Category Archives: Water pollution

What Biochemical Oxygen Demand (BOD) Measures?

What Biochemical Oxygen Demand measures are a question that all of us have in our minds while learning about water pollution. In this blog, I will walk you through Biochemical Oxygen Demand and, and its significance. The concept of Dissolved Oxygen is described in detail in the upcoming paragraphs.  In the next section, I will show you the complete details about what actually the Biochemical Oxygen Demand measures.

  1. What does the Biochemical Oxygen Demand measure?
    1. Total Biochemical Oxygen Demand Measures
    2. Biochemical Oxygen Demand Measures of Drinking Water
    3. Factors affecting Biochemical Oxygen Demand
  2. Dissolved Oxygen (DO)
    1. Dissolved Oxygen Determination
    2. Calculation of Biochemical Oxygen Demand
    3. BOD5 vs BOD20

What does the Biochemical Oxygen Demand measure?

Biochemical Oxygen Demand measures the amount of oxygen that the microbes utilize to degrade organic materials in a water body. Also, Biochemical Oxygen Demand measures the chemical oxidation of inorganic materials i.e., the removal of oxygen from water via a chemical reaction. 

The BOD value is generally expressed in milligrams of oxygen used per litre of the sample over a 5-day incubation period at 20 °C, and it is frequently used as an estimate of the degree of organic pollution in water. The reduction of BOD is used in evaluating the efficacy of wastewater treatment systems.

The main sources of BOD in wastewater include leaves and woody debris, plant and animal carcass, animal manure, effluents from pulp and paper mills, wastewater treatment plants, food-processing plants, failing septic systems, and urban stormwater runoff.

Biochemical Oxygen Demand in wastewater
Biochemical Oxygen Demand in wastewater

Biochemical Oxygen Demand has a direct effect on the amount of dissolved oxygen in rivers and streams. The higher the BOD, the faster the rate of oxygen depletion in the stream. This results in higher forms of aquatic life having less oxygen available to them. As a result, they suffocate, and eventually, die.

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Total Biochemical Oxygen Demand Measures

Total biochemical oxygen demand is the quantity of oxygen necessary to totally oxidise organic substances to carbon dioxide and water over generations of microbial development, death, degradation, and cannibalism. It has a greater impact on food webs than water quality.

Biochemical Oxygen Demand Measures of Drinking Water

A water sample having a BOD5 between 1 and 2 mg/l indicates very pure water, 3.0 to 5.0 mg/l means moderately clean water and > 5 mg/l indicates a neighbouring pollution source. The biochemical oxygen demand of safe drinking water must be 1-2 mg/l.

Factors affecting Biochemical Oxygen Demand

Temperature, nutrient concentrations, aeration and the enzymes available to indigenous microbial populations affect the BOD measurements. For instance, rapids and waterfalls will speed up the decomposition of organic and inorganic material in stream water. As a result, BOD levels at a sampling location with slower, deeper waters may be higher for a given volume of organic and inorganic material than at a similar site in highly aerated waters.

Chlorine can interfere with the BOD measurements by preventing or killing the microorganisms that break down the organic and inorganic substances in a sample. Therefore, use sodium thiosulfate to neutralise the chlorine while sampling in chlorinated waters, such as those below a sewage treatment plant’s effluent.

Algae in the wastewater affect the Biochemical Oxygen Demand measures by releasing extra oxygen into the wastewater during photosynthesis. Hence, perform the BOD test in complete darkness. Before delving into Biochemical Oxygen Demand measures we should first understand the concept of Dissolved Oxygen (DO), its significance and its measurement.

Dissolved Oxygen (DO)

Aquatic plants and algae release oxygen into the water after performing photosynthesis in the presence of sunlight. The aquatic animals breathe this dissolved oxygen. Also, some oxygen from the atmosphere continuously dissolves into the water through reaeration. These three processes are in equilibrium and maintain the level of oxygen in water bodies at the required levels.

When organic substances or pollutants enter the waterbody, it disturbs the dissolved oxygen balance since microbes utilize dissolved oxygen in its breakdown. In other words, these organic substances exert a demand on the available dissolved oxygen.

Dissolved Oxygen - Aquatic plants & Algae
Dissolved Oxygen – Aquatic plants

The greater the oxygen necessary for its breakdown, the greater would be the reduction in the dissolved oxygen in the water body. Pollution occurs when the oxygen demand exceeds the dissolved oxygen availability.

Also read: Wastewater Treatment- Stages and Process full details

Dissolved Oxygen Determination

Winkler’s method helps in determining the dissolved oxygen. The principle behind this method is the reaction between dissolved oxygen and manganese ions to precipitate out manganese dioxide.

Mn2+ + O2 —-> MnO2 

The manganese dioxide then reacts with iodide ions. This reaction liberates iodine in an amount chemically equivalent to the original dissolved oxygen.

