Air Scouring Research Articles

Recommendation on the selection of powdered activated carbon as carrier to enhance performance of polymeric UF membrane

The addition of powdered activated carbon (PAC) has been widely accepted as a solution to alleviate membrane fouling and maintain a sustainable flux in membrane bioreactor (MBR) and anaerobic membrane bioreactor (AnMBR). This is due to PAC's unique characteristics, such as serving as a supportive medium for bacterial attachment and growth, achieving adsorption of organic foulants and providing abrasion effect that minimizes the formation of cake layers on membrane surfaces. While majority of ultrafiltration (UF) membranes used in MBR and AnMBR application are polymeric, no study has yet investigated the integrity of polymeric UF membranes with the addition of PAC adsorbents. The abrasive effects of PAC on membrane surfaces under gas sparging in AnMBR or air scouring in MBR have not been studied, and no guidelines have been established for selecting a reliable PAC for such purposes. To address these concerns, this study systemically investigated the integrity of polymeric UF membranes with five different types of PACs to establish recommendations for PAC selection. Based on the integrity testing of polymeric UF membranes, a recommendation for PAC selection was formulated, highlighting bulk density and Gold Number (GN) as pivotal criteria. Five distinct types of PACs were studied to determine the optimal characteristics for polymeric UF membranes, considering variations in raw materials (coal-based and wood-based), activation methods (chemical and steam), particle morphology (sharp and granular), bulk densities (ranging from 0.23 to 0.6 kg/L) and GNs (spanning from 0.1 to > 6.0). The investigation extended beyond 100 d or until irreversible defects were observed in the UF membranes. This study provides guidelines for selecting PACs to be used with polymeric UF membranes, aiming to improve membrane performance while minimising their impact on membrane surfaces.

Simultaneous carbon, nitrogen and phosphorus removal in sequencing batch membrane aerated biofilm reactor with biofilm thickness control via air scouring aided by computational fluid dynamics

Membrane aerated biofilm reactor (MABR) is challenged by biofilm thickness control and phosphorus removal. Air scouring aided by computational fluid dynamics (CFD) was employed to detach outer biofilm in sequencing batch MABR treating low C/N wastewater. Biofilm with 177–285 µm thickness in cycle 5–15 achieved over 85 % chemical oxygen demand (COD) and total inorganic nitrogen (TIN) removals at loading rate of 13.2 gCOD/m2/d and 2.64 gNH4+-N/m2/d. Biofilm rheology measurements in cycle 10–25 showed yield stress against detachment of 2.8–7.4 Pa, which were equal to CFD calculated shear stresses under air scouring flowrate of 3–9 L/min. Air scouring reduced effluent NH4+-N by 10 % and biofilm thickness by 78 µm. Intermittent aeration (4h off, 19.5h on) and air scouring (3 L/min, 30 s before settling) in one cycle achieved COD removal over 90 %, TIN and PO43−-P removals over 80 %, showing great potential for simultaneous carbon, nitrogen and phosphorus removals.

Direct filtration of low-turbidity water by fixed-bed ion exchange treatment

In drinking water treatment, low-turbidity water (e.g., < 5 NTU) is often purified by direct filtration comprising coagulation, flocculation, and rapid sand filtration. This study assessed the potential of replacing the rapid sand filter with a fixed-bed ion exchange column. Two types of anion exchange resins, polyacrylic (Purolite A860) and polystyrene (Purolite A502PS), were used in fixed-bed columns without preceding coagulation and flocculation. After the first use of the new resins (14-h operation), the A860 column showed more significant head loss buildup than the A502PS column (151 and 101 cm, respectively). This can be attributed to the effective size of A860 resins (0.38 mm), which was smaller than that of A502PS resins (0.52 mm). A860 resins were superior to A502PS resins in filtrate quality. The A860 resins achieved filtrate turbidity (0.16–0.26 NTU) lower than the A502PS resins (0.19–0.35 NTU). Similarly, dissolved organic carbon removal by A860 resins (25–37 %) was higher than that by A502PS resin (15–25 %) due to the low hydrophobicity of the feed water. Air scouring and water backwashing were ineffective in regenerating A860 resins. In contrast, cleaning with a NaCl solution (pH = 12) partially recovered the capacity of A860 resins for turbidity removal.

