Pre-treating raw water provides protection and extends the service life of the filters downstream, reducing maintenance costs and energy consumption, and ensuring the reliability and economy of membrane filtration.
Pressure-driven membrane processes
Membrane processes are seen in increasingly diverse industries ranging from textiles (treatment of sizing chemicals), food (beverage filtration), electro-plating (pickling bath and washing water treatment), pharmaceutical (dialysis, product preparation), chemical (catalyst separation), and metal working (splitting of oil/water emulsions), to the production of drinking water and service water from seawater or surface water, right up to the treatment of industrial and municipal water and waste water (membrane bio-reactors).
While many of these applications are niche segments, the markets for seawater desalination (nanofiltration/reverse osmosis) and for decentralised waste water treatment using submerged membranes (micro/ultrafiltration) have by now increased to several thousand million Euros.
Pressure-driven membrane processes can be sub-divided into: Microfiltration (MF) for removal of suspended substances; Ultrafiltration (UF) for concentration/ fractionation of macromolecular solutions; Nanofiltration (NF) for removal of dissolved organic and inorganic substances; and Reverse Osmosis (RO) for desalination of water-based solutions.
Pre-filtration before membrane systems
All waste water and water treatment processes where membranes are used are fundamentally multi-stage processes.
Microfiltration and ultrafiltration require only pre-filtration of coarse contaminants (>50 μm), whereas nanofiltration and reverse osmosis require the raw solution to be free of particle and colloidal contamination. This explains why good pre-treatment of the raw water is so important, to be able to ensure reliable and economical membrane filtration.
In addition to mechanical pre-cleaning using conventional filters, chemical and physicochemical processes are also often used in the pre-treatment for membrane processes.
Pre-treatment generally begins with coarse filtration, which removes the coarse-grained solid material that can penetrate the system from the raw water. Screens or coarse filters in sizes 1 to 5 mm are used at this stage. In the next stage, the raw water is further treated to obtain the required raw water quality for membrane filtration. This can be done by using mechanical fine filtration and also in combination with physicochemical processes such as coagulation, flocculation and precipitation. Membrane filtration is usually followed by an after-treatment process such as disinfection.
Depending on the quality of the raw water and water quality required after treatment, the engineering processes are adapted to the special requirements.
Pre-filtration upstream of membranes is an important market sector for the Hydac Process Technology division. Hydac products such as the automatic back flush filters, AutoFilt RF3/ RF4/ RF5/ RF7; the AutoFilt TwistFlow Strainer ATF; and an array of strainer filters and inline filters are available for these applications. With its new depth filter PLF1, Hydac has the perfect high-flow fine filter, which will protect the membranes effectively.
Examples of typical pre-treatment processes
Cost reductions in water treatment are achieved primarily through well-engineered pre-treatment methods because they have a sustained impact on the downstream membrane processes. In this process, the pre-treatment consists essentially of a combination of coarse screens, back-flush filters and candle filters.
If a sand filter is used for water treatment, the RF3 can also be installed upstream of it in particular cases. However, since the sand filter is a depth filter which, when operated correctly, can remove foreign matter right down to a few micrometers nominal in size, it has to be decided on a case by case basis to what extent a back-flush filter can relieve the sand filter.
Increasingly membrane filters are also used in the pre-treatment stage. Here microfiltration or ultrafiltration membranes are used to treat the raw water to a degree where it can be fed to the reverse osmosis system.
The pre-treatment processes used in the production of drinking water, service water and process water are inherently varied and unique. Hydac filters can, in many cases, make a significant contribution to the pre-filtration process, reducing operating costs and increasing operating reliability.
Benefits of AutoFilt TwistFlow Strainer ATF
(Up to 400 m³/h; Up to 16 bar; > 200 μm)
Increases the reliability of the process during pre-treatment before membrane systems, and the service life of downstream filters; reduces maintenance costs and energy requirement; small space requirement; flexible design; self-cleaning robust stainless steel filter elements; no recurring charges; can be retrofitted; economical; and cost-effective.
Benefits of AutoFilt RF3 automatic back flush filters
(Up to 7,500 m³/h; Up to 40 bar; > 25 μm)
Extends the service life of the filters downstream; reduces maintenance costs and energy consumption; increases process reliability by thoroughly removing sand particles prior to the membrane filtration process; small space requirement; flexible design; self-cleaning robust stainless steel filter elements; no recurring charges; can be retrofitted; and economical.
Benefits of PLF1 process inline filters
Very large filter area per element (> 5 m²); filtration rating from 3 μm to 90 μm; compact design with high flow rates; clean side protected during element change by a fixed support tube; modular design ensures perfect adaptation to every application; low pressure drops due to large flow paths and filtration areas; significantly easier handling than standard disposable elements; shorter maintenance times; high dirt-holding capacity, filtration performance and fluid compatibility; and completely incinerable.
Benefits of inline filters
Long service life due to large filter areas; filter materials with high contamination retention capacity; low pressure drops because of flow optimised design construction; compact, easy-to-service filter housing; high economy due to predominantly cleanable filter elements; consistently high quality according to ISO 9001; and robust construction.
Case Study: Installation of Hydac RF3 filter before candle filters
In many water treatment plants where drinking water is produced from bank filtrates, the treatment process typically involves pre-treatment using fine filtration with downstream reverse osmosis.
Contamination retention capacity and service life of the fine filters can be crucial to the economy of the whole system.
A Hydac automatic back-flush filter AutoFilt RF3 was retrofitted upstream of the fine filters as part of a pilot project to extend the service life of the candle filters.
This study shows that the back-flush filter provides ideal protection for the candle filters. The particle contamination from fine sand can be removed continually from the raw water, helping to achieve a significantly longer service life for the fine filter candles. Retrofitting with the back-flush filter in the water treatment works will pay for itself after a short time and will demonstrate the economical and technical advantages of this concept.
Key benefits of the Hydac AutoFilt RF3 automatic back flush filter as proved in this case study, include extended service life of candle filter elements to several months; reduced maintenance costs and energy consumption (lower DP); and increased process reliability by thorough removal of sand particles upstream of the reverse osmosis process.
Additionally, it has been demonstrated that retrofitting the Hydac AutoFilt RF3 in an existing system is perfectly possible due to the small space requirement and its versatile configuration. Thanks to the self-cleaning, robust stainless steel filter elements, there are no recurring costs to consider.
The concept has already been successfully implemented in numerous water treatment plants having similar problems with fine sand contamination.