Introduction to Water Treatment Terminology and Principles (Part 2)

IX. Filtration Mechanism of Filters:

1. Mechanical Filtration: The filter layer is composed of filter particles of different sizes, with many pores between them. When wastewater flows through, particles larger than the pores are retained in the pores, causing the pores to become smaller and smaller. Subsequently, smaller suspended particles entering are also retained, purifying the wastewater.

2. Adsorption and Contact Coagulation: During the process of wastewater flowing through the filter layer, it passes through winding water flow channels, providing many opportunities for suspended particles to contact the filter material. During contact, due to the forces between molecules, adsorption and contact coagulation occur. This is especially prominent when flocculants are added before filtration. The smaller the filter particles, the better the effect of adsorption and contact coagulation.

X. Structure and Classification of Filters:

1. Classification:

（ 1) By filtration rate: slow filter, rapid filter, high-speed filter;

（ 2) By the direction of water flow through the filter layer: upward flow, downward flow, bidirectional flow;

（ 3) By filter material type: sand filter, coal filter, coal-sand filter;

（ 4) By the number of filter layers: single-layer filter material, double-layer filter material, multi-layer filter material;

（ 5) By water flow properties: pressure filter and gravity filter;

（ 6) By the supply and discharge method of influent, effluent, and backwash water: ordinary rapid filter, siphon filter, and valveless filter.

2. Structure: The external structure of a filter consists of the filter tank body, influent pipe, effluent pipe, backwash pipe, backwash water discharge pipe, and other pipes and accessories; the internal structure consists of a backwash water discharge trough, influent channel, filter layer, filter bed (support layer), and drainage system.

3. Working Process of Ordinary Rapid Filters: The filtration and backwashing processes alternate. When the filter is in operation, wastewater enters through the influent pipe, flows through the influent channel and distribution trough into the filter, and passes down through the filter layer and filter bed to be collected by the drainage system and discharged through the effluent pipe. During operation, the filter is fully submerged. During backwashing, the influent and effluent pipes are closed, and the drain valve and backwash influent pipe are opened. The backwash water flows upward through the drainage system, filter bed, and filter layer, and is collected by the distribution trough and discharged through the drain pipe in the influent channel.

Requirements for Filter Materials and Filter Layer Structure: Requirements for Filter Materials: ①The filter material should have a larger particle size, higher physical strength, stronger corrosion resistance, and lower cost; ②It should have strong impact load resistance.

XI. Filter Layer

1. Common Problems in Rapid Filters

（ 1) Air Binding: A large amount of air accumulates in the filter layer, especially when a negative head occurs in the filter layer. This part of the filter layer is in a vacuum state, causing dissolved gases in the water to escape and accumulate in the filter layer, resulting in a significant reduction in filtration rate. During backwashing, air bubbles will rush out of the filter layer surface, resulting in a large amount of air, which is the cause of filter layer cracking and deterioration of water quality. This phenomenon is called air binding or air lock. Solution: Increase the water depth above the filter layer. If the pool depth is fixed, the surface filter material can be replaced, and the filter material particle size can be increased. Sometimes, the filtration rate can be appropriately increased to make the entire filter layer more seriously fouled.

（ 2) Mud Ball Formation: Due to insufficient cleaning during backwashing, gelatinous sludge gradually accumulates in the filter layer and binds together. The main component of the sludge is organic matter, which can decompose and smell bad when severe.

2. Solutions:

（ 1) Improve Backwashing: Check the expansion degree of the filter layer and the discharge of backwash wastewater. Adjust the backwash intensity and time appropriately; in addition, the water distribution system needs to be checked, and a surface washing device or compressed air assisted backwashing can be added if conditions permit.

（ 2) Methods for Removing Mud Balls from Filters:

a. Manual cleaning of the filter, and check whether the support layer has moved and whether the water distribution system is blocked;

b. After backwashing the filter, stop using it, then keep the water depth on the filter material surface at 20-30cm, add chlorine for soaking for 12h, and then backwash. (Chlorine dosage: bleaching powder 1kg/m2, liquid chlorine 0.3kg/m2)

（ 3) Sand Leakage: Due to excessive backwash intensity or improper filter gradation, a large amount of filter material is washed away; uneven distribution of backwash water can cause the support layer to move, making the distribution of backwash water even more uneven, eventually causing a part of the support layer to be hollowed out, causing the filter material to be lost through the water distribution system. Solution: Check the water distribution system and adjust the backwash intensity appropriately. Equalization tank: A regulating tank for equalizing water quantity.

