China is the largest aquaculture country in the world. According to statistics from the Ministry of Agriculture in 2019, the total output of aquatic products in my country was 64.802 million tons. While my country's aquaculture industry has made great achievements, the direct discharge of a large amount of aquaculture wastewater has also caused surrounding Water pollution; In recent years, outbreaks of diseases and large-scale deaths of freshwater fish, marine shrimps, shellfish, etc. have occurred frequently, making people realize that the research and development of aquaculture water purification technology is becoming more and more important.

Main source of wastewater

-Autologous wastewater

-Domestic and industrial wastewater

-Overuse of Aquaculture drugs/chemicals

Aquaculture wastewater mainly includes the following substances: one is inorganic pollutants, represented by nutrients such as N and P and suspended particles. Extensive use of feed, excrement of aquatic products, algae and microorganisms; these will seriously affect the water body and adjacent waters of the dog breeder, create eutrophication of the water body, destroy the ecosystem structure and function of the breeder watershed, and lead to ecological imbalance . The second is organic pollutants, represented by disease microorganisms and chemical oxygen demand (COD). They mainly come from the part of feed and fertilizer not absorbed by aquatic products, the metabolic excrement of aquatic products, dead debris, etc., which will cause a sharp increase in the number of chemically required oxygen and pathogenic microorganisms in the water, such as Escherichia coli, Toxobacter, parasites, viruses, Fungi, etc. They can affect the longevity of aquatic products in the slightest, and cause disease pollution in severe cases, and may cause cross-infection of diseases through the discharge of wastewater.

Solutions:

The primary treatment capacity of adding coagulant shows that it improves the suspension in water. The removal rate of mechanical substances and colloids can improve the quality of primary effluent and reduce the organic loads for subsequent processing to save investment and operating costs.

The popular chemicals we usually choose are Polyaluminium Chloride(PAC), Polyaluminium Ferric Chloride（PAFC), Polyferric Sulphate(PFS), Polyacrylamide(Pam) and sodium hypochlorite(NaClO). If combined with the ultra-fine flotation equipment and the physical and chemical combination technology of front-end dosing, the decentralized and periodic aquaculture tail water treatment will become very efficient. In the case of adding 40 mg.L-1 of PAC, 2 mg.L-1 of PAM and 30 mg.L-1 of NaClO, this process technology can stably reduce COD, ammonia nitrogen and total phosphorus in aquaculture wastewater to Within 50mg.L-1, 1.0mg.L-1 and 0.3mg.L-1.

Polyaluminium Chloride (PAC) with different contents

Polyaluminium Ferric Chloride (PAFC)

Polyferric Sulphate (PFS)

Polyacrylamide (PAM)

With respect to so many flocculants for us to choose, it’s important to pick up the most cost-efficient one. So let us have a check on the data below.

In the low concentration area, with the increase of coagulant concentration, the pollutant removal rate increases rapidly and reaches a maximum value, after that, the concentration continues to increase, and the pollutant removal rate decreases. The four coagulants have different treatment effects on aquaculture wastewater: the optimal dosage of PFS is 45 mg /L, and the maximum removal rates are 37. 3%, 38. 10%, 68. 27%, and 22. 92% respectively and 32. 46%; the optimal dosage of PAC is 45 mg /L, and the maximum removal rate is 45.14%, 36. 05%, 69. 39%, 25. 73% and 42. 22% The optimal dosage of PAFC 30 mg/L, the removal rates were 52. 50%, 41.42%, 71. 78%, 27. 38% and 44. 37%.The optimum dosage of PAM was 10 mg/L, and the maximum removal rates were 44. 76%, 37. 35%, 64. 04%, 26. 67% and 38. 61%.

According to the comprehensive experimental results, the treatment effect of PAFC on aquaculture wastewater is obviously better than that of the other three coagulants, and the dosage is moderate, so it can be used as an ideal coagulant.

In today's environmental protection industry technology field, the coagulation-sedimentation method is widely used because of its simple process, high efficiency, and low cost. The technical key and core foundation of the coagulation method is the coagulant. Polyaluminum ferric chloride (PAFC) is an inorganic polymer flocculant with excellent performance. It has the advantages of less corrosion to equipment pipelines, and has the characteristics of fast settling of iron salt flocculants, easy separation, good low-temperature water treatment performance and wide pH range of water treatment. At the same time, it overcomes the shortcomings of high residual aluminum concentration in water after treatment with aluminum salt flocculant and poor stability of iron salt flocculant, and shows high efficiency in water coagulation and sedimentation treatment.