewage and that it is an oxidation product of this organic matter, but the exact function of the oxygen of the air in the production of activated sludge is as yet undetermined. It may be that it fosters the growth of micro-organisms pre-existing in the sewage or even in the air ; or that it kills off another kind of micro-organism ; or again it may be that it destroys certain constituents of the organic matter or sludge which are inimical to its use for the purpose of purifying further quantities of sewage; or the action may be a mechanical one in developing an enormous surface area on the sludge, but experiments have shown that the addition of chloroform, toluol, and other substances which kill micro-organisms, to a fully activated sludge impair or destroy its activity. We are therefore justified in assuming that activated sludge owes its activity, in great part at any rate, to the micro-organisms which it contains. [Dr. Calvert described various methods of applying the activated sludge process to the treatment of sewage, notably those used in this country at Worcester, Manchester, Bury, Birmingham, and Sheffield respectively. He continued :-]
refuse into the drains and thus into the nearest watercourse. This was the first step in rendering the dwellings more cleanly, but the resulting evils of river pollution made it necessary that further steps should be taken. At first there was little or no opposition to this growing pollution, and the general impression seemed to be that the use of rivers was to carry away sewage and trade refuse, but the burden which they were called upon to bear was beyond the power In spite of official inquiries of their purifying ability. matters were allowed to drift from bad to worse, but in 1876, as a result of a Royal Commission, the Rivers Pollution Prevention Act was passed, though it was not until county councils were formed in 1888 that any real progress was Certain Difficulties. made and the necessity for treating sewage before discharge Before the process is applied to any sewage, or indeed into rivers and streams was brought home to local sanitary before any kind of sewage treatment works are constructed, authorities. it is of prime importance that full information should be The Principle of Activation. All artificial biological methods of sewage treatment- obtained as to the volume and nature of the sewage which whether by means of contact beds, percolating filters, or may be expected to reach the works. Included in the question of volume of sewage is the activated sludge-are really developments of the " intermittent downward filtration through land " process of question of storm water, but the indications of existing that if the final settling tanks are of sufficient Frankland, and when once the principles underlying that plants arethe plants are able to deal with three times the process have been grasped the mechanism of these newer capacity weather flow for periods which are generally likely to Oxygen is the purifying dry processes is easily understood. occur in times of storm. This, of course, presupposes that agent and the air furnishes a practically unlimited supply of the first rush of storm water does not wash out of the sewers this gas. If oxygen is brought in contact with organic matter at the ordinary temperature oxidation proceeds only too much solid matter which has been deposited in the during dry weather and become septic, and also slowly, but at higher temperatures the same process takes sewers the flow of storm water is not so prolonged as to give place more rapidly. It is then termed combustion, and this that " is the process which takes place in refuse incinerators. It rise to the phenomenon which has been termed " bulking of the sludge. is obviously impracticable to evaporate sewage and to This phenomenon, about which little definite information oxidise the remaining organic matter in a refuse incinerator, and hence in sewage purification we have to aim at oxidising is available, is due to a condition which activated sludge the organic matter by introducing oxygen in the concentrated assumes under certain unknown conditions. At times it is found that the sludge, in a comparatively short space of or active form in which it occurs in various chemicals (such time, say a few hours, swells up and occupies a much larger as permanganates) or by having present along with the volume of the tank space than normally. If the normal oxygen some other substance which hurries up the process of oxidation. The former method proves more expensive amount of sludge is, say, 20 per cent. (measured after than the latter, which is the essential feature of land settlement for one hour) it is sometimes found that the treatment and artificial biological methods. The other sludge, whilst containing no more dry solid matter, will substance which hurries up the process of oxidation is a occupy a volume twice as great or even greater. Under it is often difficult to maintain the biological one, either bacteria and other micro-organisms or such circumstances requisite amount of sludge in the aerating or agitating tank even larger living organisms, but the actual mechanism of the change is not yet thoroughly understood. It would and to prevent it escaping with the effluent from the final lead us too far from our subject to discrss the various settling tanks. Included in the question of the nature of the sewage we theories which have been formulated to explain the biological oxidation of sewage, but it may serve a useful purpose have not only the variations in strength due to varying to cite the analogous process of manufacturing vinegar from water consumption and to variations due to the class of wine. Wine (for this purpose a weak solution of alcohol) the population tributary to the sewers, but also the question is allowed to trickle over shavings which have been inoculated of any trade refuse which may be admitted to the sewers. with a specific bacterium (Mycoderma aceti), and in doing Here our knowledge is not by any means definite, but the indications are that with domestic sewage the application so the alcohol in the wine is oxidised to acetic acid ; the wine is converted into vinegar. Vinegar is an oxidation of the process presents little difficulty. With trade refuse, matter is otherwise, and difficulty has been product of wine, just as carbon dioxide and nitrates are however, thewith such classes of refuse as spent gas liquor, oxidation products of the organic matter in sewage. The oil and tar, and refuse from the manufacture analogy serves to suggest that the future may teach us to I ofbrewery waste, The dyestuffs. process is able to deal with certain recognise, to cultivate, and to use specific organisms for proportions of these classes of refuse in the sewage, but it the better oxidation of the organic matter of sewage. Apart from all theories we are on safe ground if we say would appear that when once the sludge has acclimatised that the presence of organisms enables the oxygen of the itself to those proportions