Today, China requires aquaculture tailwater to meet the national discharge standard. After treatment, recycled water utilization is also in the interests of bullfrog farmers. As shown in the results described above, a significant treatment effect was achieved on the tailwater from intensive bullfrog farming by the proposed treatment system. Significant improvement was seen in the color, odor, suspended solid content, and in the contents of organic matter (CODCr and BOD5), nitrogen, and phosphorus in the treated tailwater (Table 3). The treated water quality met the requirements of Grade 1 of the national discharge standard (except for reactive phosphorus).
The results in this paper also demonstrate that the biochemical tank is a key unit in this tailwater treatment system. The biochemical tank was the predominant contributor to CODCr removal (a 67.3% removal) and the second most important contributor to ammoniacal nitrogen removal (a 40.4% removal) and total phosphorus (a 30.4% removal). In this step, organic matter was oxidized and decomposed by microorganisms in the activated sludge. Ammoniacal nitrogen was converted to nitrate nitrogen by nitrifying bacteria under aerobic conditions. Phosphate-accumulating bacteria in the system consumed phosphorus from the water under aerobic conditions and precipitated into sludge. Liu et al. (2021) reported that in a multi-stage combined process composed of three ponds and two dams, an aeration pond made the largest contribution to the removal of CODMn (18.7% removal) and NH4+-N (28.7% removal), the second greatest contribution to total nitrogen removal (67.3%), and the least contribution to total phosphorus removal (3.7%). Aeration can effectively promote the reproduction of microorganisms and improve bioactivity, thereby accelerating the metabolism of organic pollutants. Researchers have identified significant correlations between aeration intensity and organic matter removal in studies of mixed wastewater purification by constructed wetlands (Ma et al. 2011). While adequate aeration leads to thorough oxidation and decomposition of the organic matter in water by microorganisms (Sirianuntapiboon and Jitvimolnimit 2007), over-aeration destabilizes and even deactivates the microorganisms in the system, reducing their oxidative ability to less than optimal (Li et al. 2011).
In the tailwater treatment system constructed in the present study, the wetlands unit is as vital as biochemical tank that exhibited excellent performance in the removal of ammoniacal nitrogen (70.5% removal) and total phosphorus (59.4% removal). These findings were consistent with results of Liu et al. (2021) that ecological pools made the largest contribution to total nitrogen and total phosphorus removal. Boyd (1970) studied the removal of nutrients from polluted waters by aquatic vascular plants as early as 1970. Ansari et al. (2017) investigated aquaculture wastewater treatment by microalgae. Endut et al. (2011) reported that water spinach was able to reduce 79–87%, 75–85%, and 78–85% of nitrate, phosphorus, and total ammonia nitrogen, respectively, in catfish culture wastewater. Prabhath et al. (2022) reported that Spirulina (Arthrospira) platensis in aquaculture wastewater removed up to 97% of NH4+-N, 72% of NO3−-N, 96% of NO2−-N, and 93% of PO43−-P. The use of the microalga Haematococcus pluvialis effectively decreased the contents of NH4+-N, NO2−-N, and NO3−-N in culture water in an artificial culture system of whiteleg shrimp (Litopenaeus vannamei) (Wang et al 2022). The watermilfoil, water spinach, and alligator weed used in this study grew well in our constructed wetlands (Fig. 2) and realized high levels of ammoniacal nitrogen and total phosphorus removal. After one month of operation, watermilfoil plants that were initially 5 cm tall and planted at a density of 30 plants/m2 grew to 40–50 cm tall with a density of 1000 plants/m2. Alligator weed and water spinach could be harvested multiple times, making them suitable species for growing in wetlands.
