Background
The Martorell WWTP is an urban wastewater treatment plant with a design flow of 10,500 m3/day and receives wastewater from a population equivalent of 61,250 inhabitants.
This is a plant that needs to eliminate C, N and P, due to the receiving medium (Llobregat river). However, even complying with the output quality, the total nitrogen values are close to the maximum discharge limit (15 ppm), so it was necessary to seek greater control in the biological process, necessary for the elimination of the 3 elements.
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2. Description
With the idea of controlling the aerobic process more precisely to achieve greater stabilization of the process over time and to obtain more stable output parameters in compliance with the discharge limits, RUBATEC and the ACA decided to install a SENSARA SN8 on-line respirometer.
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2.1. Instalación y selección de test
In the first stage, the equipment was installed mechanically and hydraulically and the tests to be performed were optimized.
The selected tests to be performed were the Global Respirometry test (OUR/SOUR) and the Nitrification test (Rn/AUR). These tests are performed sequentially for 24 hours a day. The global respirometry tests last about 15 minutes while the nitrification tests last 75 minutes.
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The objective of the global respirometry test is the early detection of episodes of rainout, overload, toxicity, etc.
SICAIR
With the nitrification test and SENSARA’ s SICAIR algorithm, aeration control is carried out according to the Oxygen Requirement calculated by online respirometry.
The fundamental difference of this system with respect to traditional aeration control systems is that the SICAIR system takes into account, directly, a fundamental variable such as the kinetics of bacterial respiration at each moment (OUR/AUR).
Qaire (m3/h) = Fcorr*AOR / (δaire* SOTE * % O2)
With these variables, the SICAIR algorithm calculates the volume of oxygen that the bacteria need at each moment and with the technical data of the blowers, the minutes of operation of the blowers are calculated.
With the SICAIR system, a more balanced aeration is achieved, without peaks and valleys, obtaining an effective stabilization of the biomass.
where:
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Qaire (m3/h) = required air flow rate
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δair (Kg/m3) = air density
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AOR (kg O2/h) = Oxygen Requirement (Online Respirometry SN8)
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% O2 (%) = percentage of oxygen in the air
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SOTE (%) = oxygen transfer rate
In the case of rain episodes, in which dilution processes occur, the system detects them by means of the global respirometry test and the HRT data, applying a correction factor to the aeration calculation.
In the same way, when the system detects overload episodes, by means of the global respirometry test and the plant probes, a correction factor is applied to the aeration calculation.
Communication
The respirometric system (SN8-SICAIR binomial) communicates with the plant SCADA in a bidirectional way. The respirometric system sends via MODBUS all signals and in turn the SCADA shares signals with the respirometric system.
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In this way, the plant aeration system works by default by respirometry, but in case of failures, breakdowns, etc., it automatically switches to traditional control by probes, preventing the plant from being left without control at any time.
2.2. Variables compartidas por la planta
Hydraulic retention time (HRT) and ammonium/nitrate data are obtained through the plant flow rate and NH4/NO3 probes.
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Likewise, the plant SCADA shares other types of variables necessary for control such as blower status (stop/running), frequency inverter status, status (operational/out of service), aeration mode (probes/respirometer), communication failure, etc.
2.3. Variables respirómetro SN 8
The global respirometry (OUR/AOUR) and nitrification rate (Rn/AUR) data are obtained through the SN8 respirometer:
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2.4. Curvas soplantes
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3. Results
This section describes the operating results of the process obtained since start-up.
3.1. Fase de optimización
For about one month, the system was operated in pilot mode in order to optimize the tests as well as the SICAIR algorithm commands.
3.1.1. OUR/SOUR
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3.1.2. Test de Nitrificación (Rn/AUR)
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3.2. Fase de funcionamiento
Once the respirometric system was optimized and the programming was done both in the PLC of the SN8 respirometer and in the plant SCADA, aeration control was started with the SICAIR system.
The values obtained for the different parameters since start-up have been as follows:
3.2.1. OUR/SOUR
The overall respirometric rates (OUR/SOUR) of the plant in optimal operating situation are in the range of SOUR = 8 – 16 mg O2/g.h with an average solids concentration of SSVLM ≈ 3500 mg/L.
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3.2.2. Test de Nitrificación (Rn/AUR)
The nitrification rates (Rn/AUR) of the plant at optimum operating situation are in the range of Rn = 2 – 8 mg NH4/L.h with an average solids concentration of SSVLM ≈ 3500 mg/L.
4. Analysis of Results
4.1. Parámetros de Proceso
As mentioned in the introduction, the main objective was to improve the effluent quality. By means of the SICAIR algorithm, a greater stabilization of the biomass health status is achieved, thus obtaining, since the implementation of the respirometer between 2022 and 2023, a reduction in the concentration of NT and NO3 in higher outputs:
4.2. Aireación
The other main objective was, once the improvement of effluent quality stabilization had been obtained, the energy optimization of the plant.
As can be seen in the graph in Figure 10, one of the main differences of the SICAIR system is the achievement of a more balanced aeration, without such pronounced peaks and valleys, which leads to a much more effective stabilization of the biomass.
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With the intelligent SICAIR system, an energy efficiency of 14% has been achieved compared to the consumption that was achieved with the probe control.
Authors
- Jose Manuel Ochoa Martínez (SENSARA, S.L.) Iñigo Urruchi Sagredo (RUBATEC)
- Sergi Leal Arranz (RUBATEC)
- Núria Rodríguez Carrera (RUBATEC)
- Antonio Aguirre Glez. de San Pedro (INCONEF, S.L.) David Vitores (INCONEF, S.L.)
