In-depth discussion-the removal of green algae in the water treatment system
1 Coagulation and algae removal? Adding aluminum sulfate as a coagulant can simultaneously remove turbidity and algae. When the number of algae in the effluent is less than 1000 pcs/mL, the amount of coagulation required is much greater than the amount required when the turbidity is less than 3 NTU. The reason is that the clay colloid can be completely destabilized when the zeta potential = -5 mV, while the algae must be destabilized when the zeta potential = 0. If polyacrylamide or cationic coagulant is added at the same time, the amount of aluminum sulfate can be reduced. When using coagulation to remove algae, choose the agent according to the type of algae. Environmental protection bees can separately add aluminum sulfate when removing diatoms. For example, the aluminum dosage of the Shawan Water Plant in Panyu City during the high propagation period of diatoms increased from 1.2 mg/L to 3.0 mg/L, which can make the sedimentation tank effluent. The turbidity is reduced to 1 to 2 NTU to reduce the amount of algae entering the filter. The removal of green algae generally requires pre-oxidation. When pre-chlorination, the removal rate is about 95% to 98%. Without pre-chlorination, the average removal rate is 85%. Use other oxidants). Blue and green algae produce foul smells, even contain toxins, and secrete mucus, causing post-flocculation in the water distribution pipe network. This secretion may be converted into trihalomethane precursors, so it is difficult to remove in water treatment. It is also the main algae grown in most eutrophic water bodies, and it is extremely sensitive to the adjustment of the coagulant dosage. In addition, the organic matter produced by the metabolism of algae also affects flocculation and filtration. The reason is that the acidic matter in the organic matter reacts with the hydrolysate of the coagulant (iron salt or aluminum salt), and the resulting surface complex is attached to the floc. The surface of the particles prevents the particles from colliding with each other, so the dosage of coagulant must be increased to compensate for the effect of the formation of surface complexes on the destabilization and flocculation of the particles [1].
2 Direct filtration and algae removal?
Direct filtration is not suitable for processing water with extremely high algae content. At this time, a sedimentation tank or clarification tank should be added before the filter tank. However, it may also happen that the filter tank effluent contains algae content> 1000/mL, and further treatment is required.
There are many types of sedimentation or clarification structures, but the algae removal rate is different. For example, the average algae removal rate is 59% when the Thames River water is treated by the static sedimentation tank, but when it treats the Iowa River water, the algae removal rate is 37% (aluminum sulfate coagulation) to 97% (lime softening). When the clarifier is used to treat Polish river water, the average algae removal rate is 85% to 86% (no prechlorination), 95% to 97% (prechlorination), and the amount of zooplankton also decreases by 93% to 96% (no prechlorination). Pre-chlorination) and 99% (pre-chlorination), so the treatment effect of the clarification tank is better than that of the static sedimentation tank.
Direct filtration is suitable for situations where the amount of algae and suspended solids in the raw water is small. The key to this process is the filtration rate. Using a homogeneous sand filter or a double-layer filter material filter for direct filtration, the algae removal rate is about 15% to 75%. If pre-chlorination is carried out and the white coal-sand double-layer filter is used for direct filtration after the addition of coagulant (filtration rate <3 m/h), the optimal removal rate of algae is about 95%. However, when the amount of algae in the raw water is more than 1000/mL, the particle size of white coal is 0.9 mm, or the number of algae is more than 2500/mL, and the particle size of white coal is 1.5 mm, the filtration cycle is significantly shortened.
The average number of algae in the raw water of Kunming No. 5 Water Plant is 30 500/mL, and the micro-flocculation direct filtration method is used to remove algae. ～1.2 mm, height is 500 mm, filtration rate is 6～10 m/h), the average removal rate is 96.4%.
A semi-productive experiment using the Spanish river water in Madrid as raw water also yielded similar results. The algae removal rate of the double-layer filter is 63%～98%, among which 10 mg/L aluminum sulfate and 0.5 mg/L activated silicic acid have the best effect, but because the number of algae in the raw water is more than 2500 Pcs/mL, resulting in a filter's working cycle of only 6 h.
