Interview with Dr AbdelKader Gaid, Municipal Director, VW/VWS Technical Department, on the various tools and techniques for dealing with algal blooms.
Has the problem of algal blooms or red tide worsened over the years?
The term 'red tide' is often used in the US to describe a particular type of algal bloom common to the eastern Gulf of Mexico, and is also called Florida Red Tide. This algal bloom produces highly potent aerosolised toxins and the red tide toxins (known as brevetoxins) can impact respiratory function and increase respiratory symptoms in patients with asthma. Unfortunately, the intensity of this ecological phenomenon seems to be increasing more in areas like the Middle East, South Africa and US Coasts and for longer durations. For example, in the Gulf of Oman in 2009, the algal bloom lasted for more than six months instead of the usual two months.
Is coastal pollution a factor, and if
so, how?
Scientists have concluded that red tides are occurring with increasing intensity and frequency over a wider global distribution, and may be a result of human activities. Nutrient enrichment through various forms of pollution and changes induced by the greenhouse effect are thought to have influenced the intensity and frequency of red tides. Also, the transport of dormant cysts in the ballast tanks of ships is thought to have contributed to the wider distribution of red tide outbreaks.
How are desalination plants affected by algal blooms?
An outbreak of red tide can force the closure of desalination plants, and thereby, cut water supply to end-users. If the pre-treatment is not adapted to treat the red tide, seawater desalination plants, power plants and users of seawater for cooling purposes will be forced to shut down to avoid fouling and blockage problems. That's why we, at Veolia, have developed methods to control and minimise the impact of red tide through advanced monitoring tools, and to remove the red tide through the use of our Spidflow or Actiflo technology
What are the critical parameters that plant operators should be analysing to deal with red tides?
Key parameters that have to be analysed include common parameters like SDI, Total Organic Carbon, UV 254 nm and also nutrients (P, N).The evolution of the temperature is also a good parameter. Moreover, tracking (analyse once per week during the red tide season) the evolution of chlorophyll and phyto plankton can help anticipate the arrival of red tide and take the necessary measures. A correlation of these parameters obtained over some years will help the operator to anticipate the working conditions.
You were the co-author of the paper titled Use of Advanced Analytical Tools for Monitoring Performance of Seawater Pre-treatment Processes, presented at IDA Congress 2007 in Spain. Could you elaborate on its relevance to dealing with red tide?
Advanced analytical tools were developed to allow thorough characterisation of seawater samples from many aspects: inorganic content, (ii) characterisation of the natural organic matter, and (iii) enumeration of phytoplankton and bacteria. These analytical tools were used at various membrane-based desalination sites worldwide. Results obtained on raw seawater samples showed that the bacteria and phytoplankton counts in raw seawater appear to be positively correlated with the concentration of polysaccharides, organic compounds highly fouling for reverse osmosis (RO), and with the SDI values of both the raw and pre-treated seawaters.
On the other hand, other conventional water quality parameters, such as turbidity and TOC, did not show any correlation with the fouling potential of both the raw and pre-treated seawaters. Also, through the use of the newly-developed water quality parameters at various pilot-scale demonstration sites and at full-scale desalination plants, it was possible to differentiate the performance of various pre-treatment processes with greater accuracy as compared to the single use of SDI values.
This new approach allows one to anticipate the conditions of operation to be taken in the plant before the arrival of the red tide. Overall, the quantification of micro-organisms and polysaccharides concentration along seawater pre-treatment processes can be very useful, notably because those parameters directly quantify potential foulants (high molecular weight and biodegradable organics, and microorganisms).
Overall, the use of these complementary water quality parameters should provide operators of the plant to operate more efficient and sustainable seawater RO plants.
How does DAF as an effective pre-treatment against red tide compare
with alternatives?
Historically, it has been demonstrated that flotation is a technology suited for the removal of algae. Its performance is particularly high towards the elimination of different algal species present in water. This technology is applied to day as a pre-treatment of seawater to protect the RO membranes that are located downstream.
It is important to understand that the flotation step will help improve the overall quality of the pre-treated water (particularly the SDI) only for the seawater withdrawn from the open intake and not from the beach wells. The flotation process has a certain advantage over traditional settling especially when the raw water contains a large quantity of algae and other naturally-buoying compounds (for example, hydrocarbons). Raw water, that has first been coagulated and flocculated, enters the bottom of the flotation chamber where it is in contact with pressurised water distributed uniformly across the width by means of 'cloud' chambers. These are used to prevent the destruction of the flocs by the energy released when expanding the pressurised water. The sludge accumulated on the surface is removed periodically by scraping towards a trough. The clarified water is collected downstream of the work under a siphon. Air-water contact is achieved in a pressure vessel at 5 to 6 bars. This technology is proven and used for the removal of algae in surface water like lakes, dams and sea water.
The advantage of DAF is the possibility of adding high dosages of coagulant which can trap high concentrations of algae without any problem. DAF can handle red tide better than a membrane system because of extensive coagulant dose that removes all dissolved organics that occur simultaneously with algae. Moreover, high concentration of algae can clog very quickly the pores of the membranes and then reduce the production of the treated water. DAF allows a continuous working of the plant despite the fluctuation of the raw water quality. The dosage of the coagulant has to be adapted with this fluctuation.
Does the increased chemical consumption in pre-treatment during red tide events impact the environment?
The increased chemical consumption will not impact the environment because the sludge collected from the DAF is treated in a specific sludge treatment before its discharge.
© H2O 2010
