AFM tested by Danish Technological Institute

Test were conducted the the Danish Institute comparing the performance of AFM against sand, pre-coat DE Diatomaceous Earth and UF membrane filters.The results are presented below and a copy of the full article may be downloaded by clicking here.

The results confirm that AFM is the best filtration media, performing better than DE filter and membrane filters. AFM was also better than a new sand filter but comparable with a sand filter after 2 treatments.  The results again confirm that AFM is at least twice as good as a sand filter and better than membranes or DE.  Membranes should give a better mechanical filtration performance than AFM,  however in reality this is often not the situation. Also when you use coagulation as well as flocculation prior to AFM at the correct application rate with a ZPM static mixer,  the performance will increase by another factor of 2 or 3 times. In addition the coagulation reactions will remove chemicals from solution, and AFM will also adsorb chemicals from solution.  It is impossible for membranes to remove dissolved components.

The study conforms the superior performance of AFM over all other technologies tested for water treatment.

Summary and Conclusion

From 1 January 2012 to 31 December 2013, the Danish Technological Institute, Centre for Swimming Pool Technology and Centre for Chemistry and Biotechnology, has carried out a project on behalf of the Danish Environmental Protection Agency with the objective of obtaining a better understanding of filtering technology in order to optimise the operation of existing and new filter systems in swimming pool facilities. Moreover, the project aimed to develop new standards for monitoring and disinfection of pool water.

In summary, the project’s objective is:

  • Optimisation and development of methods for removal of microorganisms
  • Further development of AOT technologies as a supplement to existing methods where existing 
methods are not sufficient, e.g. for removal of large quantities of microorganisms or 
elimination of chlorine-resistant organisms such as Cryptosporidium
  • Development of a system for online monitoring of the level of microorganisms in the pool 
water, and a system to enable continuous assessment of the need for additional disinfection. 


To ensure that the overall objective of the project was achieved, project activities were divided into a number of sub-projects (SP), each focusing on different aspects of bathing water quality and monitoring hereof.


The four sub-projects are:


SP 1: Test and development of filter systems’ ability to retain microorganisms, protozoa in particular 

SP 2: Improvement and testing of an oxidation system for reduction of microorganisms

SP 3: Development and adjustment of a system for online monitoring of microorganisms in 
bathing water 

SP 4: Development of a technical solution for the combination of a microbial monitoring system for monitoring and controlling of recirculation and filtration of pool water. 


SP 1 


As part of SP 1, the purification ability of four different filter technologies, together representing all known types of fine-mesh filters for swimming pools, has been tested.

The four filter technologies are:

  1. Pressure sand filter without and with flocculation
  2. Pressure AFM filter with flocculation
  3. Pressure Pre Coat filter
  4. Membrane filter 
The test was carried out in a special test pool, built as an exact 1:10 replica of a standard 12.5 x 25 m swimming pool.


The filter efficiency test is carried out by adding to the water, before the filter, a well-defined quantity of the special fluorescent micro particles with a particle size of approximately 4.5 μm, which is equivalent to microorganisms, Cryptosporidium in particular. During the test period, samples are continuously collected before and after the filter. Subsequently, the quantity of particles before and after the filter is determined in a laboratory, and afterwards the efficiency of all tested filter systems is calculated.

The measured efficiency of the tested filtration technologies is listed below:

Sand filter without flocculation: 60.3%
Sand filter with flocculation, first treatment:
 94.1%
Sand filter with flocculation, second treatment: 99.0%
Pressure filter with AFM: 98.9%
Pressure Pre Coat filter:
 91.2%
Membrane filter: 98.7

Based on the tests, it is concluded that optimum filter technology can retain approx. 99 % of pollution particles > 4.5 μm by nothing more than a filter passage.

Further testing of filter technologies under special operation situations, e.g. after a water hammer, interruption in operation or after a backwash show significant short or long-term reduction of efficiency. It is necessary to study this particular problem, and to develop operational tools to avoid the problem. The plan is to further study and solve the problem in connection with a complementary second project, which the Danish Environmental Protection Agency has asked the Danish Technological Institute to undertake to carry out as an extension of the present first project.

SP 2

The need for water purification depends on factors such as bathing load and excretion of organic matter and microorganisms from bathers e.g. through stool, mucous secretion, sweat, skin cells etc. The use of alternative cleaning and disinfecting technologies could reduce the risk of infection in heavily loaded swimming pools.

