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University of Florida | Journal of Undergraduate Research | Volume 15, Issue 3 | Summer 2014 1 Anaerobic Digester Effluent as Fertilizer for Hydroponically Grown Tomatoes Jacquelyn Neal1 and Dr. Ann C. Wilkie2 1College of Engineering, University of Florida 2College of Agriculture and Life Sciences University of Florida Anaerobic digestion of tomato culls produces renewable energy (biogas) and a nutrient rich effluent. Using the effluent from an anaerobic digester to grow tomato plants could offset the cost of synthetic fertilizer Effluent from an anaerobic digester fed organic waste was analyzed for major plant nutrients and used as a nutrient medium to grow tomatoes hydroponically. Tomatoes grown us ing anaerobic digester effluent had a lower performance than those grown with traditional fertilizer. The predominance of nitrogen in the ammonium form, to which tomatoes are sensitive, explains the observed difference in growth. Means of improving performance of tomatoes grown in effluent are discussed. INTRODUCTION As the world population approaches nine billion, food producers will be faced with increasing food production without an increase in field space and with decreasing soil quality. In order to provide enough food for a growing population, synthetic fertilizers are used to provide essential nutrients for maximizing crop yields. Nitrogen is one of the key limiting nutrients for plant growth, which is commonly applied as a synthetic fertilizer. Atmospheric nitrogen is unusable for most plants. Nitrogenfixing bacteria maintain a symbiotic relationship with certain legumes and lightning st rikes can produce ammonia from atmospheric nitrogen, but the primary man made process for producing ammonia is the Haber Bosch process (Smil 2001). This industrial process requires a high input of energy, which presently comes from fossil fuels. An altern ative fertilizer, such as biofertilizer made from the effluent of an anaerobic digester, could potentially reduce the need for synthetic nitrogenous fertilizers and reduce the energy used in the production process. Biofertilizer would also create a complet e system within the anaerobic digestion cycle, which would create a use for the effluent in the anaerobic digestion process. Finding a use for the biofertilizer produced from anaerobic digester effluent would be beneficial to both creating a sustainable cy cle for the whole anaerobic digestion process as well as creating an alternative fertilizer that does not rely on the use of fossil fuels for its production. The nutrients within the effluent are dependent upon the substances the digester is fed (Ulusoy 2009). Tomatoes need a substantial amount of nitrogen in order to yield a tomato crop (Gould 1983). A large amount of phosphorus is also needed for tomato growth. An outdoor crop of tomatoes requires 100150 kg/ha nitrogen (N) and 2040 kg/ha of phosphorus (P), while a greenhouse crop of tomatoes requires 200600 kg/ha N and 100200 kg/ha P (Huevelink 2005). Objective The objective of this experiment is to compare the yield of tomatoes grown hydroponically with anaerobic digester effluent with that of tomato es grown using a traditional hydroponic nutrient source. Hypothesis Tomatoes grown hydroponically with anaerobic digestion will produce a comparable yield to tomatoes grown with a traditional hydroponic medium. Anaerobic digester effluent will contain the nutrient levels needed for plants to grow. Methods Anaerobic Digester Effluent. The anaerobic digester effluent (ADE) used in this experiment was obtained from a digester being fed kitchen wastes located at the BioEnergy and Sustainable Technology (BEST) L aboratory at the University of Florida, Gainesville, FL. The ADE was analyzed for total solids (TS), volatile solids (VS), pH, total nitrogen, ammonia nitrogen, and nitrate nitrogen concentrations. Nitrogen was considered the growth limiting nutrient, and therefore, applications of the media were based off the measured nitrogen content. Ammonia concentration of the ADE was measured using an Orion Research 701A meter and Orion ammonia selective probe model 9512. After calibrating the probe using three diff erent known concentrations and establishing a standard curve, samples were then measured for ammonia content according to standard methods (APHA 2005). This process was performed throughout the course of the experiment to characterize the effluent, as well as determine the amount of plant available nitrogen in the reservoirs of the hydroponic system due to the presence of ammonia.
