In the case of the waste liquors from a tannery, the use of these chemicals may often be avoided if sufficient care be taken in regulating the proportion of the various liquids which are to be mixed together and run into the settling tank. As tanning matter combines with lime and dissolved hide-substance to form a heavy brown insoluble precipitate, it is clear that if care be taken to have rather more waste lime-liquor mixed with the waste tan-liquors than is necessary to throw all the tan out of solution, a very considerable amount of purification of the effluent will have taken place without any cost whatever to the tanner. Hence, if the proportion of waste lime is small in comparison to that of the tanning liquors, an extra addition of lime may be necessary in order to precipitate the tannin.
The precipitation- or settling-tanks are usually square or rectangular vessels or pits, the size of which varies with the quantity of liquid to be treated, but the depth of which rarely exceeds six feet. They may be divided into two classes—the “intermittent,” and the “continuous.” In the former class the tank is filled with the mixed waste liquids, taking care that such a sufficiency of lime is present that the mixture is faintly alkaline to phenolphthalein paper, and is then allowed to rest until the suspended matter has settled down to the bottom of the tank, when the clear, or almost clear upper liquid is drawn off, the remainder being the “sludge”; some means must also be employed to prevent the passage of scum and floating matters. In the case of the intermittent process it is advisable to have two tanks, one of which is being filled while the other one is settling or being emptied. With the continuous process the liquids are run into the tank in the proportions calculated to give a maximum amount of purification, as described above, but as they enter very slowly the undissolved matter soon settles, and consequently the liquid may be continuously run out at the further end of the tank. This plan, though it does not yield such good results in the hands of unskilled workmen, is yet useful in many cases, as only one tank is absolutely necessary. It is desirable that in running off the tanks, the effluent should be taken as near the surface as possible, by means of a hinged pipe attached to a float, or some equivalent device; and care is required, as the tank gets low, to avoid the escape of any of the sludge.
For continuous settling the tanks are usually long and somewhat shallow rectangular ponds, into which the previously well-mixed precipitating liquid flows through a wooden trough fixed across one end and as long as the breadth of the tank, and perforated with holes to allow the uniform and quiet influx of the liquid, which finally escapes by a similar trough crossing the opposite end of the tank. In front of the exit-trough a “scum board” must be placed, which is a simple plank dipping slightly below the surface of the liquid, so as to prevent any oil, scum or other floating matter from passing out of the tank along with the clear effluent. Whether the intermittent or continuous system is employed, the effluent should in most cases be afterwards passed through a bacterial filter-bed, or treated by land filtration before it is allowed to flow into a stream or river. Tannery effluents are usually received into sewers without further treatment than mixing and settling to remove solid matter, and many authorities are satisfied with the removal of merely such coarse suspended matters as might choke the sewers. Where continuous precipitation-tanks are used, they must be emptied at frequent intervals, and the sludge run on to cinder-filters, to part with most of its water. These filters are conveniently placed at a lower level than the settling tanks, and it is generally necessary to return the effluent from them for further precipitation and settling. Several types of continuous settling tank with upward flow have been devised by Mr. Candy and others, which are very suitable for use where space is limited; but otherwise less costly constructions are often sufficient. Apart from the question of obtaining an effluent sufficiently good to satisfy the sanitary authority, the treatment of the sludge is one of the greatest difficulties in the purification of effluents. It is usually very bulky, easily putrescible, and therefore difficult to dry; it is of little value for manure; and if allowed to remain long wet, its smell is very offensive.
It has been mentioned that in most cases the liquid, and in every case the sludge, must be freed from solid undissolved matter by filtration. This may take place through open filters or through filter-presses. The open filters generally consist of a pit with an exit at the bottom for the filtered liquid. This pit is filled with either stones and sand, with clinker, ashes or coke. Most tanners use clinker and ashes, as they do not cost anything; and the material should be so arranged that while the lowest layers are very coarse, the surface of the filter-bed should be of the finest material. As soon as this has become covered with so thick a layer of solid matter that the filtration proceeds too slowly, the top surface of the filter may be removed with a rake (taking care to remove as little of the ashes or sand as possible), and burnt, or dried and used as manure. In some cases, filter-presses are used which are composed of grooved or perforated plates with cloths between them through which the liquid is forced by pressure. The solid matter remains behind in the form of a comparatively dry “cake.” The filter-cake, dried if desired, is sold as manure, for which it is in many ways very suitable. Although they work much more rapidly than do the open filters, the cloths so soon become rotten and have to be replaced, that the open ash-filter is on the whole the most convenient for the tanner’s use. It will be readily understood that apparatus of this kind, though very efficient on a small scale, is quite out of the question when many thousand gallons of liquid have to be filtered daily, and so can only be effectively applied to “sludge.”
No system of chemical precipitation has as yet proved entirely satisfactory. Undoubtedly a great deal of purification is effected by this means, but in most cases the “purified” liquid is still too impure to be turned into a stream, though for various reasons this is often permitted by the authorities.
