Secondary treatment is the second stage of wastewater treatment. In primary treatment suspended solids, colloidal particles, oil and grease were removed. In secondary treatment biological treatment is done on the waste water to remove the organic matter present. This treatment is performed by indigenous and aquatic microorganisms like bacteria and protozoa which consume biodegradable soluble contaminants like sugar, fat, detergent, food waste. These processes are sensitive to temperature and with increase in temperature rate of biological reactions increases.

Secondary treatment is divided into two different treatment processes

  1. Aerobic treatment – Aerobic wastewater treatment is a biological treatment that uses oxygen to break down organic matter and remove other pollutants like nitrogen and phosphorus. Aerobic treatment technologies are1.
    1. Activated sludge process (ASP) / Extended Aeration System (EAS)
    2. Sequential batch Reactor (SBR)
    3. Moving Bed Biofilm Reactor (MBBR)
    4. Membrane Bioreactor (MBR)
  2. Anaerobic treatment – Anaerobic treatment is a process where wastewater or material is broken down by microorganisms without the aid of dissolved oxygen. However, anaerobic bacteria can and will use oxygen that is found in the oxides introduced into the system or they can obtain it from organic material within the wastewater. Anaerobic treatment technology is Upflow Anaerobic Sludge Blanket Reactor (UASB)

Extended aeration system/Activated Sludge Process

  • The scheme of the Extended Aeration process and its mixing regime are similar to that of the completely mixed process.
  • The process employs low organic loading, long aeration time, high MLSS concentration and low F/M. The BOD removal efficiency is high.
  • Because of long detention in aeration tank, the mixed liquor solids undergo considerable endogenous respiration and get well stabilized.
  • Due to higher requirement of oxygen there is an impact on the total operating cost of the system.
  • However, operation is rendered simple due to the elimination of primary settling and separate sludge digestion.
Aeration Tank

Sequential batch reactor (SBR)

  • The Sequential Batch Reactor is a fill and draw activated sludge system for wastewater treatment.
  • Equalization, Aeration and Clarification all can be achieved using a single batch reactor. SBR system is successful for large industrial effluents.
  • The SBR eliminates the need for secondary clarifier. Although SBR has shown more maintenance in operation since the process is PLC based, it shows higher efficiency and less area requirement for the ETP.
  • SBR combines the secondary treatment process and settlement. It is used to reduce the organic matter (BOD and COD), oxygen is bubbled with the mixture of wastewater and activated sludge. After this treatment, the treated water can be discharge on surface water.
Sequential Batch Reactor

Moving Bed Biofilm Reactor (MBBR)

A Moving Bed Biofilm Reactor (MBBR) consists of an aeration tank which is similar to an activated sludge tank with special plastic carriers that provide a higher surface area where a biofilm can grow. Read More…


Membrane Bioreactor (MBR)

Membrane Bioreactor – MBR is the combination of ultrafiltration (UF) and activated sludge process. MBR produces effluent of high quality which can be discharged to surface water for reuse. It can be retrofitted in existing installations. Read More…


Upflow anaerobic sludge blanket reactor (UASB)

  • Upflow Anaerobic Sludge Blanket (UASB) technology also known as UASB reactor is a form of anaerobic digester.
  • UASB reactor is a methane-producing digester, which uses an anaerobic process forming a blanket of granular sludge and is processed by the anaerobic microorganisms.
  • In case of higher organic loads, UASB helps to reduce higher extent of organic loads and further aerobic treatment can be taken for the remaining loads.
  • Generally, during the treatment of UASB reactor, the substrate passes through an expanded sludge bed which contains a high concentration of biomass first.
  • After that, the remaining part of substrate passes through a less dense biomass which named the sludge blanket.
  • The sludge circulation and mixing is done through sludge recirculation cum disposal pump. The excess granular sludge is wasted into sludge drying beds.

Anoxic tank

  • In order for denitrification to occur, anoxic conditions must exist. Anoxic conditions are deficient in dissolved molecular oxygen but it can contain chemically-bound oxygen as found in nitrate.
  • The dissimilation process of breaking down the nitrate molecule to make it chemically-bound oxygen requires both an electron donor and an electron acceptor. Nitrate gains (accepts) electrons and is reduced to nitrogen gas and a carbon source loses (donates) electrons and is oxidized to carbon dioxide.
  • The anoxic conditions simply ensure that nitrate will serve as an “electron acceptor” in the dissimilation process instead of oxygen.
  • In pre-anoxic basin influent wastewater, return sludge from the clarifier, and nitrate-rich mixed liquor pumped from the effluent end of the aeration tanks are mixed together.