pH < 6.3: Dissolved CO₂ and H₂CO₃* dominant
Acidic conditions typical of shallow groundwater in organic-rich soils with elevated CO₂
Bicarbonate minor species, carbonate essentially absent

pH 6.3 - 10.3: Bicarbonate (HCO₃⁻) dominant species
Most natural groundwater falls in this range (pH 6.5-8.5)
Principal buffering species maintaining pH stability
Carbonate species becomes significant only above pH ~9

pH > 10.3: Carbonate (CO₃²⁻) dominant
Rare in natural systems except highly alkaline waters
May occur in contact with serpentinite or other ultramafic rocks
Sodium carbonate lakes in arid regions

Overall Reaction:
CaCO₃(s) + CO₂(g) + H₂O ↔ Ca²⁺ + 2HCO₃⁻

Stepwise Mechanism:
1. CO₂ dissolution: CO₂(g) + H₂O ↔ H₂CO₃*
2. Carbonic acid dissociation: H₂CO₃* ↔ H⁺ + HCO₃⁻
3. Calcite dissolution: CaCO₃(s) + H⁺ ↔ Ca²⁺ + HCO₃⁻
Net: One mole calcite dissolves per mole CO₂ consumed

Quantitative Results:
• Atmospheric CO₂ (PCO₂ = 10⁻³·⁵ atm): Ca ≈ 50 mg/L, HCO₃ ≈ 120 mg/L
• Soil CO₂ (PCO₂ = 10⁻²·⁰ atm): Ca ≈ 90 mg/L, HCO₃ ≈ 220 mg/L
• High soil CO₂ (PCO₂ = 10⁻¹·⁵ atm): Ca ≈ 120 mg/L, HCO₃ ≈ 300 mg/L
Values at 25°C and calcite saturation; actual field values vary with temperature and kinetics

Zone 1: Oxic (Aerobic)

Dominant Process: Aerobic respiration - organic matter oxidation by dissolved oxygen
CH₂O + O₂ → CO₂ + H₂O

Eh Range: +400 to +600 mV
pe Range: +7 to +10
Chemical Indicators: O₂ > 1-2 mg/L, NO₃⁻ stable, Mn²⁺ < 0.05 mg/L, Fe²⁺ < 0.1 mg/L, SO₄²⁻ stable
Typical Environment: Shallow groundwater near recharge, well-aerated aquifers
Metal Mobility: Fe, Mn insoluble as oxides; As, U, Se may be mobile in oxidized forms

Zone 2: Post-Oxic (Nitrate Reduction)

Dominant Process: Denitrification - nitrate reduction to N₂ gas
5CH₂O + 4NO₃⁻ + 4H⁺ → 5CO₂ + 2N₂ + 7H₂O

Eh Range: +250 to +400 mV
pe Range: +4 to +7
Chemical Indicators: O₂ depleted, NO₃⁻ declining, possible NO₂⁻ or NH₄⁺ appearance, Mn²⁺ beginning to increase
Typical Environment: Agricultural areas with nitrate contamination, organic-rich sediments
Environmental Significance: Natural nitrate attenuation, prevents nitrate discharge to surface water

Zone 3: Manganese Reduction

Dominant Process: Reduction of manganese oxides
CH₂O + 2MnO₂ + 4H⁺ → CO₂ + 2Mn²⁺ + 3H₂O

Eh Range: +200 to +250 mV
pe Range: +3 to +4
Chemical Indicators: O₂ absent, NO₃⁻ depleted, Mn²⁺ elevated (often 0.5-5 mg/L or higher), Fe²⁺ still low
Typical Environment: Moderately reducing aquifers, transition zones
Water Quality Impact: Elevated manganese may cause aesthetic problems, staining, requires treatment for water supply

Zone 4: Iron Reduction

Dominant Process: Reduction of ferric iron oxides
CH₂O + 4Fe(OH)₃ + 8H⁺ → CO₂ + 4Fe²⁺ + 11H₂O

Eh Range: -50 to +200 mV
pe Range: -1 to +3
Chemical Indicators: Fe²⁺ elevated (typically 0.5-20 mg/L, sometimes >50 mg/L), Mn²⁺ elevated, alkalinity increased from oxide dissolution
Typical Environment: Deep aquifers, organic-rich sediments, reducing zones in confined aquifers
Metal Mobility: As, U may be mobilized or immobilized depending on specific conditions; phosphate adsorption capacity reduced

Zone 5: Sulfate Reduction

Dominant Process: Bacterial sulfate reduction to sulfide
2CH₂O + SO₄²⁻ → H₂S + 2HCO₃⁻

Eh Range: -150 to -50 mV
pe Range: -3 to -1
Chemical Indicators: SO₄²⁻ depleted, H₂S or HS⁻ present (odor detectable), Fe²⁺ may decrease due to FeS precipitation, alkalinity significantly elevated
Typical Environment: Deep confined aquifers, organic-rich marine sediments, petroleum reservoirs
Metal Precipitation: Many metals precipitate as insoluble sulfides (FeS, ZnS, CuS, PbS), dramatically reducing mobility

Zone 6: Methanogenesis

Dominant Process: Reduction of CO₂ to methane
2CH₂O → CH₄ + CO₂

Eh Range: < -150 mV
pe Range: < -3
Chemical Indicators: CH₄ present (often 1-100 mg/L), very low SO₄²⁻ (< 1 mg/L), elevated alkalinity, possible elevated NH₄⁺
Typical Environment: Deep sedimentary basins, landfills, wetlands, petroleum reservoirs
Practical Implications: Methane hazard in water wells, degassing required, natural gas resource in petroleum systems

