This work presents the results of 15 months since the start of the FANGHI project (Lombardy Region: FEDER ROP 2014-2020 Innovation & Research Call Hub). The project foresees a total cost of €9,904,708.53, a duration of 30 months starting in January 2020. The pandemic has led to the postponement of some project objectives and for this reason the project will end at the end of 2022.
FANGHI proposes an integrated approach to assess the health and environmental impact of two alternative strategies of sewage sludge disposal, incineration and agricultural soil amendment, in order to identify the most sustainable from an environmental point of view. energy, economy, health and environment. Mercury (Hg) in wastewater and sewage sludge (SS) is a relevant topic that requires further studies to be understood in detail as well as to provide reliable estimates on the amount of mercury and its species released. in the environment (Wang and Mao, 2019). Moreover, there are very few studies on Hg emissions linked to the incineration of SS. In Takaoda et al, the 2012 authors reported results found for two Japanese fluidized bed type incinerators equipped with different air pollution control devices (APCDs): they found that the concentrations of total Hg (THg ) in flue gas (36.6 μg/Nm3 and 21.1 μg/Nm3 on average) were higher than those reported for other types of incineration plants in Japan. This evidence shows the importance of monitoring Hg concentrations in emissions from SS heat treatment.
This work describes a method used to quantify the THg emitted by SS heat treatment under a mono-incineration and co-incineration condition. In Europe, the reference method EN 13211:2003 prescribes isokinetic sampling for THg in order to correctly collect also the dust phase and the droplets generated by wet scrubbers. Unlike the prescribed reference method, THg speciation (i.e. Hg0 and Hg2+) and real-time acquisition are useful to better understand Hg emissions. Experimental tests were performed under isokinetic conditions , based on continuous reduction of SnCl2 and real-time CVAAS measurements (1 sec. acquisition time) on a WTE plant with a fluidized bed combustion furnace treating dried and pelletized sludge (DPS) in a single combustion or in a co-combustion regime (DPS burned with waste-derived fuel – RDF).
The work approach was rather innovative: the atomic absorption mercury analyzer (NIC SGM-9) used in this work is a real-time device based on the principle of atomic absorption spectroscopy, equipped with a gas treatment in which the SnCl2 and KOH reagents are supplied in precise concentrations. This configuration allows elemental mercury (EHg) and total gaseous mercury (TGHg) in the flue gas sample to be measured at the same time. In this configuration, the SnCl2 captures the mercury Hg and the KOH eliminates any interfering agents: These reagents are required by international standards such as ISO 21741: 2020
By simply inputting the on-site conditions (ambient temperature, measured gas humidity, and measured gas temperature), the system automatically adjusts the time and volume of discharge liquid from the gas washing cylinders. In the first configuration (first scrubber), the Hg2+ is reduced to Hg0 using a wet method (10% SnCl2 solution). Then, the sum of Hg0 is determined using the first detector. The second configuration is responsible for determining Hg0 only. Interfering substances (eg HCl, SO2) are removed from the flue gases with a 10% KOH solution (second scrubber), moisture is removed by the third scrubber, the temperature of which is reduced to 5°C.
Solutions in scrubbers can be unloaded and replaced automatically at a specified time. At set intervals (usually every 60 s), the detector monitors the background level using a gold trap that removes Hg from the fumes and corrects the analytical signal. From the probe (TECORA® TCR titanium rotating probe) to the analyzer a heated line with thermoregulator is used to avoid condensation (temperature above 180°C)
Particular attention has been paid to the calibration of the instrument.
An insulated chamber (gas box) with a flask inside with a heat-sealed thermocouple calibrated with a primary external temperature by NIST was used
The metallic mercury is removed with a gas-tight syringe and placed in the vial. From there, precise aliquots are taken from the vial and injected into a tedlar bag.
The amount of Hg contained in the air inside the chamber obtained from the gas state equation can be used to plot the amount of mercury injected into the tedlar bag which must be detected by the gas analyzer. atomic absorption
Next, a series of mercury injections were performed inside a tedlar bag.
A three-point calibration starting with a value of 4 micrograms and ending at approximately 16.50 micrograms was carried out, with a good correlation between the estimated concentration and that measured by the atomic absorption of the analyzer.
At the same time, isokinetic samples on a quartz fiber filter and by absorption on a K2Cr2O7 + HNO3 solution (EN 13211:2003) were taken in order to compare the results of the experimental tests and the reference method.
Two campaigns of approximately 5 hours each were prepared in which 100% RDF was burned in the WTE plant and 2 campaigns in which, in addition to RDF, DPS was burned with different charges.
The comparison could only be made with an analyzer as continuous as the AMS installed in the factory, a CEM which uses the photometric principle with the Zeeman effect and for the calibration uses a vaporizer generating a gas from a solution of mercury chloride
The values of the hourly averages over the sampling periods (approximately 5 hours per campaign) between AMS and SRM with the analyzer remain around 0.5 – 1.0 μg/Nm3 with a very small difference (between 0.5 and 1.0 μg/Nm3). The results appear to hold constant the AMS/SRM differences with NIC SGM-9 between 100% RDF combustion and different dry sludge combustion loads.
SRM with NIC SGM-9 generally seems to show frequent peaks unlike AMS: this may be partly due to the interaction of NIC SGM-9 with some interfering metals (molarity of KOH has therefore been increased from 1M to 2M) , partly at NIC SGM -9 higher sensitivity.
The filter analysis always provided Hg values below the instrumental sensitivity limit so as not to generate a quantifiable contribution to the analyzer values.
This work compares the results of 4 test campaigns between a validated AMS and a discontinuous system based on SRM (isokinetic sampling) coupled with a real-time (AA) system for measuring mercury in gaseous form.
The comparison already shows a satisfactory agreement and well below the LEL (50 μg/Nm³ d). However, further study of the field tests performed is needed to investigate the discrepancies found between the two systems.
Takaoka M., Domoto S., Oshita K., Takeda N., Morisawa S. Mercury emission from sewage sludge incineration in Japan. Journal of Material Cycles and Waste Management (2012), 14: 113–119.
Wang X., Mao Y. Mercury in municipal wastewater and sewage sludge. Environmental Contamination and Toxicology Bulletin (2019), 102: 643–649.