Several supplementary disinfection technologies will also be active against microorganisms with moderate or low sensitivity to chlorine, such as oocysts from the human-pathogenic protozoan Cryptosporidium. Among them is Advanced Oxidation Technology (AOT) - also referred to as Advanced Oxidation Process (AOP) - which represents a group of technologies developed for water treatment.

Several of these technologies are UV based, but differ from traditional UV systems in that, in addition to the UV effect, strong oxidising agents are also formed, primarily hydroxyl radicals and other reactive compounds of oxygen.

Such technologies are interesting in relation to the removal or inactivation of various chemicals and organisms, e.g. in drinking water and waste water.

A sequence of measurements was carried out in a laboratory setup for the evaluation of the various AOT designs. The system’s bactericidal effect due to increased turbulence in the AOT reactor was determined by means of the non-pathogenic bacterium Pseudomonas stutzeri, which belongs to the same group as Pseudomonas aeruginosa, which is relevant in relation to swimming pools.


The average reduction of live (culturable) bacteria by passage through the AOT systems is:The AOT system's effect on the concentration and size distribution of particulate matter was tested using the particle sensor from SP 3 and SP 4.

  • 'Direct ': 2.5 log reduction
  • 'Side-port': 2.8 log reduction
  • 'Flow switch': 4.1 log reduction 
The killing effect of the AOT system is estimated to be greater than the one obtained during the test, as the tests were conducted with non-chlorinated drinking water. 
Considering all AOT tests, it can be concluded that:
  • The AOT system kills >99 % of bacteria in the treated water.
  • The AOT treatment has no measurable effect on the decomposition of particulate matter.
  • The efficiency of the system can be increased by changing the flow in the reactor, which, 
however, reduces the flow rate in the system.
  • Experience and facilities from the project may possibly be transferred to other kinds of water 
treatment, where disinfection and purification is relevant, e.g. polishing of purified waste water. 
SP 3 and SP 4 
The objective of the sub-project was to uncover the restrictions of the sensor and improve the sensor platform in order to test the sensor in Danish swimming pool facilities.
The original sensor system, Biosentry 5000, was replaced during the project period, as this system was no longer commercially available. The replacement system, TCC particle counter, is a simpler particle counting system. 
The tested sensor is a TCC Particle Counter, an online sensor equipped with a flow cell through which a small stream of water is led. By means of a laser, particles in the flow cell (microorganisms, organic and inorganic particles) can be quantified and the size of the particles determined. According to the manufacturer, the sensor can measure particles at a size of 0.9-139 μm, and 120,000 particles/ml. Data is collected and displayed using the included software, and can be exported for subsequent data processing. 
By measuring the ceramic micro particles, we have recorded a reasonable correlation between concentration of particles and the sensor signal, while we have not been able to achieve a reasonable correlation between the sensor's results and the number of added bacteria. We assess that the reason for this is a significant difference between the ceramic particles and the bacteria: The bacteria consist of a considerable amount of water, while the ceramic particles are quite compact. Therefore, the light penetrability of these ceramic particles is smaller than that of the bacteria. Small particles with high transparency, similar to the individual bacterium, will therefore only to a small extent block enough light to be detected by the sensor detector. This may explain why the sensor in general emits a lower signal than expected. 
The sensor’s sensitivity to individual bacterium is, however, not necessarily a problem in swimming baths, where a large proportion of the bacteria is expected to be found in aggregates from e.g. biofilm, or together with organic particulate matter such as stool, skin cells, mucous etc.

For more detailed testing of whether the sensor is suitable for monitoring of swimming pool water, practical measurements were carried out by monitoring particle concentration in cold and warm water pools in two different swimming pool facility. Sensor data from different measurement campaigns reflects the opening hours of the swimming pool facility, number of bathers, and, in some cases, combined chlorine. Measurements thus indicate that the sensor is suitable for monitoring of bathing water from Danish swimming pool facilities.

It has not been possible to use the sensorsystem for direct control of the operation of the cleaning systems in the swimming pool facilities. Experiments in DP1 and DP3/DP4 show, that the sensor's potential in terms of operation and monitoring means that it will be suitable for monitoring of bathing water from Danish swimming pool facilities, and that it can provide a quick measurement of interruptions in operation such as filter penetration or reduced filtration efficiency as a result of e.g. backwash.

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