Introduction
A key factor in reducing global CO2 emissions is recovering energy from waste material. Anaerobic digestion is already the most widespread technology for treatment of wastewater sludge in the UK and is also being promoted as a key process to recover energy from organic material such as supermarket or agro-industrial wastes. The current drive to maximise energy recovery from waste means that the biogas energy from anaerobic digestion is increasingly harnessed by means of combined heat and power (CHP) technology. However, in many cases sludge digesters are still operated to ensure sludge stabilisation, rather than to maximise biogas output. One method of enhancing biogas potential is the supplementation of anaerobic digesters with small amounts of trace inorganic nutrients, e.g. nickel; cobalt; manganese; iron, which stimulate bacterial activity. Previous research on this topic suggests that targeted nutrient supplementation could enhance the methane content and volume of digestion biogas. The research that exists lacks information on the bioavailability of the added metals within the anaerobic digestion environment, without which it is impossible to quantify cause and effect. For example, addition of a certain quantity of trace metal might be recorded to increase biogas yield by a certain percentage, but if much of this metal was immediately precipitated and was hence biologically unavailable, this would not be a true relationship and could lead to expensive overdosing of digesters.