Hazardous wastewater contains substances that are toxic, persistent or present at very high concentrations: heavy metals, cyanide, solvents, pesticides, strong acids and alkalis or high salt levels. It comes from electroplating, chemicals, pesticides, hazardous waste collection and recycling plants, among others. Discharging it without proper treatment harms people and the environment and carries heavy legal penalties.
Why it is difficult to treat
- Composition varies widely from batch to batch
- Toxic compounds inhibit or kill biological treatment
- High salinity and COD foul ordinary membranes quickly
- Sludge and concentrate must also be handled as hazardous waste
Typical treatment steps
Treatment usually begins with segregation and equalization, so that streams of very different quality are not mixed blindly. Physico-chemical treatment follows: neutralisation, precipitation of heavy metals, cyanide destruction and advanced oxidation (Fenton, ozone) to break down persistent organics. Once toxicity is reduced, biological treatment can remove the remaining organic load.
Membrane and thermal technologies
Robust membranes are increasingly used for hazardous streams. Tubular UF handles high solids and oil, DT/ST modules work with high salinity and fouling, and acid- or solvent-resistant membranes can recover chemicals for reuse. Where discharge is not permitted, evaporation and crystallisation reduce the remaining concentrate to a solid residue.
Working with an experienced partner
Because each hazardous stream is different, treatability tests in the laboratory or with a pilot unit are strongly recommended before full-scale design. TIPA helps evaluate the wastewater, select suitable technologies and supply the membranes and equipment needed.
Legal background in Vietnam
Under the Law on Environmental Protection 2020, hazardous waste is waste containing toxic, radioactive, infectious, flammable, explosive, corrosive or otherwise hazardous components. It must be stored separately by type, transported by licensed vehicles with tracking devices, and treated only at licensed facilities with qualified staff and an environmental safety plan. Hazardous wastewater comes from rubber, welding and plating, automotive, circuit boards, chemicals, oils, solvents, medical waste and batteries, and treated water must meet QCVN 40:2011/BTNMT.
Treating mixed hazardous wastewater in practice
At a large hazardous waste treatment plant, incoming streams are split into three groups before treatment:
- Group 1 – acidic, alkaline and metal-bearing wastewater: pH adjustment, then flocculation
- Group 2 – cleaning solutions, organic and inorganic chemicals, paint: Fenton oxidation to break down organics
- Group 3 – oily wastewater: oil removal by dissolved air flotation (DAF), then treated like group 2
All streams then pass through coagulation – flocculation in the DAF tank, sand and fine filters (removing particles above 10 µm) and a Rochem PFRO membrane system. Permeate is monitored online for dissolved solids; the concentrate is incinerated, and sludge is dewatered in a filter press to 60 – 70% moisture.
- Treated water meets QCVN 40:2011/BTNMT column A even when the feed keeps changing
- Handles flow up to 20% above design and pollution 1.5 times higher
- Modular design from 15 to 2,000 m³/day, batch or continuous
- Runs 20 hours a day with 2 hours reserve and 2 hours for membrane cleaning
- About 30 – 40% less space than conventional treatment
