Process water control – A step by step guide for water treatment on site

Process water control – A step by step guide for water treatment on site


The pre-treatment of the raw water provides protection and extends the service life of the filters downstream, reduce maintenance costs and energy consumption, consequently ensures the reliability and economy of the membrane filtration.

Pressure-driven membrane processes

Membrane processes are seen increasingly in the most varied industries. The spectrum ranges from the textile industry (treatment of sizing chemicals), food industry (beverage filtration), electro-plating (pickling bath and washing water treatment), pharmaceutical industry (dialysis, product preparation), chemical industry (catalyst separation), metal working industry (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).

Whilst many of these applications are niche markets, the markets for seawater desalination (nanofiltration/reverse osmosis) and for decentralized 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 as follows:

  • Microfiltration (MF): removal of suspended substances
  • Ultrafiltration (UF): concentration / fractionation of macromolecular solutions
  • Nanofiltration (NF): removal of dissolved organic and inorganic substances
  • Reverse osmosis (RO): 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), where as with nanofiltration and reverse osmosis there is a further requirement that the raw solution should 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 above all 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.

Fig. 1: Example of using membrane processes in seawater desalination

The first step in pre-treatment generally consists of coarse filtration. This removes the coarse-grained solid material which can penetrate the system from the raw water. Screens or coarse filters in sizes 1 to 5 mm are used. In the next stage, the raw water is further treated to obtain the required raw water quality for the membrane filtration which follows. This can be done both by using mechanical fine filtration and also in combination using physicochemical processes such as coagulation, flocculation, and precipitation. Membrane filtration is usually followed by an after-treatment, e.g. disinfection.

Depending on the quality of the raw water and the water quality required after treatment, the engineering processes are adapted to the special requirements.

For HYDAC Process Technology division, pre-filtration upstream of membranes is an important market sector. 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.

The following examples show a selection of typical pre-treatment processes.

Example 1

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 the following typical process, the pre-treatment consists essentially of a combination of coarse screens, back-flush filters, and candle filters.

Example 2

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.

Example 3

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.

Other examples

The pre-treatment processes used in the production of drinking water, service water, and process water are inherently varied and individual. HYDAC filters can, in many cases, make a significant contribution to the pre-filtration process, to reduce operating costs and to increase operating reliability. See below, the main benefits of using a HYDAC filter for the pre-treatment:

Benefits of the 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
  • Increases the service life of downstream filters
  • Reduces maintenance costs
  • Reduces energy requirement
  • Small space requirement
  • Flexible design
  • Self-cleaning robust stainless steel filters elements
  • No recurring charges
  • Can be retrofitted
  • Economical
  • Cost-effective

Benefits of using the Automatic Back-flush Filters AutoFilt® RF3

Up to 7,500 m³/h
Up to 40 bar
> 25 µm

  • Extends the service life of the filters downstream
  • Reduces maintenance costs
  • Reduces 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 filters elements
  • No recurring charges
  • Can be retrofitted
  • Economical

Benefits of the Process Inline Filter PLF1

  • Very large filter area per element (> 5 m²)
  • Filtration rating from 3 µm to 90 µm
  • Compact design with high flow rates
  • The clean side is 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 à shorter maintenance times
  • High dirt-holding capacity
  • High filtration performance
  • High fluid compatibility
  • Completely incinerable

Benefits of the Inline Filters

  • Long service life due to large filter areas
  • Filter materials with high contamination retention capacity
  • Low pressure drops because of flow optimized design construction
  • Compact, easy-to-service filter housing
  • High economy due to predominantly cleanable filter elements
  • Consistently high quality according to ISO 9001
  • 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, one frequently used treatment process consists of 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. In order to extend the service life of the candle filters, a HYDAC automatic back-flush filter AutoFilt® RF3 has been retrofitted upstream of the fine filters as part of a pilot project.

This study showed that the back-flush filter provides ideal protection for the candle filters. The particle contamination which consists of fine sand can be removed continually from the raw water and consequently a significantly longer service life is achieved in 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.

Benefits in this case study

  • Extends the service life of the candle filter elements to several months
  • Reduces the maintenance costs and energy consumption (lower DP)
  • Increases the process reliability by thorough removal of the sand particles upstream of the reverse osmosis process

In addition, 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. Furthermore, thanks to the self-cleaning, robust stainless steel filter elements, there are no recurring costs to consider. In the meantime, this concept has already been successfully implemented in numerous other water treatment plants which had similar problems with fine sand contamination.

Fig.2: Comparison of the differential pressure increase across the candle filter elements, with upstream RF3, and without RF3.