Water-Energy Nexus in the Antofagasta Mining District: Options for Municipal Wastewater Reuse from a Nearly Energy-Neutral WWTP
Abstract
:1. Introduction
2. Materials and Methods
2.1. The Existing and Planned WWTP of Antofagasta
2.2. Future Scenarios
2.3. Methodology Used for the 3-E Analysis
- An ideal zero-dimension removal efficiency model, for the processes of primary sedimentation, thickening, and dewatering;
- The ASM3, for the biological processes that take place into the CAS tanks, under the both only-oxic and anoxic-oxic schemes [30,31]. The biological processes include heterotrophic carbon oxidation, autotrophic nitrogen oxidation, production of new cell materials, endogenous respiration, and carbon oxidation in anoxic conditions where applicable;
- The Takacs one-dimension clarifier model, for secondary sedimentation; that model is a non-reactive flux-based model that considers 10 horizontal layers, the 5th (from the top) of which receives the fed sludge [32];
- The surface-loading rate of the primary settlers was fixed equal to 40 m/d [28];
- The efficiency of primary settlers, in both COD and TSS removal, was fixed equal to 50% [35];
- The COD/BOD5 ratio was fixed equal to 2.8 [36];
- The COD partition, required by the ASM3, was assumed equal to that reported in Borzooei et al. [37] and detailed in the Supplementary Materials (Tables S1 and S2);
- All the stoichiometric and kinetic parameters required by the ASM3 were those reported in Henze et al. [31];
- The set of parameters required by the Takacs model for the secondary settlers was that coming from the calibration of Takacs et al. [32];
- The concentration of dissolved oxygen (DO) into the aerobic tanks of the CAS section was assumed equal to 2 mg/L;
Piece of Equipment | PEC Correlation ($) | Unit | Year | CEPCI | ER | Ref. |
---|---|---|---|---|---|---|
Primary settler | m2 | 1998 | 389.5 | 1.82 | [52] | |
Anaerobic digester | m3 | 2016 | 541.7 | 1.31 | [47] | |
Desulfurization | Nm3/h | 2011 | 585.7 | 1.21 | [47] | |
Demister | $ | 2011 | 585.7 | 1.21 | [47] | |
Gasometer | m3 | 2012 | 584.6 | 1.21 | [47] | |
Biogas blower | kWe | 2013 | 567.3 | 1.21 | [47] | |
Biogas pumps | kWe | 2011 | 585.7 | 1.21 | [47] | |
CHP unit | kWe | 2015 | 556.8 | 1.27 | [47] | |
Heat exchanger | m2 | 2005 | 468.2 | 1.51 | [47] | |
Post-thickener | m3 | 2003 | 402.2 | 1.76 | [47] | |
Back-up boiler | kWe | 2012 | 584.6 | 1.21 | [47] | |
Flare stack | m3/h | 2013 | 567.3 | 1.21 | [47] |
3. Results and Discussion
3.1. Energy Assessment
3.2. Environmental Assessment
3.3. Economic Assessment
4. Conclusions
- The introduction of an AD process (Scenario 1) to stabilize the sludge before reuse in agriculture or disposal in a landfill could save approx. 12% of the electric energy supplied to the WWTP, with an inherent reduction in CO2 equivalent emission of 660 tons/y;
- The introduction of an AD process and of a section of primary sedimentation (Scenario 2) could reduce the amount of electric energy supplied to the WWTP from external sources to only 30% of the WWTP original scheme (Scenario 0), thus avoiding the emission of 3800 tons CO2 equivalent/y. Such a benefit was made possible because of the significant increase in the produced renewable energy (+260% with respect to Scenario 1) and the decrease in the energy demand due to the aeration process (−30% with respect to Scenarios 0 and 1);
- The implementation of an oxic—anoxic partition of the CAS tank, other than AD and primary sedimentation, (Scenario 3) allowed the WWTP to reduce its electric energy demand from external sources to only 20% of that of Scenario 0, thus avoiding the emission of 4390 tons CO2 equivalent/y. The present requirement on the nitrogen concentration in the wastewater to be discharged or reused for mining activities does not require the presence of nitrification–denitrification processes. However, a nitrification—denitrification scheme makes it possible to consume an aliquot of the residual biodegradable organic substance at no free oxygen expenses, thus saving the corresponding aliquot of electric energy necessary for the aeration process.
