A constructed wetland

Yes. Research with constructed wetlands in various geographic regions shows removal rates of range from 50 to 99% for nitrogen and from 25 to 98% for phosphorus.  These results are based on specific experimental conditions and large-scale field trials.

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Pesticide removal in constructed wetlands is controlled by a variety of factors:

• the physico-chemical characteristics of the compound itself – basically predictors of how the chemical will move in the environment.  Will it stay in water? Will it bind with sediments?
• processes within constructed wetlands themselves that remove pesticides – sedimentation, microbial degradation, other chemical-reaction based degradation processes (e.g. breakdown by light), and plant uptake
• site specific factors – organic matter, pH, clay content, hydraulic retention time, plant presence (yes/no), flow conditions (intermittent vs. permanently flooded)

Some trends exist with regard to removal efficacy in constructed wetlands:

• High removal (84-97% removal efficacy): Organochlorines (e.g. endosulfan and pentachlorophenol), strobilurin/strobin (e.g. azoxystrobin, trifloxystrobin), organophosphates (e.g. chlorpyrifos, dimethoate, glufosinate, and parathion), and pyrethroids (e.g. bifenthrin, cyhalothrin, cypermethrin, esfenvalerate, and permethrin).  These groups have low water solubility.
• Low removal (24 – 50% removal efficacy): triazinone (e.g. metribuzin), aryloxyalkanoic acid groups (e.g. dichlorprop, MCPA, mecoprop), and urea-based pesticides (e.g. diuron, linuron).  Water solubility is variable and does not help to predict remediation efficacy.

Predicting whether constructed wetlands will effectively remediate the pesticides of concern to you is a challenging process – and highly technical.  Please consult with Dr. Sarah White (swhite4@clemson.edu) if you want additional information about this topic.

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Should we choose a coconut shell or coal-based filter? Should it be pre-wetted and acid washed?

In the most recent study at the University of Florida, Clean WateR³ researcher and MS student George Grant found there was no significant difference in in paclobutrazol removal between 8x30 mesh coconut or 8x30 mesh coal-based granular activated carbons. This study occurred over a short period of time with a slower flow rate than in a commercial-scale system. It is possible that activated carbon types vary in their shelf-life, but we do not know at this point.

The acid and pre-wetted treatments both reduce the percent ash content and make for a 'cleaner' carbon. The ash content on activated carbon is basically fine dust that will wash away when you first start running the system. The ash will turn water black, and an initial back flow is recommended when new carbon has been installed until this ash is washed out. It’s unknown if there are additional costs associated with these treatments, but in theory, they produce a high quality activated carbon that is easier to handle or install. 

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You can find an overview at the University of Guelph website.

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Slow sand filters (SSF) are a water treatment method that use sand on which microorganisms grow. These microorganisms are capable of degrading biological and chemical pollutants in runoff water, including Phytophthora and tobacco mosaic virus (TMV).

The disadvantage of SSFs is that slow flow rate of water through the sand is needed to attain treatment. A rule of thumb is one square foot of sand bed can treat about 0.06 gallons (0.2 liters) per minute. A large sand filter is needed to treat large volumes of water.

More information is available in this video.

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We are not aware of any research on that question. We do know that oxidizers such as chlorine are rapidly broken down as they contact peat or other organic components in growing media.  The organic matter would also deactivate a quaternary ammonium product. However, there is a possibility that a chemical with residual activity may have some impact on soil microbes - we just do not have any data.

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We know constructed wetlands remove human pathogens. But there is little definitive information regarding their remediation efficacy for plant pathogens (for example Phytophtora and Pythium). A goal of the Clean WateR³ project is to provide answers to this question.

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Many technologies are available. An effective system generally requires prefiltration, followed by a sanitizing treatment technology. Options include chemical (e.g., chlorine or ozone), physical (e.g., heat or UV), or biological (e.g., constructed wetlands or slow sand filtration) approaches. Published research on efficacy of different treatment technologies is summarized in our Waterborne Solutions tool. You can compare technologies at the University of Guelph website. 

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Select a meter that will measure free and total chlorine.

