Although the water flux for freshwater extraction stop is encouraging, the rate of absorbing water using hydrogel is significantly slower than other draw agents. To improve performance, the team envisioned running numerous desalting sponges in parallel, and further investigations need to be conducted. Overall, since the average salinity of seawater is 35 ppt, which is around 17 times more than the saline used in the test , it shows a promising application in seawater desalination on a lower NaCl concentration.There are many factors affecting the efficiency of forward osmosis, such as concentration polarization, membrane fouling, reverse solute diffusion, membrane development, and draw solution design. Concentration polarization is the most important factor among all of them. Various studies conducted regarding forward osmosis, these studies share an identical focus, which is reducing concentration polarization. The existence of concentration polarization can weaken the actual osmotic pressure difference on both sides of the membrane, which is one of the limiting factors that affect the performance of forward osmosis in water flux recovery. Pressure retarded osmosis has been defined as osmosis through asymmetric membranes. Most forward osmosis membranes used are either an asymmetric structure membrane including an active layer/a porous support layer , or symmetric structure membrane . There are two types of concentration polarizations based on the placement of the membranes: external concentration polarization and internal concentration polarization . External concentration polarization and internal concentration polarization can be further categorized into two sub-categories: dilutive and concentrative. In general applications, forward osmosis membranes are commonly placed in a way that the active layer faces feed solution,fodder growing system and the support layer faces the draw solution.
One of the exceptions is touse forward osmosis with the function of damping osmotic pressure. In this membrane orientation, when the solution is drawn from the feed solution and enters the active layer to the support layer, the feed solution can be diluted in the pores of the support layer and its surface, thus causing dilutive external concentration polarization and dilutive internal concentration polarization respectively. To be more concise, the solution that lingers in the support layer has greater osmotic pressure than that of the feed solution. When the solute from the process is transported by porous support and active layer, it can further dilute the outlier of the draw solution, causing dilutive external concentration polarization. Selecting a good draw solution is crucial for the FO process. The ideal DS should have high solubility, high osmotic pressure, and stability. Non-toxicity of the draw solution has little to no effects on the performance and structure of the FO membrane. There are three categories of DS that are generally recognized: inorganic DS, organic DS, and other DS such as nanoparticles. Currently, inorganic draw solutions are most widely used in FO technology. They usually have extremely high osmotic pressure due to the small inorganic molecular mass and high solubility, which makes them more favorable in dealing with hypersaline wastewater. However, in the reverse osmosis process, the inorganic draw solution could increase the salinity of the feed solution. The mainstream of inorganic DS is ammonium bicarbonate and sodium chloride. In 2005, McCutcheon and Elimelech et al. conducted forward osmosis experiments using ammonium bicarbonate as the draw solution and achieved ideal results; through heating, ammonia-carbon dioxide can be regenerated. Nevertheless, there can still be a certain amount of ammonia gas present in the water. As a result, in more practical applications and pilot-scale tests, ammonium bicarbonate is the most widely used draw agent.
Ammonia and carbon dioxide are evaporated in the form of gas, which is effective for recovery and re-concentration. Since the ammonium bicarbonate extraction and recovery system can make full use of low-grade waste heat and reduce energy consumption, it is especially practical for places with available waste heat, such as thermal power plants, and regions with abundant solar.As the population increases rapidly, the demand for irrigation raises correspondingly. Almost 70% of the global water is used to irrigate. At the same time, freshwater demand is raising, water reuse treatment process and drinking water treatment process became vital technologies nowadays. Under most situations, wastewater reuse and seawater reuse are a large portion of the water reuse system. However, brackish groundwater reuse became an emergent freshwater resource recently. Brackish groundwater is often located at depths of 4,000 feet or deeper under the Earth’s surface, and it has a dissolved concentration between 1,000 to 10,000 milligrams per liter . Brackish groundwater could be used for power generation, aquaculture, industry, and public drinking water supply. There are profuse brackish groundwater resources located in the United States, including Utah, New Mexico, Arizona, Virginia, Nevada, Texas, California, Idaho, and Colorado. For instance, Texas has an estimation of 2.7 billion acre-feet of brackish groundwater; In New Mexico, 75 percent of the groundwater is too saline to use without any treatment. According to what has been discussed previously, there are bountiful resources of brackish groundwater in the United States, and one of the common implements of treated brackish groundwater is direct fertigation since there are lavish nutrients in the groundwater. The combination of nanofiltration and fertilizer drawn forward osmosis is an ideal solution for brackish groundwater treatment. Since brackish groundwater has a relatively low total dissolved solid , it requires minor desalination and nutrient removal processes before direct fertigation.
