Water resources recovery facilities (WRRFs) are sources of direct emissions of greenhouse gases (GHGs) and volatile organic compounds (VOCs) produced by biological processes and indirect GHG emissions due to the energy consumed to operate the plant. The direct emissions also contribute to odour issues of WRRFs. Aeration of the biological tanks accounts for 50-60% of the total energy consumption of a WRRF and is therefore the major source of indirect GHG emissions.
The optimized management of oxidation processes is consequently associated with environmental and economic benefits. The innovative solution proposed in this study consists of an automated self-moving prototype (LESSDRONE) for real-time monitoring of oxygen transfer efficiency (OTE) and of GHG emissions from the aerated tanks during operation, and a protocol for converting LESSDRONE measures and specific WRRF data into actions aimed at minimizing carbon footprint (CF) and energy demand.
C. Carettia*, I. Duccia, F. Spennatib, S. Neric, M. Spizzirrid, S. Dugherie, R. Goria
a Civil and Environmental Engineering Department, University of Florence, Via di Santa Marta 3, 50139, Florence, Italy. * cecilia@dicea.unifi.it
b Consorzio Cuoiodepur SpA, Via Arginale Ovest 81, 56020, San Romano (PI), Italy
c West Systems Srl, Via Mazzolari, 25, 56025, Pontedera (PI), Italy
d Acea Spa, P.le Ostiense 2, 00154, Rome, Italy
e Experimental and Clinical Medicine Department, University of Florence, Largo Brambilla 3, 50134, Florence, Italy.
Competing interests: The author has declared that no competing interests exist.
Academic editor: Carlos N Díaz.
Content quality: This paper has been peer-reviewed by at least two reviewers. See the scientific committee here.
Citation: C. Caretti, I. Ducci, F. Spennati, S. Neri, M. Spizzirri, S. Dugheri, R. Gori. An innovative wireless tool for off-gas emissions and oxygen transfer efficiency assessment in WRRFs aeration tanks, 9th IWA Odour& VOC/Air Emission Conference, Bilbao, Spain, www.olores.org.
Copyright: 2021 Olores.org. Open Content Creative Commons license. It is allowed to download, reuse, reprint, modify, distribute, and/or copy articles in olores.org website, as long as the original authors and source are cited. No permission is required from the authors or the publishers.
ISBN: 978-84-09-37032-0
Keywords: aeration efficiency, energy saving, GHGs, odour, VOCs, WRRFs.
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Abstract
Water resources recovery facilities (WRRFs) are sources of direct emissions of greenhouse gases (GHGs) and volatile organic compounds (VOCs) produced by biological processes and indirect GHG emissions due to the energy consumed to operate the plant. The direct emissions also contribute to odour issues of WRRFs. Aeration of the biological tanks accounts for 50-60% of the total energy consumption of a WRRF and is therefore the major source of indirect GHG emissions. The optimized management of oxidation processes is consequently associated with environmental and economic benefits. The innovative solution proposed in this study consists of an automated self-moving prototype (LESSDRONE) for real-time monitoring of oxygen transfer efficiency (OTE) and of GHG emissions from the aerated tanks during operation, and a protocol for converting LESSDRONE measures and specific WRRF data into actions aimed at minimizing carbon footprint (CF) and energy demand.
OTE, concentrations of GHGs, VOCs and odourants, and off-gas flow rates were mapped inside the tanks under different operating conditions and their spatial and temporal variability was assessed. It proved possible to determine optimal WRRF process conditions and the parameters most affecting GHG emissions and OTE from the results obtained for different airflow rates and dissolved oxygen (DO) concentrations in the tanks.
1. Introduction
GHGs from WRRFs can be classified as direct and indirect (Mannina et al., 2016). Direct emissions are derived from biological processes occurring in the plants, while indirect emissions are associated with energy consumption for wastewater treatment. WRRFs contribute about 3% to total GHG emissions on a world basis (IPCC, 2013). Carbon dioxide (CO2) is produced during biological oxidation of biodegradable organic matter. Nitrous oxide (N2O) emissions are mainly due to nitrification and denitrification, in which N2O is an intermediate product (Foley et al., 2010). Methane (CH4) emissions originate mainly in sewer pipes and WRRF sections where anaerobic conditions prevail. However, non-negligible CH4 emissions can also be detected in the oxidation compartments, as aeration favours stripping of dissolved methane coming from other treatment sections, such as the anaerobic ones.
