DUNEDIN DRAINAGE

Otago Daily Times, Issue 17729, 13 September 1919, Page 10

DUNEDIN DRAINAGE

SEWAGE AKD SALT WATEU. ASTONISHING PUMPING OPERATIONS. "A wilful and shocking- wasto of power and money," -was the description given to a Tunes reporter of certain aspects of tho oity drainage system. Tho speukor was referring to tho largo quantity of water which, as stated by Mr J. HL Hancock at the last meeting of tlio Dunodiu Drainage Board, was finding its way into the main intercepting sewer from the harbour, and whioh was pumped out again at Lawyer's Head. It appears that on many occasions tho pumping station at Musselburgh has to deal with millions of gallons of salt water. This water makes its ingress through certain defective flood gates between Frodorick street and tho wharf, and at spring tides the incoming -volume of water is said to bo very considerable. Our reporter was informed that at the periods of neap tides a comparatively small section of 'the plant at _ tho pumping station will meet tho requirements, but when largo quantities of water aro finding their way into the sewer from the harbour, tho plant has to bo run I practically at full capacity. Over the course of a fow monthe tho waste of power and money in pumping this water back into tho sea again must bo very considerable. How does salt water get into the main intercepting sewer? is the question which will naturally be asked by the general public, and probably by a number of city councillors as well. On© councillor at a recent meeting of the City Council said it could not—that the main sewer was "sealed." The answer is simple, and, paradoxical, as it seems, it may be stated right away that the salt water gets into tho main sewer only in dry weather. There are _ about four of the old city sewers, carrying sewage and storm water, which lead into the harbour. These sewers are carried over the main intercepting sewer, and they have what are called bucket valves, which open into tho main sewer underneath. What happens in timo of rainy weather was clearly described by a member of the City Council. The buoket valves are closed, and tho sewago and storm water are carried into the harbour. As a matter of fact, it is stated that tho pumps at Musselburgh dealing with the sewage from tho main intercepting sewer are less severely taxed in wot weather than they are in dry. In dry weather, however, the bucket valves are left open, and the sewage and storm water from the four old sewers referred to run through the_ valvo into tho main sewer. There is practically no sewage and storm water, therefore, in the old sewers after they pass over the main sewer. The salt water from the harbour, however, flows, with the rising tide, up these old sewers to the bucket valves, and pours in millions of gallons into tho main sewer underneath; it is pumped into tho rising main, and thence into the ocean at Lawyer's Head, and some of it, no doubt, in one course completes what is practically a circle, again flows up the old sewers to tho main sewer, and goes through the former process all over again. Tho councillor referred to said it would

bo very interesting to know how many months had elapsed since Cr Wilson (chairman of the Works Committee of the Drainage Board) and Mr W. D. R. M'Curdie (the city engineer) had first become aware that salt water was passing on occasions in millions of gallons into the main sewer, and to learn what had been done to provide suitable sluice gates to prevent it from continuing to flow up tho sewers. Originally thero were two rising mains— that is, pipes or tunnels or sewers. They were both built of jarrah. Tho jarrah, however, was taken out of one, and it was replaced with steel piping. Tho water flows along the main intercepting sewer to tho detritus tank at the pumping station; the pumps pick it up and force it into and along the rising main, and it runs by gravitation over the last part of the journey into the ocean. Tho second jarrah main has lain unused for a number of year 3. Our informant says that, so far as he understands the the city engineer proposes to lift all this jarrah wood, clean it, and replace it, which, along with the cost of new material, would amount to about £5000. Apart altogether from whether the work can be done for the money or not, tho councillor states that tho present pumps can deal with all tho flow in the main intercepting sewer—that is, witli ordinary sewage, and also oven if salt water gets into the sewer. Ho asks, therefore, why another rising main is required, which, it should bo would reqniro also an entirely separate and new pumping plant.