Decide on the target chlorine dose. To check for research on dose response of chlorine or other sanitizers for a range of pathogens, use our Waterborne Solutions tool under Grower Tools on this website. For example, 2 ppm of free chlorine is a commonly applied dose which can control Pythium and Phytophthora zoospores.

You need a reliable meter with reagents to test for both total and free chlorine - check common brands for water quality monitoring such as Hach, Hanna, or Pulse Instruments. Although free chlorine is the most effective sanitizing form of chlorine, it is also important to measure total chlorine to avoid phytotoxicity (beware of ever applying more than 5 ppm total chlorine or 2 ppm of free chlorine).

Invest in a reliable inline dosage measurement and control system. You should have inline sensors to measure pH and oxidation-reduction potential (ORP), and possibly ppm of chlorine. Most phytotoxicity problems result from faulty equipment or calculations - don't fix one problem (pathogens) while creating another (phytotoxicity).

Check that the dilutor or injector is working correctly. Calculate how much chemical is being applied for a given volume of water. You can also use our ppm to Recipe tool in backpocketgrower.org, under Tools, to help calculate the dose.

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It depends. If duckweed is clogging your pump of filter, then yes - some treatment or pre-filter is needed to keep the floating plants away from your pump intakes. But wholesale treatment of your pond with a herbicide labeled for use over water is not the best solution. As this will not control the duckweed population long term (only changing fertility practices will) - and has potential to introduce herbicide residues to your crops when pond water is used for irrigation later. 

If control is not based on actual pump related problems - but rather to the appearance of your pond (i.e. it looks bad) - think about these plants as free labor - helping to limit algal growth (shading), cool the water, and cycle nitrogen and phosphorus - keeping your pond healthy.

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A bad case of biological clogging

Clogging of your irrigation drippers could be caused by biological, chemical, or sediment issues

Biological: The clogging material is usually slimy organic material. Soaking the drippers in a dilute bleach solution should clean up biological material (test the drippers can handle the corrosive material first).

If the material is biological, determine where the contamination is occurring (water source, or within the irrigation tanks or pipes). Send in water samples from different locations within the irrigation system to a lab for analysis of total aerobic bacteria counts. Counts higher than 10,000 colony forming units per milliliter indicate a high clogging risk from biofilm.

You will probably need to shock the system with a high rate of an oxidizer (such as chlorine, chlorine dioxide, or a peroxyacetic acid product) to clean the lines. Do not let shock solutions contact your crop. To keep the irrigation lines clean, you could then add a low concentration of a water treatment chemical such as chlorine. With any product, follow EPA label instructions and extension recommendations.

Emitter with iron deposits

Chemical: The material may look like powder or be gritty and crystalline. Soak drippers in vinegar (low pH) – if this cleans up the emitters then a chemical problem is likely. Send samples of this soak solution, your irrigation water, and the applied nutrient solution (if you use water-soluble fertilizer) to a testing lab to analyze what specific ions are reacting from your water and/or fertilizer. Knowing the specific ions is needed to take action. For example, you may need to inject acid to reduce precipitation of calcium deposits, or oxidize and filter to remove iron and manganese. A shock with a line cleaner is likely to be needed to unclog lines. Do not let shock solutions contact your crop.

Soaking drippers overnight in different solutions (bleach or vinegar) can help diagnose the problem

Sediment: The material may look and feel gritty. When you soak in warm water, solid particles may drop out. Sediment  indicates the need for more filtration,  with a finer filter pore size than the emitter size. You may need to blast out lines with high pressure, and use a line cleaner to unclog lines. Do not let shock solutions contact your crop.

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Filtration of irrigation water involves removing organic and inorganic particulate matter (debris, sediment, soil particles, algae, etc.) from the water prior to treatment for pathogens. Pre-filtration is important for two reasons. Firstly, larger particulate matter has the potential to clog the irrigation system (e.g. emitters). Secondly, the effectiveness of many pathogen treatments (e.g. all chemical treatments using oxidizers, UV) is decreased significantly in the presence of particulate organic matter. You can learn more at the University of Guelph website.

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