Filtration Technologies

Filtration is the physical removal of suspended matter through a mechanical or chemical reaction. This is one of the most important steps in the planning/design of a water treatment plant and can mean the success or failure of a plant or project.

Media Filtration

Sand and other types of specific media filtration are used for the removal of suspended matter, as well as floating and sinkable particles. The wastewater flows vertically through a fine bed of sand and/or gravel. Particles are removed by way of absorption or physical encapsulation. If there is excessive pressure loss on the filter, it must be rinsed.

Application

Sand filters are used in various sectors and processes, where removal of suspended matter from water or wastewater is required, such as in the following examples:

  • Iron-removal from groundwater using aeration and sand filtration.
  • Final purification of wastewater, follow-up to metal precipitation and sedimentation.
  • Final purification of wastewater produced in the production of iron, steel and non-ferro alloys.

Also used as final purification (or prior to active carbon filtration) to permit re-use.

Process

In continuous filters (often upward-flowing filters), the polluted sand is removed, rinsed and re-used continuously, without interrupting the filtration process. Discontinuous filters (often downward-flowing filters) are stopped, and a rinse takes place in the opposite direction. Air bubbles are blown into the sand bed to make it swirl around. Filtered water then flows through the filter bed in the opposite direction. The polluted matter is released and flows away along with the rinse water. The filtration process can then resume.

Advantages

  • Simple system and media can be replaced with relative ease.
  • System is flexible and can be used as pre-treatment or as a polishing filter.
  • One filter can have large flow capacities.

Micro-Filtration

Micro-filtration is the filtration of a liquid suspension through a membrane with pores of approximately 0.1-10 um in diameter. It is a pressure-driven process used to retain microorganisms and other suspended solid particles from a process liquid while transmitting solutes that are smaller than the selective membrane pore size.

Application

Micro-filtration usually serves as pre-treatment for ultrafiltration, or as post-treatment of media filtration methods. Suspended particles, including sediment, algae and even bacteria and protozoa such as Cryptosporidium and Giardia will be retained to some extend. It is an alternative to classic sand filtration.  Further, crossflow MF is used in the:

  • Dairy industry (cheese, milk).
  • Food industry (clarification of fruit juice, wine, beer).
  • Metal industry (oil/water emulsion separation).
  • Textile industry (effluent treatment).
  • Pharmaceutical industry (cold sterilisation).

Process

Aquest Colsen can provide various micro-filtration technologies, including bag filtration, high-flow cartridge filters, self-cleaning filters, and membranes. Depending on the application and feed quality, MF membranes can be operated in either dead-end or crossflow mode.

Advantages

  • Low operating pressure required;
  • Low energy consumption for semi dead-end set-up, compared to nano-filtration or reverse osmosis;
  • Relatively cheap;
  • No energy-consuming phase transfer needed, such as e.g. evaporation techniques;
  • Quality of the produced permeate is not determined by the management.

Ultra-Filtration

Ultra-filtration (UF) is a pressure-driven membrane processes. The ultra-filtration process uses a membrane which, only allows particles smaller than 20 nm to pass through it. The pore size varies between 20 nm and 0.1 microns.

Application

UF is suitable as pre-treatment in the preparation of drinking water or process water. It has excellent properties for removing suspended matter and bacteria. UF is implemented in the following industries and processes:

  • Chemical industry;
  • Textile industry;
  • Dairy industry;
  • Food industry;
  • Pre-treatment for nano-filtration or reverse osmosis;

Process

As feed passes through the membrane barrier, a filter cake builds up on the membrane surface, increasing the resistance to flow. Short reverse flow of filtrate removes the retained solids from the membrane fibre surfaces. When the build-up of foulants on the membrane modules increases the trans membrane pressure or reduces flow rate, a CIP cleaning cycle is usually required. Aquest Colsen provides UF systems that are fully automated, which includes the automatic chemical cleaning of the membranes, while a human interface module allows the operator to make limited adjustments.

Advantages

  • Low operating pressure required (higher than MF);
  • Lower energy consumption than nano-filtration or reverse osmosis;
  • Good permeate yield depending on the supply water and membrane choice;
  • Disinfection through removal of bacteria. To a certain extent, UF allows viruses, phage, colloids and macro molecules to be removed.

Nano-Filtration

Nano-filtration (NF) lies between ultra-filtration and reverse osmosis, with typical pore size cut-off values in the range of 150-500 Dalton, depending on the molecular structure. A nano-filtration membrane is also ion-selective. This is the ability to distinguish various ions from one another, which makes it ideal for specialized industrial effluent treatment.

Application

Aquest Colsen has implemented NF technologies in various industrial applications, such as CIP effluent recycling in the food and beverage industry, and colour removal / chemical recovery in textile effluents. NF is also used in the treatment of fresh, process and waste waters. NF membranes are also used for the removal of natural organic matter from water, especially tastes, odours and colours, and in the removal of trace herbicides from large water flows. They can also be used for the removal of residual quantities of disinfectants in drinking water.

Process

NF systems usually operates the same as any normal reverse osmosis process, but usually operates at lower pressures. Because of the wide range of NF application, the rejection of a NF membrane needs to be determined via experimental filtration tests with pre-selected molecules. Aquest Colsen offers lab-scale and pilot studies to be able to provide the best water treatment solution to the client.

Advantages

  • Reduction salt content and dissolved matter content (TDS) in brackish water;
  • Reduction in heavy metals;
  • Reduction in nitrates and sulphates;
  • Reduction in colour, tannins and turbidity;
  • Softens hard water when specific softening membranes are used;
  • chemical-free, e.g. needs no salt or chemicals during operation;
  • pH of water after nano-filtration is normally non-aggressive;

Reverse Omosis

Reverse Osmosis (RO) is a pressure-driven membrane process with a separation range between 0.1 and 1 nm. Thus reverse osmosis membranes have high retention for bacteria, viruses and micro particles. Bivalent and some univalent ions are also blocked by the membrane.

Application

Aquest Colsen uses RO technologies mainly for salt removal in brackish borehole water and sea water, in preparation for drinking water. In the industrial sector, process water with very low TDS can be produced through RO, making it ideal of boiler water supply. High quality process water is also used in the manufacturing of chemicals and food and beverage industries.

Process

Reverse osmosis is a water filtration/purification process where the membrane acts as a physical barrier to the hydraulic flow. Pressure is built up to overcome osmotic pressure, allowing selective permeation of the solvent (water) and partial or total retention of the remaining dissolved substances. The product water is demineralised or deionised water, known as permeate, and concentrate water containing rejected substances, known as brine.

Advantages

  • High rejection for salts and particular univalent ions (up to >99%);
  • Reduction in colour, tannins and turbidity;
  • Softens hard water;
  • Disinfection, including viruses.
  • Proven technology for desalination.