3 Precipitation or filtration to remove algae?
Add powdered activated carbon (PAC) to the reaction sedimentation tank as a coagulant (which can effectively remove the smell of soil), which can strengthen the reaction and precipitation effect, especially in the season of algae blooming, this method can be used as an emergency measure. In May 1995, the water supply department in Chicago, the United States, began to add PAC (approximately 2.4 mg/L) before the arrival of summer; when methyl-isoborneol (MIB) appeared in the water (in mid-July), PAC was added. The dosage of PAC gradually increased to 11 mg/L; after summer, the dosage of PAC decreased with the decrease of MIB concentration. When the dosage of PAC was reduced to 1.2 mg/L, the dosage was continued for 1 month. If the MIB concentration drops to 5μg/L during this period, the dosing of PAC can be stopped.
Japan’s Takahashi and Takashi and others monitored the inflow and outflow of water from a water plant that uses reservoir water as a source of water (using conventional treatment techniques) and found that the number of cyanobacteria is large from June to October, and diatoms are from September to the next year. There are many in April. At the same time, it is proved that as long as the number of algae is not too much, conventional treatment has better removal performance on algae [2].
Professor Pakmer from the United States studied the effect of algae on the filtration effect: when the number of algae is less than 500/mL, it will not cause the filter to block; when the number of algae is 500 to 1000/mL, the filter is slightly clogged; When the number of algae is 1000-2000/mL, there will be obvious clogging; when the number of algae is more than 2000/mL, serious clogging will occur.
The Shanghai Yuepu Water Plant draws water from the Chenhang Reservoir. Starting from late March each year, the water temperature in the reservoir is high in the upper layer and low in the lower layer. Algae multiplies quickly. It is difficult for the algae entering the water plant to settle, causing the filter to block (the filtration cycle is only For 2 to 3 h). The measures taken by the plant: one is to reduce the residence time of the raw water in the reservoir, to increase the water exchange rate of the reservoir, so that impurities in the water do not have time to settle (maintain a turbid state) and sunlight is difficult to penetrate, which makes the photosynthesis of algae difficult; Use chemical control (adding bleaching powder to the reservoir water).
4 Slow filter, biological filter algae removal
The Stockholm Water Plant in Sweden treats the effluent of the fast filter through the slow filter (the filtering area of the slow filter is 500-2400m2, the filtration speed is only 1/30 of the fast filter, and the thickness of the sand layer is 1m) to remove water The remaining tiny algae.
Israel uses straw as the filter material, and 50 kg of straw is placed in a PVC pool with a volume of 1m3 for filtration, which can remove 75% of the algae.
The biological filter process is a kind of biological algae removal, which mainly uses the flocculation and adsorption of the microorganisms on the biofilm to make it settle, oxidize or be swallowed by protozoa.
5 Pre-oxidation and algae removal?
"For some algae (such as green algae), chlorine, ozone, potassium permanganate or chlorine dioxide can be used for pre-oxidation to improve the removal effect. Some algae often produce odor after pre-chlorination. At this time, excess chlorine should be added to make free residual chlorine, and then dechlorination according to water quality requirements.
Using potassium permanganate to remove algae also has a good effect, and the algae removal effect on alkaline water is better than neutral or acidic water. Generally, the dosage of potassium permanganate is 1～3 mg/L, and the contact time is ≥1～2 h, but there are also special cases where the dosage is 10 mg/L and the contact time is 10～15 min (in order to extend the contact time , Can be administered in the water pipe). If too much potassium permanganate is used in the pre-oxidation process, it may penetrate the filter and enter the water distribution network, causing "black water" phenomenon, and the manganese content of the effluent increases, which may not meet the drinking water quality standards. Excess potassium permanganate can be removed in the sedimentation tank, as long as the light red color has disappeared in the tank, the potassium permanganate will not enter the filter tank. Some water plants use direct filtration processes (without coagulation or sedimentation), and special testing equipment is required to prevent excess potassium permanganate from penetrating the filter tank and entering the water distribution network. Sometimes powder activated carbon can also be added to remove excess potassium permanganate. The dosing point should be after the potassium permanganate oxidation reaction is completed to avoid interaction and reduce the algae removal effect. However, the powdered activated carbon may also penetrate the filter and enter the water distribution. The pipe network should be controlled on the filtration rate.
