Floc Settling Research Articles

The influence of flocculant adsorption kinetics on the dewaterability of kaolinite and smectite clay mineral dispersions

Polymer mediated flocculation is central to efficient water recovery from and impoundment volume reduction of mineral waste tailings, particularly where clay minerals comprise a significant proportion. In this study, the adsorption kinetics of four high molecular weight polymeric flocculants onto colloidal, negatively charged kaolinite and smectite clay substrates under orthokinetic conditions has been investigated. For both kaolinite and smectite surfaces, the adsorption rate decreased in the sequence: non-ionic polyacrylamide homopolymer (PAM N) > anionic polyacrylamide-acrylate copolymer (PAM A) > non-ionic polyethylene oxide (PEO) > anionic polyacrylamide 2-acrylamido 2-methylpropane sulphonate copolymer (PAM S). Upon flocculation under laminar flow conditions, optimum subsidence rates for both clay types decreased according to flocculant type in the order PAM S > PEO > PAM A > PAM N, implying slower polymer adsorption rate facilitated higher floc settling behaviour. Calculation of first order rate constants for the adsorption of PAM N, PAM A and PAM S onto kaolinite at 25 °C and 50 °C and subsequent estimation of an activation energy in the range 14–22 kJ mol−1 polymer suggest that the overall energy is distributed among the transition state hydrogen bonds which are PAM polymer structure independent. Rationalisation of the results suggests that under industrially relevant conditions, flocculants with relatively slower tails’ particle attachment kinetics facilitate improved bridging performance and markedly enhanced settling behaviour.

Size and settling velocity of suspended flocs during a Phaeocystis bloom in the tidally stirred Irish Sea, NW European shelf

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 345:51-62 (2007) - DOI: https://doi.org/10.3354/meps07006 Size and settling velocity of suspended flocs during a Phaeocystis bloom in the tidally stirred Irish Sea, NW European shelf C. F. Jago1,*, G. M. Kennaway2, G. Novarino2, S. E. Jones1 1School of Ocean Sciences, University of Wales Bangor, Marine Science Laboratories, Menai Bridge, Anglesey LL59 5AB, UK 2Departrment of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK *Email: c.f.jago@bangor.ac.uk ABSTRACT: The settling and resuspension of carbon-rich suspended particulate matter (SPM) are important determinants of pelagic and benthic productivity, and the fate of primary production, in tidally stirred shelf seas. The physical and biological properties of SPM were measured over 5 wk of a declining spring plankton bloom, dominated by the flagellate Phaeocystis, in stratified and mixed waters of the Irish Sea, NW European shelf. SPM consisted of slow-settling, small flocs with a modal settling velocity of 2 × 10–3 to 4 × 10–3 mm s–1 in long-term suspension and fast-settling, large flocs with a modal settling velocity of 2 × 10–2 to 2 mm s–1, deposited as benthic fluff at slack waters and resuspended by tidal currents; flocs reached 500 µm in diameter and settled at up to 20 mm s–1. Larger flocs were composed of living Phaeocystis and other scavenged material. Floc size and settling velocity of SPM scaled on the weight percentage in SPM of arabinose, a photosynthate produced by Phaeocystis. Over a 5-fold range of arabinose content, there was a 3 order of magnitude range of median settling velocity. Larger flocs in mixed waters, with high Phaeocystis content, were stronger than smaller flocs in stratified waters with low Phaeocystis content. Resuspended flocs in mixed waters were stronger than flocs in long-term suspension. SPM concentration in mixed waters halved during the period of observation, with most of this reduction occurring in Week 1. We conclude there was net sedimentation of Phaeocystis-enriched SPM as benthic fluff despite frequent resuspension. KEY WORDS: SPM · Phytoplankton · Phaeocystis · Flocs · Settling velocity · Shelf seas Full text in pdf format PreviousNextCite this article as: Jago CF, Kennaway GM, Novarino G, Jones SE (2007) Size and settling velocity of suspended flocs during a Phaeocystis bloom in the tidally stirred Irish Sea, NW European shelf. Mar Ecol Prog Ser 345:51-62. https://doi.org/10.3354/meps07006 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 345. Online publication date: September 13, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

