Analysis of various terms related to raw water (water treatment)

Brackish water

In a certain sense, brackish water can be defined as a low TDS water source with a large increase in TDS value due to seawater intrusion. In the RO category, brackish water can be defined as reverse osmosis feedwater that has a salt content TDE value at a low level (up to 10,000-15,000 ppm) and can be treated with a brackish water RO membrane with a maximum feed pressure of 600psi.

River water

Water that falls on the ground due to rain, flows into the river through the ground or underground. Most of the water passing through the ground contains suspended matter, while the water passing through the ground contains more soluble inorganic salts flowing through the formation. When the rainwater flowing through the surface flows into the river, the suspended matter concentration will increase sharply, so the characteristic of the river water is that the suspended matter in the river water changes greatly. Moreover, the seasonality of river water is obvious, such as changes in water temperature, the reproduction of aquatic organisms, and the content of sediments from organisms and colloidal substances from organic matter. In addition, humus and organic matter content in rivers flowing through forested and mud-covered areas will also be high. Rivers can also be polluted by municipal wastewater treatment water, factory discharges, or pesticide-containing irrigation water. Therefore, we are required to conduct sufficient investigations on the water quality, grasp the magnitude of its changes, and make necessary considerations for the pretreatment device or operating conditions.

lake

After long-term retention of river water in lakes or reservoirs, the suspended matter content will decrease due to sedimentation, but on the other hand, it will be easily affected by microorganisms. Water in lakes or reservoirs can easily cause eutrophication, causing algae with lighter proportions to multiply, resulting in poor sedimentation and agglutination functions of lakes, and sometimes blocking of filter ponds. It also consumes carbon dioxide dissolved in water due to the assimilation of carbonic acid, which causes the pH of the lake to increase.

When water stratification is formed inside the lake, the bottom of the water is beneficial to the growth of anaerobic bacteria due to the lack of oxygen, and the generation of hydrogen sulfide, including the re-dissolution of iron and manganese. Therefore, when using lake or reservoir water as raw water, it is necessary to thoroughly study the treatment method for it, and if necessary, consider not using lake bottom metamorphic water when taking water.

groundwater

The flow velocity of groundwater in the formation is extremely slow. Due to the natural filtering effect, there is almost no suspended matter, but the influence of the flow through the formation is very obvious. For example, the concentration of calcium in the water flowing through the lime salt zone is very high, and the concentration of silicon in the groundwater passing through the volcanic zone will also increase. Generally, due to the lack of oxygen in the groundwater, it is significantly reducing. It may contain reduced iron or manganese in the water, or it may contain hydrogen sulfide, barium, or strontium depending on the formation.

Groundwater has less suspended matter, and the temperature change throughout the year also appears relatively stable. As raw water for the RO system, consideration must be given to the hardness components, silicon, and metal ions that need to be removed in the pretreatment.

Municipal water

When the RO system uses tap water as raw water, check the tap water treatment process, pipeline conditions, and pay attention to residual chlorine.

seawater

The content of NaCl in standard seawater is more than 3.5%, and there are more than 10 kinds of soluble ingredients such as magnesium, calcium, potassium, sulfate, carbonate, bromine, boron and fluorine. Due to the entrance to the sea, precipitation, tide or water temperature affects the salt concentration in seawater to a certain extent. Suspended matter and organic matter content are affected by rivers and human activities, and there may be regional differences. Therefore, the water intake point and water extraction method of the desalination system are very important, which directly affect the water quality and pretreatment method of the raw water.

Waste water

In recent years, more and more municipal sewage and industrial sewage have been treated with reverse osmosis. The sewage used for reverse osmosis treatment is generally industrial cold cycle sewage discharge and tertiary treatment water that meets the reuse water standard. The COD is less than 50mg / L. After further processing, the oil content and turbidity must meet the RO water inlet requirements. Special attention should be paid to whether the raw water contains surfactants and other organic substances that cause serious pollution to the reverse osmosis membrane, and organic solvents that are incompatible with the membrane material. Due to the complexity and volatility of sewage water quality, the collection and investigation of water quality data is particularly important. Wastewater treatment reverse osmosis system requires extremely strict pretreatment, uses a very conservative low-flux design, and uses low-contamination membrane elements with excellent performance (such as LFC3-LD)

temperature

Temperature is a very critical design parameter. Design parameters such as feedwater pump pressure, water production balance in each section, fresh water quality, and solubility of insoluble salts are all closely related to temperature. As a rough algorithm: For every 10 degrees Fahrenheit drop in feed water temperature, the feed pump pressure needs to increase by 15%. The water production of each section is also affected by temperature. When the water temperature increases, the water output of the membrane element located at the front end of the RO system increases, while the water output of the membrane element at the rear end decreases. When the water temperature is low, the water production in each section is more balanced. When the water temperature is higher, the kinetic energy of ions permeating through the membrane body increases, so the salt permeability of the system increases. As the water temperature increases, the solubility of calcium carbonate decreases. When the water temperature decreases, the solubility of calcium sulfate, barium sulfate, strontium sulfate, and silica decreases.

