There are two possible approaches for reducing nutrients in wastewaters: (1) Nutrient Recovery; and (2) Nutrient Destruction. Nutrient destruction utilizes biological nitrification and denitrification reactions to destroy nitrogen and precipitates phosphorus biologically. Use of biomedia to grow biofilms, such as in Integrated Fixed Film Activated Sludge (IFAS) enables simultaneous nitrification and denitrification within the same vessel. However, nutrient destruction involves significant aeration costs, is impacted by ambient temperature and involves major equipment modifications. Further, nutrients have “value” that can be exploited for economic gain.
There are several types of biological treatment systems that can be used to achieve nitrogen removal from wastewaters. Biological treatment systems can be classified based on the contact mechanism between the wastewater and the biomass. Biomass can be either suspended in the wastewater or immobilized on a solid substrate or biomedia. The wastewater can either flow through a mixed reactor or a plug flow reactor system.
Biological removal of ammonia is generally achieved by combining anoxic and aerobic treatment system. Pre-anoxic treatment systems use an anoxic basin followed by an aerobic reactor, with recycle of wastewater from the aerobic tank to the anoxic system to denitrify the nitrate produced in the aerobic section. Post-anoxic systems combine an aerobic system followed by an anaerobic section to achieve denitrification. Post-anoxic systems have the disadvantage of also reducing sulfate to sulfides, forming hydrogen sulfide, with its associated odor issues. Sequencing batch reactors have the advantage of varying the anoxic and aerobic condition time durations to achieve the desired level of denitrification.
Membrane bioreactors can achieve better nitrification than suspended culture reactors, mainly due to retention of the nitrifiers, which are slow growing organisms and hence tend to get washed out easily. The main issue with membrane bioreactors is membrane fouling and the high cost of the membrane systems.
Alternative nutrient recovery processes includes the following:
- Recovery of struvite (magnesium ammonium phosphate) from digester supernatant (e.g., Ostara’s Pearl Nutrient Recovery Process); Only 5-15% of nitrogen is recovered through phosphate-based precipitation processes.
Production of biosolids-enhanced granular inorganic fertilizers (e.g., Unity Envirotech’s fertilizer granulation process and VitAG’s ammonium mix process).
Air stripping of ammonia followed by gas absorption to produce ammonium sulfate; Full-scale ammonia-stripping towers have been decommissioned because of operational problems and cost – (a) process has failed during cold weather, due to freezing; (2) sacling of tower packing, especially when lime was used to raise the pH; and (c) does nor achieve low ammonia concentration; and
Liqui-Cell’s Membrane process for directly converting ammonia in water to ammonium sulfate using a membrane contactor; air trapped within the membrane pores separates the wastewater from the sulfuric acid, and water is prevented from entering the pore due to membrane hydrophobicity; However, the presence of surfactants wets the membrane pore, resulting in process failure.
Clearly, there is a need for simultaneous recovery of both nitrogen and phosphorus from wastewaters