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TREATMENT OF SEWAGE.
By J. C. S. BEYNON, A.M.I.C.E.
Since writing my paper on the Septic Tank System for the Treatment of Sewage before your Society in 1904, a great advancement has been made in its adoption all over the world, and many towns in England which were in the depth of despair as to the disposal of their sewage have been enabled to get over the difficulty and to produce a final effluent so pure that it can be discharged direct into the river, and, in many cases, samples of the river water taken below the discharge of the sewage effluent have on analysis been found to be much purer than samples taken immediately above, thereby proving that instead of having a detrimental effect as heretofore, it is highly beneficial.
In this country, although slow in being generally adopted, it is gradually being accepted as the means for the disposal of sewage, but then, of course, conditions are altogether different, as here the pail system is in vogue, and to alter to the more sanitary water-borne drainage entails a large expenditure in the initial cost, but effecting a great saving in the annual charges, which is an ever-increasing
On an improvement of the financial conditions of the country, there is no doubt that a much greater advancement will be made in the proper sewering of the towns, as with the increase of population the pail system, with its accompanying disposal of slop water, the annual working costs will make it prohibitive.
For towns in this country, where land is obtainable, and which is generally the case, the better plan is to merely liquefy the solids in Septic Tanks and then irrigate the land, as water, being so valuable a commodity, it certainly should be made use of and not allowed to run away into the nearest spruit. On the other hand, if land is not procurable or suitable for irrigation, then artificial filtration is necessary for further purification. This can be done on a small area of ground by filter beds, the final object in either case being the same, namely, the oxidation of the organic constituents, and their conversion into stable products incapable of further putrefaction.
In treating the sewage bacterially, the general method is to run it into tanks, the capacity of which is generally made to hold 24 hours' supply. Latterly the view is held that this can be considerably reduced, but I think it entirely depends on the strength of the sewage; if it is in a crude state and not broken down by passing through any great length of sewers, then the full time is required ; on the other hand, if it is kept too long over putrefaction takes place, which means offensive odours. There is no need to cover these tanks unless in close proximity to dwellings, as after a short time of working a scum forms on top, which prevents any smell arising. The gases generated are carbon dioxide, hydrogen, nitrogen, and marsh gas, the latter being as much as 75 per cent. ; this gas is inodourous, and burns freely. Submerged walls and dipping slabs are constructed in the tanks, so that the course of the fluid is alternately ascending and descending.
With domestic sewage these tanks will often go for many years without any sludge requiring removal, but with town sewage, in which foreign elements enter, sludge has to be dealt with, and it is then better to build the tanks deeper, and with a conical bottom, so that all the deposit drops towards the centre. This is then discharged through a pipe at the bottom, or pumped up through a chain pump ; by this means the working of the tank is not impeded or delayed. The sludge itself can be run off into shallow trenches, the water soaking into the ground. The sun dries the remainder into a friable cake, which makes excellent top-dressing, and there is scarcely any smell with it.
After the solid matter has been liquefied in the tanks through the agency of bacteria, the final treatment of the sewage is the oxidation of the ammonia and organic compounds, the oxidation of the ammonia into nitric and organic acids, which combine with lime. soda, and other bases in the sewage to form nitrites and nitrates of the latter. This is called nitrification, and the changes are effected by ærobic bacteria. This change can be effected by applying the effluent to land (known generally as broad irrigation or intermittent, downward filtration), contact beds or percolating filters.
In contact beds the effluent from the Septic Tank is collected in tanks in which the filtering material is contained. The bed is allowed to fill, and then a period of rest is allowed when filled. It is then discharged and the effluent is allowed to flow away, or a second filtration given if further purification is required. A set of four tanks is constructed, so that when one tank is filling, another resting full, and a further one is being discharged and aerating, and a spare one is also provided, so that each filter in turn can be cut out for two or three days' complete rest, so a complete cycle is formed. The action of the filter is twofold: (1) it collects all gross particles of suspended matter on the filtering material, (2) it effects the oxidation of organic matters by bacteria. It is essential that the discharge be quick and the bed effectually drained and allowed time for aeration. The depths of these filters are constructed from two to six feet, and equally good results have been obtained from either, so that the depth of the material does not seem to make much material difference.
Various materials have been tried for filtering-broken glass, cinder, coke, coal, gravel, and broken stone, but clinker is generally used when obtainable, that from a destructor being preferable. The most important part about the material selected is that it should not break up or disintegrate, as it then settles down and becomes clogged. In contact beds, clinker broken down to a fin. mesh is as good as anything, but it must be washed and freed from all dust.
The diverting of the effluent from one filter bed to another can be done by hand, but as this entails careful watching, and has to be done at night time as well as day, it is usually effected by automatic gear, of which there are several kinds. These in their turn require a good deal of attention, but they regulate the period of rest when full and empty far better than can be done by hand.
Percolating filters, or trickling beds, are those in which the effluent is showered or sprayed on to the surface of the filtering medium, either continuously or intermittently, and allowed to slowly percolate through; these beds are constructed on a concrete floor, and the filtrate flows off this and is collected in a channel, and then flows away.
The distribution is done by revolving or rotary sprinklers, fixed sprinklers, or Stoddart's trays. The latter is continuous, while the former are either intermittent or continuous. My experience is that it does not effect the result very much, as long as the filter can at times be given a rest, say for two or three days. It is essential that as much air as possible be conducted to the interior of the bed, and for this purpose air drains are laid on the floors, which permit of the air to penetrate into the body of the material, which is aided by the downward flow of water; the exterior walls of this filter need not be built up solid, as in the case of contact beds, but can be built up to suit the natural slope of the filtering material used. This is, of course, a great saving in cost.
The depth of these filters should be at least six feet. This fall is not always obtainable, and they are sometimes constructed only three feet deep ; but the results cannot be expected to be so good, and contact beds would be better for anything under four feet.
These beds are worked up to 1,000,000 gallons per acre per diem, but the sewage would require to be weak, and 500,000 gallons is the usual maximum for any town sewage.
The filtering material in these beds is considerably coarser than that used in contact beds, being up to 2in. and zin. cubes. The usual material is clinker, but stone or coal is often used, and it is considered by many that the smoother surface gives the better results, for in clinker the effluent collects in the crevices, which becomes stagnant ; capillary attraction also helps to keep the material waterlogged, which is highly undesirable, but the most important point, as in contact beds, is to obtain material that will not disintegrate.
The advantages of Contact Beds over Continuous Filters are :-(1) The distribution need not be so carefully considered.
(2) Much less fall is lost, which is often a most important factor.
The disadvantages are :-
(2) Oxidation is limited, as the amount of air is only equal to the volume of sewage.
(3) The beds are far more liable to clogging.
(4) The capacity of beds must be much larger for the volume of effuent treated.
The advantages of percolating filters are :
(1) Percolating filters cost less, as no retaining walls of any kind are necessary.