General features. The type of inlet is not so important as for coagulating basins, provided initial horizontal velocities are suppressed. The best form of outlet is a skimming weir.

Baffles are of little utility in natural subsidence, except where basins are of irregular shape or where the inlet and outlet are so close together as to allow short-circuiting.

With the common type of rectangular basin it is important to have the bottom sloped sufficiently to allow ready cleaning by draining and use of fire-hose streams. Gutters in the bottom greatly facilitate cleaning. Some basins are arranged with hopper bottoms having individual sludge valves. A few installations have flat bottoms with a system of parallel pipe underdrains controlled by valves on individual drain lines. In some of the larger plants, where the volume of deposited sediment is great, teams with scrapers instead of fire hose, are used for cleaning.

The use of a mechanical clarifier for continuous removal of deposited sediment in connection with plain subsidence is provided for one large western plant now under construction and is projected for others.

A common fault of the older basin installations is the inadequate drain valve. This should not be less than 12 inches in diameter and in large basins at least 16 to 24 inches.

Grit chambers. Where the water supply contains a considerable amount of sand and other coarse suspended solids of rapid-subsiding value it has been found advantageous to provide a separate basin or chamber having a few minutes retention, arranged with facilities for rapid cleaning. The best example of such a grit chamber is at St. Louis, with a retention period of only ten minutes, which removes a substantial portion of the suspended solids, but little turbidity.

Operation

Observations as to the need for cleaning subsidence basins are readily made by noting the increase in turbidity in the basin effluent or failure to obtain usual performance. Except for certain western plants dealing with very turbid raw waters, the cleaning of subsidence basins is generally undertaken at relatively infrequent intervals of from one to five years depending upon the amount and character of the suspended matter in the water and the design and retention.

capacity of the basin. After drawing off the supernatant water, the deposits are usually flushed out by hose jets from high pressure water connections, either permanently installed for the purpose or temporarily arranged from fire plug or engine connections.

Subsidence basins are sometimes utilized to advantage to overcome algal troubles in the filtration plant, in that they offer an opportunity for treatment of the water by an algaecide and thus prevent the thriving of the organisms in the coagulation basins and in the filters.

The control tests of the most importance in the daily operation of subsidence basins are turbidity and bacterial content.

Performance

The efficiency of natural subsidence basins in the removal of suspended matter and bacteria is subject to limitations imposed by: (a) the detention capacity of the basin, (b) the relation of inlets to outlets, (c) the degree of quiescence within the basin, and (d) the character of the influent water. Under favorable conditions, the percentage of turbidity removed may exceed 80 and, under less favorable conditions, may be as low as 40. In the former case, the period of detention usually will exceed two days; in the latter case, it may be less than one day. Under average conditions, the removal of suspended matter ranges from 60 to 70 per cent.

In general, the percentage of bacteria removed by natural subsidence basins follows closely that of turbidity removal. For a given basin, the percentage of turbidity and of bacteria removed varies with the concentrations of these constituents in the influent water, with the character (and especially the relative coarseness) of the suspended matter, and, to some extent, with the temperature of the water. Thus, the efficiency of removal is greater with increased amounts of turbidity or numbers of bacteria in the influent water, but less with an increased fineness of the suspended matter. Basins in actual service have shown variations in turbidity and bacterial removal ranging from 30 to 80 per cent in the course of a year.

Theory

Heavy sediment like sand settles to the bottom of a subsidence basin readily; it is of high hydraulic value, settling close to the

inlets, and is not affected much by velocities. Light sediment like clay collects toward the bottom as sludge, but requires time to compact to a final resting place; it is of low hydraulic value, diffusing over a large area, and is therefore affected by velocities, especially those in a vertical plane. The line of demarcation between the sludge-forming densely muddy water and the settled turbid water is usually quite pronounced.

The direction and control of flow of water in subsidence basins is important. Theoretically, velocity of flow in a horizontal plane does not affect sedimentation of the suspended particles because they are moved downward by the force of gravity; but velocities of flow in a vertical plane and convection currents set up by temperature changes or by inlet velocities retard sedimentation. In large basins the use of multiple iniets tends to reduce the formation of certain convection currents which are caused when the heavy deposits settle around a single inlet.

The important principles to be borne in mind, therefore, in the design and operation of subsidence basins are chiefly that continual quiescence of the water is desired and that the period of subsidence be as prolonged as possible.

