GISBORNE DRAINAGE.

Gisborne Times, Volume XXVI, Issue 2280, 27 August 1908, Page 1

GISBORNE DRAINAGE.

11EPOUT BY MR. JIOKELL

The following report on a drainage schema for the Borough of O'shorne, bv Mr. Samuel Jickoll, A.jVI .1.C.J!;., M.R.5.1., was received by his W orslup the Mayor this morning:—• Gontlemen. —In submitting the loilowing report for your consideration it is iny intention r.s far as possible to divest it of padding repetitions and fanev technical terms, neither do I propose to tell you who invented this or that system of pumping, bow many microbes there are in one cubic inch* of sewage or to refer at length to the well-known fact that mghtsoil should be removed, as I consider such references are superfluous and licquently used to make one appear wise, fill in gaps, and spin out the Jengtli of such documents as these. In pursuance with instructions received from your Town Clerk, and tor the purpose of becoming acquainted with local requirements and conditions I visited Gisborne early in dune last, remaining there one week, during which period every part and street in the borough were visited, ami also inquiries made as to the direction and s*t of the currents in the Bay at different states of the tides. Mr. Robinson also supplied me with plans (three), on two of which the levels of the borough wore shown, but as the datum on these latter did not. agree I have reduced the said levels to one common datum, and these are shown on the accompanying plan. In designing n sewerage scheme, it is imperative that ample provision should be made to meet future requirements, therefore, in the present schemo all calculations are basod on an assumed ixjpulation of 20,000, each of which, it is allowed, will contribute, per day, 30 gallons of sewerage, to bo disposed of and it is estimated that hall oi this amount will bo discharged into tlie sewers in six hours, thus making tlio maximum quantity to be dealt with equal to a total of 834 gallons per minute, and upon this basis the scheme has been designed, the principle boing known as that of the separate system, in which no provision is made for admitting storm water into the sewers. Pumping and Pumps—Owing to the configuration of tho su'daco of the land upon which G’sborno stands, and to the fact that it is divid’d u to four parts by streams, also' that a great part of'such surface is init little above high-water mark, if is found that to efficiently carry olt the sewage a considerable portion of the latter will have to be pumped, but at the same time great care has been exercised that such pumping shall be limited to the smallest potsiblc quantity, with tho result that tlio sewage from fully one-third of the total area of the borough w 11 flow for all time by gravitation to the outfall ; this fact as against that of pumping the whole will effect a very considerable saving in annual cost. By reference to the plan it will be soon that tho whole of the sewage from that portion of the borough lying between the sea at Victoria Township and the Tara hern river is proposed to be pumped, whilst approximately nine-tenths of tho sewage of the suburbs of Wliataupoko and Kaiti will flow to the outfall. Tho selection of pumping machinery and apparatus in connection with same for any schemo where pumping cannot be avoided is a matter which calls for the exercise of the greatest care and consideration on .the part, of the Engineer, because not’ only is the first cost and after annual *up-keep to be taken into account, but it is to be remembered that if an inferior and costly plant is in the first place installed it is likely to remain and cause cxcessivo annual expenditure, therefore after having considered various pumping systems I have concluded that for economical reasons one pumping station only is preferable. The system of sewage pumping by means of compressed air distributed from a central power station to a number of ejector stations in a district is where adopted the best means I know of for relieving the engineer from all his troubles with regard ro localities where it is at first sight difficult to obtain sufficient fall for sewers, also from an aesthetic point of view the system % ideal. It is nice to be able to take visitors into a fine engine-room, and invite tl ■ m to observe the beautifully-kept macli'iiery. and to point out how perfectly and automically the air pressure is maintained, which latter indicates that tho far away ejectors are responding with the greatest precision. Undoubtedly all this is true, but it is quite another tiling when close inquiry is made as to tho working of the plant in which there are unavoidable losses caused by the heating of tho air, leakage in air mains and other parts, so that tho actual power available for pumping at tlie ejectors is little over onc-tliird of that generated at the central power station, the actual efficiency boing at two places known under .400, whilst other systems of pumping range from .700 to .800, therefore, both as to first cost and tlie actual after expenditure for ordinary working it is the opinion of the writer that the system should be only adopted where circumstances render such a system unavoidable. The pumping system hero recommended for Gisborne is to have one pumping station, only at, or near as convenient to, the position marked on the plan. At this station tho wells would bo of ample dimensions to accommodate two sets of hydraulic pumps and one centrifugal pump; the first-named pmnps would bo duplicates of each other, with plungers three feet in diameter, and a stroke of three feet, one pump at seven strokes per minute being capable of dealing with the sewage from a population of 20.000, whilst it proposed to actuate these pumps by water pressure at 1201bs per square inch (the present pressure with temporary intake in use is 1301bs) from the town mains, thus pumping the sowago without other cost for power than that already incurred. The proposed pumps are perfectly automatic in working, each being set in motion through the influence of the lovel of the liquid in the pump well acting upon a float, the latter being at different levels, so that tho second pump would only be brought into action in the event of stoppage of tho first one. The third or centrifugal pump is auxiliary to the others, v and would only be brought into use in tho event, of tho breakdown of the water supply of tlie borough. This pump would bo driven by a suction gas engine, and the cost would only be about Jd per horse power per hour. In pumping tlfe sewage from a population of 10,000 tho hydraulic pumps would consume about 90 gallons of water per minute, bu.: this would only be during a part of each day, whilst at night time with little sewage flowing tho pump would only make a stroke at long intervals. Some idea may be gained as to the volume of water rquired to work the I

