Effect of phosphorus removal from sewage using substrates of activated alumina in constructed wetlands

Abstract: The eutrophication caused by excess phosphorus (P) in sewage is drawing public attention increasingly. Constructed wetland (CW) is one of the most particularly useful and common ways to recover P from sewage, especially in countryside. The aim of this study was to determine the P adsorption capability of different substrates in CW in cold areas of Northern China. The paper studied four different kinds of substrates including activated alumina, steel slag, limestone and zeolite. The P adsorption kinetics and adsorption isotherms of different substrates were tested on same condition to analyze the adsorption properties of P of different substrates, and made a comparison; Then, we selected the substrate with best adsorption effect and also considered the effect of substrates' porosity on hydraulic residence time to design nine groups with different particle sizes and different volume ratios with other substrates, and carried out small-scale experiments by simulating single-stage vertical-flow constructed wetland (VFCW) to study the P removing effect in domestic sewage of low concentration (1.18-4.39 mg/L) of different combined substrates. And four kinds of combined substrates with better P removal effect were adopted to explore whether the effluent quality met the standard under high concentration of P (15.96, 38.13 and 68.22 mg/L). Finally, the optimal combined substrates for P removal effect was filtrated to evaluate the removal effect of high concentration P about 20 mg/L by the pilot-scale test of multistage VFCW system. Adsorption isotherm and adsorption kinetics tests of four substrates were carried out in the laboratory of China Agricultural University. Small-scale and pilot-scale experiments were conducted in Fangshan district, Beijing, and domestic sewage used in all experiments was collected from a factory, and the designed hydraulic load was 0.3 m3/(m2?d). The results showed that P adsorption characteristics of the four kinds of substrates could be described by the Freundlich model, Langmuir model and adsorption kinetics model commendably. The materials of activated alumina had the strongest adsorption capacity for P, followed by steel slag, limestone, zeolite. After that, nine groups of different combined substrates were applied to vertical-flow test, the removal rate of P of mixed substrates which contained activated alumina was 83.54%-93.48%, and that four substrates with different proportions of active alumina with particle size of 3-5 mm were combined with sand and limestone whose removal efficiency were good for P was 87.36%-93.48%, and their average removal rate was 89.63%, among which the removal rate of P in the combined substrates contained 75% activated alumina and 25% limestone was the highest, up to 93.48%. When the concentration of P in sewage increased to a high level, the P concentration of effluent could meet the A grade in sewage effluent discharge standard, which was 0.2 mg/L in the comprehensive discharge standard for water pollutants of Beijing (DB11/307-2013). Besides, different mixed ratios of substrates could also affect the P removal rate in sewage. No matter what the P concentration in sewage was low or high, the P removal rate of 75% aluminum and 25% limestone was about 94%-99% in pilot-scale VFCW test and the P concentration of effluent sewage was less than 0.2 mg/L, meeting the A grade in sewage effluent discharge standard. Therefore, activated alumina as a kind of substrate had commendable removal effect of P and high application value in CW.