The most important ways of introduction of human and veterinary pharmaceutical drugs into the environment are through human and animal excreta (feces and urine), waste effluents of pharmaceutical manufacturing processes, disposal of unused or expired pharmaceutical products, and accidental spills during manufacturing or distribution.
Among these, animal excreta is the major source of environmental contamination by drugs, as most of the drugs used in veterinary medicine end up in manure.
Conventionally, when the energy recovery is not necessary due to cheap energy produced from fossil fuels and the required level of efficient waste treatment is not high, the manure and slurry (urine and feces) are usually treated by low-cost options.
They are either immediately applied to agricultural fields as fertilizers or stored for some time in open basin or lagoon before land application.
Although the anaerobic digestion technology in biogas plants can generate biogas (mainly methane) for energy production, control odor and provide more efficient treatment of animal manure and slurry as well as produce less sludge than the low-cost options; this technology is not preferred due to its high investment cost.
Nowadays as more emphasis has been placed on energy recovery and renewable energy production due to rising energy prices; anaerobic digestion technology has rapidly taken over the low-cost options for treatment of animal waste.
After manure and slurry undergo digestion in biogas plants, the remaining solids (technically called digestate) are usually applied as fertilizers for crops.
When these fertilizers are dispersed on the fields, the unmetabolized drugs and their biologically active metabolites that are persistent in the digestate, may threaten the groundwater depending on their mobility in the soil system, and affect terrestrial and aquatic organisms as a result of leaching from fields.
This explains why veterinary pharmaceuticals of potentially environmental effects have recently received great concerns worldwide.