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

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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

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

Oskar Bäck

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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.

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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.

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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%.

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7. References ... 43 8. Appendices ... I Appendix 1. Enlarged flow scheme of Margretelund wastewater treatment plant ... I Appendix 2. Current situational graphs ... II Appendix 3. Dates, times and influent TSS value for experiment ... VII Appendix 4. Critical values for two-tailed test, Pearson correlation... VIII Appendix 5. Phosphorus in Margetelund WWTP ... IX Appendix 6. Experiment journal. ... X

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Table of figures

Figure 1. Effect of air-solids ratio on float concentration and subnatant suspended solids (Source: Wang,

Hung & Shammas, 2005, Physicochemical Treatment Processes, P.444) ... 7

Figure 2. Margretelund WWTP catchment area as described by Roslagsvatten (2015) ... 12

Figure 3. Location of Åkersberga town, Sweden (Map from Eniro (2014)) ... 12

Figure 4. Flow scheme Margretelund WWTP, Swedish process description. (Source: Roslagsvatten, 2015) ... 13

Figure 5. Cross section scheme of DAF basin, Swedish description. Source: Roslagsvatten (2015) ... 14

Figure 6. Teledyne ISCO 6712 sampler, used to take influent samples for TSS analysis. ... 16

Figure 7. Sampling bottles and collection bottles used for influent samples ... 16

Figure 8. HACH TSS control unit for monitoring of effluent TSS ... 17

Figure 9. Effluent point with TSS monitor and overflow in DAF unit ... 17

Figure 10. Influent TSS sampling and effluent TSS monitoring points in Margretelund WWTP's flocculation and DAF basin. ... 18

Figure 11. Pump pipe for influent water. ... 19

Figure 12. Control unit for pump used for influent water... 19

Figure 13. Daily average DAF influent TSS concentration measured during the experiment, Margretelund WWTP. ... 24

Figure 14. Removal percentage of effluent suspended solids at different levels of effluent recycle rate. 25 Figure 15. Influent TSS concentration compared to TSS removal percentage. ... 25

Figure 16. 24h average effluent suspended solids concentration with different effluent recycle rates (ERR) between 15-35%. Subfigures; A: surface load 2.5 m/h, B: surface load 4 m/h, C: surface load 5 m/h, D: surface load 6 m/h... 26

Figure 17. Effluent suspended solids concentration box plot with median and average values with different effluent recycle rates between 15-35%. Subfigures; A: surface load 2.5 m/h, B: surface load 4 m/h, C: surface load 5 m/h, D: surface load 6 m/h. ... 27

Figure 18. Effluent suspended solids concentration with different effluent recycle rate (ERR) between 15-35%. Subfigures; A: surface load 2.5 m/h, B: surface load 4 m/h, C: surface load 5 m/h, D: surface load 6 m/h. ... 28

Figure 19. Temperature during the experimental run see section 3.4 for experimental input for each trial. ... 28

Figure 20. Pressure in pressurized tank during the experimental run. See section 3.4 for experimental input for each trial. ... 29

Figure 21. Three different linear correlations showing relationship between factors chosen from Pearson correlation ... 30

Figure 22. Gas-to-solid ratio calculated with equation 5 for historical period January 2015-January 2021, DAF unit Margretelunds WWTP ... 31

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Table of tables

Table 1. Air solubility in freshwater (Source: Wang, Hung & Shammas, 2005, Physicochemical

Treatment Processes, P.448) ... 6

Table 2. Typical dry weight percentage of different compounds in prokaryote bacteria (Source: Metcalf & Eddy, 2013, Wastewater Enginerring: Treatemt amd Resource Recovery, p. 565) ... 9

Table 3. Design loads to Margretelund WWTP based on Swedish EPA’s baseline values (Source: Roslagsvatten, 2015) ... 11

Table 4. Discharge limits Margretelund WWTP (Source: Roslagsvatten 2015) ... 11

Table 5. Manufacturers design parameters of the DAF unit in Margretelunds WWTP ... 14

Table 6. Experimental design plan ... 20

Table 7. Collected historically data parameters. ... 22

Table 8. Summarized historical data of the percentages of samples exceeding 10 mg/l TSS, and percentage of ERR for samples both exceeding and not exceeding 10 mg/l TSS in DAF unit in Margretelund WWTP. Data extracted from aCurve for period Jan 2015- Jan 2021 ... 29

Table 9. Pearson correlations ... 30

Table 10. Historical yearly average values for TSS, phosphorus concentration and percentage of phosphorus in effluent TSS ... 31

Legend

Variable Description SI – Unit Abbreviation

NTU Nephelometric Turbidity Unit - NTU

Q Effluent flow rate m³h^-1 Effl. flow

Qinf Influent flow rate m³h^-1 Infl. flow

Qr Recycled effluent flow rate m³h^-1 Rec. flow

CTSS inf Concentration influent total suspended solids mg¹L^-1 Infl.conc

C TSS eff Concentration effluent total suspended solids mg¹L^-1 Effl. Conc

A Area m²h^-1 -

Vh Surface load m¹h^-1 Surf.load

℃ Temperature ℃ Temp

m Mass of 1 mg/ml air mg¹mL^-1 ^-

a Airs solubility at 1 atm pressure mL¹L^-1 ^-

f Fraction of gas dissolution at pressure p, constant value - -

P Pressure N¹m^-2 ^-

G/S Gas – to – solid ratio % -

r Pearson Correlation Coefficient - -

n Number Nr -

X Data set - -

Y Data set - -

t Student t-test - -

p Level of Significance % -

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1. Introduction

Located in Åkersberga town, Sweden, is Margretelund wastewater treatment plant (WWTP) with its receiving waters, Trälhavet, Saltsjön, approximately 300m from shore. Margretelund WWTP is designed for a population equivalent (PE) of 40 000, with 36 500 PE connected in 2021.

Built in 1956, with two subsequent major renovations, in 1974 and 1999, to meet demands regarding PE capacity and nitrogen removal. Margretelund WWTP has since 1999 been operated with the same processes for physical, chemical, and biological treatment of wastewater, with moving bed biofilm reactor (MBBR) for both biological treatment and a nitrification-denitrification process. Margretelund WWTP installed a dissolved air flotation (DAF) process in 1999 instead of a more conventional sedimentation basin for removal of effluent total suspended solids (TSS).

