Advanced Water Treatment: Tertiary Process Explained

Tertiary treatment is the final step in the wastewater management system, which further cleans wastewater after primary and secondary treatment. It is an advanced and rigorous process that targets the removal of stubborn contaminants, such as dissolved substances, metals, organic chemicals, and nutrients like phosphorus and nitrogen. This step is crucial to ensure that the treated water meets the safety standards for public health and environmental protection. The most commonly used methods of tertiary treatment are filtration, carbon adsorption, nutrient removal, disinfection, membrane technology, and chemical oxidation. After undergoing tertiary treatment, the water is purified and safe for reuse in various processes, including drinking water, agriculture, and industry.

Characteristics and Values of Tertiary Treatment for Water Pollution

Filtration

The filtration process in tertiary treatment for water pollution involves removing residual suspended matter and fine particulates from wastewater. It is the first stage of tertiary treatment and is crucial for enhancing water quality and ensuring it meets regulatory standards for discharge or reuse. Here is a detailed overview of the filtration process:

Sand Filtration: Sand is the most commonly used material for filtration in tertiary treatment. Sand filtration effectively removes residual suspended solids and fine particulates from wastewater. The sand acts as a physical barrier, allowing water to pass through while trapping the solid particles. This method is simple yet highly effective in improving water clarity and quality.

Activated Carbon Filtration: In cases where residual toxins are present in wastewater, activated carbon plays a vital role in absorbing and removing these toxins. Activated carbon has a large surface area and highly adsorbent properties, making it ideal for attracting and retaining organic compounds and other contaminants. This process ensures that any remaining toxins are eliminated, further enhancing the safety of the treated water.

Drum and Disc Filters: Drum and disc filters are mechanical filtration systems commonly used in tertiary treatment. Drum filters consist of a central drum surrounded by a woven cloth filter. Wastewater flows into the filter due to gravity, and media mounted on the drum separate the solid particles. Disc filters, on the other hand, have multiple discs with cloth filters attached to a central drum. Gravity pushes wastewater through the filters, and media mounted on the discs separate solid particles, allowing clean water to flow into a collection tank.

Natural Filtration: Some natural filtration methods are also employed in tertiary treatment. One such method is rapid infiltration, where a basin is filled with pre-treated wastewater, and the ground acts as a natural filter to remove pollutants. Overland flow is another natural process where water is forced through vegetation on a sloped surface with nearly impermeable ground, effectively removing pollutants.

Advanced Filtration Techniques: Tertiary treatment often incorporates advanced filtration techniques such as microfiltration to ensure the removal of even the smallest particulates from the water. Membrane filters, for instance, can include ultrafiltration and reverse osmosis processes that effectively remove inorganic and organic substances from the water.

The filtration process in tertiary treatment is comprehensive and employs a range of techniques to eliminate residual suspended matter, toxins, and fine particulates from wastewater. By utilizing a combination of physical, chemical, and natural filtration methods, the treated water achieves a high level of purity, making it suitable for various purposes, including discharge into the environment, industrial processes, and even drinking water in some cases.

Nutrient removal

Filtration and Lagoon Treatment

The first step in tertiary treatment is often filtration, which helps remove residual suspended matter and particulate pollutants. However, fine particulate matter may still remain in the wastewater, so it is transferred to lagoons. These lagoons, with their aerobic and biologically enriched environments, are crucial for further nutrient removal. The microbes present in these lagoons break down the remaining particulate matter, ensuring it no longer poses a threat to the environment.

Nitrification and Denitrification

To address nitrogen pollution, tertiary treatment employs nitrification and denitrification processes. Nitrification converts ammonia or ammonium, which are toxic to aquatic life, into nitrate. This process is carried out by nitrifying bacteria, which obtain energy by converting ammonia into nitrite and then into nitrate. Denitrification, on the other hand, involves the conversion of nitrate into harmless nitrogen gas, which is then released into the atmosphere.

Phosphorus Removal

Phosphorus removal is another key aspect of nutrient removal in tertiary treatment. Chemical precipitation is one method used to remove phosphorus from wastewater. Additionally, some treatment plants may employ biological phosphorus removal processes, as seen in case studies from Minnesota and the Assabet River Watershed.

