Dissolved Oxygen Control for WWTPs Efficiency with Energy Saving
(by “AUTOMAZIONE Oggi” – N.370 Marzo 2014)
WDOxy-Fuzzy is a software procedure aimed at achieve an optimal control of the biological depuration of urban/industrial wastewater by using Fuzzy Logic algorithms based on bio-process behaviors and on on-line data derived from field instrumentation. These refers to the detection of values of Ammonium NH4+ concentration (alternatively: on ORP) and on DO (Dissolved Oxygen) values in oxidation, all that by returning as a real time “output”, the optimal set-point of the process control parameters (DO, flow rate of aeration, recirculation, etc..). In this way, WDOxy-Fuzzy procedure determines the optimal value (as dynamic set-point) of DO concentration, that is able to ensure in every operative phase of depuration process the following points:
- autotrophic and heterotrophic bacteria metabolism in the depuration process;
- non-proliferation and tigger of Bulking conditions control;
- minimization of energy consumption (kWh)
The software is implemented in the typical remote control systems, also directly implemented into dedicated PLC to the Dissolved Oxygen local control loop. Since over two years WDOxy-Fuzzy is used on sewage treatment plants with nutrient removal, on small to medium in size WWTPs and it is operating in continuous or with intermittent aeration.
WDOxy-Fuzzy, designed and developed by ANOVA in Naples (Giovanni Mappa), it is used by more than two years from large international companies operating in the field of analytical instrumentation and process control, having successfully passed the Functional Testing on field already in 2012.
Principles of Operation
The principle of operation of WDOxy-Fuzzy is based on the following bio-process correlations:
1) Biological Oxygen Request and Dissolved Oxygen set-point: the wastewater aerobic biological treatment is based on the fact that the some pollutants (organics, nutrients, etc.) are the “food” (substrate) for “heterotrophic” and “autotrophic” bacteria which accrue as “biomass”, in a fixed volume (reactor). In order that the biomass can metabolize the pollutant substrate in an aqueous environment, it is necessary that it can “breathe”, or dispose of a sufficient dissolved oxygen concentration (DO) into treated wastewater. The typical used solution to provide DO in this case is, as known, to adopt systems of insufflations and diffusion of air which, exploiting the principle of the solubility of the gas (Henry’s Law), provides oxygen to the entire biomass contained in the reactor. However, considering that (under std conditions) 1m3 of air contains about 21vol %O2, it follows that it is necessary to provide an air flow rate 4-5 times that required, with respect to pure oxygen O2. In function of the different operating parameters of the process (temperature, atmospheric pressure, substrate concentration, biomass concentration, etc.), the oxygen contained in the air is transformed into DO concentration, after having used the energy required, that is about 0.5 kWh for each Kg/h of O2 required.
2) Dissolved Oxygen set-point and Bulking: dissolved oxygen concentration DO in the reactor is a parameter of a great importance also for its influence with respect to the phenomenon of Bulking and, consequently, on the sludge sedimentation. This correlation is directly influenced by the organic load F/M (Food to Microorganisms): higher is F/M, higher is the concentration of dissolved oxygen necessary to prevent the Bulking (caused by some filamentous bacteria such as S.Natans, Type 1701 and H.hydrossis)
3) DO set point and Energy Savings: usually biological treatment process takes place under dynamic conditions where input loads may change considerably in space and time. The dynamics of the DO value of concentration in water, compared to dynamics of bacterial metabolism, is an average ratio of about 5/20, that means a variation of concentration of DO in the water takes place in the order of a few minutes (starting from the activation of air insufflations), while because DO as O2 is completely used by biomass to metabolize the substrate it takes 15-20mins. All this allows to focus on the following points:
- DO is a physico-chemical parameter whose numerical value (expressed in mg/l) is to mean a greater or lesser ” availability ” of oxygen for the bacterial metabolism. This value, however, is linked in turn to the different dynamics of transfer within the biomass (temperature, oxygen gradient, size of the flakes of mud, etc.). In other words, and simplifying the concept: between being in a situation in which DO assumes high values for short periods of time, compared to the case where the DO values are lower for prolonged times, this latter case is certainly not only desirable but also energetically less costly.
- values of DO too low in relation to the actual needs of the process, i.e. the F/M ratio between the substrate (pollutant) and biomass concentration, favors the proliferation of Filamentous Bacteria, at the expense of bacteria “Floc-Forming” that are required for the proper functioning of the process (sedimentation-recirculation): that means to face to the serious outcomes both from the operational point of view, that from the standpoint of sanitary;
- DO high values (>2.5 mg/l) do not provide generally further benefits to the bacterial metabolism, because biologic dynamics of using dissolved oxygen are, as already stated, fixed and slower than DO chemism, in every case. In addition, more seriously, DO high values, besides to be an useless waste of energy, may be symptomatic of some problems related to the proper functioning of the process, i.e. a too low biomass concentration, or an insufficient organic load, or presence of toxic substances which have weakened the vitality of the biomass, etc.. Extending the concept, in a balanced system of “substrate – biomass – oxygenation”, a low value (0.4-0.8 mg/l) of average concentration of dissolved oxygen DO, it may even be a good indicator of functionality and efficiency energy, as it is to mean that the insufflations of air, produce the DO that is necessary and sufficient for the needs of the process.
In the technical-scientific literature regarding the biological purification of wastewater, is typically reported the value of set-point DO of about 1-2 mg/l, but it is a “safety value” which guarantees neither the state of health of the process, nor the protection from an energy expenditure.
In general, the advantages of WDOxy-Fuzzy system with dynamic set-point DO, compared to the traditional system (fixed set-point OD ) can be summarized in the following points:
- immediate response to peaks in the variability of hydraulic loads and pollutant input through a continuous adaptation of the set-point of dissolved oxygen;
- Increased process stability and purifying efficiency, with particular reference to the nitrification process;
- an high energy saving about 15-20 %, and simultaneous elimination of the excesses of nitrification, as it is prevented the supply of excess air by obtaining a better efficiency of the transfer of oxygen by the diffusers;
- greater control of the continuity of the performance of removal, in relation to the limits of effluent quality.
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