MnO2 + 2I + 4H+ —-> Mn2+ + I2 + 2H2O

Titration with sodium thiosulphate gives the amount of iodine liberated and thereby the equivalent dissolved oxygen content. Thus we can measure the amount of Dissolved Oxygen in a given water sample.

Now, you have got a clear idea about Dissolved Oxygen. In the next section, I will show you the complete details about what actually the Biochemical Oxygen Demand measures.

Next, let’s see the procedure to obtain the Biochemical Oxygen Demand measures.

Calculation of Biochemical Oxygen Demand

We require two samples from a location to measure Biochemical Oxygen Demand. One is immediately tested for dissolved oxygen, while the other is incubated in the dark at 20o Celsius for 5 days before being examined for the residual dissolved oxygen. 

Let me show you the detailed procedure.

  1. Fill two standard 300-ml BOD bottles with the sample wastewater. Seal the bottles properly.
  2. Immediately determine the dissolved oxygen content of one of the samples using Winkler’s method.
  3. Incubate the second bottle at 20 0C for 5 days in complete darkness.
  4. Determine the DO levels after 5 days.

The amount of BOD is the difference in oxygen levels between the first and second tests, measured in milligrams per litre (mg/L). This is the amount of oxygen that the microorganisms require throughout the incubation period to break down the organic materials in the sample container.

DO (mg/L) of first bottle – DO (mg/L) of second bottle = BOD (mg/L)

The dissolved oxygen level may be nil at the end of the 5-day incubation period. This is particularly the case for rivers and streams which have a high pollution load of organic matter. Moreover, it is impossible to determine the BOD level because it is unknown when the zero point was reached. In this situation, dilute the original sample by a factor that yields a final dissolved oxygen content of at least 2 mg/L. The dilutions should be done with special dilution water of high purity.

The dilution water consists of deionised water with sufficient nutrients, phosphate buffer, trace elements and seed organisms (mostly settled domestic sewage). Perform a blank run on this dilution water and subtract its oxygen demand from the results.

BOD5 (mg/l) = D* [(DOt=0 – DOt=5)]sample – [(DOt=0 – DOt=5)]blank

BOD5 vs BOD20

During the standard 5-day/20 0Celsius conditions, about two-thirds of the carbonaceous material undergoes degradation. In the 5-day test, compounds that aren’t easily biodegradable or soluble don’t undergo complete digestion. Incubation of 5 days gives the soluble BOD or BOD5 measures while that of 20 days gives Ultimate Biochemical Oxygen Demand measures or BOD20

Biochemical Oxygen Demand
Biochemical Oxygen Demand

It requires nearly 20 days for the complete breakdown. On the other hand, the 20-day Biochemical Oxygen Demand measures greater long-term oxygen demand from insoluble materials including cellulose, long-chain fatty acids, and grease. Always keep in mind that COD > BOD20 > BOD5.

In spite of its limitations, Biochemical Oxygen Demand analysis has wide applications in monitoring pollution. These days, Chemical Oxygen Demand analysis is gaining wide popularity for research and plant control.

That’s it about Biochemical Oxygen Demand. Hope you found it insightful. Let us know your queries in the comments.

What is Tertiary Wastewater Treatment? – Top Physicochemical Methods Explained

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.
How Reverse Osmosis Works? Credits ESP Water Products
How Reverse Osmosis Works? Credits ESP Water Products

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
Diatomaceous Earth

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.

Sand 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.

What is Sewage? – Sources, Treatment and Quality Indicators

Globally, 44% of sewage is not safely treated, according to UN-Water 2021. Releasing untreated sewage into water bodies pose a great threat to humans as well as the environment. But, how do we understand the quality of sewage and the number of pollutants in it? This blog let me walk you through the basics of sewage, its sources, types, and quality indicators.

Before heading onto the details about sewage, here are a few shocking facts about water pollution and its effects: Water Pollution – Effects and Causes. Going through this blog will help you better understand the importance of treating sewage.

Now, let’s get started.

What is sewage?

Sewage is a type of wastewater that enters the sewerage system from household bathrooms, toilets, kitchens, laundries and drains. It consists of approximately 99.6% water and 0.4% of biodegradable pollutants and small solid particles. A sewage treatment plant considers the following parameters of sewage:

  • The rate of flow
  • Physical state
  • Chemical and hazardous contents
  • Bacteriologic status are all factors to consider (which organisms it contains and in what quantities).

Components of Sewage

The major components of sewage include:

  • Greywater – wastewater from sinks, bathtubs, showers, dishwashers, and clothes washers
  • Blackwater – wastewater from toilets, mixed with the human waste flushed away
  • Soaps and detergents, and toilet paper

Also read: What are Water Pollutants? – Definition, Sources and Types

Sources and Types of Sewage

Households, municipalities, industry, and urban runoff are all sources of municipal wastewater. The following liquids and materials may be discharged into sewer systems at the household level:

  • Human excreta (faeces, urine, blood, and other bodily fluids) is frequently mixes with used toilet paper or wet wipes in case of sewage from toilets.
  • Washing water used for personal hygiene, clothing, floors, dishes, and automobiles.
  • Liquids produced in excess from domestic sources (cooking oil, pesticides, lubricating oil, paint, cleaning detergents, etc.).

