Water Research Australia Water for the wellbeing of all Australians February 2015 Page 1

Fact Sheet 1 of 2Biological fi ltrati on - will it work for you?

How can biological � ltration be used to remove tastes and odours and algal toxins?

The removal of the taste and odour (T&O) compounds MIB and geosmin through biological fi ltrati on has been demonstrated in the laboratory and at full scale, and has the potenti al to off er large operati onal and cost benefi ts for the treatment of drinking water.

Full scale evidence is diffi cult to obtain for cyanotoxins. The microcysti ns are readily biodegradable and biological fi ltrati on has been shown to be extremely eff ecti ve at the laboratory scale, but oft en aft er a lag phase during which there is no removal. Biological fi ltrati on for the removal of cylindrospermopsin is possible as this toxin is suscepti ble to biodegradati on under suitable conditi ons. However the saxitoxins are not readily biodegradable and it is unlikely that this treatment process can be used for the eff ecti ve removal of these cyanotoxins.

A major obstacle in the implementati on of biological fi ltrati on is the ti me it takes to establish a degrading biofi lm, parti cularly on new fi lter media. At the laboratory and pilot scale this “lag” phase can take over 12 months. The lag phase is due to a number of factors and will depend on a range of complex and interrelated conditi ons at the water treatment plant (WTP) which may include:

• The number of degrading bacteria present in the infl uent water

• The ability of those bacteria to colonise fi ltrati on media

• A criti cal mass of degrading bacteria being retained in the media

• The maintenance of water quality conditi ons conducive to a healthy, degrading biofi lm

• The frequent/regular presence of algal metabolites in the infl uent to the fi lters

During this period it is necessary to have additi onal barriers in place to ensure safe and palatable drinking water in the presence of cyanobacterial metabolites.

What conditions are necessary for the removal of algal metabolites by biological � ltration?

If you answer “yes” to all the statements below there is potenti al for the formati on of a degrading biofi lm on your fi lters:

• Our fi ltrati on medium has an average parti cle size < 1 mm

• There is no pre-chlorinati on or pre-fi lter chlorinati on at the water treatment plant

• Algal metabolite(s) are regularly present in raw water at the inlet to the fi lters

• There are degrading organisms in the raw water

• Degrading organisms regularly reach the inlet to the fi lters

The best way to determine whether these conditi ons are met at your treatment plant is to apply the “Biological Filtrati on Potenti al (BFP) Test” (see Fact sheet 2).

How can we improve the chances of biological � ltration e� ectively removing metabolites?

There are some operati onal conditi ons that will improve the likelihood of biological fi ltrati on occurring in full scale fi lters:

• Use smaller media parti cle size

• Cease pre-chlorinati on and/or pre-fi ltrati on chlorinati on, if usually practi ced

• Allow degrading bacteria to colonise fi ltrati on media – seed fi lters with indigenous bacteria if feasible

Once biological � ltration is established in our treatment plant what factors can in� uence its e� ectiveness?

Any operati onal change that impacts the number and/or type of micro-organisms entering the fi lters may adversely impact their ability to remove metabolites. Some examples are:

Water Research Australia Water for the wellbeing of all Australians February 2015 Page 2

• The introducti on of raw or sett led water chlorinati on

• Enhanced coagulati on practi ces

• The presence of a disinfectant in the backwash water will rapidly inacti vate a biofi lm

• Someti mes unknown and unpredictable factors can severely impact the reliability of the biological removal process

Summary

Comprehensive investi gati ons at the laboratory, pilot and full scale have suggested that biological fi ltrati on processes, while they can be very eff ecti ve, do not off er a robust, reliable barrier for cyanobacterial metabolites in drinking water. As a result, it is not possible to develop specifi c guidelines for the confi dent applicati on of biological fi ltrati on for the removal of cyanobacterial metabolites at all treatment plants. However, general recommendati ons that may increase the probability of successful biological fi ltrati on can be given, and testi ng can be undertaken to aid in the determinati on of the potenti al for the development of the process at a parti cular plant.

Laboratory, pilot and full scale investi gati ons have suggested

that granular acti vated carbon (GAC) is the best medium for the confi dent implementati on of biological fi ltrati on for the removal of cyanobacterial metabolites. The parti cles off er an ideal surface for the rapid growth of a healthy biofi lm, and the adsorbent properti es allow for the removal of metabolites during the oft en prolonged period prior to the onset of biodegradati on. However, the conditi ons necessary for biological fi ltrati on outlined above also apply for GAC – in the absence of these conditi ons adsorpti on of the cyanotoxins in parti cular will decrease over ti me due mainly to the conti nuous adsorpti on of natural organic material (NOM). The GAC may be rendered ineff ecti ve by “pre-loading” of NOM over a period of 12 months or less.

Reference: Sawade E, Ho L, Hoefel D, Newcombe G (2015) Implementati on of biological fi ltrati on for the treatment of cyanobacterial metabolites. Final report, project 1021.

Findings at a glanceFactor Operati onal Implicati onParti cle size Reducing parti cle size will increase the surface area available for biofi lm growthAcclimati on WTPs that experience regular episodes of T&Os are more likely to achieve biological removal of the

metabolitesOnce a degrading biofi lm is established operati onal changes may impact effi cacyRe-establishment of biological removal on a damaged biofi lm will occur more rapidly than on new fi ltrati on media

Water Quality Bacterial numbersEnhanced coagulati on can remove up to an additi onal log of bacteria from feed water of fi lters hence prolonging lag phase and increasing ti me to develop a biomass capable of biological degradati onReducing bacterial numbers in fi lter infl uent changes biodiversity of biofi lm and may result in producti on of T&O compoundsDisinfecti onPre-chlorinati on will inacti vate bacteria reducing chance of biological degradati onPre-fi lter chlorinati on (used to increase fi lter run ti mes) will sterilise the fi lters and prevent eff ecti ve biological removal of metabolites