Introducing Fog to Sludge – A Sticky Proposition

The acceptance and processing of fats, oils, and grease (FOG) wastes from truck haulers at municipal wastewater treatment plants (WWTPs) are of increasing interest because of the need to keep these materials out of wastewater collection systems, as well as the added energy recovery potential when FOG wastes are anaerobically digested or incinerated. The addition of FOG wastes to a WWTP usually requires the commingling of FOG waste and sludges or biosolids, and usually in a process that utilizes the energy from FOG such as anaerobic digestion or incineration. The manner in which this is done can impact the performance of the anaerobic digesters or thermal oxidation unit, as well as other solids handling processes, and so must be done properly.The objective of this paper is to present a body of experience for accepting and co-processing FOG wastes with sludges. The presentation will focus on the selection of FOG waste receiving and process flow alternatives to minimize the problems that have been associated negatively with introduction of FOG wastes into a treatment plant. These problems have included: blockage of pipes and pumps; digester foaming; grit accumulation in digesters; “stuck” digesters (curtailed methanogenesis); clogging of gas collection and handling systems; flashback and air emission exceedances in multiple hearth incineration systems; and excessive downtime for maintenance.The authors have been involved in the evaluation and development of FOG waste acceptance and transfer systems for sewage sludge incinerators at Greater New Haven Water Pollution Control Authority, CT and Hampton Roads Sanitation District, VA, and for anaerobic digestion systems at Gwinnett County, GA, Johnson County, KS, Pinellas County, FL and elsewhere. These projects have contributed to the collective knowledge base that is to be presented in this paper. Most of the referenced projects have been completed and performance experience will be summarized as well as lessons learned.Site surveys were used to gather information on multiple facilities, including those mentioned above as well as at Redwood City, Millbrae and Oakland, CA, Gresham, OR and Gothenburg, Sweden. Each facility visited was surveyed with respect to: facility size; FOG quantities received; off-loading; transfer to storage; storage; mixing; heating; digester feeding; load tracking; wash down; and odor control. The survey results were analyzed to develop a basis of design for several projects as well as to prepare this paper.