pumps recommended when l 's Mule.l that tlio maximum quantity would be approximately one-lourtli of that [lowing daily to waste from the seoiir pipe at tho upper and town side ol the Turanganui. bridge. With the system of pumping here outlined it would not be necessary (except when tho gas engine was boing used) that an attendant should bo on duty at night time, as the pumps being automatic, it would only be necessary to connect by electric alarm apparatus the engineer’s residence with the pumping station, the circuit of such alarm being clos-ed-by the increased higher level of tlio sewage in tho pump wall. OUTFALL SEWER.

The outfall sower commences near tlie pumping station at Wtiinui road, and follows the line of Hirini. street, thonco skirting tlio foot of tho hills at tho rear of tho freezing works and on past Captain Cook's .n- on. mint to the western beach, where at a point/about''66o yard's westward of the present breakwater it is carried by means of cast iron pipes on a papa roof 1000 yards out to sea, tlie sewage being deliiercd well under low water mark. At the upper end of this outfall sewer the construction is in tho form of a cylindrical brick or ooncrcto tank six feet in diameter and 1980 feet long, for the purpose of storing the sewage, the latter only being allowed to (low into tlio bay during four hours of each ebb tide, thus insuring that everything will bo carried away from tlie -Town and out to soa by the outgoing current, which travols in a south-easterly direction, as shown by the arrows on the accompanying nautical plan of Gisborno roads. This tank storage sower has a capacity of 350,000 gallons, which is ample lor ’.bug V it!’ the sewage from a population. of 20,000. LOW LEVEL AND GRAVITATION TRUNK SEWERS.

For the purpose of collecting and carrying tho sowago (to he afterwards pumped) from that portion of the borough lying to the westward of tho Turanganui river and a. very small portion of the suburb of Wliataupoko it is proposed to construct sewers as shown by blue linos on tho low ground skirting the sides of the Waikanao and Taniheru rivers. These sewers will vary in dimensions from 18 inches to 12 inches yin diameter, and where they pass under tlie rivers will be formed of east iron pipes. The two gravitation trunk sewers shown in Ormond Itoad, Whataupoko, and Harris Street, Kaiti, are for tho purpose of carrying nearly the whole of the sewage of the suburbs named (without pumping) direct to the outfall sewer in Hirini Street. These sowers will he construced generally of earthenware pipes firm 15 inches to 12 inches in diamder, but that portion which passes under tho river from Wliataupoko will lie syphoned by means of cast iron pipes. MAIN SEWERS IN STREETS. Tho whole of the main sewers in streets are proposed to be constructed of ordinary stoneware pipes, being jointed with Portland cement. These sowers vary in dimensions from G inches to 12 inches in diameter, mid the levels of same at intersections of streets, together with their gradients are figured generally on the plan, tho said gradients being such that all the sewers will be perfectly 6elf-cloansing. Provision is also made for the usual manholes and lampholes at every change or. direction of gradient- In order that all dwellings or buildings within the borough may be easily drained the inverts of the shallow or upper ends of all sewers are to be not less than 6 feet below the surfaco of the ground, as, in the experience of the writer, any less depth is insufficient. In all seworage systems it is highly necessary that proper ventilation and flushing apparatus should he adopted. Those have been provided for by arranging that all amplioles (with above-gro md shafts whore required) shall bo ventilators and that at the heads of sewers where necessary Field’s patent automatic Hushing syphons with the required tanks shall be provided. All the seivors shown on the plan both as to si e and gradient are very n tier tlie mark, but may require slight modification when the working sections are prepared. GENERAL.