Margretelund WWTP has historically had issues with increased concentration of TSS in effluent water, caused by increased flow rates and unwanted additional waters. Margretelund WWTP’s design flow rate is 600 m³/h and increases of up to 32% of the flow rate have been recorded.

Majority of TSS in wastewater is of organic heritage, and organic matter contains the base element phosphorus, which is a macro nutrient needed for production of new organic organisms, eutrophication of receiving waters could be a result if the effluent TSS emissions increase. By breaking down organic matter released into the receiving waters, the amount of available phosphorus in the water increases, and thus production of organic matter increases. While Margretelund WTTP does have discharge limits for phosphorus, there are no legal limits for TSS, and it is believed that the DAF process is operated inefficiently in removal of TSS when spikes of increased flow rates occur.

1.1 Purpose of study

The author was tasked by Roslagsvatten AB, the water utility operating Margretelund WWTP, to conduct a study about their dissolved air flotation process. A study which purpose was to optimize the present operation design for Margretelund WWTP’s DAF unit with available means was suggested, including investigation how the DAF unit should be operated during high flow rates to avoid increased effluent TSS concentrations. The study should provide Roslagsvatten AB with proposed actions to further optimize the present design of operation of the DAF unit based on a current situational analysis.

The main purpose for this study was to propose at least one method for optimization of operation of the DAF unit the DAF unit at Margretelund WWTP when increased flow rates occurs. More specifically, the objectives were to:

• analyse historic flow and TSS data for future proposed actions to the DAF process,

• investigate the effect of influent flow rate and effluent recycle rate (ERR) on the effluent TSS concentration, and to

• present suggestions on how to optimize the present design of operation of the DAF unit with available means to minimize effluent TSS discharge.

1.2 Scope of study

Theory studied in this master thesis was about dissolved air flotation and differences between the DAF process to conventional sedimentation. The focus of theory was on recycle flow pressurization configuration when considering design parameters, since Margretelund WWTP utilize this specific configuration.

Analysis of historic data was focused on historical design of operation and process efficiency, to evaluate which factors would provide a proposed impact onto the DAF process. Energy consumption, cost calculations or physical design calculations were not conducted within the study.

The numbers of experimental factors for this master thesis was determined to two and was experimentally tested during a period of one month.

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2. Theory

2.1 Dissolved Air Flotation

Dissolved Air Flotations (DAF) primary objective is to separate solid-liquid solution through flotation.

This is done by inducing a pressurized, supersaturated solution of gas-liquid mixture into the influent flow. Pressurized mixture will, when confronted with pressure release in normal atmospheric pressure, produce gas bubbles that lift total suspended solids (TSS) or colloidal solids to the surface. Bubbles produce a bubble-particle agglomerate with particulate solids. Through bubbles buoyancy force in liquid, the density of suspended solids decreases so they rise upwards and float on top off the surface (Metcalf &

Eddy, 2013;Wang, Hung, & Shammas, 2005). This process also works well for separation of oils, dissolved solutes, heavy- and light solids and grit (Wang, Hung, & Shammas, 2005).

Three more common configurations of DAF processes used for water treatment are the full flow pressurization, partial flow pressurization and recycle flow pressurization. Difference of these three is how they pressurize the saturated gas-liquid mixture (Wang, Hung, & Shammas, 2005). Design parameters deciding what process to utilize are the efficiency in floating different total suspended solids (TSS) concentration and the surfaceloading rate (Metcalf & Eddy, 2013; Wang, Hung, & Shammas, 2005).

Full flow pressurization

Entire influent feed is pressurized by a pressurizing pump and held in a retention tank before released into flotation chamber. This process is focused on low surfaceloadings (5–15 m/h) and the highest suspended solid concentration of the three systems with >800 mg/l of suspended solids. This system is best suited for water where the suspended particles flocculate rapidly.

Air is induced directly into the feed, to remove any collision impact between a pressurized and unpressurized flows that occurs in the two other systems. Without any major collision impact, no regard to shearing strains on to the particle flocs needs to be taken. Coagulating chemicals can be used at the inlet to further increase the flocculate size, but to also entrap bubbles inside the aggregates resulting in a strong air to solids bond (Wang, Hung, & Shammas, 2005).

Partial flow pressurization

A portion between 30–50 % of influent water are separated and pumped with a high-pressure pump into a retention tank for saturation before entering the DAF basin. Remaining influent water is either led by gravity or a low-pressure pump towards the DAF unit. Pressurized water is induced for production of gas bubbles from the pressure drop. Partial flow pressurization are suitable in wastewater feeds containing low concentrations of suspended solids. High shearing force are applied onto the TSS flocs from the high- pressure pump and the large pressure drop, and may break apart the flocs (Wang, Hung, & Shammas, 2005).

Recycle flow pressurization.

Clarified effluent is recycled back to the influent flow with a effluent recycle rate (ERR) between 10–

120% according to (Metcalf & Eddy, 2013) and ERR of 15–50% according to (Wang, Hung, & Shammas, 2005) of the total flow rate. Recycled effluent is pressurized in a retention tank to 3–6 times the atmospheric pressure (Metcalf & Eddy, 2013). Pressurized, semi saturated effluent is mixed with the main influent just before entry point of the flotation chamber (Edzwald, 2010). Gas bubbles produced from the pressure drop collides with flocculated suspended solids. Reycle flow pressurization have minor issues with shearing forces from collision of microbubble-particle impact. But it does not have the issues partial flow pressurization has, since pressurized water are from clarified effluent and no flocculate solids enters the retention tank. Preliminary addition of chemical coagulation and flocculation is necessary for this process to achieve bigger floc size with the suspended particle aggregates to attach more microbubbles.

(Wang, Hung, & Shammas, 2005).

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2.2 History of DAF

DAF is a separation process for liquid-solid solutions that until 1920 were used for mineral separation in mining industry (Edzwald, 2010). At first vacuum-based and full flow pressurised systems were implemented for clarification of drinking water in the 1920’s (Wang, Hung, & Shammas, 2005).

Dissolved Air Flotation clarifiers during this time had a surface load capacity between 5–15 m/h and Detention times around 20–25 minutes. Although it was not until in the 1960 that a Scandinavian company developed a DAF system that laid the foundation of how these systems are used today (Wang, Hung, & Shammas, 2005; Edzwald, 2010). This new DAF system utilized pressurized recycled effluent flow as a source to produce air bubbles, and it shortly became a clarification method competing with settling to become the primary method for surface and wastewater (Edzwald, 2010). From improvements over the years providing possibilities for production of smaller facilities with increased efficiency came in mid – 1990, a DAF system based on the Scandinavian method, that could handle surface load up to 15–

30 m/h while reducing the detention times down to 3–5 minutes (Wang, Hung, & Shammas, 2005;

Edzwald, 2010).

