Water-formed deposits usually are not homogeneous. Inorganic deposits may be formed by reaction between the water or its solutes and the containing vessel or piping, and by reaction between solutes in the water. Biological types grow by consuming the required nutrients from water and, like inorganic deposits, may be deposited where formed or carried in suspension and later deposited some distance away. Chemical and physical changes in the operation of the water system may alter the site and character of deposits either temporarily, cyclically, or permanently. Determination of the average chemical composition of the resultant mixture therefore is of limited help in diagnosing the cause, or in providing a means for prevention, of a water-formed deposit.

Pure water in an uncorrodible vessel would never leave a deposit. Although this ideal case has been very nearly reached in some instances, deposits are still found in most water or steam systems. The examination and analysis of these deposits provides the person responsible for water treatment with information about phenomena in the water system, and will frequently indicate to him the means for correcting the condition that caused the deposit. This chapter covers the nature of water-formed deposits, the sampling of deposits for analysis, and methods for identification of the constituents of a deposit. Quantitative chemical analysis is dealt with in Chapter IX. Some of the instrumental methods discussed in this chapter are not at present widely used in the examination of water-formed deposits, but brief descriptions are included because there are instances where they can be used to advantage. Deposits may be divided into three general classes: inorganic, organic, and biological. Inorganic deposits result from corrosion of containing surfaces; from precipitation by chemical reaction between two or more constituents of the water; or precipitation from physical causes, such as change in solubility with pressure or temperature, by evaporation to dryness, or by relief of supersaturation. Closely akin to these, but still somewhat distinct, is the sedimentation of suspended matter.

Pure water in an uncorrodible vessel would never leave a deposit. Although this ideal case has been very nearly reached in some instances, deposits are still found in most water or steam systems. The examination and analysis of these deposits provides the person responsible for water treatment with information about phenomena in the water system, and will frequently indicate to him the means for correcting the condition that caused the deposit.

Many of the difficulties which arise during the industrial use of water are closely related to water-formed sludges, scales, and corrosion products. The composition of such deposits can be interpreted to provide a guide for practical preventive treatment of either the water or its environment. In the proper hands, complete and detailed information concerning water-formed deposits can be useful; however, unless this information is accurate, it can be misleading. Accordingly, reliable methods should be available for the analyses of carefully selected samples.

Many of the difficulties that arise during the industrial use of water are closely related to water-formed sludges, scales, and corrosion products. The composition of such deposits can be interpreted to provide a guide for practical preventive treatment of either the water or its environment. In the proper hands, complete and detailed information concerning water-formed deposits can be useful; unless this information is accurate, however, it can be misleading. Accordingly, reliable methods should be available for the analysis of carefully selected samples. The chemical analysis of water-formed deposits presents a challenge to the analyst's skill and causes him to muster all the knowledge of his profession. The usual quantitative analysis will yield the elemental composition of the major constituents of such deposits, but other tools must be used to determine accurately the combinations of these elements. These include the spectrograph, biological and petrographic microscopes, and X-ray diffraction equipment described in Chapter VIII. It is often possible to determine the source or cause of water-formed deposits from the results of chemical and physical analyses. The treatment for removal or prevention of such deposits can thus be accomplished scientifically instead of by trial and error.

Many of the difficulties that arise during the industrial use of water are closely related to water-formed sludges, scales, and corrosion products. These water-formed deposits are the accumulations of insoluble material derived from the water, or are formed by the reaction of water with surfaces in contact with it. The composition of such deposits can be used to diagnose the reason(s) for deposition, to determine preventive treatment of either the water or its environment, and to effect their removal. A thorough and accurate analysis is an important beginning in this process.

A discussion of the composition of industrial water and water-formed deposits must include the terms used by different industries, methods of reporting results of analyses, methods of interpreting these analyses, and systems for converting results of analyses into other terms which may be better understood by the users of water. In addition, the composition of water must be classified so that a particular user can choose the best supply for his purpose or forecast what difficulties might arise from the use of the water available. A user should know the general composition ofhis water and how it varies. If deposits occur, he then should be able to identify or classify the type of deposit formed so that he can trace its source and determine remedies.