Biological Processes for wastewater treatment

Biological Processes for wastewater treatment


  • An overview of biological wastewater treatment.
  • Important aspects in microbial metabolism.
  • Principal organisms responsible for wastewater treatment.
  • key factors governing biological growth and waste  treatment kinetics.

    Biological Processes for wastewater treatment
    Biological Processes for wastewater treatment


Biological TP: a method of contact between microbes and substrate. Suitable temperature, pH, nutrients etc. are required for microbial growth. Such a growth results into the ‘removal’ of substrate.

Objective  of biological treatment

  • Coagulate and remove the non-settle able colloidal solids .
  • Stabilize the organic matter.
  • Reduce the organic matter.
  • Remove the nutrients.

In short, stabilize organic matter: convert organic matter to nonbiodegradable form so that it does not exert oxygen demand.


  • Virtually every environmental niche
  • Extremes of pH and salinity
  • Extremes of temperature and pressure
  • Without air (Anaerobic)
  • Growth on many chemical substrates
  • Attached to surfaces in biofilms
  • Geothermal vents and subterranean deposits


  • General nutritional requirements -:
  • CARBON SUBSTRATE (org. or inorg.)
  • Need for molecular oxygen.
  • Basic elements required-C,O ,N,H, P,S
  • Inorganic elements: K,Mg,Ca,Fe,Na,Cl

Types of microbes

Depending on the energy and carbon source

  • AUTOTROPHS: microbes requiring inorganic carbonaceous compounds.
  • HETEROTROPHS: microbes requiring organic compounds .
  • PHOTOTROPHS: microbes consuming light as energy source .
  • CHEMOTROPHS: microbes obtaining energy from oxidation of org. or inorg. Compounds.
  • ORGANOTROPHS: organic compounds as source of electron.
  • LITHOTROPHS: inorganic compounds as source of electron.
  • E.g. –nitrifying bacteria is an example of chemolitho-autotrophs.


  • Environmental conditions that affect microbial growth.
  1. pH  2. Temp  3. Nutrients  4. Subs conc. & composition   5. D.O.    6. Contact / extent of mixing
  • Method of contact between microbes and substrate.
  • Quantification of growth and substrate removal
  • Method of separation of microbes and substrate after desired substrate removal is achieved



µ = µm S/(ks +S)


µ=specific growth rate coff.

µm =maximum growth rate coeff.

s =conc. of limiting nutrient.

ks  =half saturation coeff.

Kinetic design approach

  • Mass balance of MO at dX/dt=0 i.e., steady state growth
  • Mass balance of substrate at dS/dt =0 i.e., steady state substrate removal
  • Above equations are required to be “performed” over the “reactor” and “separator” system

µ, µm,K, ks, kd etc. are obtained experimentally before designing.

Activated Sludge Process

Activated Sludge Process is the suspended-growth biological treatment process, based on providing intimate  contact between the sewage and activated sludge.The Activated Sludge is the sludge obtained by settling sewage in presence of abundant O2 so as to enrich with aerobic micro-organisms.

Mean Cell Retention Time

The time for which the cells remain in the system. It is given as-

θc =  Mass of solids in system   /Mass of solids leaving system/day


  • The real “heart” of the activated sludge system is the development and maintenance of a mixed microbial culture (activated sludge) that treats wastewater and which can be managed.
  • The best approach to troubleshooting the activated sludge process is based on microscopic examination and oxygen uptake rate (OUR) testing to determine the basic cause of the problem or upset and whether it is microbiological in nature.


  • Bulking sludge
  • Rising sludge
  • Foaming
  • Poor floc formation, pin floc and dispersed growth.
  • Toxicity


  • It is a stage in which an over abundance of filamentous organisms is present in the mixed liquor in the activated sludge.
  • Types of sludge bulking

Zoogloea bulking – Zoogloea occur at high F/M conditions and when specific organic acids and alcohols are high in amount due to septicity or low oxygen conditions.

Viscous bulking – it is the result of excessive extracellular polymer substances by biomass, causing poor compaction and settling of biomass in secondary clarifiers, increased effluent BOD and poor sludge dewaterability.


  • Sludge rise occurs when bacteria common in the activated sludge floc respire using nitrate in place of free oxygen when it is lacking and release nitrogen gas as a by-product. This gas is only slightly soluble in water and small nitrogen gas bubbles form in the activated sludge and cause sludge blanket flotation in the final clarifier.


  • Nocardia and microthrix parcivella are two main filamentous organisms cause foaming.


