Nutrient removal from wastewater - new challenges

Due to acute toxicity to aquatic organisms, wastewater permits have long required removal of ammonia nitrogen from wastewater. This process normally relies on Ammonia Oxidizing Bacteria (AOB) which are organisms that derive energy from the conversion of ammonium (NH4) into nitrite (NO2) and finally nitrate (NO3). The focus was to achieve full conversion to nitrate as the nitrite form was also toxic. Later in developing more advanced treatment, the nitrate/nitrite were to be removed via the denitrification process.

Many common wastewater organisms are capable of removing nitrate/nitrite from waste when dissolved oxygen falls to zero (redox potential <0) in the presence of soluble organics (we often term this BOD5). The process relies on the ability of the bacteria to harness the oxygen bound in the nitrate/nitrite when consuming soluble organics. The BOD5 is lowered and nitrate/nitrite are converted to carbon dioxide (CO2) and nitrogen gas (N2). We see this in the small bubbles that float sludge in secondary clarifiers with long residence times and deep sludge beds. It is also seen in settling ponds where in warm weather we have small nitrogen bubbles along with other anaerobic gasses.

Now that we have converted nitrogen in water to gas, another problem nutrient remained in the wastewater for treatment - Phosphorus. While not directly toxic, in most freshwater an excess of phosphate can trigger algae blooms. The algae bloom causes pH and dissolved oxygen swings and in the case of many cyanobacteria can result in toxic byproducts. To remove phosphate multiple technologies were developed including using alum and other chemical binding/solids removal technologies. These require both equipment, operator input, and produce sludge for disposal. Another option explored was to use the natural tendency of bacteria to uptake phosphate into the cell during growth. in nature phosphate is often a limiting nutrient, so when presented with high phosphate levels, bacteria tend to store phosphate for later use. After much testing, a biological process to remove phosphate was optimized. To encourage the best phosphate uptake, requires the use of a anaerobic zone at the inlet where organisms release stored phosphate to move soluble BOD across the cell wall and start anaerobic metabolism. This gives the phosphate storing cells an advantage when they get to an aerobic zone where all bacteria work efficiently. With oxygen plentiful, the Bio-P organisms, phosphate accumulators, uptake excess phosphate into their cells. This fixes free soluble phosphate into the biological solids. For removal of phosphate from the effluent, TSS or biological solids needs to be removed. A big requirement of biological phosphate removal is to keep the phosphate in the cells - which means no anaerobic storage conditions prior to discharge as this starts the natural phosphate release process.