BOROUGH DRAINAGE.

Franklin Times, Volume 9, Issue 759, 8 September 1922, Page 5

BOROUGH DRAINAGE.

PUKEKOHE SCHEME FORMULATED TO COST £20,700. ENGINEER’S REPORT. A comprehensive report on a scheme c>f drainage for the! Pukekohe Borough has been formulated by the consulting engineer, Mr T. V. Gulliver), and accepted by the Borough Council, after careful deliberation Inter alia the report states: — ‘The area dealt with is that portion bounded approximately by Victoria Street, Seddon Srreet, Hogan Street, railway line, Manukau Road. Harris Street and Helvetia Road, containing about 240 acres and having a present estimated population of' 1100. The Borough for drainage purposes may be considered as divided into two portions by ,a line running veVy approximately east and west, between King Street land Harris Street. The northern portion drains to the creek which runs parallel to the railway to Paerata and the Southern falls to the large area of flat land lying between the Show Grounds and Manukau Road, The pakt proposed to be sewered at present lies roughly on the watershed bqtween the two systems, but the greater dlrea is on the northern side. To bring . the p>art lying on the southern side into the northern outfall means deep cutting' along Harris Streqt and across Queen Street., but there is no doubt that this is the best route to adopt; firstly, because there is not, sufficient population on this portion to justify the expense of providing separate disposal wdrks ; secondly, because deepj cutting would be necessary even if such works wei’e provided ; and lastly, because the fairly large pipes due to the flushing requirements of the flat grade proposed can be used to carry the sewage from' possible extensions. . , ‘

The total asreia which can beirdrained economically to one point- on the northern area is approximately 1100 acres; ‘of this 240 acres a|re considered in the preserit-' scheme,, and a further 320-acwes c*an be brought into it. jrhesfreimaining portion of 540 acres is tße* area lying to the northwest of Victoria Street, draining by way -. of (the" creek flowing ovdr Hickey’s dam,, -and no provision has been made for this in the proposed design. The houses in Dublin Street and .Princes Street are in this area and an endeavour was made to bring them into the scheme, but the cost was found to be excessive when compared with the benefit derived.

Having derided to bring the sewage of the ar€(a under consideration $o one point of discharge on the northern side of the Borough the next thing is to fix upon the most suitable site for the purpose, and also to consider the method of disposal ,to be adopted* as this' to a great extent affects the site. The simplest .and most satisfactory method cf disposing of sewage is to dilute it with large volumes of fresh or salt wafteij, the only othe> alterna. tive being either irrigation upon a sewage farrm, treatment upon iwrtificial filters, or ia combination of some of the foregoing methods. The proposal contained in the scheme already submitted to you was,, I und«srsjtand|j, first to settle and filter the sewage and then to discharge the effluent down boreholes. This is really equivalent to .artificial treatment and disposal by dilution, for a trial boring is said to have disclosed the presence of a large body of underground water. I would like to point out that efficiency of: purification by dilution depends upon the presence of available oxygen in the diluting water and on its replacement by absorption from the air to compensate for loss as it is used up. 1 am afraid that water at the depth encountered in the trial bore would have great difficulty in getting this further supply of oxygen and, ’n the absence of this and of light, that conditions underground would speedily become equal to (those., of a very badly over-worked septic tank. It may be said that it would not matter what happened underground and that conditions in time would tend there is some possibility of a connection between this water and that used by -the Borough for domestic supply. I believe that towards stability, but I consider that the water in the bore at the pumping station rises to an elevation of 165.0, or 28 feat below the surface ard in the drainage boire, 90 chains it rises to elevation 152.0 or 16 feet from the surface.; so that if we assume the water in both bores to come from the same stratum, there would ‘appear to be a difference in level of 13 feet in favour qf the pumping station. That is to say, if there were any tendency for the water to move between the pumps and the drainage bore, it would be from the former towards the latter. However, pumping is certain to lower the level i!n the bore at the station and quite a short drop would give a reversal or gradient or a flow from the drainage bore to the pumps. On

the other hancfy whether the water comes from the same layer or not, the elevation of 152.0 for water level in the drainage bore is only 1 foot lower than the outlet .from the filters and a slight rise in water level would cause an overflow. It is not likely that the wafer in the bore comes from the swamp since /this is at an elevation of 140 0 or 12 ifeet lower.

There is the further risk of the solid matters in the effluent blocking up the surface of the bore where it is exposed) and causing an overflow at the surface. The condition of a sewage, effluent from filter’s is very different from that of fresh water, the farmer containing a small amount of fibrous and colloidal matters, which have a great tendency towards gathering around solid bodies in theijr vicinity, and I consider that in any material which forms a water bearing stratum! there is an ideal substance far this purpose* Should the underground water be in motion the material in which it is to be found would be bounds to effect a very considerable spraining action upon <the effluent and so promote blockage, while the difficulty of reaeration and risk of contjaimmation would also still be present.* I will admit that if there were a reasonable certainty at the bokes proving satisfactory, this method! would be the most convenient, for the Borough to adopt, bat as I look at the matter I think that St would! be a very temporary and rather dangerous experiment.