2.3 Design parameters

When designing and operating a DAF unit, a few important design parameters based on the density of particle flocs and viscosity of the liquid must be taken into consideration (Metcalf & Eddy, 2013). These parameters are the concentration of particulate matter, the microbubbles efficiency, air-to-solid ratio, surface load, particle rise velocity, collision of micro-bubbles and floc and how size distribution determines efficiency (Wang, Hung, & Shammas, 2005). They are further presented in text below.

2.3.1 Surface load and particle rise velocity

Surface load and rising velocity are both important for DAF and its removal of suspended particles.

Influent feed has a velocity forward through the flotation chamber, so there is a requirement of a fast enough rising velocity for the particles to be floated. To change the density of the suspended particles with chemical additives, the rising velocity of the flocs may increase enough before the flocs would reach the effluent point (Wang, Hung, & Shammas, 2005; Metcalf & Eddy, 2013).

Surface load varies between two different rates depending on DAF design. The first is a loading rate between 5–15 m/h and used mainly in conventional design. The second DAF design, called highrate DAF, has been developed since the mid – 1990s and as described in section2.2, could handle loading rates between 15–35 m/h (Edzwald, 2010). Detention times in the flotation chamber varies between 3–

60 minutes for both designs, with recycle flow pressurization configuration being within the range of 3- 5 minutes detention time (Wang, Hung, & Shammas, 2005).

2.3.2 Bubbles and airs solubility

Bubble volume and concentration has been shown to determine the efficiency of the DAF process. With smaller average bubble size comes higher efficiency gains. The small bubbles, so called micro-bubbles, are in the range of 10–100 µm with an average value around 40-50 µm (Han, Kim, & Kim, 2007; De Rijk, Van Der Graaf, & Den Blanken, 1993). De Rijk et.al (1993) showed that a relation exists between increasing the saturation pressure and flow rate for the effect of decreasing average median size of micro- bubbles. The effect is however limited, with no noticeable size decrease to micro-bubbles after reaching a pressure of 6.2 bar or above.

Production of supersaturated water works in retention tanks pressurized between 0.5–3 minutes with pressure ranging from 170 kpa–650 kpa (Edzwald, 2010; Wang, Hung, & Shammas, 2005). Amount of dissolved gas into liquid is a function of temperature, with colder water temperature yielding greater solubility of gas. Although a simpler measurement for dissolved gas saturation are to assume that a gas – liquid solution is in equilibrium. Then, the concentration of dissolved gas saturation is directly

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proportional to gage pressure in a retention tank according to Henry’s law (Vallero, 2014; Wang, Hung,

& Shammas, 2005).

Table 1. Air solubility in freshwater (Source: Wang, Hung & Shammas, 2005, Physicochemical Treatment Processes, P.448) Temperature Volume solubility Weight solubility Density

℃ mL/L mg/L g/L

0 28.8 37.2 1.293

10 23.5 29.3 1.249

20 20.1 24.3 1.206

30 17.9 20.9 1.116

40 16.4 18.5 1.13

50 15.6 17 1.093

60 15 15.9 1.061

70 14.9 15.3 1.03

80 15 15 1

90 15.3 14.9 0.974

100 15.9 15 0.949

Oxygen is the gas mainly used in DAF units, but carbon dioxide and nitrogen gas have been used in DAF processes (Wang, Hung, & Shammas, 2005).

Bubbles have mainly two mechanisms to make the flocs float, first one is inclusion where the micro- bubbles are encased into the sludge floc. The second mechanism is adhesion, when micro-bubbles are adsorbed onto flocs (De Rijk, Van Der Graaf, & Den Blanken, 1993). To improve these two mechanisms efficiency. De Rijk et.al (1993) suggests keeping the bubble size <100 µm to increase the probability for inclusion and adhesion for micro-bubbles in flocs. Smaller bubbles make it possible for smaller contact angle between bubbles and flocs aggregates. Smaller bubbles may be included into a floc more easily than a bigger bubble (De Rijk, Van Der Graaf, & Den Blanken, 1993). Furthermore, residence time in the flotation unit is dependent on bubble size. Bubble size affects the rising velocity, with larger bubbles having increased velocity. Thus, smaller bubbles having lower rising velocity would improve the possibility of collision between bubble and flocs (De Rijk, Van Der Graaf, & Den Blanken, 1993). If these bubbles become too big (> 2mm), shearing forces between flocs and micro-bubbles produced due to high velocity may break the floc at impact.

2.3.3 Gas-to-solid ratio

Gas-to-solid ratio (G/S) is one of the more frequently used methods for improving efficiency of DAF systems. With a ratio showing the percentage of gas flux in the system compared to flux of suspended solids (Metcalf & Eddy, 2013). Wang et.al (2005) showed that the ratio should be within an interval of 0.02 to 0.06 for highest concentration of floated concentration (figure 1). Although, a more typical range seen in various flotation processes from wastewater treatment is between 0.005 to 0.060 (Metcalf & Eddy, 2013).

To increase the gas-to-solid ratio would mean higher concentration dissolved gas per amount of concentration suspended solids. As mentioned in subsection2.3.2, an increase of gas volume would lead to more micro-bubbles per floc. Thus, an increased gas-to-solid ratio would increase the flotation efficiency (Han, Kim, & Kim, 2007).

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Figure 1. Effect of air-solids ratio on float concentration and subnatant suspended solids (Source: Wang, Hung & Shammas, 2005, Physicochemical Treatment Processes, P.444)

2.3.4 Collision efficiency and flocs type

Suspended particle floc size is an important factor that affects the collision efficiency of particle – microbubble collision (Han, Kim, & Kim, 2007). This collision is the process that brings the greatest impact on efficiency of the DAF system, with smaller floc size requiring smaller micro-bubbles to reach higher efficiency (Han, Kim, & Kim, 2007).

