2.1. Settling Ponds Description
The settling ponds are located in the nature site Boca Alta, inside a protected environmental area (Parque Regional del Sureste), inside the administrative territory of Arganda del Rey, in Madrid. They were caused by mining activities below the ground-water table and by discharge of petroleum sulphonate from the regeneration of waste oils until the end of the 1980s. In the area under study, there are two settling ponds: the Laguna Principal (main settling pond), with an area of 12,750 m
2 and a volume of 50,000 m
3 of petroleum sulphonate oil, and the Laguna Sureste (South-east settling pond), with an area of approximately 1989 m
2 and volume of 10,200 m
3 of solid petroleum sulphonate (
Figure 1).
Geologically, the Parque Regional del Sureste is characterized by the presence of two fundamental lithological domains: the Neogene tertiary and the Quaternary [
25]. The geological location of the settling pond is the Vega Baja del Jarama, characterized by the development of a large number of stream terraces, associated with Quaternary climatic oscillations, isostatic uplift tectonics and block adjustments, and lithological-structural controls [
26]. The main lithology of these terraces is formed by gravel bars and/or sand with limited sand fraction [
27], besides alterations in the conglomerate, silt and clay.
The ponds are located within the Parque Regional del Sureste, which is a protected natural area of the European Natura 2000 Network. For this reason, in 2008, the government of the Community of Madrid purchased the plot of land where the ponds are located in order to undertake their environmental restoration. The work to extract and recover the hazardous waste has been underway since 2014 with funding from the Community of Madrid and is expected to be completed in 2022.
2.2. Characterization of the Waste
The contaminant material of the settling ponds is fluid and semisolid petroleum sulphonate that it is estimated to come from highly viscous tar. To properly assess the semisolid material of the Laguna Sureste (LSE), trial pits have been made, and the following results have been obtained:
It is a material with an absence of stratification and without differences, but it does have punctual intercalations of calcareous filler of 20 to 50 cm in strength.
The contaminant material is black and solid, and it occasionally looks like plastic. Additionally, when in repose, its state is semisolid.
In 8 out of the 15 trial pits made, it was possible to reach the base of the settling pond, obtaining irregular depth and impermeable materials, such as gravel and sand, from those trial pits.
The material of the Laguna Sureste is similar in composition to the fluid main layer (Layer C), already extracted from the Laguna Principal, which will be described next [
28]. The material of the Laguna Sureste is more solid, and it has higher viscosity, higher quantity of volatile compounds, higher acidity, lower pH and higher sulphur content (
Figure 2). These characteristics make this pollutant harmful to the environment; it is therefore dangerous waste. The material to be extracted from the Laguna Principal is Layer D.
Layer C: this layer has the highest volume of the Laguna Principal, mainly formed by oily petroleum sulphonates, highly viscous and black. Its thickness varies depending on the topography of the settling pond, so it is not constant, reaching a depth of 6 m. Its density, fluidity and viscosity vary depending on the depth: the deeper it is, the more viscous the material becomes.
Layer D: this layer is denser and deeper than Layer C. It is black and is formed by heavy hydrocarbon that makes it possible to carry out extraction pumping at ambient temperature. It is highly viscous, even though it is also slightly fluid. The density varies, with an average density of 0.5 m. The origin of the material is the thermal de-asphalting carried out to recover oils.
Silt Layer: This is a deeper layer that occupies the bottom and part of the talus. It is formed by silt and sandy silts that make the layer impermeable and act as a containment for heavy hydrocarbons, PAHs and metals, followed by sands and gravels.
All the semisolid material of the Laguna Principal and the Laguna Sureste will be extracted through mechanical methods, that is to say, backhoe, through the 2020–2022 period. The high sulphur content of the material causes strong sulphur fumes and backhoe mining makes it a safe extraction.
Table 1 shows the characteristics of the contaminant material from Layer D and the Sureste Ponds.
Furthermore, 15% of the remaining Layer C will be extracted through pump, with a total of 6468 tonnes that were left unextracted during the first phase of the project, even though this material will not be under study for only the solid material will be quantified. The pumpable material has already been studied and quantified in [
28].
