Floc Strength Scale-Up: A Practical Approach

Transcription

1 Floc Strength Scale-Up: A Practical Approach Dr Mick Dawson Mr Brian Perkins Process Director mdawson@bhrgroup.co.uk 25 th October 2011 BHR Group 2011 BHR Group is a trading name of VirtualPiE Limited

2 Aims To highlight the importance of floc properties on effective water and wastewater treatment To outline the current state-of-the-art To describe the novel approach that WWM is taking to provide quantified, scaleable, floc breakage information for plant hydraulic design and coagulation/flocculation optimisation. BHR Group

3 Content Background Applications & importance State-of-the-art & opportunities WWM 7 Demonstration Project Approach Results Future Work BHR Group

4 Floc Strength Problem: Water & wastewater separation process performance dependent on floc properties Floc growth/breakage results from lab cannot be adequately scaled to plant BHR Group

5 BHR Expertise & Experience Micro- and Nano-particle aggregation and break-up Liquid-liquid dispersions (emulsions) Gas-liquid dispersions (bubbles) Crystallisation processes Population balance modelling BHR Group

6 BHR selection of process devices Stirred bead mill Rotor-stator Microfluidics Valve homogeniser Ultrasonic BHR Group

7 Applications Water Treatment Optimising coagulation & flocculation Post flocculation, hydraulic profile DAF, Upflow Clarifiers & Filtration performance Cold water operation Thin, upland waters BHR Group

8 Applications Wastewater Treatment Chemical Assisted Sedimentation Effective Chemical P removal Primary and Secondary sedimentation processes Chemical conditioning of sludge BHR Group

9 Background Flocs are fragile particles which break-up under conditions of high shear stress. Flocculation is an equilibrium between the formation of flocs and their break-up. BHR Group

10 Mechanisms of break up Primary Particles Aggregates Agglomerates Shattering Rupture Erosion BHR Group

11 BHR Group Modelling break-up Tensile strength of agglomerates (Tang et al, 2001) Baldyga (2007) Based on DLVO theory 2 a TOT a a T L F H T k ze T k ze H z e T Rk H HaR B B B exp 1 2 exp 1 2 exp F F F R A TOT ) ( 1 ) ( f f f f D a i D D a D a f a L L D L L

12 BREAK UP Break up occurs if hydrodynamic stresses acting on the agglomerates are sufficiently high to overcome the tensile strength Laminar flow: Turbulent flow. (depending on particle size relative to turbulent length scale): 12 2\3 L 2\3 i OR BHR Group

13 Scale-up Problems Most of the research on floc size & strength has been done with various types of jar tests where the average and local fluid stresses are not defined. This makes it impossible to scale those data to full scale or even compare different studies at small scale. Other studies have used micromanipulation of individual floc which are unrepresentative of plant conditions BHR Group

14 Estimation of mean and max Turbulent kinetic energy contours (m 2 /s 2 ) max mean 29.0 In a stirred tank the local energy dissipation rates are highly non-uniform. Flocs produced in the bulk region will tend to break in the impeller region. Floc are subjected to highly variable stresses as they repeatedly circulate in the tank BHR Group

15 Normalised average turbulent dissipation rate in plane Estimation of mean and max Distribution of dissipation rate of turbulent kinetic energy max 3 mean Inline mixers feature much more uniform local energy dissipation rates and shorter residence times. Hydrodynamic conditions better defined Static mixer length (m) Scale-up to continuous flow processes easier BHR Group

16 WWM 7 Floc Strength Demonstration Project Objectives To investigate the strength of coagulant floc when subjected to quantified shear rates or local energy dissipation rates scaleddown from real plant. To identify maximum local energy dissipation rates for a range of floc and relate the maxima to plant design. Ultimately, to relate raw water, coagulant type, dose, ph, shear profile and floc strength; enabling improved understanding of dose optimisation and hence cost reduction. BHR Group

17 Floc strength test section Following flocculation, subject formed floc to varying quantified levels of energy dissipation (W/kg) PQ AL PV L Scale-up using ε to simulate flow through clarifier distribution manifolds or DAF inlet pipework for example How far can the floc be pushed before break-up? Gradual increase in fines (erosion) or rupture? BHR Group

18 Floc Size Measurements It is important to ensure that sampling, extraction and measuring technique during flocculation-coagulation experiment meet the following criteria: Measures a good representative sample or sub-sample of the original floc Do not damage, break or alter the flocs Does not encourage further aggregation Different techniques available at BHR Group Microscopy Beckman Coulter LS230 particle sizer using laser diffraction Photometric Dispersion Analyser PDA2000 BHR Group

19 Beckman Coulter LS230 Uses Laser Diffraction technique Particle size range: 40 nm µm Laser diffraction with Polarisation Intensity Differential Scattering (PIDS) technology Particle size distribution calculation based on Mie Theory assuming spherical particles Measures the full size distribution as well as average diameters BHR Group

20 Volume % PSD Evolution by Erosion Small fragments (aggregates or primary particles) are eroded from larger agglomerates. This gives a transient bimodal PSD t=0 t>0 t>>0 t Erosion Particle Size, µm BHR Group

21 Volume % PSD Evolution by Rupture Large agglomerates broken up into smaller agglomerates of comparable size. Break up continues until aggregate or primary particle size is reached t=0 t>0 t>>0 t Rupture Particle Size, µm BHR Group

22 Volume % PSD Evolution by Shattering Each agglomerate is broken into its constitutive aggregates or primary particles in a single event t=0 t>0 t>>0 t Shattering Rupture BHR Group Particle Size, µm

23 Conclusions Overall the approach was very successful Good repeatability in floc size distributions Floc from different raw waters showed different break-up characteristics Results show distinct erosion, rupture and shattering mechanisms Degree of erosion with increasing ε (W/kg) and time quantified Threshold ε (W/kg) for rupture and shattering was determined BHR Group

24 Future Work Develop a mobile test rig that can be taken to WTW or STW sites. Extract floc directly or scale-down coagulation/flocculation for tuning Validate against floc directly from plant Optimise performance of separation processes Assist in selection of chemical coagulants or flocculants Identify potential savings in chemical dose through optimised mixing and flocculation conditions. Carry out laboratory based testing using an improved floc strength test rig to test different water types, coagulation and flocculation conditions BHR Group

25 Benefits Design values of ε (W/kg) related to floc breakage for specific plant, raw waters and operating conditions. Method of optimising treatment parameters (including mixing intensity, flocculation conditions, chemical type and dose) in relation to floc properties and separation performance. Improved plant performance Chemical and energy savings BHR Group

26 Thank you Dr Mick Dawson Enquiries: Water, Environment & Power (WEP) Fluid Systems Academy Process BHR Group 2011 BHR Group is a trading name of VirtualPiE Limited