Electrocoagulation Water Treatment: Mechanisms and Design

What is electrocoagulation?

Electrocoagulation (EC) is an electrochemical treatment in which a direct current is passed between a sacrificial metal anode and a cathode immersed in the water. The anode oxidises and dissolves, releasing metal cations (Al³⁺ or Fe²⁺/Fe³⁺) that hydrolyse to insoluble hydroxides — the same active coagulant species produced when you dose alum or ferric chloride, but generated in situ without adding a counter-anion (no sulphate or chloride load, no delivered acidity from the salt).

Three mechanisms act together: (1) coagulant generation — metal hydroxides sweep-floc suspended and colloidal matter; (2) charge neutralisation — cationic hydrolysis products compress the electrical double layer around negatively charged colloids; and (3) electroflotation — hydrogen microbubbles evolved at the cathode attach to flocs and lift them to the surface. EC therefore combines coagulation and flotation in one reactor, which is why it is often paired with, or compared to, dissolved air flotation.

How much coagulant does the current dose? (Faraday's law)

The defining advantage of EC is that dose is not a valve setting but a quantity of electric charge. The mass of anode metal dissolved is governed exactly by Faraday's law of electrolysis:

The corresponding half-reactions are, at the anode, Al → Al³⁺ + 3e⁻ (or Fe → Fe²⁺ + 2e⁻), and at the cathode, 2H₂O + 2e⁻ → H₂(g) + 2OH⁻. The cathode reaction both consumes protons — raising pH — and produces the H₂ microbubbles that drive flotation. In practice the anode often dissolves slightly faster than Faraday predicts (current efficiency commonly 100–160% for aluminium) because of chemical, "chemical dissolution" pitting alongside the electrochemical reaction.

Al or Fe: which sacrificial electrode?

Aluminium gives a strong, light hydroxide floc effective over pH ~6–8 and is preferred where colour, phosphate or fine colloids dominate. Iron is cheaper and robust for high-strength or reducing wastewaters, but Fe²⁺ must oxidise to Fe³⁺ to form the more effective ferric hydroxide, so aeration or dissolved oxygen matters, and residual soluble iron can impart colour. The choice mirrors conventional coagulation and flocculation chemistry — the same Al/Fe trade-offs apply because the active species are identical.

Whichever metal is used, floc growth still obeys classical perikinetic then orthokinetic aggregation, so a downstream flocculation and separation stage (settling, filtration, or flotation) is needed to remove the solids the reactor forms.

Worked example: dosing and energy by Faraday's law

Target an aluminium dose of 40 mg/L on a flow of 10 m³/h (2.78 × 10⁻³ m³/s), using aluminium electrodes. Work from the dose back to current, then to power.

• Metal mass rate required: 40 mg/L × 10,000 L/h = 400,000 mg/h = 400 g/h = 0.400 kg/h (0.1111 g/s).
• Current from Faraday's law: rearranging, I = (m · z · F) / (t · M). Per second, I = (0.1111 g × 3 × 96,485) / (1 s × 26.98) = 32,166 / 26.98 ≈ 1,192 A (at 100% current efficiency).
• Electrode consumption: 0.400 kg Al per hour, i.e. 0.400 / 10 = 0.040 kg Al/m³ — the sacrificial-anode replacement rate.
• Cell voltage and power: take a typical operating voltage of 4 V across the cell. Power P = V · I = 4 × 1,192 = 4,768 W ≈ 4.77 kW.
• Specific energy: 4.77 kW ÷ 10 m³/h = 0.48 kWh/m³.

So delivering 40 mg/L Al in situ needs roughly 1,190 A, consumes about 40 g of aluminium per cubic metre and costs about 0.48 kWh/m³ of electrical energy — figures you can compare directly against the cost of dosing an equivalent mass of alum. Cell voltage (and therefore energy) rises as electrode spacing widens or conductivity falls, so the specific energy is dominated by the water's ionic strength.

Current density, passivation and electrode life

Current density (A/m² of electrode area) is the key operating variable: higher density means faster coagulant generation and more vigorous H₂ evolution, but also higher voltage, more heat and faster wear. Practical designs run 10–150 A/m².

The main operational risk is passivation — an insulating oxide film (especially Al₂O₃) building on the electrodes, which raises resistance and starves the process of coagulant. Chloride ions help by pitting the film, so a modest Cl⁻ concentration is beneficial; periodic polarity reversal and mechanical or acid cleaning are also standard countermeasures. Electrode life is set directly by the consumption rate: at 0.040 kg Al/m³ a plate stack is a scheduled consumable, and replacement cost must be built into the operating budget.

How does pH evolve during electrocoagulation?

Unlike alum or ferric dosing — which consume alkalinity and depress pH — EC tends to raise pH in acidic to neutral water because the cathode generates hydroxide (2H₂O + 2e⁻ → H₂ + 2OH⁻). This is often a benefit: the process can self-buffer towards the optimum Al(OH)₃ window (~pH 6–8) without acid/base dosing. In already-alkaline water the rise can push pH above the hydroxide solubility minimum, re-dissolving coagulant as aluminate (Al(OH)₄⁻), so pH must still be monitored. The net pH trajectory is the balance between cathodic OH⁻ production and the protons released as the metal hydrolyses.

Electrocoagulation vs conventional chemical coagulation

EC and chemical coagulation share the same active chemistry but differ sharply in operation, consumables and residuals:

EC suits compact, chemical-averse or remote installations and emulsion-breaking duties (metal finishing, oily water, some food effluents). For high, steady flows where chemical logistics are easy, conventional dosing into a clarifier or a flotation and separation package is often more economical. The two are not mutually exclusive — EC is frequently used as a pre-treatment ahead of a DAF polishing stage.