Supplementary Materials
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Parameter | Unit | After Preliminary Treatments | D.S. 90/2000 Threshold Values (*) |
---|---|---|---|
Temperature | °C | >20 | N.A. |
pH | 7.7 | 6–9 | |
Fats and oils | mg/L | 75 | 20 |
TSS | mg/L | 225 | 80 |
BOD5 | mg/L | 242 | 35 |
TKN | mg/L | 51 | 50 |
TP | mg/L | 5 | 10 |
Chloride | mg/L | 999 | N.A. |
Alkalinity | mg/L as CaCO3 | 286 | N.A. |
Total coliforms | CFU/100 mL | N.A. | 1000 |
Unit of Process | Parameter and Design Criteria | Equation | Reference |
---|---|---|---|
Primary settlers | Wastewater design flow rate, QWW Surface-loading rate, SLR | Primary settler area | [28] |
Anaerobic digesters | Sludge average flow rate, Qsludge HRT anaerobic digesters Vwork/Vtot ratio | Digester—working volume Digester—total volume | [29] |
CHP unit | Methane flowrate, QCH4 Methane LHV, LHVCH4 Electric efficiency, ηel Thermal efficiency, ηth | Electric power Thermal power | [29] |
Gasometer | Biogas flowrate, Qb HRT gasometer | Gasometer volume | [29] |
Back-up boiler | CHP unit thermal power, WCHP,th | [28] | |
Heat exchanger | Max temperature sludge, Ts,max Min temperature sludge, Ts,min Max temperature hot water, Tw,max Min temperature hot water, Tw,min Heat transfer coefficient, U Thermal power to transfer to water to sludge, Wws | Heat exchanger area Logarithmic mean temperature difference, LMTD | [28] |
Flare stack | Time necessary to burn the total daily production of biogas, t | τ = 1 d | [28] |
Post-thickener storage tank | Digestate average flow rate, Qdig Digestate average mass flow rate, Mdig HRT post-thickener Solid loading rate, SoLR | Post-thickener total surface Post-thickener total volume | [28] |
Specific Electricity Consumption (Whe/e.i.·d) | Electricity Demand (kW) | ||||
---|---|---|---|---|---|
Scenario 0 | Scenario 1 | Scenario 2 | Scenario 3 | ||
Secondary settlers | 0.19 | 2.5 | 2.5 | 2.5 | 2.5 |
Pumps for WAS and RAS handling | 5.03 | 65.8 | 65.8 | 65.8 | 65.8 |
WAS thickening | 1.71 | 22.4 | 22.4 | 22.4 | 22.4 |
Anaerobic digesters | 9.51 | 0 | 124.3 | 124.3 | 124.3 |
Post-thickening | 0.02 | 0 | 0.3 | 0.3 | 0.3 |
Dewatering screw press | 4.03 | 52.7 | 52.7 | 52.7 | 52.7 |
CHP unit | 0.80 | 0 | 10.5 | 10.5 | 10.5 |
Primary settlers | 0.62 | 0 | 0 | 8.1 | 8.1 |
Anoxic tank mixing | 11.75 | 0 | 0 | 0 | 153.6 |
Total (kW) | - | 140.9 | 276.0 | 284.1 | 437.7 |
Parameter | Scenario 0 | Scenario 1 | Scenario 2 | Scenario 3 | |
---|---|---|---|---|---|
anoxic | oxic | ||||
MLSS (mg/L) | 6775 | 6775 | 3600 | 3612 | 3610 |
OD (kg/d) | 6049 | 6049 | 4675 | 0 | 3951 |
α factor (dimensionless) | 0.36 | 0.36 | 0.39 | - | 0.41 |
NH4-N (mg/L) | 1 | 1 | 1 | 25 | 1 |
NO3-N (mg/L) | 38 | 38 | 43 | 2 | 24 |
WAS (kg TSS/d) | 37,177 | 37,177 | 19,755 | 19,815 | |
PS (kg TSS/d) | - | - | 22,363 | 22,363 | |
Stabilized sludge/digestate (kg TSS/d) | 48,910 * | 33,140 | 30,310 | 30,200 | |
