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Currently electrocoagulation (EC) filtration technology is successfully applied in water purification and industrial wastewater treatment. The objective of the present study was to investigate the effects of EC in wastewater from aquaculture by providing technical means and data support for enhancing the filtration pretreatment capacity of a recirculating aquaculture system (RAS) since its application is scarce.
As an environmentally friendly and healthy aquaculture technique, recirculating aquaculture has attracted increasing attention, and the continuous improvement and development of recirculating aquaculture system (RAS) procedures are now established practices.
Generally, in the RAS, the effluent of the aquaculture tank is initially channeled through solid–liquid separation equipment to remove the majority of suspended particulate matter, and subsequently through equipment that provides biological oxidation, oxygenation, disinfection and other processes, in order to achieve recycling.
“The pretreatment of aquaculture wastewater by solid– liquid separation equipment greatly reduces the processing load sustained by subsequent water treatment units and it reduces the overall energy consumption of the system.”
Nevertheless, because of the need to ensure the smooth circulation of water in RASs and because of process and cost limitations, it is difficult to improve the physical filtration efficiency by reducing the filter pore size continuously, thus hampering the pursuit of higher filtration levels of RASs.
Electrocoagulation (EC) is also a common method for enhancing solid–liquid separation of sewage, in which the sacrificial anode releases metal cations with flocculation characteristics under the action of an external electric field, then forms flocculants to absorb pollutants in the water.
Through the flocculation initiated by EC processes, the small suspended particles in the wastewater coagulate into larger particles, improving filtration accuracy without changing the original filtration equipment and showing that EC–filtration technology has a great potential and application for sewage treatment procedures.
“However, the application of EC technology in aquaculture wastewater is rare, and the application of EC–filtration technology in aquaculture has not been reported.”
The main objective of the present research was to investigate the pretreatment effect of EC–filtration technology on aquaculture waste- water, which is expected to provide some reference for the future improvement of the filtration capacity of the RAS.
Materials and methods
In the experiment, a continuous flow EC–filtration system with a laboratory scale was designed, consisting of three sections: an EC reactor, a mixed flocculator and filtration equipment.
The effective volume of the EC reactor was 5 L, in which nine groups of electrodes were arranged in parallel: four groups of anodes were composed of Al plate or Fe plate electrodes, and five groups of cathodes contained titanium (Ti) plate electrodes.
“The mixed flocculator was arranged vertically and its effective volume was about 1.5 L. It was located between the EC reactor and the filtration equipment, adapted to an upward flow design.”
The function of the mixed flocculator was to improve the contact frequency between the flocculants and the suspended particles, thereby improving the efficiency of flocculation and adsorption.
After the flocculation treatment by EC, the aquaculture wastewater was finally filtered. The filtration equipment used was a micro- screen drum filter whose filter pore size is generally between 50 μm and 75 μm. The schematic diagram of the EC–filtration system is shown in Figure 1.
The wastewater used in the experiment originated from effluents of the aquaculture pond used for the Litopenaeus vannamei RAS. In this experiment, five different anode combinations, three different EC reactor hydraulic retention times (HRTs) and four filtration pore diameters were used.
A control group was set up without EC treatment and only relying on filtration equipment. The experiment was aimed at measuring the removal effect on the total number of Vibrio, chemical oxygen demand (CODMn), total ammonia nitrogen (TAN), nitrite nitrogen (NO–2-N), nitrate nitrogen (NO–3-N) and total nitrogen (TN) in aquaculture wastewater through the use of an EC–filtration system with different combinations of anodes, different HRTs and variable size of filtration pores.
Finally, the aquaculture wastewater post-EC treatment was filtered through filtration equipment in order to complete the pretreatment procedure.
Results and discussion
The removal effect of the EC–filtration system on the total number of Vibrio
In production, RAS uses ozone and ultraviolet synergistic sterilization to control pathogenic bacteria, which usually requires massive energy consumption. Charged ions generated by EC can neutralize the surface charge of pathogenic bacteria, reduce electrostatic repulsion and form flocs.
In addition, the flocculants produced by EC can actively absorb and flocculate the bacteria. These flocculates are eventually removed by physical filtration.