II. Coagulation

Descriptive features

Where natural subsidence fails to remove satisfactorily turbidity or where the color is objectionably high or where rapid sand filtration is employed, it becomes necessary to use chemicals to hasten clarification or to condition the water for efficient filtration. Chemicals used in this manner are referred to as "coagulants" and their action when applied to the water is called "coagulation," because they result in the clumping together of the suspended material into aggregates having a rapid subsiding value. The action is due to the formation of an insoluble hydroxide having the property of enmeshing the turbidity, coloring matter and other organic colloids together with the bacteria.

Chemicals used. There are two relatively cheap chemicals which are serviceable as coagulants, aluminium sulfate and ferrous sulfate. Sulfate of alumina or "filter alum" is available on the market in the lump or pulverized form, which is shipped in bulk carloads or

in barrels containing 400 pounds or in sacks containing 200 pounds. The value for coagulation purposes is determined by the alumina (Al2O3) content which varies from 16 to 23 per cent. The ordinary grade of filter alum contains 17 per cent alumina.

Filter alum is usually purchased from the manufacturers making a specialty of this material, but there are a number of important water works having equipment for making sulfate of alumina on the ground, using a purified bauxite ore and sulfuric acid. Examples are, Columbus, Montreal, Trenton, Kansas City, Mo., Kansas City, Kan., Omaha and the new Baltimore plant. This home-made alum is usually turned out as cakes or lumps, but the Columbus installation manufactures "syrup alum" which is diluted with water and fed in the customary manner.

Ferrous sulfate is found on the market as "sugar sulfate," "sugar of iron" or "copperas." The material in crystal form is now little used. Ferrous sulfate may be had in bulk shipment in barrels or in sacks.

Auxiliary chemicals. Aluminium sulfate can generally be used as a coagulant without the assistance of other chemicals and this fact accounts for its widespread use in comparison with ferrous sulfate which necessitates the use of lime in order to avoid soluble iron remaining in the treated water.

Within the last few years considerable study has been given to the reaction of coagulants. It has been found that some natural waters need certain adjustments towards the acid or alkaline side to bring about efficient and economical coagulation. It has been recognized for many years that the muddy western waters, though highly alkaline, are more easily coagulated with alum, if lime is also used. There are, on the other hand, some natural waters which are too alkaline for economical coagulation with alum and the change of reaction by the use of sulfuric acid with the alum has been found in a few instances economical and satisfactory.

With waters low in alkalinity it is necessary to apply soda ash or lime with alum in order to bring about coagulation and to prevent undecomposed alum remaining in the treated water.

Lime is available on the market as "quick-lime" or "hydrated lime." Quick-lime contains 85 to 90 per cent available calcium oxid (CaO) and is shipped in bulk or barrels in lump or pulverized form. Hydrated lime is water slaked quick-lime and contains

70 to 72 per cent calcium oxid (CaO). It is commonly furnished in paper bags holding 40 or 50 pounds. Hydrated lime is more suitable for small plants on account of convenience of handling but is not so economical as quick-lime. Soda ash or "dry sodium carbonate" is furnished as a powder in sacks of 200 pounds or in barrels. Chemical feeding. The oldest and most common method of feeding alum is to dissolve a known weight of the lump material in a tank, make a definite volume with water, stir thoroughly and feed by means of a graduated orifice valve set in a float box. It is customary to provide tanks with suitable gauges from which the solution level from time to time may be ascertained.

Ferrous sulfate, soda ash and lime are commonly fed in the same manner. Lime tanks have to be stirred continuously in order to preserve a uniform suspension of "milk of lime." At the larger plants where considerable lime is used, particularly in connection with water softening, equipment for crushing and handling quick lime is utilized, and the feeding is controlled by means of automatic scales dumping at specified intervals into continuous slaking tanks.

The use of dry feed machines for applying the above chemicals is threatening to supersede the older tank method, because of greater convenience, ease of changing the dose and the saving in building space. Satisfactory machines are now on the market by several manufacturers for feeding pulverized alum, lump alum, sugar of iron, hydrated lime and soda ash. There has lately been installed a machine for feeding and slaking quick-lime continuously.

Less usual arrangements for feeding alum are those where the rate of dissolving lump alum is varied at will, or where a hydrometer device automatically dilutes the alum solution to a constant predetermined strength.

Automatic control of chemical feeding for proportioning in accordance with water flow is sometimes used, though not generally regarded as advantageous.

Mixing and reaction. The thorough incorporation of chemicals with water is not difficult of accomplishment by simple means, but it is well recognized that chemical reactions in water require an appreciable time for completion and best results are obtained by keeping the water in motion after application of coagulant so as to promote thorough coagulation and flocculation before final settling. These two functions are combined in a separate structure commonly