The liquids and substances that end up in sewage at the municipal level include oils, animal faeces/manure, food waste, litter, fuel, diesel or rubber residues from tyres, soap scum, metals from vehicle exhausts, de-icing chemicals, herbicides and pesticides from gardens. These are all found in urban runoff from highways, roads, railway tracks, car parks, roofs, and pavements.

Sewage Quality Indicators

Sewage quality indicators are laboratory tests that determine if wastewater is suitable for disposal, treatment, or reuse. These tests measure the physical, chemical, and biological characteristics of sewage. Physical characteristics don’t demand complex procedures since our physical senses alone can detect them. Bioassays and aquatic toxicity tests determine the biological properties while titrations and related laboratory procedures give the chemical characteristics.

Physical Characteristics

The physical characteristics of sewage include those characteristics that our physical senses like sight, smell, touch etc can detect.


The temperature of sewage gives an idea about the level of contaminants. The temperature of wastewater fluctuates substantially based on the procedures performed at the treatment plant. When sewage becomes septic due to microbial action and chemical reactions, the temperature changes. A lower temperature implies that groundwater has entered the sewage system.

Also read: Eutrophication – Definition, Causes, Effects and Control


Colour and odour determine the age of wastewater. Fresh wastewater is light brownish-grey in colour. With an increase in anaerobic conditions, the colour changes from grey to dark grey and black. The oxidation of organic compounds causes the sewage to turn dark grey and black. The black colour indicates septic sewage.



Fresh sewage has an oily or soapy odour, however septic sewage develops an unpleasant odour due to gases produced by the decomposition of organic matter. The most characteristic odour of wastewater is that of hydrogen sulphide which results from anaerobic decomposition. Industrial wastewater also contains odorous compounds or compounds which produce odour during treatment. The following devices can measure odour:

  • H2S meter
  • Olfactometer
  • Scentometer
  • Butanol wheel


  • Measure of light-transmitting property of water.
  • Turbidity measurement involves comparing light scattered by sample to that by a reference suspension under same conditions.
  • Colloidal matter absorb light and thus prevent transmission.
  • Thus, if a sewage sample doesn’t transmit light, it indicates that the sample is turbid due to presence of suspended and colloidal substances.


Solids are those substances that remain as residue after evaporation and drying of the sewage at 103.20C.
Suspended particles are solids that have not been dissolved in wastewater. Floatable solids or scum are suspended materials that float.

Settled solids, often known as grit or sludge, are suspended materials that settle. Settleable solids refer to the solids that settle at the bottom of an Imhoff cone after the water has settled for one hour. It is a measure of the quantity of sludge that can settle by primary sedimentation.

Volatile solids are solids that burn or evaporate at temperatures between 500°C and 600°C. In a wastewater treatment plant, the sediments provide food for bacteria and other living organisms and thereby they decompose the waste. The majority of organic substances included in municipal garbage come from living plants and animals.

Also read: Activated Sludge Process – Stages and Process Control

Chemical Characteristics

Sewage comprises both organic and inorganic compounds and numerous gases produced by sewage decomposition, such as H2S, CO2, CH4, and NH3. pH, DO (dissolved oxygen), oxygen demand, nutrients, and hazardous compounds are chemical features of wastewater that are of particular interest.


The pH scale describes the acidity or alkalinity of aqueous solutions. Initially, the sewage has high pH. Further, the pH drops when it gets septic, and then rises again as it goes through the treatment process.

Dissolved Oxygen (DO)

The term “aerobic” or “fresh” refers to wastewater that contains DO. At 1.0 atm pressure, oxygen solubility in freshwater ranges from 14.6 mg/L at 0oC to roughly 7 mg/L at 35oC.

Biochemical Oxygen Demand (BOD)

BOD is the amount of oxygen that is necessary for the aerobic bacteria to decompose organic matter in a period of 5 days at a typical temperature of 20oC. We have a blog, Biochemical Oxygen Demand || Dissolved Oxygen of Water || Full Details which has all the topics that one should know about BOD. Don’t forget to check it out.

Chemical Oxygen Demand (COD)

The COD is used as a measure of the oxygen equivalent of the organic matter contents of a sample that is susceptible to oxidation by a strong chemical oxidant by definition, i.e. the COD is used as a measure of the oxygen equivalent of the organic matter contents of a sample that is susceptible to oxidation by a strong chemical oxidant.

For more details: Chemical Oxygen Demand and Total Organic Carbon Analysis

Biological Characteristics

Bacteria, viruses, and parasites are the three biological entities found in wastewater.


The typical concentration of bacteria in raw sewage ranges from 500,000 to 5,000,000 per mL. With the help of external and intracellular enzymes, these bacteria are responsible for the breakdown of complicated molecules into stable chemicals. Bacteria can be classified into three types depending on their manner of action:

  • Aerobic Bacteria
  • Anaerobic Bacteria
  • Facultative Bacteria

Along with bacteria, sewage also contains viruses, helminths, parasites etc.


