Detailed Explanation of Wastewater Treatment Chemicals (Part 1)

I. Analysis of the Mechanism of Action of Coagulants

        The stability of colloidal particles in water is due to their small size, hydrated surface, and electrical charge. After the addition of coagulants to the water, they hydrolyze into charged colloids, forming micelles with a double electric layer structure around their surrounding ions. Rapid stirring after dosing promotes the collision opportunities and frequency between colloidal impurities in the water and the micelles formed by the hydrolysis of the coagulant. Under the action of the coagulant, the impurity particles in the water first lose their stability, then aggregate into larger particles, and finally settle or float in the separation facilities.

        Mixing is the process of promoting the rapid diffusion of the coagulant into the water and ensuring uniform mixing with all the wastewater. The process by which impurity particles in the water interact with the coagulant, losing or reducing their stability through mechanisms such as double-layer compression and charge neutralization, to form microflocs is called coagulation. The process by which microflocs formed by coagulation grow into large flocs through adsorption bridging and sediment network capture mechanisms under the action of bridging substances and water flow is called flocculation. Mixing, coagulation, and flocculation together are called coagulation. The mixing process is generally completed in the mixing tank, while coagulation and flocculation take place in the reaction tank.

II. Introduction to Types of Coagulants

        Coagulants are substances that can reduce or eliminate the sedimentation and aggregation stability of dispersed particles in water, causing the dispersed particles to aggregate and flocculate into aggregates for removal. According to their chemical composition, coagulants can be divided into three major categories: inorganic coagulants, organic coagulants, and microbial coagulants.

        In practical applications, inorganic and organic coagulants are often compounded according to their different properties to produce inorganic-organic composite coagulants. Microbial coagulants are a product of the combination of modern biology and water treatment technology, and are an important direction for the research, development, and application of coagulants.

III. Types and Composition of Inorganic Polymeric Coagulants

        Inorganic polymeric coagulants of aluminum, iron, and silicon are actually intermediate products of their hydrolysis, sol, and precipitation processes, namely Al （ Ⅲ ）、 Fe （ Ⅲ ）、 Si （ Ⅳ ）hydroxy and oxy polymers. Aluminum and iron are cationic and positively charged, while silicon is anionic and negatively charged. Their unit molecular weight in aqueous solution is approximately several hundred to several thousand, and they can combine with each other to form aggregates with a fractal structure. Their coagulation — flocculation process is a comprehensive reflection of the charge neutralization and adhesion bridging effects on the particulate matter in the water. The particle size of suspended particles in water is in the nanometer to micrometer range, and most of them are negatively charged. Therefore, the positive and negative charges, charge strength, molecular weight, and aggregate size of the coagulant and its morphology are the main factors determining its flocculation effect. Currently, there are dozens of types of inorganic polymeric coagulants (main varieties are shown in Table 8--1 ), and their output has reached 30% ～ 60% of the total output of coagulants, with polyaluminum chloride being widely used.

IV. Characteristics Analysis of the Inorganic Coagulant Aluminum Sulfate

        Aluminum sulfate is currently the most widely used coagulant in the world, with an annual global production of approximately 500 tons, of which nearly half is used in the water treatment field. Commercially available aluminum sulfate exists in solid and liquid forms. Solid forms are further divided into refined and crude types according to their insoluble matter content. Alum, a solid product commonly used for drinking water purification in China, is a double salt of aluminum sulfate and potassium sulfate, but it is not widely used in industrial water and wastewater treatment.

        The applicable pH range of aluminum sulfate is related to the hardness of the raw water. When treating soft water, the suitable pH value is 5 ～ 6.6 When treating medium-hard water, the suitable pH value is 6.6 ～ 7.2 When treating hard water, the suitable pH value is 7.2 ～ 7.8 . The applicable water temperature range for aluminum sulfate is 20oC ～ 40oC Below 10oC the coagulation effect is very poor. Aluminum sulfate has low corrosiveness and is easy to use, but its hydrolysis reaction is slow and requires a certain amount of alkali consumption.

