DRAINAGE.

Marlborough Express, Volume XLV, Issue 67, 21 March 1911, Page 2

DRAINAGE.

,1. I have ca.Eef.ully examined the town of Blenheim and considered* themethods by.wßioh it could be drained. I find provision has already been

; made for storm water, for which there !is abundant !ioui|,|| i% the river run'sewa^'^riJpSrVllft., ,!»»! matter, hftMspn6W tti an ] & *nf|jsuf.ac|urers drainagelj^Ee town beVg#swate<* on aP" 'prjpniately |evel grMfd, not greatly aJjptfe nigh^Vi^te^^l^t'i" it is evident that the sewage^feanhot Be removed by gravitation alone, unless it could be discharged into the rivers in close proximity to the town. 2, This cr.rser. spears to me out ofthe q'.rsfcion. There is no strong c tiTcnt running in the Omaka in ordinary tirrr«, and the sewage flowing into comparatively still water would rapidly become an intolerable nuisance.

Two propositions present themselves :— 1. To pump the sewage at once through an iron pipe to the ocean, say somewhere near the Wairau Bar, a distance of about six miles. 2. To collect the sewage into tanks and treat it, by one of the several methods now in vogue—Jetting the filtrate run into the Omaka."

3. As the town is nearly level, and not many feet above level, it is evident that the sewage being led into some conveniently situated sump or tank—t^hat to obtain the necessary fall in the sewers the bottom of the tank will b*» at <s»ch a low level that the-sewage would have to be lifted'in any .case before it could be dealt with. '

4. If pumped away to the ocean it must be given sufficient velocity to overcome the friction of the pipe. 4." If ttreated by any systeih»'of bacteriolysis, the tanks -in which it was treated1 would have to be at such a level that the filtrate could run off into some adjoining outlet and to en- ' able this to be done it is clear that the sewage would have to be lifted at laast above the present level .V>f the surface., That being the, <ease a pumping station' would have to be provided in any case. . ~1. - The question then to be considered is whether an equally efficient scheme could be provided at less cost by say a' septfc tank, or similar treatment, than by a mam to the sea. * • '»■ 5. There area^great many, looali,ties where it is impossible to discharge the sewage direct into;, the open sea, that it has to be' treated^ before ,ijfc is desirable to disc"harge~ it either ' "oil the land or into dow flowing * * rivers, land locked harbors, etc. In such localities it becomes? watptsrative to" purify it in some manner,. ., /ifc and several variations of septic treatment ai»e now practised, but it appears to miff1 tha^Y if «|t is. possible at a^ijeasonabje! cost to discharge the. sewer into the open, ocean, that is the most desirable method; it is finally done with and can, give no more trouble. Sydney, Wellington," andDunedin are' ' .all sending their sewage into the sea. - 6. Since success has been achieved to a greater^or ~less extent by ,the septic tank treatment, the system'has J had many .-exponents -whos^ .-efforts have been far from successful—tanks -haye been put in, many places which J were neither, jnore nor Jeas_ _than. elaborate cesspits, the locality of which was readily ascertained by the smell, and were most undesirable to have in the neighborhood. It is true j the solids were broken up by the action of the *Ba^:taSi»f.i^ana" that by this; means the quantity of sludge has been lessened to an enormous extent, but:it requires not only efficient design, but skilled management to conduct thejotj^ratidnflill such a> way that the installation does not become a nuisance. '•'...■■

7. Several sanitajpy engineering firms in Europe^, ~ $hich haye ..made this bacteriolyti^lifbatment a* special study have patented' automatic appliances which are said to. ensure complete and certain workings of these installations,- and I believe are prepared to design8* Jand < erect vworks of this . nati|re«j:i T lft it could- be r shown that a substantial saving could be made in this way, and any firm of repute would guarantee to erect an installation capable of treating, say, 75,000 gals. ;or sewage per .day,' dis^ charging the filtrate at a given' standard of purity, create no nuisance in the treatment, and work the installation to the satisfaction of the Council, for, say, three months before the works are paid for, it might be well to consider the question. QUANTITY OF SEWAGE TO BE DEALT WITH. The quantities differ so greatly in different towns that it is difficult to estimate closely the probable amount, but if provision is made for double the present population, and allowing 25 gallons per head per day, I consider the estimate will be on the safe side, and sewers designed on this scale should be quite sufficient for all provable increase in the town for very many years. Where there is no water in the town other than that supplied by a water supply system, it is evident that, provided all storm water is kept out of the sewers, the amount of sewage to be pumped cannot exceed that quantity—in fact, that it must be less as all water used for_ street watering, gardens, etc., which will not be allowed to" run into the sewers, will have to, be deducted from the amount supplied by the mains. But in a town like Blenheim, where there are a considerable number of private sources of supply, a large quantity of water may be

