Understanding the influent composition of a wastewater treatment plant is one of the most important tasks for an engineer. This directly impacts the design of a treatment plant and ability to simulate a plant performance using an activated sludge model. The characterization of wastewater is done by either measuring and/or identifying the carbonaceous, nitrogen containing components, and phosphorus containing components in soluble, colloidal, and particulate fractions and their biodegradable and unbiodegradable portions. Distinguishing influent content based on biodegradable and unbiodegradable COD is important to know the mass flow of biodegradable material to a plant which can be essential in calculating carbonaceous oxygen demand at a given sludge retention time.
There are three influent types model selection options, concentration, mass flow, and state variable. The concentration and mass based have an option of selecting a COD-based, or BOD-based input. Except for state variable based all other rely on specifying the fractions as an input.
Calculates the inert inorganic solids in the wastewater. Changing this directly impacts the mixed liquor concentration.
Calculates the filtered COD of wastewater. This contains a sum of colloidal and soluble organics.
This fraction contains only soluble COD, after colloidal material is removed, both biodegradable and unbiodegradable. This fraction is always equal to or smaller than the Fraction of filtered COD. The difference between the two fractions calculates the colloidal organics present in wastewater.
Calculates the amount of VFA in the influent. Important for designing and predicting and biological P removal plants.
Calculates the soluble organics COD from the filtered COD. This fraction is not impacted by biological processes and the mass entering leaves a process at steady state conditions. This COD leaves with the effluent discharge. This number is typically identified after performing a filtered COD measurement on the effluent of a plant with minimum of 3 days of SRT.
Calculates the unbiodegradable particulates from the total COD. This fraction is not impacted by biological processes and the mass entering leaves a process at steady state conditions. This COD gets enmeshed in the sludge and accumulates in the system until a process reaches steady state and then the mass of XU leaving the system is the mass of XU entering the system. A major portion of XU leave the plant through solids wasting or sludge disposal. This value will control MLSS concentration (more so when the SRT is high, so MLSS is more sensitive to XU fraction as the SRT increases) in the aeration tanks and digester performance. The XU can be calculated by subtracting SU and all the biodegradable COD from the influent total COD measurement. Measuring XU is more difficult – a 6-week SBR test is recommended.
This fraction of biomass associated with Total COD and is essential when simulating processes with sludge retention time of less than 3 days. Also, when simulating primary fermentation, it is important to estimate or measure this fraction accurately.
This is the decay product of OHOs in the influent and have minimal impact on the performance of the process – it mostly behaves as unbiodegradable material and will show up in the sludge wastage.
This fraction calculates the colloidal unbiodegradable COD from the difference between SCCOD and SCOD.
Calculates ammonia concentration, ammonia is important for growth of biomass, and it is converted to nitrite and/or nitrate.
Calculates the orthophosphate (OP) concentration of wastewater, very essential for growth of biomass.
Calculated the soluble organic nitrogen that will undergo ammonification to release ammonia. Ammonification usually completes when the SRT is more than 3 days.
The unbiodegradable portion of particulate organic nitrogen remains unchanged in the system and leaves a plant with sludge disposal
Portion of P that is bound to the soluble organics, these organics release OP during the treatment process.
Portion of P bound to the unbiodegradable organics and remain unchanged in the system, they leave with the sludge disposal or wasting.
Using the above inputs, the influent PU model calculates the following important state variables:
This variable is metabolized at an extremely fast rate and contribute to high oxygen uptake rate; it represents the non-VFA organics. The is an important variable to know for getting oxygen uptake rate, denitrification, and biological P performance correctly.
This variable is first hydrolysed or broken down by extracellular enzymes into SB and then metabolized. This means that the oxygen uptake rate or rate of metabolization of XB is limited by the rate of hydrolysis, which is much slower than the rate of SB metabolism. Under long SRTs (longer than 3 days), the XB fraction compared to MLSS is small in the treatment system.
This variable undergoes hydrolysis to form soluble biodegradable organic nitrogen and then ammonification to release ammonia. When designing for complete nitrification and estimating the alkalinity demand it is important to take biodegradable TKN which is a sum of ammonia, and the particulate and soluble biodegradable organic nitrogen, into consideration. This variable is essential in simulating ammonia release in the anaerobic digesters.
This variable undergoes hydrolysis to for soluble biodegradable organic phosphorus and then releases P. When designing for P removal, it is essential to take biodegradable TP which is a sum of ortho-P, and the particulate and soluble biodegradable organic phosphorus, into consideration. This variable is essential in simulating phosphorus release in the anaerobic digesters and certain precipitate formation.
This variable is first flocculated to particulate XB, then hydrolysed into SB and then metabolized. These are important when simulating primary clarifier performance or a process with SRT of less than 3 days.
The maximum specific growth rate constant of nitrifying organisms and/ammonia oxidizing organisms can vary greatly with different municipal wastewater especially when a portion of wastewater contains industrial flow. This is true especially for systems with one stage BOD-nitrification process, in such systems the growth rate of nitrifiers must be considered. Correctly knowing the nitrification rate can be significant in design and operation optimization. A high nitrification rate can mean shorter SRTs meaning saving on aeration and tank volume.
These fractions or variables are not directly measured on regular basis at a plant. Certain plants might dive into specific measurements, such as VFA measurement might be necessary, and done more occasionally by plants doing biological P removal. Most of the plants will measure TSS, VSS, BOD, TCOD, SCOD, Ammonia, OP, TKN, and TP in their influent and may be some in the effluent as well. Sumo provides a tool that can be used to estimate the fractions to the influent model using these measured data.
The following states conditions always has to be kept:
Are you using monthly or yearly average measurements for inputs?
Comparing my data to usual values in US are different, why?
Negative balance means that TKN or TP is not enough to be distributed between ammonia or OP and other organics. Could be measurement issue (adjust the ammonia/TKN or OP/TP concentration based on historic data), or highly soluble wastewater. Reduce N and P fractions to balance N and P if the NHx and OP measurement is deemed reliable.
This can be done by adjusting XOHO, XU, and CU fractions:
This fraction can change based on the composition of the influent for instance:
Carbohydrates: 1.07 g COD/g VSS
Proteins: 1.53 g COD/g VSS
Lipids: 2.92 g COD/g VSS
Other fractions are relatively constant, do not change, such as:
Biomass: 1.42 g COD/g VSS
This is done by performing preliminary simulations of the plant.
Typically, a completely nitrifying plant especially a BNR plant will have very low effluent ammonia concentration. So, majority of effluent TKN is particulate and soluble organic nitrogen. The concentration of particulate organic nitrogen is decided by the removal efficiency of the secondary clarifier by matching the total suspended solids. However, in most cases soluble organic nitrogen associated with the unbiodegradable portion of COD contributes significantly to the TKN measurement. Here we take into consideration the N content of the soluble unbiodegradable COD. This parameter can be adjusted to match effluent TKN.
Nutrient load to a plant dictates its performance, when a plant has primary clarification then the primary effluent load to its biological treatment must be well reproduced. Apart from getting the primary effluent ammonia and TSS correct, it is important to match the TKN concentration and the composition. As the biodegradable particulate organic nitrogen get captured in the primary, they will not show up as nitrate in your effluent, because particulate get waste through sludge. So, it is important to look at the composition of the organic nitrogen, more biodegradable soluble organic nitrogen will mean high nitrate.
The activated sludge models are mechanistically sound for application on industrial wastewater treatment and provide a good basis for evaluation of certain industrial wastewaters, especially food processing.