4. At times surface draft avoids organisms which abound at greater depth. Clogging of screens by littoral growths may thus be remedied. Spawning fish may sometimes be avoided.

5. Surface draft sometimes provides water of less turbidity in case of disturbance. As turbidity settles the more turbid water is toward the bottom.

6. Water at a depth may lack oxygen, or have corresponding increased carbonic acid or possess odors of decomposing organic matter. Draft at a depth may then be maintained through an aerator which will correct all of these faults. Surface draft may also be employed, if necessary.

7. Frequently different organisms flourish in different basins of the same reservoir and troublesome ones may be avoided by shutting off or by-passing the offending basin and treating it or allowing it to stand until the organisms die off.

8. Variable outlets may likewise have advantages, in special instances, in the selection of waters relatively free from iron, manganese and color.

CURRENT PRACTICE WITH ALGAECIDES

The most important reasons for controlling or destroying microscopic organisms are their connection with disagreeable tastes and odors in water supply, their interference with filtration, interference with industrial enterprises, and turbidity in swimming pools. The so-called littoral growths, attached to the banks or bottoms of reservoirs, which attract quickest attention, are not especially important. Trouble is usually caused by minute floating forms, which manufacture essential oils or perfumes like those of flowers. Exceedingly minute amounts produce pleasant aromatic, geranium or grassy odors which become fishy, oily, pungent or vile in larger amounts or upon decay of the plant growths. Particular genera may fequently be identified by their odor by those who are trained in this work. Three groups of odors are distinguishable: aromatic (geranium), caused by diatomaceae and a few protozoa; grassy, caused by cyanophyceae; and fishy, caused by chlorophyceae, diatomaceae and protozoa.

The control of odors from microscopic organisms means either the control of the organisms themselves or the destruction of the odors. It is good practice to control organisms which produce taste

and odor when they amount to 1000 units and over. A few complaints are caused by 500 to 1000 units, particularly as a result of chlorination, but more general complaint occurs above 1000 and surely above 2000 units. Synura in any quantity should be attended to, since the cucumber or fishy flavor and bitter taste are persistent and its effect may be noticed following chlorination before its presence is detected by the microscope.

Organisms may be controlled by shutting off the troublesome reservoir and allowing it to stand, by shifting draft from one portion to another and from one depth to another, as previously indicated. With seasonal prevalence it is probable that temperature is the predominant factor, particularly as to the time organisms start to multiply. Once started they may persist at other temperatures. Aphanizomenon frequently lasts through the winter. Diatoms usually appear in the spring and fall, cyanophyceae in the summer and fall, and the most troublesome protozoa, synura and uroglena, prefer cold temperatures and even grow under the ice.

Aerators are employed to help remove the taste and odor-producing oils from microscopic organisms. Certain delicate organisms are partially disintegrated, as uroglena, synura, dinobryon, anabaena and asterionella.

The method of control most widely used is that of Moore and Kellerman, in which copper sulphate is applied by dragging burlap bags, containing about 50 pounds each, through the water by means of row-boats, taking a zig-zag course so as to triangulate the surface of the water. Wind, waves, diffusion and gravity admix the streaks of treated water with the remainder. Smaller dosage is effective in an open reservoir than in bottle experiments in a laboratory. On the extensive reservoirs of the watersheds it is necessary to use launches and the chemical is shoveled at frequent intervals into bags suspended from either side. In these launch treatments of large reservoirs parallel courses about 100 feet apart are usually taken. After the area has been covered in one direction it is similarly covered in courses at right angles thereto. Reservoirs of thirty billion gallons capacity have been thus treated successfully. The reservoirs should be partitioned into areas and the capacity in million gallons for total depth calculated for each area so that the dosage may be properly apportioned.

In the case of distribution reservoirs not equipped with a bypass,

The organisms where odors are expressed have been in sufficient quantity in water supply at one time or another to cause the characteristic odor. * Dosage successful in New York City's supplies.

†These organisms have been affected by chlorine and in some cases controlled by dosage ranging from 0.5 to 2 p.p.m. depending largely on amount of organism.

These organisms have caused trouble other than odor.

if they have a storage capacity of five to six days, the influent half of the reservoir may be treated. The treated water will displace the untreated water within a week and sufficient time will elapse for disappearance of taste and odor from the treated water so that complaint will not result.

Copper sulphate may be fed to an aqueduct by continuous dryfeed to destroy organisms feeding from one reservoir to another. A reservoir should interpose between the point of feed and the distribution system to allow time for removal of odor caused by the treatment. The chemical should feed through a hopper to a moving shutter actuated by a motor and adjustable in stroke or to a revolving disc with adjustable arm for scraping off the chemical. The apparatus should be of copper where subject to corrosion. Feed to the water may be through a wooden chute to a wooden crib bored full of holes, the latter floating in the water. Continuous dry-feed of copper sulphate to the water has been eminently successful. The dosage of chemical is definitely known and the application and admixture with the water are thoroughly under control. The effective dosage by continuous dry-feed is apparently the same as that in launch treatments of reservoirs.

The effect of treatment of microscopic organisms by copper sulphate is shown by an immediate production of distinctive odors, usually disagreeable, by reduction of the number of organisms in the water through sedimentation, by the appearance of the organisms under the microscope-the coloring matter being knocked to pieces, so to speak. Sometimes there is an increase in the water bacteria. which feed upon the decayed organisms. The cyanophyceae usually produce scum after treatment, of varied colors, pale blue, yellow, red or brown.

Sometimes fish are killed by the treatment by swimming into the concentrated streaks of copper sulphate, but this is more apt to occur a day or more later from smothering due to clogging of the gills with dead organisms or to reduction of oxygen by the dead organisms. Experience differs with waters of different composition, with temperature, and with amount of organisms present as to the right dosage to apply for each organism. About twice as much chemical is required in winter as in summer. It is believed that it is a waste of copper sulphate to apply an insufficient quantity at one dosage. If a smaller dosage fails, the entire larger dosage must be applied