Chapter 17

Fertigation in Greenhouse Production

Fertigation Backflow Prevention

Backflow can occur in a system due to cross connection between a water source and an irrigation system. For example, water may be turned off, but the chemical injection unit may continue to work, contaminating the water source. To protect groundwater from chemical contamination, backflow¾whether from back-siphonage or backpressure¾must be prevented. Back-siphonage is the reversal of normal system flow, caused by negative pressure (vacuum or partial vacuum) in the supplying pipe. Back-siphonage occurs due to low pressure in the water source. For example, the mainline source pipe may break at a spot lower than the irrigation system or pressure may be reduced drastically because a supply pump fails. Such situations can be avoided by installing check values, vacuum relief valves or vacuum breaker valves. Backpressure is the reversal of normal system flow due to downstream pressure increasing above supply pressure. Backpressure may occur if a system operates at higher pressures than its water supply, perhaps due to use of booster pumps or interconnection of a water source to other water systems. Such situations can be avoided by installing double check valves or special valves that combine check values with reduced pressure zones inside them (commonly known as reduced pressure principle backflow prevention valves). Most states have laws in place requiring backflow prevention equipment be installed and maintained on irrigation systems in which chemicals are injected for agricultural purposes. Some counties and municipalities have backflow prevention regulations which may be more restrictive than state law. Also, if the system is going to be used to inject pesticides, certain Environmental Protection Agency (EPA) backflow requirements must be met too. Depending on the water source, different backflow devices might be required.

Irrigation Pipeline Check Valve

A functional check valve located in the irrigation supply line between the irrigation pump and the point of injection of chemicals is required. The check valve will prevent water from flowing from a higher elevation or pressure in the irrigation system back into the well or surface water supply. It will also prevent water from being siphoned back to the water source.

Low-Pressure Drains

The low-pressure drain must be installed on the bottom of the horizontal pipe between the irrigation pump and the irrigation pipeline check valve. The drain must, in all instances, be located on the irrigation pipeline before the point of chemical injection.

Vacuum Relief Valve

A vacuum relief valve should be installed on the top of the horizontal pipe between the check valve and the irrigation pump, and opposite to the low pressure drain. The vacuum relief valve needs to have an orifice size typically 3/4-inch in diameter, and must be located upright and above the irrigation pipe so that it functions effectively.

Chemical Injection Line Check Valve

Chemical injection line check valve. A check valve must be installed in the chemical injection line between the chemical injection pump and the chemical injection port on the irrigation pipeline. Its purpose is two-fold: 1) to prevent gravity flow from the chemical supply tank into the irrigation pipeline, and 2) to prevent irrigation system water from flowing into the chemical supply tank causing an overflow.

Containment Structures

State law requires typically require stock tanks to be placed in containment structures if hazardous chemicals such as pesticides are stored. Containment can be achieved by construction of a watertight concrete pad with concrete block walls sufficiently large to hold 1.5 times the capacity of the chemical tank in the event of tank failure.

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