The next thing to consider is whether dilution can be obtained in any surface waters and all that is available is the creek flowing to Paerata which, in its present state, is quite unfit to take, the discharge! of any but a very highly purified: .effluent; At Paerata, however, the creek is in much bettefr condition and would provide very satisfactory dilution, but I think* <that the cost of a pipe line would be quite beyond the present resources of the Bdrough, and thiaft there is no guarantee that more thorough purification in the future might not be necessary and' that should such become necessary, its efficient carrying out would renddr a pipq line superfluous. There is no ax'ea cf flajji land which could be /used aS a sewage frtryn near to any satisfactory outfall site. It is possible ho carry artificial treatment, by itself, so far that the effluent produced is practically harmless, and I submitted a scheme of this kind to the Medical Officer of Health (Dr T J. Hughes) at Auckland, saying that I was of the opinion that after such purification, and a final sterilisation if found necessary, the sewage coiuldi be turned into a ditch, cut through the swamp at the head of the) creek, and that this would provide a final solution of the problem. He agreed l with me that a scheme of this type was theoretically (right, and said it would be quite satisfactory if he could be sure that the plant would be kept in an efficent condition and under expert supervision. I was unable to convince him that such would be the casq, land finally visited Pukekohe with hiim and obtained his approval of a scheme which combines Treatment with dilution. (The proposal is to pass the sewage through septic tanks and; then through trickling filters dosed by an automatic distributor. The effluent from these after passing through a settling tank, of small capacity is to be' led by a pipe to the creek and discharged the're at a point just below the swampy area on the north side of Seddon Street! by the Cape bridge. The creek is to be cleaned out and given a clear channel from this point to th,e Borough boundary, a distance of layout 60 chains. 1 think that this will give efficient disposal ; there is a considerable body of water in the creek and as i(t mostly comes from the springs above Hickey’s dam should not fall off much in volume during the summer months. The cleaning out of the channel will pat the winter in much better condition to receive the effluent at present most of its oxygen i.s taken by the growths of lillies, etq, and when cleared the water will aerate much more rapidly. It may be an objection to this scheme that there is a possibility qf the Borough having to assume responsibility far the creek ; clearing will have to be done periodically,, and it may be held liable for any damage due to floods or other alterations brought about by interference. The possibility of the proposal not proving final is an objection which can be held against all the schemes. It will be seen from the foregoing that treatment has to be applied to the sewage and this means that the disposal works have to be on land which has sufficient elevation to give the requisite fail through the filters, etc. The only position which fulfils this requirement and at the same time allows an economical arrangement of sewers with respect to the ultimate area to be served, is at title head of the swamp on the east side of Franklin Road slightly

north of the old rubbish tip. There are two possible sites here, one near the bore and the other nearer the Cape Hill Road. The first of these is the better, as it not only allows plenty of room for extension and has ample elevation, but has the additional advantage that the tanks, etc*, can be farther from, the road and may be constructed underground so as to be practically out of sight.’’ In the proposed scheme provision is made for septic tanks, 48 feet by 21 feet by 6 feet divided into two compartments ; one coarse! filter 45 feet in .diameter by 5 f.eeft deep ; and settling- tank. The report suggested the acquirement of about 2% acres of land for treatment works to provide for present and future requirements. Dealing with the proposed sewers the report adds : Main Sewer : A short length of 12 inch pipe grade, 1 in 75, from the septic tanks to the junction with the main sewers to Harris Street and West Road. Main Sewer to Harris Street: This is a 9 inch pipq, grades 1 in 58, 1 in 110, e?tc., running from /the main sewer through the Railway Reserve to a point opposite the last house in Hall Street. Here* in) the future a connection cvan be made to bring in the Totara Avenue section. It then passes under the railway line as a 9 inch pipe,, 1 in 130, to the junction with the main sewer to King Street, and continues as a 9 inch pipe, grade 1 in 21, to IfJ point at the back of Hall Street! Here it becomes al2 inch pipe, grade 1 in 600* and passes across King Street, through private property to Hall Street, along Hall Street and Harris Street to Queen Street. (Thence as a 6 inch pipe, grades 1 in 33, 1 in 110, etc., to the end of Harris Htreet. Branches serve Queen Street, Edinburgh Street,. Roulston Avenue, Mai ukau Road, Harris St. and King Street, and future extensions may be brought in along Queen Sflreet and Wellington Street.

Main Sewer to King Street: A S inch pipe, grades 1 in 16.5, 1 in 170, etc,, starting at the main sewer to Harris Sfreeti and crossing Hall St. and Edinburgh Street* turns to the south parallel to Edinburgh Street, crossing King Street and finishing in private property oni the south of King Street. Branches serve Hall, Edinburgh and King Streets, r Main Hewer to West Road: A 9in_ grades 1 in 80, 1 in 25, dtc., commencing at the main sewer and passing through private property to Edinburgh Street. It continues as a 9 inch pipe, grades 1 in 250 and 1 in 230, across by the Convent to Seddon Street, along Seddon Street and Victoria Stlreet to Harrington Avenue. Here it changes 'to a 6 inch pipq, grades 1 in 27, 1 in 50, etc., along Harrington Avenue, West Road and a short length of Helvetia Road. Branches serve Edinburgh Street, Seddon St*, Albert Ro.aid, Tobin St., and extensions can be made along Victoria Street and to bring in the lower pdrt of Dublin and Seddon Streets and Hogan Streejt. All sewers are to be glazed, stoneware socket pipes, jointed with cement mdrtar and laid on a bedding of lime concrete. The lines of pipes have been kept to the footphths in streets and, have been '.taken into private property when such appeared necessary, either to avoid deep cutting or interference with main roads.

Provision is also made .for the necessary branch drains, flushing (all sewers are designed to antic pate future growth of population, and are, in consequence too large fdr the flow through them at present) ; and ventilation. Estimate : The estimated cost of the complete scheme, including pipe laying, treatment works, outfall pipe, engineering and contingencies is £20,700. As there is only £IO,OOO available for the work it is proposed to commence, the' drainage qf the main street, Edinburgh S*reet, Tobin St., Albert St, Seddon St, and Hall St. Further matters concerning drainage will be briefly dealt with in our next issue,