Overview of Dissolved Oxygen in Greenhouse Operations
by Joel Termaat, PhD
Dissolved oxygen (DO) levels in water is a critical parameter in greenhouse and hydroponic operations. DO is required for plant respiration and thereby influences nutrient uptake. DO is also important for root health, as low oxygen levels favor harmful anaerobic organisms that cause root rot (e.g. Pythium and Fusarium). Sufficient DO levels promote an aerobic environment favoring beneficial microorganisms.
Current methods for increasing DO include hydrogen peroxide, air pump/stones to bubble thru atmospheric gases (21% oxygen), and mixers/stirrers to promote air/water surface contact. While these methods are inexpensive, they also have drawbacks. Air stones are the most highly adopted, but DO is limited by its intake air composition intake (if greenhouse air is the supply, also pulls in detrimental CO2 levels). Other techniques include air diffusers and electrolysis. Experiments have also been performed using pure oxygen.
DO levels are usually monitored by a meter, with recommended levels at about 8 mg/L which represents environmental water oxygen levels at atmospheric saturation. Note that DO saturation decreases with increasing temperature and presence of salts in the nutrient solution. Effective DO levels usually range from 6-8 ppm. Chillers are sometimes used to increase DO levels.
Critical question: Do high DO levels lead to improved plant growth or is atmospheric saturation sufficient with no benefit (law of minimum)? Plant-specific?
Soffer et al. demonstrated that DO is essential to root formation and higher DO quickens root formation (tested only up to 8mg/l). Suyantohadi et al. compared normal DO (7-8 mg/L) with an air stone and high (23 mg/ml, ~210% saturation) DO using pure oxygen enrichment. The high DO produced marked increases in lettuce growth compared to normal aeration in deep hydroponic culture. However Goto et al. found no difference in lettuce growth from 2.1 to 16 (200%) mg/L in a floating hydroponic system.
Increasing DO for cannabis has not been well-studied in the literature, but DO is clearly important in these operations as well. Aeration is a necessary practice especially in deep water culture (DWC) systems. There is a great emphasis on DO for grow operations and pure oxygen is in use (see grower Dan video). No formal statistical trials could be found to date.
Increasing DO levels
Gas saturation in water is given by Henry’s Law.
- c = pg / kH (2)
- c = solubility of dissolved gas
- kH = proportionality constant depending on the nature of the gas and the solvent
- pg = partial pressure of the gas
For oxygen at 25C:
- co = (1 atm) 0.21 / (756.7 atm/(mol/litre)) (31.9988 g/mol) = 0.0089 g/litre = 8.9 g/ml
“100%” DO saturation is for atmospheric pressure and oxygen composition of 21%. Dissolved Nitrogen in water is higher 13.8 g/ml than DO due to air composition (79%) and its Henry constant.
For example, Henry’s law suggests that:
- At 21% oxygen = 8.9 mg/L (i.e. 100%)
- At 90% oxygen = 38 mg/L (this is 427% saturation!)
Henry’s Law suggests that much greater DO levels can be achieved by using a higher % oxygen and/or increased pressure. Increasing pressure lacks practicality but higher oxygen % is easily implemented with an existing HVO system. An additional advantage is that air stones pulling in greenhouse air have a higher content of CO2 which is detrimental (outside air is recommended with air stones.
It is assumed that hydroponic DWC solution containers are fairly well-covered. Thus bubbling pure oxygen will lead to a headspace primarily of oxygen as diffusion with external air would be slow. It can be expected that higher DO levels can be achieved.
Thoughts to Date
If >100% saturation proves beneficial, it appears that minimal, if any, modifications would be needed for the existing HVO systems. As aeration is typically periodic, only a timer valve would be recommended which can be added separately with integrated greenhouse control systems. Tubing/air stones would also be separately provided. The only consideration is whether HVO systems are “overkill” in terms of cost, pressure and volume (i.e. is an oxygen concentrator ~at 1-5 lpm at 5psi) sufficient. This is likely not the case for large grow operations that would benefit from a single system with distributed tubing and head pressure requirements to support DO in an entire greenhouse.
List of Links as of 11/22/16
General DO Greenhouse Info
Discussion on DO in hydroponics with good info on DO levels and methods: http://www.just4growers.com/stream/hydroponic-growing-techniques/airing-out-the-truth-on-dissolved-oxygen-in-hydroponics.aspx
DO saturation general info: http://www.fondriest.com/environmental-measurements/parameters/water-quality/dissolved-oxygen/#7
DO hydroponics chiller: http://www.grozine.com/2014/01/31/dissolved-oxygen-in-hydroponics/
DO electrolysis system: http://www.o2grow.com/technology
Pure Oxygen Grower Dan 5K views https://www.youtube.com/watch?v=AFQDS3P-Xdg
DO Pythium and general info: http://simplyhydro.com/nutrient_temp.htm
DO wastewater system: http://gps-na.com/products/pro2-021
DO Scientific Articles / Publications
Suyantohadi high DO lettuce publication: http://www.sciencedirect.com/science/article/pii/S1474667015310727
Goto et al. DO lettuce publication abstract: https://www.ncbi.nlm.nih.gov/pubmed/11541573
Soffer DO aero-hydroponics publication: http://www.eurohydro.com/pdf/articles/gb_dissolved_oxygen.pdf
Flannery DO roses publication (small sample size, poor study design): http://lieth.ucdavis.edu/pub/Pub079_FlanneryLieth_Acta766p53.pdf
Henry's Law Links
Henry’s Law basics: http://butane.chem.uiuc.edu/pshapley/GenChem1/L23/web-L23.pdf
Solubility of gases in water CRC data: http://sites.chem.colostate.edu/diverdi/all_courses/CRC%20reference%20data/solubility%20of%20gases%20in%20water.pdf
Henry’s Law gas data and constants: http://publishing.cdlib.org/ucpressebooks/view?docId=kt167nb66r&chunk.id=d3_6_ch06&brand=eschol
Ozonation system for greenhouses: http://www.the-ozone-store.com/applications/greenhouses.htm
Ozone system greenhouse article: http://www.greenhousemag.com/article/gmpro-1210-greenhouse-upgrades-marketplace/
Ozone system greenhouse http://www.climatecontrol.com/index.php/water-filtration
Ozone greenhouse sanitation system: http://ozone-systems.com/Ozone_Applications_files/OzoneGreenhouseSanitation.pdf
Ozone liverwort control abstract: http://hortsci.ashspublications.org/content/47/3/361.full
Ozone cleaning system (Omaha co, UNMC demo day) http://cleancoretech.com/products/
Ozone treatment greenhouse pub abstract: http://hortsci.ashspublications.org/content/39/4/768.4.abstract
Ozone treatment water reuse greenhouses: http://www.lenntech.com/water_reuse_greenhouse_horticulture.htm