Phosphate Removal from Domestic Wastewater Using Coal Bottom Ashes

摘要

A very large quantity of coal ash is produced annually in the world as a result of coal combustion. Only a small portion, less than 25% for example in Canada, however is utilized, mainly by cement industries and for road construction. The majority of it is disposed of in landfills and surface impoundments. Phosphorus (P) in natural waters is a critical nutrient and often controls the productivity of aquatic plant species. An oversupply of P can cause eutrophication in surface waters. In the effort to accelerate the utilization of coal ash and to reduce P input to surface water, coal bottom ashes have been examined as sorbent of dissolved phosphate in domestic wastewater using bench-scale lab experiments as well as full-scale field tests. It was found that bottom coal ash was in general very effective in removing P from domestic wastewater. In addition to P, the ash in packed beds removed other contaminants effectively, including total suspended solid (TSS), biological oxygen demand (BOD), ammonia nitrogen (NH3-N), total Kjeldahl nitrogen (TKN) and E. coli. The removal of E. coli was close 100% in all four cases. Batch lab experiments provide information only on intrinsic properties of a coal ash and on specific P retention mechanisms. P retention processes in a batch system were quite different from that in a flow through or a packed bed system, particularly in a field environment. The packed bed system was more complicated and involved not only physical/chemical but also biological processes. As most of these processes enhanced P retention under field conditions, batch sorption experiments conducted in lab underestimated the P retention capacity of a coal ash if it was used in a field filter bed system. In general, the P retention of a coal bottom ash in a filter bed system is affected by the physicochemical properties of the ash, the hydraulic properties of the packed bed system and the conditions f wastewaters treated.