Introduction to Modeling

Performance of a wastewater treatment facility is measured by effluent water quality and total operating costs.  Process Modeling allows the operator to see the effects of changes to operating variables on process results.  Because these changes are first made only in the computer, costly mistakes are avoided.

Routines are available to optimize performance, trouble shoot problems and plan for the future. The routines are presented in a straightforward format to allow you to proceed quickly to the operational tools.

General Theory

Three interrelated factors influence evaluating and optimizing the performance of the activated sludge process: oxygen transfer and consumption, solids production, and clarification.   

The oxygen capacity of the aeration system limits the organic load capacity of a wastewater treatment plant.  The rate of oxygen consumption must be within the oxygen transfer capacity to prevent the discharge of organics and potential exceptions to the discharge permit.  Operationally, oxygen consumption can be minimized by short solids retention times (SRT) and low mixed liquor (MLSS) concentrations.  Short SRT operation, however, results in increased solids production which can offset reduced oxygen requirements.  Accordingly, minimizing oxygen requirements must consider the impact on the waste handling system. 

Solids production is minimized by long SRT operation making simultaneous optimization of the two factors mutually exclusive.  Long SRT operation produces high MLSS levels which increases oxygen requirements and the solids loading to the clarifiers.  Excessive solids loads can lead to rising sludge blankets and high effluent solids.  Evaluation and optimization of treatment plant performance must, therefore, consider the interactions of oxygen utilization, solids production, and clarification.

• Short SRT = High F/M = Low MLSS    
    Low Oxygen Requirement  ---  Increased Solids Production
    Reduced Clarified Loading  --- Lower Sludge Blanket
  
•  Long SRT = Low F/M = High MLSS  
    High Oxygen Requirement  ---  Lower Solids Production
    Increased Clarified Loading ---  Higher Sludge Blanket

Modeling of Activated Sludge is designed to provide rapid and easy analysis of these three facets of treatment plant operation.  The equations used in these analyses are classic textbook equations, but do not incorporate arbitrary constants.  Where coefficients are required, software has been provided to generate the necessary information from the database.   No "typical" coefficients from the literature have been used in the programs since their use could lead to results unrepresentative of the actual system. 

Oxygen Analysis

Modeling the aeration system of an Activated Sludge process starts with of determining the ability of the site specific aeration devices to supply oxygen.  To generate an Oxygen-Transfer Capacity curves the Oxygen transfer coefficient (KLa) must first be calculated.  Using the calculated Kla, an Oxygen-Transfer Capacity curve can be created which shows the pounds of oxygen that are available. Then, to determine oxygen needs, a statistical correlation computes the coefficients of an equation that yields the rate at which the biomass uses oxygen and the rate at which the organic load uses oxygen.  This allows us to create graphs that show the amount of oxygen required for various loads or mixed liquor concentrations.  Finally, we can ask the program to predict the D.O. given a temperature, load and MLSS.

For further technical information including the equations used, please consult Appendix B of the User Guide.

Solids Production Analysis

Solids production analysis consists of defining the quantity of solids produced under a given temperature, organic load and SRT condition.  To accomplish this objective, the solids yield coefficient, Y (mass of solids produced per mass of organics removed, e.g., BOD, COD, TOD, etc.) and endogenous decay coefficient, Kd  (fractional mass of solids converted to water and carbon dioxide per unit time) are evaluated.  Determination of these coefficients from a specific plant's database allows quantitative evaluation of solids production for that plant.  This knowledge can be used for several important purposes:

• Comparing solids production from the plant for a given range of conditions with the solids production handling capacity of the facility.  The solids handling capacity of the facility is determined from a probability plot of mass dry solids daily put through by the solids handling process train.  The number selected for the daily throughput capacity is usually taken as the 50 to 60 percent probability of occurrence.
  
• Evaluating future solids handling systems' designs based on projected Influent solids, organics and flow rate conditions.
  
• Developing (in combination with chemical, energy and final disposal costs) cost models for solids disposal to assist in the determination of minimum cost process set points. 
  
• Reaching conclusions regarding the impact of specific users on treatment plant solids handling cost by a comparison of periods before and after specific users are attached to a collection system cost.
  
• Developing daily wastage requirements to achieve a target SRT (solids retention time).

 

Solids-Flux Analysis

One of the most useful techniques for evaluating the solids handling capacity of secondary clarifiers is called the state point method.  This method utilizes a curve representing the gravitational settling characteristics and two operating lines.  The settling curve is often referred to as a Solids Flux Curve ( Solids Flux being the mass transfer rate across an area).  The program draws the curve based on RAS settling velocity test data.  See Appendix B for the lab procedure for computing settling velocity. The two operating lines represent flow into the clarifier and recycle underflow. 

See Appendix B for the theoretical basis for this model.  However, the complexities of solids-flux methods are translated into a visual game of sizing a triangle and placing it within a boundary.