Key Highlights
- Tertiary treatment of wastewater uses physical and chemical processes followed by disinfection.
- Bag filters, rotary drum filters, and disc filters are some of the filters used in the filtration processes for the physical treatment in tertiary wastewater treatment.
- Biological nutrient removal and chemical precipitation are used for the removal of excess nutrients including for the removal of nitrogen and phosphorus in tertiary treatment.
- Some wastewater treatment plants use flocculation in addition to filtration and disinfection.
- Tertiary treatment of wastewater is important to bring treated water up to the stringent regulatory requirements in some municipalities before the water can be discharged or re-used.
- Tertiary treatment of wastewater includes both physical, chemical and disinfection processes and this sets it apart from secondary treatment of wastewater that involves solely biological processes.
- Pollutants, excessive nutrients including phosphorus and nitrogen, and microorganisms are removed from the tertiary treatment of wastewater.
- The type of technologies used in tertiary treatment of wastewater is important to deal with the rising concerns of antimicrobial resistant microorganisms that can withstand disinfection and thrive on the filters.
Wastewater treatment generally involves primary treatment and secondary treatment. Primary treatment uses physical processes to remove up to 40 to 60% suspended solids. Secondary treatment involves biological processes to break down dissolved and suspended biosolids through the action of microorganisms.
And then there is tertiary treatment, that although is not used in all wastewater treatment plants, is necessary when stringent level of water purification needs to be attained to meet the regulatory standards. While primary and secondary treatments clean the wastewater enough that it can be discharged into the environment, tertiary treatment takes the water treatment one level higher to make the water safe for reuse such as for irrigation and for drinking.
What processes are used in tertiary treatment?
Tertiary treatment of wastewater uses physical and chemical processes, typically filtration followed by nutrient removal and then disinfection.
Filtration
The types of filters used in tertiary treatment are as follows:
- Bag filters: Bag filters are used to reduce contaminants to a specific micron level. They are similar to cartridge filters. A bag filter is constructed of a filter material with specific pore sizes. In general, influent water enters from the top via a high-pressure inlet and the filtrate exits from the sides and the bottom while the solid particles, depending on the pore size of the filter, stay in the bag. The bag could be made of mesh or it could be pleated.
- Rotary drum filters: A rotary drum filter is in the shape of drum laid on its side. It has a screen inside of it, and the material and the pore size depend on the wastewater treatment system. There is an inlet on one side of the drum through which the wastewater to be treated enters. As the drum rotates at high speed, the wastewater moves in a circle and the particles in the wastewater separate out. Due to gravity, these particles settle at the bottom of the drum while the liquid (the water) is discharged through the screen.
- Disc filters: Disc filters consist of a series of discs, each disc covered by a stretched material made of polyester or stainless steel with pores that act like a filter medium. The working principle is similar to the rotating drum. Wastewater enters through a feed tube into the discs and as the discs rotate, the water moves in a circle and the particles from the water settle inside of the filter material while the treated water collects on the outside of the discs into a collecting chamber.
In addition to these, there are other types of specialized filtration that are also used in tertiary treatment. These are as follows:
- Sand filtration: Sand media can trap particulates from the water. Depending on the wastewater composition and what needs to be removed, these sand media could be fine or coarse. Quartz is also used in sand filtration because silica sand as a filter material is known to result in low impurity level in the treated water.
- Membrane filtration: There are two common types of processes used in membrane filtration, microfiltration and ultrafiltration. Both of these filtration processes remove bacteria and viruses from the wastewater.
Some literature sources refer to these filters as tertiary filters to distinguish them from filtration that is utilized in primary treatment. In addition to scalability and adaptability to accommodate the volume of water to be treated, filtration in tertiary wastewater treatment is also an efficient method for removing high quantities of submerged solids. Gravity-based systems such as those that utilize drum and disc filters have low energy requirements and this feature provides advantage to tertiary wastewater treatment.
Nutrient removal
A major focus of tertiary wastewater treatment is the removal of excessive nutrients. These are nutrients that are still in the water after secondary treatment, and special emphasis is placed on phosphorus and nitrogen. Because these can lead to harmful algal blooms and deplete oxygen in the receiving water body, their removal is critical in wastewater treatment processes. Some of the ways tertiary treatment removes nutrients are as follows:
- Biological nutrient removal: Biological nutrient removal uses bacteria in aerobic and anoxic zones to remove nitrogen via the nitrification and denitrification process.
- Chemical precipitation: Chemicals such as alum and ferric chloride are used for chemical precipitation in tertiary treatment. These chemicals bind with phosphorus that is present in the water to form insoluble particles that can then be removed by filtration.
Disinfecting
The disinfecting process removes microorganisms. Chlorine, ultraviolet light, and ozone are used in the disinfection processes for tertiary wastewater treatment.
