Fats, Oils, and Grease (FOG) wastes and high-strength wastes (HSW) are frequently received at municipal water resource recovery facilities (WRRFs) as trucked-in wastes. These wastes offer significant benefits in terms of revenue from tipping fees and feedstock for co-digestion in anaerobic digesters that produce biogas, which can be beneficially used as fuel. The number of treatment plants receiving and beneficially using trucked-in wastes currently in operation or under investigation is increasing rapidly across the North America as utilities strive to remove this material from normal wastewater to avoid sewer system clogging, maintenance and backups, avoid the oxygen demand of these wastes in secondary treatment systems, and to capture and beneficially reuse the energy that is contained within the material.

Historically, trucked-in wastes have been discharged to the head end of treatment plants or to an upstream manhole in the incoming interceptor sewer to enable the material to be mixed with raw wastewater prior to treatment through the liquid stream of the WRRF. However, this approach results in loss of material and degradation of the energy value of the FOG wastes and HSW and also creates collection and maintenance issues in the preliminary and primary treatment systems. To prevent degradation of the material and retain maximum energy for the CHP system, receiving stations are being constructed for direct off-loading of the wastes to processing and storage facilities prior to their transfer to anaerobic digesters at a relatively uniform rate to minimize the potential for digester upsets while at the same time to increase biogas production. This paper presents the key components and considerations in the design and operation of modern FOG waste receiving and processing facilities.

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).

Figure 1

Former FOG waste receiving station for the South Bayside System Authority, Redwood City, CA consisting of a bar screen and debris removal basin.

Figure 1

Former FOG waste receiving station for the South Bayside System Authority, Redwood City, CA consisting of a bar screen and debris removal basin.

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

Figure 2

Modern FOG/HSW receiving station components.

Figure 2

Modern FOG/HSW receiving station components.

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).

Figure 3

Drive through FOG/HSW receiving station.

Figure 3

Drive through FOG/HSW receiving station.

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.

Figure 4

FOG receiving station truck driver interface.

Figure 4

FOG receiving station truck driver interface.

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).