Piper Diagram: Major Ion Composition Visualization

Construction Steps:

1. Convert all ion concentrations from mg/L to milliequivalents per liter (meq/L) by dividing by equivalent weight
2. Calculate percent milliequivalents for each ion relative to total cations or anions
3. Plot cation percentages on left ternary (Ca, Mg, Na+K vertices)
4. Plot anion percentages on right ternary (HCO₃+CO₃, SO₄, Cl vertices)
5. Project to central diamond field showing combined composition

Interpretation Patterns:

Evolutionary trends plot as arrays of points showing progressive change, straight-line trends suggest mixing between end members, curved trends indicate progressive reaction or exchange processes

Stiff Diagrams: Water Type Fingerprinting

Construction Method:
Horizontal axis shows concentration in meq/L, with cations plotted left of vertical centerline and anions right. Typical arrangement: Ca-Mg-Na+K on left, Cl-SO₄-HCO₃ on right. Connecting endpoints creates distinctive polygonal shape unique to each water sample.

Advantages:

• Shape provides instant visual recognition of water type
• Can be plotted on maps showing spatial patterns
• Magnitude reflected in polygon size
• Useful for comparing individual samples or small groups

Interpretation: Similar shapes indicate similar water types; changing shapes along flow path reveal developing chemistry; symmetric shapes suggest simple mineral dissolution; asymmetric shapes indicate complex processes like mixing or ion exchange

Schoeller Diagrams: Semi-Logarithmic Multi-Sample Comparison

Format: Vertical log-scale axis shows concentration (usually meq/L or mg/L), horizontal axis lists major ions in standard sequence. Each sample plots as connected line crossing ion positions at corresponding concentrations.

Pattern Recognition:

• Parallel lines indicate similar water types differing only in concentration (dilution/concentration)
• Converging/diverging lines show different evolution or mixing
• Slope changes between ions reveal chemical processes
• Can accommodate multiple samples on single diagram for comparison

Pre-Sampling Procedures:

• Verify well construction details, screen depth, geological formation
• Measure static water level before purging
• Calculate casing volume: V = π r² h (r = casing radius, h = water column height)
• Prepare clean sample bottles with appropriate preservatives
• Calibrate field instruments (pH meter, conductivity meter, dissolved oxygen probe)
• Review safety requirements and obtain site access approvals

Well Purging and Stabilization:

• Begin pumping at low to moderate rate (minimize drawdown for low-flow method)
• Measure and record field parameters at regular intervals (every 3-5 minutes typical)
• Purge minimum 3-5 casing volumes or until stabilization criteria met
• Stabilization: pH ± 0.1 units, conductivity ± 3%, temperature ± 0.5°C, dissolved oxygen ± 10%, turbidity ± 10% over three consecutive readings
• Document total purge volume and duration
• If excessive drawdown occurs, allow recovery and resume at lower rate

Sample Collection Sequence and Techniques:

Standard Collection Order (minimizes cross-contamination):

1. Volatile organics (if required) - no headspace, preserved if specified
2. Dissolved gases (methane, dissolved oxygen - if not measured in field)
3. Nutrients (nitrate, ammonia, phosphate) - filtered, preserved
4. Major ions - filtered (0.45 μm) and acidified for cations, unacidified for anions
5. Trace metals - filtered and acidified with HNO₃ to pH < 2
6. Alkalinity - unfiltered, unpreserved, analyze within 24 hours or store at 4°C
7. Stable isotopes - unfiltered, seal to prevent evaporation
8. Field parameters - measure in flow-through cell during purging

Critical Techniques:

• Filter through 0.45 μm membrane using syringe or in-line filter
• Rinse bottles 3 times with sample before filling
• Fill bottles completely, minimizing aeration and degassing
• Add preservatives immediately after collection if required
• Label bottles clearly with site ID, date, time, sampler initials

Sample Preservation and Holding Times

Note: Holding times are maximum recommended intervals; shorter times preferred for sensitive parameters. Field measurement of alkalinity, dissolved oxygen, pH, temperature, and conductivity essential as these parameters change during storage and transport.

Stage 1: Initial Feasibility and Planning Phase

Key Questions:

• What aquifer systems will be intersected by proposed mining?
• What is existing groundwater chemistry in these aquifers?
• What mineralogy will be exposed and potentially oxidized?
• What volume of water must be managed (dewatering rates)?
• What are applicable discharge standards and receiving water objectives?

Critical Investigations:

• Baseline groundwater quality monitoring (minimum 12 months capturing seasonal variation)
• Mineralogical characterization through petrography, XRD, chemical assays
• Static geochemical testing (acid-base accounting) of representative lithologies
• Kinetic testing (humidity cells, columns) for materials showing acid generation potential
• Hydrogeological testing defining aquifer properties and connectivity
• Conceptual water balance modeling estimating dewatering volumes and pit inflow rates

Stage 2: Detailed Engineering and Design

Water Quality Prediction Approach:

Treatment Infrastructure Sizing:

• Design for 90th percentile water quality (not average) to ensure adequate capacity
• Include safety factors accounting for uncertainties in predictions
• Plan for staged expansion as actual conditions are confirmed
• Consider alternative management strategies (source control, segregated drainage)

Stage 3: Operational Monitoring and Adaptive Management

Essential Monitoring Components:

• Dewatering water quality - minimum monthly, increased frequency during startup or significant changes
• Pit wall seepage quality if accessible - characterizes water-rock interaction products
• Surrounding groundwater monitoring wells - detects off-site impacts
• Discharge water quality - weekly to daily depending on regulations
• Treatment system performance - continuous instrumentation, regular sampling

Trigger-Response Protocol:

Stage 4: Closure and Post-Closure

Long-Term Water Quality Considerations:

• Pit lake water quality evolution - stratification, seasonal turnover, long-term geochemical reactions
• Post-mining groundwater recovery - potential for mobilization of stored contaminants
• Waste rock and tailings seepage - long-term acid generation and metal release
• Duration of treatment obligations - may extend decades to centuries

Closure Options and Geochemical Implications:

• Pit flooding: Submerge sulfide-bearing walls creating reducing conditions limiting oxidation
• Backfilling: Cover reactive materials, though sulfide oxidation may persist in unsaturated backfill
• Passive treatment: Wetlands or permeable reactive barriers for long-term low-cost management
• Active treatment continuation: Required if natural attenuation insufficient

Basic Speciation and Saturation Index Calculation

Required Input Data:

• Temperature (°C)
• pH (field measurement preferred)
• Major ion concentrations: Ca, Mg, Na, K, Cl, SO₄, alkalinity (as HCO₃ or total alkalinity)
• Redox indicator: pe, Eh, or redox couple (e.g., Fe²⁺/Fe³⁺, SO₄²⁻/S²⁻)
• Trace elements of interest (optional for basic calculation)

Model Output Interpretation:

• Species Distribution: Percentage of each element present as various species - guides understanding of mobility, bioavailability, toxicity
• Saturation Indices: Positive values indicate supersaturation (precipitation possible), negative values undersaturation (dissolution may occur)
• Ionic Strength: Affects activity coefficients and reaction rates
• Charge Balance: Should be within ±5% confirming data quality

Reaction Path Modeling: Simulating Progressive Mineral Dissolution

Application: Predict water chemistry evolution along flow path as minerals dissolve or precipitate

Setup:

• Define initial water composition (e.g., rainwater or recharge water)
• Specify reactive minerals and maximum amounts available for reaction
• Set reaction increments and equilibrium phases
• Model calculates composition after each reaction step

Example Application:

Simulate limestone dissolution: Start with CO₂-charged rainwater, react with calcite in small increments, track pH, Ca²⁺, and HCO₃⁻ evolution until saturation, compare predicted composition with field measurements validating conceptual model

Key Output: Saturation State Tracking

Monitor how SI values change during reaction sequence - identifies which minerals control water chemistry at different stages along flow path

Inverse Modeling: Determining Reactions from Observed Chemistry Changes

Concept: Given initial and final water compositions, calculate which mineral reactions and amounts explain the differences

Requirements:

• Chemistry of initial water (e.g., recharge area)
• Chemistry of developed water (e.g., downgradient well)
• List of potential reactive phases (minerals, gases, ion exchange)
• Uncertainty limits on analytical data

Output:

Mole transfers (positive = dissolution, negative = precipitation) for each phase; multiple solutions may exist, requiring geological judgment to select most plausible scenario

Value: Quantitative test of conceptual models, identifies controlling reactions, supports prediction of future chemistry changes

Pit Water Source Identification

Application: Determine proportions of different water sources contributing to pit inflow - critical for water balance and management planning

Methodology:

• Sample potential sources: rainfall/recent recharge, shallow aquifer, deep aquifer, process water, surface water
• Measure δ²H and δ¹⁸O for all sources and pit water
• Plot on δ²H vs δ¹⁸O diagram
• Mixing between sources plots on straight lines connecting end members
• Quantify mixing proportions using mass balance calculations

Value: Identifies which aquifers contribute most inflow, guides targeting of dewatering efforts, validates hydrogeological conceptual models

Groundwater Age Dating with Tritium and Carbon-14

Tritium (³H):

• Half-life: 12.3 years
• Age range: 0-60 years
• Natural production in atmosphere + 1950s-1960s thermonuclear testing spike
• Interpretation: ³H > 0.8 TU indicates modern recharge (< 60 years old), ³H < 0.2 TU suggests pre-1950s recharge
• Application: Distinguish recent recharge from deeper, older groundwater in multi-aquifer systems

Carbon-14 (¹⁴C):

• Half-life: 5,730 years
• Age range: 1,000-40,000 years
• Measured in dissolved inorganic carbon (DIC)
• Requires correction for dilution by dead carbon from carbonate dissolution
• Application: Age deep confined aquifer water, identify very old saline waters, assess groundwater renewal rates

Sulfur Isotopes for AMD Source Identification

Rationale: Different sulfur sources (atmospheric deposition, gypsum dissolution, pyrite oxidation, bacterial sulfate reduction) have characteristic δ³⁴S values enabling source fingerprinting

Typical δ³⁴S-SO₄ Values:

• Atmospheric sulfate: +5 to +10‰
• Marine gypsum: +15 to +25‰
• Pyrite oxidation: highly variable, often -20 to +10‰ depending on geological setting
• Bacterial sulfate reduction: produces depleted sulfide (negative δ³⁴S), residual sulfate becomes enriched

Application: Distinguish natural baseline sulfate from mining-derived sulfate, identify bacterial reduction zones in passive treatment systems, trace sulfur cycling in complex mining-affected watersheds

1.1 Baseline AMD Characterization Program (Months 1-6)

Water Quality Monitoring:

