What is Wastewater?

Wastewater is a mixture of water and a variety of pollutants, including suspended solids, dissolved organic compounds, nutrients, and microorganisms. Domestic wastewater is typically composed of human waste, food waste, and paper products. Industrial wastewater may include toxic chemicals, heavy metals, oil and grease, and other pollutants.

Suspended solids are the largest component of wastewater and can include anything from toilet paper to food scraps. Dissolved organic compounds, such as detergents, food waste, and solvents, can also be found in wastewater. Heavy metals, salts, and nutrients are also present. Nutrients, such as nitrogen and phosphorus, are the primary sources of energy for microorganisms in wastewater. These microorganisms are responsible for breaking down organic matter, which in turn produces carbon dioxide and other gases. This decomposition is essential for the removal of some pollutants, such as ammonia and phosphorus from wastewater.

Health, Environment, and Social concerns associated with wastewater:

• Health: Wastewater contains bacteria, viruses, and parasites that can cause diseases such as cholera, dysentery, hepatitis A, and typhoid. These diseases can spread through contaminated water. Wastewater also contains toxic chemicals, such as lead, arsenic, and mercury, which can cause serious health problems if ingested. Poorly treated wastewater leads to water pollution on its release, creating an even greater risk to public health.
• Environment: Wastewater can have a damaging effect on the environment if it is not properly treated or disposed of. Untreated wastewater can pollute drinking water sources and contaminate soil, which can lead to water-borne diseases and the destruction of natural habitats. Wastewater can also cause algal blooms and oxygen depletion in water bodies, which can kill off fish and other aquatic life.
• Social: Wastewater can have a significant impact on communities. Poorly treated or disposed wastewater can cause an increase in health risks to local residents. If wastewater is not managed properly, it can create an unpleasant living environment due to odor and the presence of vermin. Furthermore, wastewater has an adverse impact on the local economy, as it can contaminate agricultural land and decrease crop yields.

Precautionary measurements are taken to avoid wastewater-related issues

Precautionary measures are taken because untreated wastewater can cause a variety of negative environmental and health impacts.

To prevent these issues, precautionary measures are taken to ensure that wastewater is collected and disposed of properly. The collection of wastewater is usually done through sewer systems, septic tanks, or catchment basins. These systems allow for the safe collection and transport of wastewater to a treatment facility to remove pollutants and contaminants. Then water is released back into the environment.

Disposal of wastewater is also important in avoiding wastewater-related issues. Depending on the type of wastewater and the level of treatment it has undergone, it can be disposed of in various ways. Treated wastewater may be discharged into a local waterway, sprayed onto land, or recycled for use in industrial processes.

Wastewater collection

Wastewater collection systems typically include a network of pipes, manholes, and other structures such as pumping stations, treatment plants, and storage tanks. The collection systems are responsible for transporting wastewater from residences and businesses to a central drainage system. 

Wastewater specific treatment

STP

A sewage treatment plant is a facility where wastewater is processed to remove pollutants and produce a treated effluent that is safe to return to the environment. Treatment processes may include physical, chemical, and biological processes to remove suspended solids, nutrients, and other pollutants. The treated effluent is typically discharged to a receiving water body such as a river, lake, or ocean.

Primary, Secondary, and Tertiary treatment process in STP:

Primary Treatment

It involves the physical removal of solids from wastewater. This is typically accomplished by screening, grit removal, and primary sedimentation. This is usually done by passing wastewater through large screens or grit channels to remove large debris, such as plastic, sticks, and rags. The wastewater is then pumped through a settling tank, which allows suspended solids to settle to the bottom and the clarified water to flow out of the tank. The settled sludge is usually sent to a secondary treatment process, such as anaerobic digestion or activated sludge. Primary treatment involves the addition of a coagulant and aims at removing grits, coarse solids, oil, and grease if any are present. 

Secondary Treatment

This process uses bacteria and other microorganisms to break down organic matter from wastewater, such as food waste, soaps, and detergents, and convert it into a form to be released into the environment. The process usually involves aeration and clarification, using tanks, basins, and biological filters.

• Membrane bioreactors (MBRs): Another wastewater treatment option is MBRs. These systems use membranes to separate suspended particles from the wastewater and then allow aerobic or anaerobic bacteria to break down the organic material. The end result is a stabilized effluent that meets environmental standards for safe discharge.