Table 1

Design considerations comparison for FOG and high strength waste receiving

Design ConsiderationDLSMB WWTP, Johnson County, KSDurham AWWTF, Tigard, ORBonnybrook WWTP, Calgary, AB
Facility Size 55 mld (14.5 mgd) 76 mld (20 mgd) 492 mld (130 mgd) 
Design FOG Quantities 46,900 litres (12,400 gallons)
average day; 113,550 litres (30,000 gallons) peak day 
124,950 litres (33,000 gallons)
average day; 189,250 litres (50,000 gallons) peak day 
41,600 litres (11,000 gallons)
average day; 83,200 litres (22,000 gallons) peak day 
Material Restaurant FOG wastes and HSW from food processors Restaurant FOG wastes and HSW from food processors Restaurant FOG wastes 
Location Within plant fence enclosure Within plant fence enclosure Within plant fence enclosure 
Security Access code and security camera with DVR provided at unloading station Key card access Key card access 
Sample Collection None Collected by plant staff None 
Off-loading Single quick connect hose connection Single quick connect hose connection Single quick connect hose connection 
Debris Treatment Combination rock trap and macerator Separate rock trop and in-line macerator Separate rock trap and receiving chamber 
Transfer Pumps Rotary lobe pump Rotary lobe pump Chopper pumps 
Volume Received Measurement Storage tank volume changes Flow meter and storage tank volume changes Storage tank volume changes 
Storage Three 56,750 litre (15,000 gallon) fiberglass reinforced plastic (FRP) tanks Two 142,000 litre (37,500 gallon) concrete tanks Two 60,560 litre (16,000 gallon) FRP tanks 
Mixing Recirculation of waste using transfer pumps through the heat exchanger and grinder Single mechanical mixer per tank with recirculation of waste at low storage volumes Recirculation of waste using transfer pumps 
Heating Hot water heat exchangers to maintain temperature to above 27 °C Hot water heat exchangers to maintain temperature to above 27 °C Steam injection into receiving chamber and storage tanks to maintain temperature to above 25 °C 
Digester Feeding Progressing cavity pumps; duplicate feed lines with individual flow meters Progressing cavity pumps; duplicate feed lines with individual flow meters Progressing cavity pumps; duplicate feed lines with individual flow meters 
Load Tracking Secure panel containing key pad to activate system by company, driver and load Secure panel containing key pad to activate system by company, driver and load Secure panel containing key pad to activate system by company, driver and load 
Wash down Power sprayer and hot water available to wash down truck and unloading area; sloped concrete unloading area; wash down water drains to plant headworks Power sprayer and hot water available to wash down truck and unloading area; sloped concrete unloading area; wash down water drains to plant headworks Power sprayer and hot water available to wash down truck and unloading area; sloped concrete unloading area; wash down water drains to plant headworks 
Odour Control Biotower followed by activated carbon Biofilter Activated carbon 
Enclosures Building for process equipment and storage tanks Building for process equipment. Storage tanks incorporated in building structure. Building for process equipment 
Design ConsiderationDLSMB WWTP, Johnson County, KSDurham AWWTF, Tigard, ORBonnybrook WWTP, Calgary, AB
Facility Size 55 mld (14.5 mgd) 76 mld (20 mgd) 492 mld (130 mgd) 
Design FOG Quantities 46,900 litres (12,400 gallons)
average day; 113,550 litres (30,000 gallons) peak day 
124,950 litres (33,000 gallons)
average day; 189,250 litres (50,000 gallons) peak day 
41,600 litres (11,000 gallons)
average day; 83,200 litres (22,000 gallons) peak day 
Material Restaurant FOG wastes and HSW from food processors Restaurant FOG wastes and HSW from food processors Restaurant FOG wastes 
Location Within plant fence enclosure Within plant fence enclosure Within plant fence enclosure 
Security Access code and security camera with DVR provided at unloading station Key card access Key card access 
Sample Collection None Collected by plant staff None 
Off-loading Single quick connect hose connection Single quick connect hose connection Single quick connect hose connection 
Debris Treatment Combination rock trap and macerator Separate rock trop and in-line macerator Separate rock trap and receiving chamber 
Transfer Pumps Rotary lobe pump Rotary lobe pump Chopper pumps 
Volume Received Measurement Storage tank volume changes Flow meter and storage tank volume changes Storage tank volume changes 
Storage Three 56,750 litre (15,000 gallon) fiberglass reinforced plastic (FRP) tanks Two 142,000 litre (37,500 gallon) concrete tanks Two 60,560 litre (16,000 gallon) FRP tanks 
Mixing Recirculation of waste using transfer pumps through the heat exchanger and grinder Single mechanical mixer per tank with recirculation of waste at low storage volumes Recirculation of waste using transfer pumps 
Heating Hot water heat exchangers to maintain temperature to above 27 °C Hot water heat exchangers to maintain temperature to above 27 °C Steam injection into receiving chamber and storage tanks to maintain temperature to above 25 °C 
Digester Feeding Progressing cavity pumps; duplicate feed lines with individual flow meters Progressing cavity pumps; duplicate feed lines with individual flow meters Progressing cavity pumps; duplicate feed lines with individual flow meters 
Load Tracking Secure panel containing key pad to activate system by company, driver and load Secure panel containing key pad to activate system by company, driver and load Secure panel containing key pad to activate system by company, driver and load 
Wash down Power sprayer and hot water available to wash down truck and unloading area; sloped concrete unloading area; wash down water drains to plant headworks Power sprayer and hot water available to wash down truck and unloading area; sloped concrete unloading area; wash down water drains to plant headworks Power sprayer and hot water available to wash down truck and unloading area; sloped concrete unloading area; wash down water drains to plant headworks 
Odour Control Biotower followed by activated carbon Biofilter Activated carbon 
Enclosures Building for process equipment and storage tanks Building for process equipment. Storage tanks incorporated in building structure. Building for process equipment 

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).

