188 
REMARKS UPON THE DISPOSAL OF THE SEWAGE OF 
, HOBART, 
By A, Mautt, ConsuLtiInc ENGINEER TO THE METROPOLITAN DRAINAGE 
BoaRD. : 
Read at Special Meeting, May. 
A great deal of the discussion that has taken place upon the subject of 
the suggested methods of disposing of the sewage of the Metropolitan 
Drainage Area has been based upon ignorance of what has been pro- 
posed, and of the conditions under which the proposals have been made. 
I have suggested three methods for the disposal of the sewage: — First, 
its purification by filtration through land at an irrigation farm ; second, 
its purification by precipitation ; and third, its discharge, after being 
screened, into the tideway of the Derwent. The irrigation system 
would cost about £125,000 for works, and would entail a yearly charge 
of akout £12,000, which would necessitate a yearly rate of 10d. in the 
£1. The precipitation scheme would cost about £86,000, with a yearly 
charge of £10,500, anda rate of 8$d.in the £1. The direct discharge 
would cost £70,500, with a yearly charge of £7,500, or arate of 6d. in 
the £1. Up to a certain point there are works needed in common by all 
the systems— the sewers must be built for collecting the sewage, and 
there must be some outfall work, and some pumping power provided, 
When this common work is done, the direct discharge may be carried 
out without any additional work. But if the precipitation scheme be 
adopted, it would be necessary to add to the work already done some 
large tanks, and some more pumping power. Orif the irrigation scheme 
be adopted, it would be necessary to add yet more pumping power, 
and to provide and prepare land, and to Jay down pumping mains. 
Consequently no money would be wasted by completing the direct 
discharge system first ; that is to say, either of the other systems could 
then be adopted by expending the difference between £70,500 and. 
£86,000 in the one case, and between £70,500 and £125,000 in the other. 
I have, therefore, suggested that this direct discharge be tried first, as 
either of the others could be adopted without loss should it be found 
necessary. The following remarks are, therefore, to be taken—not as 
maintaining that direct discharge is in the abstract the best way to 
dispose of the sewage—but as showing that here the experiment of such 
discharge may be safely made ; and that it would be unwise to proceed 
to further expenditure upon works until that experiment has been 
made, and seen to be unsuccessful. 
With respect to what it is proposed to do, if the suggested plan for 
discharging the sewage without treatment into the estuary of the 
Derwent be adopted, the following are the facts:—There would be 
discharged into the tideway off Macquarie Point a quantity of sewage, 
estin.ated to amount in dry weather to 890,000gals. a day, about 4,000. 
tons. This means, if the sewage be similar in character to that of 
water-closet towns in England, that there will be there a daily discharge 
of less than three tons (6,41Slb ) of solid matters in solution, and of 
less than two tons (3,973lb.) of solid matters in suspension. Of the 
solid matters the noxious element may be said to be represented by 
692\b. of aitrogen in solution, and 1,823lb. of organic matter in suspen- 
sion. I have taken the Macquarie Point outfall as by far the principal 
one. Six times as much sewage will be discharged there as at Battery 
Point, and eight times as much as at the New Town outfall. 
With respect to the conditions under which the discharge will be 
made; in the first place, as to the matter discharged, it will be seen 
that the greater part of it is in solution; as to the rest of it, it will be 
AGien: .- -: 