Stabilized sludge/digestate, 25% TSS (ton/d) | 195.6 | 132.6 | 121.2 | 120.8 |
Pieces of Equipment | Scenario 1 | Scenario 2 | Scenario 3 |
---|---|---|---|
Primary settlers, area (m2) | 0 | 1944 | 1944 |
Primary settlers, number | 0 | 6 | 6 |
Primary settlers, diameter (m) | 0 | 20 | 20 |
Anaerobic digesters, volume (m3) | 18,700 | 21,000 | 21,100 |
Gasometer, volume (m3) | 660 | 1745 | 1778 |
Back-up boiler, power (W) | 308 | 818 | 837 |
Heat exchanger, area (m2) | 17.7 | 20 | 20 |
Digestate post-thickener, area (m2) | 375 | 420 | 420 |
Digestate post-thickener, volume (m3) | 750 | 841 | 841 |
Parameter | Scenario 0 | Scenario 1 | Scenario 2 | Scenario 3 |
---|---|---|---|---|
Biogas production (Nm3/d) | 0 | 3538 | 9333 | 9535 |
Methane production (Nm3/d) | 0 | 2229 | 5880 | 6007 |
Renewable thermal energy (kW) | 0 | 388 | 1025 | 1047 |
Renewable electric energy (kW) | 0 | 370 | 976 | 997 |
Parameter | Scenario 0 | Δ 0–1 | Δ 0–2 | Δ 0–3 |
---|---|---|---|---|
Electric energy demand (kW) | 2051 | +124 | −433 | −630 |
Produced renewable electric energy (kW) | 0 | 370 | 976 | 997 |
Energy saved from external sources (kW) | 0 | 246 | 1409 | 1627 |
Avoided CO2 emissions (ton/y) | 0 | 664 | 3800 | 4390 |
Pieces of Equipment ($) | Scenario 1 | Scenario 2 | Scenario 3 |
---|---|---|---|
Anaerobic digesters | 3,988,000 | 4,370,000 | 4,370,000 |
Biogas desulfurization unit | 14,000 | 18,500 | 18,500 |
Gravel filter demister | 1400 | 1850 | 1850 |
Gasometer | 29,000 | 75,600 | 77,000 |
CHP unit | 1,450,000 | 3,824,000 | 3,906,000 |
Heat exchanger | 204,000 | 221,000 | 221,000 |
Flare stack | 58,000 | 105,500 | 106,500 |
Back-up boiler | 61,000 | 159,500 | 163,000 |
Digestate post-thickener | 61,500 | 61,500 | 61,500 |
Primary settlers | 0 | 815,750 | 815,750 |
Total | 45,447,400 | 49,234,200 | 49,332,100 |
Increment vs. Scenario 0 | +14.8% | +24.4% | +24.6% |
Scenario 1 | Scenario 2 | Scenario 3 | |
---|---|---|---|
Sludge recovery in agriculture | 0.37 | 0.34 | 0.36 |
Sludge disposal in a landfill | 0.13 | 0.21 | 0.24 |
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Campo, G.; Ruffino, B.; Reyes, A.; Zanetti, M. Water-Energy Nexus in the Antofagasta Mining District: Options for Municipal Wastewater Reuse from a Nearly Energy-Neutral WWTP. Water 2023, 15, 1221. https://doi.org/10.3390/w15061221
Campo G, Ruffino B, Reyes A, Zanetti M. Water-Energy Nexus in the Antofagasta Mining District: Options for Municipal Wastewater Reuse from a Nearly Energy-Neutral WWTP. Water. 2023; 15(6):1221. https://doi.org/10.3390/w15061221
Chicago/Turabian StyleCampo, Giuseppe, Barbara Ruffino, Arturo Reyes, and Mariachiara Zanetti. 2023. "Water-Energy Nexus in the Antofagasta Mining District: Options for Municipal Wastewater Reuse from a Nearly Energy-Neutral WWTP" Water 15, no. 6: 1221. https://doi.org/10.3390/w15061221