JACQUE LYN NEAL AND DR. ANN C. WILKIE University of Florida | Journal of Undergraduate Research | Volume 15, Issue 3 | Summer 2014 2 Nitrate was measured according to standard methods (APHA 2005) using a nitrate ion selecetive electrode (WQ NO3 Nitrate ISE Sen sor, Nexsens Technology). A nitrate interference suppressor solution was used when measuring anaerobic digester effluent samples. The nitrate probe was calibrated with at least three solutions with known nitrate concentrations of the appropriate range. Ex perimental Setup. To investigate the suitability of ADE in hydroponic tomato production, a hydroponic system was designed to run two nutrient streams: ADE and a synthetic hydroponic medium. The system consisted of eight 5 gallon reservoir units, each cont aining an aeration stone and a reservoir lid, which functioned as a support and root anchor for the tomato plant. The reservoirs were constantly aerated using an air pump; tomato roots were submerged in the reservoir medium. The experimental design consist ed of two treatments replicated four times: Anaerobic digester effluent and MaxiGro 10 5 14 (General Hydroponics, Inc. Sebastopol, CA) as the comparative control treatment. The two treatments were applied at a rate of 60 ppm total nitrogen for the first tw o treatments (Hochmuth and Hochmuth 1990). The effluent was diluted to 60 ppm based on the total nitrogen concentration of ADE. The amount of MaxiGro used was based off the 10% total nitrogen known to be present in the nutrient source. The third application was formulated at a rate of 100 ppm N because fruits began appearing on some of the plants and nitrogen demand was assumed to be higher. Tomato seedlings, Solanum lycopersicum cv patio were chosen for this experiment because they are a short season, d eterminate breed. Seedlings were rinsed with water to remove the soil from their roots before being placed in the bucket lids of the reservoir. The entire hydroponic system was then set up in an outdoor, full sun area at the BEST Lab. Through the duration of the experiment, the average mean daily temperature for Gainesville, Florida was approximately 63 F The height of each plant was measured every 3 4 days throughout the experiment. Nitrification T est. To test whether nitrification occurred through aerati on of the ADE, samples of ADE were aerated over a 72hour period. Nitrate levels were measured before and after aeration. These were then compared. RESULTS AND DISCUSSION Plant Growth Plant growth was assessed by measuring the height of the plant every 34 days. All plants started to grow five days after the beginning of the experiment. However, between 10 and 15 days three of the plants treated with ADE stopped growing and began showing a decrease in the health and vigor. Only one of four tomato plants s urvived within the ADE, whereas all four of the control plants survived (Figure 1). Interestingly, the one plant receiving ADE that survived thrived and had growth similar to the average growth of the control group (Figure 2). Figure 1 Percent survival of tomato plants to fruition in anaerobic digester effluent (ADE) and hydroponic medium (control) Figure 2. Height of surviving tomato plants growing in anaerobic digester effluent (ADE, n=1), and hydroponic nutrient medium (Control, n=4) 0 25 50 75 100 Control ADE Control ADE 15 20 25 30 35 400 4 8 11 15 21 24 28 32Plant Height (cm) Elapsed Time (days) Control ADE
ANAEROBIC DIGESTER EFFLUENT AS FERTILIZER FOR HYDROPONICALLY GROWN TOMATOES University of Florida | Journal of Undergradua te Research | Volume 15, Issue 3 | Summer 2014 3 Characteristic Of The Cultural Medium. The initial conditions of the two treatments are quite different. The pH of the MaxiGro (control) is close to neutral and just slightly acidic initially, while the pH of the effluent treatment is more basic and greater than 8. The ammonia level of the effluent is almost triple that of the control. The nitrate concentration is essentially non existent for the effluent, while the majority of the nitrogen present in the control is due to nitrate in the medium. The pH levels for both treatments increased over the time period of 19 days, while ammonia concentrations for both decreased to below 1ppm N. The reduction of the ammonia concentrations could be due to the volatilization of the ammonia to the atmosphere or nitrification within the reservoir. At the end of the period, it is notable that the concentration of nitrate for the control decreased from 52.25 ppm to 1.07 ppm, while the concentration of nitrate in the effluent increased from less than 1 ppm to 12.42 ppm (See Table 1). The relative increase of the nitrate concentration in the ADE during the experiment suggests that nitrification could occur by simple aeration, but the nitrification rate may not sufficie nt to supply enough plant