Is city water safe for hydroponically-grown leafy crops?
Growers producing leafy greens in hydroponic systems should be wary about using city water for their operation. City water is a clean water source—it has no pathogens, agrochemicals (e.g. herbicides), and has low concentration of salts. However, city water can contain chlorine (the sanitizer commonly known as bleach). The U.S. Environmental Protection Agency allows up to 4 ppm of chlorine in drinking water to reduce the risk of human waterborne pathogens.While these chlorine levels are safe for humans, we have seen that > 0.5ppm chlorine can cause phytotoxicity in hydroponically-grown lettuce. The symptoms of chlorine toxicity are sometimes confused with Pythium root rot(uneven and stunted growth, root necrosis, and foliar wilting and bleaching). Drinking water treatment facilities do not always apply this much chlorine, they do in response to measured risks. Therefore, chlorine levels in drinking water can fluctuate without notice.
Some options to prevent the risk of chlorine toxicity are:
1. Put carbon filter at the point of water entry and inject 2.5 ppm sodium thiosulfate. Sodium thiosulfate is a fertilizer salt that neutralizes chlorine (least expensive option).
2. Put a reverse osmosis systems at the point of entry.
Contact an engineer and check your options!