High Recovery Desalination

High Recovery Desalination

Flexible Reverse Osmosis Nanofiltration

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Concentrate Management and Treatment

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Salt Harvesting

Summary & Motivation

Reverse osmosis (RO) and nanofiltration (NF) membrane technologies are used for the purification and desalination of impaired inland water sources to generate safe potable water, but are limited by salts, like gypsum and calcite, which become supersaturated due to the use of antiscalants. This can lead to concentration polarization and scaling at the surface of the membranes, affecting energy consumption and water recovery. At this point, the brine produced from the RO process can no longer be recycled back into the system for more water production. Previous studies show adding gypsum seeds to a supersaturated gypsum solution in a jar tester effectively precipitates gypsum and de-supersaturates the solution. This desupersaturated water can be filtered and recycled into the system for more water recovery. The previous study found gypsum seeds of the same surface area and different initial diameter had the same effect. Adding gypsum, calcite and silica seeds of various sizes in both gypsum saturated solutions and also calcite saturated solutions is being explored to determine the effect of surface chemistry on precipitation. This will involve using pH probes, conductivity probes, a turbidity meter, and a particle size distribution unit to collect data from solutions in a jar tester, which can then be analyzed further. We predict that surface chemistry will have an impact on the seeds’ effectiveness on precipitation, and this research will aid further studies on improving water recovery through seeded precipitation. This research was presented at the North American Membrane Society Conference in May 2024. Our objective with this research is to answer the question: “how is nucleation and subsequent crystallization affected by surface chemistry and can this be used to optimize desupersaturation impacts for water composition with differing targeted mineral salts?​”.

 

Methodology

First, the antiscalant, a compound that prevents scaling and allows for supersaturation of brine, needs to be deactivated through increasing the pH or adding limestone or soda ash. Nucleation sites (seeds) are then added to the solution and the CSTR runs continuously for a set amount of time. Measurements are taken through samples taken from the stopcock every 5 minutes. We expect to see that a larger surface area produces more precipitation given the literature, and are currently working towards experimenting with how the amount of lime added and the surface chemistry of the seeds impacts precipitation.

 

Equipment Used

  • CSTR
    • We employ a compact laboratory mixer, commonly referred to as a “jar tester,” designed for small-scale operations. This equipment features six integrated stirring paddles that can achieve speeds of up to 500 RPM, allowing it to accommodate six 1-liter clear beakers simultaneously. The mixer is versatile and supports a variety of functions, including coagulation, flocculation, and crystallization.
  • SEM (Scanning Electron Microscope)
    • A scanning electron microscope (SEM) is an advanced imaging tool that utilizes an electron beam to systematically scan and capture detailed images of a sample’s surface. The signals generated by a SEM arise from the interactions between the electron beam and the atoms within the sample at varying depths, leading to the production of secondary electrons (SEs) through ionizing radiation. These secondary electrons are highly localized at the point of impact on the sample’s surface, enabling the SEM to produce images with an impressive resolution of up to 1 nanometer. Samples are collected at specified intervals and filtered through a 5-micron filter. Subsequently, these samples are transferred onto carbon paper, which requires specialized preparation to enhance their electrical conductivity and ensure stability under high vacuum conditions, as well as to withstand exposure to high-energy electron beams.
  • Turbidity Meter
    • A turbidity meter employs light to quantify the amount of light scattered by particles present in a sample. This measurement is utilized to monitor particle growth throughout the duration of the experiment.
  • pH Probe
    • A pH probe is utilized to assess the hydrogen ion concentration in a solution, thereby determining whether the solution is acidic or basic. In this experiment, continuous pH monitoring is essential, as even minor fluctuations in pH can hinder nucleation and subsequently slow down the growth of particles.
  • Calcium Ion Probe
    •  A calcium ion probe is employed to accurately measure the concentration of calcium ions in the solution. This measurement serves to indicate whether the calcium concentration is decreasing, which would suggest that calcium is precipitating out of the solution, or increasing, which would imply that the nucleation seeds are dissolving.
  • Conductivity Meter
    • A conductivity meter is employed to assess the conductivity level in a solution. Conductivity is directly related to the concentration of ions present; therefore, this measurement can serve as an alternative method for monitoring calcium concentration in the solution.
  • Particle Sizer and Counter
    • A particle sizer and counter is employed to characterize particles in solution. Throughout the course of the experiment, samples are collected at regular intervals and subsequently analyzed using the counter to determine the median particle sizes. The resulting sample distributions are graphed, revealing a noticeable shift of the median peak to the right in the later samples. This shift indicates a change in the size distribution of the particles over time

 

Imaging Results

Conclusions

  1. Preliminary desupersaturation of calcite supersaturated RO concentrate was demonstrated using calcite and gypsum seeds.​
  2. Seeds selection should be done based on the solubility of the seeds in the supersaturated RO concentrate.
  3. Overall water recovery can be maximized via intermediated seeded accelerated desupersaturation​.
  4. Effect of sand, silica and calcite of different sizes will be tested in order to explore the effect of surface chemistry on RO concentrate desupersaturation

 

Targeted Irrigations

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