Table 1 Comparative study on LS-AD and HS-AD plants in UK and Europe [3]
Parameters | Wet anaerobic digestion process | High solids anaerobic digestion process | Remarks |
|---|---|---|---|
Feedstock solids content | The total solids content was lower but the volatile solids content was higher compared to high solids AD plants | The total solids content was higher but the volatile solids content was lower compared to wet AD plants | The feedstock TS ranged from 17 to 50% and the volatile solids (VS), as a percentage of the TS, was ranged from 50 to 96% |
Water usage | Water usage was higher | Water usage was lower | The high solids content of feedstock for high solids AD plants decreased process water consumption |
Plant footprint relative to plant design capacity | Total plant footprint of wet AD plant was influenced by the digestate storage facility | Total plant footprint of high solids AD plants was determined by the pre-and post-digestion treatments | Total plant footprints varied from 2900 to 80,000m2 while the design waste processing capacities varied from 27, 500 to 287, 500 tons per year. There was, however, no significant difference of the plant footprints relative to the plant design capacity between the wet and high solids AD plants |
Pre-treatment and post- treatment facilities | Wet AD plants in most cases do not have complex pre-treatment and post- treatment facilities but instead have storage facilities | High solids AD plants have more complex pre-treatment and post- treatment facilities | Â |
Digestate management | Residual products from the plants are bulky and are not easier to manage, handle, store, transport and use | Residual products from the plants are easier to manage, handle, store, transport and use | The digestates from the high solids AD plants have better utility values and therefore are marketable than digestates from wet AD plants |
Retention time | The overall retention time was longer in the wet AD plants than in high solids AD plants | The overall retention time was shorter in the high solids plants AD than in the wet AD plants | The overall retention time for all the AD plants varied from 10 to 100Â days |
Parasitic energy consumption | The parasitic energy consumed by the wet AD plant was not significantly different from the one consumed by high solids plants | Relatively high parasitic energy demand of high solids plant was due to extensive pre- and post treatment processes | Parasitic energy is the produced energy consumed to operate the plants. The parasitic energy consumed by high solids AD plants was similar to the one consumed by wet AD plants |
Biogas yield | Wet AD plants had greater biogas yields per tonne of waste processed than high solids plants. Specific biogas and methane yields (per tonne TS or VS) for wet AD plants were also higher than that of high solids plants | The average biogas yield for the high solids plants was 78 m3 per tonne of waste | The annual biogas yied which was dependent on plant capacity varied from 1.6 to 15.5 million m3 |
Economy performance | Specific capital cost per tonne of waste processed and specific capital cost per m3 of biogas produced by the wet AD plants were lower when compared to that of the high solids plants The average specific capital cost per tonne of waste processed by wet AD plants was £22 per tonne of waste input | The average specific capital cost per tonne of waste processed by high solid plants was £35 per tonne of waste input | Total capital cost of facilities for wet and high solids AD processes ranged from £54 to £42.7 million, The specific capital cost for the AD processes varied from £19 to £42 per tonne of waste input |