In proposing that tho sowage shall bo disposed of by discharging it below low water level into the ocean at a considerable distance from tho shore every consideration has been given t 0 the biological and chemical systems of sewage disposal, and after viewing tlio matter from every standpoint it is recommended that the method of disposal proposed is in every way the best one for Gisborne. 11l connection with this method of disposal it is frequently mjened that the breaches adjacent to the outfall will be fouled, but where t’ e outlall is well designed and its position properly selected and caro taken that the sewage is only allowed to discharge during ebb tide such fouling of beaches never eventuates, and when it is remembered that very many seaside pleasure resorts both in tho Old Country and tlie colonies, the development and existence of which depend upon tlie good sanitary arrangements, havo similar systems of disposal to that here proposed, there need bo no fear but that satisfactory results will ensue.

Tho pumping system (hydraulic) recommended is installed in the following and many other cities and towns where it has proved satisfactory over a period of many years: Millbank, London; Buenos Ayres, South America; Margate, England; "Woking, England; Esher, England. The annual cost of working the above pumping machinery (as proposed) for a town of tho size of Gisborne is estimated at £750 per year, whilst for a. eomprcs.sed air system it is probable that tlie amount- would be at least £2OOO, the difference (£1250) being_equa.l to the interest on a loan of £23,000 at 5 per cept. interest, with the further disadvantage that a loan can bo paid off but pumping charges (like the brook) go on for ever.

Tho complete sewerage scheme here recommended provides lor the whole of the present borough, but whilst it would be wise to obtain the consent of the ratepayers to borrow tlio full amount required for its completion, it is estimated that owing to many localities being very sparsely populated only about two-thirds of the whole could be completed .within the next four or five years, and arrangements could be made to take up any portion of the loan required when population had increased in the districts mentioned to such an extent as would make the working of the sewers possible. In the estimates for the scheme the question of private bouse connections (from the street sewer to the house and including w.c.. buchan, and gully traps, etc) lias not been taken into consideration,, but if it can arranged I would advise the Council to borrow

a .sufficient sum to lend to ratepayers at, say, GJ per cent for tho purpose of laying on such house connections, tho amount loaned being repayable in half-yearly instalments over a period of, say, seven years. If this were done, it would be the best incentive l know of lor rate pave re to vote in favor of tho loan, whilst at the same time the Council would run no risk, as the various amounts lent would he a charge against the property. In order to arrive at the amount to he borrowed for this purpose I may mention that there are about 1120 dwellings within the borough, the average cost being about .£l9 each. In adopting any scheme for the drainage of Gisborne, 1 would recommend that tho site lor the pumping station should he largo enough to also accommodate a refuse destructor, as the borough is fast approaching that size when such a plant could lie worked with great advantage in providing (lower for future pumping and electricity for very.considerable public lighting, whilst at tlio same time what in the future will he a nuisance would be effectually disposed of. In this connection I may state that by the installation of tlio new destructor at Wellington all the power for the pumping of the sewage on six days of the week is provided by tho city refuse, thus effecting a saving of £I2OO a year, which previously was expended in the purchase of coal.

In conclusion, I beg to say that the designing of a sewerage scheme for the Borough of Gisborne, situated and divided by streams as it is, lias 4>eon a very difficult problem, and one requiring a great amount of study and care, but at tlie same time I have confidence that if the scheme here proposed is carried out it will in every way prove economical and satisfactory. I have to thank your olHcers, Messrs Morgan and Little, for assistance during my visit, and also Captain Cumming for valuable information as to tlie set of the currents on the foreshore.

ESTIMATE. Outfall sower, including tank £ sewer, 21iii stoneware and cast iron sewers, screening and pensock chambers, and valves complete 10,090 Pumping station, including building, machinery, and wells complete 5,100 Sowers in streets, including man-holes, lamp-holes, ventilating and Hushing apparatus (37 miles) ... ... 56,411 Low level sowers, including penstocks, man-holes, lampholes, etc., 18,750 Cast iron syphon, with penstocks, etc., from 'Wliataupoko 1,480 Cast iron pipes for low level sewers under river ... 1,600 Land and easements, say ... 1,500 £94,931 Engineering and contingencies 9.493 Interest and charges on first year on £69,000, say, at 5 per cent ... 3,450 £107,874