Study On Nutrient Matching Of Fertilizer Draw Solution In Forward Osmotic Treatment Of RO Concentrated Water Based On Facility Tomato Growth Requirements

Ningxia is short of water resources,but there is a large amount of brackish water resources.The desalination and utilization of brackish water can effectively alleviate the shortage of water resources in Ningxia.In all the brackish water desalination technology,Reverse Osmosis(RO)has become the most advanced and effective membrane separation technology with the largest installed capacity,but in the process of treating water,it will discharge 20%-60%of RO concentrated water,which will lead to water waste and environmental pollution.At present,Forward Osmosis(FO)is gradually becoming the development direction of reverse osmosis concentrated water treatment technology.Forward osmosis is a membrane separation process,which uses the osmotic pressure difference formed by the concentration difference of different solutions on both sides of the membrane as a driving force,so that the water in the low osmotic pressure solution(feed solution)enters the high osmotic pressure side(draw solution)through the forward osmotic membrane,and the solute molecules are trapped in the low osmotic pressure solution.The use of forward osmosis to treat RO concentrated water can effectively solve the problem of massive discharge of RO concentrated water,but the conventional forward osmosis draw solution regeneration process requires a lot of energy consumption,in order to reduce the energy consumption of forward osmosis draw solution regeneration process,the use of chemical fertilizer solution as forward osmosis draw solution can avoid the regeneration process.The use of Fertilizer Draw Forward Osmosis(FDFO)system to treat RO concentrated water,on the one hand,can effectively concentrate the volume of RO concentrated water,reduce concentrated water emissions,on the other hand,the use of diluted fertilizer draw solution for agricultural irrigation,without regeneration,thus greatly reducing the system energy consumption.In this experiment,KCl,KNO3,20:20:20(N:P:K)and KH2PO4,four water-soluble agricultural fertilizers,were selected as the draw solution to carry out forward osmosis tests respectively.Then 100g/L,200g/L KCl,KNO3,KH2PO4 and 100g/L,200g/L,300g/L 20:20:20(N:P:K)were selected to carry out forward osmotic tests with different concentration gradients.Finally,100g/L KNO3 water-soluble fertilizer was selected to carry out forward osmosis tests with different volume ratios of feed solution and draw solution.The influencing factors on forward water flux during forward osmosis and the concentration effect of different fertilizer draw solution on RO concentrated water were studied.Nutrient changes of fertilizer draw solution during forward osmosis and nutrient matching of fertilizer solution obtained through forward osmosis based on nutrient requirements of tomato.1)KCl,KNO3,KH2PO4,20:20:20(N:P:K),four kinds of water-soluble agricultural fertilizer solutions were used for forward osmosis test.The results showed that KCl and KNO3 had the largest forward water flux as draw solutions,and were more suitable for fertilizer draw solutions.Through the forward osmotic test of 100 g/L,200 g/L,300 g/L 20:20:20(N:P:K)agricultural water-soluble compound fertilizer draw solution,the results showed that the forward water flux of 300 g/L and 200 g/L is 2.33 and 1.62 times of that of 100 g/L,respectively.The higher the concentration of fertilizer draw solution,the greater the forward water flux.Under the same concentration,the size of forward water flux mainly depended on the type of fertilizer draw solution.In the four water-soluble fertilizers,the more potassium content,the larger forward water flux was.The higher the phosphorus content of fertilizer draw solution,the smaller the forward water flux.Through the forward osmosis test of 100 g/L KNO3 water-soluble fertilizer solution with different volume ratios of feed solution and fertilizer draw solution,it is concluded that the forward water flux when the volume ratio is 5:1 is 1.13 times and 1.39 times of 4:1 and 3:1,respectively.The results show that the larger the volume ratio of feed solution and fertilizer draw solution,the greater the forward water flux.The conductivity difference of draw solution and feed solution and forward water flux of each 200g/L were fitted,and the coefficient of determination were all greater than 0.99,indicating a high degree of fitting.2)Forward osmosis test was conducted on 200 g/L of KCl,KNO3,20:20:20(N:P:K)and KH2PO4,and treatment effect of each fertilizer draw solution on RO concentrated water was analyzed.When the RO concentrated water is concentrated twice at the same time,the time used for KCl is 4.1h,the time used for KNO3 is 5.4h,the time used for 20:20:20(N:P:K)is 5.8h,and the time used for KH2PO4 is 6.9 h.Therefore,the concentration of RO concentrated water of the same multiple,the order of energy consumption from low to high is KCl,KNO3,20:20:20(N:P:K),KH2PO4.Running KCl and KNO3 for 5.5 hours,the RO concentrated water can be concentrated to the initial 19%and 24.7%.20:20:20(N:P:K)and KH2PO4 operation for 9 hours can concentrate the RO concentrated water to 18.6%and 19%of the initial absorption solution.It can be seen that the concentration efficiency in the later stage of operation is low,and about 80%of the RO concentrated water can be recycled by forward osmosis.When the system runs efficiently until 4h,the average forward water flux of KCl is 4 L/(m2·h),the average forward water flux of KNO3 is 3.54 L/(m2·h),and the average forward water flux of 20:20:20(N:P:K)is 3.53 L/(m2·h).The average forward water flux of KH2PO4 is 3.17 L/(m2·h),which indicates that the efficiency of concentrating RO concentrated water from each fertilizer extract is KCl,KNO3,20:20:20(N:P:K),and KH2PO4 in order from high to low.High efficiency of RO concentrated water treatment,short running time and low energy consumption of RO concentrated water of the same multiple are KCl and KNO3,that is,fertilizer draw solution with high K+ content.3)200g/L of KCI and KNO3 were used as fertilizer extract solution for forward penetration test.At the end of the system operation,KCl was diluted 4.5 times and KNO3 4.1 times.At this time,the K content of 360L of diluted KCl solution in each barrel of fertilizer extract solution was 8.63kg.328L of diluted KNO3 solution contains 6.4kg K and 1.87kg N.According to the fact that KCl and KNO3 need to be diluted 1000 times before they can be used for crop irrigation,it can be seen that the diluted KCl and KNO3 in the suction tank need to be further diluted before they can be used for tomato irrigation.Fertilization is divided into two types:bottom fertilizer and topdressing.Through the test soil nutrient formula of the facility tomato,when the bottom fertilizer is applied according to the standard of 500kg of decomposed chicken manure and 20kg potassium dihydrogen phosphate per mu,the topdressing amount of N is 15.65kg and K is 18.09kg.When the diluted KCl fertilizer draw solution in the forward osmosis system is used as K-containing fertilizer,2.1 barrels of 756L of KCl fertilizer draw solution and 46.72 kg of NH4NO3 are needed to meet the demand of topdressing.When the diluted KNO3 fertilizer draw solution in the forward osmotic system is used as fertilizer containing K and N,2.8 barrels of 918.4L KNO3 fertilizer draw solution and 31.1 kg of NH4NO3 are needed to meet the demand of topdressing.According to the production of 8 tons per mu,topdressing was applied four times with drip irrigation in the fruit setting period after the application of base fertilizer.When the KCl fertilizer draw solution is used as the topdressing solution,2.1 barrels of 756L of KCl fertilizer draw solution should be applied per mu,mixed with 46.72 kg of NH4NO3 and added with water to 80.32m3,and diluted to 106 times of the original to supply tomatoes.When KNO3 fertilizer draw solution is used as a topdressing solution,2.8 barrels of 918.4 need to be applied per mu.