Removal of Phosphates from Contaminated Water by Microorganisms

Yoav Bashan, Luz E. de-Bashan, and Juan-Pablo Hernandez

Environmental Microbiology, The Center for Biological Research of the Northwest (CIB) P.O. Box 128, La Paz, B.C.S. 23000, MEXICO

__________________

Wastewater, either domestic or agro-industrial originating from dairies, confined animal farms (hogs, cattle, domestic fowl), and food processing plants, is rich in nitrogen and phosphorous compounds. Discharge of these wastewaters into ponds and streams regularly produce severe environmental hazards and health risks, such as intestinal diseases, fresh lake eutrophication, and marine red tides. Re-using these waters for agriculture or their safe discharge requires that most of the nitrogen and phosphorous be removed. In the common procedure for wastewater treatment, the primary treatment eliminates mainly suspended solid particles. This is followed by treating the wastewater microbiologically in stabilization ponds or activated sludge (product of initial elimination of organic matter from wastewater), to further eliminate solids and organic matter.

Degradation of organic matter produces an excess of nutrients (nitrogen and phosphorus) in the wastewater. Inorganic phosphorous can be eliminated in wastewater treatment plants by adding aluminum and iron salts. The interactions of these salts with dissolved P (orthophosphate) produce non-soluble aluminum and iron phosphates. These salts are precipitated, removing inorganic phosphorous from the water. Organic phosphorous and polyphosphates can be removed by adding organic polymers. The polymers act as flocculants, leading to precipitation of the organic phosphorous from the water. These treatments are common in sewage treatment facilities.

In recent years, there has been a growing interest in developing biological water treatment systems (free-floating microorganisms in water or entrapped in polymers), based on adding specific bacteria and microalgae as an alternative to the common chemical treatments. These systems are less expensive and more “environmentally friendly.” Currently, phosphates are biologically removed by wastewater treatment facilities by absorption of dissolved orthophosphate, polyphosphate, and organic phosphate by living microorganisms, such as bacteria, microalgae, yeast, protozoa, fungi, and macrophytes (free-floating aquatic plants, such as water hyacinth). The quantity of eliminated phosphate depends on the net production of living biomass. Treatment of sludge removes about 30% of the phosphates in the wastewater. This is the level of elimination of many biological phosphate removal treatments. The critical factor in biological removal of phosphorous is the exposure of the microorganisms to alternate episodes of aerobic, anoxic (deficiency in dissolved O₂), and anaerobic conditions, which force microorganisms to consume larger quantities of phosphorous than normally. Phosphorus is used by these organisms for cellular maintenance, synthesis of nucleic acids, construction of cell membranes (as phospholipids), and chemical energy transfer reactions within cells (as ATP molecules). Some phosphorus is also stored for future use by the cells.

Practical biological methods used worldwide (including several patented systems) to remove phosphate are based on the assumption that the right microorganisms already exist in the wastewater or that the wastewater can be “seeded” by adding sludge from previous treatments. The engineering process of changing the incubation conditions aims to select the best and most efficient microbial communities for the task. In these cases, although the major traits of microbes can be activated, the species of the microbial population may vary among different sources of wastewater. Thus, the removal of phosphorus cannot be accurately predicted as required for an industrial process. Other methods are based on activated sludge aiming to remove both phosphate and nitrogen (another major contaminant of agro-industrial wastewater) by the same engineering process.

One of the most recent experimental technologies to remove phosphorus is the immobilization of the microorganisms in polymers. The major advantages are that the removal of the cleaning agents after treating the wastewater is easy, the microbial species involved in phosphorus removal can be controlled, the technique allows for the selection of specific, highly efficient microbes for the task rather than random selective enrichment of communities from the sludge.

Bacteriological and microalgal treatments are complementary. While the bacterial population initially degrades organic matter, the microalgae adsorb molecules (nutrients and toxins) released during the other processes. Thus, there is an advantage of combining several organisms, in tandem, to treat contaminated water. Based on this strategy, a recent conceptual innovation is the co-immobilization of microalgae (Chlorella vulgaris and C. sorokiniana, the phosphorus removal agents) with microalage growth-promoting bacteria (Azospirillum brasilense). The latter helps to increase the population of the microalagae and also increase the efficiency of phosphorus removal.

The following review will summarize the state-of-the-art in technologies and approaches designed to remove phosphorous from contaminated water.