4.1.1.1. Lead. The maximum lead dose for the fermentation process
is 340 mg dm3 [30] and [42]. According to Schattauer et al. [24],
the recommended lead concentrations for anaerobic digesters
range from 0.02 to 200 mg dm3. In this study, the lead content of
most analyzed materials was determined below 1 mg kg1 DM
(below LOQ), therefore, it was highly unlikely to exert an adverse
influence on the properties and purpose of the evaluated materials.
The highest lead concentrations were reported in the peels of both
potato varieties (3.95 and 21.73 mg kg1 DM), but they did not
exceed the toxic threshold.
4.1.1.2. Cadmium. Cadmium levels below 0.5 mg kg1 DM in plant
materials do not pose a threat, and the toxic threshold for cadmium
is set at 180 mg kg1 [42]. Beetroot peels were characterized by the
highest cadmium content of 0.99 mg kg1 DM. The above cadmium
concentrations could disrupt fermentation only if beetroot peels
were the sole substrate component. In most cases, different types of
wastes are mixed to minimize the adverse influence of heavy
metals. The contaminated material can be “diluted” only if other
wastes contain small amounts of toxic metals.
4.1.1.3. Copper. Pure copper is not toxic, but other forms of copper
can form toxic compounds (e.g. CuSO4, selected carbonates and
acetates). Copper concentrations in excess of 40 mg kg1 can inhibit
methane formation [42]. Excluding strawberry pomace from a hydraulic
press where copper levels were determined at
17.58 mg kg1 DM, copper concentrations in the remaining wastes
did not exceed 10 mg kg1 DM, which is well below the recommended
standard and could enhance biogas production. Copper
concentrations higher than 400 mg dm3 have an inhibiting effect
on biogas production [30]. According to Zhang et al. [43], the
described process is inhibited by copper concentrations of
0.1 mg dm3. The copper content of energy plants grown for
digester feedstock was determined at 0.45e15 mg kg1 FM [24].
According to them, the recommended copper concentrations for
anaerobic digesters range from 0.06 to 64 mg dm3. The cited
studies indicate that food wastes analyzed in this experiment were
characterized by optimal copper concentrations.