Effective Pretreatment for High-Purity Water Preparation – Diatomaceous Earth Filtration

Abstract: This article introduces the function of diatomaceous earth filtration and its process flow - the complete process of pre-coating, filtration, and backwashing; gives the impact of additive dosage and initial filtration rate on the filtration cycle, and points out that filtration on diatomaceous earth can be effectively used as a pretreatment method for high-purity water produced by electrodialysis and ion exchange methods.

Recently, the use of diatomite to filter beer, liquor and beverages is common in my country, but the use of diatomite to filter water has not yet been promoted. In 1999, Tongji University cooperated with the French Lian Water Company to systematically conduct research on the use of diatomite for water treatment. Recently, they jointly conducted a productive test in the water production workshop of the preparation room of the First Affiliated Hospital of the Second Military Medical University.

Filtration process of diatomite

Diatomite filtration uses diatomite as the filter medium. After pre-coating to form a membrane, the water is clarified by membrane filtration. It is a pre-coating filter, so a pre-coating stage is necessary. In this way, pre-coating, channel filtration, and backwashing are the processes of diatomite filtration. The composition of the filtration system is shown in Figure 1.

1.Pre-coating

First, use tap water or diatom filtered water to make a slurry of a certain concentration of diatomaceous earth. This slurry is called "precoat". Use a water pump to drive the slurry into the filter for circulation. The circulation flow rate is generally about twice the flow rate during filtration. After 10-15 minutes of circulation, diatomite can evenly form a 1-2mm pre-coated film on the surface of the filter element in the filter. When the turbidity of the circulating slurry reaches the required filtered water quality, the circulation stops and the system switches to filtration. Generally, 400-850g diatomaceous earth can be pre-coated per m2 of filter area. The specifications, varieties and quantity of diatomite used mainly depend on the quality of the raw water. Generally speaking, a satisfactory filtration effect can be obtained by using diatomite in two specifications, thick and fine. In the test, we used Ningbo Xiangyang Dyeing and Chemical Factory product #401 and the original soil from Shengxian County, Zhejiang, and the treatment effect was much better than using one alone.

2. filter

The mechanism of diatomite filtration has three aspects: ①Screening, when the water flow passes through the diatomite filter membrane, all the suspended impurity particles with large particle size and dry diatomite pores are intercepted. Screening often occurs on the surface of the filter membrane; ② Blocking, when some relatively small impurity particles penetrate the surface of the filter membrane and penetrate into the interior of the filter membrane, they are blocked by the winding microporous channels inside the filter membrane and the smaller pores in the filter membrane and are removed; ② Adsorption, some particles smaller than the internal pores of diatomite itself, when flowing through the filter membrane with water, collide with the internal surface of diatomite and are attracted by opposite charges. In addition, the impurity particles attract each other with different electric charges, forming chain clusters and adsorbed on the filter medium.

The main factors affecting the filtration effect of diatomite are as follows:

(1) Additives

In order to always maintain a certain porosity in the filter membrane (i.e. filter cake) and slow down the growth rate of head loss, an appropriate amount of diatomite (called an additive) needs to be added to the raw water. The main factor affecting diatomite filtration is additives. Within a certain dosage range, if the additive dosage is large, the head loss growth rate will be reduced and the filtration cycle will be extended.

The relationship curve between filtration time and head loss growth when adding different amounts of additives is shown in Figure 2.

(2)Filter speed

Filtration speed is also a very important factor affecting diatomite filtration. Constant pressure variable speed, constant pressure constant speed and variable pressure variable speed filtration methods can be used in filtration. Under the premise of ensuring the quality of filtered water and selecting diatomite specifications and varieties, the initial filtration rate used directly affects the amount of water filtered by diatomite per unit weight, that is, the cost of water production. Generally speaking, if the initial filtration rate is too large, the head loss will increase rapidly and the filtration cycle will be shortened. The specific filtration characteristics with different starting filtration rates are shown in Figure 3. It can be seen from the solid that due to the large initial filtration speed of curve D (i.e. v=3.4m/h), the filtration period is the shortest; curve A (V=2.4m/h); curve B (V=1.56m/h); curve C has the smallest initial filtration speed (v=1.46m/h). Curve C is gentle, that is, the head loss increases slowly, making its filtration period the longest.

In the case of high water quality requirements, such as pretreatment during pure water preparation, it is best to use a slower filtration speed, usually 2-3m/h is more suitable.

There are many factors that affect the filtration effect of diatomite, such as raw water quality, model and specification of diatomite used, pre-coating dosage, filtration rate, etc. But practice has proven that temperature has basically no effect on diatomite filtration.

3. Backwash

When the filtration cycle ends, the filter cake with impurities left behind needs to be rinsed out for re-precoating. There are four flushing methods: reverse water flow flushing; reverse water flow plus air to assist flushing; jet flow flushing; air impact backwashing. Reverse water flow flushing is a commonly used and easy-to-implement flushing method. Generally, with a flushing intensity of 8-10L/(m2·e), it can be flushed in only 2 minutes. The flushing water accounts for about 2% of the filtered water volume. For pretreatment of high-purity water preparation, raw water (ie tap water) can be used directly for backflushing. The backflush must be thorough and clean, otherwise it will seriously affect the filtration in the next cycle.

2. Diatomaceous earth filtration function

Removal of suspended solids and colloidal materials

Directly filtering with diatomaceous earth can remove suspended particles and a large proportion of body impurities in the water. The coarser grade diatomite can remove impurity particles above 5 μm; the coarser grade diatomite can remove particles below 50 nm.

