MR CARRUTHERS'S NEW SCHEME.

Globe, Volume VIII, Issue 937, 26 June 1877, Page 2

MR CARRUTHERS'S NEW SCHEME.

The following report by Mr Carruthcrs on the modified scheme of drainage proposed by him was read at the meeting of the Drainage Board yesterday : Wellington, 22nd June, 1877. To the Chairman of the Christchureh Drainage Board. g IR __i have the honor to acknowledge the receipt of your letter of the 29th ult, informing me that the Board would send My- Bell to hold a conference with me on the subject of the scheme of sewers designed for the drainage of

Christchurch, and enclosing a series of suggestions passed by a committee of ratepayers and adopted by the Board. I have in conference with Mr Bell gone carefully over the whole scheme to see if any modification which I could recommend would meet the suggestions of the committee. The first suggestion is that no watercloset excreta shall be removed by means of underground sewers. I regret that the Boai'd have adopted this suggestion, as I think the ratepayers will thereby lose an important part of the advantage of a. system of sewers, and will gain nothing in return.

I know how difficult it is to convince those who have not studied the subject of town sewerage that watercloset excreta are not more offensive or more difficult to deal with than an equal bulk of sewage derived from other sources.

There is however nothing better known to sanitary engineers. Nearly every writer on the subject has given the chemical analysis of the sewage of towns in which the waterclosets are connected with the sewers, and of others in which cesspits or tubs are used, and the analysis shows there is no perceptible difference between the two.

Commission after commission reporting on sewerage matters have one and all insisted on this elementary truth, which is still however persistently disbelieved by those who consider their own hastily formed impressions more trustworthy than the careful and patiently thought out opinions of others. The two largest towns in Europe have adopted the opposite systems. In London cesspits are prohibited, in Paris they are compulsory: yet the sewage of the two towns is practically identical. Watercloset excreta consist largely of carbon and other inoffensive matters, and have a very small manorial value.

Dr. Gilbert, who is perhaps the first authority on agricultural chemistry in Europe, found that the earth of an earthcloset, after it had been used three times, was poorer in organic matter than the surface soil of an ordinary wheat field. By far the greater part of the value as a manure of the human excreta lies in the urine, most of which finds its wny with the slops into thesewers, however vxrefully the less offensive waterclosets are excluded. It is true tha sewage from a closet, when fresh, is more offensive to the senses than other sewage, but in a very short time, when fermentation has begun,, the latter becomes the more offensive and injurious to health. Kitchen refuse, fat, the washings of streets, and refuse from breweries and other factories, all contain organic matter,, which, when fermenting, differ but slightly from the sewage, and are just as injurious to i health.

