In reverse osmosis (RO) desalination processes, the use of phosphonates prevents scaling, thus allowing for a higher product water recovery, which increases the efficiency of the process. However, a major concern associated with their use in RO desalination is the high cost and environmental impacts associated with the discharge of the waste brine or membrane concentrate containing phosphonates. Therefore, technologies are needed that can remove and recover phosphonate antiscalants from membrane concentrates. Chapters 2 to 5 of this thesis describe a process for the removal and recovery of phosphonate antiscalants by using adsorption technology. In Chapter 2 the phosphonate adsorption capacities of two commercially available anion exchange resins and activated carbon were compared to that of the cheap waste material iron-coated waste filtration sand (WFS). The results presented showed that, in contrast to the exchange resins, the equilibrium adsorption of nitrilotris(methylene phosphonic acid) (NTMP) on WFS is not suppressed at increasing ionic strength and is much less affected by the competitive anions carbonate and sulfate. The strong affinity of phosphonate with the iron oxy-hydroxide in the coating resulted in a relatively high adsorption capacity for NTMP of this waste material. Iron oxy-hydroxides perform very well in adsorbing phosphonates from membrane concentrates. Therefore, an iron oxy-hydroxide was selected that, in contrast with WFS, has a high purity and can be obtained commercially. Granular ferric hydroxide (GFH) was investigated as an adsorbent for NTMP in Chapter 3. Both the equilibrium and kinetics of NTMP adsorption on GFH were investigated. The adsorption kinetics were predicted fairly well with two models that considered either combined film-pore or combined film-surface diffusion as the main mechanisms for mass transport. It was demonstrated that phosphonate is preferentially adsorbed over sulfate by GFH and that the presence of calcium is beneficial for the adsorption process. Calcium causes a transformation in the equilibrium adsorption isotherm from a Langmuir type to a Freundlich type with much higher adsorption capacities. Spent GFH is reusable after regeneration with a sodium hydroxide solution, showing that NTMP can be recovered from the RO concentrate. In analogy with Chapter 3, the adsorption and desorption of NTMP from RO membrane concentrate on iron-coated waste filtration sand (WFS) has been investigated in Chapter 4. Equilibrium adsorption was described well with a Langmuir isotherm. Although the low cost and on-site availability of WFS is advantageous over GFH, the results revealed some drawbacks. WFS appeared to have a much lower adsorption capacity compared to GFH, which was related to the presence of impurities, the presence of manganese oxides, and aging of the ferrihydrite phase in the coating of WFS. The aim of Chapter 5 was to employ GFH in a packed bed adsorption column. The effective diffusivities and external film mass transfer coefficients estimated in Chapter 3 were used to predict the concentration of phosphonate in the effluent. Also, the regeneration of the saturated column with sodium hydroxide solution was investigated. In addition, it was investigated whether the regeneration solution containing the recovered phosphonate could be further concentrated by using a nano-filtration or a calcium-phosphonate precipitation step. The use of nano-filtration seemed to be more attractive. The first five chapters show that adsorptive removal of phosphonate antiscalants offers a viable way to improve RO concentrate treatment processes and enables the recovery of the phosphonate for reuse in the RO desalination process. Another way of tackling the unwanted discharge of phosphonates is minimizing their use. Smart sensors that predict the risk of scaling at an early stage can help to control the dosage of phosphonate antiscalants. This will allow for minimum usage of phosphonates without the risk of scaling. Chapters 6, 7, and 8 contribute to the development of such a sensor. Focus was on the development of the actuator part of the sensor that enhances crystal growth and precipitation by ultrasonic irradiation. In Chapter 6 the effect of ultrasonic irradiation on the crystallization of calcite was investigated. Seeded calcite growth experiments were conducted under constant composition conditions while the applied ultrasonic irradiation created cavitation bubbles throughout the suspension. In this way it was demonstrated that ultrasound enhances the crystallization rate of calcite substantially (i.e., 46 %), due to the ability of the generated cavitation bubbles altering the crystals’ habit and size. The increased surface area available for crystal growth resulted in enhancement of the observed crystallization rate. In Chapter 7, the cavitation phenomena that are responsible for the previously observed volumetric crystallization rate enhancement were visualized using high speed photography. Cavitation clusters cause attrition, disruption of aggregates and deagglomeration, whereas streamer cavitation causes deagglomeration only. Cavitation inception on the surface gave the small crystals momentum. However, it was shown that breakage of accelerated crystals by interparticle collisions is unrealistic because, upon bubble collapse, they subsequently experienced a deceleration much stronger than expected from drag forces. These direct observations contradict the general assumption that interparticle collisions always play an important role in particle attrition by cavitation. Scanning electron microscopy pictures of irradiated calcite crystals showed deep circular indentations, possibly caused by shockwave induced jet impingement. Moreover, the appearance of voluminous fragments with large planes of fracture indicated that acoustic cavitation can also cause the breakage of single crystal structures. The possibility of using ultrasound as a tool to enhance the demineralization of supersaturated calcium carbonate solutions (e.g., membrane concentrates) containing growth inhibitors was investigated in Chapter 8. The inhibiting effect of the phosphonate NTMP on crystal growth can be mitigated by ultrasonic irradiation. The results can be explained in part by breakage and attrition of poisoned crystals, resulting in an increase in fresh surface area. Mass spectroscopy analysis of sonicated NTMP solutions revealed that ultrasound can also degrade NTMP. These observations confirm in part the hypothesis that ultrasound can be used as actuator. As an alternative to the removal of phosphonates or minimizing their use by smart sensoring techniques, phosphonates may also entirely be replaced by environmental friendly antiscalants, which is the subject of Chapter 9. The effectiveness of such an alternative, carboxymethyl inulin (CMI) biopolymers, in inhibiting calcium carbonate crystallization was compared to two phosphonate antiscalants. Compared to the phosphonates, the biopolymers exhibited a stronger inhibitory effect on the crystal growth of calcite. It was shown that the ability of the biopolymers to mitigate the spontaneous precipitation of calcium carbonate is controlled by their degree of carboxylation. The biopolymers can affect the crystal habit similar to the phosphonates, which suggests that their function as crystal growth inhibitor is comparable. These results demonstrate that CMI biopolymers are effective calcium carbonate crystallization inhibitors, indicating they can replace phosphonates as antiscalant. In Chapter 10, the results presented in this work are being discussed and, where possible, placed into perspective of future desalination developments.