Effects of pH on nitrogen transformations in media-based aquaponics

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Effects of pH on nitrogen transformations in media-based aquaponics

https://doi.org/10.1016/j.biortech.2015.12.079Get rights and content

Abstract

To investigate the effects of pH on performance and nitrogen transformations in aquaponics, media-based aquaponics operated at pH 6.0, 7.5 and 9.0 were systematically examined and compared in this study. Results showed that nitrogen utilization efficiency (NUE) reached its maximum of 50.9% at pH 6.0, followed by 47.3% at pH 7.5 and 44.7% at pH 9.0. Concentrations of nitrogen compounds (i.e., TAN, NO2-N and NO3-N) in three pH systems were all under tolerable levels. pH had significant effect on N2O emission and N2O conversion ratio decreased from 2.0% to 0.6% when pH increased from 6.0 to 9.0, mainly because acid environment would inhibit denitrifiers and lead to higher N2O emission. 75.2–78.5% of N2O emission from aquaponics was attributed to denitrification. In general, aquaponics was suggested to maintain pH at 6.0 for high NUE, and further investigations on N2O mitigation strategy are needed.

Introduction

Aquaculture has become one of the fastest-growing food-producing sectors since 1980s and accounted for almost half (49%) of global fish consumption in 2012 (FAO, 2014). In order to meet the growing human demands for aquatic products, aquaculture scale is bound to continue expand. However, aquaculture is a high-polluting industry. On average, only 25% of its nitrogen and phosphorus inputs could be recovered by target organisms (Crab et al., 2007), and the rest of nutrients are discharged into surrounding water. This is not only a waste of nutrients, but also causes serious pollution to the surrounding environment.

Aquaponics is considered to have potentials to solve the abovementioned problems. Aquaponics is the combination of conventional aquaculture and hydroponics, which could achieve co-culture of fish and plants at the same time. Fish, plants and microbes are three main components of aquaponics, and microbes play the bridge role of converting fish waste to plant nutrients (Somerville et al., 2014). Usually there are three common types of aquaponics designs, i.e., floating raft, nutrient film technology, and media-based bed, mainly classified according to hydroponics (Nelson and Pade, 2007). Of which, media-based bed could act as a filtration unit and provide surface area for microbial growth at the same time. This makes it popular in currently running aquaponics. A survey conducted by Love et al. (2014) discovered that 86% of their respondents adopted media-based aquaponics.

Nitrogen is a vital element for all living organisms. In aquaponics, fish feed that contains high content of protein is added into system and digested by fish. Most of the nitrogen is then excreted in the form of total ammonia (TAN), which is toxic to fish. Fortunately, nitrifying bacteria in aquaponics could first convert ammonia to nitrite (NO2) and then into nitrate (NO3) through nitrification. Nitrate would be reduced to N2 through denitrification, but more importantly, it is an important fertilizer for plant growth. The establishment of cooperation among three components increases nitrogen utilization efficiency (NUE) and avoids nitrogen-rich wastewater discharge. To achieve higher productivity and better water quality in aquaponics, many kinds of regulation attempts have been conducted. Liang and Chien (2013) found that better fish growth, plant growth, and nutrients removal efficiency from water were obtained in aquaponics under 24-hour light than 12-hour light, and Endut et al. (2010) reported similar results at loading rate of 1.28 m/d. However, thorough study on nitrogen transformations in aquaponics is still lacking.

pH is one of the most important regulation factors for aquaponic systems, and it is needed to be balanced for fish, plants and microbes at the same time. Usually, recommended pH for plant cultivation was slightly acid (5.5–5.8) (Bugbee, 2003), while the optimal pH for nitrification was 7.5–8.0 (Kim et al., 2007). Fish can tolerate a wide pH range, and the optimal pH was different for different species (Arimoro, 2006, Lemarie et al., 2004). In aquaponics, providing the pH optima for every part is impossible, but knowing optimal pH range for the best overall performance is necessary. Tyson et al. (2008) had claimed that reconciling pH for aquaponics should be 7.5–8.0, but no difference in plant yields was detected in their research, which was unreasonable. Essential study is required to investigate the aquaponic performance under different pH conditions. In addition, to achieve the best sustainability, neither yield nor environment impacts could be ignored.

Since biological nitrogen transformations play the key role of bridge in aquaponics, it may cause environmental harms. Nitrous oxide (N2O), the third biggest greenhouse gas with a global-warming potential 296 times higher than CO2, is often generated from biological nitrification and denitrification processes. In nitrification, heterotrophic ammonia oxidation bacteria (AOB) could conduct nitrifier denitrification to produce N2O, and the oxidation of hydroxylamine, intermediate during the oxidation of TAN to NO2, would also lead to N2O production (Kampschreur et al., 2009). While in denitrification, N2O which failed to be reduced in time might be emitted to the atmosphere. Previous study has shown that 1.5–1.9% of nitrogen input was lost in the form of N2O in floating raft aquaponics, while 1.3% was found in conventional aquaculture (Hu et al., 2013, Hu et al., 2015). However, to date, no N2O emission investigation has been conducted in media-based aquaponics.

In this study, media-based aquaponics was established to investigate the effects of pH on its nitrogen transformations, and special attention was paid to N2O emission. 15N labeling experiment was used to determine the main source of N2O emission, and quantitative polymerase chain reaction (Q-PCR) technology was applied to quantify the abundance of nitrifies and denitrifies to reveal the influence of pH on microbial community.

Section snippets

Aquaponic microcosms

Experimental aquaponic systems were operated side by side under natural conditions in Jinan, China. A transparent rainproof shed was installed above the aquaponics. All systems shared same setup design. Each system was mainly consisted of two parts, fish tank and hydroponic bed. These two parts were both made of plastic box, i.e., 65 cm × 45 cm × 50 cm for fish tank and 80 cm × 55 cm × 45 cm for hydroponic bed. Fish tank was placed on ground. The effective water volume in fish tank was 100 L. There was no

Aquaponic performance

Survival rates for fish and plants were 100% in all systems, indicating that aquaponics can adapt to a wide range of pH. In the entire study period, water temperature and DO concentrations were both maintained at similar values in three pH treatments, which were 23.0 ± 1.9 °C and 6.2 ± 0.9 mg/L, respectively. Another important index for aquaponics evaluation was daily water replenishment ratio, which also didn’t show significant difference. The average water replenishment ratio of all treatments was

Conclusions

Although aquaponics could tolerate a wide pH range, its performance and nitrogen transformations were affected by pH significantly. Higher plant yield was obtained at pH 6.0, led to higher NUE. Unfortunately, this was achieved at the expense of higher N2O emission, which accounted for 2.0% of nitrogen input. In media-based aquaponics, N2O emission mainly occurred in hydroponic bed, where most of microbes grew. More importantly, 75.2–78.5% of N2O emission from aquaponics was attributed to

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 21307076 & No. 51578321), and Fundamental Research Funds of Shandong University (No. 2014TB003 & No. 2015JC056). This work is also supported by the AMIS (Fate and Impact of Atmospheric Pollutants) project funded by the European Union, under the Marie Curie Actions IRSES (International Research Staff Exchange Scheme), within the Seventh Framework Programme FP7-PEOPLE-2011-IRSES.

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