Nanofiltration with zero liquid discharge in drinking water treatment

Abstract

The topic of this thesis is a concept of zero liquid discharge for nanofiltration technology in drinking water treatment. Nanofiltration, defined as a process between Ultrafiltration (UF) and Reverse Osmosis (RO), is a rapidly emerging technology. The origin of NF membranes can be traced back to the late 1950s when it was developed to treat sea water. Now it is applied in drinking water treatment, wastewater treatment and oil separation and also in the food industry. It is even called the future process of 21st century. However, nanofiltration is also a controversial process because of the concentrate problem, especially for inland installations. In the Netherlands, this problem is more considerable due to the need of high quality of drinking water and the decreasing quality of the surface water. Concentrate contains high concentrations of dissolved organic and inorganic compounds in high concentration. The conventional approach of concentrate discharge to surface water becomes a growing problem due to the environmental guidelines from the authorities. The discharge of the concentrate is not the only problem. It is also a cost problem that 20% of the feed water is wasted. In the Netherlands, taxes are paid for extracted groundwater. Also the pretreatment is costly. So with every m3 discharged concentrate money is wasted. Therefore, there is a need for a technology by which the discharge of concentrate is not necessary. That is so-called Zero Liquid Discharge Technology. The primary problem to be solved with zero liquid discharge is the recovery. We expect through an innovative technology the nanofiltration membrane installation can be operated at very high recovery (99%) without increasing the treatment cost of drinking water. The cost for residuals treatment and disposal can be minimized because the amount of the concentrate is decreased about 20 times. A series of pretreatment processes is used for removing the scaling components from the feed water. The scaling components mainly include bivalent ions, silica and etc. Preventing fouling and scaling can guarantee a constant flux, reduce membrane area, lower chemical cleaning frequency, extend the lifespan of membrane and decrease energy consumption. In this research, a pilot experiment was performed with sludge softening, sedimentation, weak acid cation exchange and nanofiltration at Kiwa Water Research. By using this treatment process, the recovery can be handled successfully at 99% for at least 11 days. The pretreatment concept can remove the bivalent ions completely. Sludge softening is used to remove most of the bivalent ions, like calcium, magnesium and barium. The remaining bivalent ions can be effectively removed by weak acid cation exchange. In this way, the waste stream from the ion exchange is reduced. In theory, silica can be removed by sludge softening at high pH as the co-precipitation of of Mg(OH)2 and CaCO3. But in this experiment the removal efficiency of silica is low probably due to the shortage of magnesium in the feed water. After this treatment process, the remaining concentrate (1%) 3 is evaporated, only remaining salt which can be sold or discharged to a waste facility. In order to improve and guarantee the good performance of silica removal by sludge softening, a jar test is performed to define the influencing factor for silica removal. pH and the magnesium concentration can influence the silica removal efficiency. Higher magnesium concentration is necessary for silica removal. Also, a ternary ion exchange model is needed to predict the breakthrough of cation concentration in order to guarantee the good quality of the feed water for nanofiltration. A ternary system is more complex than binary system. This short report can not include all the aspects of this model. This is the first step to build a ternary model. In the first phase, the basic equation has been already found. The basic model concept has been built up, but needs to be checked and improved. And some batch experiments have been done to obtain model parameters like equilibrium constants and kinetic constants. Also two groups of column experiments have been done in order to measure breakthrough curves for Ca2+, Na+ and H+(pH). The experimental result is expected to be compared with the model result in the future. The pretreatment concept consisting of sludge softening at high pH (around 10), weak acid cation exchange in series can remove calcium, magnesium and barium completely. The calcium removal by the ion exchange is quite good. After pH breakthrough is reached, calcium still can be removed by the resin because it is exchanged with the sodium on the resin. With high magnesium concentration, silica concentration also can be reduced by the sludge softening to some extent. This combination of the treatment processes is possible to make the recovery of NF membrane reach 99% without scaling at least for 11 days. To put this innovative concept into practice needs lots of efforts on validation and testing. Stable operation of pilot experiment at 99% for a longer time is needed to check the feasibility of this concept. It is interesting and significant work. With increasing demand of drinking water, more and more people in this world needs this kind of technology to improve their living condition and environment. We expect the occurrence of this new big step of drinking water treatment technology.