Some differences between conventional activated sludge processes and those which incorporate nutrient
removal are: nutrient removal processes require a lower amount of air sludge retention times are lower for nutrient removal processes only conventional processes require primary settling conventional processes are carried out in a single stage, while the nutrient removal processes require
several stages.
Regarding the sludge produced in biofilm processes, it is true that: it is usually recirculated to the bioreactor it is a stabilized sludge it settles more easily than that of activated sludge processes its generation is lesser than that of activated sludge processes .
The rotary biological contactors: may be used in a treatment with several stages are completely submerged into the wastewater do not need secondary clarification are mainly used for large wastewater treatment plants.
The higher is the organic loading applied to a filter: the higher is the BOD5 concentration of the effluent the higher is the nitrification efficiency the higher is the nitrate concentration of the effluent the lower is the BOD5 removal efficiency.
In the biological treatment by trickling filters: the biofilm is constantly submerged in the wastewater the packing material has an active role in the biological process of wastewater treatment sludge recirculation is usually used the air supply can be done by natural convection .
The differences between the A/O process for P removal and the Ludzack-Ettinger process for N removal are
the following: the number and type of sludge recycle streams the slugde retention time the order and number of the reactors necessary to carry out the wastewater treatment the residence time of the wastewater in the reactors.
Which are the reasons to include an internal recirculation in the modified Ludzack-Ettinger process? to improve the nitrogen removal efficiency to improve the nitrate concentration in the anoxic stage to reduce the dinitrogen concentration in the anoxic stage to reduce the growth of autotrophic bacteria in the anoxic stage.
The reasons to use internal recycle in the biological processes for nitrogen removal may be: to improve the nitrate removal in the anoxic stage to increase the concentration of oxygen in the denitrification stage to enhance the BOD removal in the denitrification stage to increase the concentration of BOD in the anoxic stage.
The reasons to carry out the denitrification stage before to the nitrification stage are the following: the presence of a high concentration of oxygen can be avoided the denitrification stage requires the addition of an organic substrate when it is conducted after
the nitrification process the presence of a higher concentration of nitrate in the wastewater that enters into the
secondary biological treatment the need of alkalinity for the denitrification process.
Regarding the biological removal of nitrogen from wastewater, it is true that: the transformation of ammonium into nitrate may be considered as nitrogen removal the nitrifying bacteria do not reduce significantly the BOD of the wastewater the nitrification is done by bacteria that use inorganic carbon as substrate the denitrification process reduces the BOD of the wastewater .
Nutrient removal processes combine anaerobic and anoxic stages, which differ in: the concentration of oxygen required the concentration of nitrate required the type of biological processes developed and the predominant type of bacteria there is no difference.
Recirculation of treated water is usually carried out in trickling filter systems in order to: increase the organic loading applied to the filter increase the hydraulic load applied to the filter increase the biofilm sloughing increase the wetting of the biofilm.
The rotating biological contactors: are often used in processes of nitrification-denitrification have operating costs lower than activated sludge systems are mainly used in small WWTPs can work with natural aeration in some cases .
Regarding the hydraulic loading applied to a trickling filter, it is true that: it may be increased by decreasing the rotational speed of the distributor BOD removal increases when it is increased the organic loading increases when it is increased nitrification increases when it is increased.
Biological treatment by attached growth processes:
produces a sludge with better sedimentation properties than that of the activated sludge
processes is more frequently used than activated sludge processes produces lower amounts of sludge per kg of removed BOD than the activated sludge processes involves a higher aeration power consumption than activated sludge processes.
In the biological treatment by attached growth processes, the sloughing of the biomass:
is caused by the shear strength of the wastewater flow is related to the anaerobic conditions of the inner layers of the biofilm increases with lower hydraulic load fed to the filter must be avoided as much as possible.
Regarding the use of computer modeling methods for the design of the biological treatment, it is true that: the calibration of the models must be carried out using theoretical data they use empirical-based values of the biochemical kinetic equations to model the wastewater
biological treatment they use only empirical-based values of different parameters to model the wastewater biological
treatment they must be calibrated and validated for an specific wastewater treatment process in order to be
capable to predict accurate results .
Regarding the nutrient removal from wastewater, it can be stated that:
phosphorus can be eliminated either by biological or physical-chemical treatment nitrogen can be eliminated either by biological or physical-chemical treatment it is a treatment useful to fight against natural eutrophication it is a mandatory treatment for all the WWTPs discharging to sensitive areas.
Regarding the combination of attached growth and activated sludge processes, it is true that: the combined processes let to reach a treated wastewater with a lower pollution level than that of
the activated sludge processes the combined processes are more resistant than activated sludge processes to shock organic loads the combined processes requires more energy than the activated sludge processes the combined processes generate a sludge more difficult to settle than that of the activated
sludge processes.
Regarding the submerged attached growth processes, it is true that: they are conducted using natural aeration they do not need secondary clarification but sludge must be periodically removed Biocarbone and Linpor processes are examples of this type of biological treatment they do not need secondary clarification and sludge removal.
In the biological removal of phosphorus: the anaerobic stage necessary for P removal may be carried out simultaneously with the
denitrification stage the phosphate is mainly accumulated by bacteria in the anaerobic stage the phosphorus accumulating bacteria reproduce mainly in the aerobic stage the presence of nitrate should be avoided .
The primary settling is required for trickling filters processes in order to Reduce the hydraulic load fed to the filter The stabilization of sludge Prevent the dogging of the packing material Increase the organic load applied to the filter.
Regarding the nutrient removal from wastewater, it can be stated that: N can be eliminated either by biological or physical-chemical treatment P is removed by biological treatment as part of the wasted sludge Nitrogen removal by means of biological treatment requires the presence of autotrophic bacteria P can be eliminated either by biological or physical-chemical treatment .
The goal of the sludge treatment can be: The achieve a texture that makes sludge manageable and easy to transport To reduce pathogens, making the safe from the health point of view To decrease its nitrogen loading To reduce its water content in order to prevent fermentation.
Primary sludge thickened by gravity has a solid loading of 8% equivalent to: 8000 mg/L 8 kg/m3 80 kg/m3 800 gr/m3.
Regarding the aerobic sludge stabilization, it can be said that: Sold retention time values about 5 days are recommended Operation costs are higher than those of anaerobic digestion It is better that influent sludge was as concentrated as possible Solids concentration of influent sludge should be lower than 3,5-4%.
Regarding the anaerobic digestion of sludge, it is true that: Carbon dioxide is an important product of the biochemical reactions that occur in the digestor In mesophilic processes the power obtained from the biogas combustion is usually higher that the
power necessary to maintain the operation temperature Air is recirculated in order to agitate and thoroughly mix the sludge It is process that be carried out at 45ºC .
Regarding to sludge dewatering processes it can be said that: A final biosolid with less than 30% of water can be obtained A final biosolid with 20-30% of solids can be obtain A polyelectrolyte is usually added in order to chemically stabilize the final biosolid The final biosolid can not directly used in agriculture without an additional treatment.
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