Bacteria are the most versatile, and most important of all the organisms associated with wastewater treatment, both in terms of the conditions under which they can thrive, as well as the substrates they can metabolise. Bacteria form the basic trophic level in all biological treatment activated sludge systems and thus form the major proportion of the biomass. The dominant bacteria are the aerobic heterotrophs that degrade and eventually mineralise organic compounds present in wastewater to carbon dioxide and water. The overall population of these bacteria cannot be quantified as cultural methods do not account for the non-viable bacteria that remain active within the treatment system.
The bacterial communities of activated sludge systems are far more specialised and complex than those associated with fixed-film reactors and also have a lower diversity. There is a vast variety of bacteria in activated sludge however they generally comprise of two major types:
• Dispersed species are constantly removed by protozoan grazing and by being discharged with the effluent. The dispersed species grow faster than the second major type, the flocculating bacteria.
• Floculating bacteria, form flocs which are retained in the system. Although the activated sludge process selects the flocculating bacteria in preference to the dispersed species, the latter play a major part in substrate utilisation.
Bacteria remove nitrogen from wastewater by a 2-step biological processes: nitrification followed by denitrification. These processes occur under different environmental conditions by different microbial populations.
The nitrifying bacteria, encompasses two groups of microbes, the ammonia-oxidising bacteria (AOB) and the nitrite-oxidizing bacteria (NOB).
The preliminary stage of nitrogen transformation is when organic nitrogen in the form of urea is converted to ammonia (NH4) by a process called hydrolysis.
In the initial step, ammonia is oxidised to nitrate in 2 phases (Wong et al., 2003), i.e ammonia is oxidised to nitrite via the AOB (eg. Nitrosomanas, Nitrosococcus, Nitrosospira, Nitrospina), subsequently the produced nitrite is then oxidised to nitrate by the NOB (eg. Nitrobacter, Nitrospira) in the second phase. The nitrites are rapidly assimilated by the organisms and converted to nitrate, hence the nitrite levels in the mixed liquor are usually very low.
Nitrifying bacteria are slow growing, and are extremely sensitive to their surrounding environment. Disturbances in operational conditions (eg. pH, imbalances in the oxygen supply and temperature shifts), may cause a total loss of the nitrification population hence a breakdown of the nitrification process.
Nitrification occurs only under aerobic conditions at dissolved oxygen levels of 1.0 mg/L or more. At dissolved oxygen (DO) concentrations less than 0.5 mg/L, the growth rate is minimal. Nitrification requires a long retention time, a low food to microorganism ratio (F:M), a high mean cell residence, and adequate buffering (alkalinity). A plug-flow, extended aeration tank is ideal.
The nitrification process produces acid. This acid formation lowers the pH of the biological population in the aeration tank and can cause a reduction of the growth rate of nitrifying bacteria. The optimum pH for Nitrosomonas and Nitrobacter is between 7.5 and 8.5; most treatment plants are able to effectively nitrify with a pH of 6.5 to 7.0. Nitrification stops at a pH below 6.0.
Denitrification occurs when oxygen levels are depleted and nitrate becomes the primary oxygen source for microorganisms. The process is performed under anoxic conditions, when the dissolved oxygen concentration is less than 0.5 mg/L, ideally less than 0.2. When bacteria break apart nitrate (NO3-) to gain the oxygen (O2), the nitrate is reduced to nitrous oxide (N2O), and, in turn, nitrogen gas (N2). Since nitrogen gas has low water solubility, it escapes into the atmosphere as gas bubbles. Free nitrogen is the major component of air, thus its release does not cause any environmental
Optimum pH values for denitrification are between 7.0 and 8.5. Denitrification is an alkalinity producing process thus partially mitigating the lowering of pH caused by nitrification in the mixed liquor. Since denitrifying bacteria are facultative organisms, they can use either dissolved oxygen or nitrate as an oxygen source for metabolism and oxidation of organic matter. If dissolved oxygen and nitrate are present, bacteria will use the dissolved oxygen first thus successful denitrification will only occurs under anaerobic or anoxic conditions.oncern.
Conditions that affect the efficiency of denitrification include nitrate concentration. anoxic conditions, presence of organic matter, pH. temperature (denitrification can occur between 5 and 30°C) alkalinity and the effects of trace metals. Denitrifying organisms are generally less sensitive to toxic chemicals than nitrifiers, and recover from toxic shock loads quicker than nitrifiers.
In the biological removal of phosphorous, some microbes in activated sludge can take up larger amount of P than required for their growth and store it intracellularly in the form of polyphosphate under cyclic anaerobic and aerobic conditions which is subsequently removed from the process as a result of sludge wasting. Anreactor configuration provides the PAO with a competitive advantage over other bacteria, wherein PAO are encouraged to grow and consume phosphorous. The reactor configuration in comprised of an anaerobic tank and an activated sludge activated tank. PAO can still survive with P-limited condition by modifying their metabolism, making it as a highly adaptable group of microorganisms
GAOs are a group of functionally and numerically important bacteria found in EBPR plants that are also capable of anaerobic VFA uptake and conversion to PHA. GAOs do not release P anaerobically, nor do they take up P aerobically, hence they do not contribute to P removal. They instead hydrolyze glycogen as their sole source of energy for anaerobic VFA uptake.
Candidatus ‘Competibacter phosphatis’, and two other GAOs of the Alphaproteobacteria phylum (closely related to Defluvicoccus vanus and to Sphingomonas respectively), are distinguished by its ability to assimilate substrate anaerobically for PHA synthesis, but incapable of accumulating P aerobically.
Polyphosphate accumulating organisms (PAOs) release phosphorus anaerobically during volatile fatty acid (VFA) uptake and excessively take up phosphorus aerobically.
The selection of PAOs over GAOs would result in more efficient and reliable EBPR processes. An important factor to impact on the balance between PAO and GAO is the concentration ratio of influent organic carbon to orthophosphate (COD/P ratio). A high COD/P ratio may reduce the growth of PAOs leading to proliferation of the GAO. Thus a low COD/P ratio (10-20 mgCOD/mgP) favors the growth of PAO, although an adequate amount of COD must still be provided to achieve high P removal performance in EBPR systems.
Furthermore, each GAO community displays different preferences for acetate and propionate uptake. Thus, one strategy to eliminate GAO competition involved regular switching of the carbon sourcAnother factor which controls the population of GAO and PAO is . A high pH has been reported to advantageous to PAO and a low pH (<7.0) the GAO (Oehmen et al., 2007). It has been widely reported that EBPR performance deteriorates at elevated temperature (> 20°C), hypothesized to result from a shift in the community from PAO to GAO e from acetate to propionate.