Inside the packaging hall’s dirtiest secret: a pretreatment train that strips 70–85% of the load before it hits the plant

Peak-y, messy, and loaded with scraps: that’s the packaging hall’s wastewater profile. Think labels, cardboard, plastics, and adhesives — plus pulsed discharges from line washdowns and bottle rinses. A staged pretreatment train built around screening, equalization, pH control, and coagulation–flocculation (coag/floc) is the difference between downstream chaos and steady, predictable treatment.

The technical upside is not subtle. With properly sized screens and flocculation, more than 80% of total suspended solids (TSS, fine particulate content) can be removed, while organic load (measured as COD, chemical oxygen demand, and BOD/BOD5, biochemical oxygen demand over five days) often drops 60–80% under good conditions (MDPI). In one study, 0.4 g/L (400 mg/L) ferric chloride cut TSS from about 1000 to about 150 mg/L and COD by 85% (MDPI).

Industrial guidance also flags a cost angle: high suspended solids “increase BOD” and can incur sewer surcharges, and primary steps like sedimentation or wedge‑wire screening “are commonly used to reduce suspended organic solids loading” (Wikipedia).

Screening train and solids capture

The first barrier is screening. Coarse bar or drum screens at roughly 10–20 mm pull out bulky debris — cardboard, plastic, errant bottles — before it can clog pumps or foul mixers. Finer screening down to 1–3 mm intercepts small particles that still carry a surprising share of particulate BOD and COD. In practice, wedge‑wire and sedimentation steps at this stage are used because suspended solids “increase BOD” and “are commonly used to reduce suspended organic solids loading” (Wikipedia).

Facilities often deploy coarse, hand‑raked barriers akin to a manual screen at the headworks, then follow with a continuous unit such as an automatic screen to maintain capture without labor‑intensive cleaning. As a package, primary solids removal systems like physical separation trains cut TSS input dramatically and protect downstream equipment.

Equalization basin and flow buffering

Packaging wastewater is notoriously intermittent — batch line cleanings and bottle rinses spike both flow and concentration. An equalization tank with a mixer evens out both. Designing for sufficient volume — typically several hours’ average flow — can more than halve the peak‑to‑trough ratio of flow and BOD/COD, producing a near‑constant feed that “can significantly improve the performance” of downstream treatment and even allow smaller units, according to engineering practice summarized in the paper.

Equalization also becomes the pre‑mixing zone for chemicals, and it buffers pH swings driven by alkaline detergents or acidic rinses before any coagulant is added. Supporting hardware such as mixers, instruments, and valves is typically treated as water treatment ancillaries in design and procurement.

pH control ahead of coagulation

Because coagulants like alum (aluminum sulfate) and ferric chloride work best near neutral pH, a pH adjustment system sits at or just after equalization. Continuous monitoring targets roughly pH 6.5–7.5 ahead of coagulation, aligning both with coagulant efficiency (MDPI) and typical discharge bands (often pH 6–9). Alkalis (lime or caustic) or acids are dosed as required.

Accuracy matters here, which is why plants specify an accurate chemical dosing pump for pH control, synchronized with equalization tank recirculation so corrections are distributed quickly and uniformly.

Coagulation–flocculation performance benchmarks

Once screened and equalized, the wastewater is ready for coagulation–flocculation: rapid mixing with metal salts followed by gentle floc formation and solid–liquid separation. Typical coagulants include alum, ferric chloride, and polyaluminum chloride (PAC). The paper’s jar‑test data point to effective doses in the few‑hundreds of mg/L range: 400–700 mg/L is cited as effective overall, and a preliminary design basis of about 0.3–0.6 g/L of alum or PAC is recommended before jar tests refine it.

Specific results: dosing 0.4 g/L (400 mg/L) Al₂(SO₄)₃ at pH 6.5 removed approximately 74.5% of COD and approximately 74.5% of TSS, while 0.4 g/L FeCl₃ at pH 5.5 removed approximately 84.5% of COD and approximately 85.5% of TSS (MDPI). In a fruit‑juice stream with COD ≈ 21,000 mg/L, 0.4 g/L alum achieved 74.5% COD removal and 42.8% turbidity removal, while 0.4 g/L ferric chloride achieved 84.5% COD and 54.1% turbidity removal (MDPI).