nutrients and reduced the possible toxic effect of the high ammonia concentration. It is clear that the ADE contained enough nitrogen content to meet the tomato needs. However, as anticipated, tomatoes plants were not able to effi ciently use the ammonium. Table 1. pH, NH3 and nitrate concentration by treatment Initial Final pH NH3 N (ppm) NO3 N (ppm) pH NH3 N (ppm) NO3 N (ppm) Control 6.950.28 11.950.24 52.250.35 7.250.19 0.410.10 1.070.35 Effluent 8.130.12 31.602.45 0.770.14 8.310.10 0.190.01 12.427.86 Nitrification. Nitrification is a sequential conversion of ammonium ions to nitrites and then nitrates by aerobic bacteria (Figure). For plants that have a preference for nitrate over ammonium like tomatoes, nitrification must occur for a better fertilizer assimilation. Nitrosomonas and Nitrobacter are the two examples of bacteria genera responsible for nitrification of ammonia (NH3) within the environment (Figure 3). As they are always present in most well aerated soil, nitrification of ADE is not an issue when ADE is ap plied to the soil. However, in hydroponic production, the nitrification rate may be limited by a lack of nitrifying bacteria, which are not present in the effluent of a digester. A. 2(NH3) + 3(O22 -) + 2 (H2O) + 2 (H+) (Nitrosomonas sp.) B. 2(NO2 -) + O2 3 -) (Nitrobacter sp.) Figure 3 Stoichiometric steps in nitrification commonly driven by species of Nitrosomonas and Nitrobacter To assess whether or not fertilization did occur by simple aeration within the hydroponic reservoirs, a nitrificatio n test was setup in the lab. In a first nitrification experiment, the effluent was aerated continuously for a 48hour period. Over this time period, the NO3concentration increased from 9.13 ppm to 17.25 ppm, indicating nitrification can occur due to aer ation. Another nitrification experiment was conducted with effluent from the food waste digester over a 72 hour period. The results from this are shown in Table 2. During this 72hour period there was a drastic decrease in the NH3 concentration Ammonia dropped from 102.02 3.21 ppm to 22.68 4.69 ppm. Since there was no corresponding increase in the NO3concentration during the time period, this decrease is most likely due to volatilization of the ammonia because of the constant aeration. The concentration of nitrate actually decreased slight ly during this time period. After an additional 24 hours of aeration (total of 96 hours), the nitrate concentration in the effluent rose from 0.64 0.05 to 2.64 0.37 ppm. After another 24 hours (120 hours total) of aeration, the concentration of nitrate in the effluent continued to rise, up to 3.31 0.40 ppm. Clearly, nitrification by aeration is a process that needs time to complete before the nitrogen is available as nitrate for the tomato plants to use. Other methods of nitrifying the ammonia present in an aerobic digestion need to be tested in the future, especially methods that minimize ammonia volatilization. Nitrifying bacteria, for example, could convert ammonia to nitrate.
JACQUE LYN NEAL AND DR. ANN C. WILKIE University of Florida | Journal of Undergraduate Research | Volume 15, Issue 3 | Summer 2014 4 Table 2 Nitrification o f Anaerobic Digester Effluent ADE (Initial) ADE (Final) NO3 (ppm) NH3 (ppm) NO3 (ppm) NH3 (ppm) Control 0.91 102.02 0.64 22.68 Effluent 0.12 3.21 0.05 4.59 DISCUSSION Anaerobic digester effluent contains plant nutrients that can be used to grow tomatoes hydroponically. However, the nitrogen is in ammonium form, which may be toxic to tomato. As it is well understood, tomatoes use preferentially nitrates. The result of th is experiment showed that ADE was not as effective as a synthetic fertilizer in increasing the yield of tomatoes. As a result, to use the ADE as a nitrogen fertilizer source, the ADE has to be nitrified prior utilization. Not only the nitrification process converts nitrogen in a preferential form for tomatoes but reduces the lost by volatilization. There are several possibilities as to why the effluent plants did not grow as tall or look as healthy as the controls. The effluent plants never developed as extensive a root system as the controls. This can be seen in Figure 3, where the control plant has long, extensive, healthy roots. The effluent plant has far less root matter and they are not as long as the roots of the control plant. Also, from the beginning, there was a major difference between the pH of the control and effluent reservoirs. The control medium was slightly acidic, which allows plants to use the nutrients present in the medium. In contrast, the pH of the effluent reservoirs was slightly bas ic at a pH greater than 8. At this pH, many minerals are unavailable to the plant. Therefore, the pH of the effluent must be lowered so the plants can use the nutrients present. The effluent plants may also not have grown