left header



The solids present in the sewage are of two types viz.,

  1. Organic solids, and
  2. Inorganic solids.


Organic solids are the substances derived from living things like produces from plant and animal. Examples of organic solids are carbohydrate, protein, and fat. The organic solids undergo decomposition by the microorganisms. Inorganic solids are inert materials and they do not undergo decomposition. Examples of inorganic solids are grit, salt etc. Only 0.3 to 0.7% of solids are present in the sewage, if these solids are removed the water can be reclaimed and reused. The purpose of the sewage treatment is to remove the solids present in the sewage.


Microorganisms are unicellular microscopic living things. They multiply by binary division of cells within 10 to 20 minutes. They require oxygen for their respiration. They decompose the organic matter and convert them into cells. Examples of microorganisms are Bacteria, Fungi, Virus etc. There are two types of microorganisms. They are

  1. Aerobic bacteria, and
  2. Anaerobic bacteria.

Aerobic bacteria use dissolved oxygen (DO) from the water bodies for their respiration. They oxidize organic matter under aerobic conditions. The end products of the decomposition are water, CO2 and Cell tissues. Anaerobic bacteria use oxygen derived from chemical substances for their respiration. They multiply in the absence of DO in the water bodies. They oxidize the organic matter under septic conditions. The end products include fowl smelling gases like H2S, CH, etc.


The overall objectives of the biological treatment of domestic wastewater are:

  1. To oxidize or transform dissolved and suspended biodegradable substances into acceptable end products;
  2. To capture and incorporate suspended non-settleable colloidal solids into biological floc or bio film, and
  3. To transform and remove nutrients such as nitrogen and phosphorous.

The removal of dissolved and suspended carbonaceous BOD and the stabilization of organic matter found in wastewater is accomplished using a variety of microorganisms, principally bacteria. Microorganisms are used to oxidize the dissolved and suspended carbonaceous organic matter into simple end products and additional biomass. This is achieved by providing the favourable environment to microorganisms with food, DO, pH, temperature etc. The organic solids present in the wastewater serve as food for the aerobic microorganisms. The only thing to be provided is the DO, which is essential for the respiration of the aerobic organisms. In the biological treatment processes the DO is supplied either through natural means or by mechanical means by agitation.

Anaerobic organisms can multiply in the absence of DO and do the decomposition, but the end products are undesirable fowl smelling gases like H2S, CH, etc. Hence anaerobic decomposition process is not generally preferred. However, anaerobic treatments are also adopted in certain situations because of certain specific advantages. Examples of anaerobic treatment processes are Septic tanks, UASB, Anaerobic Sludge digesters.


The secondary treatment is designed to remove soluble organics from the wastewater. Secondary treatment consists of a biological process and secondary settling is designed to substantially degrade the biological content of the sewage such as are derived from human waste, food waste, soaps and detergent. The majority of municipal and industrial wastewater plants treat the settled sewage liquor using aerobic biological processes. For this to be effective, the microorganisms require both oxygen and a substrate on which to live. There are number of ways in which this can be done. In all these methods, the bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc particles.


Biological treatment systems are classified into (a) fixed film or attached growth system and (b) suspended growth systems.

Attached Growth System

In attached growth biological treatment systems the biomass is attached. Trickling filters and biological towers are examples of systems that contain biomass adsorbed to rocks or plastic. Wastewater is sprayed over the top of the rocks or plastic and allowed to trickle down and over the attached biomass, which removes materials from the waste through sorption and biodegradation. A related type of attached-growth system is the rotating biological contactor, where biomass is attached to a series of thin, plastic wheels that rotate the biomass in and out of the wastewater. This coating of microorganisms is able to trap and consume B.O.D. and ammonia in the wastewater.

In attached growth or fixed film systems, the microorganisms responsible for conversion of organic matter are attached to an inert packing material. Packing material used in attached growth processes include rock, gravel, sand and wide range of plastic and other synthetic material. Attached growth system can be operated as aerobic or anaerobic processes. The packing materials can be completely sumersed in liquid or not submerged, with air space above the biofilm liquid layer.

