Fruit and vegetable processors use 3–5 liters of potable water per kilogram of produce—then face high‑strength effluent loaded with soils, organics, nutrients, and pesticide residues. Engineers are converging on a compact physico‑chemical train that pairs high‑performance coagulation with DAF and carbon or AOP polishing, cutting sludge and footprint while meeting tight limits.
Industry: Agriculture | Process: Harvesting_Equipment
It starts with a simple number: fruit and vegetable processing typically uses 3–5 L per kg of produce (ResearchGate). By the end of a shift, that becomes high‑strength washwater that far exceeds common discharge standards for COD (chemical oxygen demand), BOD (biochemical oxygen demand), TSS (total suspended solids), and oil/grease. One reference point: Indonesian limits (e.g., Ministerial Decree 51/1995 for palm‑oil mills) are ~500 mg/L COD, 250 mg/L BOD, 300 mg/L TSS, and 30 mg/L oil/grease (Saka).
Processors aiming to reclaim water for irrigation or cooling often target even lower pollutant levels, which puts the spotlight on a reliable train to strip suspended solids, organics, and trace pesticides before discharge or reuse. The playbook below distills current evidence, with inline links to the datasets and pilot results that are informing design choices.
Pretreatment: screening and grit removal
Up front, large debris—skins, seeds, and sand—comes out via coarse screening or grit chambers. Tests show simple screening is highly effective for “washing‑only” wastewater rich in sand or large peelings (e.g., root vegetables) (ResearchGate). Hydrocyclones or centrifuges also remove heavy grit; one study rated hydrocycloning “very good to excellent” for TSS/DS removal in root‑crop washwater (ResearchGate). Early grit removal prevents downstream clogging and reduces sludge volumes.
Many plants standardize this step with an automatic screen for continuous debris removal; small lines may opt for a manual bar rack and modular primary separation skids that bundle screens and oil capture.
Chemical coagulation–flocculation basics
Next, chemical coagulation and flocculation (C&F) agglomerate fine solids, colloids, and a chunk of the COD. High‑performance coagulants—polyaluminum chloride, polyferric sulfate, or polymeric coagulants like polyDADMAC/polyamine—minimize dose and sludge. Bench studies indicate polymeric coagulants typically work at 5–250 mg/L, whereas conventional metal salts often need 250–5,000 mg/L (ResearchGate; ResearchGate). Cationic polyacrylamides or chitosan bind both organic and inorganic solids across a wide pH range, driving cost down.
Jar tests on diverse produce washwaters typically show >80% turbidity removal with an optimized polymer dose (ResearchGate; ResearchGate). Reductions in COD ranged from “excellent to very good” in most streams, with lower drops in some leafy‑green or fruit effluents carrying high dissolved salts/organics (ResearchGate; ResearchGate).
Operationally, dosing is standardized with inline dosing pumps and commodity reagents—plants often specify coagulants for primary charge neutralization and follow with flocculants to grow settleable or floatable flocs. Where polyaluminum chloride is preferred, packaged supply as PAC simplifies procurement.
DAF versus gravity clarifiers
Dissolved‑air flotation (DAF) injects fine bubbles (order 20–30 μm) under pressure so floc–bubble aggregates float and are skimmed, leaving clarified water below. In produce duty, DAF consistently beats settling: a comparative study on microalgae (a proxy for colloidal biosolids) reported ~98% removal for DAF versus 87% for plain settling (ResearchGate). Tests on fruit/vegetable wash water showed DAF “reduced COD more effectively than settling and C&F+settling” and removed additional oil/grease and nutrients (ResearchGate).
Sludge quality diverges too: sedimentation produced ~1.1% solids versus ~3.4% in DAF (≈3× thicker), easing dewatering and disposal (ResearchGate). DAF systems are faster (minutes‑scale detention) and more compact (often 5–10× less area) but add pressurized air recirculation and skimming. Even so, one techno‑economic analysis found a floc+DAF line delivered 33% lower total CAPEX and 20% lower OPEX per unit organics removed than floc+settling, driven by lower dewatering costs (ResearchGate).
Gravity clarifiers (sedimentation) hold flocculated water at low velocity (overflow rates on the order of 1 m³/m²·h or less) for hours‑scale detention. They are simple and handle high flows smoothly, but footprint grows and sludge is dilute. Lab tests typically show 80–90% TSS removal (ResearchGate). CAPEX for large basins can be lower, but sludge handling often pushes OPEX higher. Where land and building costs are low, clarifiers remain attractive for batch or low‑flow facilities.
Pilot‑testing on actual washwater is recommended before choosing. Some plants run hybrids—e.g., a conventional clarifier for routine solids and a smaller DAF unit for peak loads or FOG control.
Polishing pesticides: activated carbon and AOPs
After solids and bulk organics, the target shifts to trace organics like pesticides and disinfectants. Activated carbon (AC) in granular (GAC) or powdered (PAC) form adsorbs a wide range; one pesticide‑laden water case reported PAC removed ~60–65% of COD and significantly cut acute toxicity (MDPI). Well‑designed GAC beds with 10–20 min empty‑bed contact time often reduce most pesticide concentrations by >80%. Plants implement this as a packed GAC filter—sourced as activated carbon media—or dose PAC via a contact tank followed by settling or microfiltration; a simple way to remove PAC fines is a downstream cartridge filter.