We had seen the quality indicators of sewage like odour, colour, BOD, COD etc. Good quality water is essential to human health, social and economic development, and the ecosystem. Hence it’s our responsibility to ensure that all the sewage that is generated undergoes treatment. Check out our blogs Wastewater Treatment- Stages and Process full details and Secondary Treatment for Wastewater – Methods and Process if you wish to dig deep into various methods of wastewater treatment.

Here’s an interesting method of sewage and wastewater treatment: Constructed Wetlands for Wastewater Treatment. Do check it out.

Primary Treatment for Wastewater – Process and Details

Primary treatment for wastewater involves temporarily storing sewage in a calm basin where heavy materials sink and oil, grease, and lighter solids float to the top. Wastewater reaching a treatment plant through pipes first undergoes primary treatment irrespective of its source.

In the previous blog, Wastewater Treatment- Stages and Process full details, I had given an overview of the wastewater treatment process. In this blog, we go on a trip with wastewater entering the primary treatment plant. Let’s dive deep into primary treatment for wastewater and closely observe each of the processes.

Primary Wastewater Treatment
Primary Wastewater Treatment

Screening – Primary treatment for waste water

The first process in Primary Treatment for Wastewater is screening. I will show you the screening process and different types of screens used in primary wastewater treatment.

In addition to dissolved and suspended impurities, stone, rocks, and even dead animals are among the components of the treatment plant’s wastewater. These solid materials, which make up about a third of the wastewater can erode the pumps and obstruct the flow in pipelines. As a result, eliminating solid waste at the initial stage makes subsequent treatment procedures easier.

Screens and settling tanks remove the majority of the floating materials from the wastewater. Wastewater passes through bar screens consisting of parallel metal bars, wires or grating kept across the flow inclined at 30o-60o. According to the method of cleaning, the screens can be manually cleaned or mechanically cleaned screens. For manually cleaned racks the aperture size ranges from 25-50 mm and 5 to 40 mm for mechanically cleaned racks.

Based on the size of the screen opening, we have 3 types of screens as follows:

  • Coarse screens (≥ 50 mm)
  • Medium screens (25-50 mm)
  • Fine screens (10-25 mm).

Normally, domestic wastewater treatment uses medium screens. The channel approach velocities fall in the range of 0.3 to 0.6 m/s for manually cleaned racks and from 0.6 to 1.0 m/s for mechanically cleaned racks.

The wastewater moves to comminutors after screening.


Comminutors reduce bigger suspended particles to smaller sizes by cutting and grinding action. Large plants frequently employ comminutors. It consists of a fixed screen with a rotating or oscillating cutter, or a curved screen with a rotating or oscillating cutter. They are of considerable importance in treatment plants located in cold areas since they eliminate the trapping of waste on freezing screens.

Next, we are going to see grit chambers and their functions.

Grit Chambers

  • The wastewater after screening enters a grit chamber to settle the grit particles like sand, pebbles etc.
  • Grit chambers are long, narrow tanks that reduce the flow of water to allow particles like sand, stones, and eggshells to settle out of it.
  • They are highly relevant in places with combined sewer systems, which carry a significant amount of silt, sand, and gravel washed off roadways or land during a storm.
  • They protect pumps and pipelines from abrasion and prevents the deposition of grit in pipes and channels.
  • There are two common types of grit chambers – Horizontal flow and Aerated.
  • Horizontal Flow Grit chambers permit a velocity of about 0.3 m/s to settle the grit material while allowing the organic impurities to flow through the chamber.
  • The aerated grit chamber constitutes a spiral flow aeration tank. Wastewater takes a spiral path through the aeration tank. This spiralling action throws away the grit particles into a hopper located underneath.
  • Scrappers remove the grit for disposal.
Aerated Grit Chamber in Primary Wastewater Treatment
Aerated Grit Chamber in Primary Wastewater Treatment

Flow Equalisation – Primary treatment for waste water

  • Under uniform flow rates, clarifiers and mechanised secondary treatment are more efficient.
  • Equalization basins store diurnal or wet-weather flow peaks temporarily and make the water flow rate uniform.
  • Basins serve as a temporary holding area for the incoming wastewater during temporary plant shut down and maintenance.
  • It acts as a means of diluting and distributing hazardous or high-strength waste into batches.
  • Flow equalisation basins require variable discharge control which features bypass and cleaning options as well.
  • Cleaning is easier if the basin is downstream of screening and grit removal.

Also read : What are Water Pollutants? – Definition, Sources and Types

Sedimentation – Primary treatment for wastewater

The wastewater, then moves to sedimentation ponds, settling tanks, or clarifiers after the removal of settled grit. The sedimentation process removes the settleable solids by gravitational settling under quiescent conditions.