A pilot-plant study of the adsorptive micellar flocculation process: Optimum design and operation

A pilot-plant scale study of the adsorptive micellar flocculation (AMF) process is presented for the first time, and builds on the solid foundation provided by previous fundamental laboratory studies. A number of experimental tests were conducted to validate the feasibility of using an AMF pilot rig to remove phenol from aqueous effluent streams. Several key factors, including flocculation time, floc settling time, optimum air flow-rate for agitation, and flocculant dosage, were determined. Residual concentrations in treated water indicated a surfactant removal efficiency of 95–98%, and pollutant removal reached 78% in two stages of process operation. A strategy for the separate recycling of surfactant, flocculant and pollutant has already been developed. The present study has thus made further progress in developing, testing, validating and optimising the operating conditions of a pilot-scale process. As such, it has demonstrated the feasibility of scaling up from the bench-scale to a commercial, continuously operated unit.

Mass settling flux of fine sediments in Northern European estuaries: Measurements and predictions

This study describes a new flocculation model, which was based entirely on observations of settling floc spectra and turbulent shear stresses, acquired primarily from a series of in-situ experiments conducted in several European estuaries. The synthesis identified that the key components which best quantitatively describe a floc population are: the changes in the macrofloc (> 160 μm) and microfloc settling velocities (Ws macroEM and Ws microEM), together with how the suspended matter is distributed across each floc sub-population (SPM ratioEM). The Ws macroEM algorithm displayed an increase in settling velocity at low shear stresses due to flocculation enhanced by shear, and floc disruption at higher stresses for the same concentration; the transition being a turbulent shear stress of about 0.36 N m − 2 . The combination of the three algorithms into a single equation to predict mass settling flux (MSF EM), estimated the total flux of the 157 measured floc samples with a cumulative error < 4%. In comparison, the use of single Ws of 0.5 mm s − 1 and 5 mm s − 1 were both in error by an average of − 86% and + 41%, respectively. A concentration — settling parameterisation and the van Leussen techniques under-predicted the total cumulative flux by 35 and 38%, respectively. Furthermore, the analysis indicated that the various existing methods all incurred high predictive errors, at times under-estimating by over 70%, as concentration levels rose in close proximity to the bed. The findings of this study demonstrate the new Manning Floc Settling Velocity (MFSV) empirical model has flexibility in adapting to a wide range of both turbulent shear stress and suspended sediment concentration estuarine conditions.

Variable fractal dimension: A major control for floc structure and flocculation kinematics of suspended cohesive sediment

While the fractal dimension of suspended flocs of cohesive sediment is known to vary with the shear rate, electrochemical properties of the sediment and environment, geometrical restructuring, and presence of organic matter, experimental data presented in this work suggest changes in fractal dimension also during floc genesis at constant sedimentological and hydraulic conditions. A power law function is proposed to describe these changes in floc fractal dimension during floc growth and is used to analyze its impact on floc structural parameters, settling velocity, and kinematics of aggregation and breakup. An analysis of this model for the fractal dimension highlights changes of approximately a factor of 2 or more in floc porosity and aggregation and breakup frequencies and of approximately 1 order of magnitude in floc excess density and settling velocity compared to values estimated with constant fractal dimension. The results from this model compare well with prior experimental data collected in situ (Khelifa and Hill, 2006; Manning and Dyer, 1999).

