Chemical Quality Assurance

Module 5. Water, detergents and sanitizers for dairy plant operations

Lesson 16
SOFT AND HARD WATER, TEMPORARY AND PERMANENT HARDNESS

16.1 Introduction

Water is a basic renewable natural resource upon which the survival and well being of living organisms depend. Quantity, quality and availability of water are critical factor in supporting our human civilization and standards of living. Man and animals enter hydrologic cycle as users of water and producers of waste in many forms. Water is also essential to the food supply and habitat of all other living organisms. The food industry uses large quantities of water. It is the most necessary item of the food industry. In addition to use in the growing of the raw products, water is used for generating steam, cleaning, peeling, grading, and conveying products, as a heat exchange medium in heating and cooling operations, for cleaning plant and equipment, for condensing vapors, for the fire protection, sanitizing, drinking, humidification, as an ingredient in the finished products and as a means of waste disposal. To successfully fulfill this versatile role in food industry, water must be in adequate supply, safer and of high quality. Moreover due to the globalization and implementation of WHO and ISO concepts the quality of raw as well as finished products has become very important. In the present scenario the management of water, both qualitatively and quantitatively is gaining importance. Therefore, the supply of adequate quantity and safe quality of water is of vital importance to the food industry particularly to dairy industry. Management of water in food industry consists of three components: i) quality at entry level, ii) water conservation and iii) waste management.

16.2 Types of Water

Generally water is classified into two categories i) Hard water and ii) Soft water. Water hardness is basically due to the presence of di- cations including Ca2+ and Mg2+. These ions enter a water supply by leaching from minerals. Water hardness is further of two types: i) Temporary hardness and ii) Permanent hardness. Temporary hardness is caused by the carbonates and bicarbonates of calcium and magnesium. This can easily be removed by boiling of water. On the other hand, presence of sulfates and chlorides of calcium and magnesium are responsible for permanent hardness of water. This kind of hardness is not removed by simple boiling but requires some complex operations. The combined effect of temporary and permanent hardness is called as total hardness of the water. Temporary hardness and permanent hardness are also known as carbonate hardness and non- carbonate hardness, respectively. Conventionally hardness is expressed in terms of ppm of calcium carbonate. In industry, the major problem caused by hard water is the deposition of scales in and on the pipes which can clog plumbing and interfere with heat exchangers. These scale, are composed mainly of calcium carbonate (CaCO3), magnesium hydroxide [Mg(OH)2], and calcium sulfate (CaSO4). Calcium and magnesium carbonates tend to precipitate out as hard deposits to the surfaces of pipes and heat exchanger surfaces. This is principally caused by thermal decomposition of bi-carbonate ions but also happens to some extent even in the absence of such ions. In boilers, the deposits act as an insulation that impairs the flow of heat into water, reducing the heating efficiency and allowing the metal boiler components to overheat. In a pressurized system, this can lead to failure of the boiler.
The following equilibrium reaction describes the formation of calcium carbonate scales
CaCO₃ + CO₂ + H₂O ⇋ Ca²⁺ + 2HCO₃-
Hard water, form white precipitate (scum) with soap solutions, instead of producing lather. This effect arises because the di- cations destroy the surfactant properties of the soap by forming a solid precipitate. A major component of such scum is calcium stearate.

2 C₁₇H₃₅COO^- + Ca²⁺ → (C₁₇H₃₅COO)₂ Ca

The Indian standards for water quality tolerances for processed food industry are as

Table 16.1 Bacteriological tolerances

*Not applicable in the case of cooling water and of hot water supplied in dairy industry.

Table 16.2 Physical and Chemical tolerances (BIS, 1981)

Table 16.3 Tolerances for radioactivity

Table 16.4 Additional tolerances for specific operations

*For waters which are re-circulated and used. In once through and run to waste systems, carbonate hardness should be absent.
**Especially if used for washing with soap or other alkaline detergents.