To maintain a qualified number of nutrient components in brackish groundwater for direct fertigation, researchers have compared different models combining nanofiltration and forward osmosis. The first model is fertilizer drawn forward osmosis alone without nanofiltration, the results have shown that treated water samples still contain excessive nutrients for plant growth, which indicates that the water quality would not qualify for direct fertigation. The second model applies Nanofiltration as a pre-treatment. This model can remove most of the scaling and organic fouling species, enhancing the performance of fertilizer drawn forward osmosis. However, scaling became one of the major issues due to the excess amount of scaling ions . The third model applies nanofiltration as posttreatment, this system not only has the highest reduction rate of fertilizer nutrients but is also able to recycle the excess nutrients for further reuse as draw solutions. For all the models above, researchers applied an NE90 membrane with an MWCO of 220kDa. Generally, a1KDa MWCO refers to about 1.3 nm in membrane pore size, whereas 220KDa corresponds to a pore size of 3.84 nm. Out of variousfertilizers were tested, ammonium phosphate monobasic , ammonium sulfate ,chicken fodder system and mono-potassium phosphate have the highest reduction rates of nitrogen. Research has shown that ammonium sulfate contains the highest water recovery rate at 76%. Potassium dihydrogen phosphate has a second ranking water flux recovery of up to 75% while ammonium phosphate monobasic shows the lowest nutrient concentration among three of them. FDFO demonstrates its potential with fertilizer draw solution, which acts as a low-energy osmotic dilution. Researchers also proved that most fertilizers can be used as draw solutions, different combinations of various draw solutions can have numerous removal rates for a certain nutrient. . For instance, the combination of KCI and NH4H2PO4 can result in a lower concentration of N/P/K , which shows a higher nutrient removal rate than using KCI or NH4H2PO4 individually as draw solution. Moreover, different draw solutions /fertilizers have different rejection rates of nitrogen compounds. For example, Urea has a lower rejection rate compared to ionic compounds, such as nitrate and ammonium. This phenomenon indicates that Urea may have a higher nitrogen organic removal rate after ammonification. It is proven that the hybrid system of fertilizer drawn forward osmosis with nanofiltration as a post-treatment has the most effective removal rates of nutrients when it comes to brackish groundwater treatment. When nanofiltration is applied as pre-treatment, the system has a higher removal rate on scaling precursor ions and organic fouling species treating brackish groundwater. When nanofiltration is served as post-treatment, the nitrogen removal rate is the highest compared to the FO alone without NF and NF applied as a pre-treatment. The system can also recycle excess nutrients for further reuse as draw solutions when NF is applied as a post treatment. The water flux is analogously higher when this hybrid treatment process is orientated as pressure-retarded osmosis instead of normal forward osmosis mode.
Integration of nanofiltration with fertilizer drawn forward osmosis can reduce the nutrient concentration to meet the water quality standard for direct fertigation. It brings the nitrogen input in fertigation to a lower scale compared to the standard scale. This hybrid system can also adjust the input of different nutrients for varied types of crops/situations. The sources of N2O are mainly from microbial processes in soil and oxidation of NH3 in fertilizer. This research focuses on exploring the agricultural factors and providing solutions to the issue of redundant N2O emissions. At the same time, managing these controllable factors can reduce agricultural emissions by applying water treatment methods. Since California contributes 12% of the national food production, reducing N2O emissions could have a consequential effect on air quality and public health. Studies show that exposure to long-term N2O would cause ebbed lung function and asthma, especially to young ages. People that live nearby farms have a higher risk of getting respiratory diseases. Moreover, accession of N2O in water is caused by the excess nutrient runoff to the river. According to this review, N2O emissions can be reduced significantly by managing the fertigation nitrogen input appropriately. Consequently, the air quality and water quality could be improved by reducing Nitrous oxide emissions. A higher Nitrogen input fertilizer could increase the loss of certain plant species and the death of marine organisms. At the same time, low-nitrogen fertilizer would not be as nutritious as nitrogen fertilizer, it might slow down the growth rates of plants and crops. Since fertigation is commonly used in agriculture, relatively low nitrogen input could have a negative impact on the efficiency of crop production. As a result, the nitrogen amount in fertigation should be controlled to a certain amount to maintain the balance. Besides the dinitrogen and nitric oxide emission from soil denitrification, agricultural Nitrous oxide emission has the dominant contribution to the total greenhouse gas emission. According to previous studies, Nitrate oxide is 300 times more harmful than carbon dioxide towards climate change. Nitrate oxides in the atmosphere contained 270 parts per billion in 1750, and it has increased to 331 parts billion in 2018. The increasing rate of Nitrate oxide in the atmosphere breaks the record every 5 years. In the year 2021, the global temperature increased conspicuously, one of the reasons is the overt Nitrate oxides emission since the food demand is rising every year with the population growth. This review explores the relationship between agricultural factors of N2O emissions and water treatment solutions. The result of this review shows that agricultural N2Oemission is related to different factors including soil oxygen content, soil porosity, soil organic carbon content, soil temperature, PH value of soil, soil bacteria content, and Nitrogen input in soil. By adjusting these external factors, including limiting the supplement of oxygen, reducing soil water content, choosing the soil with a lower porosity , increasing the soil PH values, increasing the soil organic carbon content, etc., lowering the nitrogen input to prevent over-fertilization could be the most effective solution. As a result, the agricultural N2O emissions decrease spontaneously. Nitrous oxide has different impacts on the atmosphere. In the stratosphere, N2O depletes ozone levels by acting with halogen oxides. In the troposphere, N2O is one of the paths depleting ozone. Over 3500 measurements of N2O existences in surface water and marine troposphere, the exactitude for tropospheric, surface water, and marine measurement are 0.3%, 1.2%, and 2.2%. These numbers indicate that almost two-thirds of the worldwide flux of N2O in the atmosphere derives from sources in the northern hemisphere. Data from surface water proposes that the oceanic flux of N2O would be less than 60 Gmol/year. Deep water N2O concentration is estimated using the values of salinity of water, water temperature, water oxygen content, and the water dissolved nitrogen content. Raise of N2O concentration in water is caused by anthropogenic nitrate denitrification, resulting in tremendous depletion of marine life, especially in deep water.