Effects of chemical amendments on aquatic floc structure, settling and strength

Using a shear-cell/flow-cell combination integrated with an inverted microscope, the behaviour of Hamilton Harbour sediments was studied mixed with three different amendments: alum, chitosan (both coagulants) and a polyacrylamide (a flocculant). Samples from the shear cell were drawn into the flow cell, where floc structure and size were assessed throughout the floc formation and breakage stages using computer image analysis. Settling velocity, density and porosity were also assessed, with results suggesting that amendment addition may be an effective method for the management of high-turbidity environments, provided there are no toxicological effects. In an assessment of performance, it was found that the polyacrylamide flocculant showed the greatest promise in reducing turbidity levels as it produced the largest flocs with the highest settling velocity. Although more prone to break-up, these flocs still remained larger than those formed with alum or chitosan at the same shear. All flocs, regardless of amendment, broke up due to a fracture mechanism rather than by microscale erosion. By improving our understanding of how these amendments may influence floc properties and behaviours, more effective management tools may be developed for the remediation and control of high-turbidity aquatic environments.

Hindered settling of mud flocs: Theory and validation

This paper deals with settling of highly concentrated cohesive sediment suspensions. We evaluate a new hindered settling formula. New settling experiments on these highly concentrated suspensions are described to test this formula. For the analysis of the experiments both an analytical and a numerical method are used. Kynch's analytical theory, based on the method of characteristics is used to study the type of settling. Furthermore, a 1DV-point model is used for analysis of the settling process. We have implemented the new hindered settling formula in this model, which is tested against experimental data. It is concluded that the data are described fairly well. The analysis with the theory of Kynch [1952. A theory of sedimentation. Transactions of the Faraday Scociety 48, 166–176] and the model show that highly concentrated suspensions can settle with either one interface or with two interfaces, depending on the initial concentration and the shape of the settling flux function.

An assessment of a new settling velocity parameterisation for cohesive sediment transport modeling

An important element within the Defra funded Estuary Process Research project “EstProc” was the implementation of the new or refined algorithms, produced under EstProc, into cohesive sediment numerical models. The implementation stage was important as any extension in the understanding of estuarine processes from EstProc was required to be suitable for dissemination into the wider research community with a level of robustness for general applications demonstrated. This report describes work undertaken to implement the new Manning Floc Settling Velocity Model, developed during EstProc. All Manning component algorithms could be combined to provide estimates of mass settling flux. The algorithms are initially assessed in a number of 1-D scenarios, where the Manning model output is compared against both real observations and the output from alternative settling parameterisations. The Manning model is then implemented into a fully 3-D computational model (TELEMAC3D) of estuarine hydraulics and sediment transport of the Lower Thames estuary. The 3-D model results with the Manning algorithm included were compared to runs with a constant settling velocity of 0.5 mm s −1 and settling velocity based on a simple linear multiplier of concentration and with the above mentioned observations of suspended concentration. The findings of the 1-D case studies found the Manning empirical settling model could reproduce 93% of the total mass settling flux observed over a spring tidal cycle. The floc model fit was even better within the turbidity maximum (TM) zone. A constant 0.5 mm s −1 only estimated 15% of the TM mass flux, whereas the fixed 5 mm s −1 settling rate over-predicted the TM mass flux by 47%. Both settling velocity as a simple linear function of concentration, and van Leussen's method, did not fare much better estimating less than half the observed flux during the various tidal and sub-tidal cycle periods. When the Manning-settling model was applied to a layer with suspended concentrations approaching 6 g l −1, it calculated 96% of the observed mass flux. The main conclusions of the implementation exercise were that it was feasible to implement a complex relationship between settling velocity and concentration in a 3-D computational model of estuarine hydraulics, without producing any significant increase in model run times or reducing model stability. The use of the Manning algorithm greatly improved the reproduction of the observed distribution of suspended concentration both in the vertical and horizontal directions compared to the other simulations. During the 1-D assessments, the Manning-settling model demonstrated flexibility in adapting to a wide range of estuarine environmental conditions (i.e. shear stress and concentration), specifically for applied modelling purposes.

