Eutrophication becomes more serious with the development of society. It is difficult for the conventional sewage treatment technology to meet the requirements of environmental treatment. Efficient and economical deep nitrogen and phosphorus removal technology becomes a necessity. Biological method is often used for nitrogen removal, and physicochemical method is effective for phosphorus removal. The technology of biological denitrification for nitrogen removal is mature and widely used. For phosphorus removal, chemical precipitation usually requires additional structures and may cause secondary pollution. Physical adsorption has some disadvantages of low saturated adsorption capacity and high cost. Therefore, the potential application of some industrial solid and mineral wastes with high activity, low cost and environmental friendliness has attracted recent attentions. The development of phosphorus removal materials based on solid waste is a current research hotspot. Compared with the conventional physicochemical method, the crystallization method can reduce the structure and facilitate the recovery of phosphorus. Phosphate ore in China is rich, and the massive mine tailings after processing are piled up at will due to the improper development, resulting in environmental pollution and resource waste. Collophanite is in apatite, which can be used as a crystal seed of phosphorus removal by crystallization method. This way can reduce the cost, and realize the resource utilization of waste. In the early stage, low-grade collophanite as a mineral waste was used for deep phosphorus removal in sewage. This paper was to explore a possibility of simultaneous nitrogen and phosphorus removal in one reactor via physicochemical and biochemical processes. A low-grade collophanite was used as a denitrification filter, which acted as a carrier of denitrification microorganism attachment and growth and provided an apatite crystal seed. Effect of mineral fillers on the simultaneous nitrogen and phosphorus removal was investigated.

Low-grade collophanite, anhydrite and calcined magnesite coarser particles were crushed and classified into smaller particles with the sizes of 2-4 mm. A dynamic heterotrophic nitrogen removal column was constructed with low grade collophanite as a main component and anhydrite and calcined magnesite particles as fillers. The anaerobic sludge was cultured and enriched with heterotrophic denitrifying bacteria in a sewage treatment plant, and the heterotrophic denitrifying bacteria were inoculated for simultaneous nitrogen and phosphorus removal. The effluent nitrogen, phosphorus and other related indexes were monitored regularly to investigate the influence of combined mineral fillers on the nitrogen and phosphorus removal. The feasibility of simultaneous nitrogen and phosphorus removal with different combined filters based on low grade collophanite was verified. The microstructure of fillers, the morphology of minerals and the relationship between minerals and microorganisms were characterized by scanning electron microscopy, transmission electron microscopy and microbial community analysis.

The results show that the phosphorus removal is ineffective when low-grade collophanite is used as a filter because of insufficient calcium ions required for apatite crystallization. The addition of anhydrite to low grade collophanite can obtain efficient simultaneous nitrogen and phosphorus removal. The combined fillers of low-grade collophanite, anhydrite and calcined magnesite have a positive effect on simultaneous nitrogen and phosphorus removal. The effluent total nitrogen concentration is stable (i.e., below 3 mg/L) and the effluent phosphorus concentration is below 0.5 mg/L when the hydraulic retention time is greater than 1 h, meeting the Grade A discharge requirement (GB18918-2002).

The higher pH value is conducive to phosphorus removal, but has no significant inhibition on microbial denitrification and nitrogen removal. All the combined filters can be used as a carrier of heterotrophic nitrogen removal microorganisms, and there is little difference in nitrogen removal efficiency, compared with quartz sand.

The mechanism of efficient phosphorus removal by mineral combination is since anhydrite releases calcium ion and calcined magnesite releases hydroxyl to promote the growth of apatite when using collophanite as an apatite crystal seed. The existence of biofilm has a diaphragm effect on ion diffusion during apatite growth, and the phosphorus removal effect is not as good as that of sterile system.

Effective nitrogen and phosphorus removal in a single reactor was realized when anhydrite, calcined magnesite and low-grade collophanite as a main component of the filter materials were used. The results obtained could provide a promising potential for the resource utilization of low-grade collophanite as waste minerals.