The A to Z of industrial wastewater treatment stages

The A to Z of industrial wastewater treatment stages

The use of water for industrial, commercial and domestic purposes adds undesirable contaminants to our water bodies that alter the physical and chemical properties of water, rendering it unsuitable for use and hazardous from an environmental perspective. Wastewater therefore needs to undergo a series of treatment processes to improve the water quality, thereby bringing it up to environmental, health and safety standards.

 The treatment of wastewater usually occurs in three stages: primary, secondary and tertiary. There are series of physical, chemical and biological parameters such as colour, odour, turbidity, pH, COD, BOD, nitrogen content, TDS and pathogen content that govern the reusability and treatability of wastewater. Each stage of treatment ensures that the parameters of the wastewater come closer to being met.

1. Primary Treatment of Wastewater

Initial treatment of industrial effluents occurs during primary treatment where suspended solids likes plastic, wood, grease etc. are removed physically. Any kind of colloidal materials present in the water are subjected to chemical treatment, using flocculants and coagulants which floc together along with the contaminating colloidal material and cause it to settle to the bottom of the treatment tank. It is at this stage that the pH of effluent is neutralized using an acid or alkali before the waste water is sent for secondary treatment of wastewater. This is a necessary step to promote microbial growth at the secondary wastewater treatment stage. Microbes prefer a near neutral pH environment to feed, flourish and replicate.

2. Secondary Treatment of Wastewater

The primary wastewater treatment is followed by secondary treatment of wastewater which is the heart of the wastewater treatment process. Secondary treatment is also known as biological treatment process where the primary treated effluent is further treated using biological agents causing carbon oxidation and removal of other nutrient. The secondary treatment of wastewater is carried out using biological agents like enzymes or microorganisms.

A. Secondary Treatment of wastewater mediated by Enzymes

Enzymes are non-living biological proteinaceous molecules that catalyze a biochemical reaction. In enzyme mediated secondary wastewater treatment processes, enzymes treat the water via an enzyme-substrate reaction, where enzyme attaches to a site of the substrate and brings about degradation of the substrate. The downfall is that the use of enzymes becomes a costly affair considering continuous requirements in large volume. Due to substrate specificity of enzymes, additional organic pollutants cannot be degraded. They also remain highly sensitive to any fluctuations in the environment that can easily render them inactive and hence with the need to be replaced. They also need specific infrastructure to be employed in a treatment facility.

B. Secondary Treatment of Wastewater mediated by Microbes

Microbial degradation of wastewater is the most versatile, effective and simple method for secondary treatment of wastewater. Microbial treatment of effluent can easily degrade a majority of the organic pollutants commonly found in wastewater. Microbes use these organic pollutants as their food source breaking down the toxic, high molecular organic compounds into water, carbon dioxide, biomass and other smaller and non-toxic byproducts. The ability of microbes to survive, adapt, multiply and non-specifically degrade any organic pollutant makes it the most cost effective and efficient solution for secondary wastewater treatment. It is advisable to use a microbial product consisting of a consortium of highly tenacious and scientifically selected microbes that are able to target a wide variety of simple as well as recalcitrant organic compounds for better results during secondary treatment of wastewater.

2.1. Modes of Secondary Treatment of Wastewater

Process of secondary treatment can be mainly classified into aerobic and anaerobic treatment, depending on whether the microbes in the treatment system require the presence of oxygen to survive, and treat the water entering the system.

2.1. Aerobic treatment:

In aerobic treatment, microbial degradation of effluent takes place in the presence of air/oxygen, which is mechanically supplied by aerators. The energy consumption to continuously run the aerators and monitor the parameters of secondary tank makes it costly than anaerobic treatment. But rapid and efficient degradation of pollutants under aerobic conditions gives it an edge over anaerobic treatment. Faster degradation of pollutants ensures lesser retention time for effluent in turn reducing requirement for large land space. Aerobic technology is employed by various technologies such as:

Activated Sludge Treatment – It is the most commonly used technology in secondary treatment of wastewater. In this process the microorganisms and solids are suspended in the tank using large aerators and agitators. The microbes interact with organic pollutants and degrade them. The solids and microbes floc together to form flocs as mixed liquor suspended solids (MLSS). After achieving the desired effluent treatment the mixed liquor is discharged into the secondary clarifier where the sludge settles at the bottom and supernatant is further subjected to tertiary treatment. A part of the sludge is recycled back to the secondary treatment unit and this serves as seed for further treatment of the incoming wastewater. The microbes in this sludge are already pre-acclimatised to the organic load in the water, therefore making the degradation process faster and more efficient.