The impact of reduced phosphorus levels on microbial floc properties during biological treatment of pulp and paper wastewaters

The effect of limiting P in activated sludge was investigated in laboratory scale sequencing batch reactors (SBRs) fed effluent from a container board mill. Floc characterization included measurement of hydrophobicity, surface charge, and analysis of extracellular polymeric substances (EPS). Reactor performance was assessed by monitoring COD and inorganic P removal, MLSS, and sludge settleability (SVI and batch settling flux) over a period of eight months. Control reactors (BOD:N:P of 100:5:1) were compared to reactors run under P-limited conditions (100:5:0.3; 100:5:0.1). Reactor performance at lower temperatures (14 degrees C; control = 26 degrees C) was also studied to assess the impact of P-limitation. Changes in floc structure and the composition of EPS occurred within 1 to 3 days following a reduction in P levels. There was an insignificant increase in SVI; however, gravitational settling velocity and batch settling flux values for low P floc were consistently higher than for floc generated under control conditions. Lower temperatures (14 degrees C) resulted in a deterioration in floc settling properties at a BOD:N:P of 100:5:1. This impact on settling was significantly reduced at a BOD:P of 100:0.1. Reducing P concentrations in the wastewater treatment system has the potential to improve sludge settleability and reduce final P discharges.

Efficiency of the Coagulation-Flocculation Method for the Treatment of Dye Mixtures Containing Disperse and Reactive Dye

Abstract The coagulation-flocculation method was used to treat aqueous solutions containing varying ratios of commercial disperse and reactive dyes. The effectiveness of this method was assessed using such coagulants as alum, polyaluminium chloride (PACl) and MgCl2, and an anionic coagulant aid. The jar test method was used to measure the effects of pH and coagulant type and dosage on colour removal and chemical oxygen demand (COD) reduction. The effect of coagulant aid on floc settling time was also measured. The results showed that the coagulant dosage needed to achieve optimum colour removal and COD reduction increased as the percentage of reactive dye in the mixture increased. Alum and PACl performed effectively in a lower pH range (pH 3.8 to 5.2), whereas MgCl2 performed effectively in a higher pH range (pH 10.4 to 10.9). PACl was more effective than MgCl2 and alum, achieving &amp;gt;99% colour removal and 96.3% COD reduction using a smaller quantity of the coagulant. The addition of coagulant aid improved floc settling time. These results show that PACl is more effective than MgCl2 and alum in removing both colour and COD in the treatment of the industrial textile wastewater samples.

Settling velocity, effective density, and mass composition of suspended sediment in a coastal bottom boundary layer, Gulf of Lions, France

Particle size distribution and size-specific settling velocity are critical parameters for understanding the transport of fine sediment on continental margins. In this study, observed floc size versus settling velocity, volume distributions of particles 2 μm–1 cm in diameter, and calculated effective densities for all particle sizes provided estimates of the mass distribution in suspension, which is used to apportion mass among component particles, microflocs, and macroflocs. Measurements were made during relatively quiescent environmental conditions. Observations of size distributions based on mass demonstrate an increase in the component particle fraction through time. The increase in the percentage of component particles in suspension had implications on water column properties, as small changes in the component particle fraction affected water column optical transmission in a way that was not as easily detected by changes in the volume concentration distribution or total mass concentration. Flocs larger than 133 μm in diameter only comprised one quarter to one third of the mass in suspension. This finding may explain why suspension bulk clearance rates are often an order of magnitude lower than those predicted by other methods.

Entropy analysis of in situ particle size spectra

Entropy analysis has been used to classify in situ particle (floc) size spectra of suspended particles into groups based on similar distribution characteristics. Results revealed that the in situ spectra sorted into groups that reflected different forcing conditions (e.g. variations in turbulence). The different forcing conditions were not necessarily reflected in other commonly used distribution measures such as median floc diameter. This suggests that entropy analysis may be an effective approach for investigating the effect of changes in forcing conditions on floc size. It is hypothesized that it may be possible to derive the average shape of floc size spectra from measurement of the forcing conditions alone and subsequently derive parameters such as floc fraction, floc density, floc settling velocity and the optical properties of the water column from the average spectra.