On proper adjustment of water flow in the sedimentation tank, the suspended particles begin to fall to the bottom and form a solid mass. Raw primary biosolids, also known as sludge, is the solid mass formed out of the particles. This sludge is removed by vacuum suction or raking it to a discharge point.

Types of Primary Sedimentation Tanks

  • Rectangular Horizontal Flow Tank
  • Circular Radial Flow Tank
  • Up Flow Tanks

Rectangular Horizontal Flow Tank

  • Feed enters at one end along the width of the horizontal tank.
  • They can be economically built side-by-side with common walls.
  • Length ranges from15 to 100m and width ranges from 3 to 24m (length/ width ratio 3:1 to 5:1).
  • In rectangular tanks, the flow occurs in a horizontal, lengthwise direction.
  • Rectangular tanks, sometimes use baffle walls to prevent short-circuiting.
  • Rectangular sedimentation tanks provide reduced maintenance expenses.

Circular Radial Flow Tank

  • In circular radial flow tanks, influent is fed through a central pipe of the tank and radial flow happens.
  • They have diameters ranging from 3 to 60 metres (side water depth range from 3 to 5m).
  • Mechanical sludge scrapers gather the sludge, and a sludge pipe transports it to the bottom.
  • Circular tanks are more expensive than rectangular tanks, but they have a higher clarification efficiency.
Circular Sedimentation Tanks in Primary Treatment for Wastewater
Circular Sedimentation Tanks in Primary Treatment for Wastewater

Up Flow Tanks

  • Up Flow tanks find application in small treatment plants.
  • Feed enters through openings along the bottom side of the tank and the effluent after clarification collects at the top.
  • The flow takes place in a vertical direction.
  • A sludge blanket in the lower part of the tank acts as a filter for small particles.

The next stage is flocculation which removes the remaining suspended solids.


Flocculation is a water treatment process to remove small suspended solids which don’t settle in the sedimentation tank. In this process solids form larger clusters, or flocs on the addition of a flocculent like aluminium sulphate.

The coagulant molecules have a positive charge. Hence, they can neutralize the negatively charged solid particles that are suspended in the water. Neutralization of the particles initiates the flocculation process. The individual suspended particles come together to join and form a larger mass called a floc.

At the onset of flocculation, we add a chemical polymer. It acts as a bridge between micro and macro flocculants, increasing the mass of particles aggregating together. It also bonds the accumulated material together, preventing it from dissolving even when the water is stirred slightly. After the flocculation, the solid masses are removed either through settling or through the use of filters.

Sludge Removal

In the sedimentation tanks, sludge (the organic component of the sewage) settles out of the wastewater. Mechanical scrapers in the tank’s base continuously move accumulated sludge to a hopper, where it is pumped to sludge treatment facilities. The thickening step removes some of the water before processing the sludge in digesters.

Also read : Activated Sludge Process – Stages and Process Control

Scum Removal

Lighter materials rise to the surface as sludge settles to the bottom of the sedimentation tanks. The constituents of ‘scum’ are grease, oils, plastics, and soap. Scum is skimmed off the surface of the wastewater by slow-moving rakes. Scum is thickened before being poured into the digesters with the sludge.

Primary treatment removes about 60% of the total suspended solids and nearly 35% of BOD. It doesn’t remove the dissolved impurities. The waste must undergo secondary treatment in order to be completely free of toxic substances.

Also read : Biochemical Oxygen Demand

That’s it about primary treatment for wastewater. But, our trip doesn’t end here. Next, we move on to the secondary wastewater treatment plant – Secondary Treatment for Wastewater – Methods and Process. So, how was the trip? Let us know in the comment

Secondary Treatment for Wastewater – Methods and Process

Secondary treatment of wastewater removes the dissolved organic matter that escapes primary treatment and eliminates a higher percentage of suspended solids. In this blog, I will walk you through various biological methods used in the secondary treatment of wastewater.

Before diving deep into the biological treatment processes, make sure that you have a brief idea about the entire wastewater treatment process. So please go through our blog, Wastewater Treatment- Stages and Process full details.

What is Secondary or Biological treatment for wastewater?

Secondary or Biological treatments for wastewater remove organic pollutants using bacteria. In other words, bacteria consume organic matter as food and convert them to carbon dioxide, water, and energy towards their growth and reproduction. The elimination of soluble organic matter in the treatment plant aids in the preservation of receiving streams, rivers, or lake’s dissolved oxygen balance.

Secondary treatment of Wastewater -Flow Diagram
Secondary treatment of Wastewater -Flow Diagram

The decomposition of organic matter takes place in two ways as shown below:

  • Anaerobic Waste Water treatment
  • Aerobic Waste water treatment

Anaerobic Wastewater Treatment

Anaerobic wastewater treatment uses anaerobic microorganisms to break down and eliminate organic pollutants from wastewater. The anaerobic wastewater treatment process comprises of two stages:

  • Acidification
  • Methane generation

Anaerobes breaks down complex organic molecules into simpler, short-chain volatile organic acids during the initial acid-forming or acidification phase. The second phase that is the methane-production phase is further divided into two parts:

  • Acetogenesis
  • Methanogenesis

Anaerobes synthesise organic acids to produce acetate, hydrogen gas, and carbon dioxide by acetogenesis. These microbes then react with these newly created molecules to produce methane gas and carbon dioxide during methanogenesis.

