Analysis and optimization of a dissolved air flotation process for separation of suspended solids in wastewater Oskar Bäck Natural Resources Engineering, master's 2021 Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering Analysis and optimization of a dissolved air flotation process for separation of suspended solids in wastewater Oskar Bäck i Preface This report represents a master thesis within the master program in Natural resource engineering with focus on water and environmental science at Luleå University of Technology. The study was conducted in collaboration with Roslagsvatten AB, a Swedish water utility, about Margretelund wastewater treatment plant’s dissolved air flotation process. The study was held during a period of 20 weeks in the spring of 2021, corresponding to 30 ECTS. There are a lot of people I am thankful for helping me through this master thesis, and especially my supervisor at Luleå university of technology (LTU), Inga Herrmann, for helping me sort through all my ideas and thoughts and encourage me during these 20 weeks. I wish to both congratulate and thank my fellow classmates from LTU, and all the discussions we have had together, through both high and lows. I would also like to direct a special thank you to my supervisor from Roslagsvatten, Daniel Zetterström, for helping me with everything and anything on-site during the thesis and always came with a good answer no matter the question. Thank you, Annelie Hedström, for helping me bring out the most from this thesis as examiner. I am thankful for the opportunity given to work on this master thesis together with the process division at Roslagsvatten, and for all the help and support given by everyone at Margretelund wastewater treatment plant. ii Abstract Margretelund wastewater treatment plant (WWTP) operated by the water utility Roslagsvatten AB, was built in 1956 and is located in Åkersberga town, Stockholm County, Sweden. Margretelund WWTP was last renovated in 1999, and has been operated with the same physical, chemical, and biological processes since then. Due to issues with increased phosphorus emissions connected to increased concentration of effluent total suspended solids (TSS), Roslagsvatten would like to optimize the operation of their dissolved air flotation (DAF) process and the author was tasked to conduct a study about the subject. The specific aim of the study was to propose one method for optimization with available means to reduce effluent TSS 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 an investigation of the effect influent flow rate and effluent recycle rate (ERR) had on effluent TSS concentration. The increase of effluent TSS was believed to be caused by increased flow rates from infiltration 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 flow pressurization configuration, generated information about which parameters to take into consideration when optimizing a DAF unit. Analysis of historic effluent measurements at Margretelund showed that 42% of all samples analysed between January 2015 – January 2021 were below 10 mg/l TSS. Each historical increase of surface load has brought a decreased effluent recycle rate (ERR) and consequently an increasing percentage of samples exceeding 10 mg/l. A Pearson correlation presented a negative correlation with both ERR and surface load in relation to effluent TSS concentration. This resulted in the 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 was the base values for the experimental runs. Increases of ERR percentage was done during the experiment for four different surface loads (2.5, 4, 5 and 6 m/h), with five steps between 15% up to 35% ERR in one of the three parallel DAF units in Margretelund WWTP. TSS in the effluent was constantly monitored using a TSS sensor. Influent TSS was measured at Roslagsvatten’s accredited laboratory in a 24h 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 average effluent TSS concentrations. However, a decreased surface load was found to lower effluent TSS concentration and ERR providing only minor differences within each surface load. Largest surface load possible was found to be 5 m/h, for an ERR of 15 or 35%. Surface load less than 5 m/h provided a concentration under 10 mg/l for all ERR setting. iii Sammanfattning Margretelund avloppsreningsverk (ARV) placerat i Åkersberga, Stockholms län, byggdes 1956 och drivs av Roslagsvatten AB. Margretelund ARV har sedan 1956 renoverats vid två tillfällen senast 1999. Samma reningsprocess för fysisk, kemisk och biologisk rening har använts sedan senaste renoveringen. Roslagsvatten har haft problem med oönskat tillskottsvatten (motsvarade 32% av total volym 2020) som har påverkat deras flotationsprocess negativt gällande rening av suspenderat material. Detta har till slut lett till förhöjda utsläppsvärden av fosfor som finns bundet i det suspenderade materialet. Denna studie har utförts av författaren på efterfrågan av Roslagsvatten, med syfte att presentera optimeringsåtgärder till styrning av flotationsprocessen vid höga flöden. För att uppnå målet med studien gjordes en historisk analys av Margretelunds flotationsprocess samt undersökningar om hur variationer i inkommande flöde samt recirkuleringsgrad har påverkat koncentration av utgående suspenderat material. Teori undersöktes och information insamlades angående designparametrar gällande optimering av flotationsprocesser, mer specifikt en flotationsprocess med recirkulerat trycksatt flöde för avskiljning av susp. Analys av historiska utsläppsvärden från Margretelund ARV’s flotationsprocess visade på att 42% av proverna analyserade mellan januari 2015-januari 2021 låg under 10 mg/l för utgående suspenderat material. Varje historisk ökning av ytbelastning påvisade en minskande recirkuleringsgrad samt en ökande andel prover som översteg koncentrationen 10 mg/l. Utifrån en Pearson korrelation visades en negativ korrelationen för både ytbelastning och recirkulationsgrad gentemot koncentration av utgående suspenderat material. Både recirkuleringsgrad och ytbelastning valdes därför till denna studies experimentella faktorer. Flotationsprocessen på Margretelund ARV’s var designad för en recirkuleringsgrad på 10–15% vid ytbelastning på 4 m/h, och valdes som basvärde för experimentet. Fem olika grader av recirkulation testades för fyra olika ytbelastningar (2.5, 4, 5 och 6 m/h) i intervallet 15– 35% i en av tre parallella flotations bassänger på Margretelund ARV. Koncentration utgående suspenderat material mättes kontinuerligt med en sensor. Inkommande koncentration bestämdes genom ett dygnsprov som analyserades av Roslagsvattens ackrediterade laboratorium. Ett resultat från experimenten var att både den högsta och lägsta inställningen av recirkuleringsgrad visade de lägsta medelvärdena för utgående koncentration av suspenderat material. Dock, visade resultaten att en minskande ytbelastning resulterade i lägre koncentrationer av utgående suspenderat material. Vidare sågs att recirkuleringsgraden enbart hade en låg påverkan på koncentrationerna för varje ytbelastning. Den högsta möjliga ytbelastningen utan att överstiga 10 mg/l visades vara 5 m/h med recirkuleringsgraderna 15% och 35%. iv Table of contents Preface ................................................................................................................................................ i Abstract .............................................................................................................................................. ii Sammanfattning ................................................................................................................................ iii Table of figures .................................................................................................................................. vi Table of tables ................................................................................................................................... vii Legend .............................................................................................................................................. vii 1. Introduction ................................................................................................................................ 2 1.1 Purpose of study ........................................................................................................................ 2 1.2 Scope of study ........................................................................................................................... 2 2. Theory ........................................................................................................................................ 4 2.1 Dissolved Air Flotation .............................................................................................................. 4 2.2 History of DAF ......................................................................................................................... 5 2.3 Design parameters .....................................................................................................................