Wastewater treatment processes involve the sequential treatment of wastewater. This is done in the form of Primary, Secondary, and Tertiary Treatment of Wastewater. The third stage is the final treatment stage which involves the removal of harmful substances such as inorganic compounds, bacteria, viruses, and parasites.
The tertiary treatment removes nitrogen and phosphorous present in wastewater. It includes processes such as filtration, coagulation, reverse osmosis, ion exchange, activated carbon filter, electro-dialysis, nitrification-denitrification processes. This stage disinfects water for recycling and reuse.
Treatment steps in tertiary treatment are based on the effluent characteristics after secondary treatment and its use after the treatment.
Quality of Wastewater after Secondary Treatment
Wastewater reaching tertiary treatment has residual suspended matter, fine particulates, high levels of nutrients such as nitrogen and phosphorous, microbes, metals, turbidity, and foul odor.
Tertiary Treatment of Wastewater Processes
- Nutrient Removal
- Chemical Coagulation
- Flocculation and Sedimentation
- Filtration and Chlorination
- Ion-Exchange and Reverse Osmosis
Nitrogen and phosphorous present in secondary treated water treated using physical, chemical or combination of these processes. For example, addition of metal salts enhances precipitation of phosphorous. Nitrogen concentration reduction involves denitrification followed by activated sludge process, producing nitrified effluent. Eutrophication occurs when nutrients present in effluent are improperly treated leading to dead zones, killing fishes and harmful algal blooms. Conventional nitrogen removal involves nitrification and denitrification, which involve either releasing nitrogen gas into the atmosphere or settling organic nitrogen in bio-solids. Phosphorus settles similarly in bio-solids. Nitrogen and phosphorus are used in fertilizers and detergents. They can be recovered from wastewater via chemical or biological processes such as precipitation or biomass assimilation.
Chemical coagulation is the best method for organic removal if the coagulant dose and pH are in the optimal range. Best results for organic removal are obtained by increasing the dose of the coagulant (alum). Alum dose and pH are critical parameters in the industrial wastewater treatment process. The coagulant alum is capable of significant organic removal. The pH of the wastewater during coagulation has a significant impact on the effectiveness of coagulation for organic removal. Acidic conditions enhances organic removal. Use of various coagulants (Al2(SO4)3.18H2O, Polyaluminum chloride) (PACl), Ferric chloride hexahydrate shifts optimum pH to slightlt more acidic value. Ferric sulfate hepta hydraten enhances removal of pollutants such as color by FeCl3, turbidity by Alum, FeCl3 and Fe2(SO4)3.
Flocculation and Sedimentation
Flocculant are substances that accelerate the agglomeration of colloidal particles. Which is followed by formation of flocs in nthe water system. A good flocking agent removes impurities at the lowest possible concentration and in the shortest amount of time. Inorganic compounds (such as aluminium sulphate and iron chloride), primarily used for water and wastewater treatment because of wide availability and low cost. But, polymers (both synthetic and natural) are becoming increasingly popular flocculant. Because they increase the surface area (i.e., sorption capacity) of the flocs formed, they are especially useful in increasing the rate of slow-settling aggregates at low temperatures.
Sedimentation, also known as settling, may be defined as the removal of solid particles from a suspension by settling under gravity. It is used for removing settled sludge after the physicochemical treatment.
Filtration and Chlorination
Filtration is a physical process that removes suspended particles from wastewater. These particles can include mud, colour, organic matter, plankton, bacteria, and softening process particles, among others. Filters are classified into two types:
A) Deep Filters: Deep filters, such as gravity filters or pressure filters, separate particulate matter from fluid.
B) Surface Filters: The separation of suspended particles from liquid occurs at a very shallow depth, which is the surface of the filter, such as filtering paper.
Chlorine is a common disinfectant and disinfection is the last barrier of wastewater reclamation process to protect ecosystem safety and human health. Effluent chlorination produces mutagenic/carcinogenic disinfection by-products (DBPs) as a result of the reaction of chlorine with organic compounds in wastewater. As a result, dechlorination should be followed by chlorination, or an alternative safe disinfectant should be used.
Ion-Exchange and Reverse Osmosis for Tertiary Treatment of Wastewater
The exchange of ions between two electrolytes or between an electrolyte solution and a complex is known as ion exchange. Ion exchange is a method of purifying, separating, and decontaminating aqueous or ion-containing solutions. Ion exchange resins are used in wastewater treatment plants to exchange one ion for another in order to achieve demineralization.
RO is a process that separates and removes dissolved solids, organics, pyrogens, submicron colloidal matter, color, nitrate, and bacteria from wastewater using semipermeable spiral wound membranes. Under pressure, feed water is delivered through the semipermeable membrane, where water permeates the membrane’s minute pores and is delivered as purified water known as permeate water. The water impurities that are concentrated in the reject stream and flushed to the drain are referred to as reject water. These semi-permeable membranes reject salt ions while allowing water molecules to pass through.
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