• Sequential Batch Reactor (SBR): The SBR cycle is composed of several steps: fill, react, settle, decant, and idle. During the reaction stage, the wastewater is aerated and circulated to mix with the microbial population, allowing it to break off the organic matter. In the settling stage, the mixture is settled, allowing lighter biomass and organic matter to rise to the surface and be removed. In the idle phase, the tank is allowed to sit without any aeration or mixing, allowing the biomass to settle and the organisms to rest.

• Moving Bed Biofilm Reactor (MBBR): MBBR is a type of wastewater treatment system that uses suspended carriers to provide a large surface area for the attached growth of biofilms. The carriers used in MBBRs provide a large surface area for biofilm growth, which helps to reduce the biomass size required for effective treatment.

• Fluidized Bed Bioreactors (FBBR): FBBR is a type of secondary treatment for the removal of organic pollutants from wastewater. This allows microorganisms, such as bacteria and fungi, to colonize the particles and break down the organic pollutants. It is used in conjunction with primary treatment processes to achieve the highest level of pollutant removal possible.

Tertiary Treatment

The tertiary or chemical treatment process of sewage treatment plants typically involves the addition of chemicals such as calcium oxide, sodium hydroxide, and sodium carbonate to the wastewater. This process helps to break down organic matter and remove suspended solids and other contaminants. The process also helps to balance the pH of the water, reduce odors (Activated charcoal), and reduce the level of disease-causing organisms.

• Activated charcoal is a carbon filtration that works to remove odor and color by adsorbing and trapping contaminants on the surface of its tiny pores. Activated carbon needs replacement as its capacity to work reduces gradually.

• Chlorination: The process involves the addition of chlorine or chlorine-based compounds to the water to kill bacteria and other disease-causing microorganisms. The chlorine kills any disease-causing organisms, which helps to reduce the risk of water-borne diseases.

• Ozonation is a form of the advanced oxidation process that produces extremely reactive oxygen species. Ozone is very reactive and readily oxidizes microorganisms, effectively killing them. Ozone oxidizes the cell wall, membrane, and internal components of microorganisms, damaging their structure and function. Ozone also disrupts the microorganism`s metabolic pathways, leading to cell death.

• UV treatment – UV water disinfection technique disinfects by penetrating microorganisms and destroying their DNA. Chlorine and other disinfectants can produce toxic disinfection byproducts (THMs or Halo-acetic acids) as well as the dangers that come with their presence on site. Because UV light disinfection is a chemical-free method, it effectively eliminates any concerns about these byproducts.

The important water discharge parameters in STP are biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), total nitrogen (TN), total phosphorus (TP), and fecal coliform bacteria.

ETP

An effluent treatment plant (ETP) is a facility used to treat wastewater that is produced by industries and other sources. The treated wastewater (or effluent) is then released into the environment, usually a nearby river, lake, or ocean. ETP reduces the number of pollutants in the wastewater to the levels required by national and local environmental regulations.

In comparison with STP, ETP focuses more on reducing the number of chemical pollutants present in wastewater during its secondary treatment process.

Primary, Secondary, and Tertiary treatment process in ETP:

Primary Treatment

The primary treatment process in ETP typically involves the removal of suspended solids, oils, and other physical contaminants from wastewater. Wastewater is Treated by physical processes such as sedimentation, flocculation, clarification, and filtration.

Secondary Treatment

In secondary treatment, industrial effluent processes are modified as per the content and level of certain chemical pollutants. Following some of the industrial wastewater treatment processes will help to understand this concept better. 

Oil and petroleum industry:

A vast amount of wastewater is generated from the extraction, refining, and transportation of petroleum products. This contains a variety of contaminants, including oil, grease, heavy metals, and other hazardous substances or pollutants. It helps to reduce the concentration of pollutants and contaminants in the effluent to a level that is safe for discharge into the environment. This is achieved through coagulation, flocculation, and biological treatment. The coagulation/flocculation process involves the addition of a coagulant, such as aluminum hydroxide chloride or aluminum sulfate, to the wastewater to remove suspended solids.

The biological treatment process utilizes microorganisms to break down organic material in the wastewater and reduce the concentration of pollutants. Finally, the N:P ratio is an important parameter for the treatment of oily wastewater by using oil-degrading bacteria. By using these techniques, secondary treatment can effectively reduce the contamination level of effluent and make it suitable for discharge into water bodies.

