Gray Water Recycling Directly Through Hydroponic Systems

Task Question/Justification:

Can water be recycled in an ALS system through direct incorporation of gray water into the hydroponic system and using plant uptake / transpiration as a method of producing clean water?

Abstract:

Recycling water used for personal hygiene will be necessary in a closed, space-based system. Incorporation of human hygiene water (gray water) into hydroponic plant production systems, and subsequent recovery of the water transpired by the plants, is one potential means for water purification and recycling. The use of plants, and the active microbial community associated with their roots, for water processing would eliminate the need for physical-chemical treatment or a bioreactor and the concomitant resupply of physical components, (i.e., filters, etc.). Volunteers showered and washed clothes with predetermined amounts of soap (igepon) and deionized water to provide gray water with an approximate soap concentration of 1000 ppm. Gray water was added daily (1.5 L) to 20 L hydroponic tanks supporting wheat and soybeans (separate experiments). Tanks contained a) no gray water (control), b) soapy water additions (igepon and water), c) filtered gray water (0.2 µm filter to remove bacteria and skin cells), (soybean study only) and d) unprocessed (raw) gray water. Both the hydroponic solutions and the rhizosphere were sampled for microbial composition to determine the survival of human-associated bacteria. The rate of soap degradation was assessed throughout the plant growth cycle. Plant material was harvested and fresh and dry mass were determined. Preliminary findings demonstrated that soap degradation rates increased as plant root mass, and associated rhizosphere microbial activity, increased. Sodium levels increased in all gray water treatments over time.

Why recycle gray water through hydroponic systems?

• Approximately 40 m² growing area is needed to support human food and oxygen requirements, yet this produces about 4 times the water requirement (based on 25 L/person/day).
• If an investment in plant hydroponic systems is to be made for even a 25% food production for ALS, total water required could potentially be recycled through the system with no additional costs of water purification equipment and the resulting space, energy and materials.

Experimental Objectives:

Evaluate the effects of directly recycling human hygiene water through recirculating hydroponic systems on

• growth and development of ALS candidate crops,
• soap degradation,
• the microbial population in the rhizosphere (root zone),
• sodium increase in both the hydroponic system and plant tissue,

Experiments to date
(these are preliminary experiments and analyses are currently being completed):

• wheat (Triticum aestivum L. cv. Apogee) and potato (Solanum tuberosum L. cv. Norland): 3-11-97 to 6-3-97, 84 days
  (but due to mix cropping problems with potato, only wheat data is shown)
• soybean (Glycine max L. Merr. cv. Hoyt) 6-27-97 to 9-25-97, 90 days

Wheat and Potato (day 57) - at left, front two trays, control, back two trays, synthetic gray water		Wheat and Potato (day 57) - at left, raw gray water (back two trays infected with Pythium fungus)
Soybean experiment (day 33) - at left, front two trays, control, back two trays, synthetic gray water		Soybean experiment (day 33) - back two trays, filtered gray water, front two trays, raw gray water

Treatments:

• Control - no gray water additions
• Synthetic gray water (Synthetic GW)- soapy water (no human contact)
• Raw (unprocessed) gray water (Raw GW) - unfiltered gray water
• Filtered gray water (Filtered GW) - raw gray water filtered through 0.2 µm filter to remove human-associated bacteria and skin cells (theoretically this treatment should be identical to synthetic gray water) (this treatment only used in soybean experiment)

What soap was used?

• whole body shampoo and hand cleaner - used for both shower and laundry soap
• design specially for the International Space Station
• from Ecolabs^© (Steve Lentsch, 840 Highway 13, St. Paul, MN 55118, (612) 451-5669)
• since 98.7% of product is composed of Igepon TC-42 (sodium N-coconut acid-N-methyl taurate or sodium methyl cocoyl taurate), the term "igepon’ is commonly used to refer to the soap. It is composed of five fatty acids (lauric, myristic, palmitic, stearic, oleic)