Modeling bacteria flocculation as density‐dependent growth

Biological reactors are commonly used to remove pollutants from wastewater. One standard technology is the twostep activated sludge (AS) process. Both in the reaction and the settling tank, bacteria naturally aggregate and form flocs. It is well known—but poorly understood—that both floc formation and settling capacity strongly depend on the loading rate. To optimize this bioprocess it is, therefore, necessary to better understand the flocculation phenomenon. Mathematical modeling has proven to be a valuable tool in the study of wastewater treatment plants. The activated sludge models describe the different biological processes (for example, chemical oxygen demand removal, (de) nitrification and phosphorus removal) involved in the AS process. Its core consists of the mass-balance equations, including the reaction kinetics as a function of the limiting substrates, which read in their simplest form where x is the biomass concentration, s the substrate concentration, h(s) the specific growth rate, D the dilution rate, and sin the substrate concentration in the inflow. The description of floc formation and settling remains the weakest part of AS models. The problem has been studied by a variety of approaches (see for reviews). Population balance models (PBM) describe floc aggregation and breakage and allow to compute the floc-size distribution as a function of time. Computational-fluid dynamics (CFD) simulators describe the hydrodynamics in the clarification tank and try to predict the settling properties of the flocs. Individualbased models (IBM) take both physicochemical and biological processes into account at the level of a single floc. These modeling approaches have in common a high-dimensional parameter space. Although these parameters can be identified from experiments, the resulting model is often too complex to provide insight in the governing mechanisms. Moreover, to compute the settling properties of the ensemble of interacting flocs in the clarifier, one has to combine a CFD with a PBM approach, which leads to even more intricate models. Instead of using advanced simulators, we propose to take the simple model (Eq. 1) as a starting point. In particular, we investigate how these equations are modified when the biomass is organized in flocs. We propose a PBM-like model where both the floc interactions (as in standard PBM), and the bacteria growth are included. This qualitative model is dx dt 1⁄4 hðsÞx Dx

Seasonal and spatial variation of floc size, settling velocity, and density on the inner Adriatic Shelf (Italy)

Measurements of floc sizes, floc settling velocities and effective floc densities were obtained at three locations on the inner Adriatic shelf (Italy) in February and May/June 2003 using the in situ size and settling column tripod INSSECT (Mikkelsen et al., 2004. INSSECT—an instrument platform for investigating floc properties close to the seabed. Limnology and Oceanography: Methods 2, 226–236). Measurements were carried out in a water depth of 8–12 m and the final data set comprised 2491 flocs. Relationships of observed floc size vs. floc settling velocity and floc size vs. effective floc density were similar to those observed by other authors working in similar settings. The raw data showed significant scatter around mean trends, indicating that only a relatively small fraction of variability in floc settling velocity and effective floc density was explained by floc size. For bin-averaged data, however, much of the variability in settling velocity and density was explained by floc size. On the Adriatic shelf mean floc settling velocities varied from 0.48 to 1.35 mm s −1, while mean effective floc densities varied from 8.1 to 27.5 kg m −3; within the range reported by other authors. Analysis of variance showed significant differences in floc settling velocities, effective floc densities and floc size in space and time (seasons). Thus, floc settling velocities, effective floc densities and floc size on the inner Adriatic shelf could not be characterized by a common mean, but were more appropriately characterized by a range of values, varying in time and space.

Laboratory investigation of the phosphorus removal (SRP and TP) from eutrophic lake water treated with aluminium

Mechanisms involved in phosphorus (P) removal from eutrophic lake water with aluminium (Al) were assessed by jar tests. For this purpose, eutrophic lake water enriched with soluble reactive phosphorus (SRP), algae or sediments in order to mimic the various conditions found in shallow eutrophic lakes was studied. Total phosphorus (TP) removal was reached after floc settling, the maximal TP removal efficiency (90–95%) was obtained for an Al concentration ranging from 2 to 5 mg L −1, depending on the organic matter (OM) origin (algae or sediments). Algae appeared to limit macro-floc formation (those able to settle). In contrast, in the presence of sediments, macro-floc formation was favoured at low Al dose (2–3 mg L −1 Al). High SRP removal was obtained with the lowest Al dose (1 mg L −1 Al) and remained greater than 60% for an SRP concentration up to 350 μg L −1. SRP removal was not influenced by the OM origin. The experimental data and literature were used to suggest a hypothetical model for floc formation and P removal with Al under the conditions observed in treated lakes.

