The use of water is crucial in many industrial processes. The resulting wastewater is a potential environmental pollutant, if the respective treatment is not carried out. Hence the importance of a discharge control program for each type of industrial water.
Controlling the emission of industrial water requires knowledge of the volume of water used per unit of time and its quality. In addition, it implies knowledge of the quality standards of the water that the industry receives and that which it discards.
Depending on the use, industrial water comes from water treatment plants, boilers, heat exchangers and processes, among others. Let's see some of them.
Waste from water treatment plants
Some industries need to treat the water they receive in order to obtain the quality required for each use. Various types of wastewater from different processes used are mentioned below.
Sedimentation
Generally, uses coagulants such as aluminum or iron salts. The material settled and removed from the bottom of the sedimentation tank (mud), contains approximately 8% solids and 92% water. The water resulting from this process can be recirculated to the initial operation. The only loss of water is that which is ultimately contained in the sludge cake, usually 50 to 75% of its weight.
Softening
It consists of the removal of calcium and magnesium ions that the water contains in a cation exchanger or by treatment with lime and soda. The resulting settled sludge contains calcium carbonate and magnesium hydroxide. The final material after recycling the sludge can have a concentration of 25% solids and 75% water.
Filtration
Washing filters with a stream of clean water creates waste water which contains the solids retained in the filter. Filters are also used to separate precipitated iron from well water, which has been aerated to oxidize ferrous or ferric ion. The water used for washing these filters can be used again in this operation.
Ion exchange
Used for soft water using a cation exchange material that removes calcium and magnesium and replaces them with sodium. The exchange material is generally NaCl and the waste water after this process contains CaCl2, MgCl2 and the excess NaCl used.
Demineralization
Employs cation and anion exchangers that produce water of a similar quality to distilled water. The exchangers can be ammonium hydroxide, caustic, sulfuric acid or hydrochloric acid. The cation exchanger replaces the cations with hydrogen; Thus, acidic water is obtained that requires degassing to remove CO2 and SO2.
The anion exchanger replaces the anions with hydroxide ions. The waste water contains all the substances that the original water had and the excess exchanger material added, except acid gases. This waste water must not be recycled.
Waste water from boiler operation
Boiler blowdown
The resulting wastewater is mineralized, with a solids content of the order of 3,500 to 500 mg / l.
Water for removal of ash from coal combustion
The ashes in the boilers are removed to a well, usually with water; After use the water is alkaline and is considered waste.
Boiler cleaning Boiler
tubes must be cleaned regularly with chemicals. In low pressure boilers hydrochloric acid is used with elements that prevent the attack of iron. These cleaning substances are discharged into the ash pit; Iron precipitation and acid neutralization occur due to alkaline waste water.
High pressure boilers require more complex cleaning methods with chemicals like nitric acid. These cleaning solutions contain copper and zinc.
Waters used for heat exchange
The use of water as a heat exchange medium is not the most common application in industry. Heat is considered polluting due to the changes it causes in the animal and plant life of the water.
Cooling towers
The heat exchange water that can cause thermal pollution is recovered and reused in cooling towers, where the heat dissipates rapidly. This cold water can be recirculated, after a treatment, to prevent scaling and corrosion in the heat exchanger piping.
Heat
exchangers Heat exchangers used to cool a product must have water sampling points that are easily accessible to the product. A small leak in a tube could cause considerable product loss. The speed of the water passing through a very small exhaust hole can create a suction that causes the product to enter the cooling water stream.
Process waters
Water in contact with raw materials, intermediate products, by-products or end products is called process water.
Raw Material Cleaning Some raw materials are transported by water and require washing and removal. The food industry is an example of this use. The quality of the water used in the canning industry is directly related to the condition in which the vegetables and fruits are received.
Cleaning of containers
The waters used for containers where they are made are concentrates that are discharged intermittently and require special handling.
Refined
The extraction of solvents from materials containing water is a common industrial process. The water remaining after extraction generally
contains high concentrations of by-products, and minor amounts of products and solvent.
Gas scrubbers
The gases released from a process are usually scrubbed with water. If the gas is of value, such as hydrogen, it must be flushed to remove impurities and reused. Gases such as chlorine, hydrogen cyanide, hydrogen sulfide or phosgene are dangerous and must be removed by efficient scrubbing equipment. One way to evaluate the efficiency of this control is to analyze the wastewater.
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