The Diaper Debate Super Absorbent Polymers: To Harvest or NOT to Harvest, Diaper or Bulk, Urine Soaked or Not...

The Diaper Debate
Super Absorbent Polymers

We’ve all seen the Garden Hack where a disposable diaper is used for its absorbency crystals, to absorb and preserve moisture in potting containers. Sometimes called Petroleum (or Polymer) Absorbency Material  (PAM) or Super Absorbent Polymer (SAP), we are talking about the crystals that swell to 300 times or more their weight in liquids. The way these crystals are supposed to work, the gel absorbs moisture in the good times and transfers water to the plant when the soil dries out. SAP is also used in disposable diapers to keep moisture under the cover and away from baby, hence the garden hack - to harvest SAP. Recycling diapers is an interesting notion, but I have several questions that need to be answered before I'll be diaper mining.  Does it work that way it's supposed to work, is it cost effective to harvest SAP from disposable diapers, and if the diaper has been used, is it safe to use in plant soil?

Just how much SAP is in the average disposable butt cover anyway? It turns out that the answer to the question regarding the SAP content in disposable diapers is variable - at least two or three fold - too much for the landfill and not enough or too much for most soil mixtures. A diaper holds 10 to 15 grams of powdered SAP, enough to absorb 106 - 159 oz of liquid, so I can see where it would matter what the gel absorbed.

How much does that diaper cost though? The most absorbent diapers cost between 25 and 50 cents per diaper. Harvested from a diaper, SAP would cost about $1.25 per ounce on average (if you could actually recover all of the SAP from the diaper). I found several offerings of bulk SAP readily available online for about $10 per pound, or $0.62 per ounce. In contrast, bulk SAP is less than half the cost SAP from diapers.

Should we try to recycle the SAP in disposable diapers from an environmental and practical standpoint?  SAP is petroleum based, and not sufficiently biodegradable for environmentalists. Therefore, if the SAP were somehow harvested, recycled and packaged for use in gardening, it could be a very good thing. Disposable diapers, specifically those made for use in swimming pools, may be washed two to three times and air dried (NOT those that are soiled) before they begin to fall apart. It would be possible to harvest cleaned SAP from swim diapers, although they owe the fact that they do not swell to their lower amount of SAP. It does raise the question though... Can the home gardener clean and harvest SAP from regular disposable diapers for use in soil mixtures? (There are several academic exercises online with instructions for harvesting SAP from clean and dry disposable diapers if that is your goal.)

Yuck factor aside, can SAP harvested from used and wet diapers be used in planting mixes? Apparently, urine may be an acceptable, effective and organic fertilizer. “Urine is chock full of nitrogen, potassium and phosphorus, which are the nutrients plants need to thrive—and the main ingredients in common mineral fertilizers (as reported in Scientific America, July 23, 2010, Mara Grunbaum). An adult on a typical Western diet urinates about 500 liters a year, enough to fill three standard bathtubs. And despite the gross-out potential, urine is practically sterile when it leaves the body. Unlike feces, which can carry bacteria like salmonella and E. coli, urine poses no health risks—astronauts on the International Space Station even drink the stuff—after it was purified.” (see footnotes for citations)

My next question is, how much urine is appropriate for use in a container, like a hanging basket?A study garden shows that “The urine that one person produces can fertilize about one square meter of soil a day? An output of 1.37 liters or feeds approximately 10.67 square feet of garden area per day. Converted to English units, that amounts to 46.33 ounces or 5.79 cups of urine per day. At that application rate, that equates to just over 1/4 cup of urine for a two gallon planter. Since hydrated gel is nearly all liquid, the SAP from one diaper could hold up to 20 times more urine than is needed. Gee Wizz - that's a lot of nitrogen!

Given the variability in the urine production (how would you figure the volume of urine in the used butt cover?) how would you apply the appropriately amount of “fertilizer” for your container? I guess you can come close with calculations like those used for the two gallon planter, or you can reduce urine content by washing the gel. Are you really willing to wash disposable diapers? The main risk would be that the diaper could rupture. Water logged SAP would NOT be good for your plumbing.

A very thorough counterpoint is contained in the analysis, “Miracle, Myth or Marketing: Super-Absorbent Water Crystals” by Linda Chalker-Scott, PhD, who says,

• "Cationic PAM hydrogels should not be used in gardens and landscapes."
• "Unfortunately, on-the-ground conditions can prevent PAM hydrogels from functioning optimally. Fertilizers and other dissolved substances can interfere with hydrogel water-holding capacity. Hot, dry weather conditions can lead to increased degradation and decreased effectiveness of PAM hydrogels. And for every success story, one can find a situation where hydrogels have failed to function."
• "The best news for those of us managing a home garden or landscape is that simple changes in management practices are often superior to using polyacrylamide hydrogels. In several cases, alternative water management strategies had higher success rates than usage of PAM. These strategies were as simple as adding 2 liters of water when planting Pinus patula seedings, or providing wind protection to reduce water stress in muskmelon. More commonly, mulches (especially organic) were rated superior to hydrogels in terms of erosion control, enhancing water infiltration and conservation, plant growth and establishment, and nutrient value."