2.3. Alternative for the Use of Waste
Once the material is extracted, two alternatives for this solid waste are compared: the incineration of the material if it is unused, and the generation of fuel from the contaminant material to later be used in the cement industry. Of all the “terminal” treatment technologies, properly designed incineration systems are capable of achieving the highest degree of destruction and control for the widest range of hazardous waste streams [
29]. This alternative consists in a small treatment after the material extraction and its transport to the incinerator to be burnt.
The first alternative (incineration of the waste at the same time that the cement is produced in the two industries by using traditional fossil fuels) will be the base case used to compare the second alternative (fuel generation from the contaminant material, which will substitute the traditional fossil fuel in the two cement industries), so the energies of each alternative and the CO2 emissions avoided by carrying out the second alternative instead of the first alternative will be compared. The second alternative will distribute the derived fuel of the settling pond waste to two cement companies: Carboneras and Oural. This alternative has the following phases: first, the material extraction; second, its on-site treatment so it can be transported; third, the transportation to the plant that will generate the fuel; four, transportation and cement industry use.
2.4. Waste Treatment
The material of the settling ponds will be subjected to two treatments: an initial on-site security adaptation and a second treatment in two management plants outside the site where the settling ponds are located. The first treatment will be carried out after the material extraction, and it is necessary to adapt its characteristics and to transport it safely. The physio-chemical characteristics to be changed are the following:
Highly contaminated and acidic water.
Semisolid viscosity through all of the settling pond.
Highly corrosive material with a very low pH.
Organic and inorganic vapour emissions.
Due to these characteristics, the material must be conditioned and suitable in a mixing plant in which the pH is raised to neutral or alkaline, the aqueous phase is minimized, gaseous fumes are eliminated and the material is a non-viscous solid.
The adjustment of the mixing plant consists, first, of passing the material through a hopper with a worn drive at the bottom, whose function is to mix, degas and convey the product to the dosing hopper. The dosing hopper consists of a weighing element for lime. A premix is made and 10% by weight of lime is added. The hopper is made of stainless steel and is resistant to high temperatures because exothermic reactions will occur. From the dosing hopper, it passes to a batch drum mixer with blades and propellers for mixing, which, like the hopper, is also made of stainless steel and is resistant to high temperatures. The mixer is equipped with a degassing and outgassing system because toxic gases are released during the mixing process. These gases are fed into a chimney with a gas treatment system.
The material is discharged from the mixer into a sieve that ensures that the product is correctly mixed and has the desired granulometry. The process ends with conveyor belt transport to the stockpile areas, where analytical controls are carried out before the material is transported to the management plant. The whole treatment of the waste is shown in
Figure 3.
The second treatment will be carried out in two plants where it will be the same in both plants. According to the theoretical model, the material destined for the Carboneras cement industry will be treated at Albox (Almería), and the material destined for Oural will be treated at Arganda (Madrid), near the settling ponds. In the management plants, the material undergoes a physical–chemical treatment, where the final product is a solid fuel with the necessary characteristics to be accepted and used in the cement industry.
If the material contains concentrations of PCBs higher than 50 ppm, it will be sent to incineration. Theoretically, the material as a whole has concentrations of less than 50 ppm in PCBs and will therefore be treated in the management plants. First, when the material is received, a sample is conducted to determine the origin of the waste. Then, at the installation laboratory, all of the complementary parameters stipulated by the company, according to its own criteria of admission and expedition, are analysed. Once it is confirmed that they are settling ponds waste and that they comply with the complementary parameters, they are discharged.
The unloading area is a pit shed where the first crushing of the material is carried out with a low-speed double shaft crusher. Once this crushing is finished, the material is passed to the mixing tower. In the tower, it is mixed with other waste and sawdust to stabilize the waste. This installation is equipped with a sawdust reception hopper and a closed continuous vertical mixer with O2 analyser and nitrogen purging system.
The material is passed from the tower to the screening and refining area, where it is fed to a double screw belt that connects to the trommel inlet for the first screening; from the trommel, it is passed to the magnetic separation. The treatment is completed with storage and dispatch. The storage area consists of venting systems connected to an air treatment network, an extraction system in the lower area and a discharge area with grids connected to the contaminated water network.
Table 2 shows the tonnes extracted during the process and its heat of combustion. The heat of combustion is the average data of the material samples carried out before its extraction.
