FOG Waste receiving and processing facility design considerations

The designs of several FOG waste receiving facilities completed in the past 10 years in North America are compared in this paper. These facilities include Johnson County Wastewater's Douglas L. Smith Middle Basin Wastewater Treatment Plant (WWTP) in Overland Park, Kansas; Clean Water Services' Durham Advanced Wastewater Treatment Facility (AWWTF) in Tigard, Oregon; and the City of Calgary's Bonnybrook WWTP in Calgary, Alberta.

Key components that are typically included in a modern FOG/HSW receiving station should include:

• A debris trap to remove rocks, grit, metal items and other heavy debris

• Screens and maceration to reduce the size of rags, plastics, wood, and other debris

• Flow/volume/weight measurement systems to document the amount of material received from each load received for billing purposes

• Influent or offloading pumps to quickly remove the material from the trucks or below grade storage tanks

• Heating system to increase the temperature of the FOG wastes and prevent congealing in pumps and pipelines and pre-heat before feeding digesters

• Storage tanks for short term storage of the wastes prior to feeding to the anaerobic digesters

• Mixing system to homogenize the material from different loads and to keep debris from settling

• Odor control for the storage tanks

• Pumps with variable speed controllers to feed the digesters at a steady loading rate.

Historically FOG wastes, septage and other wastes, have been discharged at the head-end of WRRFs or in upstream manholes in the main interceptors to the treatment plants to enable the grit, rocks, and heavy debris to be removed by the treatment plant's normal bar screens and grit chambers. The debris in septage can create issues with these standard processes so some utilities have constructed separate septage receiving stations to remove the debris prior to it entering the normal wastewater flow stream. The initial FOG/HSW receiving stations appear to have been modelled after these septage receiving stations and consist of bar screens and heavy debris removal basins as illustrated in Figure 1 (Gabel et al. 2009).

Significant improvements from these early systems have been developed to provide the treatment plants' operations staff with flexibility in managing these waste streams, provide more ‘operator friendly’ approaches to maintaining receiving station equipment and facilities, and capture as much energy as possible from the FOG/HSW. Figure 2 presents the key components of a modern FOG/HSW receiving station (Gabel et al. 2014), which consist of:

• A debris trap to remove rocks, grit, metal items and other heavy debris

• Screens and maceration to reduce the size of rags, plastics, wood, and other debris

• Flow/volume/weight measurement systems to document amount of material received from each truck for billing purposes

• Influent or offloading pumps to remove the material from the trucks if needed; below grade storage tanks may not require these pumps

• Heating system to increase the temperature of the FOG wastes and prevent congealing in pumps and pipelines

• Storage tanks for short term storage of the wastes prior to feeding it to the anaerobic digesters; multiple tanks preferred to facilitate maintenance and cleaning; total volume should consider multiple peak days associated with holidays and long holiday weekends with no waste receipts but continuous digester feeding

• Mixing system to keep debris from settling and homogenize the material from different loads

• Odor control for the storage tanks

• Pumps with variable speed controllers to feed the digesters at a relatively low and constant loading rate

Other considerations include truck access, truck driver interface system, waste material sampling, decanting of the waste material, and the need (or preference) to enclose the process equipment within a building. The location of the waste receiving facility with regard to security and access, available land area, and distance to the digesters are important considerations in the planning phase. Most WRRFs are fenced with entry gates that are either locked or access controlled. If the receiving station cannot be located outside the fence, then entry onto the site will require providing the waste haulers keys, key cards, or an access code. Access by only approved waste haulers provides some level of security and also enables the WRRFs operations staff to provide standard operating procedures and training to the waste hauler staff. The location should provide sufficient room for truck movement including adequate turning radius and back- up area for the largest waste hauling truck that could use the facility. If possible, a drive through facility that slopes slightly toward the back of the truck, with a spill containment area, is preferred. Hot wash water and positive drainage to a central area drain facilitate truck cleaning that enables housekeeping in the truck unloading area is recommended. An example of a drive-through facility that accommodates three trucks simultaneously is the Des Moines, Iowa facility shown in Figure 3 (Kabouris et al. 2012).

A driver interface and facility control system, such as shown in Figure 4, is used to provide access to receiving station and initiate the unloading process (Gabel et al. 2011). Each authorized truck or driver is provided a key card or unique access code to engage the system. The system will typically record the delivery company, the driver, the date and time, and the amount of waste unloaded, and is often directly linked to the WRRF's billing system to enable billing the delivery company. For process logic controlled systems, once the key card or access code has been recognized and the truck discharge hose connected, the driver will push a button to begin an automated unloading cycle. After the truck is unloaded, an automatic flushing cycle is typically activated using non-potable water to clean the influent pipe. Periodic and random truck sampling is recommended to minimize receipt of toxic loads. Samples can be collected from each truck with only a small number analysed at random. Collecting the samples could be delegated to the truck driver.

The volume of FOG/HSW fed to the digesters is normally a small fraction of the total volume of primary and secondary sludge, and should have minor impact on the overall digester solids retention time. However, if the material is determined to be consistently weak, such as less than 3 percent total suspended solids, or the FOG waste is fed to a sludge incinerator, decanting of excess water and concentrating the FOG material in the storage tanks may be desirable (Williams et al. 2010). Decanting is typically a manual operation. Many of the first FOG/HSW receiving stations were located in warmer climates where freezing temperatures rarely occur or where heat tape wrapping of the process piping is sufficient to avoid freezing pipes. Facilities located in colder climates typically enclose the process piping and equipment and sometime even construct the storage tanks within a building to prevent freezing and provide a more operator-friendly and odour controlled environment for operations and equipment maintenance.

The key components, site conditions and owner-specific requirements of three recent designs of FOG/HSW receiving stations are compared below:

• 1.

  Johnson County Wastewater's facility at the Douglas L. Smith Middle Basin (DLSMB) WWTP in Overland Park, KS was designed to receive an average of 45,400 litres per day (12,000 gallons) per day (gpd) of FOG and HSW and started operation in December 2010.

• 2.

  Clean Water Services' facility at the Durham AWWTF in Tigard, OR was designed to receive an average of 125,000 litres per day (33,000 gpd) of FOG wastes and began operation in 2016.

• 3.

  City of Calgary's Bonnybrook WWTP in Calgary, Alberta was designed to receive an average of 41,600 litres per day (11,000 gpd) of FOG wastes and began operation in September 2016.

The design of the DLSMB WWTP facility was based on information obtained during site visits to a number of FOG/HSW receiving stations plus significant Johnson County operations and engineering staff input. The design for the Durham AWWTF was built on the design concepts of the Johnson County facility and site visits to other facilities and incorporated input from Clean Water Services operations and engineering staff. The Bonnybrook WWTP facility design built on design concepts from both of the other facilities and incorporated input from City of Calgary's operations and engineering staff. A comparison of the key design considerations for these facilities is provided in Table 1 (Gabel et al. 2014).

In addition to the design considerations above, data obtained from active facilities provides a significant database of ‘lessons learned’ to assist future designers of these facilities. Table 2 summarizes key design criteria from this database (Gabel et al. 2014).

Receiving FOG wastes and HSW offer significant financial incentives including tipping fees and reduced utility bills for WRRFs when combined with CHP cogeneration systems. However, these wastes can be difficult to manage and handle by a treatment plant's operations staff. Design considerations for state-of-the-industry receiving and management faculties are presented in this paper. Lessons learned from operating facilities are also presented to address many of the handling issues encountered with these waste streams.