THE DRAINAGE DUNEDIN.

Evening Star, Issue 10817, 29 December 1898, Page 1

THE DRAINAGE DUNEDIN.

A SCHEME WORTH CONSIDERING. [By N. Y. A. Walks.] Whatever may be the decision of the arbitration now going on between the Harbor Hoard and the City Council as to the drainage of the City into the harbor, the Council will, in the near future, have to decide upon a system of drainage and disposal of sewage, unless the surrounding boroughs agree with the Council to have a drainage board appointed, or to become a Greater Dunedin, and so relieve the present Council of the responsibility, lint the City will, in any circumstances, have to pay the lion's share of the cost. If the borougns interested do not agree to cast in their lo. : with the City, or to share proportionately in the establishment of a drainage board, they will ultimately, under force of circumstances, have to pay the City for the privilege of running their sewage through the City drains, or so much of it as cannot be otherwise disposed of. But, my object in writing now is not so much to discuss the amalgamation of the City and the boroughs as to sugge.-t a simple and inexpensive means of ending the dispute between the City Council and the Harbor Board. The idea is not original. It is an adaptation of a system that is superseding precipitation, sewage farms, and other modes of disposing of sewage. In England and in America experiments have been and are still being made in the treatment of sewage by micro-organisms. It may almost be said that the experimental stage has given place to the practical. Though the bacteriological treatment of the crude sewage has not yet been adopted by the London County Council the effluent from their precipitation channels is being treated in that way, and several provincial boroughs have it in operation and are extending their works. It would occupy too much of vour space to describe at once the working of this system in detail, but a few sentences will suffice to show how it may be applied in the disposal of the sewage o'f Dunedin, and immediately abate the nuisance so much complained of. There are at present about nine public sewers and drains discharging filth into the bay and Water of Leith. These may all be intercepted by one main sewer. Whatever system js adopted a main intercepting sewer must be | constructed. This main sewer should be I made large enough to take the drainage of

the whole of the City and slopes of the boroughs lying towards the City; but the main drainage need not be at once proceeded with. This cart of the scheme may be done leisurely, and as the authorities find themselves in a position to raise the funds. The most urgent work is the main intercepting sewer, and, though made large enough to provide for the future, it would meantime be used to convey the sewage from the present sewers to the bacteria or filtering irniks. The position of these tanks may be anywhere on the foreshore. No disagreeable smell nor nuisance comes from them to cause any inconvenience. The crude sewage contains within itself the elements of its own destruction or consumption, which develop at an enormous q. rate. It is first run into what is called a a "septic tank." It is then liquified by the * action of what are called anaerobic microbes. These do their work without air, or thev r t;ike the oxygen necessary for their active existence from the substance they feed upon. The sewage remains in the septic tank about two hours. It is then run into a filter tank nr a bacteria bed, or, as some authorities tall it, a biological filter, where the little workers called aerobic microbes feed upon the solids, and consume all, or nearly all, the organic matter, leaving nothing but the effluent, sufficiently pure to be run into any stream or into the bay. If a greater degree of purity is required the effluent may be inn into a second filter composed of smaller aggregates. This is merely an outline of the system. The details as to the best position for the tanks, their composition and area, pumping, and screening of the sewage and other matters connected therewith may lie discussed later on. As to the cost, however, I may quote from a letter received from the engineers of the works at Exeter, in England: "In England we could lay down an installation for 50.000 people with the ordinary How of sewage for from £IB.OOO to £20,000. As to the annual cost of working, we consider that at Exeter, where works provide for a population of 45,000, one man will easily be able to keep the whole in order. Outside that the expenses will be comparatively trilling, consisting merely of the occasional renewal of valves, oil, waste, etc., there being no chemicals to provide or sludge to pre*s." At Exeter the How from tanks 'o filters is regulated automatically.