Water Journal : Water Journal April 2012
wastewater treatment water APRIL 2012 139 Table 1. Key strengths and weaknesses of the available anaerobic digestion technologies. Anaerobic Digestion Type Process Summary Strengths Weaknesses Mesophilic Anaerobic Digestion (MAD) Occurs optimally around 35°C--38°C. Easy to operate. More tolerant to changes in operating conditions compared to thermophilic digestion. Medium gas production and volatile solids destruction. Larger reactor volume and higher capital cost. Potential for high-strength waste to "break through" due to incomplete digestion. Thermophilic Anaerobic Digestion (TAD) Occurs optimally around 50°C--57°C. Higher biological activity due to increased temperatures results in smaller reactor volumes and lower capital costs. Higher temperatures can facilitate increased pathogen destruction. Medium-low process stability -- requires increased operational attention. Higher energy consumption for process heating. Typically not used in small facilities. Lagoon Digestion First stage is anaerobic digestion. However, some plants are known to have aerated lagoons up front. Membrane lagoon covers capture biogas. Reduced sludge production. Lagoons have a 30-year life. No mechanical equipment required for gas production. Simple to operate. Low rate of stabilisation. Large footprint. Requires covers over the lagoons -- difficult to properly cover and seal over a large area. No process control -- therefore difficult to control/maintain. Contents of lagoons are susceptible to temperature fluctuation -- e.g. temperature drop could adversely impact lagoon health. Temperature Phased Anaerobic Digestion (TPAD) TPAD is TAD for 5 to 10 days, followed by MAD for 10 to 15 days. Increased pathogens destruction. Able to deal with co-digestion of grease and co-mingled food waste. Medium-high process stability. Mesophilic digestion following thermophilic digestion helps prevent breakthrough odours. Increased process complexity. Typically not used in small facilities. Some wastes are more challenging for stable operation as TPAD has lower stability than Two Phase Acid- Gas Digestion. Two Phase (Acid- Gas) Anaerobic Digestion Two Phase Acid-Gas Digestion is similar to TPAD in that solids are digested in two phases. First (acid) phase has a hydraulic retention time (HRT) of 1--3 days. Second (gas -- methane) phase typically has a 10--14 day HRT. Reduced foaming and improved gas yield. Suitable for small facilities. Very high process stability. Reduced digestion volume by 30%--50% results in low capital cost. More process complexity. Requires consistently high organic loading of the acid reactor.
Water Journal May 2012
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