SEWAGE ARRANGEMENTS OF CHRISTCHURCH.

Press, Volume XLVIII, Issue 7766, 21 January 1891, Page 5

SEWAGE ARRANGEMENTS OF CHRISTCHURCH.

I Remarks on the Chtistchurch district in connection with its drainage and the disposal of the sewaee, by Courtney Nedwill, M.D., and Edwia Cuthbert, M. Inst. C.E., Engineer to the Christchureh Drainage Board:— The Christchureh district is distant from the sea about five miles, above which, at high water, it stands only 16ft. The area included in the city and suburbs consists of 2800 acres, having a population of about 85,000. The ground is composed of a series of strata of clay, sand, and shingle, interspersed with water, above large water-carrying beds, which lie at a distance of from 80 to 230 ft from the surface. Springs derived from this source issue in several places over the district; and at Avonbead, four miles from Christchurch, some large ones form the headwaters of the river which flows through the town. Through Christchureh the fall ia fairly good; but thence, in its course to the sea, is very slight, tbe tide affecting it to within two miles of the eastern bouudary of the city. "Before the drainage works were put in hand the subsoil water was generally to be found within from 2ft to 3ft Of the surface, while in some places It frequently covered the land. The almost uninterrupted flow from the numerous artesian wells had a considerable influence in helping to make the district a waterlogged one. At this period Christchureh was undoubtedly very unhealthy. Slop water added to subsftsil water Saturated the ground, and common; cesspits were then to be . found in almost every quarter of the 4 4i«trict. In the yearlßJ5 4 theonew* are now sneak'" ing of, the population of the city itself was only 10,611, yet there were no less than 49 deaths from typhoid fever and 21 from phthisis, while the infant mortality rate, calculated by proportion of deaths under one year to 1000 oirth-v had reached the high figure of 184.2, the general death rate itself being 30.4. If we compare the mortality bill of 1889 with this bad record we see at once che improvement that has taken place. From a town dangerous to live in, Christchnrch ha 3 been converted Into a very healthy city. For this year the general death rate for Christchureh was only 9.77—the deaths registered from fever were only 5, those frcm phthisis only 8, while the population had in the meantime increased from 10,611 to 16,785. It is to be regretted that reliable statistics cannot be obtained for the suburbs, 80 that we might deal with the larger population of 35,000, bub there Is every reason for believing that they have fully participated in the general improvement tbat has been so manifest in the city." , The following is a short description of the works undertaken for the drainage of the district:—The sewers have been constructed by, and are vested in, the Christchurch Drainage Board, a corporate body constituted by a special Act or Parliament in the year 1875, "to provide for the drainage of the City of Christchurch and the surrounding districts." The surface drainage is collected by side channels constructed by and Under the control of the various local bodies in the district. They are for the most part of concrete, and consequently watertight, although many in the outlying portions are merely cuts in the ground. These channels discharge into natural creeks, the River Avon, and into sewers specially provided for stormwater, which discharge into the Avon and Heathcote rivers. It is much to be regretted that while the primary object of these sewers was for the caiTiage of storm-water only, they are in some cases rendered very foul owing to the ' nature of the contents of the side channels, and this a matter over which the Drainage Board has practically no control. The length of these sewers, which were constructed and maintained by the Board, is ten miles flf fceeu chains, and the cost of construction £57,000. They vary in size from 4ft Sin to 12in diameter. In abnormal rainfalls, which are of a somewhat rare occurrence, small low-lying portions of the district become flooded, but this is owing to the rise in the river, which prevents a free discharge from the sewers. On the cessation of the rain the water is quickly run off, so that it is not probable that the health ot the district is thereby affected. The system of sewers for the removal of sewaee proper was designed by the late Mr W. Chirk, M. Insc C.E., and has proved most successful. The separate system has been adopted, i.e., rain water, so far as possible, is excluded. This enables a very large percentage of the total length to be laid with sewers of small diameter. They are thus much more easily flushed, and there are not the same facilities for the generation of sewer gas as iv larger sewers. We think it is now generally admitted that both from an economic and sanitary point of view the separate system is beyond question the most suitable. The following table shows the different sizes of sewers with their .respective lengths: — Concrete egg-shaped sewers with brick arch, varying in sUe from lft 6in x 2ft 3in to 3ft Bin X 4ft 9io, 4 miles 10 chains; 18in diameter pipes, 10 chains; 15in do