"Poured hydrogen peroxide into the tank - system is clean." But hydrogen peroxide in the reservoir during active growing is not system disinfection. It is root stress if the concentration is too high, and near-zero effect if it's too low. Sanitizers - H₂O₂, PAA, and ozonation - are effective tools, but only at the correct concentration, correct contact time, and with a clear understanding of what each one is used for. The same substance at different doses is prevention, treatment, or root poison.
Quick glossary: H₂O₂ (hydrogen peroxide) - an oxidizer that breaks down into water and oxygen; used at concentrations from 0.001% (preventive during growing) to 3-5% (disinfection between cycles). PAA (Peracetic Acid) - a more powerful oxidizer than H₂O₂; more effective against biofilms and pathogen spores; breaks down into acetic acid, water, and oxygen with no toxic residues. Contact time - the minimum time the disinfectant must remain in contact with a surface or solution to achieve the intended effect; reducing contact time sharply reduces efficacy.
H₂O₂: The Logic of Concentrations
Hydrogen peroxide is the most widely used sanitizer in hydroponics precisely because it breaks down into water and oxygen with no toxic residues. But "safe" does not mean "the same at any dose."
Preventive dose during active growing: 0.001-0.003% (30-100 mL of 3% H₂O₂ per 100 L of solution). At this concentration, peroxide partially suppresses biofilm development and maintains ORP without harming roots. Apply once every 5-7 days. Monitor DO after application - H₂O₂ oxidizes organic matter and can temporarily increase oxygen consumption.
Curative dose for mild root rot or early biofilm formation: 0.003-0.01% (100-300 mL of 3% H₂O₂ per 100 L). Suppresses active Pythium and halts biofilm development. At this concentration there is a risk of mild root stress - treat for no more than 24-48 hours followed by a flush with clean water.
Disinfection between cycles (without plants): 3-5% concentration (straight 3% pharmacy solution, or 90-150 mL of 35% technical H₂O₂ per 1 L of water). Contact time 30-60 minutes circulating through the entire system. Effective against bacteria and fungi but less effective at breaking down mature biofilm matrix - mechanical cleaning beforehand is mandatory.
Important nuance: H₂O₂ decomposes rapidly in the presence of organic matter and metals. If the system is contaminated with organics, most of the peroxide is consumed oxidizing them before reaching pathogens. Zero effect at the correct concentration in a dirty system.
PAA: When a More Powerful Tool Is Needed
PAA (peracetic acid) is a mixture of acetic acid and H₂O₂ that forms an unstable but powerful oxidizer, and at equivalent concentration it outperforms H₂O₂ against mature biofilms, pathogen spores, and for system sanitation between cycles after a serious root rot infection.
Mature biofilms. PAA penetrates the polysaccharide matrix of biofilms more effectively and breaks it down from within. Where H₂O₂ treats the surface, PAA penetrates deeper.
Pathogen spores. Spores of Pythium, Fusarium, and bacterial endospores are more resistant to H₂O₂ than to PAA at comparable concentrations. With recurring root rot after H₂O₂ treatment - try PAA as an alternative.
Between cycles after serious infection. If the previous cycle ended with root rot, PAA is more reliable than H₂O₂ for system sanitation.
Working PAA concentrations for between-cycle disinfection: 0.1-0.2% (PAA is typically sold as a 5-15% concentrate - dilute to a final concentration of 0.1-0.2%). Contact time 20-30 minutes. Flushing with clean water afterward is mandatory.
PAA during growing: dose it by pure peracetic acid, not by product. In a trial with hydroponic garden cress, 2-4 mg/L of pure PAA once a week caused no phytotoxicity (details in the article on biofilms). This is less established practice than H₂O₂, since roots are more sensitive to the acetic acid produced when PAA breaks down.
What to Use and When
Use low-dose H₂O₂ 3% for routine prevention, a higher H₂O₂ dose for 24-48 hours against mild root rot, and PAA for between-cycle disinfection of mature biofilms or for shock treatment after a serious infection.
| Situation | Tool | Concentration | Duration |
|---|---|---|---|
| Preventive during growing | H₂O₂ 3% | 0.001-0.003% | Once per week |
| Mild root rot or early problem signs | H₂O₂ 3% | 0.003-0.01% | 24-48 hours |
| Between-cycle disinfection (light contamination) | H₂O₂ | 3-5% | 30-60 min |
| Between-cycle disinfection (mature biofilm, root rot) | PAA | 0.1-0.2% | 20-30 min |
| Shock treatment after serious infection | PAA | 0.2-0.3% | 30 min + mechanical cleaning |
Three Mistakes That Cost the Most
"More is better" with preventive doses. H₂O₂ at 0.1% during active growing causes chemical root burn. Concentrations for growing and for between-cycle disinfection differ by a factor of hundreds to thousands. Use the table above.
Disinfecting without mechanical cleaning first. H₂O₂ and PAA are consumed oxidizing organic matter before reaching pathogens. With heavy organic deposits and mature biofilms, no concentration compensates for skipping mechanical cleaning. Brush and flush first, then apply chemistry.
Not accounting for H₂O₂ degradation during storage. 3% pharmacy peroxide in an open bottle at room temperature loses concentration within a few weeks. Technical H₂O₂ (35%) stores longer but should be diluted on the day of use. Old or improperly stored peroxide delivers reduced efficacy with no visible sign of degradation.
How to Know the Sanitizer Worked
A sanitizer has worked when the organic load is gone from the surfaces, and that is something you verify rather than assume once the contact time has elapsed. Four checks confirm it.
- After between-cycle treatment: neutral odor with no sour or putrid smell
- Rinse water coming out is clear and EC ≤ 0.1 mS/cm
- Reservoir and pipe walls feel smooth with no sticky deposits when checked by hand
- ORP after starting the new cycle stays stable above 600 mV without oxidizers applied - a sign that organic load and microbial activity are low
If you want to go deeper: Pipeline Sanitation: Removing Organic Deposits Without Stopping the System shows how to apply sanitizers within the full system cleaning protocol.