Public Utility Infrastructure Improvements through Enhanced Grease Trap Specifications
Introduction
Wastewater utilities are experiencing increased pressure to maintain cost-effective operations while ensuring compliance with state and federal regulations. Overflows from manholes and overloads of wastewater treatment facilities often lead to regulatory non-compliances, system outages, undesired repair costs, and tarnished public image of the utility. Pervasive system problems can lead to regulatory intervention and forced remediation programs. Remediation efforts often lead to increased costs and higher rates, as well as public relations problems with utility customers.
Grease traps are the primary mechanisms and the last layer of protection in preventing the entry of grease into a sewer system and the resulting overflows and system overloads. The largest contributors of grease into sewer systems are food service establishments and commercial food processors. Use of improperly functioning or damaged grease traps by these establishments is problematic for wastewater utilities and can often be identified as the root cause of pervasive system failures.
Engineered tank design
Grease traps with inadequate storage capacity, baffle failure, and inferior sealants used in the seam of the tank will cause clogged lines. Each of these three characteristics are essential to grease trap design and are often overlooked as necessary considerations for a properly designed grease trap.
Adequate storage capacity ensures the necessary detention time is present to allow the grease to cool and separate before leaving the tank. Having sufficient capacity also protects against harmful turbulence that will not allow the separation of grease to occur. Optimal tank configurations that offer sufficient cool down time with minimum turbulence have between a 2:1 to 4:1 length-to-width ratio with baffle walls placed two-thirds the length distance away from the inlet.
To prevent baffle wall failure, baffle walls should be of a rigid design, not allowing water and grease to flow around the sides of the wall. In addition, the baffle walls should be thick enough to withstand the harsh environment experienced inside the grease trap. There are two types of baffle wall configurations: monolithic poured and slide-in baffle walls.
Monolithic type baffle walls are cast with the tank body and usually tied into the rebar reinforcing system of the tank. This type of baffle wall protects against grease and water flowing around the sides of the baffle wall and offers superior strength and long-term use.
Slide-in baffle wall designs are typically weaker and thinner than their monolithic counterparts. Slide-in baffle walls are not held rigidly in place because they are not tied into the existing reinforcing network and often rest upon materials such as bricks or wood in order to create the passage under the baffle wall needed for proper grease separation. Slide-in baffle walls must be sealed in the slot where the baffle wall slides into the tank body. Not sealing these joints leads to a less sturdy baffle wall design. Exposure to harmful grease and chemicals on three sides of the baffle wall accelerates the rate of decomposition of the concrete, which causes premature baffle wall failures.
The use of ASTM C990 compliant sealants in the joint line of a trap is critical to protect against inflow and infiltration problems and contaminating the surrounding water area with grease. Inferior sealant has often been viewed as a low priority problem; however, it is a critical component that directly affects the performance of the grease trap. Inflow and infiltration problems overload the system by introducing extra water into the grease trap, which forces unseparated grease into the sewer lines.
Wastewater utilities are experiencing increased pressure to maintain cost-effective operations while ensuring compliance with state and federal regulations. Overflows from manholes and overloads of wastewater treatment facilities often lead to regulatory non-compliances, system outages, undesired repair costs, and tarnished public image of the utility. Pervasive system problems can lead to regulatory intervention and forced remediation programs. Remediation efforts often lead to increased costs and higher rates, as well as public relations problems with utility customers.
Grease traps are the primary mechanisms and the last layer of protection in preventing the entry of grease into a sewer system and the resulting overflows and system overloads. The largest contributors of grease into sewer systems are food service establishments and commercial food processors. Use of improperly functioning or damaged grease traps by these establishments is problematic for wastewater utilities and can often be identified as the root cause of pervasive system failures.
Engineered tank design
Grease traps with inadequate storage capacity, baffle failure, and inferior sealants used in the seam of the tank will cause clogged lines. Each of these three characteristics are essential to grease trap design and are often overlooked as necessary considerations for a properly designed grease trap.
Adequate storage capacity ensures the necessary detention time is present to allow the grease to cool and separate before leaving the tank. Having sufficient capacity also protects against harmful turbulence that will not allow the separation of grease to occur. Optimal tank configurations that offer sufficient cool down time with minimum turbulence have between a 2:1 to 4:1 length-to-width ratio with baffle walls placed two-thirds the length distance away from the inlet.
To prevent baffle wall failure, baffle walls should be of a rigid design, not allowing water and grease to flow around the sides of the wall. In addition, the baffle walls should be thick enough to withstand the harsh environment experienced inside the grease trap. There are two types of baffle wall configurations: monolithic poured and slide-in baffle walls.
Monolithic type baffle walls are cast with the tank body and usually tied into the rebar reinforcing system of the tank. This type of baffle wall protects against grease and water flowing around the sides of the baffle wall and offers superior strength and long-term use.