Edzwlad (2010) assumed that microbubbles flow through the media like sand would in a filter bed, collecting particles in their path. With larger flocs achieved comes increased collision rate and additionally, flocs gain increased hydrophobic properties with increased size (Wang, Hung, & Shammas, 2005; Han, Kim, & Kim, 2007). Suspended solids have a broad size distribution that varies depending on the feed water composition. Flocculating chemicals can be added to enhance the particle agglomeration for a more homogeneous size distribution of suspended solids (Metcalf & Eddy, 2013; Wang, Hung, & Shammas, 2005). These chemicals, having a positive net charge, attracts the negatively charged suspended solids for production of flocs (Wang, Hung, & Shammas, 2005). Two common chemicals used in waste- and raw water treatment are poly-aluminium chloride and ferric chloride (Wang, Hung, & Shammas, 2005).

An increase in micro-bubble volume concentration was shown by Han et.al (2007) to be an important factor. Increased volume concentration would provide a greater amount and a wider spread in size distribution of the micro-bubbles, providing better removal efficiency for the total size range of particle flocs (Han, Kim, & Kim, 2007).

2.4 Benefits and disadvantages of DAF compared to sedimentation

According to Wang et.al (2005), a particle floc coagulated from added chemicals would have a minimum required detention time of 2–4 hours in a conventional sedimentation unit depending on the surface load. Metcalf & Eddy (2013) argues however that a sedimentation’s detention time varies between 1.5–

2.5 hours, with typical dimensioning value at 2 hours. While a DAF unit may only require 1–60 minutes of detention time depending on load and design, which is more beneficial as the DAF would have a much lower footprint (Wang, Hung, & Shammas, 2005). Detention time differences between the two

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systems depends on the settling speed. A sedimentation basin is designed to only utilize gravity, and by adding coagulating chemicals increase the density of flocs, for an increased settling velocity (Wang, Hung,

& Shammas, 2005). A DAF system, on the other hand, uses the micro – bubbles rising velocity, which is far greater than a particle settling velocity. Because of the higher velocity DAF systems has compared to a conventional sedimentation, DAF systems can be run with these low detention times.

Velocity differences allows difference in dimensioning of both systems. If both a DAF unit and a conventional sedimentation where to be designed after a specific surface load and designed flow rate, the DAF unit would require less surface area and depth than a conventional sedimentation (Wang, Hung, &

Shammas, 2005). DAF systems have been proven by Khiadani et.al (2013) to be more efficient in removal of turbidity compared to conventional sedimentation with a 25–35% difference within Nephelometric Turbidity Unit (NTU) ranges of < 20, 30–50 and 90–110. The DAF process had a requirement of less coagulation additives then what conventional sedimentation required to reach same results. DAF is thus a process more suitable for removal of low-density particles. but at a cost of more accurate control of the whole system (Khiadani, Kolivand, Ahooghalandari, & Mohajer, 2013).

According to Svenskt Vatten (2007), a DAF process would be better suited for wastewater containing a vast amount of smaller flocculated particles, a more efficient process than a conventional sedimentation basin. Flocs having a density beneath or close to waters density, requires a smaller effort to be floated.

DAF are therefore often used with chemical additives, like coagulant or flocculants, in production of the flocs (Svenskt Vatten, 2007).

One main disadvantage found to DAF compared to a conventional sedimentation process, is the high energy demand of producing a pressure of 3-6 atm and of the pumps for recycle flow, generating a greater cost and larger CO2 emission per cubic meter wastewater treated (Féris et.al, 2000).

2.5 Phosphorus in bacteria

Municipal sewage wastewater is in general rich in organic nutrients, especially nitrogen and phosphorus, which are considered macro nutrients and are essential in forming new organic matter. Prokaryotic cells utilize these nutrients in wastewater for its production of new cells. With more cells available organic particles in wastewater start to aggregate and build up flocs of suspended matter flowing with the wastewater (Metcalf & Eddy, 2013).

Physical removal of phosphorus is needed since phosphorous does not have a gaseous species like nitrogen. Therefore, phosphorus cannot be removed by evaporation into the atmosphere, but instead stays bound in particulate or dissolved species in natural systems (Metcalf & Eddy, 2013).

According to Metcalf & Eddy (2013), prokaryotes can be described with the formula C60H87O23N12P, where phosphorus has a typical dry weight percentage of 2.0 %, in prokaryote bacteria (table 2).

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Table 2. Typical dry weight percentage of different compounds in prokaryote bacteria (Source: Metcalf & Eddy, 2013, Wastewater Enginerring: Treatemt amd Resource Recovery, p. 565)

Constituents or element Percent of dry weight Major cellular material

protein 55.0

polysaccharide 5.0

Lipid 9.1

DNA 3.1

RNA 20.5

Other (sugars, amino acids) 6.3

Inorganic ions 1,0

As cell elements

Carbon 50.0

Oxygen 22.0

Nitrogen 12.0

Hydrogen 9.0

Phosphorus 2.0

Sulfur 1.0

Potassium 1.0

Sodium 1.0

Calcium 0.5

Magnesium 0.5

Chlorine 0.5

Iron 0.2

Other trace elements 0.3

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3. Methods

3.1 Margretelund wastewater treatment plant

Margretelund wastewater treatment plant (WWTP) located in Österåker municipality in northern Stockholm County, Sweden, was built in 1956 and renovated during two subsequent periods in 1974 and 1999. Margretelund WWTP has operated the same processes for wastewater treatment since the latest renovation in 1999, and these processes was still being used during the experimental runs (subsection 3.1.2) (Roslagsvatten, 2015).

There are approximately 36 500 individuals registered in the densely populated areas of Margretelund WWTP’s catchment area and Margretelund WWTP’s designed max capacity was calculated to 40 000 population equivalents (PE) with a design flow up to 600 m³/h (Roslagsvatten, 2015). Margretelund WWTP’s designed PE value is based on the Swedish Environmental Protection Agency’s (EPA) baseline value for biochemical oxygen demand (BOD7) in influent wastewater. The EPA’s baseline assumption for influent wastewater in Sweden is 70g BOD7/PE (Naturvårdsverket, 2019).

