MEASUREMENT OF TURBIDITY.
The amount of mud or turbidity in a water is often expressed as the weight of the suspended matters in a given weight of the water. Most of the data relating to turbidities of waters are stated in this way, because this was the only method recognized by the earlier investigators.
This method of statement has some disadvantages: it fails to take into account the different sizes of particles which are carried in suspension by different waters, and at different times. Thus the Merrimac River in a great flood may carry 100 parts in 100,000 of fine sand in suspension, and still it could hardly be called muddy; while another stream carrying only a fraction of this amount of fine clay would be extremely muddy. Further, an accurate determination of suspended matters is a very troublesome and tedious operation, and cannot be undertaken as frequently as is necessary for an adequate study of the mud question.
Turbidity is principally important as it affects the appearance of water, and it would seem that optical rather than gravimetric methods should be used for its determination. Various optical methods of measuring turbidity have been proposed. The general method employed is to measure the thickness of the layer of water through which some object can be seen under definite conditions of lighting. The most accurate results can probably be obtained in closed receptacles and with artificial light. Such a method has been used by Mr. G. W. Fuller at Louisville and Cincinnati in connection with his experiments, and is described by Parmelee and Ellms in the Technology Quarterly for June, 1899. This apparatus is called by Mr. Fuller a diaphanometer.
At the Lawrence Experiment Station of the Massachusetts State Board of Health as early as 1889 it became necessary to express the turbidities of various waters approximately, and the very simple device of sticking a pin into a stick, and pushing it down into the water under examination as far as it could be seen, was adopted. Afterwards a platinum wire 0.04 of an inch in diameter was substituted for the pin, and the stick was graduated so that the turbidities could be read from it directly. The figures on the stick were inversely proportional to their distances from the wire. When the wire could be seen one inch below the surface, the turbidity was reported as 1.00; when the wire could be seen two inches, the turbidity was 0.50, and when it could be seen ten inches the turbidity was 0.10, etc. This scale is much more convenient than a scale showing the depth at which the wire can be seen; and within certain limits the figures obtained with it are directly proportional to the amount of the elements which obstruct light in the water. Thus, if a water having a turbidity of 1.00 is mixed with an equal volume of clear water, the mixture will have a turbidity of 0.50. Advantage is taken of this fact for the measurement of turbidities so great that they cannot be accurately determined by direct observation. For turbidities much above 1.00 it is very difficult to read the depth of wire with sufficient accuracy, and such waters are diluted with one, two, or more times their volume of clear water in a pail or other receptacle, the turbidity of the diluted water is taken, and multiplied by the appropriate factor.
For the greatest accuracy it is necessary that the observations should be taken in the open air and not under a roof. They should preferably be made in the middle of the day when the light is strongest, and in case the sun is shining, the wire must be kept in shadow and not in direct sunlight.
The turbidities of effluents are usually so slight that they cannot be taken in this manner; in fact, turbidities of less than 0.02, with the wire visible 50 inches below the surface, cannot be conveniently read in this way. For the estimation of lower turbidities a water is taken having a turbidity of 0.03 or 0.04 and as free as possible from large suspended particles. The turbidity of this water is measured by a platinum wire in the usual way, and the water is then diluted with clear water to make standards for the lower turbidities.
The comparisons between standards and waters are best made in bottles of perfectly clear glass, holding at least a gallon, and the comparison is facilitated by surrounding the bottles with black cloth except at the point of observation, and lighting the water by electric lights so arranged that the light passes through the water but is hidden from the observer. In case the water under examination is colored, the comparison is rendered difficult, and it is often advisable to add a small amount of methyl orange to the standards to make the colors equal.
Instead of diluting a water of known turbidity for the standards, a standard can be made by precipitating a known amount of silver chloride in the water. For this purpose about one per cent of common salt is dissolved in clear water and small measured amounts of silver nitrate added, until the turbidity produced is equal to that of the water under examination. The relation of the amount of silver nitrate used to the turbidity is entirely arbitrary, and is established by comparisons of standards made in this way with waters having turbidities from 0.02 to 0.04, the turbidities of which are measured with the platinum wire, and which afterwards serve to rate the standards. The silver chloride has a slight color, which is an objection to its use, and perhaps some other substance could be substituted for it with advantage. The standards have to be made freshly each day.
One disadvantage of the platinum-wire method of observing turbidities in the open air, as compared with the diaphanometric method using artificial light, is that observations cannot be made in the night. To get the general character of the water in a stream, daily observations taken about noon will generally be sufficient; but for some purposes it is important to know the turbidity at different hours of the day, and in such cases the platinum-wire method is at a distinct disadvantage. Variations in the amount of light, within reasonable limits, do not affect the results materially, although extreme variations are to be avoided. The size of the wire also influences the results somewhat. The wire commonly used is 0.04 of an inch or one millimeter in diameter. A wire only four tenths of this size in some experiments at Pittsburg gave results 25 per cent higher; with a wire twice as large the results were lower, but the differences were much less. Wire 0.04 of an inch in diameter was adopted as being very well adapted to rather turbid river-waters. For very clear lake or reservoir waters, usually transparent to a great depth, a much larger object is preferable. Within certain limits the results obtained with an object of any size can be converted into corresponding figures for another object, or another light, by the use of a constant factor. Thus the turbidities obtained with a platinum wire always have approximately the same ratio to the turbidities of the same waters determined by the diaphanometer.
The platinum-wire method has been used in many cases with most satisfactory results. If it lacks something in theoretical accuracy as compared with more elaborate methods, it more than makes up for it by its simplicity; and reliable observations can be taken with it by people who would be entirely incompetent to operate more elaborate apparatus; and it can thus be used in many cases where other methods would be impossible.
Upon this scale the most turbid waters which have come under the observation of the author have turbidities of about 2.50, although waters much more turbid than this undoubtedly exist. A water with a turbidity of 1.00 is extremely muddy, and only one tenth of this turbidity would cause remark and complaint among those who use it for domestic purposes. In an ordinary pressed-glass tumbler a turbidity of 0.02 is just visible to an ordinary observer who looks at the water closely, but it is not conspicuous, nor would it be likely to cause general complaint; and this amount may be taken as approximately the allowable limit of turbidity in a good public water-supply. In a carefully polished, and perfectly transparent glass a turbidity of 0.01 will be visible, and in larger receptacles still lower turbidities may be seen if the water is examined carefully. In gallon bottles of very clear glass, under electric light and surrounded by black cloth, a turbidity of 0.001 can be distinguished, but a turbidity even several times as large as this could hardly be detected except by the use of special appliances, or where water is seen in a depth of several feet.