15.4 Cross-Connection Control Program Administration & Device Testing
Key Takeaways
- Cross-Connection and Backflow Devices is a named sub-topic in the Distribution System Design and Hydraulics category of the SWRCB distribution exam.
- Backflow occurs by backsiphonage when upstream pressure falls, or by backpressure when a downstream system exceeds distribution pressure.
- An air gap of at least two pipe diameters and never less than one inch is the only absolute barrier and is not defeated by pressure.
- The four mechanical assemblies in ascending order of protection are the dual check, the double check valve assembly, the pressure vacuum breaker, and the reduced pressure principle assembly.
- California requires backflow prevention assemblies to be tested by a certified tester at least annually and after any repair or relocation.
The Two Mechanisms
A cross-connection is any actual or potential connection between the potable water system and a source of contamination. Backflow is the reversal of flow through that connection, and it happens two ways:
| Mechanism | Cause | Typical scenario |
|---|---|---|
| Backsiphonage | Upstream pressure drops, creating a partial vacuum | A main break, heavy fire flow, a pump shutdown, or a hydrant used at high flow pulls system pressure below atmospheric; a hose left in a chemical tank, a filled sink, a swimming pool, or a livestock trough is siphoned back into the main |
| Backpressure | Downstream pressure exceeds distribution pressure | A boiler, a pressure washer, a booster pump, an elevated tank, a fire pump, a hydro-pneumatic tank, or an irrigation system with a chemical injector pushes back into the main |
Understanding which mechanism a given hazard presents determines which assembly is acceptable, because a vacuum breaker protects only against backsiphonage and does nothing against backpressure.
Hazard Assessment
| Hazard class | Definition | Examples |
|---|---|---|
| High hazard (health hazard / contaminant) | Could cause illness or death | Hospitals, mortuaries, dialysis facilities, laboratories, plating shops, car washes, chemical plants, wastewater treatment plants, reclaimed/recycled water users, agricultural chemigation, fire systems with antifreeze or foam additives, any auxiliary supply of unknown quality |
| Low hazard (non-health hazard / pollutant) | Aesthetic degradation only | Residential irrigation without injectors, a food-grade process, a closed loop with only potable water |
Premises isolation protects the distribution system at the service connection. Internal (point-of-use) isolation protects the building occupants at each fixture or process. A utility's program is primarily about premises isolation, because that is what protects the public main.
[!IMPORTANT] Recycled water is always a high hazard from the potable system's perspective. Any premises with a dual plumbing system - a Title 22 recycled water irrigation system alongside potable service - requires premises isolation with a reduced pressure principle assembly and a cross-connection shutdown test. California recycled water rules also require purple pipe and appurtenance marking, signage, and periodic cross-connection testing precisely because a mis-plumbed connection between purple and potable is the recurring failure mode.
Backflow Prevention Methods
| Method | Protects against | Hazard level | Notes |
|---|---|---|---|
| Air gap (AG) | Both | High and low - absolute | A physical separation of at least two pipe diameters and never less than 1 inch between the supply outlet and the flood rim. Cannot be defeated by pressure, but is easily defeated by someone adding a hose |
| Reduced pressure principle assembly (RP/RPZ) | Both | High | Two independent check valves with a differential relief valve between them that dumps to atmosphere if either check fails. Must be installed above grade and never in a pit that can flood, because the relief port must discharge freely |
| Double check valve assembly (DC/DCVA) | Both | Low only | Two spring-loaded checks with test cocks and shutoffs |
| Pressure vacuum breaker (PVB) | Backsiphonage only | High, for that mechanism | Must be installed at least 12 inches above the highest downstream outlet; not acceptable where backpressure is possible |
| Atmospheric vacuum breaker (AVB) | Backsiphonage only | Point of use | No shutoff valve downstream; not testable; not for continuous pressure |
| Dual check (residential) | Limited | Low | Not a testable assembly; used at residential meters |
Ranking by protection: air gap > reduced pressure principle assembly > pressure vacuum breaker > double check valve assembly > dual check.
[!TIP] Two rules generate most of the exam questions. First, a vacuum breaker of any kind is useless against backpressure - if a booster pump, boiler, or elevated tank is downstream, the answer is an RP. Second, an RP must never be installed where its relief port can be submerged, because a flooded vault turns the relief port into an inlet and defeats the assembly entirely.
Testing and Certification
California requires that backflow prevention assemblies be tested by a certified backflow prevention assembly tester:
- Upon installation
- At least annually thereafter
- After any repair, relocation, or replacement
Testers are certified through recognized programs, and the water supplier must maintain records of the tester's certification. The test verifies check valve tightness, relief valve opening differential (for an RP), and air inlet operation (for a PVB), using a calibrated differential pressure gauge on the assembly's test cocks. Test gauges themselves must be calibrated on a defined schedule, commonly annually.
A failed assembly must be repaired and retested, and the water supplier must have the authority to discontinue service if a hazard is not corrected.
Program Administration
A compliant cross-connection control program includes:
- Legal authority - an ordinance, rule, or tariff giving the supplier the right to inspect, require assemblies, and terminate service
- A designated program administrator, in many California programs a certified cross-connection control specialist
- A survey program to identify hazards at new and existing services, with re-survey when use changes
- A hazard assessment methodology that determines the required assembly by hazard class and mechanism
- An inventory of every assembly: location, make, model, size, serial number, hazard, installation date, and test history
- Annual test tracking and enforcement, with notices, follow-up, and service termination as a last resort
- Records retention and reporting to the State Board as required
- Public education - the hose-in-the-pool and the chemical sprayer on a garden hose are real hazards that customers create without realizing it
- Response procedures for a suspected backflow incident
Responding to a Suspected Backflow Event
- Isolate. Close the service, and if the contamination may have entered the main, isolate the affected distribution segment.
- Notify the Division of Drinking Water, the local health department, and affected customers. A confirmed backflow of a health hazard is a Tier 1 public notification situation.
- Investigate the source, the mechanism, and the extent. Was there a pressure event? What is on the premises? Is there an unprotected connection?
- Sample - bacteriological plus any analyte specific to the suspected contaminant.
- Flush and, if warranted, disinfect the affected mains per AWWA C651.
- Do not restore service to the offending premises until the cross-connection is eliminated or properly protected and the assembly has been tested.
- Document everything. Backflow incidents generate enforcement and sometimes litigation.
[!WARNING] The single most common serious cross-connection in a water system is a temporary one. A contractor's hose in a trench, a tanker filling from a hydrant without an air gap or an approved backflow device, a hydrant meter without a backflow assembly, a temporary bypass around a repair. Utilities that manage permanent assemblies rigorously and ignore hydrant meters and construction water have not closed the loop. Every hydrant meter should carry an approved backflow assembly, and construction water use should be permitted, tracked, and inspected.
A commercial building has a fire sprinkler system with a booster pump and an antifreeze additive. Which backflow protection is required?
What is the minimum acceptable air gap between a supply outlet and the flood rim of a receiving vessel?
Why must a reduced pressure principle assembly never be installed in a vault or pit that can flood?