As mentioned above, the extracted tonnes will be distributed to two cement industries equally, i.e., the alternative fuel for each industry will contain 8404.5 tonnes of waste. Similarly, the percentage of waste to be allocated to each industry in the fuel will be 50%.
2.5. Environmental Specifications in the Cement Sector
In the cement industry, there are emission limit values for pollutants, so it has become increasingly urgent to develop new methods of collecting these pollutants in order to obtain more representative samples, collected over a longer period of time [
30]. In the Directive 2010/75/UE on industrial emissions, there are legally binding limit values for the emission of dioxin and furan, namely 0.1 ng/m
3 ET (toxicity equivalents) for installations that co-incinerate more than 1 tonne per hour of dangerous waste [
31]. The measurements of the dioxin and furan emissions in the cement industry are carried out separately if co-incineration waste is used.
The RD 508/2007, 20 of April, which regulates the provision of information on emissions from the E-PRTR Regulation and integrated environmental authorisations, establishes that in industries that do not use waste, cement and/or clinker manufacturing, facilities in rotary kilns with a production capacity of more than 500 tonnes per day must comply with environmental information requirements [
32]. These requirements pertain to water, energy and fuel consumption.
Industries that do use waste are required to monitor emissions of heavy metals, dioxins and furans, as established by the RD 815/2013, which approves the Industrial Emissions Regulation [
33]. This control consists of at least four measurements per year and during the first 12 months at least one measurement every two months. These requirements are set out in the Integrated Environmental Authorisation for each installation.
2.6. Energy Balance
To calculate the energy consumed, the following energy balance, Equation (1), of the entire settling pond extraction process is carried out. This energy balance considers the different energies involved in the process, namely the gross energy of the waste (
Egross), the energy of waste extraction (
Eextr), the energy of in situ adaptation of the waste (
Etr1), the energy of transportation to the cement industry in Carboneras (
Et1), the energy of treatment in the management plants (
Etr2) and the energy of transportation to the cement industry in Oural (
Et2).
The energy balance of the case in which the waste is incinerated, the following energies involved in the process are considered: the energy of the petroleum coke used to produce cement (
, the gross energy of the waste (
), the energy extraction of the waste (
Eextr), the energy of the on-site treatment (
Etr1) and the transportation energy from some settling ponds to the incineration plant for dangerous waste (
Et). The energy consumed during the process is calculated with Equation (2).
Petroleum coke is the fuel used in cement industry kilns when waste is not used. In this case, the settling pond waste is the substitute of the petroleum coke. The waste energy (
Egross) is the same as the petroleum coke energy (
Ecoke), as shown in Equation (3).
In Equation (4) the difference between the two energy balances of the cases of study is shown, and it is shown graphically in
Figure 4. If the result is positive in this equation, it means that the energy recovery case is the energy-efficient one; if the result is negative, it means that the waste incineration case is the energy-efficient one.
2.7. CO2 Emissions
The cement industry contributes about 5% to global anthropogenic CO
2 emissions, making the cement industry an important sector for CO
2 emission mitigation strategies [
34]: in addition to combustion-related emissions, cement production also release CO
2 during the calcination of limestone [
35]. As it is a sector with high CO
2 emissions, the aim is to avoid emissions with alternatives such as the one described: using the waste as opposed to incinerating the waste. To calculate the emissions of this gas, the following emission factors for each energy have been used: emission intensity of diesel fuel type C of 2.868 kgCO
2/l [
36] for the extraction process, emission intensity of electricity of 0.181 kgCO
2/kWh [
37] in the treatment processes, emission intensity of diesel of 2.79 kgCO
2/l [
37] for transport and 0.35748 kgCO
2/kWh [
38] emission intensity of the petroleum coke for the actual case of petroleum coke burning. The last factor is used because petroleum coke is the fuel frequently used in the kilns of cement industry, so, by using the alternative fuel, it is possible to replace the petroleum coke.
Substituting waste for fossil fuels reduces the potential long-term pollution that waste can generate, both at controlled and uncontrolled sites, as well as the pollution associated with the production of traditional fossil fuels. Therefore, the CO
2 tonnes avoided are calculated with Equation (5), which is the difference between the two alternatives.