Influence of Air Scouring and Residual Fouling on Cake Build-Up and Compression in Membrane Bioreactors

Influence of Air Scouring and Residual Fouling on Cake Build-Up and Compression in Membrane Bioreactors

Optimizing Air Scouring Energy for Sustainable Membrane Bioreactor Operation by Characterizing the Combination of Factors Leading to Threshold Limiting Conditions.

Key operating variables to predict the necessary scour air flowrate in full-scale Membrane Bioreactor (MBR) systems are identified, aiming to optimize energy consumption while avoiding the limiting condition (i.e., rapid increasing total resistance). The resulting metric, referred to here as the K value, was derived by balancing hydrodynamic conditions between the particle deposit rate imposed by permeate flux normalized by fouling condition and its removal by shear stress induced from air scouring. The metric includes air scouring flow, permeate flow, Mixed Liquor Suspended Solids (MLSS) concentration, Mixed Liquor (ML) viscosity, membrane packing density, and total resistance. Long-term (year-long) data from two full-scale MBR plants were analyzed. The value of K corresponding to limiting operational operation and referred to as the limiting K value, KLim, is estimated by detecting the occurrence of threshold limiting flux from the data stream and calculating the resulting value for K. Then, using KLim, the minimum required specific air demand per permeate (SADp,Crit) is calculated, indicating a potential reduction of over half the air scouring energy in typical operational conditions. The results from this data driven analysis suggest the feasibility of employing KLim to predict the adequate scour air flowrate in terms of dynamically varying operational conditions. This approach will lead to the development of energy-efficient algorithms, significantly reducing scour air energy consumption in the full-scale MBR system.

Performance evaluation of submerged membrane bioreactor for the removal of microalgae from the source water of a water treatment plant

Freshwater algal blooms negatively impact the operations of water treatment plants when they are present in the source water. In the present study, an application of a low-cost submerged microfiltration membrane bioreactor for the removal of microalgae from a source water reservoir of a water treatment plant was evaluated. The experiments were conducted in different phases to systematically evaluate how different fouling control methods such as air scouring and water backwashing in combination with vibration technique can enhance the flux and removal efficiency of microalgae in a submerged flat sheet microfiltration membrane bioreactor. Combining backwashing with air scouring (Phase 3) positively enhanced the flux by reducing the TMP, but it did not improve the permeate quality in terms of microalgae and turbidity removal. The results of Phase 4 studies confirmed that vibration had a profound effect on microalgae removal compared to backwashing technique while reducing the TMP from 105 to 103 kPa. The average turbidity removal was increased from 76.75% to 82% when vibration is applied. Combining all fouling control methods such as air scouring, backwashing and vibration methods did not appear to be effective for the removal of microalgae and turbidity (Phase 5). In order to signify the energy input on membrane fouling control, flux in each phase was assessed with respect to total power input. In overall, air scouring at a rate of 7 L/min with 50 Hz of vibration appeared to be an optimum condition in controlling fouling without any frequent backwashing.

Design considerations for biological ion exchange drinking water filters: Resin selection, backwash, and regenerations

AbstractTwo pilot studies provide insights into the design and operations of biological ion exchange (BIEX) drinking water filters. A lab‐scale pilot with strong‐base anionic (SBA) and weak‐base anionic (WBA) resins demonstrated 35% and 31% removal of dissolved organic carbon (DOC) up to 30 months, until secondary ion exchange exhaustion. While the WBA resins exhibited 14% less removal of DOC, their larger capacity doubled the duration until filter exhaustion. WBA filters were less affected than their SBA counterparts by sulfate‐containing inlet waters. In a second pilot, while water with high DOC yielded rapid exhaustion of SBA resins, air scouring increased the breakup of filter media and improved solids removal by 30× compared to hydraulic backwash alone. Significantly, DOC removal improved by 36% for a week following air scour. Key recommendations include the use of WBA resins to extend operating life while implementing air scouring can dramatically improve short‐term DOC removal.