• Identify all AMD discharge points and seepage locations requiring treatment
• Establish sampling locations representative of treatment system influent conditions
• Conduct weekly sampling for minimum 6-month period capturing seasonal variations
• Analyze comprehensive parameter suite: pH, acidity (mineral and total), major ions (Ca, Mg, Na, K, HCO₃, SO₄, Cl), metals (Fe total and Fe²⁺, Al, Mn, Cu, Zn, Ni, Pb, Cd, As, Hg), physical parameters (TSS, TDS, conductivity, temperature)
• Calculate statistical distributions (mean, median, 90th percentile, maximum) for design basis
• Evaluate temporal patterns (seasonal, rainfall-related, operational influences)
• Deliverable: Baseline water quality characterization report with design basis recommendations

Flow Rate Assessment:

• Install flow measurement devices (weirs, flumes, or meters) at discharge points
• Record daily flow measurements minimum 6-12 months establishing flow statistics
• Correlate flow rates with rainfall, groundwater levels, operational activities
• Project future flow scenarios accounting for mine development, waste placement, rehabilitation
• Establish design flow capacity (typically 90th percentile or maximum monthly average)
• Deliverable: Hydrological assessment report with design flow recommendations

Geochemical Testing Program:

• Collect representative waste rock and tailings samples (minimum 20-30 samples)
• Conduct acid-base accounting (ABA) determining net neutralization potential (NNP)
• Perform kinetic testing (humidity cells) for acid-generating materials (minimum 20 weeks)
• Predict long-term AMD generation rates and chemistry evolution
• Assess closure scenarios and post-mining AMD continuation likelihood
• Deliverable: Geochemical characterization report with long-term predictions

1.2 Regulatory Requirements Assessment (Months 2-3)

• Review applicable Indonesian regulations (PP 22/2021, Permen LHK P.68/2016)
• Identify receiving water classification and corresponding discharge standards
• Assess AMDAL requirements for new treatment facilities
• Determine discharge permit application requirements and timelines
• Identify stakeholder consultation obligations (communities, NGOs, government)
• Review mine environmental permits for treatment system integration requirements
• Deliverable: Regulatory compliance assessment memorandum

1.3 Technology Alternatives Evaluation (Months 3-5)

• Develop treatment performance objectives based on discharge standards and AMD characteristics
• Screen technology alternatives: active chemical (lime, HDS, alternative reagents), passive (wetlands, ALD, SAPS, bioreactors), hybrid configurations
• Assess applicability considering AMD chemistry, flow rates, site constraints, operational capabilities
• Develop conceptual designs for viable alternatives including process flow diagrams, major equipment, footprint requirements
• Evaluate pilot testing needs for technologies requiring validation
• Consider resource recovery opportunities (schwertmannite, gypsum, metal recovery)
• Deliverable: Technology alternatives evaluation matrix with conceptual designs

1.4 Site Assessment and Constraints Analysis (Months 3-4)

• Conduct site reconnaissance evaluating potential treatment facility locations
• Assess topography, drainage, soil conditions, access, utilities availability
• Identify environmental constraints (wetlands, protected habitats, cultural sites)
• Evaluate land availability and acquisition requirements
• Assess proximity to AMD sources, discharge points, and operational support facilities
• Consider construction access, material delivery logistics, operational access
• Deliverable: Site assessment report with recommended facility location

1.5 Preliminary Cost Estimation and Economic Analysis (Months 5-7)

• Develop capital cost estimates (±30-40% accuracy) for viable technology alternatives including: equipment and materials, civil works and structures, electrical and instrumentation, installation and construction, engineering and project management, contingency (typically 20-30% feasibility stage)
• Estimate annual operating costs including: chemical reagents, energy consumption, labor and supervision, maintenance and parts, analytical monitoring, waste disposal, administrative overhead
• Calculate unit treatment costs (USD per m³ treated) for comparison
• Conduct lifecycle cost analysis over project duration (typically 20-50 years)
• Assess resource recovery revenue potential offsetting treatment costs
• Evaluate financing options and funding sources
• Deliverable: Cost estimate report with economic comparison of alternatives

1.6 Feasibility Study Report and Recommendations (Months 7-8)

• Integrate all technical assessments, regulatory analysis, cost estimates into comprehensive feasibility report
• Provide technology recommendation with supporting rationale
• Develop preliminary implementation schedule and project timeline
• Identify critical risks and mitigation strategies requiring attention
• Establish performance guarantees and acceptance criteria
• Define next steps for detailed design and permitting phases
• Deliverable: Comprehensive feasibility study report with executive summary and recommendations for management decision

2.1 Pilot Testing Program (Months 1-4, if required)

• Design pilot test program validating technology selection and optimizing design parameters
• Procure or fabricate pilot equipment (typically 0.5-5 m³/day capacity)
• Install pilot system at mine site treating actual AMD
• Conduct systematic testing evaluating: chemical dosing requirements, retention times, mixing intensities, settling characteristics, pH control strategies, sludge production rates, seasonal performance variations
• Optimize operating parameters minimizing costs while achieving discharge compliance
• Validate design assumptions and refine full-scale specifications
• Deliverable: Pilot test report with full-scale design recommendations

2.2 Process Design and Engineering (Months 2-7)

Process Flow Diagrams (PFD):

• Develop detailed PFDs showing all process units, piping, major equipment
• Include mass balance calculations for water, chemicals, sludge
• Specify design flow rates, retention times, chemical dosing rates
• Show instrumentation and control philosophy

Equipment Specifications:

• Reaction tanks: volumes, dimensions, materials of construction (concrete, steel, FRP), mixing equipment specifications
• Clarifiers/settlers: surface areas, loading rates, overflow weirs, sludge collection mechanisms
• Pumps: flows, heads, materials, motor sizes, redundancy requirements
• Chemical feed systems: storage capacity, dosing pumps, injection points, safety equipment
• Instrumentation: pH meters, flow meters, level sensors, turbidity meters, ORP probes
• Control systems: PLC specifications, HMI interface, SCADA integration, alarm management
• Sludge dewatering: filter press, belt filter, or centrifuge with capacity and specifications

Piping and Instrumentation Diagrams (P&ID):

• Complete P&IDs showing all piping, valves, instruments, control loops
• Specify pipe sizes, materials, pressure ratings
• Include safety devices, sample points, drain valves
• Show electrical and control connections

Deliverable: Complete process design package including PFDs, P&IDs, equipment specifications, design calculations, process narrative

2.3 Civil and Structural Design (Months 3-7)

• Site grading and drainage plans directing stormwater away from treatment areas
• Foundation designs for tanks, buildings, equipment based on geotechnical investigations
• Concrete tank designs with structural calculations, reinforcement details, waterproofing
• Building designs for chemical storage, control room, laboratory, maintenance shop
• Road and access designs for construction and operational access
• Seismic design considerations per Indonesian standards (if applicable)
• Deliverable: Civil and structural drawings package with specifications and calculations

2.4 Electrical and Instrumentation Design (Months 4-7)

• Electrical single-line diagrams showing power distribution
• Motor control center (MCC) specifications and layouts
• Lighting design for operational and safety requirements
• Grounding and lightning protection systems
• Instrumentation and control system architecture
• Control panel layouts and wiring diagrams
• SCADA system specifications for remote monitoring
• Deliverable: Electrical and instrumentation drawings and specifications

2.5 Construction Drawings and Specifications (Months 6-8)

• Complete construction drawing set including: site plans, grading plans, foundation plans, structural details, piping layouts, electrical layouts, instrumentation details
• Technical specifications covering: civil works (excavation, concrete, formwork), mechanical equipment (pumps, mixers, clarifiers), electrical systems, instrumentation and controls, protective coatings, testing and commissioning requirements
• Bill of quantities for cost estimation and bidding
• Construction quality assurance/quality control (QA/QC) plan
• Deliverable: Complete construction documents ready for permitting and bidding

2.6 Operations and Maintenance Planning (Months 7-9)

• Draft operations and maintenance (O&M) manual covering: startup and shutdown procedures, normal operating procedures, chemical dosing calculations, troubleshooting guidelines, maintenance schedules, safety protocols
• Develop staffing plan identifying operator, technician, supervisor requirements
• Prepare training program outline for operations personnel
• Establish spare parts list and inventory management approach
• Develop performance monitoring and reporting procedures
• Deliverable: Preliminary O&M manual and staffing plan

2.7 Detailed Cost Estimate and Schedule (Months 8-10)

• Develop detailed cost estimate (±10-15% accuracy class) with line-item breakdowns
• Include equipment costs from vendor quotations
• Estimate construction labor, materials, equipment rental based on quantities
• Add project management, engineering, owner's costs
• Include escalation and contingency appropriate to project risks
• Develop detailed construction schedule showing critical path activities
• Integrate permitting timeline into overall project schedule
• Deliverable: Detailed cost estimate and project schedule for approval

Electrocoagulation:

• Mechanism: Sacrificial iron or aluminum electrodes dissolve under applied current releasing Fe²⁺/Fe³⁺ or Al³⁺ ions that hydrolyze forming coagulants in situ, eliminating need for chemical addition and storage
• Reactions at anode: Fe(s) → Fe²⁺(aq) + 2e⁻ (iron dissolution), 4 Fe²⁺ + O₂ + 4 H⁺ → 4 Fe³⁺ + 2 H₂O (oxidation), Fe³⁺ + 3 H₂O → Fe(OH)₃(s) + 3 H⁺ (hydroxide precipitation)
• Reactions at cathode: 2 H₂O + 2e⁻ → H₂(g) + 2 OH⁻ (hydrogen evolution generating alkalinity), metal cations + OH⁻ → metal hydroxides (precipitation)
• Performance: 85-98% removal efficiency for iron, aluminum, arsenic, heavy metals; pH increase from 3-4 to 6-8 without chemical addition
• Operating parameters: Current density 10-100 A/m², voltage 5-20 V, retention time 10-60 minutes, electrode spacing 5-20 mm
• Advantages: No chemical handling/storage, compact footprint, automated operation, generates less sludge than chemical treatment
• Limitations: Electrode consumption requiring periodic replacement (6-18 months typical), electrical energy requirement (0.5-3.0 kWh/m³), passivation of electrodes from precipitate coating requiring cleaning
• Economics: Capital cost USD 300-600 per m³/day capacity, operating cost USD 0.40-0.90 per m³ including energy and electrode replacement

Electrochemically Enhanced Limestone Dissolution:

• Innovation: Recent Nature Communications research (2024) demonstrates electrochemical activation enhancing limestone dissolution rate 10-50 times compared to conventional systems, enabling compact reactors with 82-100% metal extraction efficiency
• Mechanism: Applied electrical current at limestone surface promotes rapid CaCO₃ dissolution through localized pH reduction at anode and enhanced mass transfer from electromigration and micro-mixing
• Configuration: AMD flows through packed bed of limestone particles serving as sacrificial anode with inert cathode, current density 20-80 A/m² limestone surface area
• Performance: Alkalinity generation 200-500 mg/L CaCO₃ equivalent in 15-30 minute retention (versus 1-4 hours conventional limestone), complete metal precipitation downstream
• Advantages: Uses low-cost limestone (USD 10-30/ton) instead of lime (USD 80-150/ton), rapid kinetics enabling compact design, selective metal recovery through staged precipitation
• Status: Pilot-scale demonstration (10-100 L/min), commercial systems under development targeting 2026-2027 market availability

Electrochemical Metal Recovery:

• Application: Selective recovery of valuable metals (copper, nickel, zinc, cobalt) from AMD through electrodeposition onto cathodes producing high-purity metal products
• Process: AMD flows through electrochemical reactor with applied voltage causing metal cations to reduce and deposit on cathode: Cu²⁺ + 2e⁻ → Cu(s), producing 98-99.9% pure copper suitable for direct sale
• Staged recovery: Sequential cells operating at controlled potentials selectively recover different metals based on reduction potential: copper first (-0.34V vs Ag/AgCl), then zinc (-0.76V), then nickel (-0.25V)
• Economics: Viable for AMD containing >100-200 mg/L copper, >200-300 mg/L zinc, or elevated precious metals (gold, silver), with product revenues offsetting treatment costs
• Integration: Often combined with upstream electrocoagulation removing iron/aluminum before metal recovery section

Upflow Anaerobic Sludge Blanket (UASB) Bioreactors for AMD:

• Design concept: Vertical reactor with upward AMD flow through dense biological sludge blanket containing high concentrations (10,000-30,000 mg/L) of sulfate-reducing bacteria, achieving much smaller footprint than horizontal wetlands for equivalent treatment capacity
• Performance: Sulfate reduction 60-85%, acidity neutralization 75-90%, metal removal 85-95%, pH increase 2-3 units, in retention times 12-48 hours versus 2-7 days for wetlands
• Operational parameters: Organic carbon dosing (ethanol, acetate, molasses) 0.5-2.0 kg COD per kg sulfate reduced, temperature control 25-35°C optimal (heating may be required in cool climates), pH 6.0-7.5 in reactor optimizing SRB activity
• Footprint comparison: 50-200 m² per 1,000 m³/day capacity versus 400-800 m² for anaerobic wetlands, enabling constrained sites
• Challenges: Requires continuous organic carbon addition (operating cost USD 0.15-0.40 per m³), process control and monitoring infrastructure, startup period 2-4 months developing active biomass, sensitivity to temperature and toxic shock loading
• Economics: Capital cost USD 250,000-450,000 per 1,000 m³/day, operating cost USD 0.25-0.50 per m³ including organics, energy, maintenance

Sulfidogenic Bioreactors with Biosulfide Generation:

• Process innovation: Separate bioreactor generating high-concentration biosulfide (H₂S/HS⁻) from sulfate reduction, then dosing biosulfide into AMD precipitation tank where metals precipitate as highly insoluble metal sulfides (FeS, CuS, ZnS, NiS) achieving very low effluent concentrations (<0.1-1.0 mg/L for most metals)
• Advantages over direct AMD treatment: Biosulfide generation reactor operates under controlled optimal conditions (pH 7-8, temperature 30-35°C, organic dosing optimized) maximizing sulfate reduction rate independent of AMD chemistry, enables separation of biological and precipitation processes for optimization, produces more stable metal sulfide products less susceptible to re-oxidation than hydroxides
• Metal sulfide solubility advantage: CuS (Ksp = 8×10⁻³⁷), ZnS (Ksp = 2×10⁻²⁵), FeS (Ksp = 6×10⁻¹⁹) are orders of magnitude less soluble than corresponding hydroxides, enabling ultra-low effluent metal concentrations
• Applications: Particularly effective for copper, zinc, nickel, and other metals forming very insoluble sulfides, beneficial for AMD requiring very stringent discharge limits (<0.1-0.5 mg/L metals), suitable for moderate AMD flows 100-500 m³/day

Bioelectrochemical Systems (BES) for AMD Treatment:

• Technology concept: Combines biological sulfate reduction with electrochemical processes in microbial fuel cell or microbial electrolysis cell configuration, where bacteria oxidize organic carbon at anode transferring electrons through external circuit to cathode where metals electrodeposit or hydrogen evolves assisting metal precipitation
• Dual benefits: Biological sulfate reduction neutralizes acidity and generates sulfide for metal precipitation, electrochemical metal recovery produces high-purity products, potential electricity generation (microbial fuel cell mode) offsetting operational costs
• Performance: Research demonstrates 70-90% sulfate reduction, 85-95% metal removal, 20-100 mW/m² power production (small but offsets sensors/controls), selective metal recovery possible through electrode potential control
• Development status: Laboratory and small pilot-scale (10-100 L/day) demonstrations, commercial systems under development, estimated 3-5 years to market availability for AMD applications
• Limitations: Complex system requiring expertise in both biological and electrochemical processes, electrode fouling management, scaling challenges to commercial capacity, high capital cost currently (estimated USD 1,500-3,000 per m³/day based on pilot systems)

Step 1: Define Analysis Parameters

• Analysis period: Mine operational period + post-closure treatment period (e.g., 15 years operations + 30 years post-closure = 45 years total)
• Design capacity: Based on AMD characterization (e.g., 1,000 m³/day design capacity)
• Utilization factor: Actual treatment volume as percentage of capacity (typically 75-90% accounting for maintenance, seasonal variations)
• Discount rate: Corporate discount rate or weighted average cost of capital (typically 8-12% real for mining companies)
• Escalation rates: Chemical costs (3-5% annually), labor (4-6%), energy (4-7%), general inflation (3-4%)
• Currency: USD with assumptions about IDR/USD exchange rate for Indonesian cost components

