Co-cultivation of microalgae in aquaponic systems
Introduction
Aquaponics, synergistically integrated aquaculture and hydroponics, is considered as a sustainable system for the future urban farming. In an aquaponic system, wastewater generated by fish is converted to high-value vegetable products (Love et al., 2015). Aquaponic systems have many advantages and are targeted solving world facing problems including population surge, soil degradation, water shortage, and food security. Comparing with traditional agriculture that uses 70% of the fresh water for irrigation, aquaponics recirculates water within the system that reduces water evaporation and infiltration. A report shows that aquaponics consumes only 1/7 of conventional agriculture water usage (Goddek et al., 2015). Furthermore, since there is no soil involved in the system, the problems associated with soil contamination and soil degradation are eliminated. Due to the controlled environment that reduces the diurnal temperature and light illumination swing, the vegetable productivity is higher than conventional in field cultivation. Using lettuce as an example, it only takes about 32–35 day for lettuce to be harvested in aquaponics while for the conventional agriculture, it usually takes 45 days for it to reach the same weight. With proper management and by following organic practice, the vegetables produced could be sold as an organic product that could potentially bring 50–100% more income than conventional products (Consumer Reports, 2015). Moreover, aquaponics offers a complete food plate on table that covers both vegetable and meat products. The aquaponic systems are especially suitable for the urban area, small islands, as well as arid places that have water shortage. However, since aquaponics needs to balance the growth condition for both fish and vegetables, the overlap of the two sets of conditions often leaves a thin margin for the system to succeed. Ammonia overshoot can be one of the multiple ‘single points of failure’ that cause death of fish.
Microalgae, as a naturally occurring microorganism in the aquaponic system, are commonly considered a nuisance because they often plug the water pipes, consume oxygen, attract insects and worsen the water quality. The decomposition of accumulated algae leads to excessive consumption of dissolved oxygen and results in a low level of dissolved oxygen (DO) that is dangerous to fish life. Algae could also cause diurnal pH swings and DO variation due to photoautotrophic growth under daytime light and respiration during the night (Storey, 2013) which shows algae have a great impact in an ecological system.
However in this study, instead of eliminating algae in the aquaponics, the beneficial aspects of the algae were intended to be utilized when algae were properly managed. Under a controlled circumstance, the algae may be used to 1) further remove nutrients and improve water quality in the aquaponic system, 2) control pH drop caused by nitrification process, 3) generate dissolved oxygen in the system, 4) produce polyunsaturated fatty acid as a value-added fish feed, and 5) add diversity and improve resilience to the system.
It is known that in the fish waste, ammonia nitrogen is the main form of nitrogen pollutant (90%) in the exit water (Wongkiew et al., 2017). According to the nitrogen cycle in the aquaponic system, the ammonia is firstly converted to nitrite (NO2−) by ammonia oxidizing bacteria that are represented by the “nitrosomonas” genus and then nitrite-oxidizing bacteria, represented by the “nitrobacter” genus, convert nitrite further to nitrate (NO3−) (Goddek et al., 2015). Although some fish species can detoxify ammonia to urea through the ornithine-urea cycle (Ip and Chew, 2010), high levels of ammonia/ammonium are still very toxic to fish due to the accumulated NH4+ displaces K+ and depolarizes neurons and the excessive Ca2+ influx finally leads to cell death in the fish’s central nervous system (Randall and Tsui, 2002). In general, the total ammonia/ammonium level should be controlled less than 3 ppm (Rakocy and Brunson, 1989). For tilapia fish, the un-ionized ammonia, which is a temperature and pH related parameter, is recommended to be less than 0.04 ppm (Nelson and Pade, 2008). Once the nitrogen is converted to nitrate nitrogen, it will be far less toxic to fish population and ready to be utilized by the vegetables.
Differing from most of the aquaponics vegetable that could only use nitrate nitrogen, algae can utilize both nitrate and ammonia nitrogen (Shi et al., 2000, Xin et al., 2010). In many studies, algae were used for aquaculture wastewater treatment (Sfez et al., 2015, Kuo et al., 2016). Studies indicate that some algal species even prefer ammonia uptake when both ammonia and nitrate are presented that nitrate use is suppressed at even low level of ammonium with 0.018 mg/L (Raven et al., 1992, Syrett and Morris, 1963). It can be very beneficial for ammonia reduction in the aquaponic system. Differing from pH sensitive nitrification process that is optimized at pH of 7–8, algae can adapt to a wider range of pH variation from 5.5 to 9. In case that the nitrification process fails, algae can still act as a backup system for ammonia removal.
The algae growth under autotrophic growth condition generally increases pH value in the water; however the nitrification process in the aquaponic system lowers the water pH value due to the nitric acid generated. The pH value should be controlled at 7.0 or slightly above 7.0 because the nitrification slows down when pH drops below 7 and stops when pH is less than 6.0 (Nelson and Pade, 2008). The algae component turns out to be the counterpart of the nitrification process for the pH adjustment.
Algae produce oxygen under photoautotrophic condition. According to the simplified photosynthesis equation 6CO2 + 6H2O → C6H12O6 + 6O2, for every one mole of carbon dioxide fixed from the atmosphere, one mole of oxygen will be released. Since there is barely organic carbon in the water, all carbons accumulated in the algae biomass primarily come from the atmosphere. Assuming the carbon content in microalgae is 50%, for every gram of algae produced there will be 1.3 g of oxygen generated. Although decomposition of dead algae cells consumes oxygen, this problem could be mitigated if the algae cells are harvested from the system periodically.