  • The washing of cement or lime trucks to a manhole, dumping of congealed diesel fuel to the sewer system, and overload of small systems with septage (which contains a high amount of organic acids and sulfides which can be toxic).
  • Sulfide toxicity to activated sludge is more common than currently recognized. Sulfide may originate from outside the activated sludge system, from septic influent wastewater or from septage disposal, or it may originate “in-house”, from anaerobic digester flows or from aeration basins or primary or final clarifiers with sludge build-up and anaerobic conditions.
  • Hydrogen sulfide toxicity is highly pH dependent, due to the H2S form being the toxic agent and not HS

Modifications of ASP

Conventional plug flow:- Settled water and recycled activated sludge enter the head end of the aeration tank and are mixed by diffused air or mechanical aeration. During the aeration period adsorption, flocculation and oxidation of organic matter occurs.

Modified aeration:- It is similar to conventional plug flow except that shorter aeration time and higher F/M ratio are used.

Tapered aeration:- Varying aeration rates are applied over the tank length depending on the oxygen demand. Greater amounts of air are supplied to the head end of the aeration tank, and the amount diminish as  the mixed liquor approaches the effluent end.

Step feed aeration

Generally three or more parallel channels are used. The settled waste water is introduced at several point in the aeration tank to equalize the F/M ratio, thus lowering peak oxygen demand.

Extended aeration

It operate in the endogenous respiration phase of the growth curve, which requires a low organic loading and long aeration time.

Deep shaft process

  • It is a Process having a mechanism of great depth aeration (depth of 40 to 150 m as an aeration tank) and it is practiced where land  is in short supply.
  • It can treat the waste water at higher rate.
  • It is also known as a space efficient and energy efficient biological process.

Deep Shaft Advanced Biological Treatment Process

  • High intensity aerobic liquid effluent treatment process with oxygen transfer rate typically 10 times higher than conventional process.
  • Well suited for treatment of high strengths wastes with low operating and capital cost

Operational advantages of Deep Shaft

  • Oxygen rate transfer is 10 times higher than for conventional processes.
  • Full automation possible with minimal instrumentation.
  • Resistance to changes in flow rates, BOD loadings and toxins results in robust and reliable performance.
  • Capable of treating a wider range of liquor strengths (typically 1 – 30 kg BOD/m3 day compared with 0.4 -1.3 for conventional plants).
  • Can operate at higher MLSS concentration (3-10 g/l compared to 2-5 g/l) for conventional plants.
  • Design sludge loading (kg BOD per day/kg of MLSS) is higher, (0.7 to 3.5 compared with 0.1 to 0.5), and this reduces reactor size.
  • Limited growth of filamentous organisms means improved sludge settling and smaller clarifiers.
  • Less sludge produced per kg BOD removed.
  • No moving parts with low maintenance costs.
  • Overall cost effective high performance.

General advantages of Deep Shaft

  • Proven and reliable technology with more than 80 plants in operation.
  • Low capital and operating costs.
  • Low land area requirements.
  • Mechanical simplicity.
  • High energy efficiency i.e. 1-4Kg BOD/kwhr.
  • Environmental  friendly.


  • Lagoons are deep waste stabilization ponds -like bodies of water or basins designed to receive, hold, and treat wastewater for a predetermined period of time by artificial means of aeration.
  • In the lagoon, wastewater is treated through a combination of physical, biological, and chemical processes.


According to the microbial activity in the aerated lagoons-

  •   Aerobic aerated lagoons.
  •   Facultative aerated lagoons.


  • Dissolved oxygen is present throughout much of the depth of aerobic lagoons.
  • They tend to be much shallower than other lagoons.
  • They are better suited for warm, sunny climates, where they are less likely to freeze.
  • HRT = 3 TO 6 days.


  • Three types of zones are present

Aerobic Zone.

Anaerobic Zone.

Facultative Zone.

  • HRT is higher than aerobic lagoons because time requires for the solids to settle and for many pathogens viruses to either die off or settle out.

 Operation And Maintenance

For Facultative Lagoons

  • Most facultative lagoons are designed to operate by gravity flow. The system is not maintenance intensive and power costs are minimal because pumps and other electrically operated devices may not be required.
  • Earthen structures used as impoundments must be inspected for rodent damage.

For Aerobic Lagoons

  • Any earthen structures used as impoundments must be periodically inspected. If left unchecked, rodent damage can cause severe weakening of lagoon embankments.
  • In submerged diffused aeration, the routine application of  HCl gas in the system is used to dissolve accumulated material on the diffuser units
  • The use of submerged perforated tubing for diffused aeration requires maintenance and cleaning on a routine basis to maintain design aeration rates



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