Table 3. Design loads to Margretelund WWTP based on Swedish EPA’s baseline values (Source: Roslagsvatten, 2015)

Parameter SI-unit Value

Design size PE 40 000

Design flow m³h^-1 600

BOD7 kg¹d^-1 2800

Total phosphorous kg¹d^-1 120

Total Nitrogen kg¹d^-1 520

Table 4. Discharge limits Margretelund WWTP (Source: Roslagsvatten 2015)

Parameter SI-unit Value Time period Definition

BOD7 mg¹L^-1 10 Average value per quarter Guide value

BOD7 mg¹L^-1 10 Average value per year Limit

Total phosphorous mg¹L^-1 0.3 Average value per quarter Guide value

Total Phosphorous mg¹L^-1 0.3 Average value per year Limit

Total Nitrogen mg¹L^-1 15 Average value per quarter Guide value

Total Nitrogen mg¹L^-1 15 Average value per year Limit

Total Nitrogen % >50% Removal

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3.1.1 Catchment area

Margretelund WWTP catchment area is seen highlighted within the circle (figure 2). Wastewater is mainly collected from the town of Åkersberga located in Österåker municipality (center). Although, the catchment also includes in the north and south edges, smaller areas of Vallentuna municipality (north) and Vaxholm municipality (south).

Total combined length of sewer piping is 240 km over the total area, consisting of a mix between low pressure sewers, sea pipeline and self-flow sewers, with the latest having a large majority.

Figure 2. Margretelund WWTP catchment area as described by Roslagsvatten (2015)

Figure 3. Location of Åkersberga town, Sweden (Map from Eniro (2014))

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3.1.2 Processes in Margretelund wastewater treatment plant

Margretelunds process scheme (figure 4) presents the mechanical, biological and chemical treatment processes and the flow scheme of the WWTP. The processes are further described below in written text.

Mechanical treatment starts at incoming section of the WWTP with a screen and grit chamber, followed by pre – sedimentation and in the last step in the WWTP where the water is treated in a dissolved air flotation (DAF) reactor.

Biological treatment in Margretelund WWTP is conducted in moving bed biofilm reactors (MBBR) in an aerobic environment. For removal of nitrogen in the wastewater, a pre-denitrification – nitrification – denitrification process is used in combination with MBBR, see figure 4. Recirculation of water occurs between the nitrification – denitrification step.

Chemical treatment is conducted in four different steps during the whole process. Coagulants are added to thicken sludge before dewatering. Flocculants are added both in the grit chamber and in the flocculation chamber. Phosphoric acids are added to prevent phosphorus shortage in the nitrification process and external carbon source are added in the last denitrification process.

Figure 4. Flow scheme Margretelund WWTP, Swedish process description. (Source: Roslagsvatten, 2015)

3.1.3 DAF reactor

Margretelunds WWTP had three parallel DAF reactors utilizing recycle flow pressurization configuration (section1.1), with a surface area of 50 m² and a water depth of 4.0 m each. Four wooden walls are located at the start of the DAF basin (figure 5,). With inlets in different heights to reduce flow rate peaks and allow a more constant water depth through the DAF reactor. The fourth wall, see figure 5, was tilted to a certain degree to guide the pressurized flow upward. Micro-bubbles produced during pressure release was then concentrated to a small area of the DAF basin to increase the bubble volume concentration (subsection2.3.4) (Roslagsvatten, 2015). According to the DAF reactors manufacturer, Purac, the design values for Margretelund WWTP’s DAF process was a surface load of 4 m/h with a percentage of total flow rate being recycled, called effluent recycle rate (ERR), of 10-15%. Surface load of 4 m/h corresponds to a flow rate of 200 m³/h. (Roslagsvatten, 2015).

All three DAF basins used one pressurized tank for pressurization and saturation of recycled effluent.

Treated effluent water used as recycled water for the pressure tank was collected by two pumps at the outlet of the DAF basins, with a capacity of 65 m³/h each (Roslagsvatten, 2015). One pump ran constantly to refill the pressurized tank, and the second pump started when the first pump was insufficient to keep a constant volume in the tank. Volume of the pressure tank was 4 m³ with a constant target

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pressure at 6 bar. If the pressure exceeds 6 bar, a pressure relief valve opened automatically to reduce the excess pressure.

Two sets of sludge scrapers was installed in each basin, one for the floated sludge blanket and one for the heavier sludge blanket that settles at the bottom of the basins.

Surface sludge scrapers was run in intermittent mode with an interval of 0-40 minutes between each start. According to Purac (2000), normal run time value was set to 5 minutes but could manually be changed within an interval of 0–10 minutes. Bottom sludge scraper started once a day and could be set to run for 0–120 minutes, with normal runtime at 30 minutes (Purac, 2000; Roslagsvatten, 2015).

The valve regulating ERR opens the nozzle to max capacity momentarily before closing again to set ERR value at 3 a.m. every night to prohibit build-up of calcium carbonate. This sequence releases a burst of recycled water into the DAF basin, producing turbulent flow as a result. The turbulent flow can reduce the DAF process ability to float flocs and it could swirl up sediment from the bottom of the basin.

It was noticed that this re-occurring process increased the effluent TSS concentration at 3 a.m. with a concentration decreasing slowly over time afterwards. These values where removed from experimental results and seen as an anomaly due to an unnatural increase of effluent TSS concentration.

Table 5. Manufacturers design parameters of the DAF unit in Margretelunds WWTP

Parameter SI-unit Value

Effluent recycle percentage % 15 Pressurized flow m³h^-1 30

Influent flow m³h^-1 170

Total flow m³h^-1 200

Area m²h^-1 50

Surfaceloading rate m¹h^-1 4

Figure 5. Cross section scheme of DAF basin, Swedish description. Source: Roslagsvatten (2015)

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3.2 Gathering relevant theory and information

Relevant theory and information used for this project was collected from different sources. The literature used consisted of different scientific articles and relevant books in the subject. Roslagsvatten, the water utility operating Margretelund WWTP provided a process description of the DAF process from the manufacturer Purac AB, a floor plan for the DAF unit and self-monitoring program about Margretelund WWTP.

3.3 Data collection

Data used in the thesis was collected through water samples, online monitoring, and extraction of historical data via computer software’s.

3.3.1 Sampling

During each experimental run samples of influent total suspended solids (TSS) were taken once every hour with a Teledyne ISCO 6712 Full-Size Portable Sampler that was programmed to do sequence test over 24 hours, where one sequence was 200ml per bottle and hour. All samples were mixed and then poured into a 2 – liter bottle to give an estimated average concentration during the latest 24 hours.

The sampler stopped working around hour 15-16 caused by an unknown error and required a hard reset of the sampler to function again, occurred for two out of 19 experimental runs. This resulted in that these two samples would not represent the full planned 24 hours, but the collected sample provided enough water for analysis.