used and poured' into the sewer by persons who are not connected with, the public watefj; supply. It will, therefore ? injytEei ;case of Blenheim, be necessary j^'^^ke aX liberal allow^ ance, an&^jthAk that 25^gallons- per|;; head per ;<say wijlf be a very fair■".estimate, .v.i/.- a%i kT: .> H . j;, $' The drainage/of i&e^town should be carried in stoneware 'pipes of suit- ' able size to some position preferably near the eastern, boundary of the ' town, the sewer pipe being laid at • such gradient as will ensure velocity j sufficient to carry the sewage without, allotting deposit; the pipes discharg- j ing into a sumi*, 1 from which it can j be pumped to the outfall. In conse- [ quence of the Omaka and Opawa dividing the _town into three ■ j>artsy the| sewage will have to be carried by sew^er pipes in each of these two northern parts to sumps, from which it can be pumped over the bridges int6 $be sewers in the southern part. That part of, the town lying between the Omaka and Opawa will some day carry a very considerable population^" an^ that must be borne in mind when designing the sump for that part; buis that part lying in the bend of ttiei Opawa is effyefy; small area, and th©| sump will bnly require to be of small size.;-'■'-- :i;. v*^.-/: ■~ ■'; ■ • •-. Tjhe_ exact position of these two subsidiary sumps'is not a matter of yer^s much importance, as the ground is so1-level; but the-* nearer they can1 be fplaced -fco JhQjaißngine-hduse front* whdnqe the power, is derived:-the; bettdf. The amount"of land required will be very small in either case, and the|most desirable site,, both for these and £he main sump^ can best becdetermined by a careful study of the local;conditions, cost of the required land;, and thei (trroximity, of houses, the qwner&.pC ,whij|h might be likely to raise objections and make claims for! compensation. . I tfai&l the position of the'sumps is fixedsjand plans prepared showing the j levelftjof all the streets, the planning of tlieS sewers cannot be undertaken, as theisige-'O&athe sewer. ipipe in- «ach street will to a certain extent be governed by "the fall oßtaiaable. But although it is not t s*6«sible to definitely plan the sew.jgisftj^Vpresent, it is not difficult to^Jkp^oximate the mileage of the various sizes that will be required. .iitYe*!!* I estimate thatf^lie-ffolWing will practically be quite sufficient to meet all requirements :-£ >„ ( 12in diamvsteui^i^e, ... 6 mil^s. ' 9in diambtec p?pe ... 9 miles. 6in diameter pipe ... 10 miles.

,25 ~ . ■ The sewer pipes should be glazed stoneware pipes, laid with cemented joints and with manholes at all juno.tions, and at a distance of not more than 330 feet apart. It, is very probable that the deeper sewers will have to be laid in concrete if the depth to which they have to be laid takes them into soft wet ground, and I have allowed a sum of money in my estimates for this purpose. PUMPING PLANT. I would recommend a suction gas plant the engine, to be 25 h.p., driving a generator which would work three pumps, one} at the main sump, the itwo others at the two subsidiary stations. These pumps would all be motor driven, the two subsidiary ones being actuated automatically by floats in the sewage wells, as is done vi some parts of Sydney. If these works were installed at the same installation, one staff would manage the whole, and the cost of a second set of buildings^ wduld be saved; and there would also be a saving on the plant. ->^_>?rv j If; worked separately, the machin cry should be at the main sump; here the largest pump would be re • jjuired to lift the whole of the sewage andJorce it through the outfall pipe . to the sea. • v The two other pumps would not hav^ much to do, but I have allowed fully for every contingency in my estimate of 25 h.p. By making the sump of sufficient capacity to hold 16 hours' sewage the pumps' would only need to be run eight hours per day. OUTFALL MAIN. This should be, Bin diameter C.I. main* It would- be underground till near the pullet of the lagoon, where for a distance of a few, chains it would be on trestle work; the seaward end should be placed at about half-tide level. ■.-; «j I estimate the total cost of the work when completed at £36,141, and the yearly cos^at. £2580 10s; but this is for the. wbjole scheme when com-! pleted, and the running costs are for dealing with sewage from a population of 3000. It will doubtless be quite unnecessary to complete the whole of these works for some years. Many of vne streets are so little occupied that it would be money wasted to lay the sewer on them at present. , The exact miles of sewage that should b© laid at present is chiefly a matter of policy for your Council to decide. It should, of course embrace all the closely-built part of the town, trenerally speaking, I should think that about ten miles would cover all that require laying at present, and the working expenses would be proportionately reduced.

m This would require a rate of W "• o ™e.£ on "the unimproved value of £303,604.

ESTIMATED COST OP DRAINAGE WORKS. ■ £ 1. Land, say 400 0 0 2. Mam sump and two subsidiary do 1500 0 0 3. Pumping plant 1700 0 0 4. Sewers— 12in dia. 6 miles ... 4800 0, 6 9in dia. 9 miles ... 6380 0 0 6m dia. 10 miles ... 6400 0 0 Manholes ........ 4080 0 0 5. One mile electric connection .... ... ... 300 0 0 6. Outfall pipe, Q. miles 6000 0 0 7. Buildings- ......:.. 500 0 0 8. Contingencies to allow for probable bad ground when laying the sewer, ' 5 per cent, on £17^680 ... 879 0 0 Engineering, raising loan, clerical, work; ;<}tc..;. 1646 0 C First year's interest; 4J ; per cent, on £34,585 1550 0 0 ■■-■: ;■ . ;:: = :•. -£36,141 0 0 Interest at 4| per cent., sinking fund 1 per \ «ent;—51 per cent.; oi per cent. on £36,141 ... 1987 0 0 Working costs— Wages, fuel, oil, re- _ Pairs ... ■ ... 423 10 0 Depreciation in plant and machinery ... 170 0 0 £2580 10 0