Disinfection and Odour Removal

The final step in the tertiary treatment of wastewater is the removal of undesirable microbes and pathogens through disinfection processes. This includes the use of ultraviolet radiation to destroy harmful pathogens and prevent waterborne diseases. Additionally, odour removal is an important part of this step, ensuring that the treated water is suitable for reuse or discharge.

Regulatory Compliance and Water Reuse

Tertiary treatment ensures that the treated water complies with regulatory standards for wastewater effluent quality, such as the Clean Water Act in the United States. By achieving high-quality effluent, the treated water can be reused in various operations, including industrial processes, agriculture, and drinking water supplies. This promotes sustainability and resource recovery while also protecting public health and preserving the integrity of aquatic ecosystems.

Disinfection

One of the most commonly used methods for disinfection is ultraviolet (UV) radiation. UV light effectively destroys pathogens and reduces their presence to trace levels, making the water potable and suitable for recreational purposes. However, for UV light to be effective, the wastewater must be clear, and proper maintenance of lamps and equipment is essential for long-term use.

Another powerful disinfectant is ozone, a highly reactive compound formed by adding a third oxygen atom to standard diatomic oxygen molecules. Ozone can be generated on-site using electrical energy or by passing oxygen through a high-voltage current. It is highly effective in destroying most microorganisms and is considered safer than chlorine as it does not form hazardous by-products. However, the equipment required for ozone treatment is relatively expensive to maintain.

Tertiary treatment also targets the removal of undesirable microbes and odour-causing bacteria. Carbon reactors, regulated amounts of chlorine, hydrogen peroxide, and calcium nitrate can be used to manage hydrogen sulfide levels, reducing unpleasant odours.

The disinfection process in tertiary treatment plays a vital role in ensuring that the water is safe for human consumption and environmental discharge. By eliminating harmful pathogens and microbes, this stage contributes to maintaining water quality, protecting public health, and preserving the integrity of aquatic ecosystems.

Advanced oxidation processes

AOPs are effective in breaking down contaminants that are not removed by conventional methods. They involve the use of oxidants and UV light combinations to degrade complex organic pollutants into safer, simpler compounds. The oxidants used in AOPs can readily degrade complex organic pollutants and remove certain inorganic pollutants in wastewater. Hydroxyl radicals (OH-) or sulfate radicals (SO4-) are generated in sufficient quantity to remove refractory organic matters, traceable organic contaminants, or certain inorganic pollutants, or to increase wastewater biodegradability as a pre-treatment before biological treatment.

The specific mechanisms and treatment efficiencies of AOPs depend on the type of AOP, the physical and chemical properties of the target pollutants, and the operating conditions. The engineering challenges inherent to these processes are significant. For example, there is a limited active reaction volume, which refers to the small area where the plasma comes into contact with the water that is being treated. As a result, it is crucial to design efficient reactors to ensure sufficient mass transfer and to create feasible solutions.

Successful pilot testing paves the way for the full-scale application of new AOPs, which ideally complement the spectrum of well-characterized and ready-for-use AOPs to provide optimal treatment solutions for a wide range of pollutants and scales.

Water reuse

One of the primary goals of tertiary treatment is to achieve water purification levels that make the water suitable for reuse in various processes. This includes water-intensive operations such as industrial and manufacturing processes, oil and gas extraction, utilities cooling, and agricultural practices like irrigation. By reusing treated water in these sectors, we can conserve freshwater resources and promote sustainability.

The treatment methods employed in tertiary treatment vary depending on the composition of the wastewater and the required quality standards. Common processes include advanced filtration techniques, such as microfiltration and membrane separation (e.g., ultrafiltration and reverse osmosis), carbon adsorption using activated carbon or zeolite, nutrient removal, and disinfection through ultraviolet radiation or ozone treatment. These advanced technologies ensure that the treated water meets stringent safety and environmental standards, making it safe for human consumption and protecting public health.

The importance of water reuse is heightened by the growing population and increasing generation of wastewater, especially in developing countries. By effectively treating and reusing wastewater, we can address water scarcity issues and reduce pressure on natural sources of water. Additionally, the by-products of wastewater treatment, such as treated sludge, can be converted into fertilizers, providing nutrients to improve plant growth and yield without relying on harmful chemicals.

Overall, water reuse in the context of tertiary treatment for water pollution is a vital component of sustainable water resources management. It ensures that treated water can be safely reused in various sectors, promoting conservation of freshwater resources, environmental protection, and public health.

Frequently asked questions