nutrient removal from sewage
5.2 rEgulAtory BAckground
5.4.1 description of alternative conventional Bnr processes and configurationsprocesses and configurations
5.4.1 description of alternative conventional Bnr processes and configurations
Biological nutrient removal (BNR) has emerged as the preferred approach for nutrient removal as it has much lower operating expenses compared to physico chemical processes. BNR is currently integrated within the biological treatment of municipal wastewater; nitrogen is removed through the biological processes of nitrification and denitrification, and is finally converted into a gaseous form and escapes into the atmosphere. In the nitrification bioreaction, ammonium (NH4+) is oxidized first to nitrite (NO2−) by the autotrophic ammonium oxidizing bacteria (Nitrosomonas) and then to nitrate (NO3−) by the autotrophic nitrite oxidizing bacteria known as Nitrobacter and Nitrospira. These reactions take place strictly under aerobic environment. During denitrification, nitrate is biologically reduced to nitric oxide (NO), nitrous oxide (N2O), and finally to nitrogen gas (N2) by heterotrophic bacteria in an anoxic environment (absence of oxygen, presence of nitrate/nitrite) (Metcalf & Eddy, 2003). Figure 5.1 summarizes the nitrogen related bioreactions that can take place in a WWTP. The two stage oxidation of ammonium to nitrate consumes alkalinity (7.14 g of CaCO3 are required to oxidize 1 g N) and requires oxygen. According to stoichiometry, 4.57 g O2 is required to oxidize 1 g N, if all the nitrogen was oxidized. However, the bacteria also require nitrogen for growth, thus reducing the oxygen requirements to 4.33 g O2 per g N removed.
The denitrification process requires anoxic conditions and the presence of organic carbon as electron donor. To denitrify nitrate to gaseous nitrogen the amount of organic carbon required is given by 2.86/(1 – YH) where YH is the heterotrophic biomass yield.
1. Aerobic ammonium oxidation to nitrite
2. Aerobic nitrite oxidation to nitrate
3. Nitrate reduction to nitrite 4. Nitrite reduction to nitric oxide 5. Nitric oxide reduction to nitrous
oxide
6. Nitrous oxide reduction to N2 7. Anoxic ammonium oxidation to
N2 (Anammox reaction).
Figure 5.1 Nitrogen bioreactions which can take place in a WWTP.
Phosphorus removal from wastewater effluents within WWTPs can be achieved in two fundamentally different ways: by chemical precipitation and
Integration of energy efficient processes 75 by enhanced biological removal. In both ways, phosphorus is trapped in the solid matrix and is then separated from the liquid through the subsequent secondary sedimentation process. Chemical removal of phosphorus is accomplished through the addition of aluminium and iron coagulants or lime to form phosphorus precipitates. It is a flexible process for phosphorus removal which can be implemented at various stages, in the WWTP. Specifically, the chemicals can be applied (i) before primary sedimentation, (ii) directly inside the biological treatment process or (iii) at the secondary effluent, as a tertiary treatment process. In the first case, the phosphorus precipitates in the primary settling tank and is thus removed with primary sludge. In the second case phosphorus precipitates in the mixed liquor and is removed with waste activated sludge. The last case is less often practiced due to increased costs and the need for an additional separation stage (Morse et al. 1998). The chemical removal of phosphorus has the disadvantages of having higher operating expenses, producing more sludge and resulting in the addition of chemicals compared to biological phosphorus removal (US EPA, 2007). In the enhanced biological phosphorus removal (EBPR) process, phosphorus accumulates in activated sludge by phosphorus accumulating organisms (PAOs). These bacteria have the ability to accumulate much more phosphorus (up to 10% of their dry weight) compared to normal bacteria. This is accomplished through the implementation of a sequence of anaerobic and aerobic conditions. Under anaerobic conditions, the PAOs break the polyphosphate chains stored in their cells to generate energy;
as a result phosphorus is released from the solid to the liquid phase. Then PAOs use the generated energy to convert the readily biodegradable organic matter (volatile fatty acids) that is present in the liquid into organic carbon which is stored internally in the form of polyhydroxyalkanoates (PHAs) as an energy and food reserve. Then, under aerobic conditions the PAOs utilize PHA as an energy reserve to uptake phosphorus (Metcalf & Eddy, 2003). Figure 5.2 depicts diagrammatically the EBPR process.
Figure 5.2 Diagrammatic representation of the enhanced biological phosphorus removal process.
