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What is flocculant used for?
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In key processes such as industrial wastewater treatment,municipal water supply purification,and sludge dewatering,flocculants,as the core agent for enhancing solid-liquid separation,always play the role of"invisible guardians".It condenses small particles in water into separable large particles through physical and chemical reactions,directly determining the efficiency and cost of water treatment systems.This article will start from technical principles and systematically analyze the uses of flocculants in six core areas,providing scientific selection basis for industry users.
1.The technical essence of flocculants:practical application of coalescence theory The mechanism of action of flocculants is based on the"coalescence theory",which means that colloidal particles(particle size 1nm-100μm)in water lose stability and aggregate through charge neutralization,adsorption bridging,and net trapping and sweeping.Taking polyaluminum chloride(PAC)as an example,the Al(OH)3 colloid produced by its hydrolysis is positively charged,which can neutralize the surface charge of negatively charged colloids(such as clay and organic matter)in water,reduce the zeta potential to the critical value(usually<25mV),eliminate the repulsive force between particles,and then form micro flocs through van der Waals forces.
This process requires strict control of the GT value(the product of the velocity gradient G and the stirring time T),generally maintained within the range of 10⁴-10⁵.For example,in the primary sedimentation tank of a municipal sewage treatment plant,PAC needs to be uniformly mixed through rapid stirring(G=300-500s⁻¹)after being added,and then slowly stirred(G=30-50s⁻¹)to promote floc growth,ultimately achieving solid-liquid separation through sedimentation or air flotation.
2.Core application one:Multi scenario application in industrial wastewater treatment 1.Treatment of heavy metal wastewater In the wastewater of electroplating and metallurgical industries,flocculants remove heavy metal ions through chelation precipitation.Taking polyferric sulfate(PFS)as an example,the Fe(OH)∝colloid produced by its hydrolysis can adsorb heavy metals such as Cd²⁺and Pb²⁺,forming insoluble hydroxide precipitates.When used in conjunction with precipitants such as sodium sulfide,PFS can further reduce the concentration of heavy metals in the effluent to below 0.1mg/L,meeting the requirements of the"Emission Standards for Electroplating Pollutants"(GB 21900-2008).
2.Organic wastewater treatment For high concentration organic wastewater in the papermaking and printing and dyeing industries,flocculants remove COD through adsorption bridging.The positively charged groups on the molecular chain of cationic polyacrylamide(CPAM)can neutralize negatively charged dye molecules(such as reactive dyes and direct dyes)in wastewater.At the same time,by sweeping suspended particles with a long-chain structure,the COD removal rate can be increased to 60%-80%.
3.Oilfield wastewater treatment In the treatment of oilfield produced water,flocculants need to simultaneously address the triple challenges of emulsified oil,suspended solids,and bacteria.Reverse demulsifiers(such as polyethers)can destroy the oil-water interface facial mask,making emulsified oil drip and float;When used in conjunction with inorganic flocculants such as aluminum sulfate,an oil removal rate of over 90%and a suspended solids concentration of less than 10mg/L can be achieved.
3.Core use two:Safety barrier for municipal water supply purification 1.Raw water pretreatment In high turbidity water sources such as the Yangtze River and Yellow River,flocculants are the first line of defense to ensure water supply safety.Taking the Yellow River water as an example,its turbidity can reach 1000-3000NTU.When PAC is used alone,the optimal dosage is 30-50mg/L,which can reduce the effluent turbidity to below 5NTU.If combined with activated silica flocculants,larger flocs can be formed,reducing precipitation time by more than 30%.
2.Deep processing In the treatment of micro polluted water sources,flocculants can be used in conjunction with processes such as activated carbon and ultrafiltration membranes to remove organic matter.For example,in a water plant in the the Taihu Lake Lake basin,the adoption of the"PAC+powdered activated carbon"process can reduce the permanganate index(CODMn)from 6 mg/L to 3 mg/L,and the olfactory threshold(TON)from 15 to below 5,significantly improving the sense of drinking water.
3.Emergency response In sudden water pollution incidents,flocculants can quickly reduce the concentration of pollutants.In the 2014 Lanzhou tap water benzene exceeding standard incident,a combination process of activated carbon adsorption and PAC flocculation was used to reduce the benzene concentration from 18.9μg/L to<1μg/L within 48 hours,restoring water supply safety.
4.Core use three:Efficiency booster for sludge dewatering The residual sludge produced by activated sludge process has a moisture content of up to 99%,and the direct treatment cost is high.Coagulants can change the surface properties of sludge particles from hydrophilic to hydrophobic through charge neutralization and bridging,thereby improving dewatering efficiency.
1.Regulation mechanism The extracellular polymeric substances(EPS)in sludge contain a large number of hydrophilic groups(such as carboxyl and hydroxyl groups),which are the main factors hindering dehydration.Cationic flocculants(such as CPAM)can neutralize the negative charges on the surface of EPS,disrupt its hydration layer,and reduce the sludge specific resistance(SRF)from 10¹³-10¹⁴m/kg to 10¹⁰-10¹¹m/kg,creating conditions for subsequent mechanical dewatering.