Table 2

FOG and HSW handling issues encountered and resolutions identified

FOG HandlingDesign or Operational Resolution Issue Encountered
Corrosiveness of material to metal and elastomer components 
  • Use stainless steel for most internal pump and grinder components (e.g., rotary-lobe wear plates and grinder cutting blades)

  • Use Buna-N elastomer rather than EPDM for seals, gaskets, etc., as EPDM will swell and soften when exposed to FOG.

  • Some organic acids may be present in high concentrations depending on the waste source (acetic, lactic, and fatty acids); testing of waste material prior to system design is recommended.

  • Line concrete tanks with high performance epoxy polyamide coating system to accommodate pH 2.5 – 11 material.

 
Rocks and inert debris 
  • Locate rock traps near quick-connect truck coupling to capture heavy material as soon as possible

  • Use robust macerating system for wood and plastic debris and locate after rock traps

  • Consider ease of emptying debris and cleaning in design

 
High viscosity of material at lower temperatures 
  • Install hot-water/FOG heat exchangers or steam injection to pre-heat FOG entering storage tanks

  • Insulate piping; heat trace if needed

  • Use heating systems designed to keep FOG temperatures >30 °C at all times to minimize clogging and prepare to feed digesters

 
Flow meter inaccuracies 
  • Flow meters can be inaccurate if waste material is not homogenous

  • Use ultrasonic or radar-type level indicators in storage tanks to determine quantity of material received

  • Load cells under storage tanks or truck scales also appropriate

 
Truck traffic control and orderly receiving 
  • Implement computerized card-key system to identify and log in trucks

  • Use signage and driveway markings for clear directives to drivers and one-way traffic control

  • Use quick-connecting couplings for truck FOG hoses and build robust spill containment systems

  • Include high pressure hot water cleaners for trucks and facility

  • Install multiple off-loading stations to handle high truck traffic

  • Consider impact of schedule and frequency of truck traffic on area roads/neighbours.

 
Potential for clogging in pumps and pipes 
  • Implement manual hot-water flush systems with operator input

  • Feed FOG into heated parts of the digester piping system such as digester feed or recirculation loops

  • Automated storage tank effluent sump flushing system

 
Potential to upset digestion process 
  • Include multiple FOG storage tanks in system to allow for separate storage of FOG and HSW loads and metered feeding to digestion

  • Establish maximum allowable feed rate based on hydraulic and organic loadings (no more than 33% of total COD feed to digesters)

  • Perform laboratory testing of potential material prior to system start up and establish on-going testing protocols for continued operations

  • Use variable-speed metering pumps to proportion FOG feed to sludge feed

  • Feed FOG into digester sludge feed lines or sludge storage tanks for mixing prior to digestion

  • Feed FOG to several digesters at once or in sequence

 
Accessibility to system components for maintenance 
  • Design with mostly above-ground facilities and components for easier access for maintenance, cleaning, etc.

  • Robust and convenient provisions for cleaning equipment and piping

  • Locate cleanouts to allow for thorough flushing of system

  • Design floor for wash down with adequate slope and drains

  • Install man-ways near ground level of elevated storage tanks

 
Control of odours 
  • Completely contain FOG receiving and storage system

  • Exhaust head space of storage tanks into odour control system

  • Consider biofilter or bio-trickling filter followed by activated carbon depending on available space

  • Use grease filters upstream of fans

 
Excessive wear on pump materials 
  • Use rotary lobe, progressing cavity, chopper pumps or hose pumps depending on rates required. Select robust metallurgy and appropriate elastomers for pump materials to accommodate low pH and higher operating temperatures.

