Cleansing of Filters.—Every two or three months (according to the kind of water) air should be blown through, and if the charcoal be in the block form it should be brushed. Then four to six ounces of the pharmacopœial solution of potassium permanganate, or twenty to thirty grains of the solid permanganate in a quart of distilled water, and ten drops of strong sulphuric acid, should be poured through, and subsequently a quarter to half an ounce of pure hydrochloric acid in two to four gallons of distilled water. This plan would be useful on foreign stations where the filter cannot be sent home, or taken to pieces; if it can be taken to pieces, the charcoal should be spread out in a thin layer, and exposed for some time to air or sun, or heated in an oven.
The Average Composition of Thames Water, before and after Filtration
through Spongy Iron.
| Description. | Dissolved Matters. | |||||
| Total solid impurity. | Organic carbon. | Organic nitrogen. | Ammonia. | Nitrogen, as nitrates and nitrites. | Total combined nitrogen. | |
| As delivered from Chelsea Waterworks | 28·04 | ·198 | ·042 | ·0009 | ·117 | ·220 |
| The same water filtered through spongy iron | 16·8 | ·069 | ·018 | ·019 | ·018 | ·049 |
| The mean of the 14th and 15th taken after the spongyiron filter had been in operation in the Rivers Commission Laboratory forupwards of eight months.[305] As supplied from Waterworks | 24·47 | ·170 | ·055 | ·001 | ·098 | ·154 |
| After filtration through spongy iron | 14·26 | ·083 | ·016 | 0 | 0 | ·016 |
[305] The figures demonstrate that the purifying action of spongy iron, if at all altered, has been increased, as regards the most important impurities of water, viz., nitrogenous matters and hardness.
| Description. | Dissolved Matters. | |||||
| Previous Sewage or Animal contamination. | Chlorine. | Hardness. | No. of samples analysed. | |||
| Temporary. | Permanent. | Total. | ||||
| As delivered from Chelsea Waterworks | 1·464 | 2·01 | 15·5 | 6·2 | 21·7 | 15 |
| The same filtered through spongy iron | ·177 | 2·00 | 6·8 | 4·9 | 11·7 | 15 |
| The mean of the 14th and 15th samples taken after the spongyiron filters had been in operation in the Rivers Commission Laboratory for upwardsof eight months. As supplied from Waterworks | ·675 Analysis of the 15th sample. | 1·95 | — | — | 19·1 | — |
| After filtration through spongy iron | 0 | 1·95 | — | — | 9·6 | — |
If sponges are at all used, they should be removed from time to time, and thoroughly washed in hot water.[306]
[306] Parkes ‘Practical Hygiene.’
Oils are filtered, on the small scale, through cotton-wool, or unsized paper, arranged in a funnel; and on the large scale, through long bags, made of tweeled cotton-cloth (Canton flannel). These bags are usually made about 12 or 15 inches in diameter, and from 4 to 8 feet long (see engr.), and are inclosed in bottomless casings, or bags of coarse canvas, about 5 to 6 or 8 inches in diameter, for the purpose of condensing a great extent of filtering surface into the smallest possible space. A number of these double bags (from 1 to 50 or 60) are connected with corresponding holes
in the bottom of a block-tin or tinned-copper cistern, into which the oil to be filtered is poured. The mode in which these bags are fastened to the cistern is of the utmost importance, as on the joint being close and secure depends the integrity of the apparatus. Three methods of doing this are figured in the engraving, which, with the references, will explain themselves, the same letters referring to the same parts of each.
