Pesticide Wastewater: Matching Five Very Different Streams to the Processes That Can Handle Them
Pesticide wastewater is what a plant discharges during pesticide production, and its quality and quantity are unstable by nature. Treating it as one stream is the first mistake. The category splits into several chemically distinct wastes, and each has a different workable route.
Five streams, five problems
Benzene-containing wastewater comes from organochlorine production: producing 1 ton of BHC (hexachlorocyclohexane) discharges 3-4 tons of wastewater with benzene content of 1,500-2,000 mg/L, which can be handled by distillation followed by adsorption on coal-gangue slag. Organophosphorus-containing wastewater runs COD above 10,000 mg/L with around 1,000 mg/L of organophosphorus; dimethoate, methanol and dimethylamine are worth recovering first by extraction or distillation, after which the residue can be rendered harmless biologically. High-concentration phenol-containing wastewater is treated by recovering phenol through extraction to bring content below 300 mg/L, then biological or chemical oxidation once appropriately pretreated. Mercury-containing wastewater is acidic with mercury in dissolved form, so sulfide precipitation works; reverse osmosis and the activated-carbon biofilm method have also been applied. Several countries have banned organochlorine production and are researching microbial pesticides, which is the fundamental way to prevent the pollution in the first place.
Why the stream is so hard
Pesticides come in great variety and the wastewater is chemically complex. Pollutant concentration is high, with chemical oxygen demand (COD) reaching tens of thousands of milligrams per litre. Toxicity is high: besides pesticides and intermediates the water carries phenol, arsenic and mercury along with many compounds organisms cannot degrade. It smells foul enough to irritate the human respiratory tract and mucous membranes. And quality and quantity are unstable, so a process tuned on Monday can be off-spec by Friday. The environmental impact when it goes wrong is correspondingly serious.
Biology: necessary, rarely sufficient
Most domestic pesticide manufacturers have built biological treatment facilities, and almost none achieve ideal results. Research continues because it has to. Fungi, bacteria and algae all degrade pesticides well, and the biofilm method — fixing cells on packing medium where they attach, grow and reproduce into a film-like biological sludge — offers high volumetric biomass concentration, long survival generations and a rich variety of microorganisms. That makes it especially suitable for introducing specialised strains into a wastewater system, which is often the only way to attack a specific compound.
Micro-electrolysis and the physical routes
Iron-carbon micro-electrolysis works through flocculation, adsorption, bridging, sweeping, co-precipitation, electrodeposition and electrochemical reduction together. Freshly formed iron surfaces and the large amount of nascent Fe2+ and atomic H produced are highly active, changing the structure and properties of many organic compounds through chain scission and ring opening. The electric field around the micro-cells also drives charged ions and colloids to aggregate and deposit on the electrodes, while the Fe2+, Fe3+ and their hydrates formed add strong adsorption and flocculation. The practical outcome is better biodegradability, which is usually the real objective.
Ultrasound — sound above 20 kHz — degrades organics through cavitation: the negative-pressure phase creates extreme conditions that cause bond cleavage, aqueous-phase combustion, high-temperature decomposition or free-radical reactions. Beyond a certain concentration the degradation rate changes little, and solution temperature should be held at 15-60 C. Applied as ultrasonic flotation pretreatment on monocrotophos and trimethyl phosphite wastewater, it cut COD and toxicity and improved biodegradability, with subsequent photosynthetic-bacteria-dominated biological treatment bringing COD down to 200 mg/L. Most ultrasonic work remains at laboratory scale, with little systematic or pilot data.
Advanced oxidation: where the results are
Advanced oxidation generates highly reactive OH radicals through oxidant combinations and is regarded as the best technology for refractory organic pollutants. Anatase TiO2 under ultraviolet irradiation generates hydroxyl radicals that oxidise organics to CO2, H2O and inorganic substances — fast degradation, no secondary pollution. The open questions are the influencing factors and transformation pathways, the immobilisation of nano-TiO2 and integration of reaction with separation, and raising the catalytic efficiency of prepared catalyst. Most photocatalytic systems run on high-pressure lamps, high-pressure xenon lamps, black-light lamps or ultraviolet germicidal lamps, all of which consume substantial energy. If nano-TiO2 can be stably sensitised to broaden its absorption spectrum so sunlight works directly as the source, cost would fall sharply.
Introducing ultraviolet light and hydrogen peroxide together while controlling reaction pH can improve ozone advanced oxidation further. On 2,4-dichlorophenoxyacetic acid (2,4-D) wastewater, four ozonation processes were compared — ozonation alone, ozone/UV, ozone/hydrogen peroxide and ozone/hydrogen peroxide/UV — and ozone/UV advanced oxidation was the best. For a 2,4-D sample at 200 mg/L, reaction for 30 min achieved complete 2,4-D degradation, and mineralisation exceeded 75% at 75 min. An alkaline environment favours the ozonation reaction. Separately, aeration promotes photolysis, particularly for the UV/Fenton process: after 2 h of photolysis the COD removal rate improved, and adding H2O2 at a dosage of 75 mg/L raised the COD removal of the sample further. In practice, H2O2 dosing is easier to retrofit than a UV bank, which is why peroxide-based routes often get tried first on existing pesticide plants despite the stronger laboratory showing of ozone/UV.