After the tertiary treatment, the wastewater is ready for discharge such as for irrigation and for reuse including for operations that require clean water such as in oil and gas extraction, refining, and in industrial and manufacturing processes. Tertiary treatment also makes the water safe for drinking.
Interestingly and notably, the color purple for purple pipes is the international color for recycled water.
Other processes utilized in tertiary treatment
Some wastewater treatment plants utilize other processes in addition to filtration and disinfection, most notably flocculation.
Flocculation: Some wastewater treatment plants use flocculation in their tertiary treatment before using filtration and disinfection. Flocculation could involve the use of polymers, which when mixed with the water, bind dissolved organic matter together that forms clumps known as flocs. The flocs are then filtered in the filtration step.
Filtration, nutrient removal processes, and disinfection are the basic steps in tertiary wastewater treatment. However, these could be supplemented with other treatments. For example, a study showed that the use of activated carbon in tertiary-treated wastewater led to the highest adsorption of micropollutants compared to wastewater that was just treated by filtration. The study attributed the higher adsorption of the micropollutants in the tertiary-treated wastewater to be possibly related to a lower concentration of dissolved organic carbon.
Why is tertiary treatment important?
Tertiary treatment is important for meeting strict regulatory requirements and reducing or eliminating the issues of algal blooms in receiving waterways.
Meeting stringent regulatory requirements
The regulatory requirements for wastewater discharge are stringent depending on what the treated wastewater is used for. If the municipalities have specific requirements for the discharge of treated water and the resulting treated water from primary and secondary treatments do not meet those requirements, then tertiary treatment is applied. This ensures the resulting water meets the specific requirements to be discharged in those municipalities.
Additionally, several industrial processes such as oil and gas extraction and refining, cooling for data centers, and irrigation in agriculture require clean water and they use water from tertiary treatment. To meet the level of cleanliness for such uses, tertiary treatment must treat the water using physical and chemical processes to eliminate harmful microorganisms.
Preventing algal blooms
Algal blooms are caused by the presence of nutrients including nitrogen and phosphorus in the water and the ratio of these two chemicals dictates the type of algal bloom that occurs. The optimal ratio of N:P for algal growth is 16:1 according to a study and this ratio can vary depending on the physical conditions and chemical composition of the affected water body.
A research study emphasized on the importance of water purification to minimize the risk of algal bloom in the receiving water body. In this study, secondary treated water was discharged into an urban river and the risk of algal bloom was evaluated. While the ammonia nitrogen level in the river was below standard, the concentration range of the total phosphorus was high (between 0.42 to 0.86 mg/L), showing that discharging secondary treated water in the river affected the water quality. This calls for the need of tertiary wastewater treatment to reduce the phosphorus level in the wastewater to prevent algal bloom when the treated water is discharged into the water body.
How does tertiary treatment differ from secondary treatment?
Secondary treatment occurs after primary treatment and before tertiary treatment. It involves biological processes that utilize aeration and activated sludge treatment for the breakdown of biosolids that occur via the action of microorganisms.
Secondary treatment involves solely biological processes whereas tertiary treatment does not involve those processes. Specifically, tertiary treatment includes physical and chemical processes followed by the disinfection processes.
What contaminants are removed in tertiary treatment?
Tertiary treatment removes excess nutrients from water that remains after primary and secondary treatment such as nitrogen, phosphorus, and other pollutants. It also removes microbiological contaminants. While excess nutrients are removed using physical and chemical processes, microorganisms are removed by the disinfection processes.
We want to think of wastewater having undergone tertiary treatment as having fewer microorganisms than through secondary treatment only. Theoretically, this should be the case because microorganisms that are not removed in secondary treatment should have been removed in tertiary treatment. However, there is a pressing concern pointed out in a study regarding antibiotic resistance bacteria. This study showed that although the tertiary wastewater treatment processes reduced chemical pollutants in the wastewater, the use of the denitrifying biofilter significantly increased the absolute abundance of selected antibiotic resistant bacteria.
As industrialization and urban population increase and there is increase in the quantity of wastewater containing pharmaceuticals and other chemical pollutants coming into wastewater treatment plants, these plants need to make choices in the technologies to use for tertiary wastewater treatment to deal with antibiotic resistant microorganisms that can withstand the disinfection processes and settle on the filters, ultimately causing human and environmental health concerns.
About the Author
Saleha Kuzniewski
Saleha Kuzniewski, Ph.D. has authored several publications in the fields of scientific research, biotechnology, and environmental regulations. She is the winner of the 2023 Apex award for publication excellence. She is also the founder of Environmental Remediation & Innovations, LLC. Kuzniewski can be reached at [email protected].