V. Characteristics Analysis of Inorganic Polymeric Coagulants

IPF The advantages of IPF are reflected in its superior performance compared to traditional coagulants such as aluminum sulfate and ferric chloride, and its lower cost compared to organic polymeric coagulants ( OPF ). It has been successfully applied in various treatment processes for water supply, industrial wastewater, and municipal sewage, including pretreatment, intermediate treatment, and deep treatment, gradually becoming a mainstream coagulant. However, in terms of morphology, degree of polymerization, and corresponding coagulation — flocculation effects, inorganic polymeric coagulants still occupy a position between traditional metal salt coagulants and organic polymeric coagulants. Their molecular weight and particle size, as well as their flocculation bridging ability, are still much lower than those of organic coagulants, and there are also problems of instability to further hydrolysis reactions. IPF These weaknesses of IPF have promoted the research and development of various composite inorganic polymeric coagulants.

VI. Characteristics Analysis of the Inorganic Coagulant Ferric Chloride

        The applicable pH range of ferric chloride is 9 ～ 11 The formed flocs are dense, easily settle, and remain effective even at low temperatures or high turbidity. Solid ferric chloride is highly hygroscopic, highly corrosive, easily corrodes equipment, has high anti-corrosion requirements for dissolution and addition equipment, has a pungent odor, and has poor operating conditions.

        The mechanism of ferric chloride is to use various hydroxyl iron ions generated by the stepwise hydrolysis of trivalent iron ions to achieve flocculation of impurities in the water, and the formation of hydroxyl iron ions requires a large amount of hydroxyl groups in the water, so a large amount of alkali will be consumed during the process. When the alkalinity of the raw water is insufficient, it is necessary to supplement alkali sources such as lime.

        Ferrous sulfate, commonly known as green vitriol, forms flocs quickly and stably, has a short sedimentation time, and is suitable for situations with high alkalinity and high turbidity, but it is difficult to remove color, and it is also highly corrosive.

VII. Introduction to Types of Inorganic Flocculants

        Generally speaking, inorganic flocculants have the characteristics of readily available raw materials, simple preparation, low price, and moderate treatment effect, so they are widely used in water treatment.

        Traditionally used inorganic flocculants are low-molecular-weight aluminum salts and iron salts. Aluminum salts mainly include aluminum sulfate ( AL2 （ SO4 ) 3∙18H2O ), alum ( AL2 （ SO4 ) 3∙K2SO4∙24H2O ), and sodium aluminate ( NaALO3 ), while iron salts mainly include ferric chloride ( FeCL3∙6H20 ), ferrous sulfate ( FeSO4∙6H20 ) and ferric sulfate ( Fe2 （ SO4 ) 3∙2H20 ).

VIII. Analysis of Flocculant Characteristics

        When using traditional flocculants, coagulant aids can be added to enhance the flocculation effect. For example, using activated silicic acid as a coagulant aid for ferrous sulfate and aluminum sulfate inorganic flocculants and adding them in sequence can achieve a good flocculation effect. Therefore, in layman's terms, inorganic polymer flocculants IPF actually combines the coagulant aid and the flocculant together for preparation and then combined addition to simplify the user's operation.

        Coagulation treatment is usually placed before solid-liquid separation facilities, and combined with separation facilities to effectively remove suspended solids and colloidal substances with a particle size of 1nm ～ 100μm in raw water, reducing effluent turbidity and CODCr It can be used in pretreatment and deep treatment of wastewater treatment processes, and can also be used for the treatment of residual sludge. Coagulation treatment can also effectively remove microorganisms and pathogens in water, and can remove emulsified oil, chroma, heavy metal ions and other pollutants in wastewater. When using coagulation sedimentation to treat phosphorus in wastewater, the removal rate can be as high as 90 ～ 95% ，which is the cheapest and most efficient phosphorus removal method.