The world houses more people every year. Those people continue a trend of gathering in urban centers. Environmental sensitivity is increasing steadily. These factors add up to demands for higher quality effluent from wastewater treatment plants (WWTPs). In some cases, primary and secondary treatment are adequate, where the outflow from a plant is to a less sensitive environment. From the first paragraph above, we can see that fewer of those low-sensitivity areas are available all the time. Enter tertiary treatment, an enhanced cleaning and clarification of the effluent from the secondary stage of treatment. In this third stage, further removal of turbidity, metals, nitrogen, phosphorus, organics, and pathogens takes place. This article will further detail the considerations of cleaner water and discuss the processes involved in achieving cleaner effluent. Transcend Water’s role in designing plants and processes to reach that goal will also be considered.
What is Tertiary Treatment, and How Does it Improve the Environment?
Traditional primary and secondary wastewater treatment can leave certain undesirable substances in the effluent, such as non-biodegradable compounds, and pathogenic organisms.
The outflow from a secondary treatment plant can be discharged into rivers or lakes, where dilution and natural filtration render it suitable for wildlife and possible other uses, like irrigation or industrial feedwater.
Tertiary treatment of the effluent from secondary wastewater management processes aims to remove most of the remaining contaminants. The goal is to render the tertiary effluent compliant with stricter standards.
The Processes Used in Tertiary Treatment
The processes available for tertiary treatment are many, and some of the more common ones are discussed in the following sections.
Transcend Water’s Transcend Design Generator (TDG) can help you decide which processes are appropriate for your wastewater management project.
Advanced Filtration Methods
Filters used in tertiary treatment can use a variety of materials. Sand, activated carbon and woven cloth are among the more common media. Some of the filter configurations employed:
- Bag filters are useful in applications where contaminants need to be reduced to a specified micron level. Felt and fine mesh are among the common media used in bag filters. Since bag filters can be placed in a variety of housings and containers, they are quite flexible in how they can be incorporated into the process.
- Drum filters have a cloth filter medium wrapped around a hollow drum. Water enters the drum and is filtered as it exits the sides of the drum through the filtering media. This configuration allows quick and easy backwash to clean the filter periodically.
- In a disc filter, several discs, each covered in a cloth filter, are attached to a central hub. That assembly is partially submerged in a trough with feedwater entering one end. The discs fit closely to the trough wall so that water passing longitudinally through the tank must pass through the filter discs. The disc assembly is rotated slowly. At the top of its rotation, mechanical or hydraulic cleaning can be accomplished in a separate chamber. Filtrate exits the tank at the opposite end to feedwater entry
The TDG can automatically add disc or cloth filters depending on the TSS (Total Suspended Solids) limit specified or you can select them as the preferred tertiary treatment unit.
Disinfection can be accomplished by various processes.
Chlorine is effective in killing bacteria, and hypochlorite compounds are its most common form. The resulting effluent must then be dechlorinated since chlorine can interact with other elements in the water to produce undesirable compounds.
Chlorine gas or chlorine dioxide can also provide disinfection, but sodium or calcium hypochlorite is easier to handle and store, though it has limited shelf life.
Ultraviolet (UV) light can be an effective tertiary treatment to render bacteria and viruses harmless by damaging their genetic structure. Though it usually doesn’t kill them, it makes them powerless to infect humans or wildlife. While the sun’s rays contain an ultraviolet component, it takes a long time to have much effect. Concentrated ultraviolet lamps work much faster. A caveat is that any remaining solids in the feedwater create UV shadows, so the feedwater to the UV process must have been previously well-filtered.
Transcend Water has built the Transcend Design Generator to automate the process of wastewater treatment design, including electrical, chemical, civil, mechanical, architectural, process, control and automation considerations. The tertiary treatment processes described here can be pre-selected as input to the Design Generator, or it can select them automatically depending on effluent specifications.
If this design option appeals to you for your treatment needs, contact Transcend Water’s experts through their website to explore their approach and capabilities.