A great advance was made in the purification of effluents when manufacturers were compelled by law to allow the effluent from the precipitation-tank to filter through land set apart for that purpose. In this case certain hardy cereals were sown on the land, which was watered as often as possible with the effluent. This latter, after soaking through the land, was drained off into the nearest stream. Although in many ways this treatment was satisfactory, it had the disadvantage of being very expensive, especially in the neighbourhood of large towns where the price of land is high, and, in addition to this, the conditions necessary for success were far from being correctly understood, so that the land often became “sewage-sick” or waterlogged, and ceased to purify the effluent. It was not until the researches of bacteriologists proved that the purification by land-filtration was mainly due to the bacteria in the soil, that any really satisfactory solution of the problem could be found, but the question has now been to a considerable extent simplified by the introduction of “bacterial treatment.”
Bacteria, considered from the point of view of their action on organic matter, are often classified as “anaerobic” and “aerobic,” though many species are capable of existing under both conditions (Cp. L.I.L.B., Section XXIV.). The anaerobic bacteria thrive only in the absence of air, and their chemical action consists in breaking down the organic matter on which they feed into simpler, and generally more soluble forms, by processes which do not involve oxidation. The aerobic bacteria, on the other hand, require air or oxygen for their existence, and produce changes which are generally of a less complex character, but result in the complete oxidation and conversion of the organic matter to simple compounds, such as nitrates and carbonic acid, which are perfectly harmless and inoffensive. The two classes therefore are to a large extent complementary to each other, the anaerobic bacteria converting the animal or vegetable substances into more soluble and simple compounds which are adapted to the needs of the aerobic, which complete the destruction of the organic matter.
In harmony with what has just been said, bacterial treatment of sewage is of two kinds, each of which may be used alone, or in conjunction with a preliminary precipitation-process, but which are generally best used successively. The oldest form of bacterial purification depends mainly on the action of anaerobic bacteria, and is known as the “septic tank.” This originally consisted of a tank sometimes filled with small pieces of coke, but generally containing the liquid only, and which was tightly closed to prevent access of air and escape of foul gases. It has, however, been found that if deep tanks (6 to 10 feet) are employed, they soon become in continuous use so covered with scum and floating matter as effectually to prevent access of air and light, or any serious escape of smell. The liquid to be purified is allowed to flow very slowly through a tank or series of tanks of this description, entering about a foot below the surface through a distributing trough at one end, and flowing out similarly at the other, at such a rate as to change the contents of the tank about once in twenty-four hours; and when the tank is in working order, the liquid is much purified by the process, and most of the solid organic matter has become liquefied and disappears. It not unfrequently happens, especially where the septic tank treatment is not very prolonged, that the liquid which escapes has a stronger and more offensive odour than it had on entering the tank. It is nevertheless really purer than before, the increased smell being due to the volatile products of the partially decomposed organic matter; and, by passing the liquid through an open coke-filter, the smell will be effectually removed. In all cases it must be borne in mind that as septic tanks and bacterial filters depend for their efficiency on the organisms they contain, time must be allowed for these to develop and accumulate before good results are obtained; and for this about six weeks’ use is generally necessary, after which they will continue to act for an indefinite period until they become choked by sand and inorganic matter.
It must not be supposed that the action in the septic tank is wholly anaerobic; and with weak sewage, most of the organic matter may under favourable circumstances be converted into nitrates and carbonic acid by this means only; but generally a much more complete purification is effected by the subsequent use of “bacterial filters.” These in their simplest form consist of tanks of about 4 feet deep, filled with coke, broken bricks, or clinkers, and fitted with drain pipes at the bottom, by which they can be easily emptied. These tanks, often known as “contact-beds,” are filled with the sewage or septic tank effluent, which is allowed to remain on them two hours, and the tank is then emptied, and allowed a rest of six hours for oxidation and aeration. In most cases the sewage requires two such treatments, the last often through a bed with finer coke, in order to be completely freed from putrescible matter. In place of the intermittent process, as applied on the contact-beds, continuous aerobic filtration is often employed, the bed being so constructed as to allow of free admission of air at the bottom and sides, and the liquid to be purified being distributed on the surface by a sprinkler, or some similar device, and allowed to trickle through the bed. The continuous process seems likely to supersede the intermittent one, as the beds are not only capable of treating a much larger quantity of sewage in proportion to their area, but are also less liable to choke. About six weeks is required, with either contact-beds or continuous filters, before the material they contain becomes coated with the necessary bacterial layer and they get into full working order. The results as regards the effluent are perfectly satisfactory, and the great difficulty and cost consists in the slow but inevitable choking of the beds, which involves the replacement of the porous material. This is considerably delayed by the use of a settled or precipitated sewage, and in this respect, beside its bacteriological function, the septic tank serves a useful purpose in settling insoluble matter, which is much more cheaply removed from it than from the filter-beds. It will be obvious that ordinary settling-tanks, if deep, fulfil many of the functions of the septic-tank, and both lead to the production of a much more uniform liquid from the different effluents which the tanner produces, which is important in the subsequent bacterial purification. A good deal of interesting information on these subjects will be found in a paper by Mr. W. H. Harrison on the ‘Bacteriological Treatment of Sewage.’[188]
[188] Journ. Soc. Chem. Ind., 1900, p. 511.