sewage); (5) sludge from septic tank; (6) "settled" sewage sludge (no chemicals used); (7) "Globe Fertiliser " (previously dealt with).

In Series I.--Experiments with turnips, mangolds, and swedes. Twenty-two plots of land in duplicate, each onetwentieth acre, were selected, on which it was decided to use such quantities of each of the sludges as would supply, as near as possible, 40 lbs. of nitrogen and 36 lbs. of phosphoric acid per acre, because of the inability of each to supply equal quantities of the manurial constituents. With sludge No. 1, 2,072 lbs. supplied this amount. On the first seven duplicate plots each received an amount of one of the sludges equal to 2,072 lbs. per acre. Twelve other duplicate plots were used for comparative experiments with No. 1 sludge, mixed with either sulphate of ammonia, superphosphate, or both; and also for experiments with sulphate of ammonia, superphosphates, and fish meal, either together or alone. The whole of the experimental plots received an equal dressing of potash salt, supplying 30 lbs. of potash per acre. "The season of 1905 was somewhat abnormal in character, and, on the whole, not very favourable to the action of manures, ," reads the report; and, comparing the results obtained with sludge No. 1 alone and mixed with either superphosphate or sulphate of ammonia, Dr. Somerville shows: (1) That the phosphate of the sludge is less active than the phosphate in the superphosphate, because the addition of a further quantity of phosphoric acid in the form of superphosphate produced a greater relative effect in the former than in the latter. (2) That there are similar indications with regard to the nitrogen of the sludge." "From the direct comparison of the effects of equal weights of nitrogen (20 lbs. and 40 lbs. respectively) and phosphoric acid (18 lbs. and 36 lbs.) derived from sludge No. 1, and from sulphate of ammonia with superphosphate and with fish meal," the report shows that the sludge-grown crop is in nearly every case considerably inferior in weight to the crop grown with artificial manure.

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His conclusion is: That no consistent manurial effect was produced by the sludges on the root crops grown in the experiments-mangolds, swedes, and turnips in the 1905 season, which was of a character which militated against the growth of such crops, though allowing artificial manures to exert a considerable influence on the growth of this crop." These experiments are, however, being continued, as it is thought that sludge may be slow acting as a manure, and

therefore further experimenting may produce results of considerable importance.

Series II. comprised experiments with sewage sludge upon hay at the centres shown in the above Table, and also on grass land at Woburn. They were similar to those of

Series I.

At the Northern experimental centres, the wet summer was favourable to slow-acting manures, and the application of the sludges seems to have been useful. In the Southern counties, where the hay is, of course, cut much earlier, the sludges produced no results whatever.

Comparing the action of the nitrogen and phosphates supplied in the ordinary artificial manures with those chemicals present in sewage sludge, it would appear that for root crops and grass the action of the latter is very slow, so that it seems very likely "sludge supplied in proper quantities would form a good dressing for the slow-growing plants of many permanent pastures and meadows." Sludges precipitated by the addition of lime to the crude sewage seem to be more valuable as manures than those resulting from precipitation with salts of alumina or iron. This is only, perhaps, to be expected bearing in mind phosphate of lime is more readily assimilated by plants than phosphate of iron or alumina. The above experiments are being continued, as before, under the direction of Mr. Middleton. Others were carried out on similar lines by Dr. J. A. Voelcker, at Woburn. Eleven plots of one-fortieth acre were employed, seven of which were dressed with each of the seven sludges set out above in quantities sufficient to supply 40 lbs. of nitrogen per acre. The soil of the field, which is on the junction of the Lower Greensand with the Oxford clay, partakes of the character of both formations. The grass was only moderate in quality. One of the plots was left unmanured; the other three also received a dressing of lime.

"In all experiments conducted on grass land, it is unfair, and often misleading," says Dr. Voelcker, 'to take the results of a single year as conclusive." The dryness and absence of warmth which characterised the season of 1907 militated against satisfactory experiments. Here, again, it would seem that their continuation is called for, because it is considered in every way likely that the sludges require a longer time to show their influences.

