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.

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