pH value

The pH of the feedwater defines its acidity and alkalinity. It is neutral at pH 7; acidic at 0-7; alkaline at 7-14. In analytical chemistry, pH is the negative logarithm of the hydrogen ion concentration. In water chemistry, it is important to define the pH balance of carbon dioxide, bicarbonate, carbonate, and hydroxide ions. The pH value of concentrated water is generally higher than the pH value of the feed water. This is because the concentration of bicarbonate and carbonate ions is higher than the concentration of carbon dioxide. [Rodesign] The software allows the user to adjust the pH value of the feedwater with hydrochloric acid and sulfuric acid, lower the pH value of the feedwater with acid to reduce the LSI (Langrier) index, and reduce the possibility of calcium carbonate precipitation. The pH value of feed water and concentrated water also affects the solubility and pollution degree of silicon, aluminum, organic matter and grease. The change of the pH value of the feedwater also affects the removal rate of ions. When the pH value decreases, the removal rates of fluorine, boron and silicon decrease accordingly.

Conductivity

Electrical conductivity is an indicator of the conductivity of dissolved ions in water. Ideal pure water without ions and no current flow. Conductivity is measured with a conductivity meter and its unit is micro Siemens / cm (μs / cm). Conductivity is also a convenient method for measuring ion concentration in water, but it does not accurately reflect the ion species. The conductance value varies with the ion composition; however, the conductance value increases with increasing ion concentration. The TDS (total dissolved solids) meter uses a conversion factor to convert the conductivity value into a TDS value. In water quality analysis, the conductivity value can be estimated using different conversion coefficients corresponding to different ions or a single conversion coefficient corresponding to the total dissolved solids (TDS). The electrical conductivity can be calculated by multiplying the square root of the carbon dioxide's ppm concentration by 0.6; silicon ions have no effect on the change in electrical conductivity. The most accurate conductivity value of RO high purity water is measured online. Otherwise, exposure of high-purity water to the air will change its carbon dioxide content.

TDS (Total Dissolved Solids)

In the water treatment process, TDS is the remaining inorganic matter after filtering out suspended matter and colloids and evaporating all the water. TDS is in ppm or mg / l. In [IMSdesign] software, TDS is the total of all positive and negative ions and silica. [IMSdesign] The TDS of feedwater and freshwater in the software can be calculated by their respective conductivity. You can also use a TDS meter to measure TDS on the spot. The TDS meter measures the conductivity of water and multiplies it by a conversion factor to get the TDS value of a known reference solution (such as sodium chloride and potassium chloride). It is worth noting that the TDS value of the aqueous solution mixed with various ions indirectly measured by the conductivity value is not the same as the TDS value obtained by adding the concentration of various ions. A rough algorithm is: For a sodium chloride reference solution, every 1 ppm of TDS value corresponds to a conductivity of 2ms / cm. Conductivity / TDS is 102-1.7

Alkalinity

Alkalinity mainly refers to carbon dioxide, bicarbonate, carbonate and hydroxide. In nature, land is alkaline, and its pH value does not change much during the neutralization of acid rain. The pH of the carbon dioxide and bicarbonate solution is 4.4 to 8.2; when the pH is 4.4 or lower, the alkalinity exists as carbon dioxide; when the pH is 8.2, there is no carbon dioxide, and all alkalinity is bicarbonate. Bicarbonate and carbonate solutions balance each other at pH values of 8.2 to 9.6. At pH 9.6, carbon dioxide and bicarbonate are absent, and all alkalinity is carbonate. When the pH is above 9.6, hydroxide basicity appears due to the presence of hydroxide ions. Most natural sources of water have a pH of 6.0 to 8.4, so the appearance of hydroxides is artificial. Alkalinity (especially for boiler water chemistry) can be expressed as M alkalinity and P alkalinity. M alkalinity refers to the total alkalinity of water expressed in ppm value of calcium carbonate (using methyl orange as an indicator, the endpoint of acid titration is pH = 4.2). P alkalinity measures the amount of bicarbonate, carbonate and hydroxide (using phenolphthalein as an indicator, the end point of acid titration is pH = 8.2).