Figure 2(a) shows the effect of different composite anodes and HRTs on the removal of Vibrio in the EC reactor. When the HRT increased, the removal efficiency increased. It follows that the EC reactor had a significant removal effect on the total number of Vibrio.
Also, without a solid–liquid separation, the removal efficiency increased with the addition of the iron electrode proportion in the combined anode and with the increase of HRT. Furthermore, the experiment proved that the sterilization of Fe as anode during EC was more effective than that of Al anode, which is consistent with results reported.
Figure 2(b1)–2(b3) show the effects of different composite anodes and filter pore sizes on the Vibrio removal by EC–filtration. Compared to the control group, the removal efficiency of experimental groups for Vibrio was obviously greater.
Effects of the EC–filtration system on CODMn removal
In the RAS, the increase of CODMn in the water body favors bacterial growth, increasing the oxygen consumption rate and thereby adding costs. As a sewage filtration pretreatment technology, the EC–filtration system can remove CODMn from wastewater by electro-oxidation and flocculation–filtration, which have a better removal effect on CODMn than traditional physical filtering methods.
Figure 3(c) shows the effect of different composite anodes and HRTs on CODMn removal in the EC reactor. The removal efficiency increased with the addition of the Fe electrode proportion to composite anodes and with the increase of HRT.
Figure 3(d1)–3(d3) show the effects of different composite anodes and filter pore sizes on COD-Mn removal in the EC–filtration system. Removal efficiency increased by 14.73 times, 8.78 times, 6.56 times, and 5.39 times compared to the control group.
Therefore, the EC–filtration system has an excellent removal effect on CODMn in aquaculture wastewater, and without changing the original filter aperture, EC can immensely enhance the capacity of the physical filtration equipment to remove CODMn.
Effects of the EC-filtration system on TAN, NO–2-N, NO–3-N and TN removal
EC–filtration technology can remove TAN, NO–2-N and NO–3-N at the same time as removing residual bait and feces. Applying EC as a pretreatment process can not only reduce the load of subsequent biological oxidation equipment, but also improve the activity of microorganisms, thereby improving the nitrogen removal capacity of RAS and reducing the threat of nitrogen pollutants to cultured organisms.
Energy consumption analysis
The application of composite anodes in the pretreatment of aquaculture wastewater through the EC–filtration system was evaluated in terms of electric energy loss. From Table 1, results show that during the EC process, the energy consumption of the system increased with the addition of the Fe electrode proportion to the composite anodes and with the increase in the HRT of the EC reactor.
The study found that the electro-oxidation of the EC reactor was stronger when Fe was used as anode than when Al was sed.
Conclusions
EC process could enhance the capacity of the subsequent physical filtration equipment. The EC– filtration system was found effective for the removal of the total number of Vibrio, CODMn, TAN, NO–2-N, NO–3-N and TN in aquaculture wastewater.
Compared with the Al electrode, when the Fe electrode was used as anode, the EC–filtration system had a stronger electro-oxidation capacity, which was conducive to the removal of TAN and NO–2-N. However, the flocculation effect was relatively weaker and the energy consumption was increased by 2.59 times.
In comparison with the single Al or Fe anode, the Al-Fe combined anode had more advantages. Considering both removal efficiency and energy consumption, the optimum anode combination for the EC–filtration system was 3Al + Fe.
With the increase of HRT and the decrease of filtration pore size, the enhanced effect of the EC process on the filtration equipment was more obvious. When the HRT was 4.5 min and the filter pore size was 45 μm, the maximum removal efficiency for Vibrio, CODMn and TN increased by 3.31, 5.39 and 4.57 times compared with the control group, respectively.
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “APPLICATION OF HYBRID ELECTROCOAGULATION-FILTRATION METHODS IN THE PRETREATMENT OF MARINE AQUACULTURE WASTEWATER” developed by: JI-ANPING XU, YISHUAI DU, TIANLONG QIU, LI ZHOU, YE LI, FUDI CHEN* and JIANMING SUN- Institute of Oceanology, Qingdao and *University of Chinese Academy of Sciences.
The original article, including tables and figures, was published on MARCH, 2021, through WATER SCIENCE & TECHNOLOGY.
The full version can be accessed online through this link: doi: 10.2166/wst.2021.044