Microalgae are known for high lipid content with enriched omega-3 fatty acids which are uncommon in many aquaponics vegetables. It was reported that many algal species contain about 20% of lipids and among them many fatty acids were essential fatty acid (Li et al., 2011, Zhou et al., 2012). Adding suitable algae to the fish feed could improve both fish health and their nutritional value (Cheunbarn and Cheunbarn, 2015, Tocher, 2010). Furthermore, the algae production might add additional economic value for the feed because the market values of algae are high, e.g., Spirulina is about $10/Lb and Chlorella is nearly $20/Lb which is more expensive than vegetable.
However, the presence of algae can be contradictory since algae are in a competing position with in-system vegetable for nutrition, space and sunlight. On one hand, too many algae may indicate the nutrients in the water body are not well consumed by vegetables that the water quality might in a poor quality and affect the fish health. On the other hand, too little algae would diminish the benefits mentioned above. In addition, algae growth in the system will rely mainly on photosynthesis that requires light. If the surface area that could be used for growing vegetable is replaced by microalgae, it will inevitably reduce the vegetable productivity. However if the microalgae productivity could surplus vegetable production, due to higher market value and nutritional benefit, it might be worthwhile to grow algae instead of growing vegetables. The question of how to use the algae in a beneficial way without consuming too much resource in the system has not been addressed before and remain unclear. Therefore, the objective of this research was to evaluate the algae effect on the aquaponic system and to determine if algae were worthwhile to be added into the system.
Section snippets
The aquaponic system
Two aquaponic systems (F-5 Fantastically Fun Fresh Food Factory, Nelson and Pade Inc.) were purchased from Nelson and Pade Inc. and set up in a greenhouse on Saint Paul campus at the University of Minnesota. The overall dimension of the system is 80″ × 100″ (Fig. 1). The water discharged from the fish tank (110 gallon) enters the clarification tank (30 gallon) where the fish solid waste can be separated out. The central baffle forces the water to flow downwards and then upwards so that the solid
First study
Since the water and the fish were introduced from another aquaponic system, the fish had not experienced too much shock. At the startup stage, the ammonia levels in both systems were high due to the nitrifying bacteria had not been established. In Fig. 2, a t-test between NP1 and NP2 during the time period of 9/7–9/11 was conducted and found that the NP2 system without the algae had higher ammonia level (4.25 ± 0.43 ppm) than the NP1 system (2.18 ± 1.45 ppm) with the p-value of 0.042. Two fish died
Conclusions
The algae component has many proven positive effects in the aquaponic system. In daily operations, algae can help balance pH value, add oxygen, and control ammonia in the system. Although algae have lower productivity comparable to vegetable and economically unfavorable to grower, but algae can remove nitrogen more efficiently than vegetable due to higher nitrogen content in algae. Moreover algae are unlikely to compete with vegetable for nitrate nitrogen but compete for total nitrogen resource
Acknowledgements
The funding for this project was provided in part by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative Citizen Commission on Minnesota Resources (LCCMR), the University of Minnesota Grand Challenge Program, and the University of Minnesota Center for Biorefining.
References (30)
- et al.
Simultaneous microalgal biomass production and CO2 fixation by cultivating Chlorella sp. GD with aquaculture wastewater and boiler flue gas
Bioresour. Technol.
(2016) - et al.
Commercial aquaponics production and profitability: findings from an international survey
Aquaculture
(2015) - et al.
Integration of algae cultivation as biodiesel production feedstock with municipal wastewater treatment: strains screening and significance evaluation of environmental factors
Bioresou. Technol.
(2011) - et al.
Ammonia toxicity in fish
Mar. Pollut. Bull.
(2002) - et al.
Environmental sustainability assessment of a microalgae raceway pond treating aquaculture wastewater: from up-scaling to system integration
Bioresour. Technol.
(2015) - et al.
Heterotrophic production of biomass and lutein by Chlorella protothecoides on various nitrogen sources
Enzyme Microb. Technol.
(2000) - et al.
The inhibition of nitrate assimilation by ammonium in Chlorella
Biochim. Biophys. Acta (BBA)-Specialized Section on Enzymological Subjects
(1963) - et al.
Growth and nutrient removal properties of a freshwater microalga Scenedesmus sp. LX1 under different kinds of nitrogen sources
Ecol. Eng.
(2010) - et al.
Environment-enhancing algal biofuel production using wastewaters
Renewable Sustainable Energy Rev.
(2014) - et al.
Effects of pH on nitrogen transformations in media-based aquaponics
Bioresour. Technol.
(2016)
Cultivation of algae in vegetable and fruit canning industrial wastewater treatment effluent for tilapia (Oreochromis niloticus) feed
Survival
Renewable Diesel From Algal Lipids: An Integrated Baseline for Cost, Emissions, and Resource Potential From a Harmonized Model (No. ANL/ESD/12-4; PNNL-21437; NREL/TP-5100-55431)
Use of methane fermentation digestate for hydroponic culture: analysis of potential inhibitors in digestate
Eco-Engineering
Critical chlorophyll, total nitrogen, and nitrate-nitrogen in leaves associated to maximum lettuce yield
J. Plant Nutr.
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