These cannot be separated from the water' with which they are mixed, and must of necessity be carried away by sewers whether open or 1 underground. There can be no advantage in excluding from these sewers the waterclosets which do not contain more organic matter than an equal bulk of the other sewage which they carry, but if the Board decide to do so I see no great objection beyond the loss of money and comfort which the ratepayers will sustain. There are a good many towns where earthclosets are used which are not less healthy than those where the more civilised and convenient water-closet is in use, and although the balance of evidence is clearly in favor of the watercloset in point of healthiness, the earth-closet in some of its various forms appears to be nearly as good, if managed with skill and care, and a complete disregard of cost. The Ratepayers' Committee appear to have adopted the opinion of their engineer, or to have been misled by a wrong generalization that the solid matter which sinks to the bottom of sewers and causes so much trouble to the sanitary engineer, consists of the solids contributed by the water-closets, and that if these were excluded the necessity for flushing and for considerable velocity in the floor of the sewers would disappear. This is a very common opinion amongst those who know nothing or very little of tha subject, but I do not know of any engineer besides Mr Bray who holds it. Water-closet excreta are lighter than water, and are carried to the outfall without difficulty. Sewage sludge consists of inorganic matter, principally sand and clay in mechanical contact and mixture with organic sewage. So much is this the case tint one of the most successful of the " precipitation" systems, that of Scott's Sewage Company, owes its success entirely to this fact. The sewage is by this process treated with lime, the effect of which is, that the sewage solids and the lime are together mechanically precipitated, as the lees of wine are mechanically precipitated by white of egg. The precipitate consists of the lime thrown in, of tli3 clay and other solids contained in the sewage, and of organic matter. When these are burnt together the lime and clay form Portland cement, by the sale of which part of the cost of manufacture is defrayed. The opinion of the committee that watercloset solids have any important influence on sewage is quite opposed to all modern sanitary enquiry. They form a comparatively unimportant part of the whole sewage to be got rid of, and a system of sewers properly designed for the purpose of carrying away other matters may, without any change in size, form, or inclination, carry these away also. In designing a sewer system for CLristchurch I should not in the slightest degree have changed the plans had I known that waterclosets would have been prohibited. On the whole, I regard the first suggestion! of the committee that watercloset excreta shall not be removed by means of undei-ground l sewers as comparatively unimportant; as far as its effects would extend it would be, I think, injudicious to adopt it, but it would not have any serious effect on the health of the town. It would only cause a loss of money. The second suggestion of the committee that no portion of the excreta shall be placed in the estuary is practically the same as the first. I have shown that watercloset excreta do not increase the foulness of ordinary sewage, and if any sewage is allowed to go to the estuary there would be no harm in allowing this also togo there. The Board have, however, decided not to allow waterclosets, and none of the excreta can therefore be brought to the estuary. A great deal has been said and written against allowing any sewage into the estuary at all, but I have not seen any proposal by which this can be avoided. The sewage must go somewhere. At present it goes partly to the Avon and partly to the estuaiw, and a great deal is allowed simply to soak into the ground. If it is not allowed to go into the Avon, nor into the Heathcote or estuary, where is it to go ? The sea is three miles away, and the sewage, would have to be pumped to get it to flow there. If it were worth while going to the expense l , it might all be thrown into the sea, but the cxj>enscwould be beyond the means of the present population. By letting it flow into the estuary no harm will be done for many years, and when the town becomes larger it will bo very easy to pump the sewage as proposed in my report into the neighbouring sandhills, or even into the sea.

Professor Bickerton's analysis of the water in the estuary shows that although the sewage of half the town has been flowing into it for years, it has not been in the slightest degree polluted. The next suggestion ol the committee is that an estimate of the cost of the scheme of drainage to be adopted shall be first submitted. I have prepared complete estimate s, which I will hereinafter submit.

Suggestion fourth consists of two _ distinct parts. Ist. That a system of drainage shall bo adopted avoiding as much as possible

