Webinar #6 - Chlorination of recycled water in nursery production (March 2014)

By: Chuan Hong

Webinar Recording

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►Hello, everyone, welcome to the webinar! I'm Chuan Hong, a professor of plant pathology at Virginia Tech.

Before we begin the webinar, I would like to ask you to do me THREE favors!
First, answer two survey questions on the computer screen if you haven’t.
Second, MUTE your phone to avoid background noise.
Third, post your questions in the CHAT box. We will only use phone for better quality audio. Thanks.

►Today’s webinar begins with a presentation, followed by a panel discussion. 

►On the panel are Mr. Bill Daughtry, Mr. John Lancaster and their colleagues from Lancaster Farms and Bennett’s Creek Nurseries, both in eastern VA; they have a lot to share in terms of irrigation water treatment. Bill was the MAN who introduced chlorination technology from drinking water systems into nursery production in 1983 (31yrs ago).

►This is the sixth webinar in a series organized by the multi-institutional research team on the Specialty Crop Research Initiative Project: Integrated Management of Zoosporic Pathogens and Irrigation Water Quality for a Sustainable Green Industry. This project is sponsored by the USDA National Institute of Food and Agriculture. We greatly appreciate the Society of American Florists and the AmericanHort, previously known as the American Nursery and Landscape Association, the advisory panel and many farmers including Bill, John for their continuing support. They have played key roles in every stage of this project from proposal development to field studies and educational programs.

A major goal of this SCRI project is to gain a better understanding of the biology of plant pathogens in irrigation water and educate people about the topic. Through these research efforts and education programs, we hope to help farmers BETTER manage these pathogens.

This project has two major educational programs.

► One is an international book project reviewing and summarizing the existing knowledge about plant pathogens in irrigation water, their biology, detection and management.

Here is the book cover with some quick facts. This book has been sent to print with anticipated release in June 2014.

►The other program is this 14-session webinar series. The recordings of previous webinars along with the transcript and related materials have been posted at the project website (www.irrigation-pathogens.info). If you missed any, please visit this site to view the recording. All you have to do is to click this Webinar tab, then click the recording you like to view or review.

►We learned from Dr. Moorman’s first webinar that ALL major groups of plant pathogen have been found in irrigation water. Here is a brief summary of those pathogens.  They include 38 species of Phytophthora, 29 species of Pythium, … and many other groups.

These numbers increase every day. We will not be able to cover all groups in this webinar. We will use Phytophthora species as an example.

►Why Phytophthora?

The major reason for this selection is economic significance. Phytophthora species attack a HUGE number of ornamental plants and other horticultural crops. They could wipe out entire crops within weeks.
The other reason is that Phytophthora species have been regarded as WATER MOLDS since 1944 when Dr. Elizabeth Blackwell published her note in Nature, a very prestigious journal (70 yrs).

As Dr. Moorman pointed out during the last webinar, before treating irrigation water, a farmer MUST make sure that pathogen causing disease problem is REALLY from water, not from soil, potting mix or any other source(s).  

►Back in 2000 and 2001, we did several field trials comparing recycled water with well water. In this study, annual vinca plants irrigated with recycled water developed severe foliage blight within 2.5 months, while the other half irrigated with well water remained healthy until the end of growing season. What caused this difference was a Phytophthora pathogen called P. nicotianae in recycled water but not in well water.

Pathogen populations in irrigation water typically are small or even undetectable with current technology. However, they can still cause severe diseases and crop losses.

►Why pathogens become more destructive once entering an irrigation system?

Phytophthora species produce several types of structures for different purposes:

►So, what type of spores is mainly disseminated through water?

According to the studies performed under different crops and production systems, zoospores accounted for AT LEAST 94% of propagules in irrigation water. Thus, zoospore should be the target of water treatment and pathogen mitigation!

►There are a number of options for mitigating pathogens in irrigation water. Some water treatments are listed on the left column, while a few best practices are highlighted on the right.

