Sustainable disposal of brine

To meet growing water demand, more water is being harvested from nontraditional sources like brackish water from deep underground aquifers. Since these sources contain salts, drinkable water is produced by separating salt and other minerals (TDS, Total Dissolved Solids) from water through a process commonly known as desalination. A typical desalination plant produces 50-60% potable water from brackish water and the remaining water (brine) is disposed in evaporation ponds or by injecting it below the ground surface. To maximize the limited water supply, inland desalination plants have developed technologies to reduce brine production commonly known as zero liquid discharge (ZLD) technology. Although the technology allows maximum water recovery, the produced brine consists of very high TDS (more than 10,000 mg per liter of water) which makes current disposal practices unsustainable. The disposal of such a large quantity of salt in an economical and sustainable environmental friendly manner can only be achieved by using it as a construction material and was the main focus of this research. In this study, it is proposed to use TDS in place of sand to prepare mortar for application in highway infrastructure like vertical moisture barrier or embankment fill material. However, addition of TDS (mainly highly concentrated sodium chloride) weakens the integrity of cement matrix, thus, resulting in lower strength and durability of mortar. To improve durability and reduce leaching of TDS, fly ash and aerobic bacteria were used. The use of fly ash increases the long term strength and durability while reducing required cement content. Any reduction in cement content translates into reduction in carbon footprint because fly ash is a byproduct. In addition, fly ash creates optimum environment for bacterial growth by lowering pH of mortar matrix. The aerobic bacteria were also used to increase compressive strength and stabilize salt by calcite precipitation and by minimizing porosity. Since addition of salt increases pH of the mortar environment, the survivability of bacteria becomes an issue. To survive in high pH (around 12) mortar environment, the bacteria were mutated by exposing them to ultraviolet rays. The advantage of mutation is that the bacteria can withstand higher pH as well as helps in formation of more calcite than normal bacteria. The use of mutated bacteria in association with fly ash not only creates finer pores for bacterial growth, but also lowers the pH of mortar matrix by consuming free lime or calcium hydroxide, formed during hydration of cement. The mutated bacteria, fly ash and salt/TDS were used to prepare mortar specimens. These specimens were subjected to strength and durability tests (such as freeze thaw test, water permeability test and absorption test). In addition to strength and durability tests, micro-level analysis of mortar was performed using X-ray diffraction and scanning electron microscopy techniques. This type of experiments provided the crystallography and mineral information to explain the behavior of samples from micro-scale point of view. The test results indicated that fly ash and microorganism application not only improves the strength and durability, but also stabilized TDS by sealing or reducing the void space of specimens