Ozone is the only effective oxidant that does not increase the total solids in the treated water, and the dosage is 0.5-5 mg/L. Due to the relatively high capital and operating costs of this law, it is rarely applied in China.
The combination of ozone and activated carbon to remove algae has attracted people's attention. The raw water from the Fukuma Town Water Plant in Japan is taken from a reservoir, which has a small capacity and shallow depth, so the algae are easy to multiply (sometimes have peculiar smell). The water plant only uses ozone-activated carbon treatment equipment when the reservoir water level is low, there are many algae, and the smell is strong, so that the algae can be controlled.
Beijing Tiancunshan Water Plant's water source is taken from Huairou, Miyun and Guanting Reservoirs. The effluent after conventional treatment during the peak period of algae growth does not meet the drinking water standard. When the advanced ozone-activated carbon treatment is added, satisfactory results have been achieved.
There are regulating reservoirs near many water plants in the United States. There are a lot of algae in the reservoir, and three methods are often used to remove algae: ①When the number of algae is large, special personnel add copper sulfate solution to the reservoir every day. The dosage is generally 0.5～0.7 mg/L; ②When the amount of algae is medium, add potassium permanganate to the raw water; ③When the amount of algae is small, use pre-chlorination to remove. In recent years, people believe that chlorine dioxide can be used as an effective algaecide. The algae removal mechanism is that the pyrrole ring in the chlorophyll of algae is very similar to the benzene ring. Chlorine dioxide has a certain affinity for the benzene ring, which can make the benzene ring change without odor and taste. Chlorine dioxide can also act on the pyrrole ring to oxidize chlorophyll, causing the algae to die due to the termination of metabolism and interruption of protein synthesis. Chlorine dioxide reacts very fast with algae, which can effectively control musty and fishy odors. Chlorine dioxide is currently not widely used in China, and it needs to be produced on-site just like ozone. There are stable liquid chlorine dioxide supply stations abroad, but they are expensive.
6 Air flotation and algae removal?
In recent years, the dissolved air flotation method has been widely used to remove algae. This method has relatively satisfactory effects in terms of solid-liquid separation speed (5-8 m/h), sludge concentration and chemical consumption saving. The main advantages of air flotation method over clarification method are: ①When the number of algae in the raw water is (3～5)×104/mL, the number of algae in the effluent of the air flotation tank and the clarification tank is both 1000-1200/mL mL, but the air flotation method can save 20%-40% of the coagulant; ②The dry solids concentration of sludge in the air flotation tank is 25-30g/L, and the dry solid concentration of the clarification tank is only 1/10 of that of the air flotation tank. The air flotation method can save the sludge thickening stage during mud treatment and reduce the investment of treatment equipment; ③The air flotation process saves operating costs, but requires that the suspended solids content of the raw water is not high, and the equipment can be repaired in time when the equipment fails. Water plants in Kunming, Wuhan and Wuxi in my country have adopted air flotation ponds, which can achieve the purpose of clarification and algae removal at the same time.
7Micro filter method
Using a microfilter with a mesh pore size of 10 to 45 μm (mostly 35 μm), the algae removal rate is about 50% to 70%, but the turbidity can only be reduced by 5% to 20%. The average removal rate of algae is 40% when a cloth screen with an aperture of 25-35μm is used to treat the Nile water in Cairo; it is 55% when it is used to treat the water of the Seine in Paris, and it is 50%-65% when it is used to treat water from lakes and reservoirs. The Slipplingen water plant in Germany uses a microfilter to remove most of the algae and other particulate matter in the raw water before administering the raw water.
The removal rate of algae by the microfilter varies greatly with the type of algae. The finer the algae, the more difficult it is to remove, sometimes only 10% is removed, but this type of algae consumes the largest amount of coagulant. And because the turbidity that can be removed by the microfilter is not much, it is almost impossible to reduce the dosage of coagulant by using the microfilter.
In some special circumstances, such as when zooplankton (worms, crustaceans, etc.) need to be removed, a microfilter can be used to remove algae.