WRRFs are also major sources of other air pollutants, such as VOCs and odourants (Agus et al., 2012; Shaw and Koh, 2014). VOCs include all the odourous compounds generated by degradation of organic matter in wastewater; they can form in the sewer due to anaerobic processes and are more readily released into the atmosphere at points of turbulence or due to stripping in aerated treatment tanks. The emissions from urban and/or industrial wastewater treatment plants can be particularly complex and foul-smelling due to the complexity of the wastewater. The malodourous emissions associated with wastewater treatment have negative effects on the health of communities living near WRRFs and complaints about air pollution from such communities have constantly increased (Latos et al., 2011). Thus, in recent years, the monitoring of odourous emissions has also become important in the management of WRRFs.
Since aeration is responsible for a large share of indirect emissions, its optimization is crucial for minimizing the CF of WRRFs. In this context, the LIFE LESSWATT project (LIFE16 ENV/IT/000486), co-financed by the European Union, has the main objective of designing and implementing an innovative tool to assess and minimize direct and indirect contributions to the CF due to oxidation processes in WRRFs. The proposed solution includes a prototype (LESSDRONE) for monitoring OTE and emissions of GHGs during operation, and a protocol that translates the information collected into actions aimed at minimizing WRRF carbon footprint. LESSDRONE also collects oxidation tank off-gas samples in order to measure VOCs and odour.
Six measurement campaigns were conducted at the Cuoiodepur WRRF (Tuscany, Italy) for the prototype testing phase and other measurement campaigns are being carried out in five other WRRFs in Italy (Florence, Rome and Reggio Emilia) and in the Netherlands (Eindhoven and Tilburg) to evaluate technology transferability and versatility in different contexts. Here we describe the prototype and its functions, and by way of example, the main outcomes of the measurement campaigns at the Cuoiodepur WRRF (OTE and GHG emission results) and at the East Rome WRRF (VOC and odour results).
2. Materials and methods
LESSDRONE (Figure 1) consists of a removable foldable steel support frame, carrying six independent inflatable cylinders, one on each arm of the frame. A hood mounted at the center of the frame conveys gases rising from the liquid surface into a collection pipe. On the upper part of the frame there is a housing for two boxes containing the analysis instruments and the control/positioning devices. The gas passes through a condensate collection system and then through four parallel cartridges containing silica gel to remove residual moisture. It then passes through infrared absorption sensors for the measurement of CO2, CH4, N2O and through a fluorescent optical sensor for the measurement of O2. The instrument also measures humidity, temperature and gas pressure in the circuits and has a probe for measuring DO and temperature in the mixed liquor. LESSDRONE is propelled by eight motors via remote control, or else a measurement path can be set by GPS.
LESSDRONE was designed and built to evaluate OTE by the off-gas method (Redmon et al., 1983), which is based on a mass balance in the gaseous phase between the oxygen content of the reference gas (atmospheric air) and the off-gas. During the experiments, two different tests were carried out: a point test for determining the spatial distribution of the parameters measured and a stationary test for prolonged monitoring (usually at least 7 days) of the temporal variability of the data in relation to process conditions (inlet loads, air flow, DO, night/day, weekdays/holidays), at a fixed point in the tank. An aliquot of off-gas captured by the hood can also be conveyed outside the tank via a silicone tube to inflate nalophan (or tedlar) bags (1-5 liters). These off-gas samples are subsequently analyzed in the laboratory for VOCs (by gas chromatography-mass spectrometry), hydrogen sulphide (H2S, by gas chromatography and pulsed flame photometric detector) and odour units (via portable automatic olfactometer SM100i).
Fig. 1.: On the left, 3D view of LESSDRONE; on the right, LESSDRONE in the oxidation tank during a measurement.
The Cuoiodepur WRRF (850000 PE, 130 gCOD/d/PE) is at San Miniato (Pisa) in a major European tanning district. The tests were carried out in two of the seven oxidation tanks of the plant (tanks 5 and 6, Figure 2). The tanks (51 m x 13,5 m) have uneven membrane disc diffuser distribution (three different diffuser density zones) and differ in diffuser age: tank 5 diffusers were replaced in August 2018, while those of tank 6 were replaced in September 2017. Six measurement campaigns (in oxidation tanks 5 and 6) were conducted from May 2019 to July 2020. Two point tests and one 10-day stationary test were performed for each campaign. Oxygen transfer efficiency and GHG emission concentrations were measured at nine points in each tank in the point tests, and at a fixed point in the center of the tank in the stationary tests.
The East Rome WRRF is one of the largest in Italy and treats 900000 PE (about 280000 m3/day of municipal wastewater). The tests were performed in one of the plant's seven oxidation tanks (Figure 3). The tank (91 m x 21 m) is divided into three sections of equal area but with different numbers of air diffusers. In January 2021, three point tests (at 12 points in the tank) and one 7-day stationary test (at a central point in the tank) were carried out. In addition to measurement of OTE and GHG emissions, off-gas samples (in nalophan bags) were taken at 6 out of 12 points for VOCs, H2S and odour analysis.