Slide-in baffle wall designs are typically weaker and thinner than their monolithic counterparts. Slide-in baffle walls are not held rigidly in place because they are not tied into the existing reinforcing network and often rest upon materials such as bricks or wood in order to create the passage under the baffle wall needed for proper grease separation. Slide-in baffle walls must be sealed in the slot where the baffle wall slides into the tank body. Not sealing these joints leads to a less sturdy baffle wall design. Exposure to harmful grease and chemicals on three sides of the baffle wall accelerates the rate of decomposition of the concrete, which causes premature baffle wall failures.
The use of ASTM C990 compliant sealants in the joint line of a trap is critical to protect against inflow and infiltration problems and contaminating the surrounding water area with grease. Inferior sealant has often been viewed as a low priority problem; however, it is a critical component that directly affects the performance of the grease trap. Inflow and infiltration problems overload the system by introducing extra water into the grease trap, which forces unseparated grease into the sewer lines.
Installation of grease traps that have superior engineered design features and that are constructed of durable watertight materials will reduce sanitary sewer overflows and clogged lines due to grease infiltration.
Tank durability
Grease traps that are built with durable watertight materials increase the long-term effectiveness and minimize regular maintenance costs. Characteristics of durable grease traps include the use of proper material composition, the testing of freshly mixed concrete, appropriate use of traffic and non-traffic rated tanks, and the use of solid construction techniques.
Every load of concrete should be tested for basic properties such as slump, temperature, density, and air content. This testing should be performed by a certified technician, such as an American Concrete Institute (ACI) Certified Field Technician. Testing basic properties of concrete ensures that the concrete received from the producer meets the requirements needed to produce watertight grease traps. Concrete varies from load to load, and deficiencies in the concrete cannot be addressed without prepour concrete testing.
An important property that best describes the impermeability of concrete is density. Mix designs of 5,000 psi and greater are more dense and less porous than mix designs less than 5,000 psi because of increased cement content (water-to-cement ratio). Less dense concrete and insufficient concrete coverage around the steel allows chloride, oxygen, and moisture to easily reach the reinforcing steel, thereby accelerating the breakdown of concrete and the corrosion of the steel. Steel corrosion will lead to cracking and chipping (spalling) of the concrete, which lessen the useful life of the tank and make the tank subject to inflow and infiltration.
Proper placement of reinforcing steel results in sufficient concrete cover and ensures that corrosion of the steel is delayed and proper strength requirements are met. Steel reinforcement generally does not have sufficient coverage because support chairs are not used or are too tall for the application. Liberally using correctly sized support chairs ensures that reinforcing steel has sufficient concrete coverage. With the presence of steel corrosion, spalling is accelerated, and incidental damage from regular service events, such as pumping grease out of the tank, will compromise the watertight integrity and structural soundness of the tank over time.
The common grease trap is not designed for areas with vehicular traffic or where deep burial is required. The use of standard grease traps in vehicular traffic areas, such as parking lots, drive-throughs at food service establishments, and roadways, exceeds the safe load-bearing capacity of the tank. Tanks that are not designed for the load-bearing capacity for use in traffic areas are stressed, causing cracks to form in the tank due to the structural overloading. A common, but poorly designed, workaround when placing non-traffic rated grease traps in areas with vehicular traffic is to construct a concrete slab over the top of the grease trap. This adds to the overloading of the grease trap because of the additional weight of the concrete slab, typically 10,000 – 15,000 pounds.
Quality control program
Requiring plant certification for manufacturers of grease traps, such as the National Precast Concrete Association (NPCA) certification program, provides two major advantages for utilities. First, a Quality Control Manual is supplied to precasters to be used as a guide for developing a plant specific manual. The precast plant’s manual is used to establish guidelines and procedures for material specifications, testing, record keeping, and proper concrete construction techniques. This ensures that production practices remain unchanged over time.
The second advantage to utilities is that an unannounced physical in-plant inspection of the precast plant is performed by an independent engineering firm to confirm compliance with the procedures in the Quality Control Manual. Unbiased inspections are performed by certified inspectors in the field of precast concrete and eliminate the scenario for a Quality Control program to be established and then “forgotten” over time.
Recommendations
Utilities can achieve improved regulatory compliance and a more cost-effective operation leading to improved customer satisfaction by requiring the installation of grease traps with the following features:
- Monolithic tank baffles provide improved tank operating life for specified functionality.
- Traffic-rated (H-20) tanks provide long-term durability and continued operation in the high traffic areas where grease traps are typically installed.
- Engineered tank designs ensure watertight, structurally sound tanks that create environmentally friendly grease traps.
