Waste Water Treatment Sequential Process

  Overview : Wastewater Treatment in STP

  Primary Wastewater Treatment



Bar Screens are generally used for screening process. Screening is a preliminary step in wasteater treatment.

 Bar Screens are Parallel steel bars placed at 2-7cm apart it is followed by wire mesh screen with small opening to remove all floatable material from the wastewater. Screening is basically done to: 


  • To remove large floatable material
  • To prevent clogging and damage of pumps

Screening is followed by Comminuter.



Comminuter is a sort of grinder which grind all course material into small pieces. The step makes screening process more effective as it reduce any chance of clogging in further treatment steps.


  Grit Chamber

Grit is the course sand, gravel or any sharp granules whicg are abrasive for treatment process. They are heavy in nature thus can be settled by reducing the speed of wastewater flow.

Grit particles which are smaller than the aperture of the coarse screen will pass through and may cause abrasive problems to pipes and pumps.Detention time long enough to settle sand, grit and other heavy materials to settle but too short to let organics and lighter particles to settle.

In grit removal process, velocity is so controlled that grit may settle but most of the organics are retained in suspension.

Grit is often removed by

  • Mechanically cleaning
  • Manually cleaning

In mechanically cleaned grit chamber, scraper blades collect the grit settled on the floor of the grit chamber.The grit so collected is elevated to the ground level by several mechanisms such as bucket elevators, jet pump and air lift. Manually cleaned grit chambers should be cleaned at least once a week.The simplest method of cleaning is by means of shovel.

Aerated Grit Chamber

An aerated grit chamber consists of a standard spiral flow aeration tank provided with air diffusion tubes placed on one side of the tank. The grit particles tend to settle down to the bottom of the tank. Settling rates dependant upon the particle size and the bottom velocity of roll of the spiral flow.


  Primary Settling Tank

It is often known as Sedimentation Tank/Primary Clarifier/ Primary Settling Tank.

Objective is the removal of settelable organic and inorganic solids by Sedimentation, and the removal of the material that will float (scum) by skimming.

Flow speed is reduced to allow settling of Suspended Solids due to gravity. Detention Time : 2-3 hours.

Primary Treatment by Bar screens, comminuter, grit chamber and primary settling tank reduce:

  • Around 25-40% of the incoming BODs,
  • 50-65% of the total suspended solids and
  • 65% of oil and grease

Solids that settles in primary settling tank is called Primary Sludge/Raw suldge

Overflow rate = 15-30 m3/day per m2

Detention Time = tank volume/influent flow rate     

  Secondary Wastewater Treatment

Secondary Treatment is a biological process and can be done by either of the methods in comparison to Primary Treatment which involved physical and chemical methods for removal of pollutants. Conventional wastewater treatment plants (STP/ETP) in India use ASP as it is one of the most effective methods. We will be studing all the methods in details.

Biological Treatment

Secondary treatment is designed to substantially degrade the biological content of the sewage which are derived from the human waste, food waste, soaps and detergents etc.

Aerobic bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugar, fats, short chain carbon molecules etc.) and binds much of the less soluble fractions into floc

Some common techniques are:

  • Trickling Filters , Activated Sludge Process and Rotating Biological Contactor only after primary treatment
  • Oxidation Ponds



  Trickling Filters (TF)

Trickling filter is an attached growth process i.e. process in which microorganisms responsible for treatment are attached to an inert packing material. Packing material used in attached growth processes include rock, gravel, slag, sand, redwood, and a wide range of plastic and other synthetic materials.

The wastewater in trickling filter is distributed over the top area of a vessel containing non-submerged packing material

Air circulation in the void space, by either natural draft or blowers, provides oxygen for the microorganisms growing as an attached biofilm.

Bed is covered with layer of biological slime that adsorbs and consumes waste. Consist of bacteria, fungi, algae, protozoa, worms, insects, larvae, snails etc. The organic material present in the wastewater metabolised by the biomass attached to the medium. The biological slime grows in thickness as the organic matter abstracted from the flowing wastewater is synthesized into new cellular material

Slime periodically slides off individual rocks and collect at bottom along with treated water, from where it passes to Secondary settling tank

General dimensions of TF are Diameter 30m, Depth 3m

Plastic media replacing gravels, provide more surface area, are lighter, less land requirement.

TF are often called as Biological Towers


  Rotating Biological Contactor (RBC)

The rotating biological contactor (RBC) is a fixed film biological secondary treatment device. The basic process is similar to that occurring in the trickling filter. In operation, a media, consisting of a series of closely spaced circular, plastic disks (3.6m in diameter) mounted side by side on a common shaft is rotated through the wastewater flow

The surface of the disk is covered with a biological slime similar to that on the media of a trickling filter. RBC units are usually installed in a concrete tank so that the surface of the wastewater passing through the tank almost reaches the shaft. This means that about 40% of the total surface area of the disks is always submerged. The shaft continually rotates at 1 to 2 rpm, and a layer of biological growth 2 to 4 mm thick is soon established on the wetted surface of each disk. The organisms in the slime assimilate (remove) organic matter from the wastewater for aerobic decomposition.