the construction of pumping stations; and secondly, that such system shall be independent of any system of underground drains, and connected with the latter only when necessary. The wording of the suggestion is very involved, but 1 understand it to have the meaning above given. If the word " possible" in the first part of the suggestion be taken to mean "expedient, as was probably intended, the suggestion becomes one that no one would willingly disregard. _ It may in any system be possible to avoid pumping, and yet inexpedient to do so on account of the cost; and in such a case the committee would not, I presume, recommend pumping to be adopted. When I recommended a system with two pumping stations I was not aware that the existing outfall drain was so badly built that it was unfit to form part of a permanent system of sewers. The Board having, however, now informed me that such is the case, I have prepared estimates of a new outfall drain at a level which would carry off all the sewage of the town withont pumping. The extra cost of this sewer over that or pumps would be ,£41,000, and I have great hesitation in recommending the Board to incur so great an expense. The cost of pumping would be .£I7OO per annum, if no restriction to the flow of artesian wells be made ; and this sum would be sufficient to pump the sewage of 120,000 people if waste of water were prevented, or at least means takea to keep overflow water out of the sewers. As however the existing sewer is found to bo defective, it would be better to incur the extra expense at once, and to build a new outfall, as sooner or later it will have to be done, when the old one becomes too wrecked to be of use. This alteration in the outfall would make only the slightest difference in the system of sewers already designed. The main sewers of the northern system, which was designed to fall into the pumping station at the corner of Kilmore street and the East belt, would be continued silong the East belt to the outfall sewer, and the main sewer of the Ferry road system would turn tip the East belt to the same place, none of the sewers of these two systems would be in the least changed in size, level, or inclination. The inclination of the sewers of the high level system would be modified to suit the lower outfall, but the alteration would be so slight and obvious that the present plan may be regarded as unchanged. The outfall sewers along the Ferry road would be of course clone away with and a smaller one at a higher level substituted to drain the road itself and the neighboring country. If the Board should decide to incur the ex. pense of .£41,000 this suggestion of the committee will be fully met. The second pari of the committee's suggestion is practically a recommendation that a system of surface drainage be adopted. I cannot recommend such a system for any town of the population of Christchurch, and for a flat town like Christchurch least of all. It means that all sewage, except the comparatively unimportant watercloset dejecta, are to stagnate in the side gutters of the streets until washed by rain into the natural watercourses. Throughout the greater part of the town the section's are scarcely, if at all, higher than the gutters. Sewage matter cannot even get to the gutters, but lies in the yards where it soaks into the ground, it with decomposing organic matter, and breeding fever and other diseases. In the houses of the wealthy little barm is done ; there is plenty of land to deodorise the sewage, and little trouble is necessary to have it carried to the garden, where it is useful; but in the houses of the poorer classes the _ case is very different. There no opportunity exists for -utilising the sewage, and even where there is the owner seldom has time or knowledge to apply it to <p»»oper use. In some of the most thickly peopled parts of the town there is scarcely any fall to the ground, and the side-ditches stand fall of stagnant sewage, the level of which is scarcely below that of the floors of the houses. When rain falls the houses are flooded, and are often under water for days together, the water being simply dilute sewage. The surface drainnge system is in fact that which is now in use in Christchurch, and which has made it the most unhealthy town in New Zealand. In spite of its fine climate, its great extent compared to the population, the practical absence of poverty, and the large proportion of men and women in middle life due to immigrafcio.u, its death-rate is higher than that of London, and is double the average of the rural •parts of New Zealand. In otSjer words, nearly half the people who die in Christchurch are murdered by the " surface system of drainage." I think the gentlemen who recommend the perpetuation of such a system cannot have seen the back streets of the town. The number of deaths does not measure the loss to the community ; for every death it has been «alculated that thirty people are laid up with illness, and the number who suffer from general Joss of health and strength, lassitude, and the other less deadly forms of sewage illness is much greater. A surface drainage system for a town is radically bad, and should never be adopted. Sewage may be said to begin at the sink outside the kitchen door, where kitchen refuse, slops, and other putresciblc matters are thrown. If allowed to run in an open gutter they soon putrif y, they must therefore go through a closed pipe. A less fall than one in 50 is not admissible for such a pipe as would be used, and as the centre of the street is seldom less than 150 feet away, the street drain must be at least three or four feet below the level of the kitchen sink. In a flat town this meana three or four feet below the street, and an open drain of that depth is out of the question. We are therefore driven to an underground sewer at the very beginning of our course, and the necessity of maintaining a proper vlocitv of flow in the sewers increases the depth below the surface at which they are placed, unless the ground has also the proper inclination, which in Christchurch it has not. Complaint has been made that the system of sewers I have designed for the Board is placed at too deep a level, but they areas shallow as they can be made.

