Many wastewater treatment plants use more than 50% of their total energy budget in the aeration basin and, when nutrient removal is included in the activated sludge process, it is very common that the majority of oxygen uptake goes to the nitrification process. For that reason, in most wastewater treatment plants, energy optimization is focused on nitrification management. This information can be obtained in different ways, but perhaps the most direct way is based on the correct control of the minimum dissolved oxygen (DOmin) and sludge retention time (SRT) without impairment of process efficiency.
The present paper describes a relatively simple, fast procedure whereby the use of an advanced and high sensitivity respirometry system, both parameters are obtained throughout a novel method to accurately determine values of the half-saturation DO constant (KOA) and active nitrifier concentration (XA) from the results of a single respirometry test.
The procedure and methods here described do not aim to be a scientific document. On the contrary, one of its aims is to be accessible to any plant operator with access to a respirometer provided with necessary features to quickly obtain information about critical parameters on which the nitrification process can be calibrated so that it can be developed in the framework of its best energy optimization.
The influence of dissolved oxygen (DO) on nitrification is well known. Nitrification increases with the DO concentration until reaching a limiting-DO (between 3 and 4 mg/L) where the maximum nitrification rate is achieved (Figure 1).
It is understood that nitrification can take place within different DO ranges; but the approach of many plant operators is to select the minimum DO range and operate within a sludge retention time (SRT) to get the maximum energy optimization in the aeration tank corresponding to the process temperature, pH and effluent ammonium requirements.
Figure 1. Effect of DO on the % maximum nitrification rate
Historically, the influence of dissolved oxygen on the nitrification rate has been controversial. One of the main reasons comes from evidence that the half-saturation coefficient (KOA) applied in kinetic parameters determination, despite being critical, is not a well-defined value. This is evidenced by reported concentrations ranging between 0.15 and 2 mg/L O2.
Table 1 – Example of some reported half saturation values
According to Monod kinetics, the specific growth rate (μA) and the maximum nitrification rate (AURo) for any nitrification limiting DO concentration is dependent on temperature, pH and DO. But when those parameters are obtained under equivalent conditions of substrate, temperature, pH and oxygen to the actual process, this correlation only relies on the DO and half-saturation coefficient (KOA). Therefore, it is evident that KOA plays a critical role, but due to the variability in reported ranges (see Table 1), in many cases it can be too risky to use the default values from bibliography or simulation software. For that reason, it is more than justifiable to develop an affordable method that could calculate the KOA value for a specific nitrification process.
The major nitrification driver is the SRT. This value can be obtained from the reciprocal value of the actual autotrophic growth rate and is dependent on the nitrification rate (AUR) and active nitrifier concentration (XA). Therefore, to determine the corresponding SRT it is necessary to determine the XA beforehand; this is possible by taking advantage of the principle that the endogenous respiration rate is directly proportional to the biomass concentration (James C. Young. 2004; Peter A. Vanrolleghem. 2002).
2. BM RESPIROMETER
The methods and case study described in this paper were carried out with parameters and calculations resulting from a test performed using an advanced respirometer. This respirometer is able to conduct tests within different ranges of temperature, pH and DO. It also gives the option to change these variables, if required, during the test.
Figure 2. BM-Advance respirometry system
1. Stirring motor - 2. Oxygen & pH controller - 3. Oxygen & pH sensors - 4. Inlet & Outlet recirculation tubes - 5. Peristaltic pump. 6. Stirring paddles - 7. Air diffuser - 8. Automatic cooling & heating system - 9. Console display - 10. PC with BM software
The BM respirometer is programmed with three different operation modes: OUR, Cyclic OUR and R; in the present study the operation mode utilized was R.
The R mode is based on a modified LFS batch respirometry type where the dissolved oxygen is measured in liquid which is continuously aerated, stirred and recirculated. The respirometer is already calibrated from the factory to run in R mode. The exclusive feature of this operation mode is based on the fact that, when sludge under endogenous respiration is used, the stable resultant dissolved oxygen of the sludge without adding any substrate is taken as base line. Then, when substrate is added, the test actually begins and the software is able to calculate the exogenous respiration rate directly related to the biological substrate removal for a maximum DO concentration over time. In our case, the substrate is ammonium and therefore the exogenous respiration rate is exclusively related to nitrification.
3. MAIN APPROACH AND OBJECTIVES
- Energy optimization by operating within minimum DO range and SRT.
- Calculation of the specific KOA
- Calculation of the minimum and maximum DO in which nitrification can operate in its actual ammonium range.
- Calculation of the SRT in which the process should operate.
The common condition for all of them is to get a short, simple and reliable method.