One-way flow and clean–dirty zoning: plant layout against cross-contamination

AgrifoodV · Published 27 October 2026

The layout decides whether a plant can control cross-contamination, even before the equipment arrives. A buyer’s audit team only needs to look at the drawing to see whether dirty raw material crosses the path of finished product, or whether workers can take a shortcut from the receiving area to the filling area. Correcting flows after construction almost always means knocking down walls. Article 7 links site selection in Article 6 with finishing materials in Article 8: draw the one-way flow and the clean–dirty zones on the very first drawing.

The principle: product moves one way, from dirty to clean

In the chapter “Design Principles” of the Handbook of Food Factory Design (2013), P.J. Wallin defines a “clean area” as one where food has already been heat-treated or preprocessed. This is also where the product is most vulnerable to microorganisms and contamination. A HACCP study will identify the highest-risk areas and, according to Wallin, one critical control point usually occurs exactly where the product changes from unprocessed to processed. The book therefore advises separating the ingredient preparation area, the unprocessed (dirty) area and the clean area with walls.

Volume 4 – Food Industry (draft 2026), section 10.2.3.5, turns this principle into a specific requirement: the chain Raw-material receiving → Pre-processing → Processing → Packing → Finished-goods store follows one continuous direction, with no crossing points or backtracking between dirty and clean areas. The book divides the layout into three levels:

Sources: Volume 4, section 10.2.3.5 (three zone levels); Wallin (2013), sections 12.4.3–12.4.6, pp. 311–312 (design requirements).

Wallin gives four reasons for providing a high-risk area: the product is microbiologically sensitive; it is not naturally stable; there is no kill step; or contamination from other areas or from raw materials is possible. His list of facilities with higher than normal risk includes flavours and spices. In AgrifoodV’s assessment, cut fruit and vegetables that are frozen without any heating fall squarely under the “no kill step” criterion.

Where to put partitions, and when to use enclosed systems

Volume 4, section 7.5.3.2, names three typical locations for partitions: after the product is filled or sealed in primary packaging; after chilling in a chilled or frozen food plant, where the chilling or quick-freezing equipment itself acts as the partition; and after the dryer, once water activity is too low for microorganisms to grow. In the same section, the book classes primary packing as high risk and casing and palletising as low risk: the packing hall is usually cut in two by a partition.

The book also notes the two sides of the problem: to keep flexibility for future expansion, internal partitions, columns and drains should be kept to the necessary minimum, but partitions needed for food safety must still be there. In AgrifoodV’s assessment, this is why the zoning drawing must be fixed before the structural drawing: hygiene partitions are a constraint, not an option.

Walls are not the only method. Wallin notes that, with improved cleaning-in-place (CIP) and sterilisation-in-place (SIP), some liquid processors have moved to totally enclosed processing systems. US regulation also places the separation methods on an equal footing. Section 117.20(b)(2), Part 117, Title 21 of the US Code of Federal Regulations (21 CFR), requires the plant to permit adequate precautions against contamination and allergen cross-contact through food safety controls, operating practices or design, including separating operations where these risks are likely by location, time, partition, airflow or dust control, enclosed systems or other effective means. The investor has a choice; what is mandatory is that every operation has a separation method clearly shown on the drawing.

The flow of people: changing rooms and the hygiene lock

According to Wallin, people are the greatest cause of contamination in a factory, carrying dirt and disease on their clothing, hair and hands, yet the flow of people is often only considered as an afterthought. The book asks four questions for shift changeovers: how many people move in and out in a given time window; are the corridors large enough; are the locker rooms suitable; are the cleaning facilities adequate.

The location of amenities is also a zoning matter. According to Wallin, changing rooms, eating areas and other staff facilities must be laid out so that the segregation between high- and low-risk, dirty and clean areas is not compromised; in some cases facilities have to be duplicated. Regulation (EC) No 852/2004, Annex II, Chapter I, adds three requirements: lavatories must not open directly into rooms where food is handled; there must be an adequate number of suitably located washbasins with hot and cold running water, designated for cleaning hands; and adequate changing facilities for personnel must be provided where necessary.

Volume 4, sections 7.6.7.2 and 10.2.3.5, specifies the entrance to the clean zone as a four-step hygiene lock: (1) change into clean protective clothing, hair cover and dedicated boots; (2) wash hands at a touch-free sensor tap; (3) disinfect hands with 70% alcohol; (4) pass through a boot-disinfection bath or an automatic sole cleaner. In AgrifoodV’s assessment, a quick test is to ask of every door on the drawing: can a person from the receiving area reach the filling area without passing through the hygiene lock? If so, the drawing is not yet acceptable.

The flows of materials, air and wastewater

The simplest check proposed by Wallin: on the layout drawing, mark the path of each material in a different colour. Where the coloured lines are dense, there is congestion and crossing. The book uses packaging as an example: if the packaging store is far away and on the wrong side of the factory from the packing lines, every replenishment trip takes longer and space must be found for the truck or conveyor.

