Water is an essential raw material that is widely used in industries such as cooking, brewing, and generating electricity, which means that thousands of gallons of water are used in these processes. However, industrial processes cannot utilize water directly from the city or a well.
The water that enters the factory carries all kinds of contaminants that can cause issues in the machinery. It contains dissolved minerals, gases, bacteria, and solids among other impurities. Therefore, industrial water treatment is concerned with eliminating all those contaminants to ensure proper operation of equipment.
This article explains the treatment processes in the order in which water is treated in an industry: pre-treatment of incoming water, purification, boiler operation, cooling, and then wastewater disposal.
Why raw water cannot go straight into the plant
When it comes to contaminants, engineers always have to keep four types in mind.
First, there are dissolved minerals. Water hardness is attributed to the presence of calcium and magnesium in the water, which can be measured through the use of parts per million (ppm) of calcium carbonate. When such water is heated up or concentrated, the minerals will come out of solution and form scale, causing problems with boiler and radiator operations. Total dissolved solids (TDS) is the broader term that encompasses all these minor particles.
Second, we have suspended solids. This is comprised of silts, clays, rust particles, organic debris, and so on. These materials build up in places of low flow, clogging filters and damaging seals.
The third type of contaminant is dissolved gases. Oxygen and carbon dioxide will contribute to corrosion and make the plant go offline.
Fourthly, we have live organisms such as algae or bacteria. These bacteria will form biofilm, which creates a protective layer under which corrosion can occur.
These four types of contaminants will exist in different concentrations in water taken from different sources, meaning that they will need to be treated in different ways. Therefore, each treatment company builds custom industrial water treatment solutions based on the particular plant for which it is designed. The process is known as the treatability test and will come up later in this guide.
Step one: pretreatment and filtration
The pretreatment phase involves the removal of large, identifiable materials.
Coarse screens and strainers serve to remove all large visible impurities. The clarifiers then eliminate the smaller suspended contaminants through a coordination of chemical means resulting in coagulation (the use of chemicals such as aluminum sulfates to neutralize the charge that keeps suspended particles apart) and flocculation (gentle stirring which allows the particles to settle into floc).
Following clarification, filtration is performed through multimedia filters. These filters employ layers of anthracite, sand, and garnet to prevent contaminants that are approximately 10 microns in size from passing through.
The cartridge filters provide further purification of the water through filtration to approximately 5 microns to ensure that the subsequent membranes and resin beds are protected from contamination.
Step two: softening, demineralization and reverse osmosis
Raw water can be sufficiently clean to be used for washing and cooling cosmetic products; however, it is usually necessary to remove a greater portion of the dissolved minerals to ensure appropriate temperature control in boilers and many other manufacturing processes.
The solution to this problem is the use of the ion exchange water processing method. This process relies on the water softening system, which uses special resin beads loaded with sodium ions that will exchange their positions with calcium and magnesium while passing through, thereby ensuring the removal of hardness from the water.
The next stage is the reverse osmosis water processing system. The system uses the pressure pump to create high pressure and make the water go through a special membrane that removes almost all the dissolved salts, silica, and many organic substances.
The process needs purification treatment after the reverse osmosis stage for producing the cleanest water (.e.g boiler feedwater at high temperatures or rinsing water in electronics production). It is necessary to get rid of all remaining traces of dissolved ions.
Boiler water: fighting scale and oxygen
A boiler concentrates all of the elements that enter into it. When steam exits, it is pure vapor and the minerals remain in the boiler water. As a result, the concentration of the boiler water increases with each passing hour.
Two concerns predominate. The first is the formation of scale on the fire-side of the tubes where the metal is at the highest temperature. Even a small amount of scale on the tube surface causes the burner to work harder, resulting in tube overheating. The second issue is the oxidation of steel, which occurs as a result of the presence of oxygen in the boiler.
The answer to both problems lies in the treatment program. The de-aerator is a container for heating and spraying feedwater in order to get rid of most of the dissolved oxygen. Sodium sulfite, which is a chemical scrubbing agent for the oxygen, can be used to remove the oxygen.
Phosphate or polymer programs keep the remaining hardness in a soft sludge, which is removed by blowdown. This means that a portion of boiler water will be drained off in a controlled manner so as to manage the concentration levels. The operational procedures include monitoring the cycles of concentration, which is the number of minerals that are present in the boiler water compared to the minerals that are in the feedwater.
Cooling water: the open loop problem
Cooling towers operate on principles that differ from those governing closed systems. In this system, warm water flows down a tower and is blown through the air, resulting in evaporation and cooling while concentrating minerals such as salts in the water because of the evaporation process, thus leading to contamination of the water with dust, pollen, and bacteria.
Three risks follow. Scale forms on condenser tubes where the water is warmest. Corrosion attacks piping and heat exchangers, and it becomes galvanic when dissimilar metals touch in the same system; a quick refresher on how different metals behave explains why copper tubes and steel piping make a troublesome pair. Biological growth coats the tower fill and the basin.
The cooling process ensures that inhibitors of scale are introduced in addition to inhibitors of corrosion that coat the metal with a protective layer. Biological growth is also inhibited through the use of biocides added to the water based on a pre-conceived schedule. Side-stream filtration aids in the creation of a path through which part of the circulating water passes through the filter continuously to manage quantities of solids in the water.
Step three: wastewater, discharge and reuse
Every procedure causes water to be released, and that water goes from the plant either to a sewer, to a river, or back into the procedure.
Discharges to surface water in the United States require a permit, and the Environmental Protection Agency sets the limits for industrial facilities through the National Pollutant Discharge Elimination System, described on the EPA’s industrial wastewater page. Discharges to a municipal sewer fall under local pretreatment rules instead.
Having to meet those limits requires its own treatment train. Acid and alkaline streams are neutralized to a safe pH level. The dissolved metals from plating and finishing lines are precipitated as hydroxides and settled. The organic load expressed as biochemical oxygen demand is treated by bacteria in aeration basins or membrane bioreactors employing a combination of biological treatment and filtration.
Recovery is the new last step in the treatment process. The treated waste-water which previously was disposed of is now put back into operation as cooling tower water or rinse water reducing not only the costs of fresh water but also the quantified volume of discharge, which is the goal of a zero liquid discharge system.
How a treatment program is designed
Each plant has its own program since not all plants have identical water or process. The treatability study is the first step in the design process.
The initial step in the process is bench-scale testing, where water samples from the actual plant are treated with chemistry in the laboratory to determine what works best. After that, there is pilot testing, which involves running a skid-mounted system over the actual water stream so engineers can observe actual results.
That allows an equipment, chemistry, and service program to be established. The heavy removal process will be done by reverse osmosis and softeners and the chemical feed system will protect the cooling and boiler system day by day through standard testing.
For an engineer who is new to the field, it is best to keep in mind that this entire process involves one of four contaminants and the plant will determine which one based on the source of its water and the needs of its process.