Membrane filtration
Membrane filtration is a process in which substances are removed from water by separation based on particle size and pressure difference. By applying a specific membrane (semipermeable material) with a specific pore size, the desired separation can be obtained.
Ultrafiltration, also referred to as UF, is a membrane filtration technique in which a liquid is forced through a semi-permeable membrane under pressure. In ultrafiltration, this membrane has a pore size that varies roughly from 0.1-0.01 μm (10-100 nm). Ultrafiltration membranes are mainly used for the removal of suspended and colloidal substances, bacteria and viruses. Ultrafiltration membranes are supplied in a range of configurations. Possible configurations include:
Besides the specific membrane configurations, there are also a number of different operating methods. The two most commonly used methods are dead-end (this is the method Logisticon generally uses) and crossflow. The names refer to the way the nutrient is supplied to the membrane. With dead-end UF, the supply is fed into the hollow membrane and stopped at the end. This way, the water passes through the membrane and the dirt accumulates. The dirt is regularly washed away by means of a backwash. The dirt is then released and can be removed. This is called semi-dead-end operation. If the dirt is too compressed or is sticking too much to the membrane, the backwash may no longer be sufficient. In this case, chemical cleaning will be performed. In cross-flow operation, the liquid is directed along the membrane surface. The permeate passes through the membrane and the dirt stays behind. This creates a concentrated current. In this operation, the dirt is continuously removed by the supply current flowing past.
Reverse osmosis, also called hyperfiltration, is mainly used to remove salts and minerals and therefore to reduce the conductivity. Another effect is that reverse osmosis partly blocks other substances, such as pesticides, heavy metals, medicine residues and more. Osmosis is a natural process involving flow through a semi-permeable membrane. When pure water of the same temperature is present on both sides of a membrane and the pressure on both sides is the same, no water will flow through the membrane. When salt is dissolved in the water on one side, it will stimulate a water flow through the membrane from the pure water to the water with salts. Nature tries to eliminate the difference in concentration, as it were. When pressure is applied to the side where the salts have been added, a new equilibrium is created. Due to this pressure, water will pass through the membrane whereas the salts cannot. This phenomenon is called reverse osmosis. The driving force behind reverse osmosis is the applied pressure minus the osmotic pressure. The energy consumption of reverse osmosis is directly related to the salt concentration, given that a higher salt concentration produces a higher osmotic pressure.
This technique is used, among other things, for the production of process water and boiler feed water and the preparation of drinking water (from brackish or salt water), demineralised water and ultra-pure water. The possible options also include the softening and decolourisation of water, the upgrading of process flows to a higher quality and the purification of waste water aimed at reuse.
Logisticon uses capillary nanofiltration or CapNF membranes. These inside-out hollow fibre membranes are made of polyethersulfone (PES), which, unlike normal spiral wound membranes, can also be backwashed. The advantage of using PES is that it is more resistant to oxidative substances such as active chlorine.
With capillary nanofiltration, surface water or (tertiary) waste water can be purified in one step into high-quality process water or even drinking water. CapNF combines the beneficial properties of capillary ultrafiltration (UF) with the beneficial properties of nanofiltration (NF). This technique is very suitable for the removal of organic dissolved substances (large molecules), colour, micro-pollution, pesticides and partly also ions (calcium, sulphate, phosphate, etc.). And just like a UF membrane, a CapNF membrane also removes bacteria and viruses.
A MBR consists of two parts: a bioreactor and a membrane installation. The biological processes that break down the pollution take place in the bioreactor (activated sludge). The membrane installation separates the water from the active sludge, after which it can be discharged or reused.
The biological processes in a membrane bioreactor (MBR) are almost identical to an active sludge system with the difference that the separation between active sludge and the purified water is achieved by means of membranes rather than settling and extraction with the conventional biological treatments. The advantages of a membrane bioreactor (MBR) over conventional techniques are:
Different types of membranes are used with MBR: Submerged membranes and crossflow membranes.
The submerged MBR membranes work on the basis of negative pressure. The flux (capacity per m² of membrane surface) is relatively low, which means that more membrane surface is required in relation to overpressure (crossflow) membranes. However, the advantage is that a submerged membrane consumes much less energy. So a shift from Opex to Capex. Another advantage is that the membranes are operated from the outside in. In this way, the dirt stays on the outside and cleaning is easier in the event of calamities or extreme pollution.
Besides the use of submerged membranes, there is also the option of applying overpressure MBR membranes, which is the case when using a crossflow membrane unit. The advantage of such a system is that much higher fluxes can be realised (50 – 80 l/m².h) so that the required membrane surface remains limited and a relatively compact installation can be built. On the other hand, one major disadvantage is the higher energy consumption compared to the use of immersion membranes.
One of the newer techniques for removing certain dissolved gases from water uses membranes. With these special membranes, it is possible to remove gas from water in an energy-efficient way and without the addition of chemicals.
The degassing membrane consists of thousands of micro-porous polypropylene hollow fibre membranes (similar to straws) wound around a distribution pipe. Because the hollow fibre membranes are hydrophobic (water-repellent), the aqueous stream cannot penetrate through the pores but the gas can. The gas can be removed at the pores by applying a higher pressure on the aqueous side than on the gas side. If very low values of dissolved gases need to be realised, it is possible to apply a vacuum on the suction side – where relevant, in combination with a stripping gas (usually nitrogen). This creates a driving force that moves dissolved gases from the water phase into the gas phase. In some cases, for example, oxygen levels of <10 ppb can be achieved.
Besides removing known gases such as ammonium, oxygen and carbon dioxide, it is also possible to remove methane from water. Various studies show that methane can be removed from groundwater to achieve low values. In favourable situations, this methane can even be recovered and used as an energy source.
Electro deionisation (EDI) or continuous electro deionisation (CEDI) are used interchangeably and are techniques for making very pure water (demineralised water). The technique consists of a combination of membrane filtration and ion exchange. Unlike a traditional ion exchanger, this regeneration system does not require any chemicals. The ion exchange resin is located alternately between a cation selective and an anion selective membrane. These are the product and reject compartments.
By applying a DC voltage to the CEDI module, the ions in the feed water are transported in the direction of the ion-selective membranes. This conveys them to the surface of the ion exchange resin. The ions pass through the membrane and end up in the reject compartments. They are then disposed of. What remains is very clean water. The direct voltage causes the water to split into H+ and OH- ions. These in turn regenerate the ion exchange resin. Because the selective membranes only allow ions to pass through, a small part (5-10%) of the feed water is used to remove the ions from the reject compartments as a concentrated stream.