Floc structural characteristics using conventional coagulation for a high doc, low alkalinity surface water source

Removal of natural organic matter (NOM) is well established using metal salt coagulants. In addition, flocculant aids are also commonly used to improve solid removal. The objectives of this paper is to describe the impacts of both NOM and polymer on floc structure. The study offeres a comparison of floc physical characteristics for coagulant precipitate flocs, organic-coagulant flocs and organic-coagulant-polymer flocs for optimum coagulant and polymer doses. A ferric sulphate-based coagulant was used as the primary coagulant and the polymer selected was a high molecular weight (MW) cationic polydiallyldimethylammonium chloride (polyDADMAC). Floc size, breakage, re-growth and settling characteristics were measured. Precipitate flocs were larger than organic flocs and had better settling characteristics when compared to NOM-coagulant flocs. When polymer was added, floc size and compaction was seen to further reduce. An explanation was offered in terms of the mode of flocculation involved. Floc breakage behaviour showed that polymer reduced the rate of floc degradation but did not greatly improve floc re-growth potential after breakage, which was generally poor for all of the suspensions.

Turbidity maximum in the macrotidal, highly turbid Humber Estuary, UK: Flocs, fluid mud, stationary suspensions and tidal bores

The macrotidal Humber system, comprising estuaries of the Humber, Trent and Ouse, and their various tributaries, is one of the largest and most turbid in the British Isles. This paper presents detailed spatial and temporal data on the estuarine turbidity maximum (ETM) within the Ouse Estuary of the Humber system during quiescent summer conditions, when freshwater runoff was very low and approximately steady. Under these conditions, an extremely turbid ETM existed in the low salinity reaches of the upper Humber, within the Trent and Ouse Estuaries. Longitudinal surveys of salinity, temperature and turbidity were obtained at approximately local high water (HW) or low water (LW) between the tidal limit of the Ouse and the upper Humber. Tidal-cycle stations were worked between the upper Ouse and the coastal zone. In situ median floc sizes were measured at some stations. Tidal water levels were very asymmetric and currents were flood dominant in the upper estuary, especially at spring tides. Frictional drag on the currents was approximately balanced by water-level slope forcing, which led to a large reduction in tidal amplitude as the tide propagated into the estuary. A tidal bore, 0.1–0.2 m high, formed at spring tides in the upper estuary, but did not cause suspension of fine sediment at locations up-estuary of the ETM. Generally, salinity was fairly well mixed vertically, despite strong SPM stratification in the ETM region. However, large salinity inversions did occur in the presence of underlying, stationary sediment suspensions (∼90 g l −1). The ETM core region, in which near-bed SPM concentrations exceeded 16 g l −1, extended over a longitudinal distance of 35 km at HW, both at spring and at neap tides. It was separated by nose and tail regions from much lower turbidity waters. The nose was much sharper than the tail and was located 15 km into the tidal river at spring tides, where salinity was less than 1. Except at very small neap tides, when fluid mud layers and stationary suspensions formed in the tail region of the ETM, maximum near-bed SPM concentrations (∼50 g l −1) occurred close to the nose in the upper core region. The ETM was displaced down-estuary by ca. 12 km during the transition from spring to neap tides. It also was displaced down-estuary between HW and LW. Floc settling led to pronounced SPM stratification over the HW, HW-slack and early ebb period. Estimates of settling velocity, corrected for hindered settling, ranged from 1.2 to 2.1 mm s −1. At HW slack, hindered settling prevented appreciable deposition of flocs to the bed in the main channel of the nose and upper core regions and at LW of spring tides there was little time for deposition. In the water column, in situ floc sizes maximised around mid-depth at HW slack. Therefore, slack water and low current shears led to increased flocculation, greater sizes and enhanced settling. These maximum median sizes typically were 300–500 μm, whereas sizes during other states of the tide were in the range 70–300 μm. A strong, negative correlation existed between depth-averaged median floc size and bulk vertical current shear in the water column for flocs that were greater than a station-dependent size. The largest median floc sizes occurred within the near-bed stationary suspensions at neap tides, where floc sizes could exceed 1 mm. Entrainment of these flocs led to floc breakage, which reduced their median sizes to less than 200 μm.

