How to improve the effectiveness of Phoslock in seawater.

Phosphate in aquatic systems can cause excessive growth of algae. Addition of the product Phoslock ^TM to freshwater effectively removes phosphate (Figure 1). In brackish or marine systems application of Phoslock ^TM is unsuccesful in removing phosphate (Figure 2) unless larger and subsequently more expensive amounts than normally recommended for freshwater are applied.

Application of Phoslock ^TM combined with a method to reduce the pH of the water body allows the Phosphate removal process to work as effectively in marine and brackish water as in fresh water. The method of reducing pH described here is inexpensive, safe for aquatic life and does not involve the use of acids. Once pH has dropped to 7.4 - 7.7, application of Phoslock ^TM will effectively remove phosphate (Figure 3).

Phoslock ^TM does not work effectively in seawater because the bicarbonate:carbonate buffer system maintains pH around 8.3. At this pH carbonate competes with phosphate for the lanthanum complex which is the active ingredient of Phoslock ^TM. Thus the activity of Phoslock ^TM is rapidly neutralised and is rendered ineffective.

The method of reducing pH is by application to the water of a non toxic soluble organic carbon source with a high carbon :nitrogen (C:N) ratio such as sugar or molasses. Such application will result in rapid proliferation of heterotrophic bacteria which will produce CO₂ causing the water pH to drop (Figure 4). Consequently the proportion of carbonate drops as well and is replaced by bicarbonate . This reduction in the carbonate concentration allows Phoslock ^TM to work effectively.

An example of how the process is applied is given here. A fish pond manager determines that phosphate in his seawater pond of 1 million litres, with pH of 8.3, must be reduced from 2.5 mg PO₄^3- per litre to 0.5 mg per litre. Molasses is added to the pond at a rate that does not allow the dissolved oxygen in the pond to fall below a level considered critical by the manager. Users of this process must be aware that if too much molassses is added at one time, the subsequent proliferation of bacteria may overwhelm the capacity of the pond system to maintain the required levels of dissolved oxygen. Conversely, if too little is used then there will be no appreciable impact on pH (see Figure 4 where initial lower doses had little effect on pH). A typical application is 5 litres of molasses per 1 million litres of water per day. The pH reduction is cumulative, such that an effective dose of molasses added each day will reduce pH to the required range. Typically the pH reduction takes about a week. When pH is in the 7.4 – 7.7 range, 40 kg of Phoslock ^TM is applied for every kg of excess PO₄^3- . In this example, removal of 2 kilograms of phosphate will require 80 kg of Phoslock ^TM,