Fig. 2.: On the left, Cuoiodepur WRRF oxidation tanks; on the right, the measuring points and the three diffuser density zones in the two tanks.
Fig. 3.: On the left, East Rome WRRF oxidation tank; on the right, the measuring points and the three diffuser density zones in the tank.
3. Results and discussion
Regarding the Cuoiodepur WRRF, average air flow in the point tests was 3,4 Nm3/h/m2; air flow was higher near the inlet of the tank and lower near the outlet, in line with diffuser density. Considering the total area of the tank, average air flow calculated by LESSDRONE was about 2200 Nm3/h, very close to that measured by the air flow meter (about 1900 Nm3/h) installed in each oxidation tank. Therefore, LESSDRONE not only highlighted the spatial distribution of air flow in the oxidation tanks, but also assessed total air flow by extrapolating values measured at different points in the tanks. This is important information for plants, which unlike Cuoiodepur WRRF, do not have an air flow meter in every oxidation tank.
Average OTE under standard conditions and in process water (αSOTE) was 30,3% and 28,7% in tanks 5 and 6, respectively; this is in line with the ages of the diffusers in the two tanks (older membranes are less efficient). According to the literature, αSOTE varies between 5 and 6% per meter of depth (Rosso et al., 2005). An air flow increase is associated with an OTE reduction, since efficiency is affected by bubble dynamics; this was confirmed by most of our experimental results. The average direct load of CO2 during the point tests was 174 gCO2/h/m 2; the corresponding average direct loads of N2O and CH4 were 0,02 gN2O/h/m2 and 0,08 gCH 4/h/m2, respectively. Considering the global warming potential of N2O (298 CO2 equivalent) and CH4 (25 CO2 equivalent), the average total CO 2 equivalent emissions were about 20000 kgCO2,eq/day. The indirect GHG emissions related to oxidation are due to the energy consumption of the aeration system blowers. In 2019, blower energy consumption was 9430 kWh/day; considering an electricity production CO 2 emission factor of 0,316 kgCO2/kWh (Caputo, 2019), indirect CO2 emissions were 2980 kgCO2,eq/day, i.e. about 1/6 of direct emissions.
The air supplied to the tanks follows the trend of the industrial wastewater entering the Cuoiodepur WRRF: the greater the loads, the greater the air flow supplied. As the air flow increases, OTE decreases. The trend of CO2 emissions follows that of inlet chemical oxygen demand, and therefore that of the load of organic matter entering the tank. Emissions of N2O are mainly influenced by DO concentration and air flow.
Regarding VOC and odour measurements at the East Rome WRRF, only 26 of the 130 VOCs analyzed (halogen-derived compounds, nitrogen compounds, aliphatic and aromatic hydrocarbons, oxygenated compounds) in the oxidation tank were detected. In any case, all values were very low, and the VOCs detected are ubiquitous in outdoor places. The compounds detected in the highest concentrations were toluene (256 µg/m3), m+p-xylene (28 µg/m 3) and o-xylene (12 µg/m3). The average concentration of total VOCs was about 480 µg/m3, which is an acceptable level in air for human health (Standard ASHRAE, 2016). The average concentration of H2S was 0,17 ppm and the average odour unit was 5 ouE/m3, which is also very low.
4. Conclusions
The optimization of oxidation in WRRFs is a concrete possibility for reducing the energy consumption and carbon footprint of wastewater treatment plants. Aeration systems are responsible for 50-60% of total energy consumption and their optimized management is associated with environmental and economic benefits. Our experimental results indicate that the LESSDRONE prototype, which was designed and built to monitor oxygen transfer efficiency and direct emissions from oxidation compartments, is useful for carbon footprint assessment and minimization in WRRFs. LESSDRONE also makes it possible to collect off-gas samples for analysis of VOCs and odours. The drone's ability to move around inside the aerated tank with all the necessary equipment for measurements on board, ensures complete automatic mapping of the tanks. The data acquired was consistent with plant process conditions and provided vital insights. LESSDRONE output values proved to be reliable, repeatable and accurate. The experimental outputs and WRRF data made it possible to assess the optimal air flow rate and DO to ensure removal of pollutants and minimization of the aeration carbon footprint and of emissions of VOCs and odour. When less air is needed, the aeration system benefits as oxygen transfer efficiency increases. Monitoring the oxygen transfer efficiency under standard conditions and in process water makes it possible to optimize the methods and frequency of cleaning and/or replacing the air diffusers.
5. References
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