- Tanks manufactured under a certified manufacturing program, such as the NPCA, ensure that experienced professionals using proven manufacturing procedures are producing strong, durable, and superior finished products.
To provide long-term system improvement, utilities must ensure that properly designed, durable tanks are installed for grease trap applications. These recommendations will help prevent faulty grease trap installations from occurring in the future. There is no cost to the utility for specifying high quality grease traps. The aforementioned recommendations will lower costs to the utility by reducing system outages, minimizing undesired repair costs due to grease clogs, and reducing inflow and infiltration of wastewater treatment facilities.
Grease traps that are built with durable watertight materials increase the long-term effectiveness and minimize regular maintenance costs. Characteristics of durable grease traps include the use of proper material composition, the testing of freshly mixed concrete, appropriate use of traffic and non-traffic rated tanks, and the use of solid construction techniques.
Every load of concrete should be tested for basic properties such as slump, temperature, density, and air content. This testing should be performed by a certified technician, such as an American Concrete Institute (ACI) Certified Field Technician. Testing basic properties of concrete ensures that the concrete received from the producer meets the requirements needed to produce watertight grease traps. Concrete varies from load to load, and deficiencies in the concrete cannot be addressed without prepour concrete testing.
An important property that best describes the impermeability of concrete is density. Mix designs of 5,000 psi and greater are more dense and less porous than mix designs less than 5,000 psi because of increased cement content (water-to-cement ratio). Less dense concrete and insufficient concrete coverage around the steel allows chloride, oxygen, and moisture to easily reach the reinforcing steel, thereby accelerating the breakdown of concrete and the corrosion of the steel. Steel corrosion will lead to cracking and chipping (spalling) of the concrete, which lessen the useful life of the tank and make the tank subject to inflow and infiltration.
Proper placement of reinforcing steel results in sufficient concrete cover and ensures that corrosion of the steel is delayed and proper strength requirements are met. Steel reinforcement generally does not have sufficient coverage because support chairs are not used or are too tall for the application. Liberally using correctly sized support chairs ensures that reinforcing steel has sufficient concrete coverage. With the presence of steel corrosion, spalling is accelerated, and incidental damage from regular service events, such as pumping grease out of the tank, will compromise the watertight integrity and structural soundness of the tank over time.
The common grease trap is not designed for areas with vehicular traffic or where deep burial is required. The use of standard grease traps in vehicular traffic areas, such as parking lots, drive-throughs at food service establishments, and roadways, exceeds the safe load-bearing capacity of the tank. Tanks that are not designed for the load-bearing capacity for use in traffic areas are stressed, causing cracks to form in the tank due to the structural overloading. A common, but poorly designed, workaround when placing non-traffic rated grease traps in areas with vehicular traffic is to construct a concrete slab over the top of the grease trap. This adds to the overloading of the grease trap because of the additional weight of the concrete slab, typically 10,000 – 15,000 pounds.
Quality control program
Requiring plant certification for manufacturers of grease traps, such as the National Precast Concrete Association (NPCA) certification program, provides two major advantages for utilities. First, a Quality Control Manual is supplied to precasters to be used as a guide for developing a plant specific manual. The precast plant’s manual is used to establish guidelines and procedures for material specifications, testing, record keeping, and proper concrete construction techniques. This ensures that production practices remain unchanged over time.
The second advantage to utilities is that an unannounced physical in-plant inspection of the precast plant is performed by an independent engineering firm to confirm compliance with the procedures in the Quality Control Manual. Unbiased inspections are performed by certified inspectors in the field of precast concrete and eliminate the scenario for a Quality Control program to be established and then “forgotten” over time.
Recommendations
Utilities can achieve improved regulatory compliance and a more cost-effective operation leading to improved customer satisfaction by requiring the installation of grease traps with the following features:
- Monolithic tank baffles provide improved tank operating life for specified functionality.
- Traffic-rated (H-20) tanks provide long-term durability and continued operation in the high traffic areas where grease traps are typically installed.
- Engineered tank designs ensure watertight, structurally sound tanks that create environmentally friendly grease traps.
- Tanks manufactured under a certified manufacturing program, such as the NPCA, ensure that experienced professionals using proven manufacturing procedures are producing strong, durable, and superior finished products.
To provide long-term system improvement, utilities must ensure that properly designed, durable tanks are installed for grease trap applications. These recommendations will help prevent faulty grease trap installations from occurring in the future. There is no cost to the utility for specifying high quality grease traps. The aforementioned recommendations will lower costs to the utility by reducing system outages, minimizing undesired repair costs due to grease clogs, and reducing inflow and infiltration of wastewater treatment facilities.
Installation of grease traps that have superior engineered design features and that are constructed of durable watertight materials will reduce sanitary sewer overflows and clogged lines due to grease infiltration.