Fats, Roots, Oils, and Grease (FROG) in Centralized and Decentralized Systems

Abstract

Fat, oil, and grease (FOG) is generated everyday by food preparation and cleaning activities conducted at commercial establishments and, on a smaller scale, by residential sewer usage. Another common problem, known to all plumbing and utility maintenance personnel, is the process of increasingly dense mats of root hair. FOG or root accumulations in the sanitary sewer collection system result in reduced capacity that may lead to sanitary sewer overflows (SSO) if not periodically cleaned. As the mat or accumulation commence, the effect of slowed wastewater flow exacerbates the rate of accumulation and deposit of FOG materials onto the pipe walls, thereby reducing capacity. The primary means of controlling FOG blockages is to capture and retain FOG materials through passive grease interception devices. Limited scientific studies have been done evaluating these devices, and many claims of enhanced performance made in marketing strategies by manufacturers of grease and oil interception devices need to be verified by objective and unbiased research protocols. This report performed an evaluation of field grease interceptors through their separation and cleaning cycles, performed controlled laboratory scale grease interceptor tests and numerical simulations to assess their removal efficiency at different residence times and under different geometric configurations. Currently, no study has performed any analysis of the chemical and physical makeup of FOG deposits formed in the sewer collection system. This type of information would be a crucial first step in understanding what compounds should be limited in the effluent discharge of grease interceptors. This report identifies the major chemical constituents and physical characteristics of FOG deposits retrieved from sewer collection systems across the different EPA zones. Measurements included total oil and grease, fatty acid profile, metals and mineral content, shear and compressive strength. In addition, surface chemistry analyses have been performed on sewer pipes and tree roots to determine if there is a preferential selection of FOG deposits on specific surface types. Finally, the invasion of sewer pipes by tree roots is a major cost to both the corporations managing urban infrastructure and to private landowners. When a root branch encounters improved sewer pipe conditions (high moisture and nutrients), the root system tends to envelope that section of sewer pipe, particularly when the surrounding urban soils are typically poor in nutrients or low in water content such as during drought conditions. Roots grow by elongation, which allows roots to enter into small openings in sewer pipe joints and cracks. While root control strategies that involve chemical and or mechanical methods are currently being utilized by municipalities, few studies have performed control studies in sewer systems. This report compares two chemical root control methods and a mechanical control method. Root control tests were performed in a pilot-scale sewer system that included three types of trees. This title belongs to WERF Research Report Series . ISBN: 9781843395232 (Print) ISBN: 9781780403649 (eBook)