As the leading manufacturer of Sodium Metabisulfite, Tianya Chemical has been supplying highly efficient Sodium Metabisulfite Food Grade and Industrial Grade (Technical Grade) products for water treatment industry. Let’s introduce the principles of wastewater treatment by sodium metabisulfite.

1. Reduction of Hexavalent Chromium

Chemicals Principles Theoretical mass ratio
Sodium sulfite HCrO+3NaSO+3HSO=Cr(SO4)+3NaSO+4HO 3.64
Sodium Metabisulfite HCrO+NaSO+14HSO= 4Cr(SO4)+6NaSO+7HO 2.74
Sodium Bisulfite HCrO+3NaHSO= Cr(SO4)+3NaOH+2HO 3.06
Sodium Hydrosulfite NaSO 1.67
Sodium Thiosulfate NaSO 1.34
Sodium Sulfate (CrO)+3(SO)+8H=2Cr+3(SO)+4HO

2. Removal of cyanide

The alkaline chlorination method for cyanide removal is divided into two stages:

The first stage is to oxidize cyanide to cyanate, which is called “incomplete oxidation”. The reaction formula is as follows.

CN-+ClO-+H2O    CNCl+2OH-
CNCl+2OH-CNO-+Cl-+H2O

The reaction between CN – and OCl – first generates CNCl, and the reaction rate of hydrolysis of CNCl to CNO – depends on pH value, temperature, and effective chlorine concentration. The higher pH value, the higher the water temperature, and the higher the effective chlorine concentration, the faster the hydrolysis rate. Moreover, under acidic conditions, CNCl is highly volatile, so the pH value must be strictly controlled during operation.

The second stage is to further oxidize and decompose cyanate into carbon dioxide and nitrogen, called “complete oxidation”, and the reaction formula is as follows:

2CNO-+3ClO-+H2O    2CO2  +N2  +3Cl-+2OH-

2CNO-+3Cl2+4OH-2CO2  +N2  +6Cl-+2H2O

The ratio of the dosage of cyanide breaker to cyanide is 8:1.

3. Removal of Copper (by sodium metabisulfite water treatment)

a) Sulfide precipitation method:

Heavy metal ions and S are prone to form insoluble or insoluble precipitates, and the adding of NaS can completely precipitate heavy metal ions in wastewater. CuS has a tendency to form colloidal solutions (which can pass through filter paper), and flocculants need to be added to form large flocs and settle together. If NaS is added excessively, it will produce a foul odor, and ferrous salt needs to be added to precipitate it.

b) Heavy metal capture method:

Heavy metal collectors are water-soluble chelates that can form stable insoluble substances with a variety of heavy metals, mainly including xanthate and dithiocarbamate derivatives (DTC), which are widely used.

c) Oxidation-reduction method:

Oxidation method: add strong oxidant to the wastewater to oxidize the coordination ion of copper to release copper, and then add alkali to coprecipitation it. Common oxidants include NaClO, fenton reagent, etc., pH=9~10.

Iron powder reduction method: under acidic conditions, iron powder with high chemical activity is added to the wastewater as a reducing substance to replace copper, and then the pH value is raised to generate Fe (OH) and copper coprecipitation, with large sludge production and easy caking.

Aluminum catalytic reduction method: Metal aluminum releases 3 electrons under alkaline conditions and oxidizes itself into (HAlO), while copper obtains 2 electrons and is reduced to elemental copper. Due to the fact that aluminum is an amphoteric metal, it can also dissolve and form colloidal precipitates of Al (OH) in highly alkaline chemical copper wastewater and wastewater.

Principles of Wastewater Treatment by Sodium Metabisulfite

4. Removal of Zinc (by sodium metabisulfite water treatment)

Zinc-containing hydroxide compounds are amphoteric compounds, and high or low pH values can cause precipitation and back-dissolution. Therefore, they are collected and treated separately, with a pH of 7-8

5. Removal of fluoride

Fluoride has strong corrosiveness, especially under acidic conditions, producing hydrofluoric acid, which corrodes the glass probe of the instrument. Generally, calcium chloride or calcium hydroxide is used to precipitate, but the resulting precipitate is milky and difficult to precipitate. Moreover, when the concentration of fluoride ions reaches 15mg/L, it is difficult to precipitate again, and the generated calcium fluoride will wrap around calcium hydroxide, preventing it from coming into contact with wastewater. Generally, two-stage sedimentation is required, with the secondary reaction sludge returning to the primary reaction, and the unfinished sludge serving as the reactant to reduce the dosage. If entering the membrane system in the future, the impact on the membrane should be considered.

6. Chemical phosphorus removal (by sodium metabisulfite water treatment)

Al+PO→AlPO↓ pH=6~7

Fe+PO→FePO↓ pH=5~5.5

In acidic environments, Al and Fe can form precipitates with PO for removal, and Al and Fe have a flocculation effect, which is beneficial for the flocculation generated by sedimentation.

Al+OH→Al(OH)↓ pH=6

Fe+OH→Fe(OH)↓ FepH=8,FepH=4.5

The reaction that competes with precipitation reaction is the reaction between metal ions and OH, so it must be controlled in a slightly acidic environment.

5Ca+3PO+OH→Ca(PO)OH↓ pH≥8.5

During the precipitation process, the main effect on the formation of insoluble calcium phosphate is not Ca, but OH ions. As the pH value increases, the solubility of calcium phosphate decreases. However, in the range of pH 8.5 to 10.5, in addition to calcium phosphate precipitation, calcium carbonate is also produced, which can cause scaling.