The disk continues to rotate, leaving the wastewater and moving through the air. During this time, oxygen is transferred from the air to the slime. As the slime re-enters the wastewater, excess solids and waste products are stripped off the media as sloughings. These sloughings are transported with the wastewater flow to a settling tank for removal

Typically, a single contactor is not sufficient to achieve the desired level of treatment, so a group of contactors are used in series. Each individual contactor is called a stage and the group is known as a train. Most RBC systems consist of two or more trains with three or more stages in each. One major advantage of the RBC system is the level of nitrification that can be achieved if sufficient stages are provided.

During operation, observations of the RBC movement, slime color, and appearance are helpful in determining system performance; that is, they can indicate process conditions. If the unit is covered, observations are usually limited to that portion of the media that can be viewed through the access door. The following may be observed:

Gray, shaggy slime growth - indicates normal operation

Reddish brown, golden shaggy growth - nitrification

White chalky appearance - high sulfur concentrations

No slime - severe temperature or pH changes

In regard to typical performance, a well-maintained, properly operated RBC typically produces a high quality effluent with BOD at 8-95% and Suspended Solids Removal at 85-95%. The process may also reduce the levels of organic nitrogen and ammonia nitrogen significantly if designed for this purpose.

  Activated Sludge Process (ASP)

Activated sludge refers to biological treatment processes that use a suspended growth of organisms to remove BOD and suspended solids. The process requires an aeration tank and a settling tank

After primary clarifier the effluent goes to aeration tank which also received sludge from the secondary settling tank known as activated sludge

Primary effluent (or plant influent) is mixed with return activated sludge to form mixed liquor. The mixed liquor is aerated for a specified length of time (6-8 hrs). During the aeration the activated sludge organisms use the available organic matter as food producing stable solids and more organisms. Wastewater is now known as mixed liquor flows to the secondary settling tank.

The suspended solids, bacterial mass are separated by subsidence. A portion of bacterial mass is returned to the aeration tank to maintain the proper bacterial population and remainder is disposed off.


There are a number of factors that affect the performance of an activated sludge treatment system. These include:

Temperature, return rates, amount of oxygen available, amount of organic matter available,pH, waste rates, aeration time, wastewater toxicity

To obtain desired level of performance in an activated sludge system, a proper balance must be maintained between the amount of food (organic matter), organisms (activated sludge) and oxygen (dissolved oxygen). It offers great contact time between microorganisms and wastewater in a given volume of space, thus less land area requirement

Less expensive to construct and fewer problems of flies, odour, and have high BOD removal But high energy requirement for blowers thus high operating cost

  • Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new wastewater entering the tank
  • This fraction of the floc is called return activated sludge (R.A.S.). Excess sludge is called surplus activated sludge(S.A.S.) or waste activated sludge(W.A.S)
  • S.A.S is removed from the treatment process to keep the ratio of biomass to food supplied in the wastewater in balance
  • S.A.S is stored in sludge tanks and is further treated by digestion, either under anaerobic or aerobic conditions prior to disposal


  ASP over TF and RBC

Trickling filters and rotating biological contactors systems are effective unit processes in treating wastewater. However, trickling filters and RBCs are temperature sensitive, remove less BOD, and trickling filters cost more to build than activated sludge systems. Although they are more expensive to build, the activated sludge systems are much more expensive to operate because of the need for energy to run pumps and blowers.


  Removal Efficiency


  Oxidation Ponds

Oxidation ponds, also called lagoons or stabilization ponds, are large, shallow ponds designed to treat wastewater through the interaction of sunlight, bacteria, and algae.

Algae grow using energy from the sun and carbon dioxide and inorganic compounds released by bacteria in water. During the process of photosynthesis, the algae release oxygen needed by aerobic bacteria. Mechanical aerators are sometimes installed to supply yet more oxygen, thereby reducing the required size of the pond. Sludge deposits in the pond must eventually be removed by dredging. Algae remaining in the pond effluent can be removed by filtration or by a combination of chemical treatment and settling.

Oxidation ponds are shallow ponds, 1m – 2m deep, raw or partially treated sewage is decomposed by microorganisms.

Aerobic plus anaerobic conditions prevail then called Facultative Ponds. Deeper ponds which are mechanically aerated are called Lagoons (1Acre per 100 persons). They are easy to manage and are low in cost. Foul smell in winters due to less oxygen and photosynthesis activity

Oxidation Ponds may not always meet permissible limits and can also used to augment secondary treatment then they are called to be Polishing Ponds