Today’s webinar will focus on the chlorination. This water treatment technology was introduced from drinking water system to nursery production WITHOUT adequate research data from agricultural settings. I still have a vivid memory of a conversation with several farmers over a lunch table during my first visit to Lancaster Farms and Bennett’s Creek Nurseries in the summer of 1999.  (15 yrs)

The very first question they asked me was “What chlorine concentration and contact time are required to kill Phytophthoras in water?” This was a very intelligent question. It showed that my farmers knew not only what are the major pathogens and diseases they have to deal with, but also where the major knowledge gap was.
►To answer this question, we did a number of studies in the lab and in the field as well.

In a lab study, we evaluated how chlorine concentration may affect zoospore survival of seven major Phytophthora species with 2 minutes of treatment time.

As you can see in this chart on the left, zoospore mortality increased with increasing FREE chlorine concentration up to 1.0 or 2.0 ppm, depending upon the species.

The bottom line is that NO zoospore of any species survived at 2.0 ppm free chlorine.

In the field studies, we took a total of 40 water samples at sprinklers from six commercial nurseries that used chlorine for water treatment over a 2-year period either monthly or quarterly. Each sample was checked on-site for free chlorine level and water pH using portable meters, followed by lab assays to determine whether it contained any viable spores of Phytophthora species.

These species included P. citrophthora, P. megasperma, P. nicotianae, and P. pini (=P. citricola).


►Based on these data, we recommend farmers to maintain 2.0 ppm free chlorine at sprinklers providing that water pH is between 5.0 and 6.5.

► To understand the importance of water pH here, we need to introduce some terminology.

There are three species of chlorine: all chlorine molecules in water are called total chlorine. The amount of chlorine that have reacted with different organic and inorganics thus tied up is called combined chlorine. The leftover (or those NOT tied up) is called residual chlorine or free chlorine. It is Free chlorine that is capable of killing pathogens. So, free chlorine is of the most interest and importance to farmers.

It is very important to note that FREE chlorine could be in three MAJOR forms in water


►These three forms of free chlorine have very different pathogen-killing power. It was estimated that hypochlorous acid is approximately 20 times as powerful as hypochlorite ion. What is really tricky here is these three forms of chlorine CO-exist in equilibrium; and their dynamics depends largely upon water pH as illustrated in the chart.

That is why it is very important to make sure water pH between 5.0 and 6.5 to get the most out of chlorine dollars.

►This table presents a few scenarios of chlorine performance at different pH levels, assuming that hypochlorous acid is 20 times as efficacious as hypochlorite ion.

From this table you can see how quickly your chlorine dollars could be watered down with increasing pH. So, it is very important to know water pH when using chlorine to treat irrigation water.

You might wonder what is the water pH range in my irrigation pond(s)? Is it good for chlorine performance?

►In a recent study published in Irrigation Science, we monitored water pH in an irrigation pond 24 hours a day and 7 days a week for 3 years and we found THREE THINGS by surprise.

Apparently, this is NOT good a pH range for chlorine performance.

Are these findings exceptions or indeed common in other irrigation ponds and production facilities?

►To answer this NEW question, we made water quality monitoring and study AN important research objectives in this SCRI project. Specifically,

Here is a summary of some data from these expanded monitoring in the form of PERCENTAGE of hourly or quarter hourly water pH readings under EACH of the three categories: acidic (<7.0), slightly basic (7.0-9.0) and highly basic (>9.0).

As you can see, seven of the nine ponds monitored had at least 61% of the pH readings at 7.0 or above.

In fact, water pH in one pond NEVER went below 7.0.

These data confirmed our previous findings and this is NOT good news for chlorine performance.

Bad news is always hard to swallow and digest, but no need to be panic. There are a number of things that farmers can do to improve chlorine performance. Here I will show you THREE of those possible action items.

►The first action item is to irrigate plants in early- to mid-morning instead of evening.

Our studies have consistently shown that water pH fluctuates diurnally with the lowest level in the early to mid-morning. Water pH increases sharply after 10 AM and reaches the peak of the day in the evening. As shown in this chart, diurnal water pH fluctuation extremes could be as great as 3.5 units.

Irrigating crops in early to mid-morning when water pH is at its LOWEST point of the diurnal cycle allows chlorine to do a better pathogen-killing job WITHOUT any extra cost!

