Keeping the Filter Plant Up to the Mark
Should Be Washed With Filtered Water—Getting Rid of “Mud Balls” and Slime—Special Attention Should Be Given to Control Apparatus
THE filtration plant is an important link in the water purification chain, and to its care and upkeep much thought must be given by the superintendent. The article herewith gives some excellent hints along this line.
The modern water purification plant bears a certain similarity to the human system. The latter consists, in part, of organs, each of which is interrelated. These organs are charged with important functions to be performed if the human body is to “carry on” efficiently. The water purification plant is an assemblage of appurtenances, each having a specific function in logical relation to the other parts of the plant. Plant efficiency is only obtained when these functions are carried out and the proper relations logically maintained.
The heart is commonly considered the most important organ in the human body. The filter is probably the most important appurtenance in most water purification plants. Other appurtenances, while very important in such a plant are, in the writer’s opinion, usually subsidiary and adjunctive to the filter. In the treatment of most waters, where suspended matters must be removed, the filter is, like the infantry of an army, the man power and main element of defense.
Up-keep Should Have Special Attention
If then the filter is of great, or possibly greatest importance in the operation of a water purification plant, it is worth while to give special attention to its upkeep.
The purpose of upkeep, as the writer sees it, is two fold.
First, and most important, to obtain and maintain a creditable standard of efficiency in the operation of the plant, and
Second, to always maintain equipment so as to present a tidy, attractive and workmanlike appearance.
Parts of the Modern Filter
Every rapid filter of the modern standard type for producing sanitary drinking water, even if only moderately well designed, consists in the main of the following:
- A rectangular or circular filtering cell, or unit, with heavily constructed floor and walls usually of concrete or wood.
- An under-drainage system consisting of a multitude of orifices distributed horizontally over the whole area of the filter floor, through which filter water is drawn from the filter in one direction, and wash water is applied to the filter in the opposite direction.
- A horizontal layer of coarse heavy particles lying above the strainer system.
- A horizontal layer of filter sand resting on the coarser particles.
- Troughs and gutters with their horizontal upper edges about one and one-half feet above the sand surface for carrying away waste water and foreign matter when the filter is being washed.
*Sanitary Engineer, Proximity Manufacturing Company, Greensboro,
- Pipe connections, valves and controllers for delivering unfiltered water to the filter, conducting filtered water and waste water from the filter; and delivering pressure water (and sometimes compressed air) to the filter manifolds for washing purposes.
The Filter Cell
The filter cell itself will usually require little attention. If it is of wooden construction, commonly known as the “tub type,” it is important that it be kept nearly full of water in order that as much of the staves as possible tie kept saturated. In no case should any wooden filter he allowed to remain only partially filled with water or entirely empty for long periods, as drying and shrinkage of the staves would occur, resulting in leakage at the joints and possible rotting of the wood. It is well to keep the steel hoops of these tanks painted with two coats of Asphaltum paint. Unless for some special purpose, do not paint the outside of wooden tanks, as they will last longer unpainted. If appearances should demand it, use a special oxide paint which does not clog the pores of the wood.
Concrete filter cells will probably need no attention except care to keep that portion of the inside walls above the sand level free from slime by flushing. This is desirable in either concrete or wooden filters. This cleaning or flushing can be done with a hose stream, or in some designs from fixed perforated manifold pipes hung on the inside of the walls above the flow line.
The Under-Drainage System
The under-drainage system is, of necessity, the most inaccessible appurtenance of the filter. It is usually never accessible for inspection, or even visible, except when the filter is being erected or constructed or unwatered and dismantled of filtering material. For this reason, designers and manufacturers of filter equipment have given much thought to so arranging under-drainage systems and using in them such materials as to secure freedom from depreciation and any action which might result in an unequal flow of water either to or from the drainage orifices. As far as the writer has learned, most filter under-drainage systems installed in recent years by experienced designers are giving little or no trouble, and require no special attention or upkeep.
It is hardly necessary to counsel against washing filters with water containing any considerable amount of suspended matters, either mineral or organic. Keep an adequate supply of filtered water always available for filter washing purposes, and use no other. Avoid anything which may introduce solid or fibrous matters into the under-drainage manifold or clog the strainer orifices. It is obvious that the clogging of orifices by solids from the pressure water side will be difficult to rectify.
“The purpose of upkeep of the filter plant is two fold. First, to obtain and maintain a creditable standard of efficiency in the operation of the plant and, second, to always maintain equipment so as to present a tidy, attractive and workmanlike appearance.”
Filtered Water for Washing Filters
The filtering material is commonly of silica stone, gravel and sand. This is a practically indestructible material and will last indefinitely with no significant change or any necessity of replacement in most plants. There are some exceptions to this, as for example, where lime is used in softening or in preliminary treatment. A deposit of lime carbonate often takes place on the filter sand and in the orifices of air and water manifolds. In some cases, the said grains have grown by accretion of lime to three times their original size, and also become cemented together in large masses, necessitating the removal of the sand from the filter to break it up. and treating it with acid to remove the lime.
