Fenton Oxidation Process

The Fenton oxidation process is an advanced oxidation process (AOP) that combines ferrous iron (Fe²⁺, typically dosed as ferrous sulfate) with hydrogen peroxide (H2O2) under acidic conditions to generate hydroxyl radicals (OH•) — among the most powerful oxidants available in water treatment. These radicals attack and break down complex, recalcitrant organic molecules that resist conventional biological treatment, converting them into simpler, more biodegradable, or fully mineralised compounds. Fenton oxidation is widely used where activated sludge or anaerobic systems alone cannot achieve the required COD reduction.

The process follows a defined sequence. Raw effluent is first acidified to pH 3-4, the optimal range for hydroxyl radical generation. Fenton's reagent — ferrous sulfate and hydrogen peroxide — is then dosed into the acidified stream, and the reaction is allowed to proceed for a controlled reaction time of roughly 30-60 minutes while the radicals oxidise dissolved organics. Once the reaction is complete, the effluent is neutralised back to pH 7-9. This neutralisation step precipitates the dosed iron as ferric hydroxide sludge, which conveniently co-precipitates some residual colour and heavy metals along with it. A clarification stage then settles out the iron sludge, leaving a clarified, substantially lower-COD effluent.

Fenton oxidation is most commonly deployed for high-COD recalcitrant industrial effluents from pharmaceutical, pesticide, dye-intermediate, and specialty chemical manufacturing, where complex or toxic organics survive biological treatment untouched. It is also used as a pretreatment step to raise the BOD/COD ratio of an effluent — breaking large refractory molecules into smaller, more biodegradable fragments — before the stream is routed to a biological treatment system. In some cases, particularly where discharge COD limits are very strict, Fenton oxidation is applied as a standalone polishing step ahead of final discharge. Typical COD reduction achieved is 60-90%, depending on effluent characteristics, reagent dosing, and reaction conditions, with actual performance best confirmed through bench-scale jar testing on the specific effluent.

Two operating realities shape how Fenton oxidation is deployed in practice. First, the process generates iron hydroxide sludge that requires dewatering and disposal — a real, ongoing operating consideration that adds to plant footprint and operating cost. Second, reagent costs for hydrogen peroxide, ferrous sulfate, and the acid and alkali needed for the pH swing are significant. For this reason, Fenton oxidation is often applied selectively to a high-strength or recalcitrant side-stream rather than the full effluent flow, concentrating treatment cost on the fraction of flow that actually needs it while the balance of plant effluent is handled by lower-cost biological treatment. This side-stream approach is a key design decision Spans Envirotech evaluates during effluent characterisation and treatability studies for Indian industrial clients facing tightening CPCB/SPCB COD discharge norms.