I. What are flocculants, coagulants and conditioners?
These agents can be classified according to their use in sludge filter press treatment as follows.
1, flocculant: sometimes also known as coagulant, can be used as a means of strengthening solid-liquid separation, for primary sedimentation tank, secondary sedimentation tank, flotation tank and tertiary treatment or depth treatment process links.
2, coagulant: auxiliary flocculants to play a role in strengthening the coagulation effect.
3. Conditioning agent: Also known as dewatering agent, it is used for conditioning the remaining sludge before dewatering, and its varieties include some of the flocculants and coagulants mentioned above.
II. Flocculants
Flocculant is able to reduce or eliminate the precipitation stability and polymerization stability of dispersed particles in water, so that the dispersed particles coalesce and flocculate into aggregates and remove a class of substances.
According to the chemical composition, flocculants can be divided into inorganic flocculants and organic flocculants.
The traditionally applied inorganic flocculants are low molecular aluminum and iron salts, the aluminum salts mainly include aluminum sulfate (AL2(SO4)3∙18H2O), alum (AL2(SO4)3∙K2SO4∙24H2O), sodium aluminate (NaALO3), and the iron salts mainly include ferric chloride (FeCL3∙6H20), ferrous sulfate (FeSO4∙6H20) and iron sulfate ( Fe2(SO4)3∙2H20).
Generally speaking, inorganic flocculants have the characteristics of easy availability of raw materials, simple preparation, low price, moderate treatment effect, etc., so they are more often used in water treatment.
2、Inorganic polymer flocculant
Both hydroxyl and oxy polymers of Al(III) and Fe(III) are further combined into aggregates, which are kept in aqueous solution under certain conditions with particle size roughly in the nanometer range, and thus exerting coagulation-flocculation will give the result of low dosage and high effect.
If their reaction polymerization rates are compared, the aluminum polymers react more gently and have a more stable form, while the iron hydrolysis polymers react rapidly and tend to destabilize and precipitate.
3、Organic polymer flocculant
Synthetic organic polymer flocculants are mostly polypropylene and polyethylene substances, such as polyacrylamide and polyethyleneimine. These flocculants are water-soluble linear polymeric substances, each macromolecule consists of many repeating units containing charged groups, and are therefore also called polyelectrolytes. Those containing positively charged groups are cationic polyelectrolytes, those containing negatively charged groups are anionic polyelectrolytes, and those containing neither positively nor negatively charged groups are called nonionic polyelectrolytes.
III. Coagulants
In the coagulation treatment of wastewater, sometimes the use of a single flocculant can not achieve good coagulation effect, often need to add some auxiliary agents to improve the coagulation effect, this auxiliary agent is called coagulant. Commonly used coagulants are chlorine, lime, activated silicic acid, bone glue and sodium alginate, activated carbon and various clays, etc.
1、The types of commonly used coagulants
There are many types of coagulants, but they can be broadly divided into two categories in terms of their role in the coagulation process, as follows.
1) Agents that regulate or improve coagulation conditions
2) Coagulant that increases the particle size, density and solidity of the alum flower
IV. Conditioning agents
Conditioners, also known as dewatering agents, can be divided into two categories: inorganic conditioners and organic conditioners. Inorganic conditioning agents are generally suitable for vacuum filtration and plate and frame filtration of sludge, while organic conditioning agents are suitable for centrifugal dewatering and belt filter press dewatering of sludge.
1. Types of commonly used sludge conditioning agents
1) Inorganic Conditioning Agents
The most effective, cheapest and most commonly used inorganic conditioners are mainly iron salts and aluminum salts. Iron salt conditioners mainly include ferric chloride (FeCl3∙6H2O), ferric sulfate (Fe2(SO4)3∙4H2O), ferrous sulfate (FeSO4∙7H2O) and polymeric ferric sulfate (PFS) ([Fe2(OH)n(SO4)3-n/2]m), etc. Aluminum salt conditioners mainly include aluminum sulfate (Al2(SO4)3∙18H2O), aluminum trichloride (AlCl3), alkaline aluminum chloride (Al(OH)2Cl), polymeric aluminum sulfate (Al(OH)2Cl) and aluminum sulfate (Al(OH)2Cl). Aluminum trichloride (AlCl3), alkaline aluminum chloride (Al(OH)2Cl), polymeric aluminum chloride (PAC) ([Al2(OH)n∙Cl6-n]m), etc.
2) Organic Conditioning Agents
There are many types of organic synthetic polymer conditioning agents, which can be divided into low polymerization (molecular weight of about 1,000 ~ tens of thousands) and high polymerization (molecular weight of about hundreds of thousands ~ millions); cationic, anionic, non-ionic, anionic and cationic according to ionic type. Compared with inorganic conditioning agent, the organic conditioning agent dosage is less, generally 0.1% to 0.5% of the dry solids weight of the sludge, and there is no corrosion.
2. Selection of sludge conditioning agents
1) Variety characteristics of conditioning agents
As far as the commonly used inorganic conditioning agents of aluminum and iron salts are concerned, the dosage of the agent is larger when using aluminum salts, and the flocs formed are less dense and less effective in conditioning, which will block the filter cloth in the dewatering process. Therefore, in the selection of inorganic conditioning agent, iron salt is used as far as possible; when the use of iron salt will bring many problems, then consider using aluminum salt.
2) Nature of sludge
For sludge of different nature, the type and dosage of conditioning agent are also very different. For sludge with high organic content, the more effective conditioning agent is cationic organic polymer conditioning agent, and the higher the organic content, the more suitable to use cationic organic polymer conditioning agent with higher polymerization degree.
V. Relationship of flocculants, coagulants and conditioning agents
A dewatering agent is an agent that is added before dewatering the sludge, which is also known as a sludge conditioner, so dewatering agents and conditioners have the same meaning. The amount of dewatering agent or conditioning agent added is generally measured as a percentage of the dry solids weight of the sludge.
Flocculants are used to remove suspended matter from wastewater and are important agents in the field of water treatment. The dosage of flocculants is generally expressed in terms of the amount to be applied per unit volume of water to be treated.
The dosage of dewatering agent (conditioning agent) with flocculant and coagulant aid can all be referred to as dosing. The same agent can be applied both as a flocculant in the treatment of wastewater and as a conditioning agent or dewatering agent in the residual sludge treatment process.
Coagulants are called coagulants when used as additives to flocculants in the water treatment field, and the same coagulant is generally not called a coagulant in residual sludge treatment, but is collectively referred to as a conditioning agent or dewatering agent.
When using flocculants, since the amount of suspended matter in water is after all limited, in order to achieve full contact between flocculants and suspended particles, mixing and reaction facilities are required, and both should have sufficient time, such as mixing takes tens of seconds to several minutes, and reaction takes 15 to 30 min. while sludge dewatering often has only a few tens of seconds from the application of conditioning agent to the sludge entering the dewatering machine, i.e. only the equivalent of This means that there is only the equivalent of flocculant mixing and no reaction time, and experience has shown that the conditioning effect decreases as the residence time increases.