Step 2: Capital Cost Estimation (Year 0)

• Direct costs: Equipment and materials, civil works, electrical and instrumentation, installation and construction
• Indirect costs: Engineering and design (8-12% of direct costs), project management (5-8%), owner's costs (3-6%), commissioning (2-4%)
• Contingency: Construction contingency (10-15% for detailed design), scope contingency (5-10% for uncertain elements)
• Example capital cost breakdown (1,000 m³/day active HDS system):
  • Equipment (reactors, clarifier, pumps, mixers, instrumentation): USD 320,000
  • Civil works (foundations, structures, piping, tankage): USD 280,000
  • Electrical and controls (MCC, SCADA, power distribution): USD 140,000
  • Installation and construction: USD 180,000
  • Subtotal direct costs: USD 920,000
  • Engineering and project management (15%): USD 138,000
  • Contingency (12%): USD 127,000
  • Total capital investment: USD 1,185,000

Step 3: Annual Operating Cost Estimation

• Chemical reagents: Annual quantity × unit cost (e.g., 640 tons lime/year × USD 110/ton = USD 70,400)
• Energy consumption: Annual kWh × electricity rate (e.g., 730,000 kWh/year × USD 0.093/kWh = USD 67,900)
• Labor: Operators, technicians, supervisors with benefits (e.g., USD 135,000 annually)
• Maintenance materials and services: 3-5% of capital cost annually (e.g., USD 40,000)
• Laboratory analysis: Number of samples × average cost (e.g., USD 20,000)
• Insurance and overhead: 2-4% of capital (e.g., USD 30,000)
• Total Year 1 operating cost: USD 363,000 (USD 0.99 per m³ at 1,000 m³/day, 365 days)

Step 4: Major Maintenance and Replacement Costs

• Mixer/pump overhauls: Year 7, 15, 23, 31, 39 (USD 45,000 each occurrence)
• Clarifier mechanism replacement: Year 12, 28 (USD 65,000)
• Instrumentation replacement: Year 10, 20, 30, 40 (USD 35,000)
• Structural rehabilitation: Year 18, 36 (USD 85,000)
• Control system upgrade: Year 15, 30 (USD 55,000)

Step 5: Calculate Net Present Value and Levelized Cost

• Discount each year's costs: PV = Cost / (1 + discount_rate)^year
• Sum discounted capital, operating, and maintenance costs over analysis period
• Calculate levelized cost per m³: Total lifecycle cost NPV / Total discounted water volume treated
• Compare alternatives: Technology A total lifecycle NPV versus Technology B, sensitivity to key assumptions

Example Lifecycle Cost Comparison: Active vs Passive vs Hybrid Treatment

Scenario: 1,000 m³/day AMD capacity requirement, moderate AMD (pH 3.5, 650 mg/L Fe, 90 mg/L Al, 850 mg/L acidity), 15-year operational period + 30-year post-closure = 45 years total, 10% discount rate

Analysis conclusions: Passive wetland shows lowest lifecycle cost but requires large land area (0.65 hectares) and may face performance challenges during seasonal variations. Active HDS demonstrates highest reliability and compact footprint but highest cost. Hybrid approach provides balanced solution with 76% of active-only cost, superior reliability versus passive-only, and operational flexibility valuable for long-term sustainability. Sensitivity analysis should evaluate impacts of discount rate variation (8-12%), treatment period uncertainty (30-60 years), chemical cost escalation scenarios, and performance risk monetization.

Step 1: Determine AMDAL vs UKL-UPL Requirements (Month 0-1)

• AMDAL (Analisis Mengenai Dampak Lingkungan) required if: New mining operation, major expansion requiring substantial new waste management, treatment capacity >10,000 m³/day (indicative threshold), significant environmental sensitivity (protected areas, critical habitats, water source protection zones)
• UKL-UPL (Upaya Pengelolaan Lingkungan - Upaya Pemantauan Lingkungan) acceptable if: Treatment facility within existing permitted mine, moderate scale (<5,000 m³/day typical), low environmental sensitivity, no protected area impacts
• Determination process: Consult with provincial environmental agency (Dinas Lingkungan Hidup/DLH), submit project description, receive screening decision (typically 2-4 weeks), appeal to Ministry of Environment and Forestry if disagree with determination
• Strategic consideration: AMDAL provides stronger legal foundation but requires 9-15 month timeline and higher cost (USD 50,000-200,000 depending on project scale and baseline studies required), while UKL-UPL enables faster approval (3-6 months, USD 10,000-30,000 cost) suitable for straightforward projects

Step 2A: AMDAL Process (if required) (Months 2-16)

