Why Activated Sludge is Negatively Charged

Activated sludge surface with negative charge, physicochemical sludge with positive charge, is known to many people, so the sludge produced by biochemical selection of cationic flocculant, physicochemical sludge selection of anionic flocculant. But why is activated sludge negatively charged? This is related to the main components of activated sludge bacteria, the charge of bacteria determines the charge of activated sludge, this article will give you a comprehensive analysis of the principle of bacterial charge!

1. What is isoelectric point?

Isoelectric point (pI): In a certain pH solution, the tendency or degree of dissociation of amino acids or proteins into cations and anions is equal, and they become facultative ions and are electrically neutral. At this time, the pH of the solution is called the isoelectric point of the amino acid or protein.

The role of isoelectric point

In a certain pH solution, the trend and degree of amino acid dissociation into cations and anions are equal, and the net charge is zero, which is electrically neutral. At this time, the pH of the solution is called the isoelectric point of the amino acid.

The charge of zwitterion varies with the pH value of solution. When the positive and negative charges of zwitterion are equal, the pH value of solution is its isoelectric point.

When the pH of the external solution is greater than the pI value of the zwitterion, the zwitterion releases protons with negative charge.

When the pH of external solution is less than the pI value of amphoteric ions, the protonation of amphoteric ions is positively charged.

When the isoelectric point is reached, the solubility of amino acids in solution is the lowest.

2. What is acidity and alkalinity?

Acidity and alkalinity refer to the degree of acidity and alkalinity of solution, which is generally expressed by PH value.

PH value < 7 is acidic, pH value = 7 is neutral, and pH value > 7 is alkaline.

PH value, also known as hydrogen ion concentration index and acid-base value, is a measure of the activity of hydrogen ion in solution, that is, the acid-base degree of solution in general sense. This concept was proposed by Sø Ren Peter Lauritz Sø rensen, a Danish biochemist, in 1909. P stands for German Potenz, meaning strength or concentration, and H stands for hydrogen ion (H). Sometimes pH is also written as pondus hydrogenii in Latin.

Usually (25 C, 298K), when the pH < 7, the solution is acidic, when the pH > 7, the solution is alkaline, when the pH = 7, the solution is neutral.

Note: The pH value is allowed to be less than 0, such as - 1 for hydrochloric acid (10 mol/L).

3. The difference between isoelectric point and acidity-basicity

Amino acids, containing both amino and carboxyl groups, are amphoteric electrolytes and exist in the form of facultative ions or polar dipole ions in aqueous solutions. Facultative ions of amino acids can accept protons to form cations in acidic solutions and release protons to form anions in alkaline solutions. When the net charge of amino acid is zero, the pH becomes isoelectric point.

That is to say, the isoelectric point of amino acid has little relation with the acidity and alkalinity of its environment, and it only determines its form of existence in aqueous solution.

Isoelectric point may be acidic or alkaline, not medium state!

4. Charge Properties of Bacteria

50-80% of the dry weight of bacteria is protein, which is composed of 20 amino acids linked by peptide bonds in a certain order. And the main component of bacterial cell wall also has protein, so the isoelectric point of bacteria is similar to that of amino acids.

The isoelectric point of known bacteria is 2-5 in Ph. Gram-positive bacteria are 2-3 and Gram-negative bacteria are 4-5.

Generally, the cultivation, dyeing, testing and utilization of bacteria are higher than the isoelectric point of bacteria under alkaline (7-7.5), neutral and acidic (6-7), so they all have negative charges.

Therefore, the charge of bacteria determines the charge attributes of activated sludge. The suitable PH of activated sludge in sewage treatment can only survive under neutral or alkaline condition. Therefore, in normal sewage treatment, our activated sludge carries negative charge.