►The second action item is to carefully select water source for irrigation.

This is a computer screen shot (or print screen) showing real-time water pH readings in four ponds over a 6-month period from April to October of 2011. These four ponds are color coded with the light blue and orange ponds in a central VA nursery while the dark blue and red ponds in another nursery in eastern VA.

Let’s look at water pH in the light blue and orange ponds. The PERCENTAGE of quarter hourly water pH readings falling below 7.0 was 93% for the light blue pond while only 21% for the orange pond. The difference between these two ponds at this nursery is 72%. This means that the light blue pond is much better a choice of water for irrigation than the orange pond in terms of chlorine performance. In fact, water pH in the light blue pond only occasionally rose to the 8 range and these readings were during the evening time. So, water pH issue in the light blue pond could be EASILY taken care of by shifting irrigation to morning.

►The third action item is to regularly check water pH and acidify water as needed prior to chlorination. This ONLY applies to production facilities where water source selection and irrigation timing is NOT ENOUGH to bring water pH down to the range for the chlorine performance.

Acidifying water will cost some money, but the GOOD NEWS is that water alkalinity or buffering capacity in most irrigation ponds is relatively low, so it will not take much acid to bring down water pH to the range for the best chlorine performance. I will discuss water acidification in details in a later webinar of this series scheduled for the month of October.

In addition to water quality, a number of other factors could affect the translation of chlorine dollars into more and better quality horticultural products and more profit margins. I will use next few slides to show you some important factors affecting the chlorination economics.

►The first factor is the choice of chlorine product.

Among the three types of chlorine product, chlorine gas is the most cost-effective choice. Theoretically, it costs less than 3 cents for treating 1000 gallons of water. Comparatively, it costs 4 cents with bleach and 9 cents with tablets.

Following we will use chlorine gas as an example to discuss chlorination system setup and other aspects that also affect the economics of chlorination.

►The second factor is way of chlorine injection.

The top photo shows chlorine gas tanks and a regulator. In the bottom of this slide, the photos show three different ways of setup for chlorine injection.

►The third factor is chlorine dose control system. There are two major choices from which farmers can choose, depending on the flow rate and budget.

►For fixed flow rate, farmers set up their chlorine dose at the gauge on the top of a chorine tank. This is done through trial and error to make sure that free chlorine level at sprinklers is 2.0 ppm.

►For production facilities with variable water flow rates, a smart valve system is required to dose chlorine injection.

On the far left, John was showing where the rotometer is installed in the pump house and the next two photos show what the rotometer looks like. When water is being pumped, it turns the paddle wheel, which generates a 4- to 20-mA signal.

The computer mounted on the wall translates that signal to an amount of water is being pumped and determines how much chlorine gas is needed to treat the flowing water in the system.

On the far right is a chlorine dosage indicator showing how much chlorine is being injected into the system within a 24-h period.

►The fourth factor is to check whether your chlorination system is working properly by taking water samples and measuring the free chlorine level.

Here is one chlorine test kit and two meters that are being used by local nurseries in Virginia. Colorimetric titration kit is one of the simplest and least expensive options to measure chlorine. Two meters are more sophisticated and cost most as well (around $200 to $600 each). These units use the same principles, and provide digital and more accurate chlorine readings.

When checking chlorine concentration, water sample MUST be taken at SPRINKLERS.

►Like all other water treatment technologies, chlorination has its own share of potential problems. Two most common concerns are phytotoxicity, and health/environmental hazards if not used properly.

►Here are a few take-home points

► The take-home points (cont’d)

►With that, I would like to thank you for your attention! Now, it is TIME for a panel discussion. You can post your questions in the Chat box or reactivate your phone and ask by phone.

If there is no more quick question, I would like to thank the panel members for their times and expertise.

I also like to thank you ALL again for your participation and questions. If you have further questions, please drop me a note by email at chhong2@vt.edu.

►Our next webinar will discuss alternative water treatment technology by Dr. Warren Copes. Warren is an expert in chlorine dioxide and other disinfestants. We look forward to your attendance on April 8.

Thanks and bye.