Analysis and optimization of a dissolved air flotation process for separation of suspended solids in wastewater

Abstract: Margretelund wastewater treatment plant (WWTP) operated by the water utility Roslagsvatten AB, wasbuilt in 1956 and is located in Åkersberga town, Stockholm County, Sweden. Margretelund WWTP waslast renovated in 1999, and has been operated with the same physical, chemical, and biological processessince then. Due to issues with increased phosphorus emissions connected to increased concentration ofeffluent total suspended solids (TSS), Roslagsvatten would like to optimize the operation of their dissolvedair flotation (DAF) process and the author was tasked to conduct a study about the subject. The specificaim of the study was to propose one method for optimization with available means to reduce effluentTSS concentration during high flow rates for the present DAF process at Margretelund WWTP.Achieving the aim required an historical analysis of Margretelund WWTP’s DAF process and aninvestigation of the effect influent flow rate and effluent recycle rate (ERR) had on effluent TSSconcentration. The increase of effluent TSS was believed to be caused by increased flow rates frominfiltration and inflow (recorded to 32% of total volume the year 2020) affecting the dissolved air flotation(DAF) process.The literature study design parameters for a dissolved air flotation process, specifically the recycle flowpressurization configuration, generated information about which parameters to take into considerationwhen optimizing a DAF unit. Analysis of historic effluent measurements at Margretelund showed that42% of all samples analysed between January 2015 – January 2021 were below 10 mg/l TSS. Eachhistorical increase of surface load has brought a decreased effluent recycle rate (ERR) and consequentlyan increasing percentage of samples exceeding 10 mg/l. A Pearson correlation presented a negativecorrelation with both ERR and surface load in relation to effluent TSS concentration. This resulted inthe selection of the experimental factors ERR and surface load to be investigated in this study.Margretelunds WWTP’s DAF design of ERR being 10-15% and the design surface load of 4 m/h wasthe base values for the experimental runs. Increases of ERR percentage was done during the experimentfor four different surface loads (2.5, 4, 5 and 6 m/h), with five steps between 15% up to 35% ERR inone of the three parallel DAF units in Margretelund WWTP. TSS in the effluent was constantlymonitored using a TSS sensor. Influent TSS was measured at Roslagsvatten’s accredited laboratory in a24h composite sample with 1 hour for each sub-sample.The results showed that both the highest and the lowest ERR settings tested provided the lowest averageeffluent TSS concentrations. However, a decreased surface load was found to lower effluent TSSconcentration and ERR providing only minor differences within each surface load. Largest surface loadpossible was found to be 5 m/h, for an ERR of 15 or 35%. Surface load less than 5 m/h provided aconcentration under 10 mg/l for all ERR setting.