How to control algae growth in stabilization ponds and settling basins remains one of the most challenging tasks towards maintaining viability and effectiveness of the whole wastewater treatment system. This has always been a major concern because most standard pond designs that provide long detention time, with some up to days or even weeks will allow good opportunity and provide suitable condition for the algae to develop. Discharge of wastewater high in algae concentration has been the topic of discussion that demands strict regulatory control because sudden die-off will produce decaying material, not only is it toxic in nature but will also cause the BOD to increase substantially and therefore totally depriving the much needed oxygen to sustain living organisms in the receiving streams. This can cause major pollution especially those activated sludge effluents that contain mixture of fecal discharge as well. Hence this presents the needs to minimize and control algae development at all cost.
Extensive studies that focused specifically on the algae growth rates have concluded that usually HRT (hydraulic retention time) of less than two days will not pose much issue unless the discharged wastewater has been retained longer than that period. More over in lagoons whereby there is no mechanical aerator to prevent stratification of the water bodies, this has actually presented a perfect opportunity for the algae to further develop. With that in mind, some researchers even suggested installing some form of aerator unit just to prevent this stratification condition. The logical explanation behind all these is that moving waters help to disperse the carbon dioxide generated during the night going to the atmosphere so that this will not get utilized for the algae growth. Also with aeration in place, dead spots in the ponds are avoided. However, selection of aerator is extremely important because it must not be too large and it must be located at a certain level not too deep in order to avoid strong current until this prevents solids from settling at the bottom.
Apart from aeration, the design of the pond and basin can also have a major impact particularly on how much and how high the total suspended solids (particularly those contributed by the presence of algae) still remains in the final discharge. First of all, the final withdrawal point should have a depth of at least 0.5 meter from the top surface so that algae will not get carried over as the wastewater flows out. On top of that, to further improve quality of the wastewater discharge, construction of baffled plates to create a series of partitions or cells can further help to isolate the amount of floating detritus particularly the algae so that it remains at one partition. Another approach that is often used to limit algae growth is by reducing the light exposure but since stabilization and settling basins with wide surface areas have no means to overcome or prevent this, achieving control using this method is not viable. For example, in wetland systems, algae growth has never been a problem because presence of dense vegetation will prevent light from penetrating through to the water surface.
Despite the control efforts, sometimes the focus needs to shift towards the removal aspects. To take note, usually high concentration of TSS (total suspended solids) is usually attributed and caused by the presence of algae and other floating detritus and some of the removal methods which include using intermittent sand filters are possible. The approach is very simple as the particulate material will get trapped and removed via physical straining and then biologically degraded. These are usually used in effluent discharge for polishing purpose to produce clear water clarity but however, the main weakness associated with using it is the clogging problem which can be time consuming (sometimes taking days) and also difficult to handle. When the need arises to produce better effluent quality, this calls for the use of finer sand particle size and further improvements can be made by using a two-stage system. Usually for better efficiency, two parallel filtration units can be employed with one spare unit standing in while the other one undergoes cleaning operation. Likewise, the same working principle also applies to rock filters and other granular media filtration designed specifically to remove and trap the algae. Finally, coagulation–flocculation system based on chemical treatment using mainly alum has also been investigated to be effective for use in wastewater stabilization pond effluent discharge for algal removal.
Related topic: Although algae in discharge remains a big concern, one can actually make full use of it to harvest and turn the algae into biofuel production