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 may vary by region and need to align with local regulations. The following aspects need to be considered:

• Preceding treatment and stabilization
• Pathogen reduction
• Heavy metal limits
• Nutrient concentrations
• Land application planning
• Buffer zones and setbacks
• Monitoring and record keeping

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.

Biosolid Characteristics

The treatment processes shall ensure that compliance with local, national, and international legislation or standards is reached. The specific guidelines and limits concerning the use of biosolids vary in the different regions in various applications. When no regulation exists, internationally existent guidelines are recommended to be consulted.

Generally, the following characteristics need to be assessed:

• Pathogen levels
• Heavy metal limits
• Nutrient concentrations
• Organic matter content
• pH level
• Salinity

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

Land Application Planning

Buffer Zones and Setbacks

Monitoring and Record Keeping

For further information, please click on the Materials tab at the top of the page.