ENStarting point
A washing line should be assessed as a system of material, water, contaminants and energy. Recirculated flow is not the same as fresh water consumption, and visual cleanliness does not demonstrate polymer quality or wastewater suitability.
Water, solids and output quality
Complementary control areas to evaluate together.
Define the problem washing must solve
The first step is to identify what arrives attached to or mixed with the plastic: soil, paper, oil, product residue, fines or other materials. Record the source and variability between batches. A washing system sized for packaging with surface dirt may not address a stream of technical parts with persistent adhesives or substances incorporated into the formulation.
Define the required output condition with the buyer or next processing stage. This may be impurity content, moisture, odour, extrusion behaviour or another measurable characteristic. Herbold documentation presents the integration of cleaning and water treatment as part of recycling line performance. This is a manufacturer's description that must be validated with the project's material, without transferring commercial results from other installations. [1]
Distinguish washing, separation and water removal
Although they appear together on the line, these operations have distinct objectives. Washing aims to remove dirt; separation distributes components between fractions; and dewatering reduces the water carried by the material. One machine may contribute to more than one function, but the performance protocol must distinguish each outcome and its corresponding measurement point.
Herbold describes friction washers, tanks and hydrocyclones as solutions with specific functions. [2] Do not choose a stage based solely on its commercial name. Compare composition, particle size, bulk density, throughput and required quality. Density separation depends on the particles' actual characteristics: fillers, bonds between materials and trapped air can alter their behaviour. Purity must be verified by analysing the resulting fractions, not inferred solely from their position in the tank.
Draw up two balances: water and solids
In the water balance, show fresh water, moisture entering with the waste, internal recirculation, blowdown, evaporation, water in the product and water in the sludge. In the solids balance, track recovered polymer, removed contaminants, fines, treatment residues and changes in inventory. Use the same observation period and state which streams were measured and which were estimated.
Recirculation disappears from the external balance when the boundary encompasses the whole circuit. Counting it as consumption artificially multiplies the indicator. Nor should all mass reduction after washing be treated as plastic loss: some may be removed dirt or water. Interpretation requires sampling and, where necessary, dry-basis results. This distinction affects the technical assessment, payment and estimates of the destination of generated waste.
Review a simplified water balance
Consider a hypothetical steady-state example with no change in water inventory. Over one hour, 3.0 m³ of fresh water and 0.4 m³ associated with the material enter. Outflows are 1.7 m³ in blowdown, 0.9 m³ with sludge, 0.5 m³ with the product and 0.3 m³ through evaporation. Inputs and outputs each total 3.4 m³. The circuit internally recirculates 20 m³ during that hour.
If accepted production over the period is 2.0 tonnes, specific fresh water consumption is 3.0 ÷ 2.0 = 1.5 m³/t. Dividing recirculation by production would produce a different indicator: circulation intensity, which does not represent water abstraction. In actual operation, level changes, leaks and instrument uncertainties may prevent an exact balance; investigate the discrepancy before presenting a rate as a confirmed result.
| External stream | Input | Output |
|---|---|---|
| Fresh water | 3.0 m³ | — |
| Feed moisture | 0.4 m³ | — |
| Blowdown | — | 1.7 m³ |
| Water in sludge | — | 0.9 m³ |
| Water in product | — | 0.5 m³ |
| Evaporation | — | 0.3 m³ |
| Total | 3.4 m³ | 3.4 m³ |
Control recirculated water quality
Closing the circuit further may reduce make-up water and increase contaminant concentrations. The water keeps circulating, but its ability to contribute to the outcome may change. Define sampling points at the feed to critical stages, in the return and in the blowdown. Select parameters based on the material and treatment, such as solids, pH, conductivity and organic load where relevant.
An indirect parameter needs to correlate with the problem being investigated. Conductivity, for example, cannot by itself identify all relevant contaminants. Relate water trends to plastic quality and treatment performance. Monitoring frequency must allow action before an entire campaign is compromised. If the input composition changes, reassess the suitability of the chosen parameters and previously established operating limits.
Include wastewater, sludge and fines in the design
A contaminant removed from plastic moves into another stream; removing it from the product does not mean it disappears. Identify where fines, dirt and the agents used become concentrated. The European best available techniques reference document for waste treatment brings together references on stream and emission management. It can support engineering assessment without replacing Brazilian licensing requirements and conditions. [3]
The treatment system and stream destinations must be defined by qualified professionals based on characterisation and local conditions. Include capacity, contingency arrangements, maintenance access and traceability of collections. The term “closed loop” must describe the actual arrangement: there may be blowdown, make-up water and concentrated waste. Avoid turning it into a promise of no wastewater or impacts without specific evidence.
Compare cost per accepted material
Add up energy, water, consumables, labour, maintenance, treatment, waste management and economically significant losses. Divide by the volume that meets the intended destination's requirements. Comparing only tonnes fed favours an operation that processes a lot but delivers little usable material. Maintain a second indicator per tonne received to understand how waste composition affects the effort required.
Test changes with comparable batches and record temperature, time, flows and feed conditions. Increasing washing intensity may raise costs and generate more fines without improving the attribute that limited the application. The decision should identify which improvement in quality or yield justified the additional resource. Include cleaning and maintenance downtime, since the best result during a short demonstration may not persist throughout the campaign.
Close the campaign with an operational decision
At the end of the pilot, bring together balances, test reports, consumption records, incidents and labelled samples. Explain the relationship between incoming dirt and output condition. Define the accepted feed range, circuit monitoring, responsibility for treatment and what to do when the water or product falls outside the established conditions.
The result should inform the solid and industrial waste management and the specification of recycled resins. A well-assessed line delivers more than visually clean plastic: it shows where the water, contaminants and losses went, which product was accepted and what conditions make that performance repeatable at scale.
Sources and references
References accessed on 5 October 2026. The numbering in the text indicates the concepts supported by each source.
- Herbold Meckesheim. Efficiency and Quality in Plastic Recycling. Manufacturer's description, without independent performance validation.
- Herbold USA. Washing Line Systems Components. Washing, separation and dewatering principles presented by the manufacturer.
- European Commission, Joint Research Centre. Best Available Techniques Reference Document for Waste Treatment, 2018. European technical reference.
Circuvia corporate content to support project discussions. Methods, acceptance criteria and applicable requirements are defined within the scope of each operation.
