Electrocoagulation: From Electrical Charge to Separation

One Operation, Two Different Outcomes

Electrocoagulation, EC, generates metallic species through consumable electrodes and promotes aggregation or incorporation of water constituents into a separable phase. Its assessment must distinguish coagulant production from subsequent removal. Producing flocs does not mean extracting them from the circuit. A sample collected before settling, flotation or filtration may contain solids originating from the electrode itself and show more suspended solids than the feed, even though their separability has improved.

Within Aguas SB's proprietary commercial T1 method, this distinction leads to an operational question: how much improvement comes from transforming the dispersion, and how much from removing material? Answering it requires examining both water and residue. A decrease in chemical oxygen demand, COD, in the supernatant does not, by itself, identify oxidation, much less mineralization. It may represent organic matter incorporated into sludge, a useful outcome that needs the correct name.

Faraday's Law and Units

For an ideal anodic reaction, each mole of metal released requires a defined amount of electrons. Accumulated charge is calculated by integrating the current actually measured:

Qe = integral I(t) dt
m_theoretical = M * Qe / (z * F)
D_electrical = Qe / V

Qe is electrical charge in coulombs, not flow rate; I is expressed in amperes and time in seconds. F = 96485 C/mol is the constant used here; z denotes electrons per atom released, and M is molar mass in grams per mole. Consequently, M*Qe/(z*F) has units of grams. At constant current, the integral becomes I*t. One ampere-hour equals 3600 C: omitting this conversion introduces an error of three orders of magnitude.

The mass-to-volume ratio also requires care. One gram distributed through one cubic metre corresponds to one milligram per litre. This result is an equivalent concentration of added metal, not necessarily dissolved metal at the outlet, active coagulant concentration or dry sludge mass. Precipitates incorporate other elements and may retain contaminants and water.

Current density, j = I/A_active, adds a different coordinate. Two cells operated at the same current for the same time receive equal total charge but may have different current densities, reaction distributions and surface responses. The definition of wetted active area must be stated, including participating faces and electrical connections. Multiplying power-supply current indiscriminately by the number of plates is incorrect.

Synthetic Example 1: 2 A for One Hour in 5 m3

Consider a purely stoichiometric balance, without assigning it to any equipment or actual water. Assume a constant current of 2 A, a duration of 3600 s, a volume of 5 m3 and ideal dissolution of the specified metal:

Qe = 2 * 3600 = 7200 C
D_electrical = 7200 / 5 = 1440 C/m3
m_Al = 26.9815 * 7200 / (3 * 96485) = 0.67115 g
m_Fe = 55.845 * 7200 / (2 * 96485) = 2.08366 g

The difference follows from M/z, not superior treatment performance by iron. The Fe calculation assumes release as Fe(II); it does not establish subsequent speciation. Nor does it select an optimum dose: composition, treatment target, chemical availability, capture and separation are still unknown. The decimal places make the arithmetic auditable but do not represent precision achievable in a weighing campaign.

When current varies, its recorded history must be integrated; the setpoint is not a substitute. When sampling or purging changes volume, the relevant mass and volume must be reconstructed before reporting an equivalent concentration.

Aluminium, Iron and Matrix

The schemes Al -> Al(III) + 3e- and Fe -> Fe(II) + 2e- count electrons. The species interacting with colloids cannot be reduced to those symbols: hydrolysis, solid formation, complexation and local chemical evolution are involved. For iron, transformation between oxidation states also matters. Tank pH does not necessarily describe the immediate environment of each surface.

Selecting aluminium or iron should be treated as a process comparison. Relevant outcomes include residual metal, sludge volume and handling, consistency of separation and compatibility with downstream operations. Matching amperes alone does not match metal mass or chemistry. An informative comparison states what is held constant: charge, moles of metal, equivalent concentration or final treatment outcome.

The matrix can simultaneously change conductivity, coagulation demand and surface condition. A lower voltage after changing the water therefore does not demonstrate better coagulation; it may reflect lower electrical resistance. An experimental study using formulated water and Al/Fe electrodes found different passivation responses to carbonate, chloride and polarity reversal. It supports investigating matrix dependence, not prescribing a universal addition. Passivation study, 2025.

Measured Efficiency and the Meaning of Weighing

Indicators should be defined without combining different observables:

eta_loss = electrode_mass_loss / theoretical_mass
eta_recovered = corrected_recovered_metal / theoretical_mass

The second ratio requires subtracting metal initially present and adding the phases included within the boundary. The first may include detached material, chemical corrosion and errors associated with deposits or cleaning. A ratio above one therefore does not prove that more electrons were generated than supplied; it indicates that weighing does not represent the selected Faradaic reaction exclusively. An experimental study of cork-boiling wastewater compared observed electrode loss with Faraday's prediction and identified a chemical dissolution contribution. EC in a real matrix, 2022.

Calling a ratio “Faradaic efficiency” requires defining the mass and mechanism being counted. Total loss matters for maintenance engineering; explaining the process additionally requires knowing which fraction enters the liquid, precipitates or remains attached. No single indicator answers both questions.

Synthetic Example 2: Reconciling Loss and Recovery

Assume a theoretical mass of 10.0 g, an electrode mass loss of 12.0 g and 9.0 g of recovered metal, corrected for the initial background, across liquid and solids. Then eta_loss = 1.20 and eta_recovered = 0.90. These invented calculation inputs do not demonstrate a chemical efficiency of 120%.

The 3.0 g gap between loss and recovery calls for examination of deposits, unsampled fractions or cleaning and digestion biases. It must not automatically be assigned to dissolved metal or an unknown reaction. Repeating the balance with conditioned electrodes and deposit recovery can distinguish these explanations. The testable prediction is that a more complete boundary will reduce the discrepancy if retained material was its main cause.

Mini-Protocol 3: Surface and Separation

A practical comparison starts with a homogenized batch divided between EC treatment and a hydraulic control without current. Current, voltage, temperature, conductivity and pH are recorded over time; electrode area, initial condition and cleaning procedure are documented. Weighing must follow a reproducible procedure with a blank that estimates losses caused by cleaning itself.

Samples are then collected before and after a defined separation, using identical settling time or filtration procedure in both arms. Relevant metal, solids and COD fractions are analysed, and the residue is retained to close the balance. Analytical filtration must not be presented as an installed industrial separation. If the advantage appears only after separation, the conclusion concerns EC plus separation, not the isolated cell.

A voltage rise at constant current guides inspection but does not alone diagnose passivation: changes in conductivity, temperature, connections and geometry must be excluded. As an inferred recommendation, successive operating cycles should also be compared, because an initial test with freshly prepared surfaces can conceal operational drift.

Evidence and Design Questions

The 2011 thesis by Pablo Santiago Hernández Lehmann describes a 144 L/h plant in its abstract and addresses treatment, separation and energy. The record and abstract were consulted here, not the complete equations or full experimental conditions. The supervisor must not be confused with the author. UPM thesis.

For T1, testable questions are whether recovered mass follows charge, whether the surface retains its response, and whether contaminants removed from water appear in a controlled phase. Changing pH, material or cleaning may justify another experiment; it does not automatically change the engine's rules. EC's contribution is better described through charge, metal, surface condition and separation than through a single percentage.