any very material variation either by depriving air to oxidise the organic matter in sewage very rapidly, up or down is likely to cause difficulty, either " and it remains to consider what are the best methods of the sludge of its activity or by giving rise to bulking." These difficulties can be counteracted to some extent by bringing the three reacting substances-the organic matter of the sewage, the oxygen of the air, and the organisms- the preliminary treatment to which the sewage is subjected into as intimate contact as possible. In land filtration each before it enters the activated sludge tanks. Such preliminary particle of the soil becomes coated with a gelatinous film treatment should always include screening and the removal of derived from the organic matter of the sewage. This film grit in detritus tanks. The final settling tanks, following the aeration, agitation, forms a nidus for organisms and at the same time such an extensive surface that it attracts more organic matter out or activated sludge tanks, serve the purpose of separating of the sewage and a partially purified liquid passes on to the activated sludge from the purified sewage and concenthe next particle of soil, where the process is carried a stage trating the former for return into the process tanks or for further. Following the sewage comes a supply of air, and disposal of the surplus, for more is always produced than is we thus have the three reacting substances together. required in the process tanks. The form of construction Oxidation takes place and the gelatinous film on the soil which has been almost always adopted for these tanks is that particle is ready for another dose of sewage. In the case of the Dortmund or hopper-bottomed tank.
I experienced
1150 The volume of sludge which is returned from these tanks into the process tanks is regulated according to the amount which it is desired to maintain in the process ; 15 or 20 per cent. (measured after settlement for one hour) may be regarded as an average amount. It is difficult to give figures relating to the cost of the process, the power consumption, and the size of plant which may be regarded as the smallest economical unit, since local circumstances vary so enormouslv. The capital constructional costs of a plant to deal with a daily dry-weather flow of a million gallons of average domestic sewage, and with three times this volume in wet weather, would be under average conditions, approximately £30,000, and the annual working costs from £1200 to £1400. The power required has been found to vary from 15 to 30 h.p. per million gallons, but the price of electrical energy varies at the several plants from d. to 4d. per Board of Trade unit. Skilled supervision is necessary where the process is installed, and it is not usual, though it might be advantageous, to provide such supervision at small works where the daily dry-weather flow is less than, say, 500,000 gallons. A Modified Process. The methods of application of the activated sludge process aim at what is termed the complete purification of the sewage-that is, the production of an effluent which is nonputrescible and which is not likely to give rise to a nuisance when discharged even into the smallest watercourse. We will now turn our attention for a moment to a modification of the process which has been worked out at Birmingham and which promises a wider applicability. At Birmingham the basic idea is to stop the purification process at an early stage, separate the partially purified liquid from the partially deactivated sludge, and reactivate the sludge separately for re-use in the process. The partially purified liquid is then passed on to existing percolating filters at a rate of two or three times the ordinary rate of feed. The short period of agitation enables the plant to be constructed correspondingly smaller, but the reactivation or reaeration of the sludge in the concentrated form is a more difficult matter than in the dilute form, so that the size of plant necessary for this part of the process is not reduced strictly in proportion to the concentration of the sludge. A large-scale plant to deal with 21; million gallons per day has been operated on these lines for some time at Birmingham, and the size of the plant is now being trebled. Local conditions make the process peculiarly applicable. The settlement tanks are situated some four or five miles higher up the valley than the percolating filters, and in traversing the carrier between the two portions of the works the tank liquor becomes septic and is liable to give rise to nuisance from smell when it is sprayed on to the percolating filters. The partial treatment, or bio-flocculation, as it has been termed, for it consists essentially in a flocculation of the colloidal organic matter in the sewage, deprives the sewage of its septic character, so that no nuisance from smell is caused when the sewage is sprayed on to the filters, and owing to the high rate of feed of the filters the normal extension of filter area to meet the needs of increasing population is avoided. It is also probable that with this partially purified liquor the question of fly nuisance on the percolating filters will be lessened. The process is a peculiarly inviting one, for there are few large towns in a position to afford absolutely new sewage disposal works, and most of the activated sludge plants installed in this country have been adjuncts to existing works which required extension. In most cases they deal with a portion of the whole flow of sewage and not with the whole flow and its daily variations. The process in use at Birmingham affords an opportunity of extending existing works, especially where percolating filters exist, without separating the works into two parts and without constructing further expensive percolating filters. The Nature of Activated Sludge. When seen in the aeration tanks activated sludge has a brown flocculent particulate appearance, and the experienced eye can judge its activity, knowing that a greyness and ultimately a blackness comes over it when it loses its activity. Under the microscope great differences are seen in activated sludge produced at different places. From its mode of formation activated sludge contains the whole of the mineral matter present in the settleable and colloidal solids of the sewage, and such portions of the organic matter as have not been oxidised to gaseous or soluble forms. The particles of sludge are heterogeneous masses which have a tremendous surface per unit of weight, and they teem with life. Many forms of bacteria and other organisms have been recognised in the sludge, but up to the present our knowledge of these forms of life and their functions is very limited. It would appear that the phenomenon of bulking," to which reference has been made, is associated with the preponderance of protozoal forms. Activated sludge has a higher percentage nitrogen content than ordinary sewage sludge, and percentages of "
5
or
7, measured
on
the
absolutely dried sludge,
are
not
uncommon.