do, 2 miles 47 chainsi; 12in do do, 3 miles 3S chains ; 9in 'do do, 25 miles 22 chains. Total length, 35 miles 47 chains. Owing to the topographical features of the district it was found impossible (even had it been desirable) to obtain a natural outfall, so that pumping has had to be resorted to for the removal ot the sewage to the irrigation farm.The sewers vary in depth from §ft to 13ft below the surface. The pipes are jointed with cement mortar, and are laid on concrete, which is brought half way up the pipe, Manholes and ventilators are provided at frequent intervrls. As a general rule the gradients are so arranged as to obtain a velocity of 150 ft per minute. House connections are laid with pipes of 4in diameter, and soil pipes of 3in diameter. The area served is about 2800 acres in extent, and consists of the city of Christchureh, and surrounding suburbs of Sydenham, St. Albans, Linwood and a portion of the Heathcote, Biccarton and Spreydon dwtricts, with a population of say 30,000. There is ampie margin, however for a large increase in the population without involving any extra outlay in the existing sewers orpumping station. The hours worked at the pumping station for the year 1889 averaged 10-4 hours per and the amount pumped per diem to 62ab tons. Of this a proportion of say 80 per cent, is

subsoil water, which of course will not i materially increase for all time. From I tbis it will be at once seen to what extent i further connections can be made before I extra power will be required. The expenditure incurred for these sewers, machinery, buildings, tank, iron main to sandhills, land and contingencies, was £l_7,ooo.lnseverallocalitlea tbeqnicksands proved to be very troublesome, and special and costly provisions had to be made to overcome the consequent dangers and difficulties. Had it not oeen for tbis fact, the cost wouid have been considerably less than as before stated. Flushing Is regularly and systematically attended to, the water being derived from artesian wells and collected in tanks, which are placed in the most suitable positions. The pipes are as clear and free from obstruction as when first laid. The system is well ventilated by openings in the centre of the streets, and very few, if any, complaints are made on account of smells at these ventilators. That all important portion of the scheme, viz., house connections and internal and external sanitary appliances receives every care. Tbe work is done by plumber 3 and pipelayers licensed by the Board for the purpose, and is subject to the inspection and approval of the Board's Engineer. A code ot Sanitary Rules and Regulations for the guidance of licensees and the public, based on the most approved methods for work of this nature, has been in force for some years with the most satisfactory results, and licensees are bound by such rules and the conditions contained therein. The whole of the Bewera on the north side of the river are collected at a point on ita northern bank and passed through a cast iron syphon under its bed. On the south side they are collected by the main intercepting sewers (which of course also take the northern sewage) aud discharge into the tank at the pumping station. During the night, tbe main sewers do duty as reservoirs, for the sake of economy, but it is hoped that the Board will soon be in a more prosperous condition financially, when this system will give way to continuous pumping. In addition to these sewers there U a length of one mile twenty six chains of duplex sewers, in the suburb of Richmond, which take both storm water and liquid sewage—these discharge into the Kiver Avon, about one mile below tbe city." The total length of covered sewers under the Board's control is 47 miles 8 chains, and the number of houses connected, to 31st December last, was 1291. As each house is connected a survey is made of the premises, showing the exact position of tbe service pipe, syphons, junctions, &c. These details are plotted on block plans kept for the purpose, and all particulars are periodically entered up in the connection register. The collecting tank is 30ft diameter by 2lft deep. It is built of brick in cement, with a concrete invert, and was constructed under most unfavorable circumstances in a boiling quicksand. It is, however, of a most substantial character, and Is not in the least likely to cause trouble by settlement or otherwise. From this tank the sewage is pumped. The pumping plant consists of two compound surface condensing horizontal engines, fitted with expansion gear of forty-eight indicated Horsepower each. They are run with a consumption of about 3lb of coal per I.H.P. per hour. The pumps are four in number, two to each engine. They are double acting, and of the usual type for sewage pumping. Each pair is capable of throwing 3000 gallons per minute. The boilers are three in number, of the usual Lancashire type, and fitted with Galloway tubes. It is usual to work only one engine and one boiler at a time. The working pressure is 501b, and the vacuum generally stands at 27in. The cost of working, including coals, general stores, wages, and repairs is, say, £730 per annum. In times of excessive floods the sewage is sometimes