Comparison of fine bubble scouring, backwash, and mass air load supply for dynamic membrane maintenance and steady operation in SFD MBR for wastewater treatment

The self-forming dynamic membrane bioreactor (SFD MBR) is an evolution of the conventional ultrafiltration-based MBR where filtration occurs through a cake layer (dynamic membrane, DM) supported by a filtration net. The effluent quality levels that may be achieved through this technology are comparable to those of conventional MBR and suitable for effluent reuse, as shown in previous research. In addition, SFD MBR are more sustainable, both in terms of investment costs (filtration net materials are cheaper than UF membranes), and operation. Indeed, the typical operating pressure of these systems is below 100 mbar, thus compatible with gravity-driven operation. Nevertheless, as all membrane-based processes, also SFD MBR require periodic maintenance cleaning of the supports holding the filtering layers, in order to ensure high and constant effluent quality and steady operation. In this paper, different strategies for continuous or temporized cyclic DM maintenance aimed at limiting the frequency of on-site manual cleaning interventions are compared. In particular, five tests were performed at the bench scale adopting the same process conditions except for the type of DM maintenance strategy. Continuous fine-bubble air scouring of the membrane surface was compared with two intermittent cyclic procedures based on backwash with permeate (with and without simultaneous air scouring) and mass air load supply (with two cycles of different durations), respectively. The results have confirmed the overall high performance of this technology under all the tested conditions, with COD removal efficiencies always exceeding 86 %, effluent turbidity values normally well below 5 NTU (and often much lower), complete nitrification and partial nitrogen removal. Furthermore, the comparison among the different DM maintenance strategies has shown that the mass air load supply is the most efficient approach towards sustainable operation. In particular, shorter mass air load cycles provided the best performance in limiting the manual cleaning needs, which under this condition were as low as once every 3 weeks. The key finding of this study is the demonstration of cyclic air mass load as an effective preventive cleaning strategy allowing good performance of a bench-scale SFD MBR, strongly reducing the manual maintenance requirements. More experiments are required to confirm these results in larger scale installations.

The Effect of Washing and Air Scouring on The Performance of Immersed Membrane Microfiltration

The quality of raw water in Surabaya is decreasing, and conventional treatment units are no longer efficient in treating raw water to become drinking water. Microfiltration membranes applied in sedimentation units can produce better quality results on limited land that uses a small number of chemicals. The material will be deposited on the membrane’s surface throughout the filtration process. The deposition of these materials can cause fouling, leading to a decrease in membrane productivity. This study involved a detailed investigation into the membrane washing method and air scouring implication in the microfiltration process to understand the most efficient technique to increase membrane productivity. Washing was done manually with a gentle scrubbing method and backwashing with time variation for 20 minutes and 10 minutes. Air Scouring was done with air pressure variations of 0,3 bar and 0,6 bar. The study showed that water flux was affected by the washing and air scouring method. Permeate quality was also found to be affected by washing and air scouring variation. Permeate quality analyses in this study are turbidity, TSS, micro-plastics, E-Coli, and organic substances.

Effects of Viscosity on Submerged Membrane Microfiltration Systems.

Submerged microfiltration has a wide range of applications in water and wastewater treatment. Membrane fouling is a major problem, resulting in a severe decline in flux, high energy consumption and frequent membrane cleaning and replacement. The effect of viscosity was not previously studied under controlled conditions to relate it to the air scour. Hence, this study investigated the effect of viscosity on membrane fouling during the operation of submerged membrane microfiltration by adding predetermined amounts of glycerol to a kaolin clay suspension. The addition of glycerol increased the viscosity (from 0.001 to 0.003 Pa·s), resulting in a 3-fold higher transmembrane pressure (TMP) development. An increased airflow (air scour) rate by 3 fold (from 0.6 m3/m2/h to 1.8 m3/m2/h), reduced TMP development by 65%. Membrane fouling quickly developed during the initial stage of microfiltration operation. Therefore, special precautions to control fouling during the early stages of filtration could significantly enhance the operation of the microfilter. Higher airflow caused a reduction in average specific cake resistance, whereas higher viscosity increased this value.

Mitigation of membrane fouling of alginate with combined actions of aeration and powdered activated carbon: Fouling behaviors and mechanisms.

