Sewage As A Fertiliser

New Zealand Journal of Agriculture, Volume 63, Issue 2, 15 August 1941, Page 136

Sewage As A Fertiliser

Practical and Economic Method Yet To Be Found

-By-

W. DONOVAN

late Director of the Dominion Laboratory.

SUGGESTIONS have been made that sewage should be utilised as a fertiliser, but ‘ there are difficulties because of its dilution. Where i suitable areas exist near a centre of population sewage can be applied directly to the land, as is the case in Christchurch, but, as far as can be seen,’ this method would not be practicable in any other large, centre of population .in New Zealand. It has also to be taken into account that there is a prejudice against vegetables grown on a sewage farm, and probably this alone would make the development of this method of sewage disposal inadvisable in this country. It must be appreciated that this method is used primarily for disposing, of sewage and not for fertilising the land. For the latter purpose it is probably not economic. The alternative would be treatment to recover manurial constituents, but this would utilise at the most only part of the material in the solution. In the activated sludge process, which is being increasingly adopted, a considerable amount of sludge is produced which gives great. difficulties in its disposal. Activated sludge con-

tains 90 and up to 99 per cent, of water, and it is mainly for this reason that its disposal is so very difficult. ■ln a report of a conference on “Treatment and Disposal of Sewage” by A. Parker,. D.Sc., held at Glasgow in 1938 by the International Union of Local Authorities it is stated in regard to activated sludge: — “What is required is a cheap method of drying sludge to reduce the moisture to less than 10 per cent, which could be adopted at most sewage disposal works. The dried sludge could then be converted into a powder which could be transported to farms and used as a fertiliser at a reasonable cost. During recent years various methods of producing dry sludge have been proposed and tried. These include centrifuging, treatment in various types of mechanical filters, including vacuum filters, sometimes after . heating , the sludge and final treatment in heated ■ driers. . Some of these methods are in operation on a large i scale. For example, at Milwaukee the sludge is filtered in rotary filters and dried in rotary driers. It is then pulverised and sold as a fertiliser. None of the methods so far tried, however, have reached the stage of a proved economic

proposition suitable for adoption at a large number of places.” . It has to be taken into account that the adoption in New Zealand of any method for utilising sewage as a source of fertiliser would involve very heavy capital expenditure and : the- solution of difficult engineering problems, and in view of the experience of other countries would not be economic. . It would appear that if the application of human excreta to the land becomes essential the most practicable method would be to abandon our present water-borne sewage systems and return to the old system of privy and then disposal of the nightsoil by what is known as the Indore process, which has been > successfully adopted in parts of India. This proposal is so revolutionary, however, that it is highly improbable that it would be

adopted. Finally, it is possible that the total amount of fertilising material in sewage is perhaps, over-rated by many people. In “Sewage and Sewage Treatment,” by Babbitt fan American authority) is the following comment: ; “The popular idea, possibly started by Victor Hugo in ‘Les Miserables,’ that there is value in sewage is not well founded. The extraction of - values from sewage sludge, is discouraging as much from an economical as from a scientific viewpoint.” It is very significant that, as far as can be seen, in no country have practical economic methods been adopted for recovering fertiliser from sewage.

Answer to Correspondent

Sweet Clover As Green Manure

R.C. (MATAURA):— Would you please advise me through the columns of the “Journal of Agriculture” if sweet clover is grown or has been tried in New Zealand, • as the

writer notes, that in the 1935 edition of Root’s “A B C and X Y z Z • of • Bee Culture”’ it has the following to say on page 709: — “Sweet clover sown in wheat or oats in the spring and ploughed in for corn in late April or early in May a year later will turn to the soil anywhere from 75 to as much as 250 lb. of nitrogen per acre. It will require 20 tons of .ordinary farm manure to add 200 lb. of ■ nitrogen to the - soil.”. • On-page 707 it. gives the acreage in sweet clover in the U.S.A, in 1930. South Dakota alone had 133,384 acres. FIELDS DIVISION:— ' There are two main species to which the name “Sweet Clover” are given, namely, the white flowered melilotus ■alba and the yellow flowered- melilotus officinalis'. Both of these are to be found sparingly on roadsides and on sandhills ■ throughout. .the Dominion. Seed of both species can be obtained from certain of the larger seed merchants. Sweet clover has been tried many years ago in New Zealand but as it is eaten., only - sparingly by stock, and hence is chiefly of value as a green

manure its use has not become general. It is sometimes grown as a source of nectar, for bees. Sweet clover is of distinct value as a green crop for ploughing in, arid has been given a good deal of attention in other countries, particularly North America. On farms in New Zealand, however, where red clover, white clover, and lucerne thrive, there is not the same incentive to grow special' leguminous crops for green manuring. Each of the above crops is far more valuable than sweet clover for fodder, and each also returns a considerable amount of nitrogen to the soil when ploughed under. Sweet clover may be worthy of a trial for green manuring in the orchard or garden, but for this purpose lupins appear to be generally satisfactory.