Homogenization tank: A regulating tank for equalizing water quality.

Purpose of regulation: Equalize water quality or quantity.

Working principle of heterogeneous homogenization tank: constant water level, gravity flow ——The flow path of each particle in the sedimentation tank varies from short to long, and combined with the coordinated arrangement of the inlet and outlet tanks, the water of the previous and subsequent time periods can be mixed with each other to achieve a random homogenization effect.

Alkaline wastewater: Alkaline content greater than 1%-3% high-concentration alkaline wastewater is called waste alkali liquid.

Acidic wastewater: Acid content greater than 3%-5% high-concentration acidic wastewater is called waste acid liquid.

Ordinary neutralization filter: It is a fixed bed, and the water flow direction is divided into horizontal flow and vertical flow (also divided into upflow and downflow). The filter material particle size is generally 30-50mm, no powdery impurities are allowed. When the wastewater contains substances that may block the filter material, pretreatment should be carried out. The filtration speed is generally no more than 5m/h, the contact time is not less than 10min, and the filter bed thickness is generally 1-1.5m.

Upflow expanded bed neutralization filter: The water flows upward, and the flow rate is as high as 30-70m/h, coupled with the action of generating carbon dioxide gas, causes the filter media to collide and rub against each other, constantly renewing the surface, thus resulting in a better neutralization effect.

Purpose of neutralization: acid-base neutralization, using waste to treat waste.

Principles and applicable conditions of chemical neutralization and filtration neutralization: Principle and applicable conditions of chemical neutralization: For example, the lime milk method involves dissolving lime into lime milk before adding it. Because Ca(OH)2 has a coagulation effect on impurities in wastewater, it is therefore suitable for acidic wastewater with high impurity content.

Principles and applicable conditions of filtration neutralization: This refers to the neutralization reaction of wastewater passing through filter media with neutralization capabilities. Applicable to wastewater with sulfuric acid concentration greater than 2-3mg/L and various acidic wastewaters that produce easily soluble salts for neutralization treatment.

Coagulation: Generally, coagulation refers to the process of coagulation where colloidal particles aggregate due to the double electric layer effect and the process of coagulation where colloidal particles adhere to each other due to the adsorption bridging effect of high molecular polymers.

Double electric layer: The surface of the colloid nucleus has a layer of ions, which become potential ions. The potential ion layer attracts ions with opposite charges in the solution to the colloid nucleus through electrostatic action. The attracted ions are called counterions, and their total charge is equal to that of the potential ions but with the opposite sign. In this way, a so-called double electric layer is formed in the interfacial region of the medium around the colloid nucleus.

Destabilization of colloidal particles: To make colloidal particles settle, it is necessary to destroy the stability of the colloid, causing the colloidal particles to contact each other and become larger particles. The key is to reduce the charge of the colloid. This can be achieved by compressing the thickness of the diffusion layer and reducing the ξ potential. This process is called the destabilization of colloidal particles.

Coagulant: A substance that can cause colloidal particles in water to adhere to and aggregate is called a coagulant.

Basic aluminum chloride: ( PAC) is a multivalent electrolyte that can significantly reduce the colloidal charge of clay-like impurities in water. It has a large molecular weight, strong adsorption capacity, and excellent coagulation ability. The formed coagulum is larger, and its coagulation and sedimentation performance is superior to other coagulants.

Coagulant aid: In wastewater coagulation treatment, using a single coagulant cannot achieve good results, and auxiliary agents need to be added to improve the coagulation effect. The added auxiliary agent is the coagulant aid.

Clarifier: A type of equipment used for coagulation treatment, in which mixing, reaction, and sedimentation separation processes can be completed simultaneously.

Principle of coagulation: Double electric layer effect (low molecular weight electrolytes cause neutralization of colloidal particles to cause coagulation of colloidal particles) and chemical bridging effect (colloidal particles have a strong adsorption effect on high molecular weight substances, and various particles rely on the connection of high molecular weight substances to form a certain aggregate, combining into flocculent matter).