Gray water production: laundry (wheat only) and shower water gray water stored at 4 C for up to 2 days (soybean) or 7 days (wheat) both experiments used daily additions of 1.5 L gray water to a 20 L hydroponic system supporting 0.5 m2 growing area experiment based on system in which plant growth provides 25% of food requirement (10m2 /person), but receives 100% of gray water (25 L /person /day)

Plant Growth:

Wheat:

• although all three treatments had similar head dry mass, number of heads was lower in the raw gray water, indicating larger head size
• seed dry mass appeared to be greater in the gray water treatments

Soybean:

• due to zinc toxicity problem with the control, a soybean control from a previous experiment in the chamber is shown
• seed and vegetative dry mass data are shown: results appear slightly less than previous experiment control
• this may not be a direct effect of the gray water, but an indirect effect caused by pH decline in gray water treatments and resulting nutrient imbalances

Sodium Accumulation:

• all gray water treatments sustained an increase in sodium
• igepon is a sodium-based soap, as are almost all soaps today
• sodium is not a usual plant element and can be toxic to plant growth
• as some plants may not uptake sodium, continual addition of igepon to the system could allow sodium to reach toxic levels within a year (detrimental levels at ~460 mg L^-1 for soybean and potato, ~1400 mg L^-1 for wheat)
• elemental analysis on soybean tissue shows some incorporation into roots, but minimal into any other section

Soap Degradation:

• The graph represents three time course studies depicting that the degradation rate of Igepon in raw gray water increases throughout the growth cycle of soybean. The same trend is found in synthetic and filtered gray water.
• DAP 10, the first day when soap was added to the system, soap concentrations remain high 12 hours after addition of gray water.
• DAP 17, the soap is completely degraded in all three treatments after 12 hours.
• DAP 73, complete degradation is observed by 6 hours after the addition of gray water.
• Although the initial concentration added to the tanks was essentially the same throughout the experiment, the decreasing concentration at time zeros as plants grow is due to the soaps immediate adherence to the large root mats.
• Soap analysis makes use of the cationic dye methylene blue, which binds the anionic surfactant Igepon TC-42, then is extracted into chloroform and intensity of blue color measured.

Microbial Responses:

Soap degraders

• Organisms capable of using soap as a sole carbon source (i.e., soap degraders) were recovered from all treatments, including the control. These results indicate that the normal rhizosphere microflora has a significant potential for soap degradation.
• Numbers of soap degraders increased more rapidly in the unfiltered gray water treatment, suggesting an inoculum effect. However, characterization of the isolates showed that the raw gray water treatment did not contain any unique soap degrading organisms (i.e., of potential human origin).
• After 83 days, numbers of soap degraders were more than a log higher in the synthetic and gray water treatments than in the control.

Total and respiring cell numbers

• Total and respiring cell numbers were greater in both the synthetic and gray water treatments than in the control. This suggests that soap is a more important source of carbon than the human-associated component of gray water (i.e., fatty acids, skin cells, etc.).

Human-associated bacteria

• Current analysis involves evaluation of results from selective media and molecular studies. Preliminary results suggest that human-associated organisms do not survive in the hydroponic system.

Conclusions

These are two preliminary gray water studies that have not yet had a chance to be replicated. From these studies, new experiments will be designed to better answer the questions that arose from gray water additions.

• Inclusion of gray water seems to have minor effects on plant growth, although indirect effects on pH led to additional problems in the soybean plant system.
• Soap degradation rate increases with time. This is probably due to increases in root mass and microbial density.
• Soap degraders were recovered from the control as well as the soap treatments, suggesting that normal rhizosphere microflora has a significant potential for soap degradation.
• Sodium levels increased in the hydroponic solution over time from the continual addition of soap:
  • these levels could eventually reach toxic concentrations if the plants do not uptake the sodium
  • tissue analysis shows little incorporation of sodium into soybean tissue
  • the sodium question (whether from soap or urine) is being studied separately.
  • use of a potassium-based soap would prevent the potentially toxic build-up of sodium (potassium is a plant nutrient)
• Need to fully develop molecular methods for monitoring survival of human-associated bacteria
• Although soap appears to be rapidly degraded within the hydroponic system, preprocessing though a bioreactor will be studied as part of solid waste recycling