7.4 How to properly apply biosolids?

Besides the appropriate application rates, other best practices can be followed when applying biosolids for agricultural purposes.

Some other recommendations are as follows:

• Proper covered storage: In case biosolids cannot be applied directly, but need to be stored, it is essential to establish a covered storage facility to prevent access by vector insects and minimize odour release. Nonetheless, timely application is important to reduce the attractiveness of biosolids to insects.
• Effective application methods: Options include injection into soil, surface spreading followed by incorporation, or surface application, depending on the biosolids consistency and site conditions. Liquid biosolids with more than 94% water content can be injected into the soil or applied to the land surface. Usually, specialized vehicles or equipment are required to inject biosolids into the soil. Dewatered biosolids (up to 30% dry matter) do not require any specialized equipment and can be applied with conventional agricultural equipment. Utilising application methods such as injection or incorporation, which bury biosolids beneath the soil surface, can deter insects and minimize odour release. This method can also promote better soil incorporation and reduce surface runoff.
• Application timing: Biosolids are applied conveniently for crop production, typically before planting or after the harvest for agricultural lands. Timing is less critical in forest applications when nutrients can be incorporated into the soil throughout the growing period. Winter application is less desirable in many locales. Applications can be made as long as the ground is not saturated or snow-covered and whenever livestock can be grazed on alternate lands for at least 30 days after the application. The timing of single applications in land reclamation programs is less critical and may only be dictated by regulatory compliance schedules.
• Site characteristics: Before applying biosolids, the soil slope is recommended to be assessed. Steeper slopes increase the risk of runoff and erosion. In the case of surface application, slopes of 0-3% have no or only slight limitations. In case of incorporation of biosolids into the soil, slopes of 0-8% are desirable. Further, the distances from wells and waterways are important to be considered. Typically, a minimum buffer zone of 30 meters from surface water bodies such as rivers, streams, wells, and lakes is recommended.
• Soil characteristics: Soil testing is recommended before the application, as well as regularly during the application. Records of applied loads, field locations, application rates, etc. should be kept. The soil structure, permeability, and cation exchange capacity (CEC) are indicators of the suitability of soil for biosolid application. Soils with clay content below 20% are not recommended for the application of biosolids. Further, the soil pH should be maintained at 6.5 or above at all times to limit the mobility of metals that potentially could leach through the soil profile. The measurement of the concentration of nutrients, trace elements and metals before the application will give a baseline to determine the effects of the application.
• Continuous monitoring: Monitoring and inspection of storage areas and application sites for signs of insect activity, odour release, and biosolid quality can help identify issues early and implement corrective actions. Prompt responses to insect infestations or odour complaints can prevent further problems.

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