Mechanism of body destabilization: To make colloidal particles settle, it is necessary to destroy the stability of the colloid, causing the colloidal particles to contact each other and become larger particles. The key is to reduce the charge of the colloid. This can be achieved by compressing the thickness of the diffusion layer and reducing the ξ potential.

Factors affecting coagulation: a. PH value. b. Temperature (35-40 is optimal) c. Type and dosage of chemicals; d. Stirring: Appropriate.

Twelve, Classification of coagulants and their approximate application range

Inorganic and organic.

Stages of the coagulation process and the role of each stage: Chemical addition, mixing, reaction, and sedimentation separation. The role of the mixing stage is to quickly and evenly distribute the chemicals to all parts of the wastewater, to compress the double electric layer of colloidal particles in the wastewater, reduce or eliminate the stability of the particles, so that these particles can aggregate into flocs. The role of the reaction stage is to promote the collision and aggregation of unstable colloidal particles into visible flocs.

Working principle of hydraulic circulation clarifier: It utilizes the kinetic energy of the raw water, under the action of the water injector, to suck the activated sludge in the pool and mix it thoroughly with the raw water, thereby strengthening the contact and adsorption between solid particles in the water, forming good flocs, accelerating the sedimentation speed, and clarifying the water.

Physical adsorption: Adsorption produced by intermolecular forces between the adsorbent and the adsorbate is called physical adsorption.

Chemical adsorption: Adsorption caused by chemical bond forces between the adsorbent and the adsorbate is called chemical adsorption.

Ion exchange adsorption: Adsorption caused by electrostatic attraction between the adsorbent and the adsorbate is called ion exchange adsorption.

Adsorption equilibrium: If the adsorption process is reversible, after the wastewater and adsorbent are fully contacted, on the one hand, the adsorbate is adsorbed by the adsorbent, and on the other hand, some of the adsorbed adsorbate can leave the surface of the adsorbent due to thermal motion and return to the liquid phase. The former is adsorption, and the latter is desorption. When the two speeds are equal, that is, when the amount of adsorption per unit time is equal to the amount of desorption, the concentration of the adsorbate in the liquid phase and the concentration on the surface of the adsorbent no longer change and reach adsorption equilibrium.

Adsorption capacity: Refers to the weight of adsorbate adsorbed per unit weight of adsorbent.

Adsorbent: A porous solid material with adsorption capacity.

Adsorbate: The substance adsorbed from wastewater.

Static adsorption operation: Adsorption operation performed under non-flowing conditions of wastewater.

Dynamic adsorption operation: Adsorption operation performed under flowing conditions of wastewater.

Classification and characteristics of adsorption: Physical, chemical, ion exchange.

Factors affecting the adsorption process: Properties of the adsorbent, properties of the adsorbate, and operating conditions of the adsorption process.

Requirements for adsorbents: Porous or finely ground substances with a large surface area.

Forms and characteristics of adsorption operations: static (batch operation), dynamic (fixed bed is semi-continuous, moving bed and fluidized bed are continuous)

Ion exchangers: inorganic and organic types. Inorganic types include natural zeolite and synthetic zeolite. Organic types include sulfonated coal and various ion exchange resins (a type of organic polymeric polyelectrolyte with ion exchange properties, a loose porous solid spherical particle).

Ion exchange capacity: a standard for the size of the resin's exchange capacity. It can be expressed by weight method (the number of ion exchange groups in a unit weight of dry resin) and volume method (the number of ion exchange groups in a unit volume of wet resin).

Selectivity of ion exchange resin: Since the ability of ion exchange resin to adsorb various ions in water is not the same, some ions are easily adsorbed while others are difficult to adsorb. During regeneration, some ions are easily replaced while others are difficult to replace. This property of ion exchange resin is called selectivity.

Single-bed ion exchanger: A single-bed structure using one type of resin.

Multi-bed ion exchanger: An ion exchange system using one type of resin and consisting of two or more exchangers.

Double-bed ion exchanger: A series system of two exchangers using two types of resins.

Mixed-bed ion exchanger: Both cation and anion resins are filled in the same exchanger.

Combined-bed ion exchanger: Combined use of double-bed and mixed-bed.

Co-current regeneration and counter-current regeneration: If the direction of the liquid flow in the regeneration stage is the same as the direction of the water flow during exchange, it is co-current regeneration; otherwise, it is counter-current regeneration.