For soft or medium hard wastewater, when calcium precipitation is used, the amount of calcium required to achieve the required pH value is very small, while wastewater with strong buffering capacity requires a larger amount of calcium addition.

Removing 1g of P requires adding 2.7gFe or 1.3gAl.

Mud production Δ Y=1.42 × n × [P0-P]+1.91[Fe]+2.89[Al]

N is the efficiency of chemical phosphorus removal, calculated in mg or kg for [P0-P], [Fe], and [Al]

Principles of Wastewater Treatment by Sodium Metabisulfite

7. Denitrification (by sodium metabisulfite water treatment)

Organic nitrogen accounts for 60% and ammonia nitrogen accounts for 40% in wastewater, which can be converted into ammonia nitrogen through biological processes.

a) Blow off method

NH+HO→NH+OH

Under alkaline conditions, lime is usually added to regulate pH=10.5~11.5, but it is prone to generate CaCO on the surface of the filler. The hydraulic load is set at 2.4~7.2m/mh, the gas-liquid ratio is set at 3590m/m, the wind speed is<168m/min, and high temperatures are conducive to the reaction. It cannot operate at temperatures below 0 ℃ and cannot handle nitrate nitrogen.

b) Point chlorination

2NH+3HOCl=N↑+5H+3Cl+3HO

It can control the ammonia nitrogen in the effluent within 0.1mg/L, which is not suitable for high flow and high ammonia nitrogen wastewater. There are other substances that consume chlorine in the wastewater, and the actual dosage is greater than the theoretical value of 7.6. A neutral pH value is conducive to the reaction, with NO generated if it is too high and NCl generated if it is too low. The remaining chlorine is removed using sulfur dioxide or activated carbon.

c) Chemical precipitation

Adding Mg and PO to form a insoluble complex salt MgNHPO6HO with ammonia nitrogen is particularly suitable for high concentration ammonia nitrogen wastewater. pH<9.5, the longer the sedimentation time, the larger the grain size, and the better the effect. n (Mg): n (N): n (P)=1.3:1:1.08 (mass ratio), pH=9, and the removal rate is 98%.

d) Biological denitrification

Nitrification reaction: NH+2O → NO+2H+HO

DO>2mg/L, pH value ↓ pH=7-8, 20-30 ℃ C/N should not be too high

Complete nitrification of 1g ammonia nitrogen requires 4.57g oxygen (3.43g Nitrosation, 1.14g nitrification) and 7.14g alkalinity (calculated by CaCO).

Item nitrosobacteria nitrobacteria
cell shape Ellipsoid or rod-shaped Ellipsoid or rod-shaped
Cell size 1~1.5 0.5~1
Gram’s stain negative negative
Generation period (h) 8~36 12~59
Aerobic capacity Strictly aerobic Strictly aerobic
Maximum growth rate 0.96~1.92 0.48~1.44
yield coefficient 0.04~0.13 0.02~0.07
saturation constant 0.3~3.6 0.3~1.7

Denitrification:

6NO+2CHOH→6NO+2CO+4H0

6NO+3CHOH→2N+3CO+3H0+6OH

BOD/TKN=4-6, methanol can be used as the carbon source for dosing

Reducing 1g NO can provide 2.6g oxygen, generate 3.47g CaCO and 0.45g denitrification bacteria, consume 2.47g methanol (about 3.7g COD), pH=6.5~7.5 DO<0.5mg/L at 5~40 ℃.

e) Biological zeolite for ammonia nitrogen removal

Zeolite is a porous hydrated silica aluminate crystal with a tetrahedral framework structure, which has good adsorption and ion exchange properties. At the same time, zeolite has a large specific surface area, is non-toxic to organisms, and is easy to attach microorganisms as biological carriers. In the biological zeolite ammonia nitrogen removal process, on the one hand, zeolite is used as a biological carrier to enrich nitrifying bacteria, on the other hand, zeolite adsorbs ammonium in water through ion exchange, and on the other hand, it is very important that the nitrifying bacteria in the biofilm on the surface of zeolite convert the ammonia nitrogen adsorbed on the zeolite into nitrate, forming a self absorption and self digestion cycle. Through biological methods, zeolite can not only be continuously regenerated, It can also improve the nitrification performance of ammonia nitrogen removal and effectively remove ammonia nitrogen through the action of microorganisms. The process of removing ammonia and nitrogen from biological zeolite essentially involves three processes: chemical adsorption, ion exchange, and biological nitrification.

8. Removal of Aniline (by sodium metabisulfite water treatment)

a) Oxidation of ClO into simple organic and inorganic acids: aniline to ClO concentration ratio=3:10 30min, pH=6, removal rate 71.35%

b)Fenton: pH=3 HO: Fe molar ratio=3:1 5h removal rate 91.17%

c) O: Dosage 300mg/h, initial aniline concentration 200mg/L, pH=9 10min, removal rate 99%

d) Microorganisms: Within 30 days, 10 mmol of aniline by Fusarium was degraded by 70%, 10 mmol of aniline by Rhizopus was degraded by 65%, pH=7, 30 ℃, 48 hours, and the removal rate of Arthrobacter was 94.7%. The initial concentration of aniline was 1022 mg/L.

9. Oil removal (by sodium metabisulfite water treatment)

Oils in COD generally have negative charges and need to carry positive charges such as H, Fe, Fe, Al, etc. for neutralization and coagulation, acidification and coagulation demulsification.