Biological Wastewater Treatment: Processes and Applications
Biological Treatment
Biological treatment processes (BTPs) involve the destruction and conversion of organic chemical and biological pollutants. Inorganic chemicals cannot be destroyed by biological treatment, but they can potentially be converted in valence or oxidative state into compounds that meet permit levels, or inorganic pollutants that can be stripped from organic compounds biologically. In biological treatments, organic or hydrocarbon food serves as an energy source, or electron donor, to the bacteria, and then oxygen, nitrite, nitrate, sulfate, or carbon dioxide serves as the electron acceptor.
Why Use Biological Treatment?
This integral part of any wastewater treatment plant has economic advantages, such as reductions in soluble BOD5 or nutrient concentrations.
- Microorganisms are used for removing contaminants from wastewater through biochemical reactions.
Types of BTPs
- Suspended growth: Microorganisms are maintained in a liquid form; for example, activated-sludge process (floc size: 50–200 mm) and aerated lagoons.
- Attached growth (or biofilm process): Microorganisms are attached to an inert packing material (i.e., a biofilm) as part of aerobic or anaerobic processes (e.g., trickling filters and rotating biological contactors).
Treatment Functions
- Nitrification: ammonia (NH3) → nitrite (NO2) → nitrate (NO3).
- Denitrification: Nitrate (NO3) is reduced to nitrogen and other gaseous end products.
- Stabilization is known as aerobic or anaerobic digestion.
Factors for Optimum Biological Growth
- pH is 6.5–7.5.
- No toxic materials are present.
- Mixing is critical, especially in suspended-growth systems.
- Any deficiency in nutrients or environmental factors inhibits biological growth and causes a loss of process efficiency.
Applications to Practical Aerobic Biological Treatment Systems
F/M ratio:
- In suspended-growth systems: organism concentration = MLSS (mixed liquor suspended solids) or a volatile fraction (MLVSS):
F/M =
Factors to Consider in Aerobic Biological Treatment Processes
- Residual food (substrate, S); mass of residual bacteria (MLVSS or Xv) → F/M ratio
Anaerobic Wastewater Treatment
- Utilizes anaerobic microbes to biologically degrade organics in wastewater.
- Speed of treatment depends on the slowest stage of bacterial reactions.
- Methane gas production is a rate-limiting and controlling factor.
- This wastewater treatment is used to stabilize biological sludge prior to dewatering and/or final disposal; it is also used to treat a variety of industrial wastes.
- Organic matter is biologically decomposed to CO2 and methane in the absence of O2.
- Anaerobic wastewater treatment is designed to treat high concentrations of organic waste matter and is the most economical method for treating concentrated wastes.
- This treatment is also used for organic sludge removed from wastewater in primary sedimentation and in final sedimentation following aerobic biological treatment.
Advantages and Disadvantages of Activated Sludge Processes
| Resistant to organic and hydraulic shock loads | High energy consumption/electricity |
| Can be operated at a range of organic and hydraulic loading rates | High capital and operating costs |
| High reduction of BOD and pathogens after secondary treatment | Needs to be operated and maintained by skilled technicians |
| High nutrient removal possible | Complicated chemical and microbiological problems |
| High effluent quality | Not suitable for applications at a community |
| Little land required compared to extensive natural system (e.g., waste stabilization ponds) | Needs experts for designs and construction |
| Can be modified to meet specific discharge limits | Necessary for further treatment of sludge |
Suspended Growth Biological Treatment Systems
- Process design considerations: reactor type; SRT (solids retention time); loading criteria, nutrients; sludge production; oxygen requirements and transfer; settling characteristics of biosolids; and effluent characteristics
SRT (Solids Retention Time): an average period of time during which the sludge has remained in the system!