Fixed film systems are more able to cope with shocks in biological loading and provide higher removal rates for BOD and suspended solids than suspended growth systems.

Advantages of attached growth systems include

  • maintain a high density of biomass population,
  • increase the efficiency of the system without the need for increasing the mixed liquor suspended solids (MLSS) concentration, and
  • eliminate the cost of operating the return activated sludge (RAS) line.

Suspended Growth System

    In suspended growth systems the microorganisms responsible for treatment are maintained in liquid suspension by appropriate mixing methods. Typically, suspended growth systems require smaller footprints than fixed film systems for an equivalent capacity. There are a number of biological processes. The most common is activated sludge process in which microbes, also known as biomass, are allowed to feed on organic matter in the wastewater and remain in suspension. The make-up and dynamics of the microbial population is a function of how the ASP is operated.


    There are two types of biological treatment process; aerobic and anaerobic. Aerobic process means that oxygen is present for the microbes for respiration. Anaerobic process means that the process proceeds in the absence of DO. Aerobic and anaerobic biological systems are available in both attached and suspended growth configurations. Examples of the aerobic suspended growth systems are trickling filter and RBC. Aerobic suspended growth systems are activated sludge process, waste stabilization ponds etc. Anaerobic attached and suspended growth systems are, respectively, anaerobic filters and upflow anaerobic sludge blanket units.

    The end-products of aerobic and anaerobic processes are different. Under aerobic conditions, if completely oxidized, organic matter is transformed into non-hazardous products. But an anaerobic process can produce methane (CH4), which is explosive, and ammonia (NH3) and hydrogen sulfide (H2S), which are toxic. Some materials are better degraded under anaerobic conditions than under aerobic conditions. In some cases, the combination of anaerobic and aerobic systems in a series provides better and more economical treatment than either system could alone.

    Because the biomass has a specific gravity slightly greater than that of water, the biomass can be removed from the treated liquid by gravity settling. It is important to note that unless the biomass produced from the organic matter is removed on a periodic basis, complete treatment has not been accomplished because the biomass, which itself is organic, will be measured as BOD in the effluent. Biomass generated during biological treatment is settled in secondary sedimentation tank. This settled biomass or sludge is then piped to sludge-management systems. In activated sludge process part of the settled biomass is returned to the biological reactor in amounts needed to maintain the appropriate biomass level.

    Options in the Biological Processes

    Available options in the biological treatment processes of domestic sewage options are

    1. Trickling Filter (TF),
    2. Activated Sludge Process (ASP),
    3. Oxidation Ditch (OD),
    4. Aerated Lagoon (AL),
    5. Waste Stabilization Ponds (WSP),
    6. Up flow Anaerobic Sludge Blanket System (UASB),
    7. Moving Bed Biological Reactor (MBBR), and
    8. Membrane Biological Reactor (MBR)

    The first two methods are Conventional treatment processes, the next four methods are Low cost methods and the last two methods are emerging technologies.

    Trickling filters

    Trickling filters are intended to treat particularly strong or variable organic loads. They are typically circular filters filled with open stone or synthetic filter media to which wastewater is applied at a relatively high rate. The design of the filters allows high hydraulic loading and a high flow-through of air. On larger installations, air is forced through the media using blowers. The resultant liquor is usually within the normal range for conventional treatment processes.

    Activated sludge process

    The activated sludge process (ASP) is an aerobic biological wastewater treatment process that uses microorganisms, including bacteria, fungi, and protozoa, to speed up decomposition of organic matter requiring oxygen for treatment. In this process, microorganisms are thoroughly mixed with organics under conditions that stimulate their growth and waste materials are removed. Activated sludge plants use a variety of mechanisms and processes to use dissolved oxygen to promote the growth of biological floc that substantially removes organic material. A portion of the settled sludge is returned to the aeration tank (and hence is called return sludge) to maintain an optimum concentration of acclimated microorganisms in the aeration tank to break down the organics. It also traps particulate material and can, under ideal conditions, convert ammonia to nitrite and nitrate and ultimately to nitrogen gas.

