Tertiary wastewater treatment, also known as advanced wastewater treatment, is the third step of wastewater treatment. After secondary treatment, tertiary treatment of effluent entails several extra procedures to minimise organics, turbidity, nitrogen, phosphorus, metals, and pathogens. Tertiary treatment of wastewater makes it ready for reuse. Some examples of reuse include:
- Reclaimed water finds use in cooling systems, boiler feed, process water, and other industrial applications.
- Reuse in agriculture, horticulture, lawn watering, golf courses, and other applications
- Finds use in groundwater recharge to supplement groundwater resources for downstream consumers or to keep saline water out of coastal areas.
Tertiary Waste Water Treatment Methods
Most methods used in tertiary treatment include physicochemical methods such as coagulation, filtration, adsorption on activated carbon, reverse osmosis, and further disinfection. We also use some biological methods like constructed wetlands and membrane bioreactors for nutrients removal. If you wish to go on a trip to a constructed wetland, make sure that you check out our blog: Constructed Wetlands for Wastewater Treatment.
The properties of effluent after secondary treatment and the quality of water required at the end of the treatment determine the treatment options in tertiary treatment. For example, filtration and disinfection are the desirable tertiary treatment methods if we require potable water.
In this blog, let me walk you through various physicochemical methods used for tertiary treatment. Before diving deep into these methods, have a look at our blogs on the secondary and primary treatment of wastewater: Secondary Treatment for Wastewater – Methods and Process.
The various methods used in tertiary treatment include reverse osmosis, electrodialysis, filtration etc. Let’s begin with one of the most commonly used methods, reverse osmosis.
Reverse Osmosis -Tertiary Wastewater Treatment
Reverse Osmosis produces demineralized water by forcing water through semipermeable membranes at high pressure. We apply a pressure greater than the osmotic pressure across a membrane separating a concentrated solution and dilute phase in this process. This forces the solvent or water to move towards the dilute phase.
The concentration of the solute or impurity increases on one side of the membrane. At the same time, pure water (solvent) travels through the membrane into the dilute phase. The Reverse Osmosis process requires high pressure of the order of 4000 to 7000 kN/m2 to ensure sufficient solvent flux across the membrane
The most crucial element in the reverse osmosis process is the permeable membrane. They are usually made from a mixture of cellulose acetate, formamide and magnesium perchlorate. These membranes need large surface areas for effective treatment and to compensate for the low water flux. We use membrane modules instead of a single membrane sheet to reduce the space requirements.
Reverse osmosis finds applications in the following:
- Separation of toxic ions from plating wastes.
- Desalting seawater to produce drinking water.
- Concentration of radioactive wastes.
- Removal of organics from vegetable and animal wastes.
Electrodialysis – Tertiary Wastewater Treatment
Electrodialysis is another popular tertiary wastewater treatment method that employs the removal of the solute from the solution instead of removing the solvent. This process uses selectively permeable membranes and an electric potential difference to separate ions from a solution. The electric power required depends on the number of ions removed from the water.
An electrodialysis cell contains anionic and cationic membranes arranged alternatively. This kind of arrangement creates many compartments between the electrodes placed at either end. Anions from the solution migrate to the positive electrode and cations migrate to the negative electrode on the application of an electric voltage across the cell. Consequently, solutions in alternate compartments become dilute while that in the others turn more concentrated. After reaching the desired degree of separation we can remove the solutions.
The removal of total suspended solids (TSS) by tertiary treatment entails the removal of components that have remained after a secondary clarifying process. Before we proceed with filtration, pretreatment is required. The concentration of suspended particles in the influent must be less than 100 mg/l for effective filtration.
The most common types of filtration include diatomaceous earth filtration, pressure filtration, sand filtration with standard and multimedia units, ultrafiltration, and the moving-bed filter. All these processes involve the physical straining of the finely separated particles.
Diatomaceous Earth Filtration
Diatomaceous earth filtration is a type of mechanical separation that involves filtering wastewater with diatomaceous earth, a powdered filter aid, on a supporting media. As the filtration progresses, the solid material that will not pass through the diatomaceous earth accumulates on the filter. Eventually, this builds up pressure that prevents filtration. After that, we backwash the filter and remove the accumulated material to prepare it for the next round of filtration.
The standard method of filtration consists of sandbeds with graded sand placed on a supporting medium with an underdrain to collect the filtered effluent. Solids will build up and eventually block the holes as wastewater containing solids passes through this type of filter. This results in excessive head loss and/or poor effluent quality. As a result, sand filtration demands some provisions for the removal of the accumulated material. Backwashing the sand, or reversing the flow with air scour, helps to keep the sand in suspension while washing away the lighter material.
Ultrafiltration (UF) is a method of water purification that involves forcing water through a semipermeable membrane. Water and low-molecular-weight solutes filter through the membrane to the permeate side, while suspended particles and high-molecular-weight solutes remain on the retentate side.
UF removes most organic compounds and viruses, as well as a variety of salts. It is popular because it generates consistent water quality regardless of the source water, has a small physical footprint, removes 90-100% of pathogens, and does not require chemicals (except for membrane cleaning). Also, it uses considerably lower pressure compared to reverse osmosis. Ultrafiltration uses pressures on the order of 50 lb/in2, whereas reverse osmosis uses pressures above 500 lb/in2.
Shall we wrap up?
In this blog, we had a short discussion about some of the tertiary wastewater treatment methods like reverse osmosis, electrodialysis, ultrafiltration etc. Depending on the end-use of the wastewater we use a single method or a combination of the above-mentioned ones. Tertiary treatment ensures that the water is safe for release into water bodies or for irrigation.