7.2 What are the preconditions and requirements for the safe reuse of sludge for agricultural purposes?
The safe reuse of sludge for agricultural purposes is subject to several preconditions and requirements to ensure environmental protection and public health. These conditions must align with regulatory standards, which vary between countries. When no regulation exists, internationally existent guidelines are recommended to be consulted. The following aspects are commonly to be considered regarding the sludge:
- Preceding treatment and stabilization
- Pathogen reduction
- Heavy metal limits
- Nutrient concentrations
Preceding Treatment and Stabilisation
The mandatory preceding treatment and stabilisation of sewage sludge reduces pathogens, eliminates, or minimises contaminants, and transforms the sludge into biosolids suitable for reuse. We learned about a variety of treatment methods, including, amongst others, aerobic digestion, and composting. Most essentially, sludge/biosolids shall be applied where the manner, quantity, and time of application are adjusted to the nutrient requirements of the plants.
Pathogen Reduction
One important aspect that is considered in all guidelines is the pathogen concentration limits. If not properly managed, pathogens can cause crop contamination, human health risks during agricultural activities and environmental contamination. The following table lists international and national guidelines that provide pathogen limits that need to be respected.
| Organization | Guideline Requirements | Source |
|---|---|---|
| World Health Organization | Helminth egg count: 1 egg per gram of total solids E. coli: 1,000 count per gram of total solids | WHO (2006) |
| US Environmental Protection Agency (Part 503 biosolids rule) | Class A biosolids: faecal coliform density 1,000 per gram of total dry solids, or Salmonella subspecies (spp) density 3 per 4 grams of total dry solids Class B biosolids: faecal coliform density 2,000,000 per gram of total dry solids | US EPA (1994) |
| Department of Environment, Bangladesh | Salmonella: none Helminths egg count: <0.25 mg/kg of dry matter (or 1 viable egg per 4 grams of dry matter) | DoE Bangladesh (2015) (Link) |
| Water Research Commission & Department of Water Affairs and Forestry Resource Protection and Waste, South Africa | Microbiological class A (unrestricted use quality): faecal coliform <1,000 per gram of dry matter (5 log reduction), and helminth egg count <0.25 eggs per gram of dry matter (1 egg per 4 grams) Microbiological class B (general use quality): faecal coliform <1×106 per gram of dry matter (2 log reduction), a helminth egg count <1 egg per gram of dry matter Microbiological class C (limited use quality): faecal coliform >1×107 per gram of dry matter (no reduction), and helminth egg count >4 eggs per gram of dry matter | WRC (2009) (Link) |
Heavy Metal Limits
As with pathogen requirements, there are regulations for heavy metals. These regulations can differ between regions. Exemplary, the following table shows the limits for heavy metal concentrations in sludge and soil to be respected in Bangladesh.
| Parameter | Limit in Sludge (mg/kg of dry matter) | Limit in Soil (mg/kg of dry matter) |
|---|---|---|
| Arsenic (As) | 40 | 40 |
| Cadmium (Cd) | 10 | 1.5 |
| Chromium (Cr) | 900 | 100 |
| Copper (Cu) | 800 | 60 |
| Lead (Pb) | 900 | 100 |
| Mercury (Hg) | 8 | 1 |
| Nickel (Ni) | 200 | 50 |
| Zinc (Zn) | 2500 | 200 |
Nutrient Concentrations
Acceptable nutrient concentrations in biosolids for agricultural application vary depending on regulatory standards and specific crop needs. Excessive nutrients can lead to an imbalance in soil nutrient levels that can negatively affect plant growth and soil health. High nitrogen levels can result in nitrate leaching into groundwater, delayed crop maturity, and reduced crop quality. Phosphorus runoff into surface waters can contribute to eutrophication and algal blooms.
Typical nutrient concentrations in biosolids that are considered desirable and beneficial for crop production are as follows:
- Nitrogen: 0.8 – 9.0 %
- Phosphorus: 0.4 – 4.0 %
- Potassium 0.01 – 1.8 %
For further information, please click on the Materials tab at the top of the page.
