Again, where the sewage has been emptied into the sea, tide-locked drains are objectionable, and the sewage,'when mixed with salt water, generally gives off more stench than ever. We may briefly say that all attempts at deodorization by chemical processes have hitherto failed, and as far as our present knowledge goes, are not to be relied upon. The utilization of the sewage on the fields by irrigation is, therefore, the true solution of the problem, and we must, arrive at the simplest, cheapest, most certain, and most perfect system of accomplishing this. When sewage and rain-fall all go together in the same drains, its they do in all the older systems, n11. is uncertainty; while when the two are separated, rain and surface-water can be discharged at any point into the natural water-courses of the country, and a fixed quantity of sewage, with household and flushing water, would be passed to the main outlet., to be there dealt with. The opponents of this system say that it is too expensive and 'troublesome to plan; that it is unnecessary, as it is sufficient if engineers provide for the dry-weather flow of the sewage, and use that for irrigation; and that when the overflows come into action in floods, the whole is so much diluted, that no harm is done to any one. The advocates of this double system of drainage have proved that the total separation of the two is the most sanitary system. because the street-gratings and rainwater pipes, which at present let clown the rain-water into the sewage drains, aet, in fact, as so many venti lating shafts, and discharge the stench in the midst of the inhabitants; while, under a separate system. the would be entirely sealed up, and only ventilated at such places as could safely done; that the rain-water as a flushing power ought to be entirely discarded, as it fails in dry weather, just when it is most wanted; that in wet weather, and winter again, when the discharging of the sewage on to the surface of land is carried out, the great quantity of water sent clown through the drains by the present system is agriculi urally a serious injury; that when pumping has to be employed for lifting the liquid for irrigation, as it is in most cases, all is uncertainty, and 'that no machinery can be economical and efficient under such circumstances, and that the plan ning of the irrigation also becomes difficult to manage, and irregular. With regard to the expense, it been proved that, as the rain-water and surface-water can be dis charged at the nearest point, all the drains may he much lessened in- size; and further, that the power of the water in the sewage-drains will be much more efficient, while the corresponding lessening of the expense- in carrying out the process of utili zation will completely compensate any additional outlay that may lie incurred in laying the drains in towns. if we take the case, which is a common one, of a pop. of l0,000 people living upon a sq.m., the first-mentioned system, where rain and sewage-water go together, would require pumping machinery, in dry weather, of, say, five horse power, to lift the liquid; and it would further be necessary, for wet weather, to have in reserve a lifting power of 150 horses; while, on the separate system, where the sewage, alone would have to be dealt with, the five horse-power engine would be and constantly employed, and its work would bealmost entirely confined to the daytime, whereas the other must be ready at any time, and for every emergency. The systemsof sending sewage and rain-water together has been hitherto adopted in all towns: but except in one or two eases where gravitation -has been available to utilize the discharge from the drainage, all engineers have failed to prevent the pollution of rivers, and it is obvious that something else must he tried, as that cannot be permitted to go on nnich longer. The system of separating the sewage and rain-water has been carried out in several large asylums and public buildings, many barracks, the town of Eton, and Windsor castle—wlfere every consideration, both of expense and sanitary influence, was brought to bear on the subject. Reading, Oxford, and several other towns arc fast following on the same principles, and the results are hitherto most satisfactory. Great economy has resulted from the process.
4. Pie Utilization, of Sewage.—The whole of the sewage of a house or town having
been conveyed away in the manner we have described, the next important. step is to know what to do with it. Above all things, it is desirable to add to the productiveness of the soil, so as to compensate in some agree for the constant supply we are drawing from that source.
The liquid nature of sewage, adopting as we may the ordinary amount of dilution in dry weather at the rate of 25 gals. per head, has been a great obstacle in the way; while also the vast quantities of road-grit, and the great gluts of rain that come down along with the sewage when there is only one system of drains in a town, have upset all arrangements and caleulatithas. Many attempts have been made; especially at Leicester, some years ago, to precipitate all the valuable • qualities of the sewage by impregnating the whole with milk of lime; but the process was unremundative to those who did it, as so much sand was precipitated at the same time, that the product obtained was almost worthless as a. manure; while, as the greater part of the ammonia escaped in the water, the discharging Of it into any stream was still, strictly speaking, quite illegal. As far as chemical knowledge can guide us, there seems at present to be no hope in this direc•-• tion.
At Edinburgh, again, and at Croydon, the irrigation of land by gravitation has ren dered the process a simple one, because the whole has been poured over the land with many excellent results. These, however. are clearly exceptional cases, and we must look to pumping as being necessary in by far the greater proportion of towns; while for the two places we have mentioned, the results would, in all probability, have been better still if the strength of the sewage had been more concentrated. Agriculturally speaking, any dilution above 25 gals. per head of the population is not desirable, but is injurious and expensive to distribute; while, again, human•ftecal matter is too strong to be applied to land unless diluted in something like 10 gals. of water: The Chinese teach us an important lesson in this respect. They place all the solid matter, when they remove it from the towns,. in small wells in their fields, and then take a scoopful and mix it in about ten or twelve times its volume of water before it to their crops. If any one attempts utilizing sewage when mixed with rain-water, and has to pump the whole all the Tear throbgh, he will findhimself in endless difficulties.
Presuming, then, that we can arrive at a fixed quantity of 20 gals. per head of the population, or what may be taken as the dry-weather flow of the drainage from a town, the first step is to pass the whole through a strainer, so that all materials may be inter cepted which will be likely to interfere with the pumping, or choke the smaller pipes used for irrigation. This is necessary, also, because in its unstrained state we cannot depend upon sewage going down and up again, and so passing over a valley, and the sphere of operations then becomes more limited.
Great part of the solid matter can also be removed by this process. and common house-ashes are the best mixing and deodorizing material to facilitate the stuff being carried away.
A piece of land should then be sought ought, with a slope, if possible, of 1 ft. in 30 at least, and the filtered liquid, which will be full of strength, conveyed either by pumping or -gravitation to the highest point of that land. • Iron pipes should not be used, if possible; and when the land is very flat, it must be ridged and leveled. From the highest point of the land selected, the liquid must be conducted by open channels or through common drain-pipes laid on to the surface to all the different points where it is wished, and utilized for irrigation. The land adopted should be moderately porous, and then for every 100 people an acre may be allowed, but this varies much according to the nature of the soil. The laud must be thoroughly drained and prepared. The best crops to be grown are Italian rye-grass, with alternately crops of vegetables, such as potatoes, cabbages, rhubarb, mangold. All these will luxuriate on the liquid, and we think we may safely say that the command of such liquid would be worth to any persoefrom £5 to £10 an imperial acre, according to local circumstances.