pumped to the Estuary direct for the sake of economy. It is then, of course, largely diluted with subsoil water and rain water, which gets through the ventilators, and house connections in low-lying localities. The iron main to the Sandhills is a mile and threequarters long, of 24in diameter. It has been laid with turned and bored joints, which have never shown any- sign of leakage. The mean lift is 30ft. At tne end of the rising main the sewage enters a wooden flume for a short distance, and thence into a settling pond, thence along an open carrier, about one mile in length, and through a second settling pond before it is distributed in the brancn carriers and I applied to the land. A large quantity of. sewage sludge, which proves injurious to the land by choking the pores, is caught m the settling ponds. These are cleaned out periodically. Having arrived at the farm the sewage is distributed by carriers over seventeen paddocks laid down with English and Italian rye grasses, and there treated by surface irrigation. The liquid ! has not only to pass over the surface, but 1 must percolate through the land before it is caught by the eflfuent water drains, through which it is discharged into the Estuary. The area of land prepared for irrigation is at present only thirty-alx acres, and consists of a very sandy loam. Jt has not been found necessary as yet to provide subsoil drains, owing to the extremely porous nature of the soil. The growth of grass is very prolific. Only two cuttings of hay are made each year, as it is found more profitable to feed cattle and dispose of tbem in the shape of beef. None of the grass paddocks have as yet been ploughed since they were formed, but this may shortly be found necessary in some of those at first laid down owing to a tendency to rankness iv the grass and the presence of Yorkshire fog. They are formed level on their longitudinal section, and slope each way from the branch carrier, with a fall of 1 in 150. Each paddock contains about two acres. The operations at the farm for the last year show a profit of £54 5s Bd, and it is expected that better results will be obtained when the supply of sewage increases and more land is prepared for treatment. Of all modes of disposing of sewage in a healthy and proper manner none can compare with filtration through and . irrigation over the land. As each year passes, the tendency seems more and more to favor this method. It is only necesi sary to refer to the recent sanitary congress at Brighton, and the able paper on the purifying qualities of tbe earth, read by Mr Vivian Poore, to be convinced of this fact. Charges have been made against sewage farms as being injurious to health, but when the sewage, is delivered In moderate quantities, and in a state before important putrifactive changes have occurred, no danger need be feared. The milk and butter from cows fed on sewage-raised food, compares favorably with other samples, while vegetables grown oh sewage farms are of excellent quality. When the members of the Association visit our farm they will see for themselves that cattle thrive amazingly on the grass raised by sewage off common sandhills. These sandhills, which were perfectly worthless, will in time produce large quantities of excellent crops, although at gresent it cannot be contended thac the ltration is as effectively performed as would be the case In good mould. The following analysis, made for us by the Colonial Analyst, shows that in tbe passage of the sewage through the sand a certain amount of ammonia has escaped conversion, which would not have happened in better land. ANALYSIS. 1. Sewage—Colorless and turbid with the odor of putrefaction. 2. Effluent Water—After No. 1 had been passed through the paddocks, and as discharged into the estuary. The following results were obtained upon them as computed in grains per gallon :— No. 1. No. 2. Ammonia ... ... .16 .02 ♦Organic matter ... 10.21 4.21 tMineral salts ... 10.81 10.12 21.18 1_35 * No. 1, containing nitrogen, .51 grain ; No. 2. do do, .16 grain. ■"* t No. 1 contains .21 grain of phosphoric acid and .76 .'grain of potash; No. 2 contains traces of this acid only and .31 grain of potash. " The precise nature of the rest of the mineral substances in these waters has not been determined, but as far as was shown by qualitative analysis they appeared to consist in each case mainly of alkaline chlorides and sulphates. These results snow tbat the sewage has been very largely deprived of its more valuable constituents, and that tbe soil through which it has been passed has gained those substances that were most deficient in it after having been exhausted by cropping, j In conclusion, we can unhesitatingly I assert that the sewage system has so far been an unqualified success. Its benefits are directly felt by the large amount of sewage which is daily removed from the district, and the indirect benefit hy the lowering of the level of saturation, which has conduced so largely to the health of the people. As bouses are more freely connected with the sewers, so will £heir direct benefit be more appreciated and felt, and we can only express the hope that the ratepayers at large will be sensible of this fact ,,r )