    Oxidation ditch

    Oxidation ditch is an extended aeration ASP. It is a large holding tank in a continuous ditch with oval shape similar to that of a race-track. The ditch is built on the surface of the ground and is lined with an impermeable lining. With a detention time of more than 24 hours, the wastewater has plenty of exposure to the open air for the diffusion of oxygen. The liquid depth in the ditches is very shallow, 0.9 to 1.5 in, which helps to prevent anaerobic conditions from occurring at the bottom of the ditch.

    Aerated lagoon

    An aerated lagoon is a suspended-growth process treatment unit. The aerated lagoon system consists of a large earthen pond or basin that is equipped with mechanical aerators to maintain an aerobic environment and to prevent settling of the suspend biomass. Initially, the population of microorganisms in an aerated lagoon is much lower than that in an ASP because there is no sludge recycle. Therefore, a significantly longer residence time is required to achieve the same effluent quality.

    Waste stabilization ponds

    Waste Stabilization Ponds (WSPs), often referred to as oxidation ponds or lagoons, are holding basins where decomposition of organic matter is taking place naturally. A WSP is a relatively shallow body of wastewater contained in an earthen man-made basin into which wastewater flows and from which, after certain retention time a well-treated effluent flows out. The activity in the WSPs is a complex symbiosis of bacteria and algae, which stabilizes the waste and reduces pathogens. The algae produce oxygen during photosynthesis by utilizing carbondioxide and solar energy derived from sun light. The bacteria utilize oxygen for the biological process to convert the organic content of the wastewater to more stable and less offensive forms and release carbondioxide.

    Upflow anaerobic sludge blanket reactor

    UASB reactor is an anaerobic treatment system. In a UASB-reactor, the accumulation of influent suspended solids and bacterial activity and growth lead to the formation of a sludge blanket near the reactor bottom, where all biological processes take place. Two main features influencing the treatment performance are the distribution of the wastewater in the reactor and the “three-phase- separation” of sludge, gas and water.

    Moving Bed Biological Reactor

    Moving Bed Biological Reactor (MBBR) involves the addition of inert media into existing activated sludge basins to provide active sites for biomass attachment. This conversion results in a strictly attached growth system.

    Membrane Biological Reactors

    Membrane Biological Reactors (MBR) includes a semi-permeable membrane barrier system either submerged or in conjunction with an activated sludge process. This technology guarantees removal of all suspended and some dissolved pollutants. The limitation of MBR systems is directly proportional to nutrient reduction efficiency of the activated sludge process. The cost of building and operating a MBR is usually higher than conventional wastewater treatment.

    Secondary sedimentation

    The final step in the secondary treatment stage is to settle out the biological floc or filter material in a secondary sedimentation tank (SST) or secondary clarifier and produce sewage water containing very low levels of organic material and suspended matter


    5 Responses to “SECONDARY TREATMENT”

    1. Mehreen says:

      nice answer
      i want to know about difference between septic tank and activated sludge process can you send me plz


      Get above answer from http://www.sewagetreatment.us

    2. Sachin says:

      Dear sir
      please write swimming pool water treatment system and design.

    3. Water Treatment says:

      I am appreciate your interest in this field.Keep it up.We will update swimming pool water treatments in feature post.

    4. jabun says:

      Dear Sir,
      If coaltar is used in the sediment of aeration tank of activated sludge process?
      what iis the appropriate ratio or depth of microbial biofilm in raw or contaminated water?

    5. nancy tao says:

      Dear Sir:
      Very good explanation. It makes us very clear about BIOLOGICAL PROCESSES.
      nancy from Shanghai MegaVision Membrane

    Leave a Reply