THE USE OF STRAW TO CONTROL BLUE-GREEN ALGAL GROWTH: FINAL REPORT ON WORK UNDERTAKEN BY THE AQUATIC WEEDS RESEARCH UNIT 1991-94
Report No FR0461
H A JAMES
Decomposing barley straw produces an agent which has been shown to be effective in inhibiting the growth of blue-green algae. Provided that it can be shown that this agent produces no detrimental effects on water quality, the development of an effective method for dosing water bodies may offer an attractive alternative to other algal control measures, such as dosing with ferric salts.
The objectives of this research programme were to:
Although there are effective methods available for blue-green algal control, which are either direct (e.g. dosing with copper, which is toxic to the algae) or indirect (e.g. ferric/ferrous dosing, which results in a reduction in nutrient (phosphate) availability), there is concern regarding undesirable side-effects of such methods. Algal control by rotting straw appears to provide a `green' alternative provided it is demonstrated that it is effective, some means of dosing large water bodies can be devised and it can be shown that there are no deleterious effects on water quality, particularly in the case of raw water storage reservoirs.
Blue-green algae can be controlled by the active agent produced during the decomposition of barley straw. This has been demonstrated in laboratory and field experiments. A digester design in which water is trickled through a straw bed is more effective than one in which the straw is constantly submerged, as aerobic conditions (essential for maximal production of the active agent) are more easily maintained. Field installations are prone to vandalism, and successful deployment will need to take this into account.
The effectiveness of the more successful digester design needs to be established in field conditions. Ideally this would involve installation at a secure site on a large water body where blue-green algae have regularly caused problems in the past, and investigation of the design of a distribution system for the effluent from the digester to maximise the algal control achieved. Water quality parameters should be regularly monitored, and if it is intended to deploy digesters at raw water storage reservoirs potential deleterious effects following water treatment (e.g. production of tastes or odours after chlorination) need to be investigated.
VI RESUME OF CONTENTS
It has been demonstrated that the growth of blue-green algae is inhibited, both in laboratory and field experiments, by an agent produced during the decomposition of barley straw. For field situations where direct application of loose straw can be used, a dosage rate of 5 g m-3 is suggested. Two designs for bankside digesters for the production of the anti-algal agent have been investigated. In the first, water was passed through submerged straw. Although initial results based on laboratory experiments were promising, in field experiments it appeared that anaerobic conditions (primarily due to persistent vandalism of the equipment) produced sub-optimal levels of the anti-algal agent. The second design, in which water is trickled through straw supported on a grid, ensures aerobic degradation and shows more potential for scaling up to treat large water bodies. However, more work is necessary to investigate the optimal design of a distribution system to effectively disperse the anti-algal agent.
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