ETP - Biological Process

Biological Unit Process
The objective of biological treatment of industrial wastewater is to remove, or reduce the concentration of, organic and inorganic compounds. Biological treatment process can take many forms (Table 4) but all are based around microorganisms, mainly bacteria. These microorganisms use components of the effluent as their “food” and in doing so break them down to fewer complexes and less hazardous compounds. In the process the microorganisms increase in number. There are two main types of processes, these involve suspended microbial growth (e.g. activated sludge) and attached microbial growth (e.g. fixed film). With both approaches large populations of microorganisms are brought into contact with effluent in the presence of an excess of oxygen. In both systems the microbial population has to be retained in a tank referred to as the reactor. With suspended growth systems microbes grow in small aggregates or “flock” (this is known as activated sludge). Activated sludge (AS) leaves the reactor with the treated effluent but is settled out in a clarifier and returned to the aeration unit to recycle the bacteria. If the amount of AS is excessive some may be disposed of rather than being recycled. In fixed film systems the microbial population grows as a thin layer (a “bio-film”) on the surface of an inert support medium. The classical fixed film system is known as a percolating or biological filter and uses small stones as a medium to support microbial growth. In the more modern system microbes grow on plastic supports. In the traditional percolating filters effluent is sprayed over the medium and trickles through a packed bed with oxygen entering from the air. In more recent reactor designs, the medium (usually plastic) is submerged in effluent and air is blown into the base of the reactor. Traditional percolating filters require large areas of land and are unlikely to be of use in Bangladesh due to land costs. Submerged fixed film reactors using plastic media require much less land. Fixed film systems require a final clarifier to remove particles of bio film that become detached from the medium. However, this material is not recycled to the reactor. While most of the activated sludge is recycled some may be surplus to requirements and needs to be disposed of, as does detached bio film from fixed film reactors. This material must be disposed of appropriately so that the pollutants now present in this sludge do not enter the water cycle. The treated liquid is discharged to the environment or taken for further treatment depending on the desired standard of effluent quality or the required use of the wastewater. Biological treatment plants must be carefully managed as they use live microorganisms to digest the pollutants. For example some of the compounds in the wastewater may be toxic to the bacteria used, and pre-treatment with physical operations or chemical processes may be necessary. It is also important to monitor and control pH as adverse pH may result in death of the microorganisms. The ETP must be properly aerated and must be operated 24 hours a day, 365 days a year to ensure that the bacteria are provided with sufficient “food” (i.e. wastewater) and oxygen to keep them alive. Like humans, microorganisms need a “balanced diet” with sources of carbon, nitrogen, phosphorus and sulfur. While textile wastes have enough carbon and sulfur (sulfate) they are generally lacking in nitrogen and phosphorous containing compounds. If the microorganisms are to grow and work effectively they are likely to need addition of nutrients. Normally materials such as urea and ammonium phosphate are added. It is possible to replace these nutrients by substituting the liquid portion of effluent from toilets, which is rich in nitrogen and phosphorus containing chemicals (the solid portion may cause problems). Both activated sludge and fixed film systems can produce high quality effluent but both have advantages and disadvantages. In the AS process, the settling and recycling of AS to the aerobic reactor is vital, and the settling process can be difficult to accomplish. Fixed film systems do not require recycling of biomass and so do not have this problem.