  • Use rock trap to capture large debris and grinder to macerate solids

 
Volume measurement and billing 
  • Municipal regulations to require record keeping of grease trap cleaning quantities, haulers and generators

  • Automatic logging and billing systems preferred by operations staff

 
FOG HandlingDesign or Operational Resolution Issue Encountered
Corrosiveness of material to metal and elastomer components 
  • Use stainless steel for most internal pump and grinder components (e.g., rotary-lobe wear plates and grinder cutting blades)

  • Use Buna-N elastomer rather than EPDM for seals, gaskets, etc., as EPDM will swell and soften when exposed to FOG.

  • Some organic acids may be present in high concentrations depending on the waste source (acetic, lactic, and fatty acids); testing of waste material prior to system design is recommended.

  • Line concrete tanks with high performance epoxy polyamide coating system to accommodate pH 2.5 – 11 material.

 
Rocks and inert debris 
  • Locate rock traps near quick-connect truck coupling to capture heavy material as soon as possible

  • Use robust macerating system for wood and plastic debris and locate after rock traps

  • Consider ease of emptying debris and cleaning in design

 
High viscosity of material at lower temperatures 
  • Install hot-water/FOG heat exchangers or steam injection to pre-heat FOG entering storage tanks

  • Insulate piping; heat trace if needed

  • Use heating systems designed to keep FOG temperatures >30 °C at all times to minimize clogging and prepare to feed digesters

 
Flow meter inaccuracies 
  • Flow meters can be inaccurate if waste material is not homogenous

  • Use ultrasonic or radar-type level indicators in storage tanks to determine quantity of material received

  • Load cells under storage tanks or truck scales also appropriate

 
Truck traffic control and orderly receiving 
  • Implement computerized card-key system to identify and log in trucks

  • Use signage and driveway markings for clear directives to drivers and one-way traffic control

  • Use quick-connecting couplings for truck FOG hoses and build robust spill containment systems

  • Include high pressure hot water cleaners for trucks and facility

  • Install multiple off-loading stations to handle high truck traffic

  • Consider impact of schedule and frequency of truck traffic on area roads/neighbours.

 
Potential for clogging in pumps and pipes 
  • Implement manual hot-water flush systems with operator input

  • Feed FOG into heated parts of the digester piping system such as digester feed or recirculation loops

  • Automated storage tank effluent sump flushing system

 
Potential to upset digestion process 
  • Include multiple FOG storage tanks in system to allow for separate storage of FOG and HSW loads and metered feeding to digestion

  • Establish maximum allowable feed rate based on hydraulic and organic loadings (no more than 33% of total COD feed to digesters)

  • Perform laboratory testing of potential material prior to system start up and establish on-going testing protocols for continued operations

  • Use variable-speed metering pumps to proportion FOG feed to sludge feed

  • Feed FOG into digester sludge feed lines or sludge storage tanks for mixing prior to digestion

  • Feed FOG to several digesters at once or in sequence

 
Accessibility to system components for maintenance 
  • Design with mostly above-ground facilities and components for easier access for maintenance, cleaning, etc.

  • Robust and convenient provisions for cleaning equipment and piping

  • Locate cleanouts to allow for thorough flushing of system

  • Design floor for wash down with adequate slope and drains

  • Install man-ways near ground level of elevated storage tanks

 
Control of odours 
  • Completely contain FOG receiving and storage system

  • Exhaust head space of storage tanks into odour control system

  • Consider biofilter or bio-trickling filter followed by activated carbon depending on available space

  • Use grease filters upstream of fans

 
Excessive wear on pump materials 
  • Use rotary lobe, progressing cavity, chopper pumps or hose pumps depending on rates required. Select robust metallurgy and appropriate elastomers for pump materials to accommodate low pH and higher operating temperatures.

  • Use rock trap to capture large debris and grinder to macerate solids

 
Volume measurement and billing 
  • Municipal regulations to require record keeping of grease trap cleaning quantities, haulers and generators

  • Automatic logging and billing systems preferred by operations staff

 

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.

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