Colloids are very small particles with sizes ranging from 0.3 to 1.0 um. Colloids are charged and suspended, and they repel each other. Colloids may come from any of the following sources: 1) Aluminum silicate colloids may be present when water enters or passes through clayey structures; 2) iron colloids may exist in corrosion products of carbon steel pumps, pipes and filters; 3) Aluminum hydroxide colloids may be present in water pretreated with alum or improper clarifiers.
Colloidal fouling of reverse osmosis membranes is due to the coagulation of colloids and the deposition of condensates on reverse osmosis membranes during the reverse osmosis treatment process during the operation of reverse osmosis membrane equipment.
I. Sludge Density Index
The colloid content (n) is determined by SDI. SDI is derived from the clogging rate of water through 0.45 um microporous filter under 207 kPa working pressure. Although SDI measurement is not absolute, it is the best method to measure colloid content that has been found. The relationship between SDI and colloid deposition rate in permeators has been obtained. Another method for measuring colloid content (turbidity and particle calculation) is unsatisfactory. Particle size calculation can not give the results of Yawei particle size range. Turbidity is a function of particle size, shape and concentration. Therefore, there is no correlation between turbidity and colloidal fouling from one location to another.
Well water SDI is usually about 1.0 and does not require any pretreatment for removing colloidal fouling. If the SDI of well water exceeds 1.0, the possible reasons are: surface water intrudes into shallow wells; iron corrosion products exist; biological pollution exists; colloidal sulfonation exists. Surface water contains a large amount of colloidal substances, and SDI values range from 10 to 175.
II. Colloid Pretreatment of Reverse Osmosis Membrane
The empirical relationship between SDI and fouling indicates that SDI must be 3.0 or less in order to minimize the fouling rate of reverse osmosis equipment so that the reverse osmosis system can run stably for a long time. It must be noted that surface water treated by municipal water treatment plants usually has SDI greater than 6. Therefore, if the water is not further treated, colloidal adhesion will occur on the reverse osmosis membrane elements.
1. Diatomite (DE) filter
If the original SDI is within 4-5, diatomite filter can be used to reduce SDI to 2.0; if the colloid content is very high, the pressure drop in the filter will increase rapidly.
2. Pressure Media Filter
A small scale test can be used to determine the degree of colloid reduction and to select an optimal filter material or combination for a given water quality.
In pipeline neutralization, coagulation, flocculation and filtration are two basic processes to reduce colloid content and make SDI less than 3.0. Therefore, it is necessary to use suitable coagulants, adjust the concentration of coagulants, and ensure the proper mixing and retention time. Field tests must be carried out to determine the optimum operating conditions. It is important to monitor and control the pretreatment system so that the quality of finished product water can be continuously produced. The seasonal variation of colloid content may also involve the change of coagulant concentration.
4. Colloidal stability
The rate constant K2 is directly related to the stability of colloids. If the stability of colloid is improved, the fouling rate of colloid will decrease. There are several methods of stabilizing colloids in theory. However, the only practical method found is water softening. That is to say, raw water is exchanged with sodium through cation exchange resin, divalent and a small amount of trivalent iron ions. Softening increases the colloid thickness by two times and the effective charge. Both of them can effectively increase the stability of colloids and delay coagulation. Alternating stability can be determined by measuring colloid_potential. _potential usually doubles after softening water. In order to soften, it is extremely important to give effective stability to the recently completed size and proper regeneration.