FAQ: Chlorine, Tap Water, and Tribus®

By Michael Key | 11 Nov 2019

One question we get all the time at Impello is: “Can I use Tribus with tap water?” Most growers ask this question because they’re concerned about the concentration of chlorine in municipal water sources.

The short answer is this: It’s fine to use tap water when mixing up nutrients and Tribus.

Here’s why.

 Bacillus species are spore-forming gram-positive bacteria, which differentiates them from gram-negative bacteria like E. coli that commonly contaminate water sources. Gram-positive bacteria like Bacillus are notoriously difficult to kill compared to most gram-negative bacteria primarily because of their ability to survive as super tough spores (technically “endospore” is the correct term, but we say “spore” for short).

 Tribus is a blend of three Bacillus species that are all in endospore form in the bottle. These spores are tough to kill and very resistant to disinfectants like chlorine.

 While most drinking water contains about 0.2-0.5 mg/L of chlorine (1) at the tap–and never more than 4mg/L (2)–most Bacillus spores require well over 20mg/L to be effectively killed. Bacillus subtilis, one of the species in Tribus Original, requires 160mg/L of chlorine to be effectively killed (3).

 In other words, it takes almost 1,000 times more chlorine to kill B. subtilis than what’s in most tap water.

 Knowing that, it’s clear that a little bit of chlorine in tap water won’t harm Tribus straight out of the bottle.

 Things do get a little more complicated when the bacteria in Tribus emerge from their dormant spore form and begin actively growing. Despite their hardiness as spores, when the environmental conditions are right—like when they’re added to the root zone of a plant—Bacillus spores will germinate and grow rapidly. And like all bacteria, the actively growing vegetative cells are far less resilient to stress and disinfectants like chlorine than when they’re in spore form.

 Fortunately, the chlorine in tap water becomes less of an issue by the time it’s made its way into the rhizosphere. Chemical and biological reactions are constantly occurring, and in fact, recent research shows the rhizosphere has naturally elevated levels of chlorination compared to the surrounding bulk soils (4). It’s also important to remember that plants uptake chlorine too: the minimum requirement for growth is 0.2-0.4 mg/g (dry mass), which helps support photosynthesis and gas exchange in the plant (5).

 On top of that, the concentration of free chlorine in tap water generally decreases by about 60% over a 24-hour period after it’s left the faucet (6), so nutrient reservoirs that stay filled for more than a few hours lose a lot of ‘killing’ potential even before the Bacillus species in Tribus emerge from their dormant spore form.

 So, for the vast majority of growers using Tribus, there’s not much to worry about when it comes to using tap water. For growers adding chlorine and other sterilizers or disinfectants to their irrigation systems, keep in mind the concentration and degradation rate of these chemicals and consider when and where they are being added to the irrigation system in respect to when and where Tribus (or other bacterial products) are being added.

 If you ever have any questions about product compatibility or other questions about Tribus, please reach out to us over social media or email and we’ll get your question answered!



(1) WHO. Guidelines for Drinking Water Quality, 3rd edition. WHO, Geneva (2004)

(2) ACC. Chlorine Chemistry. American Chemistry Council (2019)

(3) Tonney et al. The Minimal ‘Chlorine Death Points’ of Bacteria. American Journal of Public Health 20(5), 503-508 (1930)

(4) Montelius, M. et al. Radiotracer evidence that the rhizosphere is a hot-spot for chlorination of soil organic matter. Plant Soil 443(1-2), 245-257 (2019)

(5) Marschner P. Marschner’s mineral nutrition of higher plants. Academic Press (2012)

(6) Sheiki R. et al. Decay of Free Residual Chlorine in Drinking Water at the Point of Use. IJPH 43(4): 535-536 (2014)

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