Water Journal : Water Journal July 2012
refereed paper environmental concerns water JULY 2012 57 Treatment Plant (WWTP) is a conventional municipal activated sludge treatment facility with an annual average treatment capacity of approximately 47.3ML/d and currently does not have a phosphorus removal process. The typical secondary effluent orthophosphate (OP) ranges from about 1 to 5mg-P/L. The pilot system was fabricated in two separate skids to help facilitate shipment and testing at future locations. Figure 2 shows the two pilot skids in Lawerence, Kansas. Carousel operation The Lawrence pilot system was configured with two adsorbent columns in series and a third column on standby, as shown on Figure 3. The system was operated in a "carousel" fashion: when phosphorus breakthrough is predicted in the first column, influent flow is diverted directly to the second column, which now serves as the primary column, and the third column is placed into service as the secondary column in series, while the original first column undergoes phosphorus desorption and media regeneration. After regeneration, the original first column is placed on standby to await the next cycle. With this arrangement, the secondary column is always the most highly regenerated one, serving as a polishing unit to the primary column to prevent phosphorus from bleeding through the system. Column switches in the carousel operation were set to occur when either of the following occurred: (1) the accumulated mass of OP removed per litre of media in the lead column reached its setpoint; or (2) the OP concentration from the second column reached its setpoint. Two phosphate analysers were used on the pilot system. One was used to continuously monitor the influent OP concentration to calculate the theoretical mass of OP accumulated on the lead column, since this was a critical control parameter for the automatic column changeover in the carousel operation. A separate low-range phosphate analyser was used to measure effluent OP concentrations from both the lead and polishing columns, automatically alternating back and forth between the two sampling points. Testing phases After startup and commissioning operations of the pilot system, four different testing phases with different influent phosphorus target concentrations and operational set points were conducted as summarised in Table 1. The first two phases (1A and 1B) of the pilot operations involved steady-state flow at a rate of 0.40m3/h (or a space velocity of 20 h-1). During Phase 1A, the primary changeover setpoint was increased from 1.5 to 2.0 g-OP/L of media. During Phase 1B, this setpoint was further increased to 2.5 g-OP/L, which agreed favourably with the estimated capacity from previous bench-scale testing using Lawrence wastewater samples. In subsequent pilot operations (Phases 2 and 3), the influent flow rate to the columns was varied every two hours from 0.26 to 0.48m3/h in a pattern that matched the average diurnal flow pattern observed at the Lawrence WWTP. The diurnal flow pattern corresponded to space velocities ranging from 12.8 to 23.9 hr-1 while maintaining a daily average space velocity of 20 h-1. Additional peak flow scenarios with flows as high as 1.8 times the average flow (0.72m3/h) were simulated during a two-day period from June 22 to June 24. The diurnal flow pattern was continued during Phase 3, while ferric chloride was fed continuously upstream in the aeration basin. Table 1. Lawrence pilot testing phases. Phase Description Target Influent OP (mg-P/L) Period (Year 2010) 1A Steady-state flow at SV = 20 hr-1; Lead column changeover set at 1.5 to 2.0 g-OP/L of media 1to5 April 19 to April 30 1B Same as Phase 1A, except lead column changeover increased to 2.5 g-OP/L of media 1to5 May 24 to June 8 2 Same as Phase 1B, except diurnal flow pattern with daily average SV = 20 hr-1 1to5 June11to June 25 3 Same as Phase 2, except with ferric addition to upstream activated sludge process 0.5to1 June25to July 9 SV = Space velocity Calcium Phosphate Secondary Effluent Filter Treated Water Solids-Liquid Separator pH Control Tank Waste Precipitation Reactor Desorption Tank Ca(OH)2 Tank Acid Tank NaOH Tank Adsorption Towers Adsorption Desorption Recovery Neutralization Carousel Operation Two columns in series Third column stand-by Figure 3. Carousel operation of the adsorption columns. Figure 2. Lawrence pilot skids.
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