Process water treatment : membrane technology - membrane filtration - water reuse - reuse of process water - degassing of liquids - membrane degasification - RO after MBR
Water treatment - membrane filtration
Membrane filtration techniques are used for very specific applications in the field of industrial waste water treatment. Membrane filtration is often applied as final step in the treatment of wastewater, usually within the scope of water recuperation or reuse. However, this technology is also used for the treatment of very specific wastewater and processwater flows, such as for the treatment of oil-emulsions and other specific emulsions. Based on the separative capacity of the membranes, there are mainly four different steps to be distinguished in the field of membrane filtration :
- microfiltration : separation of approx. 0,1 to 1 μm size particles (big proteins, yeast cells, microorganisms, …). The principle of micro-filtration is physical separation. The extend, to which particles are seperated depends on the membrane pore size. Particles bigger than the membrane pore size will be completely separated, particles smaller than the membrane pore size will only be partly removed, depending on the dirt layer on the membranes. Conventional pressures between 0,1 and 3 bar. Possible applications in the field of wastewater treatment are e.g. : microorganism separation from wastewater, separation of oil/water emulsions, pretreatment of waste and process water prior to nanofiltration or reverse osmosis.
- ultrafiltration : separation of approx. 0,01 tot 0,1 μm size particles (organic substances, oil emulsions, protein molecules such as als gelatin, bacteria, etc.). Conventional pressures between 3 and 5 bar.
- nanofiltration : separation of approx. 0,001 tot 0,01μm size particles (bacteria, viruses, sugars, pigments, sulfur, water softening).
- reverse osmosis : separation of approx. 0,0001 tot 0,001 μm size particles (salt solutions, metal ions, nitrates, etc.). Conventional pressures between 10 and 60 bar.
Most filtration techniques can only separate undissolved particles from the water. Microfiltration and ultrafiltration can only separate the suspended solids from the pretreated wastewater. This way for instance, dissolved salts, pigments, metal ions (influencing the conductivity of the wastewater) and sugars remain present in the wastewater. Consequently the COD/BOD values of the wastewater treated with ultrafiltration and microfiltration techniques will remain too high to be reused in the production process. Only the last step, reverse osmosis, will allow to effectively separate salts, metal ions etc. During the reverse osmosis process the membranes will have to support very high pressures. Only high-technological and pressure resistant membranes are having the right features for this kind of application.
Membrane filtration - reverse osmosis
This technology is allowing to obtain the highest degree of purification : physical, chemical, organical and bacteriological. For this technique a semipermeable membrane is used. In the area of wastewater treatment, the reverse osmosis technology is used for the production of ultrapure water for industrial processwater, for nitrate separation, desalination of briny water and seawater, …
Reverse osmosis technology : working principle
Osmosis is based on a natural phenomenon. When two liquids with a different concentration are divided by a semipermeable membrane, the pure water will flow through the membrane from the least concentrated liquid to the most concentrated liquid. This natural action restores the osmotic balance of the two solutions.
The reverse osmosis technology involves the application of mechanical pressure on the most concentrated solution, this pressure is to be higher than the osmotical pressure. This way, the osmotic pressure redistribution phenomenon through the membrane will reverse itself, allowing to obtain pure water.
Membrane filtration : different types of membranes
To counteract the fouling problem of the membranes (surface and pores) , every manufacturer has its own range of high-tech features (ao. asymmetric membranes, special properties of the membrane surface, hydrophilic versus hydrophobic membranes, crossflow filtration). Moreover, the membranes often have to be permanently resistant to solvents, high temperatures and very diverse pH degrees. That's the reason why the new generation membranes have become much more performant in comparison to the old generation membranes; this way the necessary replacement of the membranes can be postponed for longer and longer periods, allowing to compensate increasingly for the high purchase price of the membranes. There are 3 main types of membranes : tubular membranes, hollow fiber membranes and spiral wound membranes. Each type has it's own preferential application area.
Membrane techniques in wastewater and process water treatment : advantages
- material reliability
- less chemicals required
- relatively simple follow-up, once the installation is correctly set
- efficient use of energy
- no change in state of aggregation necessary
Membrane technology in wastewater and process water treatment : disadvantages
- high purchase price of the membranes
- residue (very concentrated filtrate) has to be collected or further treated
Membrane filtration in wastewater and process water treatment : possible application areas
- oil/water separation, treatment of lyophilic substances
- recycling of car wash water
- treatment of liquid manure
- treatment of wastewater from the cosmetics industry
- treatment of wastewater from the food and beverage industry
- filtration of suspended solids out of wastewater
- biomass separation
- cooling oil separation
- ink separation in the flexographic printing industry
- reuse of process water
- rain water recycling
- reuse of several kinds of effluent*
- membrane degasification of water and liquids*
Degassing of process waters - deaeration of liquids - degassing of liquids : * Special techniques
More and more, membranes are used for the degassing of liquids and water, with very good results : this technique allows to reduce the oxygen content of various kinds of process waters to ppb level. In a degassing installation, the gasses/air dissolved in the water, mainly dissolved oxygen, but also CO2 and NH3 can be separated from the water/liquid by means of so-called membrane contactors. The degassing process is driven by creating a vacuum and/or use of strip gas (N2) allowing to change the equilibrium between gaseous and liquid state. Using membranes for this purpose allows to fit a compact skid installation in an industrial environment, without big adjustment costs and/or important factory space consumption for the customer. Important economies can be realized at operational cost level; for this technique for instance industrial nitrogen can be used, rather than the pure (and more expensive) version, providing a considerable cost reduction on a yearly basis. In an industrial environment, the degassing membranes are mainly used in following applications :
- reducing oxygen/O2 levels of various kinds of process waters to ppb level
- CO2 separation from various kinds of process waters
- ammonia separation from various kinds of process waters
- separation of gasses from liquids/beverages in the food/beverage industry
- corrosion control in industrial environments
- controlled and advanced carbonation and nitrogenation of beverages
Wastewater reuse – combined RO (reverse osmosis) and MBR (membrane bio reactors) installation – cost saving measures : * Special techniques
Water is getting more and more expensive anywhere in the world. Companies are paying more and more for waste water disposal. This is leading to an increasing demand for reuse of waste water, certainly for the big water consumers, such as slaughter houses, laundries, food and beverage industry, cosmetic industry, textile industry, etc.).
Inserting a RO installation in addition to a MBR unit, is allowing to reuse a substantial part of the wastewater (approx. 60 to 80 %) in the production process. For the companies concerned the savings on water supply and wastewater disposal are considerable, allowing to recover the investment relatively soon. Knowing that water cost will continue to increase for companies all over the world, measures taken to reduce this water cost to the maximum are absolutely crucial. The exploitation cost for such a combined RO – MBR installation, according to the customer’s specific needs, can e.g. only represent a third of the cost of city water.
For each specific project there is a previous feasibility check, by means of performance and dimensioning test on site. Whenever appropriate a test installation is installed for a lost check-up. The final full scale installation is entirely custom-made, in function of the wastewater type, the aimed quality for reuse and taking into account the economic aspects of the project. The MBR unit in container, as well as the RO unit mounted on skid, come as compact entities, easy to integrate in an existing water treatment environment. The RO installation is fully automated and completely remote controlled.
We dispose of 30 years of knowhow in the field of membrane technology and can present four specific references so far (at the end of 2015 to first installation has been put into service successfully, 3 more installations are to follow at the beginning of 2016).