Free Chlorine at 2 ppm: Recycled Irrigation Water and Gel

The site had done everything by the book. Tertiary-treated recycled water, 1.8 mg/L free chlorine at the outlet, drip lines flushed monthly, gel dosed at the specified rate into 600 street tree pits. Two seasons later, the gel in the wetted bulb had turned into a soft, cloudy gel that fell apart between the fingers, while the gel 150 mm outside the drip line was still firm. The difference was not the soil. It was the water.

Recycled water is now standard on municipal and commercial landscape projects, and it brings three things that change how a polymer behaves: oxidant residual, a higher salt load, and a biological load that likes a wet, warm, carbon-rich environment. Only the first one is usually discussed, and it is not the one that causes most of the trouble.

What chlorine does to the polymer

Free chlorine oxidises the polymer backbone. At 2 mg/L and 30 minutes of contact, the effect on a single pass is small; the polymer chains break slowly, and the visible result is a loss of elasticity rather than a loss of mass. In a buried application, contact is not a single pass. The wetted bulb around an emitter is flushed every irrigation cycle for months, and the gel at the emitter face sees a cumulative dose that is hundreds of times the single-pass figure.

Measured in the lab, repeated exposure to 2 mg/L free chlorine at pH 7.5 for 30 cycles reduces swelling capacity by 10 to 20 percent. That is survivable. The same exposure at pH 9, where hypochlorite is the dominant species and the soil is calcareous, is harsher. The failure on the site above was not a 20 percent loss of capacity; it was surface degradation to a soft gel that collapsed under the weight of the backfill, which is a structural failure rather than a capacity one.

The salt load matters more

Recycled water typically runs an EC of 1.0 to 2.2 dS/m, against 0.3 to 0.6 for town water. Every milligram of calcium and magnesium that enters the gel crosslinks it a little further, and every milligram of sodium that enters it reduces the osmotic gradient that drives swelling. Double the EC and you should expect a 15 to 25 percent drop in absorption, on top of whatever the chlorine is doing.

This is why two sites with identical product and identical rates can look completely different. The one on recycled water is effectively under-dosed by a fifth, and the one on town water is not. If you are converting a site to recycled water mid-project, the dose should go up, not stay the same.

Practical rules for recycled water

• Pre-swell with clean water where you can. A 1,000-litre tank of town water costs very little and gives you a known starting point.
• If you must pre-swell with recycled water, test free chlorine first with a DPD kit, and let the water stand in an open tank for 24 hours if the reading is above 1 mg/L. Chlorine dissipates; salt does not.
• Dose up 15 to 20 percent against the town-water rate, and check the first 50 pits at 60 days rather than at 12 months.
• Flush drip lines at the start of each season, not only at the end, because biofilm in the line is a bigger emitter problem than the gel is.

The counterintuitive part

The pits that failed worst on that site were the ones with the highest irrigation run time, because they received the most chlorine. The crew had increased run time on the sandy pits, which was correct for the soil and wrong for the polymer. Two variables moved in opposite directions, and the fix was not more gel everywhere, it was shorter, more frequent cycles on the sandy pits and a pre-swelling step with clean water.

There is a second finding from the same site that is worth passing on. The gel outside the wetted bulb, which had never seen chlorinated water, was intact but dry. It had absorbed its last water at planting and had been sitting in soil at 4 percent moisture ever since, because the emitter wetted a bulb that never reached it. The product was fine; the layout was not. When you audit a site, sample both zones before you blame the chemistry.

For any project on recycled water, put three numbers in the file: free chlorine at the outlet, EC, and the dose adjustment applied. Then take a sample from inside and outside the wetted bulb at 90 days and compare texture. The specification data for our potassium polyacrylate water retaining gel includes performance in 1.5 dS/m water so you can plan the adjustment before the first irrigation.