Biological Treatment Services


The principal objective of wastewater treatment is to allow human and industrial effluents to be disposed of without danger to human health or unacceptable damage to the natural environment. Wastewater from households and industry, commonly together with water runoff from urban areas, is conveyed by the sewerage system to sewage treatment plants for safe and economic treatment of sewage, and treatment and disposal of the resulting sludge.

Conventional wastewater treatment consists of a combination of physical, chemical, and biological processes and operations to remove solids, organic matter and, sometimes, nutrients from wastewater. The concept of all biological methods of wastewater treatment is to introduce contact with bacteria (cells), which feed on the organic materials in the wastewater, thereby reducing its BOD (Biochemical Oxygen Demand) content. In other words, the purpose of biological treatment is BOD reduction. Typically, wastewater enters the treatment plant with a BOD higher than 200 mg/L, but primary settling has already reduced it to about 150 mg/L by the time it enters the biological component of the system. It needs to exit with a BOD content no higher than about 20-30 mg/L, so that after dilution in the nearby receiving water body (river, lake), the BOD is less than 2-3 mg/L. Thus, the biological treatment needs to accomplish a 6-fold decrease in BOD. Simple bacteria (cells) eat the organic material present in the wastewater. Through their metabolism, the organic material is transformed into cellular mass, which is no longer in solution but can be precipitated at the bottom of a settling tank or retained as slime on solid surfaces or vegetation in the system. The water exiting the system is then much clearer than it entered it.

A key factor in the operation of any biological process is an adequate supply of oxygen. Indeed, cells need not only organic material as food but also oxygen to breathe, just like humans. Without an adequate supply of oxygen, the biological degradation of the waste is slowed down, thereby requiring a longer residency time of the water in the system. For a given flowrate of water to be treated, this translates into a system with a larger volume and thus taking more space.

General terms used to describe different degrees of treatment, in order of increasing treatment level, are preliminary, primary, secondary, and tertiary and/or advanced wastewater treatment. The biological wastewater treatment is achieved in three stages; primary, secondary and tertiary treatments.

  1. Primary Treatment :
    The first stage of treatment is known as primary treatment which also includes certain preliminary operations such as flow equalization, grease removal, flow measurement, etc. The unit operations in primary treatment are screening to remove larger floating objects, grit removal for removing inert sand and inorganic particles, and settling for removing settleable suspended organic solids. The main purpose of the primary treatment is to produce a generally homogeneous liquid capable of being treated biologically and a sludge that can be separately treated or processed.
  2. Secondary Treatment :
    The next stage of treatment is secondary treatment, which is designed to remove soluble organics from the wastewater. Secondary treatment consists of a biological process and secondary settling. Secondary treatment is designed to substantially degrade the biological content of the sewage such as derived from human waste, food waste, soaps and detergent. 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.
  3. Tertiary Treatment :
    Third stage treatment is referred to as tertiary treatment or advanced treatment. More commonly used advanced systems are adsorption to activated carbon, filtration through sand and other media, ion exchange, various membrane processes, nitrification-denitrification, coagulation-flocculation, and micro-screening.

Types of Biological Treatment based on Process

There are two types of biological treatment process; aerobic and anaerobic.

Biological Aerobic Treatment

Biological wastewater treatment is an extremely cost effective and energy efficient system for the removal of BOD (Biological Oxygen Demand), since only micro-organisms are used. These feed on the complex materials present in the wastewater and convert them into simpler substances, preparing the water for further treatment. Aerobic wastewater treatment is a biological process that takes place in the presence of oxygen. Aerobic wastewater treatment encourages the growth of naturally-occurring aerobic microorganisms as a means of renovating wastewater. Such microbes are the engines of wastewater treatment plants. Organic compounds are high-energy forms of carbon. The oxidation of organic compounds to the low-energy form (carbon dioxide) is the fuel that powers these engines. Understanding how to mix aerobic microorganisms, soluble organic compounds and dissolved oxygen for high-rate oxidation of organic carbon is one of the fundamental tasks of wastewater engineers.

Since anaerobic treatment is preferred when the dissolved organic concentrations of untreated wastewater are high, aerobic treatment is often used as a secondary treatment process and follows an anaerobic stage. Aerobic treatment consists of activated sludge processes or oxidation lagoons. The size of these can be reduced and tolerance against fluctuations and toxics increased by adding a step with moving bed bioreactors (MBBR) prior to the active sludge treatment.


  • Energy efficient
  • Cost effective
  • Can be used in combination with anaerobic processes

Biological Anaerobic Treatment

Anaerobic wastewater treatment is the biological treatment of wastewater without the use of air or elemental oxygen. Many applications are directed towards the removal of organic pollution in wastewater, slurries and sludges. The organic pollutants are converted by anaerobic microorganisms to a gas containing methane and carbon dioxide, known as 'biogas'. Anaerobic treatment is a slow process and can take up to 3 months, also due to septic decomposition, unpleasant odours may occur.

There are a variety of different technologies that can perform biological wastewater treatment. As technology advances, these systems are becoming increasingly convenient, more efficient, and smaller in size.

Most common available options in the biological treatment processes of domestic sewage options are

  • Trickling Filter (TF)
  • Activated Sludge Process (ASP)
  • Submerged Aerated Filter (SAF)
  • Moving Bed Biological Reactor (MBBR), and
  • 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 ultimately to nitrogen gas.


Submerged Aerated Filter

Submerged Aerated Filter (SAF) is a well proven technology for wastewater treatment. Technology is seen as the simplest and most cost effective method of commercial and residential sewage treatment / waste water treatment, particularly for small to medium sized treatment plants where available land is limited, and where full time operational manning would be uneconomical. A well-built Submerged Aerated Filter plant has no moving parts within its main process zones, any serviceable items will be positioned to access easily without disrupting the ongoing sewage treatment particularly when using random filter media. Submerged Aerated Filter (SAF) technology is a process used to reduce the organic loading of residential and commercial sewage / waste water, and in doing so will reduce the Biological Oxygen Demand (BOD) and a significant quantity of Suspended Solids (SS) which if otherwise untreated would contaminate river and sea outfalls, in other words it is used to substantially improve effluent discharge quality.

As with traditional effluent treatment, Submerged Aerated Filter technology uses three stages of dealing with commercial and residential sewage / waste water:

  • Primary Settlement Where larger solids settle into the bottom of the primary tank and are removed periodically as sludge, and where other buoyant materials float upwards to be removed usually by a scraping/screening method.
  • Secondary Treatment Where larger solids settle into the bottom of the primary tank and are removed periodically as sludge, and where other buoyant materials float upwards to be removed usually by a scraping/screening method.
  • Final Settlement / Clarification Where remaining solids (Humus) are settled out of the biological treated effluent.

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.

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