Based on the above biological principles of BNR various technologies and configurations have been successfully adopted for biological nitrogen and phosphorus removal. Figure 5.3 shows the various configurations, for both suspended and attached growth processes, that can be used to remove nitrogen from wastewater. The existing conventional nitrogen removal technologies can decrease the treated effluent nitrogen concentrations to levels lower than 10 mg/L. The modified Ludzack Ettinger (MLE) is a suspended growth process, with an anoxic reactor where denitrification occurs followed by an aerobic reactor where nitrification takes place. The recirculation of mixed liquor to the anoxic reactor provides the required nitrates for denitrification. During aerobic conditions, the aerobic biodegradation of the organics takes place together with nitrification. The organic source required for denitrification of nitrate is provided by the influent sewage. The MLE can also be combined with simultaneous nitrification/denitrification (MLE/SND). In SND micro-aerobic conditions prevail, resulting in aerobic conditions at the exterior of the flocs and anoxic conditions in the interior. Thus, nitrification and denitrification takes place within the same reactor simultaneously. The four stage Bardenpho process is a suspended growth process with the sequence of anoxic/aerobic/anoxic/aerobic which could also include a SND stage. An external organic carbon source should be added in the second anoxic reactor to accomplish denitrification at a significant rate. This configuration having two aerobic and two anoxic zones results in higher nitrogen removal and in an effluent with lower nitrogen concentration than the two zone. The moving bed biofilm reactor (MBBR) is an attached growth process where many biofilm carriers are suspended inside the biological reactor. The biofilm develops on the surface of these carriers. Suitable anoxic and aerobic conditions can be maintained to favour the nitrogen removal processes. The biological aerated filter (BAF) is an attached growth process consisting of submerged media filter. Wastewater is pumped through the filter media and is treated as it comes into contact with it. The media filter provides a surface for microorganisms to grow on. Aeration is provided from the bottom part of the reactor. The BAF is a flexible system where filtration and biological treatment of wastewater are combined. The rotating biological contactor (RBC) is an attached growth process in which biofilm develops on parallel rotating disks mounted on a rotating shaft (Barnard, 2006). The sequencing batch reactor (SBR) process is a suspended growth process in which all processes take place within the same reactor in a sequencing manner. The sequence that is followed is: fill, reaction phase, sedimentation and decant. In the reaction phase aerobic and subsequently anoxic conditions can be maintained for nitrogen removal.
The oxidation ditch is a continuous flow suspended growth process with uses looped channels to create time sequenced anoxic and aerobic zones to remove nitrogen. Nitrogen removal is also integrated in membrane bioreactor (MBR) processes. In these cases, suitable anoxic and aerobic reactors are integrated with the membrane modules. The latter can be placed within the aerobic
Integration of energy efficient processes 77 tank, in a separate reactor or as an external membrane module in the external configuration.
Figure 5.3 Attached and suspended biomass configurations which can be applied to remove nitrogen (SND = simultaneous nitrification/denitrification, C = dosing of external organic carbon).
Several configurations for the EBPR have been invented, developed and implemented (Figure 5.4). In practice, the EBPR is successfully implemented when the alternation of anaerobic/aerobic is ensured. Any bioprocess in which the nitrate/
nitrite is prevented from entering the anaerobic bioreactor and an adequate readily biodegradable COD is provided during the anaerobic phase can accomplish EBPR (Barnard, 2006). Therefore, EBPR can be integrated within any of the configurations described above for nitrogen removal, provided a dedicated anaerobic phase is established. The phosphorus removal configurations shown in Figure 5.4 place the anaerobic reactor upstream of the biological process. This has the advantage that volatile fatty acids present in sewage can be used by the phosphorus accumulating organisms (PAOs). The simplest configuration is the Phoredox (A/O) process with one anaerobic reactor placed before the aerobic one. In the SBR process, EBPR can be accomplished by including an anaerobic phase just after the filling and before the aerobic reaction. Thus, the sequence of anaerobic, aerobic, anoxic phase in an SBR can successfully remove phosphorus and nitrogen from wastewater. The two and four stage Bardenpho processes can be modified to remove phosphorus by inserting an anaerobic reactor upstream. These configurations are known as the three and five stage modified Bardenpho processes (Wentzel et al. 2008).
In the University of Cape Town (UCT) process and its modified version (which has two anoxic tanks), an internal recycle of nitrate is carried out from the aerobic tank to the anoxic one in order to achieve effective denitrification. Furthermore, to minimize the entrance of nitrates in the anaerobic zone, the mixed liquor is recycled from the anoxic tank to the anaerobic one and the settled sludge is returned to the anoxic tank.
By manipulating the nitrate recycle ratio, the nitrate concentration in the anoxic tank Figure 5.3 (Continued)
Integration of energy efficient processes 79 can drop to zero. As a result, the anoxic recycle will not introduce any nitrate into the anaerobic tank. The modified version includes two anoxic tanks in order to have better control of the recycle. In the Johannesburg configuration, the elimination of nitrates in the mixed liquor recycle to the anaerobic tank is accomplished by a second anoxic tank which removes the nitrate from the recycle. The Biodenipho/Biodenitro process is a phased isolation ditch system where nitrogen removal is accomplished by the phased aerobic/anoxic reactors which are interchanged. The anaerobic conditions are accomplished upstream in a separate tank (Wentzel et al. 2008).
Figure 5.4 Configurations applied in WWTPs to removal phosphorus biologically (most schemes also integrating nitrogen removal).