due to ammonia toxicity, as tomatoes prefer nitrate and are sensitive to ammonia.The nitrification experiments showed nitrification by aeration is a slower process, so simply aerating effluent without inoculation of nitrifying bacteria may not have been enough to convert ammonia to nitrate Perhaps the effluent needs to be aerated for several days prior to use as a hydroponic growth medium. Other methods of nitrification beyond aeration will be needed for effluent to be used as a hydroponic media for tomatoes. It is clear tomatoes can grow in effluent, as tomatoes have appeared on some of the effluent plants, but the leaves, stems, and roots of the plants did not thrive like the control plants did. There also could have been nutrient deficiencies in the effluent keeping the plants from gro wing. More comprehensive study of the nutrients available in effluent is needed if it is to be used as a fertilizer for hydroponics. Liedl et al. (2004) faced a similar problem with the growth of tomatoes while using anaerobic digestate from poultry wa ste. After characterizing the effluent, the researchers concluded ammonia toxicity was most likely the cause of the poor growth of the effluent plants. After heating and sparging the effluent, 25% of the original ammonia had been removed. The researchers a dded calcium nitrate to increase the nitrogen levels to match that of the commercially grown tomato controls. This seems to defeat the purpose of the effluent as an organic fertilizer, as it involves adding chemicals to the effluent in order for it to be used. It also seems to be small return 25% ammonia removed, but sparging and heating the effluent requires energy inputs. Nitrification of the ADE using naturally occurring, or inoculated nitrifying bacteria may be a more viable alternative. CONCLUSION Hyd roponically grown tomatoes can be produced using anaerobic digester effluent as the growth media, but the plant growth is limited. The most likely reason for this lack of growth was due to the high ammoniacal nitrogen content of the effluent and tomato sensitivity to that ammonia. The low nitrification rate by simple aeration was the limiting factor of the tomato growth in the ADE. Therefore ADE must be nitrified prior to utilization to avoid toxicity, especially to ammonia sensitive plants under hydroponic cultivation. Though there are challenges, the use of effluent as a fertilizer for hydroponic cultivation is a low hanging fruit. FURTHER RESEARCH The major hurdles for using anaerobic digester effluent as a hydroponic growth media are the variability between batches of effluent and the ammoniacal nitrogen content. Because the nutrients available in effluent differ based on the inputs, the ammonia concentration must be measured for each batch. Once this is completed, the ammonia in the effluent must be nitrified, and the concentration of nitrate must be monitored as well. Further research should focus on the nitrification process by using large scale aeration over longer time periods or nitrifying bacteria. Additionally, characterizing other nutrients in effluent, such as phosphorus and potassium, could lead to a better
ANAEROBIC DIGESTER EFFLUENT AS FERTILIZER FOR HYDROPONICALLY GROWN TOMATOES University of Florida | Journal of Undergradua te Research | Volume 15, Issue 3 | Summer 2014 5 understanding of the effluent and its potential as a fertilizer. REFERENCES American Public Health Association., American Water Works Association., & Water Environment Federation. (1999). Standard methods for the examination of water and wastewater S.l.: American Public Health Association. Gould, W.A. (1983) Tomato production, processing, and quality evaluation Westport, C T : AVI Pub. Co. Hochmuth, G.J., Hochmuth, R.C. (1990). Nutrient Solution Formulation for Hydroponic (Perlite, Rockwool, NFT) Tomatoes in Florida (HS796). Gainesville: University of Florida Institute of Food and Agricu ltural Sciences. Retrieved October 10, 2012, from http://edis.ifas.ufl.edu/ cv216 Huevelink, E. (2005). Tomatoes Cambridge, MA. : CABI Publishing Liedl, B.E., J. Bombadiere, and J.M. Chatfield (2006) Fertilizer potential of liquid and solid effluent from thermophilic anaerobic digestion of poultry waste. Water Science and Technology 53 6979. Liedl, B. E., Cummins, M., Young, A., Williams, M. L., & Chatfield, J. M. (2004). Liquid effluent from poultry waste bioremediation as a potential nutrient source fo r hydroponic tomato production. Acta Horticulturae 659, 647-652. Smil, V. (2001). Enriching the earth: Fritz Haber, Carl Bosch, and the transformation of world food production. Cambridge, M A : MIT Press. Ulusoy Y., A.H. Ulukardesle r, H. Unal, and K. Alibas (2009) Analysis of biogas production in Turkey utilizing three different materials and two scenarios. African Journal of Agricultural Research. 4 996 1003.