6.2 Cross-Connection Control & Backflow Prevention

Cross-connection control is the cornerstone of distribution system water quality protection. Without effective backflow prevention barriers, hazardous contaminants can enter public drinking water mains, threatening public health.

1. Cross-Connection Fundamentals & Hazard Classification

A cross-connection is defined as any actual or potential physical connection or arrangement between a public potable water system and any non-potable water supply, sewer, conduit, drain, storage tank, plumbing fixture, or industrial fluid system containing unapproved water, chemicals, or waste.

Hazard Classifications

Cross-connections are categorized by public health risk under federal and state standards:

• High Hazard (Health Hazard): Any condition, device, or water quality impairment involving a substance that could cause illness, disease, toxic poisoning, or death if ingested into the public water system. Examples include industrial chemical plating tanks, boiler feed lines treated with toxic corrosion inhibitors, medical laboratory sinks, agricultural chemical chemigation injectors, sewage lift stations, and auxiliary unapproved well systems.
• Low Hazard (Non-Health Hazard): Any condition or substance affecting the aesthetic quality of water (taste, odor, color, turbidity) that does not pose an actual health threat or toxic hazard to humans. Examples include food processing sugar syrup lines, fire sprinkler systems containing non-toxic water without chemical additives, commercial ice makers, and stagnant water storage tanks.

2. Hydraulic Mechanics of Backflow

Backflow is the unwanted reversal of flow of water or liquids into the potable distribution piping system. Backflow occurs through two distinct hydraulic mechanisms:

Back-Siphonage

Back-siphonage is backflow caused by negative or sub-atmospheric pressure (less than 0 psig or 14.7 psia) within the supply water main. Sub-atmospheric pressure creates a vacuum effect, pulling liquid from downstream non-potable sources back into the supply piping.

• Operational Causes: Main breaks, high-volume fire suppression pumping, line draining for emergency maintenance, or severe undersizing of supply mains during peak hourly demand.
• Classic Example: An open garden hose submerged in a bucket of toxic pesticide while a main break occurs down the street. Sub-atmospheric line pressure siphons the pesticide into the home and public distribution grid.

Back-Pressure

Back-pressure is backflow caused when downstream pressure exceeds the operating supply pressure in the potable distribution main.

• Operational Causes: High-pressure downstream booster pumps, elevated thermal expansion in closed-loop heating boilers, elevated storage tanks connected downstream, or pressurized industrial process lines.
• Classic Example: A commercial boiler operating at 60 psi connected directly to a city water supply main whose pressure drops to 45 psi. Higher boiler pressure forces boiler water containing toxic chemical additives back into the city main.

3. Backflow Prevention Assemblies & Devices

Protecting water networks requires selecting the correct backflow prevention assembly or device based on hazard level and backflow hydraulics.

1. Air Gap (AG)

An Air Gap is an unobstructed vertical physical separation through the free atmosphere between the lowest opening of a supply pipe and the flood-level rim of a receiving tank, plumbing fixture, or vessel.

• Engineering Specification: The vertical distance must be at least two times (2×) the effective internal diameter of the supply pipe, and in no case less than 1.0 inch (25 mm).
• Protection Level: High Hazard & Low Hazard; Back-Siphonage & Back-Pressure. Provides the highest degree of backflow protection. However, air gaps can be bypassed or defeated by attaching temporary hoses to supply outlets if not monitored.

2. Reduced Pressure Principle Assembly (RP or RPBP)

An RP Assembly consists of two independently acting, spring-loaded check valves separated by a hydraulically dependent, spring-loaded differential pressure relief valve located between them. The unit includes two resilient-seated shutoff valves and four test cocks.

• Operational Mechanics: The differential relief valve maintains an intermediate zone pressure at least 2.0 psi lower than the supply pressure. If either check valve leaks or if supply pressure drops, the relief valve opens automatically and vents water to the atmosphere, maintaining the low-pressure zone.
• Protection Level: High Hazard & Low Hazard; Back-Siphonage & Back-Pressure.
• Installation Requirements: Must be installed above grade or floor level (minimum 12 inches above floor/grade, maximum 60 inches). Must NEVER be installed in a pit, vault, or location subject to flooding, as submergence of the relief valve air gap causes cross-contamination.

3. Double Check Valve Assembly (DCVA)

A DCVA consists of two independently acting, spring-loaded check valves located between two tightly closing shutoff valves, equipped with four test cocks.

• Operational Mechanics: Holds water pressure between check valves. If one check valve fails, the second check valve prevents backflow. However, it lacks an atmospheric relief valve to signal valve failure.
• Protection Level: Low Hazard ONLY; Back-Siphonage & Back-Pressure. Strictly prohibited on high-hazard installations because particulate debris holding a check seat open permits undetected contamination.
• Installation Requirements: Can be installed in subterranean vaults or above grade with adequate clearance for testing.

4. Pressure Vacuum Breaker Assembly (PVB)

A PVB consists of an independently acting, spring-loaded check valve and an independently acting, spring-loaded air inlet valve, bounded by two shutoff valves and test cocks.

• Operational Mechanics: Under normal flow, internal water pressure holds the air inlet valve closed. If sub-atmospheric pressure occurs, the internal spring forces the air inlet valve open, admitting atmospheric air to break the vacuum.
• Protection Level: High Hazard & Low Hazard; Back-Siphonage ONLY.
• Constraints: Must NOT be subjected to back-pressure (downstream pressure holds the air valve closed). May be subjected to continuous supply pressure. Must be installed at least 12 inches above the highest downstream outlet or overflow pipe.

5. Atmospheric Vacuum Breaker (AVB)

An AVB contains a floating check disc that rises under line pressure to seal an atmospheric vent port. When pressure drops to zero, the disc drops, admitting air to break back-siphonage vacuums.

• Protection Level: High Hazard & Low Hazard; Back-Siphonage ONLY.
• Constraints: Must NOT be subjected to continuous pressure (must not remain pressurized for more than 12 continuous hours in a 24-hour period). Must NEVER have shutoff or control valves installed downstream of the device. Must be installed at least 6 inches above the highest downstream outlet.

Summary Comparison of Backflow Assemblies

4. South Carolina Regulation 61-58 Compliance & Administration

Under South Carolina State Primary Drinking Water Regulations (R.61-58.7(F)), every public water system (PWS) in South Carolina is legally mandated to establish and execute an active Cross-Connection Control Program.

Regulatory Mandates & Tester Certification

• Hazard Inventory: Utilities must complete facility surveys to classify customer connections as high hazard or low hazard and mandate appropriate assembly installations.
• Assembly Certification: Installed assemblies must be approved by the Foundation for Cross-Connection Control and Hydraulic Research (USC FCCCHR) or ASSE.
• Certified Tester Requirement: Testing, maintenance, and field repair of backflow assemblies must be performed exclusively by individuals holding an active South Carolina DES Backflow Prevention Assembly Tester Certification.
• Mandatory Testing Frequency: All testable assemblies (RP, DCVA, PVB) must be field-tested using calibrated differential pressure gauge kits:
  1. Immediately upon initial installation,
  2. Immediately following any repair, re-seating, or relocation, and
  3. At least annually (every 12 months) thereafter.
• Record Retention: Water utilities and certified testers must maintain official test reports for at least 5 years. Reports must record initial differential pressure readings, specific repairs performed, and final passing test metrics.
• Enforcement Authority: Public water systems are authorized by SC R.61-58 to issue notices of non-compliance and terminate water service to any facility failing to install, test, or repair backflow assemblies within specified regulatory timelines.