A laboratory-scale flat-sheet ceramic microfiltration membrane system was developed to investigate the membrane fouling behaviors and mechanisms of sodium alginate (SA) in the presence of aeration and powdered activated carbon (PAC). When the SA concentration increased from 20 to 500 mg/L, the permeate flux decreased by 81.7%, and the transmembrane pressure (TMP) and resistance increased 1.7 and 24.5 times, respectively. At an SA concentration of 500 mg/L, it was found that the membrane fouling tended to decrease with the increase in the aeration rate, indicating high control of the fouling by air scouring, while PAC-aeration scouring produced a significant improvement in the permeate flux with substantially reduced fouling. In the microfiltration of 500 mg/L SA at an air flow rate of 2.2 L/min and PAC concentrations of 40, 100, and 250 mg/L, the flux increased by 179.3%, 238.0%, and 302.7%, the TMP decreased by 32.6%, 34.8%, and 45.7%, and the cake and pore blocking resistance decreased by 78.0%, 85.1%, and 87.9%, respectively, compared to the corresponding values without PAC-aeration scouring. Intermediate blocking and complete blocking models were confirmed to elucidate the effect of aeration scouring and PAC-aeration scouring on the mitigation of membrane fouling by SA. PRACTITIONER POINTS: Air scouring was effective at mitigating membrane fouling of sodium alginate. The addition of PAC could alleviate membrane fouling of SA. Synergistic scouring by aeration and PAC offers a promising means for more-efficient and cost-effective control of membrane fouling. The fouling mechanisms in various scenarios were elucidated.

Periodic fouling control strategies in gravity-driven membrane bioreactors (GD-MBRs): Impact on treatment performance and membrane fouling properties

This study aims to assess the effects of periodic membrane fouling control strategies in Gravity-Driven Membrane Bioreactor (GD-MBR) treating primary wastewater. The impact of each control strategy on the reactor performance (permeate flux and water quality), biomass morphology, and fouling composition were evaluated. The application of air scouring coupled with intermittent filtration resulted in the highest permeate flux (4 LMH) compared to only intermittent filtration (i.e., relaxation) (1 LMH) and air scouring under continuous filtration (2.5 LMH). Air scouring coupled with relaxation led to a thin (~50 μm) but with more porous fouling layer and low hydraulic resistance, presenting the lowest concentration of extracellular polymeric substance (EPS) in the biomass. Air scouring under continuous filtration led to a thin (~50 μm), dense, compact, and less porous fouling layer with the highest specific hydraulic resistance. The employment of only relaxation led to the highest fouling formation (~280 μm) on the membrane surface. The highest TN removal (~62%) was achieved in the reactor with only relaxation (no aeration) due to the anoxic condition in the filtration tank, while the highest COD removal (~ 60%) was achieved with air scouring under continuous filtration due to the longer aeration time and the denser fouling layer.The results highlighted the importance of performing in-depth fouling characterization to link the membrane fouling properties to the hydraulic resistance and membrane bioreactor performances (i.e., water quality and water production). Moreover, this work proven the versatility of the GD-MBR, where the choice of the appropriate operation and fouling control strategy relies on the eventual discharge or reuse of the treated effluent.

Startup and initial operation of an MLE-MABR treating municipal wastewater.

A 630 m3/d pilot plant was installed at Subiaco WRRF to determine design and operational parameters of a hybrid Modified Ludzack-Ettinger - Membrane Aerated Biofilm Reactor (MLE-MABR) configuration. Two commercial ZeeLung MABR cassettes were installed in series in the anoxic zone and the pilot was fed with primary effluent (averaging COD 601 mg/L, TKN 68.5 mg/L and 17-29 °C). A nitrifying biofilm was developed within 3 weeks and the nitrous oxide (N2O) gas emissions from the MABR exhaust gas proved to be a reliable parameter to assess biofilm development. Both MABRs achieved the average nitrification rate (NR) of 3.7 gNH4-N/m2.d when air flow was 8.6 and 11.2 Nm3/h to MABR1 and MABR2 respectively, which reached a maximum oxygen transfer rate of 17.4 gO2/m2.d. Biofilm thickness was controlled via air scouring and intermittent coarse bubble mixing (90 s on/90 s off). This paper discusses the startup strategy, minimum requirements for process monitoring, impact of different air flow conditions, ORP and mixing patterns on performance efficiency over a 22-week period.