Anaerobic systems are commonly utilised to treat waste streams containing significant levels of organic pollutants, as well as warm wastewater stream. It has several advantages over aerobic treatment systems in that it produces less overall sludge and generates valuable byproducts.

Aerobic Wastewater Treatment

The aerobic wastewater treatment systems use oxygen-feeding microorganisms to clean water. These systems take advantage of the natural microbial decomposition process to break down industrial wastewater pollutants and remove them.

Biochemical Oxygen Demand

The biochemical oxygen demand (BOD) gives a measure of the organic pollutants decomposed by the bacteria. BOD refers to the amount of dissolved oxygen required by aerobic organisms to break down organic matter into smaller molecules. BOD values beyond a certain threshold indicate a high concentration of biodegradable material in the wastewater.

Aerobic digestion is preferred for large quantities of dilute wastewater with BOD5 < 500 mg/L. For highly polluted wastewater (BOD> 1000 mg/L) anaerobic digestion is recommended.

Types of Secondary or Biological Treatment Methods

The three most commonly used aerobic secondary treatment procedures for wastewater are listed below:

  • Trickling filter
  • Activated sludge process
  • Oxidation pond

Now, let’s dig deeper into their features and working. Off, we go.

Trickling Filter

A trickling filter is an aerobic secondary wastewater treatment system that uses a biofilm of microbes attached to the filter media to remove organic pollutants. These systems are also known as attached-growth processes in contrast to the suspended growth systems wherein microbes are suspended in the effluent.

  • A trickling filter consists of a fixed bed of rocks, coke, gravel, slag, polyurethane foam, sphagnum peat moss, ceramic, or plastic media.
  • As the wastewater trickles down, bacteria collect and proliferate the media and form a layer of microbial slime (biofilm) to grow.
  • The constant flow of sewage over these growths allows bacteria to consume dissolved organics.
  • They release carbon dioxide gas, water, and other oxidised end products as wastewater pass over the media.
  • This decreases the sewage’s biochemical oxygen demand (BOD).
  • In addition, air moving upward via the crevices between the media supplies the necessary amount of oxygen for metabolic activities.
  • The microbial biofilm layer absorbs and adsorbs organic chemicals as well as some inorganic species such as nitrite and nitrate ions from the wastewater stream.
  • The bio-film layer requires dissolved oxygen for the biological oxidation of organic compounds.
  • As the thickness of bio-film increases, all the obtained oxygen depletes before reaching its base.
  • Thus, anaerobic conditions prevail at the base of the slime layer.
  • As a result, microbes enter into a decay phase and lose their attaching ability.
  • Subsequently, the film detaches and becomes part of secondary sludge. This process is called sloughing.
  • Trickling Filters find wide applications in milk processing, paper mill and pharmaceuticals.

Ever heard of a pond which treats wastewater? Let’s look at what’s happening inside such oxidation ponds.

Oxidation Ponds

Oxidation Ponds are man-made ponds that treat wastewater through the combined action of sunlight, microbes and oxygen to reduce the organic content and pathogens. It refers to a stabilisation pond that uses microbes to stabilise residential, commercial, and industrial wastes. It appears to be a vast shallow pond with a water body that is 2-6 feet deep. 

The industrial or domestic wastewater influents enter the oxidation pond via the inlet system. Subsequently, the bacteria transform the biodegradable organics into inorganic molecules through microbial interaction along with producing carbon dioxide. The bacteria that predominate in the stabilisation pond are Achromobacter, Proteus, Alcaligenes, Pseudomonas, Thiospirillum, Rhodothecae, etc.

Firstly, anaerobic bacteria transform insoluble organic waste into soluble organic acids such as ethanol in the absence of oxygen. Anaerobic bacteria decompose organic acids further, releasing H2S, NH3, CH4, CO2, and other gases. The non-biodegradable or solid organic wastes settling to the bottom of the stabilisation pond forms sludge.

Most ponds require both bacteria and algae to maximise the breakdown of organic materials and the removal of other contaminants. Algae produce oxygen during photosynthesis and consume it through respiration. But, they leave an excess of oxygen. Aerobic bacteria utilise this oxygen for respiration and organic matter oxidation activities.

The treated water exits through a stabilisation pond’s outlet system. The dredging process separates sludge deposits from the stabilising pond. Filtration method or a combination of chemical treatment and settling separates the algal and bacterial biomass.

Now, let’s move on to the various configurations of oxidation ponds.

