Chemical behavior of orthophosphate and total phosphorus in phosphorus removal process

Abstract

Removal of phosphorus in wastewater was performed in lab scale to determine the chemical behaviors of orthophosphate and total phosphorus. Firstly, coagulation/precipitation test was conducted for wastewater which was collected after a secondary treatment process at a wastewater treatment plant. Alum and Ferric chloride were used as coagulants in different mole ratio concentrations which are 1:1, 2:1 and 3:1 (Al or Fe : P). Orthophosphate and total phosphorus concentration were analyzed to identify their chemical behaviors. Results indicate that the removal of phosphorus species is influenced by the concentration of the coagulants and the reactivity of the phosphorus species. It is believed that the reactivity differences between orthophosphate and polyphosphate does not play a significant role in chemical reaction in low concentration of coagulant (1:1). Polyphosphate has greater molecular weight and larger particle size than orthophosphate, therefore, the polyphosphate has higher possibility to participate in chemical reaction with coagulants than the orthophosphate. On the other hand, the reactivity differences might play a significant role in high concentration of coagulants. The orthophosphate which is the most reactive species of phosphorus was dominant in chemical reaction in high mole ratio concentration (2:1 and 3:1). Adsorption/precipitation column test was conducted by feeding wastewater upward through the column with various sizes of slag (0.5-1 mm, 1-2 mm and 2-4 mm) as absorbent. The effluent samples were collected after up to 100 bed volume for the analysis of orthophosphate and total phosphorus concentrations. According to the results, phosphorus was removed rapidly in the initial phases of the adsorption/precipitation column test and the slag of which size is between 0.5 and 1 mm showed the highest phosphorus removal efficiency. With the coarser sized slag samples, adsorption and precipitation tends to occur less than with the finer sized slag. In addition, during the initial phases, the conditions of the wastewater were appropriate for precipitation in terms of a high pH and a high concentration of cations, such that both orthophosphate and polyphosphate were removed completely by precipitation. However, the proportion of orthophosphate was high in the initial phases and it indicates that the polyphosphate was dominant in adsorption because it has larger particle size than the orthophosphate and, therefore, the polyphosphate was adsorbed on the surface of the slag in initial phases of the experiment more than the orthophosphate. In later phases, the proportion of orthophosphate becomes lower because orthophosphate participated in the adsorption reaction more than in the initial phases, and both orthophosphate and polyphosphate were adsorbed on the surface of slag in later phases.