3.3.2 Analysis of total suspended solids in the influent and effluent of the DAF

Analysis for influent TSS in wastewater was conducted by Roslagsvatten ABs accredited laboratory on site at Margretelund WWTP. The sample was first brought to room temperature at 20 + 2°C before filtration. The filtration process was done with vacuum filtration in 1.6 µm filter, to later dry the filter in an oven at 105 + 2°C for minimum 1 hour and maximum 14–16 hours before weighing. The result was transformed and presented as mg total suspended particles per liter water.

Effluent TSS concentration were continuously analysed with a TSS sc Sensor from HACH. The TSS sensor was cleaned before every experimental run according to descriptions provided from HACH. Figure 8 depicts the control unit used for monitoring the effluent TSS concentration, and measured data was stored and collected using the software aCurve (subsection3.3.4).

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Figure 6. Teledyne ISCO 6712 sampler, used to take influent samples for TSS analysis.

Figure 7. Sampling bottles and collection bottles used for influent samples

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Figure 8. HACH TSS control unit for monitoring of effluent TSS

Figure 9. Effluent point with TSS monitor and overflow in DAF unit

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3.3.3 Points for influent TSS sampling and effluent TSS monitoring.

Influent TSS sample point was positioned in the second (out of two) flocculation basins discharge point, at a depth of 0.5 m from the surface (figure 10).

Effluent TSS monitoring point was positioned at the end of the DAF basin at a depth at 0.2 m (figure 10) and before the overflow for treated effluent wastewater (Figure 9).

Figure 10. Influent TSS sampling and effluent TSS monitoring points in Margretelund WWTP's flocculation and DAF basin.

3.3.4 Software

aCurve, a software developed by gemit Solutions AB was used to collect, review, and extract raw data in real time from Margretelund WWTP’s different control systems. Resolution of data can be manually set between second, minute, hour, day, or week. Extracted data were further presented in excel, for calculations and assumptions for this study. Data extracted from aCurve during this project was effluent TSS, pressurized flow rate, influent flow rate, and effluent flow rate from Margretelund WWTP (table 7).

Software Labware was used to analyse sampling data and lab data from Roslagsvatten ABs accredited laboratory. Data about concentration total suspended particles and total phosphorous in incoming and effluent water was extracted from this software for the period of January 2015 – January 2021 to use in calculations.

Influent TSS sampling point Effluent TSS sampling point Waters flow path

Influent TSS sampling point.

Effluent TSS monitoring point.

Waters flow path.

(A). Cross section of Margretelund WWTP’s DAF unit

(B). Overview of Margretelund WWTP’s flocculation and DAF basin Flocculation basin

DAF basin

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3.4 Experimental design

Based on results from section 4.2, the investigated factors were selected to be the effluent recycle rate (ERR) and the surface load (equation 1). The ERR was set to vary between different percentages between 15-35%, with 5% increase for each step. These ERR were tested on four different surface loads, 2.5, 4, 5 and 6 m/h. These four was chosen based on table 10, with the most historically common surface load being 2.5 m/h, the designed surface load was 4 m/h, and to test if improvements could be achieved in surface loads above the designed loading rate, 5 and 6 m/h were chosen. The time for each test was set to 24 hours, allowing the WWTP one hour for stabilizing between flow changes and so daily variations of incoming SS could be seen for each ERR. See table 8 for full design plan.

To keep a constant influent flow rate in the DAF line that was for the experiment, the section was sealed off from the distribution path and the influent water was pumped into the experimental line during the full time-period. Figure 12 depicts the control unit for the pump used for influent water, and the flow rate was changed by changing the Hz value by trial and error.

Figure 11. Pump pipe for influent water.

Figure 12. Control unit for pump used for influent water.

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Table 6. Experimental design plan

Experimental design

Test ERR

Pressurized flow rate

Influent flow rate

Total flow

rate Area Surface load

nr % m³h^-1 m³h^-1 m³h^-1 m² m¹h^-1

1.1 20 60 240 300 50 6

1.2 25 75 225 300 50 6

1.3 30 90 210 300 50 6

1.4 15 45 255 300 50 6

2.1 15 30 170 200 50 4

2.2 20 40 160 200 50 4

2.3 25 50 150 200 50 4

2.4 30 60 140 200 50 4

2.5 35 70 130 200 50 4

3.1 15 18.75 106.25 125 50 2.5

3.2 20 25 100 125 50 2.5

3.3 25 31.25 93.75 125 50 2.5

3.4 30 37.5 87.5 125 50 2.5

3.5 35 43.75 81.25 125 50 2.5

4.1 15 37.5 212.5 250 50 5

4.2 20 50 200 250 50 5

4.3 25 62.5 187.5 250 50 5

4.4 30 75 175 250 50 5

4.5 35 87.5 162.5 250 50 5

3.5 Analysis of historical DAF data

Historical data on TSS contents, ERR and flow rates of the WWTP were analysed to investigate if there were any faults in the current design of operation of the DAF unit and to evaluate the unit’s efficiency.

First, information was collected about the existing DAF unit and what available design parameters that may be changed without any reconstruction work. This was done by ocular inspection on–site and by using information about Margretelunds WWTP, the DAF reactor and construction schemes of the DAF basin available at Roslagsvatten AB.

By assuming a phosphorus concentration of 2% dry weight in Margretelunds TSS, it would correspond with discharge regulations of 0.3 mg/l phosphorus to an effluent TSS concentration of 15 mg/l.

Calculations and assumptions for the historical data analysis was instead based on a limit of 10 mg/l TSS to allow room for fluctuating concentration up to 15 mg/l TSS, and a varying phosphorus dry weight concentration depending on TSS constituents.

Data used for calculations (table 7) was extracted from the software’s LabWare and aCurve, further described in subsection 3.3.4. From data gathered, surface load and removal percentage of TSS was calculated with equation 1 and equation 2, respectively.

𝑉_ℎ=^𝑄^𝑟^+𝑄^𝑖𝑛𝑓

𝐴 (eq.1) Where 𝑉_ℎ is surface load, 𝑄_𝑟 is the recycled flow rate in the DAF basin, 𝑄_𝑖𝑛𝑓 is the influent flow rate to the DAF basin and 𝐴 is the cross-section area of the DAF basin.