Advanced oxidation processes (AOPs) chemically degrade pesticides using radicals (•OH). Common options include ozone, O₃/H₂O₂, UV/H₂O₂, and Fenton’s reagent. In a fresh‑cut produce pilot seeded at 0.1 mg/L of various pesticides, ozonation at pH 6.3 achieved >80% pesticide removal only after ~120 minutes (total O₃ dose ~180 mg/L); ozone was extremely fast at disinfection, reaching >5‑log E. coli kill in 5 minutes (MDPI). Fenton’s reagent achieved ~95% COD removal and virtually eliminated acute toxicity in a pesticide‑plant wastewater, outperforming ferric coagulation and PAC, but it needs tight pH control and produces iron sludge (MDPI).
Design note: AOPs belong after solids removal to avoid radical scavenging. For ozone, typical injection rates are 0.1–0.2 g O₃/L·h in a pressurized contactor, with 30–60 min detention for multi‑log pesticide decay (MDPI). UV/H₂O₂ is a practical route when UV transmittance is high; for non‑potable reuse loops, compact UV systems are commonly dropped in as the terminal barrier. A simple, robust configuration pairs GAC with a short ozone or UV polish to reach >90% pesticide removal at moderate chemical use.
Scalable plant design parameters
1) Equalization and screening: a small equalization tank with 1–2 h retention dampens pulses and stabilizes dosing. Coarse screening at 5–10 mm removes debris; facilities standardize with rugged grates or automated screens. Where space is tight, an inline automatic screen simplifies maintenance.
2) Coagulation/flocculation: a rapid‑mix tank (around 0.5–1 m³ in small skids) doses coagulant at 20–200 mg/L (polyaluminum or polymeric coagulants are typically 10–100 mg/L in commercial wash‑water treatment) (ResearchGate). Rapid mix is sized at G≈100–200 s⁻¹; the floc zone runs 15–30 minutes at G≈20–50 s⁻¹. Skids use metered dosing pumps and commodity reagents (e.g., PAC).
3) Primary separation—clarifier or DAF: a settling basin runs at surface overflow ~0.5–1 m³/m²·h (detention ~1–2 h). A DAF unit runs at ~2–5 minutes float time; cylindrical DAFs typically recirculate at ~4–6 bar to form 20–50 μm bubbles. Designs target ≥90% TSS removal and a large COD drop, with pilot tests setting final doses and setpoints.
4) Sludge handling: DAF scum is thicker (3–5% solids) versus ~1% in clarifier underflow, with the former often skipping thickening and the latter needing it. Dewater via filter press or centrifuge as volumes justify. DAF’s thicker sludge aligns with the ~3.4% versus ~1.1% solids contrast seen in algae systems (ResearchGate).
5) Polishing—AC or AOP: a packed GAC column with 5–10 min empty‑bed contact time is common, sized to site‑specific pesticide loading. Alternatively, dose PAC at 20–50 mg/L in a small contact tank and remove it by settling or by a downstream cartridge filter. For AOPs, an ozone contactor at ~0.1 g/L·h with 30–60 minutes detention is a representative setup; UV/H₂O₂ is used when UVT is improved by prior clarification (MDPI). Plants sometimes pair activated carbon with occasional ozone spikes for sanitation.
6) Post‑treatment and reuse: adjust pH as needed after ferric/alum or Fenton. Add residual chlorine only if required. For internal recycling (e.g., produce washing), many lines add final filtration and UV disinfection. Final effluent should meet local limits for BOD, TSS, and NH₃.
Performance and economics snapshot
With the above train and proper dosing, sites typically see >90% removal of TSS and BOD/COD, enabling irrigation reuse in many contexts. Coag+DAF often cuts COD by 50–80% depending on influent; AC/AOP polishing removes >80% of pesticides. DAF with aeration has been observed to reduce some COD by oxygen transfer, and nutrient removal (N, P) is modest but >50% TN/TP reduction has been observed with DAF plus aeration (ResearchGate).
On costs, a parametric view suggests chemical (coagulant + PAC) spend of roughly $0.05–0.10 per m³ treated, with power for pumps and blowers of similar order. For small plants, CAPEX often lands in the mid‑four‑figures (USD) per installed m³/day. Coagulant choice is a lever—polymeric flocculants cost more per kg than alum but drive overall chemical consumption down (ResearchGate). For high‑strength, continuous loads, DAF’s denser sludge can reduce lifecycle cost materially; one study quantified a ~$0.20 reduction per kg of biomass harvested for DAF‑based systems (ResearchGate).
A complete analysis should tally sludge disposal, reagent usage, energy, and the upside of water reuse. The decision most plants face—DAF or clarifier—still benefits from a short pilot on actual washwater, with hybrid setups bridging day‑to‑day loads and peak events.