Removal of bacteria

Fort, Virginia, USA Work by the Army Engineering Research and Development Laboratory in Belvoir demonstrated that filtration using finer grade diatomaceous earth not only removes 100% of amoebic dysentery cysts but also effectively removes other pathogenic microorganisms; work by Rutgers University states that Yuan et al. High-grade diatomite can only remove more than 50% of E. coli (100 ml of incoming water contains 210-1300 E. coli), while the finest grade of diatomite can completely remove all E. coli in the incoming water, even thousands of E. coli in 100 ml of incoming water can be removed.

We also found in the test that the bacterial removal rate of diatomite filtration has always remained in a high range, sometimes as high as 100%.

Remove iron and manganese

Iron-containing water without any oxidation treatment can be filtered directly with diatomaceous earth, and about 30% of the iron content in the raw water can be removed. If the water contains low-priced iron and germanium (such as groundwater), it should first undergo oxidation pretreatment. After the low-priced iron and manganese in the water are oxidized into high-priced iron and manganese, the water should be filtered and removed with diatomite. However, compared with other iron removal methods, the oxidation does not need to be too complete, which is the advantage of diatomite iron removal. Diatomite filtration technology has been used abroad to remove iron and manganese from groundwater for thirty years.

Removal of other substances

Diatomaceous earth filtration can also remove algae, color, humic acid and other organic matter. The color removal effect can generally exceed about 60%; diatomaceous earth still has an adsorption effect on organic matter such as humic acid.

Characteristics and benefits of diatomite filtration as electrodialysis and ion exchange pretreatment

As mentioned earlier, the main objects of pretreatment are suspended substances, colloids, microorganisms, organic matter, etc. The biggest feature of diatomite filtration is that it is very effective in removing the above impurity particles. Tests in the preparation room of Changhai Hospital showed that the turbidity of the water filtered by diatomite is 0.2 to 0.4 degrees lower than that of the honeycomb tubular filter made of cotton fiber, and the water quality is stable. It can extend the regeneration cycle of the ion exchange resin by about 20%, and the sterilization rate can reach more than 90%.

Diatomite filtration pretreatment has the following beneficial effects on electrodialysis and ion exchangers:

1. Effect on electrodialysis

Diatomaceous earth filtration removes turbidity thoroughly and removes bacteria efficiently, which not only extends the service life of the ion exchange membrane but also reduces its resistance, which is beneficial to the quality of the effluent water. This is because the suspended solids adhering to the membrane surface become an obstacle to ion migration. Bacteria use the membrane as organic nutrients and colonize the membrane surface, which will increase the membrane resistance and worsen the water quality.

The diatomite filtration process has the characteristics of removing iron and germanium, which can prevent the poisoning of the ion exchange membrane by high-priced gold ions of iron and germanium.

2. Effect on ion exchange resin

Diatomaceous earth filtration removes turbidity and bacteria more thoroughly, which prevents various impurities from adhering to the surface of the exchange resin particles to reduce its exchange capacity and block the pores of the resin layer, causing increased head loss. When backwashing is poor, the above-mentioned impurities may penetrate deep into the interior, causing the exchange agent layer to agglomerate and cause biased flow, thereby deteriorating the quality of the effluent water.

The characteristics of diatomite filtration and color removal are beneficial to ensuring the quality of effluent water. Generally speaking, water containing more sediment is yellow; water containing a large amount of sediment and humus is yellow-brown; water containing colloidal iron is light yellow or green; water containing algae is bright green or brown or dark brown. Chromogenic substances will contaminate the exchange resin, block the pores of the resin layer, colonize bacteria, and worsen the quality of the effluent.

The characteristics of diatomite filtration in removing iron and reducing manganese are more conducive to ion exchange resin. Because the iron and manganese ions in the water are easily adsorbed by the resin and are not easily replaced by low-concentration regenerants, they accumulate inside the resin particles, reducing the resin exchange capacity. If hydroxide precipitates form, they will clog the resin micropores and pores and increase head loss.

In short, diatomite filtration is used as a pretreatment for producing high-purity water, which can increase the service life of ion exchange membranes and exchange resins, and improve the quality of effluent water.

3. Things to note

(1) When the head loss of the diatomite filter reaches the end-of-period allowable value (generally the end-of-period allowable head loss value is 0.15MPa), the pump needs to be stopped to interrupt filtration. After stopping the pump, it can be restarted immediately. After a cycle of 10-15 minutes, when the filtered water reaches the required water quality, it can then be transferred to normal filtration.

(2) When a filtration cycle ends, the diatomaceous earth is not actually fully utilized. There are diatomite (precoating agent and additive) and impurity particles in the filter cake. In order to make full use of diatomite, the filter cake that should be discharged can be used to restart a cycle according to the method in (1); or the filter cake that should be discharged can be flowed back into the precoating barrel, and then the slurry can be used for precoating. This practice is called the reuse of diatomite.

Generally, the initial head loss of a reused filter membrane is higher than that of a filter membrane formed by clean diatomite pre-coating, and as the number of reuses increases, the initial head loss becomes higher each time; while during the filtration process, the growth rate of the head loss increases slowly with the number of reuses. Under the premise of ensuring the quality of the filtered water, when the raw water turbidity is high, the number of reuses can be 2-3 times. For raw water with low turbidity, it can be reused 6-7 times or even 10 times.