Series III. consisted of experiments with wheat culture in pots holding 40 and 32 lbs. of soil, the last fifth of which

was in each case incorporated with the sludge or artificial manure. The same seven sludges were used.

The experiments were divided into three groups :

(1) A comparison of the manurial effect, on the wheat crop, of one kind of sewage sludge as against another, when these were respectively applied in quantities calculated to supply 40 lbs. of nitrogen per acre.

(2) A comparison of the effect of each individual sludge with that of an artificial dressing composed of superphosphate, rape dust, and lime, calculated to supply the same amounts of phosphoric acid, nitrogen, and lime as did the corresponding sludge.

(3) A comparison of the cost of increase of crop, obtained by the addition of sludge and by artificial manures of known cost. In this experiment the sludges were used in equal weights (two tons per acre), and compared with dressings of artificial manures such as a farmer would be likely to use.

On comparing the rootlets of the plants four months after planting (in April, 1907) but little difference was shown from the various sludges; on the other hand, the artificials had in each case given the larger and stronger root growth. In the end sludge No. 2 (see above) showed the best results of the sludges.

Dr. Voelcker arrives at the following conclusions:

"(1) That the different sewage sludges, when used in sufficient quantity to supply 40 lbs. of nitrogen per acre, are capable of increasing the yield by 10 to 12 per cent. above the unmanured produce.

"(2) That artificial equivalents of the different sewage sludges will similarly give 16 to 17 per cent.

straw.

(3) Sewage sludges mostly produce greater length of

(4) Those sludges which contain the most moisture and lime do the best among them. For wheat-growing none would be worth 10s. per ton on the farm.

(5) That the value of the sludge does not turn mainly upon the amount of nitrogenous organic matters contained, but that these may indeed be in a form incapable of ready assimilation and requiring lime for their decomposition.

From the total results of the experiments so far conducted, the Commissioners conclude that sewage sludge has undoubtedly a manurial value, but the manurial constituents, being of necessity mixed with such a large proportion of grit,

etc., the question of its economic use as a manure depends, to a large extent, upon the cost of carriage.

The results of the extended experiments will be awaited with much interest. In the meantime the experiences of individual agriculturists would doubtless make welcomed additions to these official experiments if they were conducted upon scientific lines, and based upon analyses of the sludges used.

CHAPTER XXXIX.

Automatic and other Appliances,

In works connected with the treatment and disposal of sewage by any satisfactory method, and whether on a large or small scale, the employment of various mechanical appliances is a necessity. Briefly, they are required for controlling and measuring the flow, admitting and releasing it from tanks and "beds," lifting" it in cases where it is not possible to rely on gravitation, and distributing the sewage on to filter beds, etc.; the ventilation and flushing of the sewerage system must also be a factor for consideration, and calls for the employment of some mechanical means.

The Value of Automatic Gear.

There seems no question but that the employment of skilled supervision of the various appliances used in sewage treatment is to be preferred to mechanical means. In large works this can always be ensured. There is no doubt but that automatic appliances save a large amount of labour in regulating the filling and emptying of tanks, contact beds, and filters; but, at the same time, it is not desirable to rely entirely upon such apparatus, owing, chiefly, to their liability to get out of order.

In the case of smaller works, especially those of a private character, cost of maintenance is a feature which must always be considered, and consequently the retention of a capable man cannot always be ensured on account of the outlay which it necessitates. Automatic appliances have therefore to be relied upon; though occasional attention at least is then necessary. If this can be given daily, so much the better.*

Measuring Appliances.

It is essential in so many cases to have an accurate knowledge of quantities under various conditions, that a satisfactory measuring meter is an essential feature in properly-planned sewage installations. Among the uses of such an appliance may be mentioned the ascertainment of

*A magnifying glass will be found useful in studying most of the illustrations in this book.