Moving bed biofilm reactors (MBBRs): MBBR are also a type of activated sludge process but the microbes are attached on the surface of polyethylene carrier as a biofilm. These inert carriers can easily move inside the aeration tank providing easy access for microbial and waste interaction due to its larger surface area. Their movement facilitates a higher degradation rate.

Sequential batch reactors (SBRs): It works on filling and drawing of activated sludge system at specific intervals during the treatment process. After treatment the aeration and agitation is stopped and sludge settling is allowed. The treated supernatent is then drawn from the outlet. In this process for secondary treatment of wastewater, equalization, aeration and clarification can be achieved in single system.

Membrane bioreactors: The combination of biological wastewater treatment like activated sludge along with membrane processes like microfiltration (MF) or ultrafiltration (UF) can be achieved with membrane bioreactos. The filtration unit is placed inside the biological reactor and effluent is drawn out through the filtration unit maintaining the microbial biomass inside the reactor.

2.1.2. Anaerobic Treatment-

Microbial treatment of effluent in absence of air/oxygen is known as anaerobic treatment. These degradation is carried out by anaerobic microorganisms. It is cost effective method for secondary treatment of wastewater as it does not require high energy input. Though the energy input for systems like agitators and real time data monitoring is minimal, the space requirement for treating large volume of effluent with higher retention times is much higher as compared to its aerobic counterparts. The technologies used in anaerobic treatment includes:

 Anaerobic lagoons: Manmade earthen basin such as ponds that provides anaerobic conditions for effluent treatment is called as anaerobic lagoons. The effluents are maintained in lagoons and added with potential anaerobic microbes for degradation of pollutants. This technology requires higher retention time compared to any other anaerobic system.

Anaerobic sludge blanket (ASB) Reactors: In this process, the influent is passed through a blanket of flocs or granular sludge under anaerobic conditions The flow of effluent can be downward or upward. Upflow anaerobic sludge blanket (UASB) reactor is the most commonly used ASB technology.

Anaerobic filter reactors.  In this process, effluent is allowed to pass through anaerobic filters  with biofilms retained on its surface. The filter contains inert carrier which can be of synthetic or natural origin. The filters can be operated in upward or downward flow.

Nitrogen removal by nitrification and denitrification:  This method of secondary treatment of wastewater uses a combination of aerobic treatment followed by anaerobic treatment for nutrient removal such as nitrogen. The nitrogen is usually present in water in the form of ammonia in alkaline pH, ammonium ion in acidic pH and ammoniacal nitrogen at neutral pH. The process of nitrogen removal is a two- step process called as nitrification and denitrification.

Tertiary Treatment of Wastewater

Tertiary wastewater treatment is the final treatment process that utilizes a combination of physical and chemical processes to remove harmful microbial pathogens and higher life forms from wastewater. It may involve a physical method of separation such as filtration followed by additional physical/ chemical disinfecting treatment. Other methods may also be additionally employed. Sometimes due to lack of resources, this part of the treatment is entirely skipped altogether. This makes it all the more important for the secondary treatment of wastewater to be as efficient as possible.

Organica Biotech is an environmental biotechnology company that has been a pioneer in the wastewater treatment industry for over twenty years. Organica Biotech has helped industries across the spectrum, step up their wastewater treatment processes by providing cutting edge secondary treatment solutions that are tailor made to optimize effluent treatability at an affordable cost. Organica Biotech has an array of wastewater treatment solutions that cater to the different types of wastewater as well as industrial wastewater treatment processes. Visit our website to know more about us.

John Norwood
John Norwood is best known as a technology journalist, currently at Ziddu where he focuses on tech startups, companies, and products.