Oxidation Pond Configurations

Waste stabilization ponds consist of man-made basins comprising a single or several series of anaerobic, facultative or maturation ponds. The main configurations of pond systems are:

  • A single facultative pond.
  • Anaerobic pond followed by a facultative pond.
  • Facultative pond followed by maturation ponds in series.
  • A series of maturation ponds preceded by an anaerobic pond and a facultative pond.
Oxidation Pond
Oxidation Pond

Anaerobic Pond

Anaerobic ponds are deep ponds (usually 3.0 to 5.0 m) that receive raw wastewater. Most of the solid matter in the wastewater settle to the bottom as sludge. Due to the depth of the pond, oxygen can’t penetrate to the bottom of the pond. Thus the sludge digestion takes place under anaerobic conditions.

Facultative Pond

After coming out of an anaerobic pond, the remaining solid particles in the wastewater settles into a larger but shallow pond called a facultative pond. Air and sunlight kill the harmful germs in the wastewater and makes it less dangerous to the aquatic flora and fauna.

Maturation Ponds

Maturation Ponds are two or three ponds in series wherein sunlight and oxygen destroy more harmful germs and make the liquid fit enough to be released for irrigation or into a river. The higher the number of maturation ponds, the cleaner is the effluent.

Activated Sludge Process

The Activated Sludge process employs aerobic microorganisms that can digest organic substances in sewage. Also, they have the ability to cluster together via flocculation. The flocculated particles settle out as sludge. As a result, the liquid coming out is relatively free of suspended solids and organic matter.

The sludge blanket becomes Return Activated Sludge (RAS) once it has settled. The RAS returns back to the primary clarifying tanks, where the bacteria in it aid in the breakdown of organic waste in the entering sewage.

activated sludge
activated sludge

Anaerobic sludge blanket reactors

  • A popular method used in the anaerobic secondary treatment for water.
  • The wastewater is carried across a free-floating “blanket” of suspended sludge particles in sludge blanket reactors, which are a type of anaerobic treatment.
  • Anaerobes in the sludge multiply and accumulate into larger granules that settle to the bottom of the reactor tank and can be recycled for future cycles as they decompose the organic contents in the wastewater.
  • The treated effluent rises and exits the unit.
  • Throughout the treatment cycle, collection hoods collect biogases produced by the degradation process.

Shall we wrap up?


We took a short trip through various secondary treatment methods for wastewater namely trickling filter, oxidation pond, activated sludge process and anaerobic sludge blanket reactors. The sludge coming out of these secondary wastewater treatment plants undergoes dewatering and digestion. Finally, the dried sludge finds uses in landfills and fertilizers.

That’s it about secondary treatment methods for wastewater. Hope you found it informative.

Activated Sludge Process – Stages and Process Control

Activated Sludge Process is a globally used wastewater treatment technique. In the previous blogs, I had shown you the various secondary wastewater treatment techniques. In this blog, we will dig deep into this widely used technique of activated sludge process, its configuration, process control and aeration methods.

Before starting make sure that you take a quick glance through the blog, Wastewater Treatment- Stages and Process full details for better understanding

The activated sludge process is a type of wastewater treatment that uses aeration and a biological floc made up of bacteria and protozoa to clean sewage or industrial waste waters. It is a biological process that finds applications in a variety of purposes, including oxidising carbonaceous biological matter and nitrogenous waste in the biological matter (mostly ammonium and nitrogen).

The activated sludge process employs aerobic microorganisms that can digest organic substances in sewage. Also, they have the ability to cluster together via flocculation. The flocculated particles settle out as sludge. As a result, the liquid coming out is relatively free of suspended solids and organic matter.

Activated Sludge Process Basic Configuration

The configuration of an activated sludge process for eliminating carbonaceous pollution consists of the following:

  • Aeration Tank: Air or oxygen is introduced into a mixture of primary treated sewage or wastewater combined with microbes (mixed liquor) in an aeration tank.
  • A settling tank: It is also known as a “secondary settling tank”. It separates biological sludge by allowing biological flocs to settle. A part of the sludge goes back to the aeration tank while the other for disposal.
  • Sludge Recycling System

Now, how about looking deep into what happens inside an activated sludge process?

Activated-sludge-Process - Flow diagram
Activated-sludge-Process – Flow diagram

Activated Sludge Process Steps

  • After primary treatment, wastewater enters into an aeration tank. A portion of sludge from the secondary settling tank also enters.
  • Organic matter comes into close contact with sludge from the secondary settling tank. Sludge is densely populated with microorganisms that are actively growing.
  • Diffusers or surface aerators inject air in the form of bubbles into the sewage-sludge mixture.
  • Microorganisms break down organic matter into stable chemicals like NO3, SO4, and CO2 while also producing new bacterial cells.
  • The effluent along with the actively growing microbial population passes to the secondary settling tank.
  • The secondary settling tank separates the aeration tank’s effluent, which contains flocculent microbial matter into supernatant and sludge. The treated supernatant undergoes further treatment before discharge.
  • This sludge from the settled waste returns to the aeration system’s inlet to re-seed the new wastewater reaching the tank. Return activated sludge (R.A.S.) is the fraction of the floc that returns to aeration tank.
  • The remaining sludge goes to sludge digesters for further treatment and safe disposal.