The data on efficacy is mixed. According to multiple sources, including one published by Global Post, SAP can be so good at absorbing moisture that it can even over dry skin causing contact rashes and dry cracked skin in diapered babies. If that’s the case, can we depending on these crystals to impart moisture to plants? Maybe. I think it would depend on the plant structures, whether they are soft or woody. Other research has shown that SAP can be effective in delaying moisture evaporation and in supplying moisture to plants. Another study showed that results depend on how much SAP is used - more is not better and can cause plant damage as the swelling gel forces out oxygen, filling all voids with water logged gel. “Eventually, regarding many studies done about super absorbent polymers in green space, it can be concluded that using this substance lonely, or with incorporation of other xeriscaping methods, in particular in arid and semi arid green space has many merits." It turns out that there are many factors determining the amount of gel required for the best performance - more than soil volume and urine content (Sivapalan 2006; Zohurianmehr 2010 as cited in the Iran study). 

Finally, returning to the questions posited in the beginning, “To Harvest or NOT to Harvest, Diaper or Bulk, Urine Soaked or Not…” After careful review of the issues, I will not be diaper diving to harvest SAP from unused diapers at twice the cost of SAP purchased in bulk. Nor will I be gathering urine soaked SAP from used diapers. I may not even use SAP purchased in bulk now that I realize how variable the results can be and how many factors need to be considered before measuring out the first teaspoon full of dry, urine free crystals.

I have high hopes for a market response to the very real potential for producing recyclable absorbent materials that are earth friendly.

If (when) you find errors, or have comments, I really appreciate feedback, especially if you can think of an easier way to present the information. Believe me, I did not imagine the issues would be this complicated.

Footnotes

1- Carnegie Mellon university exercise, http://www.cmu.edu/gelfand/k12-teachers/polymers/polymer-and-absorption/super-absorb-powder.html

2- http://www.peggyomara.com/2014/01/16/a-tale-of-two-diapers/

3- 21 Disposable Products Toy Can Reuse, by Paul Michael on 14 March 2012, http://www.wisebread.com/21-disposable-products-you-can-reuse

4- http://www.scientificamerican.com/article/human-urine-is-an-effective-fertilizer/

5- If I assume a depth of about 1/2 foot of urine penetration into the soil to approximate the volume of soils for the purpose of comparison, the volume were talking about is approximately .152 cubic meters or 5.37 cubic feet. Restated, the urine produced by an adult fertilized 5.37 cubic feet of soil. There are 7.48 gallons in one cubic foot, more than I thought. Relative to the soil in a two gallon planter, if 5.79 cups of urine was added to 5.37 cubic feet of soil per day, how much would that be for just two gallons? 5.79 cups is the amount of urine that was added to 5.37 cubic feet of soil per day. By dividing the gallons by the volume, we arrive at a rate of 1.08 cups of urine applied to each cubic foot of soil. A two gallon container holds approximately .267 cubic feet. .267 cubic feet per planter X 1.08 cups per cubic feet in the research = .29 cups of urine per day for a two gallon pot.

6- Linda Chalker-Scott, PhD, Master Gardener, WSU editor, Extension Urban Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University, Puyallup, Washington: published in MasterGardener, Autumn 2007, www.MasterGardenerOnline.com,http://puyallup.wsu.edu/~linda%20chalker-scott/horticultural%20myths_files/Myths/magazine%20pdfs/MythsHydrogels.pdf.

7- http://everydaylife.globalpost.com/gel-balls-diapers-harmful-4431.html, Are the Gel Balls in Diapers Harmful? by Lori A. Selke, Demand Media

8- http://www.mdpi.com/2073-4441/6/7/2056/htm, Experimental Assessment of the Use of a Novel Superabsorbent polymer (SAP) for the Optimization of Water Consumption in Agricultural Irrigation Process, Giuseppe Cannazza , Et.al., Department of Engineering for Innovation, University of Salento, Complesso Ecotekne, Edificio Corpo O, via Monteroni, Lecce 73100, Italy

9- http://www.ijfse.com/index.php/IJFSE/article/download/IJFSE-Vol%202%281%29-2012-4/28, S. Shooshtarian, J. Abedi-Kupai, A. TehraniFar (2012). Evaluation of Application of Superabsorbent Polymers in Green Space of Arid and Semi-Arid Regions with emphasis on Iran, International Journal of Forest, Soil and Erosion, 2 (1):24-36.