What does a wastewater treatment plant do

Wastewater Treatment Plants are essential facilities that clean and purify wastewater before it is released back into the environment. They play a crucial role in protecting public health and the ecosystem.

Here's a breakdown of the typical processes involved in wastewater treatment:

Pre-treatment:

Removes large objects like debris, rags, and grit.

May involve screening or grinding.

Primary Treatment:

Separates solids from the wastewater.

Often involves settling tanks where solids settle to the bottom.

Secondary Treatment:

Biological process that breaks down organic matter using bacteria.

Typically involves aeration tanks or activated sludge processes.

Tertiary Treatment:

Optional step to remove additional nutrients (nitrogen and phosphorus) and disinfect the wastewater.

May involve filtration, chlorination, or ultraviolet disinfection.

Disinfection:

Kills harmful bacteria and viruses.

Often uses chlorine, ultraviolet light, or ozone.

Effluent Discharge:

Treated wastewater is released back into the environment, often into rivers, lakes, or the ocean.

The specific processes and technologies used can vary depending on the size of the plant, the quality of the incoming wastewater, and local regulations.

In wastewater treatment, tertiary treatment (also known as deep treatment or advanced treatment) aims to further remove nutrient pollutants, dissolved substances, fine suspended solids, non-biodegradable organic matter, and salt from wastewater. This process usually includes a variety of filtration and disinfection methods. The following is an overview of different types of tertiary filtration and disinfection methods:

Third-stage filtration methods

Sand filtration (sand filtration)

Sand filtration is a physical filtration method that filters out suspended particles and colloids in wastewater through a sand layer. Sand filters are often used in the final stage of wastewater treatment to improve effluent quality.

Coagulation

Coagulation is the process of adding coagulants to aggregate colloidal particles and tiny suspended matter in water into larger flocs for subsequent sedimentation or filtration removal. The coagulation process is widely used in wastewater treatment to improve treatment efficiency and effluent quality.

Microfiltration

Microfiltration (also known as microfiltration) is a precision filtration technology that can filter out micron- or nano-sized particles and bacteria in a solution. Microfiltration membranes are often used in tertiary treatment to remove tiny suspended matter and colloids from wastewater.

Activated carbon adsorption

Activated carbon adsorption uses the porous structure and strong adsorption capacity of activated carbon to remove pollutants such as dissolved organic matter, residual chlorine, and heavy metals from sewage. This method is particularly effective in deep treatment and can significantly improve the quality of effluent.

Reverse osmosis

Reverse osmosis is a membrane separation technology that uses the selective permeability of a semipermeable membrane to separate solvents (such as water) from solutes (such as salt, organic matter, etc.) under pressure. Reverse osmosis technology is often used in seawater desalination and deep sewage treatment, and can effectively remove salt and most organic matter from sewage.

Ion exchange

Ion exchange is the exchange of ions on the resin with ions in sewage to achieve the purpose of removing or recovering specific ions in sewage. This method has significant effects in removing heavy metal ions from sewage and softening water.

Disinfection method

Chlorination disinfection

Chlorination disinfection is one of the most commonly used disinfection methods. Chlorine gas, sodium hypochlorite and other chlorides are added to sewage, and the strong oxidizing property of chlorine is used to kill pathogenic microorganisms in sewage. Chlorination disinfection has the advantages of good disinfection effect and low cost, but it may cause residual chlorine problems.

Ultraviolet disinfection

Ultraviolet disinfection uses the photochemical effect of ultraviolet rays to destroy the DNA or RNA structure of pathogenic microorganisms, making them lose their reproductive ability and achieving the purpose of disinfection. Ultraviolet disinfection has the advantages of no chemical residues and no by-products, but it may be affected by water quality, resulting in unstable disinfection effects.

Ozone disinfection

Ozone disinfection uses the strong oxidizing properties of ozone to kill pathogenic microorganisms in sewage. The oxygen atoms and hydroxyl free radicals produced by the decomposition of ozone in water can quickly destroy the cell structure of microorganisms to achieve a disinfection effect. Ozone disinfection has the advantages of thorough sterilization and no secondary pollution, but the equipment investment and operating costs are high.

Other disinfection methods

In addition to the common disinfection methods mentioned above, there are also disinfection methods such as thermal disinfection, radiation disinfection, ionizing radiation sterilization, and hydrogen peroxide plasma sterilization. These methods have their own advantages and disadvantages and are suitable for different sewage treatment scenarios and needs.

Types of Media Used in Water Treatment

The type of media used in water treatment depends on the specific process and the desired outcome. Here are some common types of media:

Filtration Media

Sand: Used in sand filters to remove suspended solids and reduce turbidity.

Anthracite: A type of coal used in filters due to its high porosity and low density.

Gravel: Used as a supporting layer in filters and can also be used as a filter media itself.

Diatomaceous Earth (DE): A natural material made up of fossilized diatoms, used in DE filters to remove very fine particles.

Activated Carbon: Used to remove organic matter, taste, odor, and certain chemicals.

Ion Exchange Resin: Used to remove dissolved minerals, such as hardness, from water.

Membrane Filters: Used in membrane filtration processes to remove particles and microorganisms based on size.

Disinfection Media

Chlorine: A common disinfectant added to water to kill bacteria and viruses.

Ultraviolet (UV) Light: Used to kill microorganisms by exposing them to UV radiation.

Ozone: A powerful disinfectant that can oxidize contaminants and kill microorganisms.

Lime: Used to raise the pH of water and can also be used to remove hardness.

Soda Ash: Used to increase the alkalinity of water.

Biological Treatment Media

MBBR Bio Filter Media: These are plastic carriers with a high surface area that support the growth of biofilm. They are used in moving bed biofilm reactors (MBBRs) for biological wastewater treatment.

Bio Block Filter Media: These are plastic blocks with a honeycomb structure that provide a large surface area for biofilm growth. They are used in biofiltration systems for wastewater treatment.

Random Tower Packing: These are irregularly shaped materials that are used in packed bed reactors for biological wastewater treatment. They provide a large surface area for biofilm growth and promote good gas-liquid contact.

Other Media

Tube Settler Clarifier: This is not a media per se, but rather a device that uses a series of tubes to improve the settling of suspended solids in wastewater.

MB Membrane Bio-Reactor: This is a type of membrane bioreactor that combines biological treatment with membrane filtration. The membranes can be made of different materials, such as polymers or ceramics.

Disc Bubble Diffuser: This is a type of diffuser that uses a series of discs to distribute air bubbles evenly into the water. It is used in aeration systems for wastewater treatment.

Bubble Tube Diffuser: This is a type of diffuser that uses a tube with small holes to release air bubbles into the water. It is used in aeration systems for wastewater treatment.

Spiral Mixing Aerator: This is a type of aerator that uses a spiral blade to mix the water and introduce air. It is used in aeration systems for wastewater treatment.

Sludge Dewatering Machine: This is not a media per se, but rather a machine that is used to dewater sludge. It can use different technologies, such as belt filters or centrifuges.

The choice of media depends on factors such as the water quality, the desired treatment goals, and the cost-effectiveness of different options. For example, sand and gravel are relatively inexpensive and can be used for a variety of filtration applications, while activated carbon and ion exchange resins are more specialized and may be more costly.