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% 𝑟𝑒𝑚𝑜𝑣𝑎𝑙 = 1 − (^𝐶^{𝑇𝑆𝑆 𝑒𝑓𝑓}

𝐶_{𝑇𝑆𝑆 𝑖𝑛𝑓}) (eq.2) Where 𝐶_{𝑇𝑆𝑆 𝑖𝑛𝑓} is the influent TSS concentration and 𝐶_{𝑇𝑆𝑆 𝑒𝑓𝑓} is the effluent TSS concentration.

3.5.1 Regression analysis

Pearson correlation was conducted on data chosen from historical data analysis to test how strong correlations between different design parameters and effluent suspended particle are in Margretelunds WWTP DAF reactors. Number of tests done was set to 312, with a 310 degree of freedom.

Equation 3 was used in excel to produce the correlation coefficient r. The coefficient may vary between -1 to +1, indicating a negative or positive correlation, with values closer to zero having low to no correlation, and values between 0.3-0.5 having low and 0.5-0.7 having moderate correlation (Berman, 2016).

𝑟 = 𝑛(∑ 𝑋𝑌)−(∑ 𝑋)(∑ 𝑌)

√(𝑛 ∑ 𝑋²−(∑ 𝑋)²)(𝑛 ∑ 𝑌²−(∑ 𝑌)²) (eq.3) Where 𝑛 was the amount of data sets, 𝑋 and 𝑌 were different sets of data that was tested against each other to see the correlation between them.

Critical values used for comparing Pearson correlation coefficient was used with a two tailed student t-test and a significance level alpha equal to 0.05, see Appendix 4 for table with critical values. For the first correlation test, where the number n of data sets exceeds 100, the critical value for 100 was chosen (Berman, 2016; Siegle, 2015).

To test the null hypothesis of each correlation, equation 5 was used to gain the t – statistic value. The higher t-statistic one statistic set gains; the stronger correlation exists. Excels built-in statistical function T.DIST.2T was further used for two-tailed t-test to calculate the probability p for each t – statistic and compare the null hypothesis against alpha 0.05.

𝑡 =^𝑟^𝑥𝑦^√𝑛−2

√1−𝑟_𝑥𝑦²

(eq.4) Where 𝑡 was the t-statistic value, 𝑟 was the Pearson correlation coefficient calculated from eq.3, 𝑋, 𝑌 and 𝑛 were the same parameters as in equation 3.

3.5.2 Gas – to – solid ratio

Gas to solid ratio was calculated based on equation 3, to get a visual representation on how Margretelunds WWTP gas–to–solid ratio (G/S) lays compared to theory (figure 1).

The ratio is calculated from data gathered from aCurve and LabWare during the period of January 2015 – January 2021.

𝐺

𝑆 = ^(𝑚𝑎)(

𝑄𝑟 𝑄)(𝑓𝑃−1)

𝐶_{𝑠𝑠 𝑖𝑛𝑓} (eq.5) Where 𝐺 is mass flow rate of gas, 𝑆 is the mass flow rate of solids, 𝑚 is the mass of 1 mg/ml air, 𝑎 is the airs solubility at 1 atm pressure, 𝑄_𝑟 is the recycled effluent flow rate, 𝑄 is the total effluent flow rate, 𝑓 is a constant value between 0.1-1 for the fraction of gas dissolution at pressure P, assumed to be 0.5. 𝑃 is the pressure in atm, and 𝐶_{𝑠𝑠 𝑖𝑛𝑓} is the influent TSS concentration.

3.5.3 Amount of phosphorus in wastewater

Theory proclaims that dry weight of phosphorus in prokaryotic bacteria is up to 2% (section 2.5) and while wastewater contains an inhomogeneous TSS mixture. Assuming that majority is organic matter, an estimated phosphorus concentration could be calculated based on concentration of TSS (Metcalf & Eddy, 2013).

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With 312 datasets of TSS concentrations and phosphorus concentrations collected from Margretelund WWTP effluent wastewater between January 2015 – January 2021, yearly average values for effluent TSS, phosphorus and percentage of phosphorus to effluent TSS was calculated (Table 10). These 312 datasets were extracted from LabWare.

3.5.4 Processing of historical data

Chosen data parameters extracted from aCurve and LabWare for the period January 2015 to January 2021 are presented below in table 7.

Table 7. Collected historically data parameters.

Parameter SI-unit

Concentration influent total suspended particle mg¹L^-1 Concentration effluent total suspended particle mg¹L^-1

Influent flow rate m³h^-1

Effluent flow rate m³h^-1

Pressurized flow rate m³h^-1

Concentration effluent phosphorus, total value mg¹L^-1

Resolution of data collected was set to weeks from days and hours to reduce the amount of data sets down to 312 points. The resolution scale was increased to match different sets of data that had been sampled during different days but the same week. Some data sets had multiples for the same week, so an average value was based on these samples to use as one single data point.

Historical concentration of influent TSS was sampled before the grit chamber (figure 4). A calculated removal of 60% from sample value was done to match the concentration that theoretically would be achieved after the grit chamber and pre-sedimentation (Svenskt Vatten, 2007).

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4. Results

4.1 Experiment results

The setting for the investigated factors off each experimental run may be found in table 6.

4.1.1 Influent suspended solids concentration

Figure 13 presents the sampling results for influent TSS concentration for each day during the experiment.

Highest value was 695 mg/l, lowest was 56 mg/l and average value was 165 mg/l.

Figure 13. Daily average DAF influent TSS concentration measured during the experiment, Margretelund WWTP.

4.1.2 Removal efficiency of total suspended solids

Surface load of 2.5 m/h kept a removal between 96-98% throughout every change of effluent recycle rate (ERR).

Surface load 4.0 m/h have two peaks at 15% respectively 35% ERR with both values close to 96%

removal. ERR 20-30% are kept almost constant at 92-93% removal.

Surface load of 5 m/h has a highly fluctuating values between 77-92% removal. A removal close to 77% is seen in figure 17 for 25-30% ERR. 5 m/h peaks at 92% removal with 35% ERR.

Surface load of 6 m/h reached a plateau between ERR 20-25% with 92-93% removal, but both decreasing to 15% ERR and increasing to 30% ERR reduced the removal efficiency.

A influent TSS concentration above 100 mg/l showed to provide an increased TSS removal efficiency (figure 15).

0 100 200 300 400 500 600 700

Influent TSS concentration [mg/l]

Date

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Figure 14. Removal percentage of effluent suspended solids at different levels of effluent recycle rate.

Figure 15. Influent TSS concentration compared to TSS removal percentage.