Those numbers translate to an expected 70–85% reduction in organic load (BOD/COD) for high‑strength, particulate‑rich packaging waste — think sugary rinses — and a similar reduction in TSS. Where organics are mostly soluble, results are smaller: a brewery study found that 675 mg/L PAC at pH ≈ 7 cut COD by only about 30% in real effluent (MDPI). That contrast underscores coagulation’s strength on colloidal/particulate BOD, with soluble BOD largely passing through.

In practice, plants dose a primary coagulant — often selected from coagulant chemistries including alum, ferric salts, or PAC (for wastewater service, see PAC for industrial wastewater) — then add 1–5 mg/L of a polymeric aid to build larger, faster‑settling flocs (flocculant polymers). Jar tests (bench‑scale beaker trials) finalize the type and dose.

Clarification, flotation, and sludge handling

After flocculation, separation is typically by settling or flotation. A conventional clarifier handles the settled floc stream, while dissolved air flotation (DAF) is favored for lighter, grease‑associated flocs; compact DAF units are available as DAF systems. The paper notes that TSS removal is commonly ≥80% (with residual TSS and BOD then handled by the main WWTP), and that organic load often drops by 60–80% under good conditions (MDPI), though highly soluble organics may see smaller reductions (MDPI).

Two anchor examples illustrate the scale of change. If influent TSS is 200 mg/L, 85% removal leaves roughly 30 mg/L — a big assist toward final limits like <50 mg/L TSS. And one ferric test case cut TSS from roughly 1000 to roughly 150 mg/L while reducing COD by 85% at 0.4 g/L (MDPI).

Sludge output rises with chemical treatment — on the order of 0.5–1 kg of dry sludge per kg of coagulant — so the design includes sludge collection and handling (thickening and dewatering). The paper cites collected sludge solids of about 2–5% before dewatering, with disposal to be planned (for example, landfill of solids). Lower organic loading to the biological plant also reduces its downstream, biologically generated sludge.

Load reduction to the main WWTP

Beyond TSS, pretreatment delivers a more stable, cleaner feed to the biological plant. Jar tests suggest BOD5 (five‑day biochemical oxygen demand) reductions on the order of 70–80%. As a simple sizing context, a packaging flow of 50 m³/day at 300 mg/L BOD5 corresponds to about 15 kg/day of BOD5; after pretreatment, the paper estimates about 3–5 kg/day — a cut that often translates to lower sewer charges and reduced O&M.

The summary in the paper ties these gains to compliance in Indonesia, noting that the combined train helps meet Indonesian effluent limits and supports outcomes like <50 mg/L TSS and <20 mg/L BOD in final discharge (MDPI).

Design assumptions and monitoring

The proposed train — coarse plus fine screening, equalization/pH control, and coagulation–flocculation — is backed by bench data and standard practice. Typical coagulant doses are a few hundred mg/L; start around 0.3–0.6 g/L for alum or PAC, then refine with jar tests. Polymer doses of 1–5 mg/L often follow the coagulant. pH is continuously monitored and held near 6.5–7.5 ahead of coagulation to match coagulant efficiency and typical discharge bands of pH 6–9 (MDPI).

Operationally, the paper recommends regular performance checks on TSS, BOD, and pH and using jar tests to finalize coagulant type and dose. Instrumentation, mixers, and solids‑handling gear are specified as wastewater ancillaries to keep the system dialed in.

Key performance summary

With properly sized screens and flocculation, >80% of TSS can be removed, and up to 70–80% of organic load (COD/BOD) can be abated (MDPI, MDPI). Residual TSS and BOD are then handled by the main WWTP. For context, one study reported that 0.4 g/L ferric chloride cut TSS from about 1000 to about 150 mg/L and COD by 85% (MDPI).

Sources and references

Industrial wastewater treatment overview (Wikipedia).

G. Musa et al., Industrial Wastewater Treatment by Coagulation–Flocculation and Advanced Oxidation: A Review, Water (MDPI) 17(13), 2025 (removal efficiencies cited) (MDPI, MDPI).

M. C. Miino et al., Treatment of a Real Brewery Wastewater with Coagulation and Flocculation, Appl. Sci. 13(6), 2023 (MDPI).