UNDERGROUND DRAINAGE.

Timaru Herald, Volume XC, Issue 13317, 20 June 1907, Page 7

UNDERGROUND DRAINAGE.

HINTS FROM WELLINGTON. The Wellington City Council has_ just, received from their engineer, Mr W. H. Morion, a report on underground drainage for outlying portions of the city, which contains" threo passages that -will be of interest to the people of Timaru, as having a bearing upon their own .scheme. The tirst, referring to the thinly populated port ions, suvs :

"It, .'hould ba [understood that some sewers designed to cany the maximum flow of .>*-wage will for some time be delivering but a. small proportion of the ultimate quantities to be dealt with. The velocity of the flow through such sewers, will corwequently, bo much less than when running full or half full. In cases considerable care will be required to prevent obstruction by deposits of &olid matter. Long lengths of sewers without a sufficient number of house:; discharging sewage therein are likely to ba a. source of danger through want of sufficient flushing and proper ventilation."

In dealing with Island Bay, the report saj:s: The ultimate discharge of sewage from this district, as proposed, may cause apprehension in the miridwof some citizens that such method will be productive of nuisance, but, after the sewage has been passed through a " septic tank in which tho solids have been broken down and the sewage robbed of the greater portion of its most offensive characteristics, there should bo very little that is offensive, resulting from the effluent, which would be discharged below low water level. I have not considered it necessary to provide nitrification tanks to more thoroughly purify the effluent from the septic tanks, as, in my opinion, a sufficient degree of purifi cation will be effected in the septic tanks themselves to'allow the effluent to b?. discharged, into the sea without nuisance." In dealing with another 'suburb of the city, Koseneath, Mr Morton proposes to, pass the drainage through a septic, tank in Evans Bay, and discliarge the effluent into the harbour below low-water mark. "To do this will require the consent of the Harbour Board, which I anticipate no difficulty in obtaining." Mr Morton gives the following estimate of the quantity of sewage to be dealt with, per head of population: "In arriving at the quantity of sewage to bo discharged I have allowed five persons -to each dwelling, and the number of dwellings per acre to be from 3 to 6 in various portions of each district, according to the position and. configuration of the ground. Tb/ 5 - quantity of sewage per head to ba .discharged has been taken at 40 gallons per day, and, as is vsually done-, the maximum rate of dry weather flow is> assumed to be half of this quantity discharging in six hours. Although it is intended that the whole, of Uie sewerage work row proposed shall be on what is known as the " separate system "—that :c, rainfall to be kept out of he sewers as much as possible or advisable —I have allowed a proportionate amount of rainfall, equal' to 300 superficial feet i>er house, bting discharged into the fewers at the rate of 2 inches per hour of Tainfall in 24 hours. It is difficult to arrive at the exact amount of rainfall to be provided, bu I consider thk provision to be ample. . The total maximum discharge of se.wage and rainfall allawed for on the above figures is at the rate of 22.8 cubic feet per head per day."