The rate of wash water application to a filter should probably not exceed 24 inches per minute vertical velocity. Too high rates of wash water may remove a considerable portion of the fine or even medium fine sand and leave too coarse a filtering layer. For average conditions the effective size of the filter sand should not be far from 0.4 millimeter. In the larger filter plants, possessing apparatus for making sand analyses, the condition of the filter sand as regards size and uniformity can be determined at intervals at the plant. At smaller plants where sand testing apparatus is not available, an occasional test of the filter sand by a consultant, or by the State Board of Health, can well be made if any change in the filter sand is suspected. As the sand in a filter is usually stratified horizontally with reference to size of grains, particular care should be exercised in obtaining representative samples. If the sand be found deficient, with reference to size, for local conditions, it should be an easy matter to rectify this by the replacing of a proper proportion of the old sand by new sand of suitable diameter as determined by the analyses.
Freeing Filters from “Mud-Balls” and Slime
In some plants a considerable amount of upkeep may he necessary in freeing the filter sand from slime and “mud balls.” The latter will sometimes appear even when the distribution and rate of wash water seems to be efficient. The presence and location of mud balls should be ascertained by the frequent use of a thin wooden test rod thrust into the suspended sand while washing the filter. Frequently they can be dissipated by the vigorous use of an iron hand rake with vertical motion while the wash water is on. If mud balls persist, they can be removed by draining the filter, excavating the sand at that with a spade, and distributing the execated material on the sand surface.
The presence and distribution of slime in the filter sand can be determined by sampling the sand at various depths in the sand layer. Open a test pit or trench in the sand layer down to a point near the gravel, keeping one side as nearly vertical as possible. Cut out approximately equal samples of sand at various depths from the surface. After placing each sample in a half gallon glass jar, fill each jar half full of water and shake vigorously. Allow them to stand several minutes. A complete separation of the sand and slime or mud will thus be effected, the latter forming a layer on top of the sand. Some slime will probably be always found in the upper layers of the sand bed. If the amount of slime appears excessive or to be penetrating to the lower layers of sand, it probably can be reduced by using more wash water or a higher rate of wash water or both.
In some cases it may be necessary to have recourse to special means of removing the slime coating from the sand. At Baltimore, temporary relief from serious sliming of the sand was reported last Fall by Mr. James W. Armstrong, by removing the sand from the filter and passing it through a Nichols sand washing machine. At Little Falls, N. J., Mr. Frank W. Green reported the reconditioning of filter sand, which had become badly slimed, by the use of solutions of Caustic Soda and Soda Ash applied to the filters and kept near the boiling point with live steam for a period of two days. After this treatment the filters were washed, and the sand reported as sharp and clean as when new.
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Keeping the Filter Plant Up to the Mark
(Continued from page 792)
Wash water troughs and gutters, if entirely of concrete, should require no upkeep other than occasional washing to keep them free from slime and sand. If of steel, however, they should be protected from corrosion by scraping and painting with two coats asphaltum or graphite paint about once a year.
Care of Control Devices
All filters are more or less completely equipped with valves for controlling the inflowing coagulated water and pressure water for washing; the outflowing filtered water and waste water; together with recording flow meters, loss of head devices, automatic effluent controllers and other hand or hydraulically operated control devices. A considerable part of the operator’s time can be profitably used in watching, inspecting, testing and adjusting all of the appurtenances just mentioned, if they are to be kept up to a desirable standard of efficiency.
Most of the automatic devices depend for their actuation on very small water pressures or heads. Consequently the moving parts must be hung and balanced with great delicacy to prevent sticking and attain accuracy. A bent controller stem, a tight gland, a stretched gauge cable, a loose gauge pointer or an air pocket in a controller may entirely vitiate the accuracy of these important appurtenances and lead to improper operation or incorrect records. The accuracy of most of these appurtenances can be verified by measurements with a twofoot rule, together with simple calculations, and the operator should check them at frequent intervals. If any particular piece of apparatus gives trouble which cannot be overcome, do not hesitate to get in communication with the manufacturer or State Board of Health, as these organizations will cheerfully render valuable aid to the operator from their large experience in these problems.
All parts of control appurtenances on the filter operating floor should receive special attention to keep them in a clean and attractive condition. Hand wheels and standards should be kept neatly painted and all finished metal work polished. Some superintendents have added largely to the attractiveness of the interior of their plants by placing large potted plants or ferns on the operating floors.
In conclusion, the writer would call attention to the fact that water, as delivered to the consumer, is in the nature of an edible product, and in this age of sanitation, it would seem that the plant and appurtenances producing it should present at all times a wholesome and attractive appearance.
(Excerpts from paper read before the Second Annual Conference of Filter Plant Operators, at Asheville. N. C., at annual convention of North Carolina section, A. W. W. A.)
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