2.Equipment adaptation Different dehydration equipment has different requirements for flocculants:
Belt filter press:requires CPAM with low molecular weight and high charge density(molecular weight 3-6 million)to form uniform flocs;
Centrifugal dewatering machine:requires CPAM with high molecular weight and low charge density(molecular weight 8-12 million)to resist high shear forces;
Plate and frame filter press:requires CPAM with medium molecular weight and medium charge density(molecular weight 6-8 million)to balance floc strength and permeability.
5.Core use four:Expanding applications in emerging fields 1.Soil remediation In the remediation of heavy metal contaminated soil,flocculants can fix heavy metal ions and reduce their bioavailability.For example,adding phosphate flocculants to lead contaminated soil can convert lead from an exchangeable state to a residual state,reducing its bioavailability by more than 70%.
2.Desalination of seawater In reverse osmosis(RO)pretreatment,flocculants can remove colloids,microorganisms,and organic matter from seawater,preventing membrane fouling.The combination process of"iron sulfate+polyacrylamide"can reduce the SDI(sludge density index)from 10 to below 3,and extend the service life of RO membrane by more than 30%.
3.Food industry In the fields of food processing such as juice clarification and beer filtration,flocculants need to meet food safety standards.For example,chitosan(extracted from chitin),as a natural flocculant,can be used for juice clarification.Its effect on removing pectin and protein is better than traditional gelatin,and it does not introduce harmful residues.
6.Technological development trend:dual drive of green and intelligent With the increasing demand for environmental protection,flocculant technology is developing towards green and intelligent direction:
Greening:Microbial flocculants(such as NOC-1)have become a research hotspot due to their biodegradability and lack of secondary pollution.It is secreted by Streptococcus pyogenes and has a flocculation efficiency of up to 98%for kaolin suspension,which is stable within the pH range of 4-10.
Intelligence:Real time adjustment of flocculant dosage through online monitoring(such as flow current detector)can reduce agent consumption by 20%-30%.For example,after a certain sewage treatment plant adopted an intelligent dosing system,the PAC dosage was reduced from 40mg/L to 30mg/L,saving an annual cost of 500000 yuan.
From industrial wastewater treatment to municipal water supply purification,from sludge dewatering to soil remediation,flocculants have become indispensable core materials in the field of water treatment due to their unique technological advantages.In the future,with the development of green flocculants and the application of intelligent dosing technology,they will play a greater role in ensuring water resource security and promoting sustainable development.For industry users,a deep understanding of the mechanism and application scenarios of flocculants is the key to optimizing treatment processes and reducing operating costs.
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1.The technical essence of flocculants:practical application of coalescence theory The mechanism of action of flocculants is based on the"coalescence theory",which means that colloidal particles(particle size 1nm-100μm)in water lose stability and aggregate through charge neutralization,adsorption bridging,and net trapping and sweeping.Taking polyaluminum chloride(PAC)as an example,the Al(OH)3 colloid produced by its hydrolysis is positively charged,which can neutralize the surface charge of negatively charged colloids(such as clay and organic matter)in water,reduce the zeta potential to the critical value(usually<25mV),eliminate the repulsive force between particles,and then form micro flocs through van der Waals forces.
This process requires strict control of the GT value(the product of the velocity gradient G and the stirring time T),generally maintained within the range of 10⁴-10⁵.For example,in the primary sedimentation tank of a municipal sewage treatment plant,PAC needs to be uniformly mixed through rapid stirring(G=300-500s⁻¹)after being added,and then slowly stirred(G=30-50s⁻¹)to promote floc growth,ultimately achieving solid-liquid separation through sedimentation or air flotation.
2.Core application one:Multi scenario application in industrial wastewater treatment 1.Treatment of heavy metal wastewater In the wastewater of electroplating and metallurgical industries,flocculants remove heavy metal ions through chelation precipitation.Taking polyferric sulfate(PFS)as an example,the Fe(OH)∝colloid produced by its hydrolysis can adsorb heavy metals such as Cd²⁺and Pb²⁺,forming insoluble hydroxide precipitates.When used in conjunction with precipitants such as sodium sulfide,PFS can further reduce the concentration of heavy metals in the effluent to below 0.1mg/L,meeting the requirements of the"Emission Standards for Electroplating Pollutants"(GB 21900-2008).
2.Organic wastewater treatment For high concentration organic wastewater in the papermaking and printing and dyeing industries,flocculants remove COD through adsorption bridging.The positively charged groups on the molecular chain of cationic polyacrylamide(CPAM)can neutralize negatively charged dye molecules(such as reactive dyes and direct dyes)in wastewater.At the same time,by sweeping suspended particles with a long-chain structure,the COD removal rate can be increased to 60%-80%.
