Algal control

This comprehensive definition makes it elear that a wide number of substances may be eonsidered to be pestieides, and that the eommonality among all pestieides is their ability to provide eontrol over pests. A variety of classifications for pesticides have been developed that are specific for the type of pest controlled. Insecticides, for example, are pesticides that control insects, while herbicides control weeds and fungicides control plant diseases (molds). In addition to these major classifications of pesticides, there are many other classifications. These include nematicides (for nematode control), acaracides (mite control), rodenticides (rodent control), molluscicide (snail and slug control), algacides (algal control), bacteriocides (bacterial control), and defoliants (leaf control). 

S. Mitrovic and L. Bowling, Literature review Algal Control and Removal from Tertiary Sewage Treatment Ponds—Management Options for Algal Control and Removal, Department of Land and Water Conservation, NSW, Anstralia, 1995. 

Chemicals available for bacterial and/or algal control may be inorganic or organic. Chlorine (inorganic) is widely nsed for biocidal control, either injected as gas or generated in the system from bleach (sodium hypochlorite solution - nsnaUy snpplied as a 12% available-chlorine solution). 

Economic evaluations of algal production indicate that production costs vary from 0.15 to 4.00/kg of algal product, depending on type of bioreactor, culture technique, and operating conditions (51). For systems with controlled agitation and carbonation, including raceways and tubular reactors, production costs ate estimated to range from 2.00 to 4.00/kg. 

The most commonly used physical method for long-term eutrophication control in lakes is that of artificial destratification. This method is well tried and understood and uses either jetted water or compressed air bubbles to break down the lake stratification in the summer months. Algal growth is also affected by an increase in circulation. This is due to the artificial shading effect which results from the algae spending less time near the surface and consequently less time in the light. This technique also reduces the redox-dependent phosphorus release from sediments because the sediment surface remains aerobic. 

In most cases, artificial destratification is carried out throughout the summer. However, work in the Lund Tubes in Blelham Tarn suggests that intermittent destratification could provide further enhancement to this method of control by creating an unstable environment. This instability causes disruption of growth patterns of the algal community and, consequently, no species becomes dominant. 

Because of their surfactant and filming properties fatty amines such as coco-alkylamine acetate (and more especially diamines, such as tallow propylenediamine) are also occasionally employed in other types of water treatment programs. For example, they may be used as corrosion inhibitors for steel cooling systems, especially those smaller units where minimal operational control is provided. The amines must be continuously dosed to ensure good film formation (and thus corrosion protection), typically at 5 to 10 ppm active amine. They also tend to have good biostatic control properties, which provide a benefit of algal and bacterial control at no extra cost. 

II. Chemostat In a chemostat, the medium is delivered at a constant rate, to keep constant growth rate. The nutrient medium is supplied to the culture vessel at a constant rate by a peristaltic pump used to control the washout rate. The rate of media flow is often set at approximately 20% of culture volume per day. Air is pumped into the algal culture vessel through an air compressor controlled by a flow meter and carried in two flasks of sterile water. This bubbling air has three effects (i) it supplies C02 and 02 to the culture, (ii) allows circulation and agitation of the cultures and (iii)... 

Biovolume of benthic algal communities was reduced at both dose levels when compared to controls (Carder and Hoagland 1998) Altered species composition (DeNoyelles and Kettle 1985)... 

Algal densities and biomass reduced, diversity decreased, and species composition altered. Within 16 days after removal of atrazine stress, net productivity was indistinguishable from controls, but community structure remained altered at day 21 (Hamala and Kollig 1985)... 

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