Example: Microorganisms to remove oil contaminants such as Bacteria, Pseudomonas aeruginosa: P. aeruginosa bacteria are able to break down oil and petroleum products due to their ability to produce enzymes that are specific to hydrocarbons. Pseudomonas bacteria are usually introduced into wastewater as slurry. This slurry is made up of a combination of Pseudomonas bacteria, nutrients, and a carbon source. The carbon source is important because it provides the bacteria with the energy they need to break down the oil and petroleum-based pollutants.

Other common microorganisms used in the oil and petroleum industry for wastewater treatment include bacteria such as Acinetobacter, and Bacillus, and fungi such as Aspergillus and Trichoderma.

Textile industry (Dyes and paints, colorants,):

The main purpose of secondary treatment is to provide BOD removal beyond what is achievable by simple sedimentation. It also removes appreciable amounts of oil and phenol. The dissolved and colloidal organic compounds and color present in wastewater are removed or reduced to stabilize the organic matter. Textile processing effluents are amenable to biological treatments.

Textile waste also contains significant quantities of non-biodegradable chemical polymers. For non-biodegradable pollutants, filtration technologies are utilized in the textile industry to clean wastewater. Traditional membrane processes in textile wastewater treatment include the use of ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) membranes. The selection of membrane technologies for textile effluent relies on costs based on the balance between water flux and solute retention. RO also becomes less effective when osmotic pressure, is caused by high salt concentration in the feed wastewater. Becomes too high to obtain a reasonable transmembrane permeate flux without applying excess transmembrane hydraulic pressure.

The membrane filtration system typically consists of a series of membranes with differing pore sizes and compositions that are used to remove particles from the wastewater. The membranes are typically made from polymeric materials such as polyvinylidene fluoride (PVDF), polysulfone, or polypropylene.

Metallurgic, chemical, and Fertilizer industry:

This is usually done through the application of an activated sludge process, trickling filter, rotating biological contactors, and oxidation ponds. The activated sludge process is used to remove organic pollutants from wastewater by using a variety of microorganisms. Trickling filters use a bed of media on which microorganisms grow to degrade the organic pollutants. Rotating biological contactors use a series of rotating plastic disks on which microorganisms grow and degrade organic pollutants. Finally, oxidation ponds are used to provide long-term biological treatment.

Tertiary Treatment

The tertiary treatment or biological treatment process in an ETP is designed to remove organic and inorganic pollutants from wastewater. This process is typically used in applications where the effluent needs to be treated to a high level of purity. The water is then treated with chlorine, ultraviolet light, or ozone to kill any remaining bacteria, viruses, and other pathogens before it is released into the environment. Primary and secondary treatment typically gets wastewater only clean enough to discharge safely into the environment. Tertiary treatment can achieve levels of water purification that make the water safe for reuse in water-intensive processes or even as drinking water.

Tertiary wastewater treatment often works by using a combination of physical and chemical processes to remove harmful microbiological contaminants. The process usually involves filtration followed by additional disinfecting treatment. In some cases, tertiary treatment may also use other specialized treatments like lagoon storage, biological nutrient removal, and nitrogen and phosphorus removal.

Final water disposal activities:

All water either from STP or ETP is finally safely disposed into the environment or reused depending upon the quality of wastewater as follows.

• Filters: Tertiary filtration components can contain a few different materials. Sand and activated carbon filters are common, and filters can also contain fine woven cloth. The filters come in a few types, including bag filters, drum filters, and disc filters. Backwash cleans the media components to ensure their continual functioning.

• Disinfecting: The process of tertiary disinfection may take a few different forms. Chlorine is one of the most commonly used disinfectants in wastewater treatment. Ultraviolet light is a common disinfectant in tertiary treatment. Ozone is highly reactive and can destroy most microorganisms it comes into contact with.

• Discharge: Once the wastewater has undergone tertiary treatment, it is ready for discharge back into the environment. Many municipalities have specific requirements for the discharge of treated water. Tertiary treatment should be sufficient to meet those standards, keep the environment clean and preserve human health, experts say.

• Reuse: Many treatment plants use tertiary treatment specifically to make the water safe for human ingestion. Water that has received tertiary treatment is also suitable for numerous operations that require clean water. These include industrial and manufacturing processes, oil and gas extraction and refining, utilities cooling, and agricultural practices like irrigation.