Sewage Sludge, Metals, and Agricultural Spreading: Between Resource and Vigilance

Wastewater treatment is essential for protecting aquatic environments. But it produces a by-product that cannot be ignored: sewage sludge. Each year in France, about one million tons (dry matter equivalent) of sludge are generated. A large share of this volume is recycled on farmland through spreading. This practice is useful, regulated… and under scrutiny. This article looks at three simple questions:

• Why do we still spread sludge?

• Where are the real risk zones?

• What role does (and will) real-time monitoring such as Klearia’s PANDa+ play?

Why Spread Sludge?

Agricultural spreading remains today the main method of sludge reuse in France. And it’s not just about “getting rid” of waste: sludge has genuine agronomic value. It provides:

• Organic matter : useful for soil structure, carbon retention, and biological activity.

• Major nutrients : nitrogen, phosphorus, potassium, calcium… all essential for crop growth. Some limed sludges also help correct soil acidity.

• A partial substitute for mineral fertilizers : recycling phosphorus and nitrogen instead of importing or synthesizing them is both economical and aligned with circular-economy principles.

  
  

The economic advantage for municipalities is clear: spreading is generally less costly than incineration or landfilling, and it remains technically feasible, particularly for medium-sized treatment plants. (Encyclopédie de l’Environnement, “Boues d’épuration : une aubaine pour les terres agricoles ?”, October 23, 2025.)

The Sensitive Issue: Metals

Sludge doesn’t only contain nutrients. It also concentrates trace metals such as cadmium, lead, nickel, copper, and zinc. These so-called “heavy metals” come not only from industrial sources but also from daily life: urban runoff, corrosion of materials, cleaning products, cosmetics, and more.

The feared risks are twofold:

• Long-term soil accumulation, if spreading is repeated on the same plots.

• Transfer to crops, and potentially into the human or animal food chain.

  
  

This issue has become key for social and market acceptability. Some agri-food industries, buyers, and even quality labels reject sludge spreading upstream of their production chain, even when regulatory thresholds are respected. In other words, it’s not just a toxicological issue, it’s also about reputation and traceability. (Source: ENGREF / AgroParisTech, “Des métaux dans les boues de stations d’épuration ? Conséquences, origines et prévention”, 2010.)

What Do Field Studies Show?

Agronomic experiments carried out in France on plots treated with sewage sludge yield three key insights.

1. Agricultural productivity: When properly managed, sludge applications can enhance plant growth (yield, biomass), primarily due to their organic and mineral enrichment.

2. Metals in soil: After spreading, there is indeed an increase in trace metal content in the surface soil horizon. This increase depends largely on:

   - the sludge composition,

   - the application rates,

   - and the field’s historical management.

   
   

The strong accumulation cases reported in the 1970s–80s were mostly linked to sludges highly loaded with metals and heavy application rates. Today, authorized fluxes are much lower, and sludge intended for spreading is much cleaner than it was 30 or 40 years ago.

3. Transfer to harvested plantsAnalyses on harvested plant organs (e.g., wheat grains) show no critical exceedances for elements like cadmium or nickel when spreading follows the regulatory framework. In other words, metals are found mainly in the surface soil, not necessarily in the edible part of the plant.

   
   

However, context matters. On acidic or fragile soils, the mobility of metals is higher. Conversely, on calcareous or limed soils, metals are less available to plants. Soil pH is therefore a critical parameter to monitor. (Revue SET / Ingénieries, “Épandage en agriculture de boues de station d’épuration : impact sur la productivité et les transferts en éléments traces métalliques”, 2008.)

A Very Strict Regulatory Framework

In France, sludge spreading is governed by a strict regulatory system that sets out:

• Maximum thresholds for certain metals in sludge before reuse.

• Trace metal limits in receiving soils, verified before spreading.

• Mandatory periodic analyses of both soil and sludge.

• Minimum pH requirements (to avoid excessive acidity, which increases metal mobility).

• Exclusion distances (for drinking-water catchments, inhabited areas, steep slopes, etc.).

  
  

This regulatory pressure has had a real impact: metal concentrations, particularly cadmium, have sharply decreased in urban sludge since the 1980s–90s, largely thanks to reduced industrial discharges into sewer networks. Today, sludge intended for agriculture no longer has the same profile as it did thirty years ago. At the same time, some crop sectors and quality labels maintain stricter standards than the law itself (organic farming, high-reputation circuits, etc.), creating local tensions, especially when spreading sludge from large urban areas.

The Real Challenge Now: Proof and Traceability

Technically, it’s now possible to produce sludge of acceptable quality for spreading. Legally, the framework is well established. What matters now is demonstrating, continuously, that the entire chain is under control. Three main expectations are emerging among local authorities, wastewater plant operators, and agricultural partners:

1. Control inputs at the source: Limiting metal inflows upstream (industrial discharges, polluted urban runoff) remains the most effective way to ensure “cleaner” sludge that can be more easily reused.

2. Document sludge quality: Agricultural stakeholders no longer want a single test result; they want to understand trends, variability, and be reassured about the absence of abnormal spikes.

3. Move from occasional testing to dynamic monitoring: As long as analyses rely on occasional lab samples, only an “average state” is visible. Deviations and one-off incidents remain hidden, which is exactly what undermines local acceptance.

How Klearia Responds: Toward Intelligent Monitoring with PANDa

Klearia develops microfluidic instruments for in situ pollutant analysis in water, capable of semi-automating the measurement of critical parameters, including certain metals. The goal is not just to analyze a sample “at a given moment,” but to monitor the evolution of pollutant flows over time.

Our PANDa instrument fits perfectly into this logic of continuous supervision:

• Early detection of anomalies: Rapidly identifying the unusual appearance of a metal or pollutant in effluents before it becomes concentrated in sludge.

• Protection of the sludge chain: Reducing the risk of producing, over a short period, a batch of sludge that becomes unsuitable for spreading due to an abnormal industrial discharge.

• Enhanced traceability: Providing actionable data to local authorities, farmers, technical partners (agricultural chambers, engineering firms), and eventually downstream buyers.

  
  

In short, better water monitoring today means more secure and socially acceptable sludge reuse tomorrow.