Pilot Scale Co-Digestion of FOG

This research evaluated the operation of pilot scale anaerobic digesters used for co-digestion of wastewater generated solids with fats, oils, and grease (FOG) waste. The pilot digesters had an operational volume of 1200 L each. Both the control and the test digesters were fed 60 L per day, to provide an operational solids retention time of 20 days. The test digester was fed an increasing percentage of FOG by adding more FOG and reducing the amount of wastewater solids. The FOG had a COD concentration that on average was 33 times that of wastewater solids, thus, the COD load to the test digester increased although the volumetric load did not change.This research showed that anaerobic digesters can handle a significant load of FOG. The feed to a digester can be two parts FOG and one parts COD in terms of the total COD load without any observable impact on the performance of the digester. Further pilot digesters were shock loaded such that the COD load in the feed was three parts FOG to one part wastewater solids and the digester appeared to be stable, with only small indications that the limits of process stability may be near, e.g. reduced digester gas production and a higher carbon dioxide concentration in the digester gas. Even at the maximum ratio of the FOG COD to wastewater COD, volatile acids in the digester were within acceptable ranges and organic removal rates were still consider sufficient.The pilot facility confirmed practices that many full scale plants have been observing that digester gas production increases due to FOG addition. The research found that FOG addition on the order of about 5 to 7.5% volumetrically would be acceptable without impacting digester stability. This would convert to an acceptable FOG loading of between 3,000 and 5,000 gallons per day per 1 million gallons of digester volume. However, volumetric loadings are likely not the best method of estimating loading to the digester as FOG samples can be significantly different depending on where they are received and if any decanting of water is completed to concentrate the FOG.