By: Jianping Xu, Yishuai Du, Tianlong Qiu, Li Zhou, Ye Li, Fudi Chen and Jianming Sun
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 pre-treatment 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.
One traditional method to enhance the efficiency of sewage filtration, involves adding chemical flocculants such as aluminum (Al) or iron (Fe) salts to the sewage ahead of filtering. Ebeling et al. used chemical flocculants (alum) to improve the filtering capacity of the belt filter system on the tail water of the RAS.
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. Compared with traditional chemical flocculation, EC has the advantages of causing less secondary pollution and less sludge output and having more controllability.
At present, EC–filtration technology has been successfully applied to surface water purification, industrial wastewater treatment and in other fields. 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.
Experimental set up
In this 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. The filtration equipment used was a microscreen drum filter whose filter pore size is generally between 50 μm and 75 μm. 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) (1.5 min, 3.0 min, and 4.5 min) and four filtration pore diameters (75 μm, 63 μm, 54 μm, and 45 μm) were used.
The current density of the EC reactor was 19.22 A/m2 . 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 (NO2 -N), nitrate nitrogen (NO3 -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.
Samples were collected at the water inlet of the system to measure the initial water quality index. Based on the change in water quality detected, it was possible to evaluate the treatment capacity of the EC reactor under different test conditions.
The total number of Vibrio was detected immediately using the plate counting method (TCBS plate) after the water sample was collected. For the detection method of CODMn followed. And TAN, NO2 -N, NO3 -N and TN were measured using standard methods. Salinity, pH and conductivity were measured by a multi-parameter water quality analyzer (YSI-556, USA). Reported results are based on mean and standard deviation (mean ± SD).
Result 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. Although ozone has strong bactericidal ability, it will threaten the health of cultured organisms, so the use of ozone in RAS needs to be strictly controlled EC–filtration technology, as a pretreatment method of aquaculture water, can eliminate pathogenic bacteria while removing residual bait and feces; its application in RAS can reduce the work intensity and energy consumption of subsequent ozone and ultraviolet sterilization equipment.
These flocculates are eventually removed by physical filtration. 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. Removal efficiency for Vibrio by EC–filtration showed different trends with the addition of the Fe electrode proportion in composite anodes. This was determined by the two sterilization procedures characterizing the EC–filtration system.
When the electrode, electric field and electro-oxidation played a major role, the removal efficiency for Vibrio increased with the addition of the Fe electrode proportion in composite anodes. When the electric neutralization/flocculation– filtration became predominant, the removal efficiency increased initially and then decreased with the addition of the Fe electrode proportion in combined anodes.
This was due to the fact that the flocculants produced by the Al-Fe combined anodes in the EC process were superior to those of the single Al or Fe electrode in terms of structure and strength. A portion of microorganisms in the aquaculture wastewater attaches to the suspended particles, which act as a substrate for microbial growth.
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. 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. It was also due to the lack of solid–liquid separation, so that the EC reactor mainly relied on the effect of electro-oxidation to remove CODMn, and the removal efficiency depended on the amount of oxidizing substances present in the water. Compared to Al, Fe as anode can generate more oxidants during the EC process.
The removal efficiency of CODMn through filtration equipment depends on the size of organic particles in the water when the filtration aperture is constant; and the larger the particle size, the higher the removal efficiency is. In the EC–filtration system, flocculants can be released into aquaculture wastewater by the EC reactor, which can increase the size of organic particles, thus improving the efficiency of the subsequent physical filtration.
Effects of the EC–filtration system on TAN, NO2 -N, NO3 -N and TN removal
EC–filtration technology can remove TAN, NO2 -N and NO3 -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 (such as moving bed biofilter, fixed film aerobic bioreactor), 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.
The experiment showed that the removal efficiency of TAN and NO2 -N increased with the increase of HRT and the addition of the Fe electrode proportion to composite anodes, The suspended solids in the aquaculture water contain a certain amount of TN, accounting for 10–40% of total TN, which can be removed by solid–liquid separation.
Therefore, in addition to electroreduction, the EC–filtration system can also remove TN from aquaculture wastewater by flocculation–filtration. The TN removal efficiency of the filtration system clearly increased after EC treatment, and the higher the HRT of the EC reactor, the greater its enhancement effect on the subsequent use of filtration equipment.
Energy consumption analysis
The study found that the electro-oxidation of the EC reactor was stronger when Fe was used as anode than when Al was used; it also found that the treatment efficiency in terms of CODMN, TAN and NO2 -N removal was higher. When Al was used as sacrificial anode, the adsorption/flocculation capacity of the EC reactor was greater than when Fe was used, and the removal efficiency for Vibrio and TN was higher.
However, under the same operating conditions, the energy consumption of the EC reactor with the anode combination of 4Fe was 2.59 times greater than that of the EC reactor with the anode combination of 4Al.
By utilizing Al-Fe composite anodes, the running energy consumption of the EC–filtration system was reduced, while combining the advantages of Al and Fe anodes.
When 3Al + Fe was used as anode, the removal efficiency of the system for Vibrio, CODMn and TN was inferior to that observed when 2Al + 2Fe was used as anode, and the difference was relatively small, less than 5%. However, compared with the 3Al + Fe anode, the energy consumption of the system increased by 19.43% when 2Al + 2Fe was used as sacrificial anode. Therefore, when using the EC–filtration system to treat aquaculture wastewater, and considering the procedure from the perspective of treatment efficiency and energy loss, the optimum anode combination method was 3Al + Fe.
In this experiment, EC–filtration technology was used to pretreat aquaculture wastewater, and the effects of anode combinations, EC reactor HRTs and filter pore sizes on pollutants removal were studied. 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.
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: Jianping Xu, Yishuai Du, Tianlong Qiu, Li Zhou, Ye Li, Fudi Chen and Jianming Sun. The original version was published in 2021 through Water Science & Technology.