“Mixed liquor” refers to the combination of the liquid and microorganisms in the aeration tank. The suspended solids are called “Mixed Liquor Suspended Solids” (MLSS).

In the next section, we will find out the basic process control parameters in an activated sludge process.


Process Control in Activated Sludge Process

The general process control method monitors the following variables:

  • Sludge Volume Index (SVI)
  • Mean Cell Residence Time (MCRT)
  • Food to Microorganism Ratio (F/M)
  • Dissolved oxygen (DO)
  • Biochemical oxygen demand (BOD)
  • Chemical oxygen demand (COD)

Let me explain these parameters in detail.

Sludge Volume Index

Sludge Volume Index is the volume of settled sludge in milliliters occupied by 1g of dry sludge solids after 30 minutes of settling in a 1000 milliliter graduated cylinder. It gives a measure of the settling ability of the sludge. SVI ranges from 40 to 100 for a good sludge which settles down easily. Bulking Sludge is a biomass consisting of filamentous organisms with very poor settling characteristics. For a bulking sludge, SVI value can exceed 200. Sufficient pH control, adequate aeration and addition of hydrogen peroxide to the aeration tank prevents bulking.

Mean Cell Residence Time

Mean Cell Residence Time is the ratio of total mass (lbs) of mixed liquor suspended solids in the aerator and clarifier to the mass flow rate (lbs/day) of mixed liquor suspended solids leaving as final effluent.

Food to Microorganism Ratio

Food to Microorganism Ratio is the amount of organic matter fed to the microorganisms each day relative to the mass of microorganisms under aeration. In other words, it is the ratio of the amount of BOD fed to the aerator (lbs/day) and the amount (lbs) of Mixed Liquor Volatile Suspended Solids (MLVSS) under aeration. 

Main Control Parameters

The mean cell residence time and F/M Ratio are the main control parameters used industrially since both are directly related to the effluent quality. However, it is tedious to control the plant on the basis of the F/M ratio since it necessitates a lot of laboratory work to find the BOD and MLSS in the system. Therefore, the mean cell residence time is the best choice for controlling an activated sludge system.

Now you got an idea about the entire process and its important parameters. Next, we move on to the various aeration methods.

Aeration Methods in Activated Sludge Process

The decomposition of organic waste requires a very high concentration of oxygen at the initial stages of contact between microorganisms and the organic matter. The conventional systems usually maintain a plug flow hydraulic regime and keeps aeration and a mixing at an uniform rate along the entire tank. As a result, the oxygen concentration drops rapidly in the inlet and this can harm the microbes.

At the outlet, there is a surplus of oxygen which is not necessary and leads to economical losses. In order to match the oxygen supply and demand along the entire journey of wastewater from inlet to outlet, the mode of aeration needs some modifications. Let’s have a look at the different aeration methods in an activated sludge process.

Diffused Aeration

Sewage liquor is pumped into large tanks with floor-mounted diffuser grid aeration devices. Passing air creates a curtain of bubbles that oxygenates the liquor while also mixing it. An air blower usually creates the air. Oxygen replaces air for unusually strong and difficult to treat sewage.

Diffused aeration
Diffused aeration

Tapered Aeration

The organic waste needs more oxygen at the inlet. As it degrades progressively its oxygen demand decreases. Tapered aeration works on this principle. Aeration is intense at the inlet and decreases progressively along the length of the aeration tank. As this method involves the more efficient use of air, it results in savings in the pumping costs too.

Step Aeration

This method aims to equalize the oxygen supply and its demand. It introduces fresh feed at several points in the aeration tank, while keeping the rate of oxygen supply constant. This ensures a more even oxygen distribution over the entire tank and throughout the aeration stage. Baffles divide the aeration tank into several channels with each channel representing one step of the process.

Complete Mix Activated Sludge Process

In complete mix process, the aeration tank receives a mixture of fresh feed and recycled sludge at several locations within the tank. This ensures a constant supply and demand of oxygen along the length of the tank.

Contact Stabilisation

The microbial mass comes in contact with wastewater for short durations of time, approximately 0.5 to 1 hour in the biosorption unit. An anaerobic digestion unit stabilizes the resulting sludge after a retention period of about 2-3 hours. In the digestion unit, microbes consume the organic wastes removed in the biosorption unit. Since we stabilize the return sludge with higher solid concentrations, this reduces the volume of the aeration tank.

Pure Oxygen Activated Sludge Process

This type of activated sludge process supplies and recirculates pure oxygen in place of air into well mixed and converted chambers. Instead of the 5-10% oxygen utilization in conventional processes, the pure oxygen activated sludge process ensures about 90% utilization of oxygen. Further, it results in higher bacterial activity, lower sludge volume and sludge with better settling characteristics.

That’s it about activated sludge process. Let us know in the comments if you wish to know more.