3.Oilfield wastewater treatment In the treatment of oilfield produced water,flocculants need to simultaneously address the triple challenges of emulsified oil,suspended solids,and bacteria.Reverse demulsifiers(such as polyethers)can destroy the oil-water interface facial mask,making emulsified oil drip and float;When used in conjunction with inorganic flocculants such as aluminum sulfate,an oil removal rate of over 90%and a suspended solids concentration of less than 10mg/L can be achieved.
3.Core use two:Safety barrier for municipal water supply purification 1.Raw water pretreatment In high turbidity water sources such as the Yangtze River and Yellow River,flocculants are the first line of defense to ensure water supply safety.Taking the Yellow River water as an example,its turbidity can reach 1000-3000NTU.When PAC is used alone,the optimal dosage is 30-50mg/L,which can reduce the effluent turbidity to below 5NTU.If combined with activated silica flocculants,larger flocs can be formed,reducing precipitation time by more than 30%.
2.Deep processing In the treatment of micro polluted water sources,flocculants can be used in conjunction with processes such as activated carbon and ultrafiltration membranes to remove organic matter.For example,in a water plant in the the Taihu Lake Lake basin,the adoption of the"PAC+powdered activated carbon"process can reduce the permanganate index(CODMn)from 6 mg/L to 3 mg/L,and the olfactory threshold(TON)from 15 to below 5,significantly improving the sense of drinking water.
3.Emergency response In sudden water pollution incidents,flocculants can quickly reduce the concentration of pollutants.In the 2014 Lanzhou tap water benzene exceeding standard incident,a combination process of activated carbon adsorption and PAC flocculation was used to reduce the benzene concentration from 18.9μg/L to<1μg/L within 48 hours,restoring water supply safety.
4.Core use three:Efficiency booster for sludge dewatering The residual sludge produced by activated sludge process has a moisture content of up to 99%,and the direct treatment cost is high.Coagulants can change the surface properties of sludge particles from hydrophilic to hydrophobic through charge neutralization and bridging,thereby improving dewatering efficiency.
1.Regulation mechanism The extracellular polymeric substances(EPS)in sludge contain a large number of hydrophilic groups(such as carboxyl and hydroxyl groups),which are the main factors hindering dehydration.Cationic flocculants(such as CPAM)can neutralize the negative charges on the surface of EPS,disrupt its hydration layer,and reduce the sludge specific resistance(SRF)from 10¹³-10¹⁴m/kg to 10¹⁰-10¹¹m/kg,creating conditions for subsequent mechanical dewatering.
2.Equipment adaptation Different dehydration equipment has different requirements for flocculants:
Belt filter press:requires CPAM with low molecular weight and high charge density(molecular weight 3-6 million)to form uniform flocs;
Centrifugal dewatering machine:requires CPAM with high molecular weight and low charge density(molecular weight 8-12 million)to resist high shear forces;
Plate and frame filter press:requires CPAM with medium molecular weight and medium charge density(molecular weight 6-8 million)to balance floc strength and permeability.
5.Core use four:Expanding applications in emerging fields 1.Soil remediation In the remediation of heavy metal contaminated soil,flocculants can fix heavy metal ions and reduce their bioavailability.For example,adding phosphate flocculants to lead contaminated soil can convert lead from an exchangeable state to a residual state,reducing its bioavailability by more than 70%.
2.Desalination of seawater In reverse osmosis(RO)pretreatment,flocculants can remove colloids,microorganisms,and organic matter from seawater,preventing membrane fouling.The combination process of"iron sulfate+polyacrylamide"can reduce the SDI(sludge density index)from 10 to below 3,and extend the service life of RO membrane by more than 30%.
3.Food industry In the fields of food processing such as juice clarification and beer filtration,flocculants need to meet food safety standards.For example,chitosan(extracted from chitin),as a natural flocculant,can be used for juice clarification.Its effect on removing pectin and protein is better than traditional gelatin,and it does not introduce harmful residues.
6.Technological development trend:dual drive of green and intelligent With the increasing demand for environmental protection,flocculant technology is developing towards green and intelligent direction:
Greening:Microbial flocculants(such as NOC-1)have become a research hotspot due to their biodegradability and lack of secondary pollution.It is secreted by Streptococcus pyogenes and has a flocculation efficiency of up to 98%for kaolin suspension,which is stable within the pH range of 4-10.
Intelligence:Real time adjustment of flocculant dosage through online monitoring(such as flow current detector)can reduce agent consumption by 20%-30%.For example,after a certain sewage treatment plant adopted an intelligent dosing system,the PAC dosage was reduced from 40mg/L to 30mg/L,saving an annual cost of 500000 yuan.
From industrial wastewater treatment to municipal water supply purification,from sludge dewatering to soil remediation,flocculants have become indispensable core materials in the field of water treatment due to their unique technological advantages.In the future,with the development of green flocculants and the application of intelligent dosing technology,they will play a greater role in ensuring water resource security and promoting sustainable development.For industry users,a deep understanding of the mechanism and application scenarios of flocculants is the key to optimizing treatment processes and reducing operating costs.
https://www.mysdchemicaltech.com
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