Dynamics of Micro/Macro Elements and Heavy Metals during Anaerobic Treatment of Organic Fraction of Municipal Solid Waste (OFMSW)

In this study, the dynamics of macro and micro elements (including heavy metals) during anaerobic treatment of organic fraction of municipal solid waste (OFMSW) was investigated to determine the effect of seasonal variation on the performance of the treatment process. The first anaerobic digestion (ADH) process was conducted during the dry season (between March and May, 2016) while the second anaerobic digestion (ADC) process was conducted during the rainy season (between July and early October, 2016). OFMSW was collected and subjected to anaerobic treatment inside one-stage 250 litre – capacity batch-type mesophilic reactors with useful volume of 230 litres, substrate (OFMSW) concentration of 5.53%, rumen juice as the source of microbial inoculum and a retention time of 84 days. To monitor the treatment process, temperature, pH, macro and micro elements including heavy metals were estimated with time. Result showed that average process temperature inside the reactors (ADH and ADC) during the dry and rainy season ranged from 29.7ºC to 39.3ºC and 26.8ºC to 30.8ºC respectively after 84 days. Inside the reactors (ADH and ADC), cumulative biogas increased to 34.8 L and 26.5 L while nickel, iron and cobalt ranged from 6.80 to 1.52 mg/l and 6.62 to 0.93 mg/l, 21.40 to 1.25 mg/l and 23.70 to 5.7 mg/l, and 2.14 to 0.63 mg/l and 2.21 to 0.64 mg/l respectively after 84 days. Cadmium, chromium and mercury ranged from 2.91 to 0.05 mg/l and 2.64 to 1.1 mg/l, 1.84 to 0.06 mg/l and 1.81 to 0.8 mg/l and 0.58 to 0.02 mg/l and 0.57 to 0.08 mg/l respectively after 84 days. This suggests that anaerobic digestion could be applied to reduce the concentration of heavy metals in biodegradable wastes to some degree before the wastes can be disposed into the environment. Nevertheless, it appears that the anaerobic digestion (ADH) process conducted during the dry season may have performed better than the anaerobic digestion (ADC) process conducted during the rainy season in terms of reducing the concentration of these elements as was biogas production with time.