Assessment by Microscopy of the Coverage of Sludge Flocs by a Nano‐scale Surface Layer

AbstractFlocs are irregularly shaped suspended particles of complex structure and composition that occur naturally in aquatic systems. In water treatment facilities, the settling of flocs is extremely important. Transmission electron microscopy was used to quantify the formation of a nano‐scale surface layer on flocs from two laboratory bioreactors modelling a biological wastewater treatment facility. By comparing different floc populations, we demonstrated that flocs that settled quickly out of wastewater had a significant amount of this layer, whereas those with little nano‐scale surface layer had poor settling properties. This morphological feature of floc ultrastructure may permit engineering manipulations that promote floc settling.

Removal of Dyes by Aqueous Mixed Coagulants

Alum, MgCl2, and mixtures of alum and MgCl2 have been used as coagulant to remove coloring matter in the reactive dye Levafix Brill Blue EBRA. The jar test was used to determine the effectiveness and the optimal condition of the coagulation process in terms of pH of the coagulation process, coagulant dosage, percentage color removal, and floc settling time. Coagulant dosages of 1,000-5,000 ppm were studied. The treatment of textile industry wastewater was done in the treatment plant of a printing mill. The results showed that coagulation by MgCl2 and alum occurs at the removal pH ranges of 10.5-11.0 for MgCl2 and 4.0-6.0 for alum. The mixtures of MgCl2 and alum gave two distinct effective color-removal zones at pH 4-5.5 and pH 8-9.5. The addition of alum to MgCl2 shifted the optimal pH of MgCl2 from 10.5-11. 0 to 8. 0-9.5. The alum dose of 1,000-3,000 ppm in the coagulant mixture increased by 10-25 % the color removal in the alkaline zone. Floc settling time was found to (a) increase with coagulant dose and (b) take longer in the acidic color-removal zone than in the alkaline zone. The presence of alum increased the floc settling time in the alkaline zone. The study done at the treatment plant of a printing mill showed similar results. A mixture of 60% MgClz and 40% alum at an average dosage of 2,800 ppm gave a color removal of 97% and reductions of 88% for COD and 69% for suspended solids. Keywords: Chemical coagulation, color reduction, mixed coagulants, and textile waste.

The flocs generator reactor-FGR: a new basis for flocculation and solid–liquid separation

This work presents the design and the basic development of an innovative in-line mixing helical (coiled) reactor for the flocculation and solid–liquid separation of suspended particles. The device system was named flocs generator reactor (FGR), a compact flocculation system whereby the flocculation of particles is assisted by the kinetic energy transfer from the hydraulic flow through the reactor. The flow mixing, characterized as a plug flow regime with a slow dispersion, promoted the required hydrodynamics conditions to disperse the polymer flocculant (cationic polyacrylamide) and also to generate “strong” flocs of Fe(OH) 3 (colloidal suspension model system) at pH 7.5. The solid–liquid separation of the flocs was then achieved either by settling or by flotation with microbubbles (30–70 μm). Studies were carried out with different models of FGRs (varying the length/volume ratio), and process efficiency (mainly kinetics) was evaluated as a function of polymer concentration and operating parameters, namely, feed flow rate, air-to-solids ratio and residence time. Results showed that the aerated flocs are readily floated with rising rates of the order of 120 m h −1, values higher than those obtained for the settling of the nonaerated flocs (18–24 m h −1). The FGR requires short residence times and generates dense, well-structured flocs (with a mass fractal dimension, d F of about 3) which withstand high shear forces. Due to the high process efficiency and high loading capacity shown, it is believed that the FGR has a good potential as an in-line flocculation (or flotation) separator device in applications requiring high rate solid–liquid separations.