• Phase 2.1: Terms of Reference (KA-ANDAL) Preparation (Months 2-4)
  • Engage accredited AMDAL consultant team (LP-AMDAL certification required)
  • Prepare scoping document identifying environmental components, study boundaries, methodology
  • Submit KA-ANDAL to AMDAL Commission for review
  • Revise based on commission comments (typically 1-2 revision cycles)
  • Receive KA-ANDAL approval (target 75 working days but often longer)
• Phase 2.2: Environmental Baseline Studies (Months 5-10)
  • Water quality: minimum 6-12 month monitoring covering wet and dry seasons
  • Aquatic ecology: benthic macroinvertebrates, fish surveys, habitat assessment
  • Social surveys: community demographics, water uses, stakeholder mapping
  • Air quality and noise (if relevant): ambient measurements at sensitive receptors
  • Other components per approved KA-ANDAL (terrestrial ecology, soils, etc.)
• Phase 2.3: Impact Analysis and Document Preparation (Months 11-13)
  • Prepare ANDAL (Environmental Impact Analysis) document: impact prediction modeling, significance assessment, alternative comparison
  • Prepare RKL (Environmental Management Plan): mitigation measures, monitoring programs, organizational responsibilities
  • Prepare RPL (Environmental Monitoring Plan): parameters, frequencies, locations, reporting
  • Total document package typically 300-800 pages depending on project complexity
• Phase 2.4: Public Consultation (Month 13)
  • Conduct public consultation meeting presenting project and AMDAL findings
  • Announce consultation 30 days in advance through newspapers and community notification
  • Facilitate question and answer, receive written comments
  • Prepare consultation report documenting attendance, comments, responses
• Phase 2.5: AMDAL Commission Review and Approval (Months 14-16)
  • Submit ANDAL, RKL, RPL with public consultation report to AMDAL Commission
  • Commission conducts technical review (75 working days target, often longer)
  • Present to commission meeting answering questions and concerns
  • Revise documents based on commission feedback (1-3 revision cycles typical)
  • Receive environmental feasibility approval (kelayakan lingkungan hidup)
  • Obtain environmental permit (izin lingkungan) from provincial governor or regency head

Step 2B: UKL-UPL Process (if AMDAL not required) (Months 2-7)

• UKL-UPL Preparation (Months 2-4): Engage environmental consultant (LP-AMDAL certification preferred but not always required), prepare environmental management and monitoring plan document (typically 50-150 pages), describe project activities and environmental management measures, specify monitoring parameters and frequencies, identify organizational responsibilities
• Baseline Data Collection (Months 3-5): Conduct abbreviated baseline surveys (1-3 months typical), water quality sampling minimum 2-4 sampling events, social assessment through interviews and secondary data, prepare baseline conditions summary
• Submission and Approval (Months 6-7): Submit UKL-UPL to regency/city environmental agency, agency technical review (30 working days target), revise based on comments if needed, receive environmental permit (izin lingkungan) for project implementation
• Advantages: Faster approval timeline (4-6 months total versus 9-15 months AMDAL), lower cost (USD 10,000-30,000 versus USD 50,000-200,000), less extensive baseline studies, simpler document preparation

Step 3: Discharge Permit Application (Concurrent with design, Months 6-14)

• Document Preparation: Treatment system design description with process flow diagram, discharge location map and coordinates, effluent quality predictions with supporting calculations, receiving water characteristics and classification, monitoring and sampling plan (parameters, frequencies, methods), emergency response and spill prevention plan
• Supporting Documents: Environmental permit (AMDAL or UKL-UPL approval), company environmental documents (SPPL - environmental management statement), business license and mine operating permits, laboratory accreditation if using internal lab
• Submission and Review: Submit application to provincial or regency environmental agency (jurisdiction depends on receiving water classification), agency conducts technical review and site inspection (60-90 working days target but often longer), may request additional information or clarifications, negotiate permit conditions if proposed limits problematic
• Permit Conditions: Specific discharge limits (pH, TSS, metals per applicable standards), monitoring frequency (typically weekly to monthly depending on parameters), reporting requirements (monthly reports, annual summary), permit validity period (typically 5 years renewable), modification procedures for changes in discharge quality or volume
• Fees: Application fee (typically IDR 500,000-2,000,000), annual monitoring fee based on discharge volume and pollutant load

Step 4: Additional Permits and Approvals (Various timelines)

• Building Permit (IMB - Izin Mendirikan Bangunan): Submit to regency/city public works department, requires site plan, structural drawings, 2-4 month timeline
• Water Abstraction License (if using water): Provincial/regency water resources agency, water requirements assessment, 3-6 month timeline
• Construction Safety Plan: Labor department approval for safety plan and procedures
• Forest Use Permit (if in forest area): Ministry of Environment and Forestry, land clearing authorization, can be lengthy (6-12+ months)

Ongoing Compliance Obligations During Operations:

• Self-Monitoring: Conduct sampling and analysis per discharge permit frequency, use accredited laboratory (ISO 17025), maintain chain of custody documentation, record operational parameters (flows, chemical dosing, equipment hours)
• Reporting: Monthly discharge reports to environmental agency via SILApop online system within 15 days of month end, quarterly statistical summaries, annual compliance summary reports, immediate incident reporting for any violations or upsets
• PROPER Program: Participate in Ministry of Environment's performance rating program (voluntary but strongly encouraged), submit annual environmental performance report, may receive public rating (Black/Red/Blue/Green/Gold), good ratings benefit corporate reputation and regulatory relationships
• Record Keeping: Maintain all monitoring data minimum 5 years, laboratory reports and chain of custody, equipment calibration records, operational logs and maintenance records, chemical purchase and consumption records, training documentation
• Inspections and Audits: Facilitate periodic regulatory inspections (typically annual or triggered by complaints), provide facility access and records as requested, address inspection findings promptly with corrective action plans, conduct internal audits verifying compliance status
• Permit Renewals: Initiate discharge permit renewal 6-9 months before expiration (5-year validity typical), update permit if substantial changes in operations, flows, or chemistry

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