TROUBLESHOOTING A GRIT REMOVAL SYSTEM

Like many municipal wastewater treatment facilities, and especially those that serve combined collection systems, King County's West Point Treatment Plant experiences significant operational impacts due to inefficient and unreliable grit removal systems. Grit that passes through the aerated grit chambers creates pervasive problems throughout the treatment plant, from high maintenance for solids handling equipment to frequent cleaning of the anaerobic digesters. High grit loads, usually resulting from storm events, cause immediate problems for the treatment plant operators due to frequent of plugging of various components of the grit removal system. This paper describes a troubleshooting procedure that can be applied to any grit removal system, although in this case aerated grit chambers are evaluated. The recommended procedure for troubleshooting a grit removal system begins with forming a project team that includes key personnel from the plant operations and maintenance staff and laboratory staff (for grit testing support). These individuals should be involved in all phases of the investigation, as there are many different tasks involved, and the assistance of treatment plant personnel is critical to efficiently accomplishing these tasks. The knowledge that these individuals have with dealing with the grit problems on a daily basis should be tapped in brainstorming sessions with the lead investigators. The lead investigators must also review available design criteria for the particular grit removal system in question. Many treatment plants have been expanded without performing necessary upgrades to the grit removal system. The increased flows reduce detention time and increase turbulence in the grit removal system, both of which can significantly decrease grit removal efficiency, especially in an aerated grit chamber.In order to identify the “weak link” in the grit removal process it is necessary to perform grit tests at strategic locations on both the liquid and the solid streams. Locations for liquid stream testing include the grit basin influent and effluent channels (if turbulent enough to insure representative grit samples), and the influent and effluent streams of the grit separation and dewatering system (e.g., the cyclone/classifier system). Solids stream testing involves sampling the grit that is removed from the influent by the existing system (i.e. the “captured” grit that is hauled from the plant). It also includes sampling the primary sludge to determine the amount and size of the grit that is not removed by the existing system. This grit testing program allows the estimation of the grit removal efficiency of the various components of the grit system, as well as the removal efficiency of the overall process. This information can reveal which component is most responsible for the poor grit removal, permitting more cost-effective solutions to be developed.