70%

75%

80%

85%

90%

95%

100%

0 100 200 300 400 500 600 700

1.1 1.2 1.3 1.4 2.1 2.2 2.3 2.4 2.5 3.1 3.2 3.3 3.4 3.5 4.1 4.2 4.3 4.4 4.5

Removal percentage

Influent TSS [mg/l]

Experimental run ID

Influent TSS concentration TSS removal percentage 70%

75%

80%

85%

90%

95%

100%

15% 20% 25% 30% 35%

Removal percentage

Effluent recycle rate

2,5 m/h 4 m/h 5 m/h 6 m/h

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4.1.3 Effluent suspended solids concentration.

Average effluent TSS concentration increases with increased surface load (Figure 16). Both surface loads of 2.5 and 4 m/h had their average values beneath the limit of 10 mg/l TSS for every ERR tested. While surface load 5 m/h had two ERR values beneath 10 mg/l, ERR 20 and 35%, and ERR 15, 25 and 30%

exceeding 10 mg/l TSS with only a few mg/l.

Only trend visible between each surface load for changes of ERR, where the concentration increases for each step in ERR up until ERR 35%.

Surface load 6 m/h (Figure 16.D) has an average concentration reaching 52 mg/l with ERR 20%. All four surface loads presented in Figure 16 had their lowest average value when their ERR were at highest, which is 30 or 35%.

With a phosphorus dry weight percentage of 1.86% (table 10), an effluent TSS concentration of 16.13 mg/l would be allowed without exceeding any discharge regulations. Thus, the only surface load exceeding legal regulations are 6 m/h (figure 16.D)

Designed ERR of 15% is presented low average values for each surface load, except in surface load 5 m/h (Figure 16.C). There it is presented as the median value of the five ERR tested.

Surface load of 6 m/h (Figure 16.D) exceeded the set limit value of 10 mg/l TSS for every ERR tested.

Figure 16. 24h average effluent suspended solids concentration with different effluent recycle rates (ERR) between 15-35%.

Subfigures; A: surface load 2.5 m/h, B: surface load 4 m/h, C: surface load 5 m/h, D: surface load 6 m/h.

(A) (B)

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Figure 17. Effluent suspended solids concentration box plot with median and average values with different effluent recycle rates between 15-35%. Subfigures; A: surface load 2.5 m/h, B: surface load 4 m/h, C: surface load 5 m/h, D: surface load 6 m/h.

4.1.4 Effluent suspended solids concentration over time

All four surface loads showed that the TSS concentration varies over time (figure.18). Increases between hour 12-20, except for the surface load of 6 m/h and ERR 20%, where the largest peak is between hour 4-10.

At a surface load of 2.5 m/h, ERR of 20% shows the largest concentration value during the whole experiment (figure 18.A). ERR 30-35% do not show a concentration that increases over time as strong as with 15-25% ERR, but instead keeping a more constant concentration (figure 18.A).

ERR 30% was the only ERR to exceed 10 mg/l TSS for several hours at 4 m/h surface load (figure 18.B). 25% ERR exceeds 10 mg/l effluent TSS in hour 18 and was close to the limit 10 mg/l during several hours.

Every ERR tested during surface load 5 m/h exceeded 15 mg/l TSS during different periods. 35% ERR provided the lowest average TSS concentration and 20% ERR as second lowest. Both 25% and 30%

ERR had concentrations <25 mg/l TSS.

No ERR tested for surface load 6 m/h had a TSS concentration below 10 mg/l, with 20% ERR reaching 88.6 mg/l during hour 6. 25% ERR showed strongly fluctuating values between 17-36 mg/l TSS during the day, but the average was 23 mg/l TSS.

Effluent recycle rate 15% and 30% showed similar concentrations, not having a value higher than 25 mg/l TSS and average values at 15 mg/l TSS.

At surface load 5 m/h and an ERR of 25% (figure 18.C) a sharp increase of effluent TSS concentration occurred at hour 15. This was caused by an operation technician at Margretelund WWTP, who, by habit, lowered the ERR in the morning because it was visible turbulent flow in the DAF unit.

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Figure 18. Effluent suspended solids concentration with different effluent recycle rate (ERR) between 15-35%. Subfigures; A:

surface load 2.5 m/h, B: surface load 4 m/h, C: surface load 5 m/h, D: surface load 6 m/h.

4.2.5 Temperature and pressure during experimental run

The temperature during the experimental run were between 7.5-9.3°C with average temperature 8.3°C.

Figure 19. Temperature during the experimental run see section 3.4 for experimental input for each trial.

6 6.5 7 7.5 8 8.5 9 9.5

1.1 1.2 1.3 1.4 2.1 2.2 2.3 2.4 2.5 3.1 3.2 3.3 3.4 3.5 4.1 4.2 4.3 4.4 4.5

Temperature [ ℃]

Experimental trial id Temperature

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

flotation process for separation of suspended solids in wastewater

Oskar Bäck

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

Oskar Bäck

Preface

Abstract

Sammanfattning

Table of contents

Table of figures

Table of tables

Legend

1. Introduction

1.1 Purpose of study

1.2 Scope of study

2. Theory

2.1 Dissolved Air Flotation

2.2 History of DAF

2.3 Design parameters

2.3.1 Surface load and particle rise velocity

2.3.2 Bubbles and airs solubility

2.3.3 Gas-to-solid ratio

2.3.4 Collision efficiency and flocs type

2.4 Benefits and disadvantages of DAF compared to sedimentation

2.5 Phosphorus in bacteria

3. Methods

3.1 Margretelund wastewater treatment plant

3.1.1 Catchment area

3.1.2 Processes in Margretelund wastewater treatment plant

3.1.3 DAF reactor

3.2 Gathering relevant theory and information

3.3 Data collection

3.3.1 Sampling

3.3.2 Analysis of total suspended solids in the influent and effluent of the DAF

3.3.3 Points for influent TSS sampling and effluent TSS monitoring.

3.3.4 Software

3.4 Experimental design

3.5 Analysis of historical DAF data

3.5.1 Regression analysis

3.5.2 Gas – to – solid ratio

3.5.3 Amount of phosphorus in wastewater

3.5.4 Processing of historical data

4. Results

4.1 Experiment results

4.1.1 Influent suspended solids concentration

4.1.2 Removal efficiency of total suspended solids

4.1.3 Effluent suspended solids concentration.

4.1.4 Effluent suspended solids concentration over time

4.2.5 Temperature and pressure during experimental run