7.2 What are the preconditions and requirements for the safe reuse of sludge for agricultural purposes?

Sludge and Biosolid Management 7 Reuse of Sludge 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 must 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)

Recommended Pathogen Requirements for Biosolids Reuse from Various Sources

Heavy Metal Limits

As with pathogen requirements, there are regulations for heavy metals. Different countries may have varying limits for metals like zinc, copper, nickel, cadmium, lead, mercury, and others. 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

Limits for Heavy Metal Concentrations in Sludge and Soil for Agricultural Purposes in Bangladesh (Source)

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.

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