The operation of household filters is necessarily, with rare exceptions, left to the kitchen-girl and luck. Scientific supervision is practically impossible. With a large filter, on the other hand, concentrating all the filters for the city at a single point, a competent man can be employed to run them in the best-known way; and if desired, and as is actually done in very many places, an entirely independent bacteriologist can be employed to determine the efficiency of filtration. With the methods of examination now available, and a little care in selecting the times and places of collecting the samples, it is quite impossible for a filter-superintendent to deliver a poor effluent very often or for any considerable length of time without being caught. The safety of properly-conducted central filtration is thus infinitely greater than that from even the best household filters. Further, it may be doubted whether an infected water can be sent into every house in the city to be used for washing and all the purposes to which water is put except drinking, without causing disease, although less than it would if it were also used for drinking. The use of household filters must be regarded as a somewhat desperate method of avoiding some of the bad consequences of a polluted water-supply, and they are adopted for the most part by citizens who in some measure realize the dangers from bad water, but who cannot persuade their fellow-citizens to a more thorough and adequate solution of the problem. Such citizens, by the use of the best filters, and by carefully watching their action, or by having their drinking-water boiled, can avoid the principal dangers from bad water, but their vigilance does not protect their more careless neighbors. CHAPTER XII. REMOVAL OF IRON FROM GROUND-WATERS. The filtration of ground-waters is a comparatively recent development. Ground-waters are filtered by their passage through soil generally much more perfectly than it is possible to filter other waters, and any further filtration of them is useless. Such waters, however, occasionally contain iron in solution as ferrous carbonate. Waters containing iron have been used as mineral waters for a very long time. Such waters have an astringent taste, and have been esteemed for some purposes. As ordinary water-supplies, however, they are objectionable. The iron deposits in the pipes when the current is slow, and is flushed out when it is rapid, and makes the water turbid and disagreeable; and still worse, the iron often gets through the pipe-system in solution, and deposits in the wash-tub, coloring the linen a rusty brown and quite spoiling it. An organism called crenothrix grows in pipes carrying waters containing iron, and after a while this organism dies, and decomposes, and gives rise to very disagreeable tastes and odors. It thus happens that ground-waters containing iron are unsatisfactory as public water-supplies, and are sources of serious complaint.