Experiments have demonstrated that activated sludge a higher manurial value than ordinary sewage sludge and that its nitrogen is as available for plant life as the nitrogen of sulphate of ammonia. For horticultural work and the dressing of golf greens and in market gardens the activated sludge produced at Milwaukee and Chicago has been dried and put up in 100 lb. bags and has found a ready sale. The problem of using activated sludge as a fertiliser has not yet been tackled seriously in this country, and at the various works where it is produced it has been dealt with much on the lines in which ordinary sewage sludge has been dealt with-by shallow ploughing into the land, by lagooning, or by drying on special drying beds and then spreading on the land. At most works it is mixed with the ordinary sewage sludge produced at another part of the works, but in no case has a preliminary dewatering in presses or filters with subsequent drying by heat in mechanical driers proved successful. At Milwaukee and other places in America very large plants have been constructed to deal with the sludge on these lines, but whether the success which is reported from these plants is due to some difference in the American sludges or to a want of vision on our part is not clear. There is still a chasm of ignorance concerning the activated sludge process of sewage treatment. Much more research will have to be done before we are in a position to define the future possibilities of the process, but there is every reason to expect that these possibilities will be large. possesses
OPHTHALMOLOGY IN EGYPT. MODERN Egyptian ophthalmology, which owes its inception to the late Sir Ernest Cassel, who endowed the ophthalmic hospitals, and to Mr. A. F. MacCaIlan, who during his twenty years’ residence in the country organised their work, continues to flourish. This year’s report of the Ophthalmological Society of Egypt records an increase of three permanent and three travelling hospitals during the years 1924 and 1925, the number of new patients treated in the latter year having increased to 236,903. The main cause of blindness, as in all Eastern countries, is trachoma, but it is encouraging to read that whereas in 1919 the percentage of blindness in one or both eyes among the patients was 15-3, this percentage has been gradually reduced until in 1925 it was 10’4. There is an entertaining paper in the current Bulletin of the Society by Dr. Max Meyerhoff entitled New Light on the Early Period of Arabic Medical and Ophthalmological Science. It contains photographs of anatomical sketches from an Arabic manuscript of the year 1003 A.D., now preserved in a private library at Cairo. One of the diagrams shows the external ocular muscles, and another the decussation of the optic nerves at the chiasma. DENTAL CARE OF MOTHERS AND YOUNG CHILDREN. AT the quarterly meeting of the National Baby Week Council, held at Carnegie House on Nov. 17th, a discussion on Dental Care of Expectant and Nursing Mothers and Children under Five was opened by Dr. Helen P. Dent, of Leicester. She said that anaemia, digestive troubles, and debility followed an unhealthy mouth, and it was quite possible that pyorrhoea might have a share in the production of puerperal sepsis. There was much ignorance and prejudice amongst mothers in regard to the need and the safety of dental treatment during pregnancy. The provision of dental treatment for mothers should always be part of the local authority’s maternity and child welfare scheme, and it should be available at least for all those attending the welfare centres, whether they were expectant or nursing at the time or not.-Dr. Harry Campbell said that caries was absolutely preventable, and pyorrhcea largely so; if teeth were kept perfectly clean they would never decay. The food given to young children was often not of a kind to develop mastication ; the greater part of the right sort of food belonged to the vegetable kingdom, and a considerable proportion of farinaceous food which compelled grinding should be provided.-Dr. Vincent Denne emphasised the importance of the hereditary factor in connection with teeth. Many women suffered from malnutrition owing to, diseased teeth. Toxins from the teeth were absorbed, and’ the infant was not only badly nourished but poisoned. A number of women were unable to suckle their infants ;y they could not elaborate the ingredients of breast-milk because they were damaged by bacterial infection. There was no reason why fillings should not be carried out at any time during pregnancy and extractions until quite an advanced stage, if proper precautions were taken. A CHEQUE for 100 guineas has been sent to the New Health Society by a firm of cigarette manufacturers as an expression of regret for having inadvertently included in an advertisement a photograph of Sir W. Arbuthnot Lane.