MR CLARK’S REPORT.

Globe, Volume IX, Issue 1268, 11 April 1878, Page 3

MR CLARK’S REPORT.

The following letter appears in this day’s “Press”:— Sir, —I agree . entirely with the remarks made by yourself, in one of your articles, as to the fact that the Drainage Board allowed Mr Clark to depart without obtaining from him some definite instructions as to dealing with the sewage after it has arrived at its destination, viz., the Sandhills, I trust I may assume that the Board will admit that there are in existence other persons as well as Mr Clark who know something about drainage. I hope also that the Board is prepared to admit that facts arc really stubborn things, and are not to bo disposed of by the mere report of an engineer, however eminent he may be. We are told to pump our sewage on to the Sandhills, and there filter it. One would suppose from this that the filtration of sewage was the simplest thing in the world, even where the filtering grounds are on that side of the city from whence the most prevalent winds blow all through the year.

I have therefore taken the liberty of writing to ask you in the interests of the public, and (may I with due humility say ?) for the instruction of the Drainage Board, to publish the following extract from a work published as late as 1877, by two engineers of good repute, residing in London. The book is a compilation of statistics, and also of the actual results of the various modes of dealing with sewage. I think a perusal of these facts will satisfy the most self-confident amateur drainer that there may be, and probably will be, very serious results arising from pumping the sewage on to the Sandhills, with the object of filtering it, and then allowing the efiluent water to flow into the estuary. There is at any rate quite sufficient in these facts to make the question of dealing with our sewage, by filtration alone, a matter for very grave consideration. Tours, &c., Ratepayer, FILTRATION. Excepting in laboratory experiments, and a few instances upon a very small scale, filtration of crude or untreated sewage, apart from irrigation, has never yet succeeded. Experiments were tried by the Rivers Pollution Commissioners, in 1868, with filtration of sewage through various soils, and they came to the very natural conclusion that the process of filtration through sand, gravel, chalk, or certain kinds of soil, if properly carried out, is the moA effective means for the purification of sewage. It should however, no longer be lost sight of that the Commissioners’ experiments were all ppon a very limited scale, and were not carried on uninterruptedly for any length of time. In dealing continuously with sewage of a town by filtration, two great difficulties are, in practice, found to arisej which are not perceivable in merely experimental measures. Whatever the filtering medium may be. laud, gravel, charcoal, or other substances, the result in the end is the same, viz.;—i. The pores of the filter become choked after a time with the solid matter contained in sewage; and sooner or later, when they are choked, the filter acts only in a partial manner, censing to purify what sewage may find its way through it, and eventually is unable to filter at all. 2. The collection of solid foecal and other matter deposited from sewage upon the surface of the soil or filter, does give rise to a formidable nuisance. A ttempts have been made to meet the first of these difficulties by removing and replacing the filtering medium, and, when charcoal lias been employed, cither to utilise it, thus fortifipd by the solid matters from sewage, as a manure or to revivify it by burning out the organic matter it has arrested from the sewage, and using it over again for the same purpose. However seemingly successful either of these may be on a very small scale, they have completely failed when put into practice; no ready sale for the charcoal as a manure occurs. It is moreover too expensive an article to _ keep in large quantities on hand, and, in addition, the cost of labor to constantly remove the large amount of filtering medium necessary for either purpose is so great as to make such an undertaking, even if it were satisfactory in other respects, quite impracticable ; attempts have also been made to meet the second difficulty by the cultivation of the su-facc of the filtering beds, the suppqsiticm being that the vegetation grown thereon will take up and utilise the organic matter deposited from the sewage. In practice, however, it is found that such an expedient will only answer as an auxiliary to irrigation or sewage farming, where the overdosing of the vegetation growing upon the filters may be avoided. Far Hi being a filtering medium easily obtained, and chemical as well as mechanical in its action, is of much value for this purpose, if carefully used. It may be safely employed, when the sewage is weak or very dilute, in combijnaipop with irrigation, and for the ti'lfratk>,n pf efiluent waters from precipitation processes, but with risk if used apart f rom either of these. Though many trials have been made, no instance can be adduced of a successful attempt to purify crude sewage by filtration alone. From tbe following places (tlurty-eight in number), where various modes of filtration have been adopted or tried, there appears scarcely any but unsatisfactory results in a sanitary point of view.

Abergavenny. Monmouthshire —Two sets of wickerwork filled with gravel have been employed. The pillars are cleaned out every six weeks, and the residue, dined, sells for manure at 3s fid per ton, but a nuisauce arises from it in hot weather.

Alton, Hampshire, tried subsidence and filtration through coarse shingle, but it proved unsuccessful.

Ashby-de la-Zouch, T eicestersbiro—The sewage was received in tanks, divided into compartments, between which strainers were placed, composed of perforated boards, 3ft. apart. The upper strainers were filled with coarse, the lower ones with finer pebbles, and the last with fine gravel, but the plan was abandoned. Atherton, Lancashire, employed three settling tanks and two filter beds, composed of broken stones, gravel and sand, but these are ah Out to be replaced by some other method'. Chelmsford —Previous fo irrigation subsidence and filtration through chqrcoal were Vied, but failed.

Cheltenham —Previous to irrigation filtration through gravel, witfi lime treatment, was tried, and abandoned. Chesterfield, Derbyshire—At the outlet cl each sewev is fixed a lilior, consisting of three screens composed of perforated metal plates, forming hoses, filled with gravel and animal charcoal, but a nuisance arises from the process. Coventry—Previous to the process now in use, subsidence tanks with lateral filters formed of perforated planks, inclosing walls of gravel 3ft. thick, were constructed at considerable expense and used, but completely failed. _ Croydon—Previous to irrigation tmed filtration through (h.rccal out it failed. Ji_y, Cambridgeshire Upward filtration through sand, &e., has boon tried and abandoned, and catch-pits with upward chequered woodwork sieves, are being tried with no bettor success. Tewkesbury, Gloucestershire Filters the sewage through three filtering beds of difi event sized gravel. It is said to answer thr p.mpnse. The following are the .cnvuning tihvty'-csght places where filtv.fi.ion has been tried and - Badwg, MhVttoaex ; EafHd. M ;^Aosex : Fast illabral; I Lincolnshire ; Harborne. Staffioyd. stiiro; Hurstpicrpoint; Litchurch, pcrby slrrc; Ludlow, Shropshire; Motley, Yorkshire fi Fc,v-castle-under-Lyuo, StalYoidshirc ; Oswestry, Shropshire 1 ; 6»t.' Thomas the Apostle, Devon ; Safirc.u Walden, Essex; Sent .am, arwickI st,ire ; Dlvei-stone Lancashire; Uxlnudgo. Middlesex; Walton-on-the-Hill, Lancashire; Wellingborough, Northamptonshire. Other places might be mentioned where nitration experiments have been tried, but it is considered that sufficient evidence Ins been obta’iiea to prove conclusively that whatever be me filtering medium employed, and whether, in conjunction with subsidence tanks or not, for crude vg untreated sewage the system lias been unsfiC" cessful.