Handling equipment must also follow the zones. R.C. Kill, in the chapter “Movement of Materials” of the same book, notes that plastic pallets are now a common requirement in food production areas because they are easily washed and non-absorbent. In clean rooms, according to Wallin and Volume 4 (section 7.6.8.7), incoming materials are transferred into washable containers or cleanable plastic pallets, with containers specifically coloured for use only in that room. Kill also warns that food dusts cause cross-contamination of taints and odours between products and attract insects, birds and rodents.

Air and wastewater are two invisible flows, but they are inspected closely. Regulation (EC) No 852/2004, Annex II, Chapter I, point 5 requires mechanical airflow from a contaminated area to a clean area to be avoided; point 8 requires fully or partially open drainage channels to be designed so that waste does not flow from a contaminated area towards or into a clean area, in particular an area handling foods likely to present a high risk to the final consumer. The table below sets the two markets side by side.

Sources: Regulation (EC) No 852/2004, consolidated version of 24 March 2021; 21 CFR 117.20(b)(1)–(2), eCFR consulted on 27 October 2026.

Vietnamese conditions: holding the clean zone with air

Volume 4, section 7.3.1, frames hygienic design in a tropical monsoon climate with relative humidity typically above 80%, and gives the example of condensation on ceilings and walls of storage buildings causing mould when moisture extraction ventilation and roof insulation are missing. Under these conditions, walls are not enough; the clean zone has to be held with air. Volume 4 (sections 7.6.8.8.3 and 10.2.3.5) gives parameters for clean zones in Vietnam:

• Positive pressure difference of +15 to +30 Pa to keep out dirty outside air and fine dust.
• Temperature of 18–20 °C and relative humidity of 45–60% so that water vapour does not condense on ceilings and floors, a common cause of mould.
• Wet and dry areas fully separated: wet areas have floors sloped at 1:40–1:80 towards stainless-steel drains with odour traps; dry areas are cleaned by vacuum or dry cleaning.
• Aseptic filling: HEPA filtration and a cleanliness class under ISO 14644-1 chosen according to risk, for example local ISO Class 5 at the aseptic filling zone.

In AgrifoodV’s assessment, the consequence for investors is that the pressure, temperature and humidity of each zone must be on the zoning drawing from the basic design stage. If they are left as an add-on mechanical and electrical item after walls, doors and ceilings are built, achieving the pressure difference becomes much harder and more expensive. Article 11 will cover ventilation, air conditioning and positive pressure in detail.

What companies should do

AgrifoodV’s recommendations:

• Ask the design firm for separate flow diagrams for raw materials, packaging, finished goods, people, waste, air and wastewater, overlaid on the same layout; every crossing point must have a solution.
• Fix the dirty–clean boundary from the HACCP study before fixing the positions of walls, doors and changing rooms; for each operation, state whether separation is by partition, time, airflow or enclosed system.
• Give the clean zone a single entrance through a hygiene lock; two zones sharing one set of amenities is a gap.
• Write the target pressure difference, temperature and humidity of each zone on the zoning drawing, so that the air-conditioning design follows it rather than chasing it.

AgrifoodV supports companies in preparing the design brief, reviewing zoned layouts and setting the design direction for processing plants.

References

1. Wallin, P.J. “Design Principles”, chapter 12 in Baker, C.G.J. (ed.), Handbook of Food Factory Design. Springer Science+Business Media New York, 2013 (p. 307, sections 12.3.1–12.3.2 movement of materials and people; pp. 311–312, sections 12.4.3–12.4.6 wet and dry areas, ingredient preparation, clean area, high-risk area; pp. 313–314, clean rooms; pp. 315–316, section 12.6.2 amenities). DOI: 10.1007/978-1-4614-7450-0_12.
2. Kill, R.C. “Movement of Materials”, chapter 5 in Baker, C.G.J. (ed.), Handbook of Food Factory Design. Springer Science+Business Media New York, 2013 (p. 120, plastic pallets; p. 143, food dusts). DOI: 10.1007/978-1-4614-7450-0_5.
3. Tập 4 – Công nghiệp thực phẩm [Volume 4 – Food Industry], in the series Ứng dụng khoa học và công nghệ thực phẩm trong phát triển kinh tế [Applying food science and technology in economic development] (draft 2026), sections 7.3.1 (humid climate), 7.5.3.2 (partition locations), 7.6.7.1–7.6.7.2 (movement of materials and people; hygiene lock), 7.6.8.4–7.6.8.8.3 (preparation, clean, high-risk areas, clean rooms, parameters for Vietnam), 10.2.3.5 (three-level hygiene zoning).
4. Regulation (EC) No 852/2004 of the European Parliament and of the Council of 29 April 2004 on the hygiene of foodstuffs, Annex II, Chapter I, points 2–5, 8, 9; consolidated version of 24 March 2021. eur-lex.europa.eu.
5. 21 CFR 117.20 “Plant and grounds”, paragraphs (b)(1)–(2), Part 117 Subpart B (Current Good Manufacturing Practice), Electronic Code of Federal Regulations, consulted on 27 October 2026. ecfr.gov.

Article by AgrifoodV, compiled and interpreted from the sources above (accessed 27 October 2026). This content is not a substitute for design advice or legal advice for any specific project. Cover image: Anna Shvets / Pexels.

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