In every case the sewers at the upper end of the system are brought as near the surface as possible, and the inclinations are as flat as would allow the sewers to be self-cleansing. The system is, in fact, as near an approach to surface drainage as is admissible in a town. If the Board should attempt to keep the sewers nearer the surface by lessening the inclination and following more closely the ground line, they will most certainly incur a complete failure. The sewers will choke, and no amount of flushing will clear them. They will in a short iame have to be dug up, after spreading disease •aixt death in the town. It would be better to '{maintain the present system of so-called surface drainage, which is less deadly than the other -vvould be, and the high death rate would be due ito an act of omission instead of an act of comMiissiou as in the other case. The words used by the committee to convey their suggestion might be taken to mean that they recommend the separation of the rainfall from the sewage. In my former report, I so fully explained why this system is quite inapplicable to the conditions of the case, that I need not repeat my arguments. It is a system which can very seldom be advantageously used, and would be very costly and ineffective if adopted for Christchurch. The following are my estimates of the cost ol the works proposed ■— F<# a complete system, with a sewer in every street in the town, and surrounding district, duuips, (tanks, man holes, ventilators, syphon, flushing stakes, storm outlets, and all other expenses,'but making use of the existing outfall

drain as shown on the lithographed map, .£170,000. . For the same, but with new outfall dram to estuary, and without pumps, .£211,000. I do not think any complete system can be designed which would cost less than the above sums, and be at the same time effective. It is not necessary to carry out the whole at once, and a great deal of the work should certainly be postponed until the population increases. The most important parts are the main arterial sewers. If these were completed the minor sewers could be made from time to time as might be required. The cost of the primary sewers of the system utilizing the present outfall sewer would be £G5,000, and would include the following sewers : South District.—Ferry road, Gasworks road to Jackson's creek, Madras street, south belt, to Pound road. High Level.—Tuam street, east belt to Manchester street; Manchester street, Tuam street to south belt; south belt, Manchester to Colombo street; Colombo street, south belt to Pound road; St. Asaph street, Manchester street to Lincoln road ; Montreal street, south belt to river overflow. Northern District.—Madras street, river to north belt; north belt, Madras street to Papanui road ; Papanui road, north belt to creek, twentyfour chains ; Northern road, Edgeware road and Crescent road from north belt to St. Albans creek; Springfield road, north belt to creek, twenty-two chains. The same system of arterial sewers with a new low level outfall would cost ,£129,000, and would include all the sewers above enumerated and also a sewer along the east belt, from Kilmore street to the south belt, and along Kilmore street from Madras street to the east belt with a syphon across the river. The advantage of adopting cither of these proposals would be that the most unhealthy and flooded parts of the town would be at once relieved, and that whenever a new sewer was required it could be built as part of the complete scheme, so that money would not be wasted by building sewers to be afterwards pulled up as not fitting in with other sewers in line and level. Of the two proposals I recommend the more expensive, although, as I said before, I have groat hesitation in recommending the Board to increase the cost of the sewerage of the town by so large an amount as £ 11,000, and should not do so if the present outfall were well constructed and likely to last. I may mention that the above estimates include engineering and all other expenses. To summarise what I have above said in special reference to your letter under reply, I beg to say—1. That I do not think any system can be designed which would fully serve the town and suburbs, and cost less than that I have submitted to you, but that it is unnecessary to complete the whole at once. A great deal of the sewers would in any case not be wanted for years. The .arterial sewers of the system would cost £G5,000 with the present outfall, and £129,000 with a new low level outfall.

Seeing that the present outfall sewer is so badly constructed.that it will probably have to be rebuilt in a few years, I think it would be better to undertake the more expensive scheme at once. 2. I think any system of sewers which arc properly constructed to carry ordinary sewage may be allowed without any change to carry watercloset dejecta. The Board do not propose to allow waterclosets to be adopted, but this will have no influence whatever in the design of the sewers. 3. The pumping stations originally proposed were only adopted because the expense of avoiding them was too great. Seeing however that n°a few years a large expense will havo to be incurred to rebuild the present outfall drain, I recommend that the extra expense be incurred at once. , . 4. The smaller branch and pipe drains may be all left out for the present. The main arterial drains will greatly improve the health of the town, and will relieve the flooded districts where at present fever, diseases, &c, are very prevalent, and from time to time new sewers can be added where most urgently required. I have the honor to be, Sir, Your obedient servant, John Carkuthers.