In the final effluent, although the removal of ammoniacal nitrogen and nitrite nitrogen reached 83% and 92%, respectively, the total nitrogen removal (39.3%) was not high (Table 3). It has been reported that about 36% of the feed is excreted as a form of organic waste in aquaculture, while about 75% of the feed N and P are unutilized and remain as waste in the water (Burford et al. 2003; Piedrahita 2003; Gutierrez-Wing and Malone 2006). The main sources of nitrogen compounds, including NH3, NH4+-N, NO3−-N, NO2−-N, and organic nitrogen, are the decomposition of bait residues and the excretion of aquaculture organisms. In water, the sum of ammonia (NH3) and ammonium (NH4+) is called ammoniacal nitrogen. The ammoniacal nitrogen removal process is NH4+-N→NO2−-N→NO3−-N→N2 (Crab et al. 2007). Nitrification is a process in which NH3 + NH4+ is oxidized and converted to NO2−-N and NO3−-N under the action of nitrifying bacteria. Denitrification refers to the process in which NO2−-N and NO3−-N are converted to N2 under the action of denitrifying bacteria. Nitrification processes only occur under aerobic conditions, whereas denitrification process only occurs under anoxic conditions, and an organic carbon source as an electron donor is required to complete the denitrification process. Nitrifying bacteria are sensitive to a wide variety of environmental factors, such as high concentrations of ammonia and nitrous acid and low dissolved oxygen levels. Nitrifying bacteria are especially sensitive to even traces of sulphides present in the sediments and sludges accumulated in aquaculture systems. Moreover, nitrification necessitates a low C/N ratio because for higher C/N ratios, the heterotrophic bacteria out-compete nitrifiers for available oxygen and space. We believe that the relatively low removal of total nitrogen compared with ammoniacal nitrogen and nitrite nitrogen found in the present study is due to unsuitable factors for the denitrification process, which delayed and blocked the step of NO3−-N to N2.
In this study, the removal of reactive phosphate and total phosphorus reached 86.7% and 76.5%, respectively. However, the level of reactive phosphate still slightly exceeded Grade 1 of the national discharge standard (Table 3). Reactive phosphate includes all soluble forms of phosphates (PO43−-P). Phosphate-accumulating bacteria in activated sludge can release polyphosphate in their cells under anaerobic conditions (anaerobic phosphorus release), while under aerobic conditions, these bacteria can absorb phosphorus from water (aerobic phosphorus absorption) and convert it into polyphosphate in their cells, thus forming phosphorus-rich biological sludge. Therefore, the release and absorption of phosphorus in activated sludge is a dynamic balance. Only the phosphorus-rich sludge is discharged from the system by precipitation, and the effect of phosphorus removal from wastewater is achieved. In this study, removal of the sludge was not conducted during operation, and treated effluent collected in the purified water tank was reused to replenish water in the breeding pond. Liang et al. (2016) suggested that the various plant species in constructed wetlands have significantly different nitrogen and phosphorus removal effects. Gichana et al. (2019) reported that in a small-scale recirculating aquaponic system, phosphorus removal was substantially higher in pumpkin (Cucurbita pepo) (75.5%) than in sweet wormwood (Artemisia annua) (47.36%) and amaranth (Amaranthus dubius) (40.72%). Paolacci et al. (2021) found that the removal of nitrogen and phosphorus by Lemna minor was the highest at the lowest plant density when uptake rates were calculated per square meter of water area covered by Lemna fronds. Integrated multi-trophic wetlands including phytoplankton, algae, hydrophytes, as well as bioflocs may be further developed in the future.
Finally, we strongly recommend that the ecological principle of food chain relationships should be introduced into the design of wastewater treatment system in intensive aquaculture. In the design of processing units, people should pay more attention to the concept of "utilization" of the chemical substances in the system rather than "removal" from the system, which causes secondary pollution. Taking this experiment as an example, the active sludge in sedimentation tanks, which is rich in nutrient elements and organic matter, can be cleaned regularly and used to manufacture organic fertilizer. Ammoniacal nitrogen and soluble phosphates should be absorbed and utilized in wetlands by ammoniophilic or phosphatic-preferring organisms with economic benefits, or the harvested products can be used as carbon sources and organic matter in the manufacture of organic fertilizer. Ammoniacal nitrogen should not be converted into nitrogen and released into the atmosphere, which would cause secondary pollution. The anionic residuals, such as nitrate, phosphate, and other inorganic anions, can be adsorbed and precipitated using cationic bioflocs and then used for organic fertilizer manufacturing. The treated effluent can reach the national standard and be returned to the breeding pond for water replenishment. In this manner, intensive aquaculture can finally realize the ideal of an ecological aquaculture cycle with zero emissions and no waste production.