With more urban dwellers every year and more environmental sensitivity, governments everywhere set more robust quality requirements for water discharged into the environment.
In the past, primary and secondary processes in WWTPs could meet requirements for water quality. More and more operators are turning to tertiary treatment processes to meet more stringent limits on contaminants in the discharges from their plants. their discharges.
Effluent from secondary treatment may include these contaminants:
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Suspended particles,
Dissolved and non-biodegradable organics,
Matter departing color and odor,
Dissolved inorganics, and
Microorganisms which can include pathogens.
The tertiary processes selected for a particular plant depend on which of these contaminants are of concern and in what concentration they exist.
This article will discuss several methods for further treatment of secondary process effluent.
Transcend Water has developed their automated design generator to assist water engineers in designing efficient wastewater treatment facilities, including tertiary processes.
Moving Bed Biofilm Reactors (MBBR)
One of the popular tertiary treatment processes is the moving bed biofilm reactor or MBBR.
This process involves dispersing thousands of tiny particles into an aeration tank, sufficient to fill 50-70% of the tank’s volume. These particles have geometric designs that present a huge surface area and provide surfaces that encourage beneficial microorganisms to settle on them. The resulting biofilm decomposes the organic matter in the wastewater.
Diffusers maintain continuous aeration throughout the tank, further promoting the growth of microorganisms and their consumption of waste in the feedwater. Several types of media can be used for the particles depending on influent and desired effluent characteristics, temperature variation in the tank, fluctuation in contaminant concentrations, and the metabolisms of the microorganisms in the tank.
Screens at the tank’s exit prevent the media particles from leaving the tank with the treated water.
Adsorption Techniques
In a WWTP, adsorption is a process in which molecules dissolved or suspended in water are transferred to the surface of a material called an adsorbent.
Adsorption is a tertiary treatment favored by many plant operators for its low investment requirements, both in process setup and use of land space.
Activated carbon is a very effective adsorbent. It can be produced from many materials such as nutshells, agricultural and industrial waste, and any other material containing carbon. Activated carbon is effective because of the massive surface area it offers for molecules to collect on.
Phosphorus Removal
Phosphorus is one of the primary nutrients in organic and inorganic fertilizers, and it has many positive uses. However, in WWTP discharges into environmental waters, it can foster the growth of algae and other undesirable plant growth, negatively affecting marine life.
A common method for removing phosphorus from wastewater is to encourage it to form phosphate salts, using calcium, iron or aluminum as the metal component of the salts. These salts can then precipitate out of the water and be removed mechanically.
Disinfection and Dechlorination
Adding chlorine to secondary stage effluent is an effective tertiary treatment to kill bacteria, viruses and pathogens that can cause severe human diseases.
Calcium and sodium hypochlorite are commonly used compounds to impart chlorine to the wastewater.
High chlorine concentration can harm many forms of aquatic life and reduce biological water quality. Through the use of sodium bisulfite, the chlorine ions can be removed from the water after they’ve accomplished their disinfection task, making the water safe for discharge into the environment.
Ion Exchange
Another tertiary treatment method to accomplish demineralization is ion exchange. Depending on the valence of the minerals to be removed, cation or anion resins can stimulate the exchange of harmful ions for harmless compounds, further rendering the effluent suitable for discharge into the environment or for agricultural or industrial use.
Other Processes
Here are a few additional processes that engineers can use in the tertiary treatment of wastewater in WWTPs.
Advanced Oxidative Processes
The hydroxyl radical can speed up the oxidation process to reduce organic contaminants in WWTPs.
Ozone (O3) is another effective method of oxidation.
Ultraviolet (UV) Light
Ultraviolet irradiation is an effective disinfectant process, excellent for killing pathogens and rendering treatment plant effluent safely usable for discharge into the environment or reuse.
Transcend Water’s Transcend Design Generator is a unique SaaS app available to wastewater treatment plant designers to help them select process options, including tertiary treatment. Using this tool, they can design a plant that meets the plant owner’s needs and the environmental requirements placed on the plant.
Visit Transcend’s website to learn more about wastewater treatment processes and discuss your project with their experts.