Reclamation of pharmaceutical wastewater by UAF, contact oxidation, and auto brush-cleaning aiding nano ZrO2 coated ceramic membrane MBR - an industrial-scale practice and functional microbial diversity

This project aimed to assess the feasibility of applying UAF/contact oxidation followed by ceramic membrane MBR in reclaiming 1400 m3/day of pharmaceutical wastewater with COD and NH4+-N of around 8400 and 135 mg/L, respectively. UAF and contact oxidation system employed novel porous lightweight ceramsite as microbial carriers to facilitate the biomass retention. Results revealed that the integrated biological treatment experienced excellent and stable performance in the operation of successive 160 days in terms of COD and NH4+-N abatement, around 96.8% and 81%, respectively. With the aid of auto mechanical bush and air scouring in submerged flat-sheet nano ZrO2 coated ceramic membrane MBR for mitigating the membrane fouling, the MBR exhibited stable and better performance to remove suspended solid and soluble microbial products with around flux of 25 LMH and no severe fouling was observed during the whole treatment process. The MBR effluent can be reclaimed for road cleaning, urban landscaping, etc. Miseq sequencing analysis in the sludge-day 160 indicated that UAF was rich in Firmicutes with 42.02% relative abundance, followed by Thermotogae (22.91%) and Bacteroidetes (13.59%), which can decompose macromolecules into small molecules. Moreover, some species in Thermotogae and Bacteroidetes accounted for generation of biofilm in the ceramsite in UAF. Genus Petrotoga played a key role in conspicuous COD removal in UAF. The bacterial species composition in contact oxidation exhibited substantially discrepant. Appearance of Proteobacteria, as the dominant phylum (41.7%) can be responsible for COD abatement. Genus Thauera, Acinetobacter and Aeromonas detected had the capacity in NH4+-N removal.

Chemical cleaning procedures on permeability recovery and lifespan of MBR membranes treating petroleum refinery wastewater: From bench- to pilot-scale applications

Different cleaning strategies for membranes from membrane bioreactors (MBR) were investigated and applied to a pilot MBR treating real refinery wastewater. It was assessed different cleaning agents (sodium percarbonate, dodecyl sulfate, citric acid, oxalic acid, and sodium hypochlorite) at different concentrations (500–5000 ppm), temperatures (22–40 °C), pH (without adjustment and pH 12), with and without air scouring, for membranes' permeability recovery. The best cleaning sequence was defined to be oxalic acid (pH 2.5) followed by NaOCl (5000 ppm), which was responsible for the inorganic fouling removal and allowed the oxidant agent to remove more effectively the organic fouling layer. Improvement in permeability recovery at 40 °C (92.7% of the initial permeability) and higher NaOCl concentration (5000 ppm) was found due to the increase in chemical oxidation kinetics and the diffusion rate of the cleaning agents within the fouled layer and membrane. Oxalic acid followed by sodium hypochlorite (40 °C) had a lower effect on membranes' morphology and thermal resistance, proven by thermogravimetric analysis and Fourier transform infrared spectroscopy. The pilot-scale operation was used to estimate the membrane lifespan which was approximately 1916 days of monitoring – about five years.

Enhanced anti-fouling performance in Membrane Bioreactors using a novel cellulose nanofiber-coated membrane

• The fouling of TFNC-CNF membranes is reversible when filtering wastewater. • Enhanced flux was achieved in the submerged TFNC-CNF membrane bioreactor. • TFNC-CNF membranes showed higher rejection potential of bio-foulants. • Air scouring enhanced the flux of TFNC-CNF membranes when filtering wastewater. The filtration performance, fouling and flux recovery efficiency of a novel thin film nanofibrous composite-cellulose nanofiber (TFNC-CNF) coated membrane was studied and compared with polyvinylidene fluoride (PVDF) membranes treating domestic wastewater. The enhanced anti-fouling property of the TFNC-CNF membrane was associated with the super-hydrophilic nature and negative charge on the membrane surface. The dead-end cell filtration tests demonstrated the TFNC-CNF membrane recovered more than 90% of the initial flux after mechanical cleaning, compared with 26–43% recovery by PVDF membranes. Continuous wastewater ultrafiltration tests with mechanical air scouring confirmed the outstanding permeability (71.3–138.9 LMH/bar) and the superior anti-fouling characteristics (59.2–86.8% recovery of the initial flux) of the TFNC-CNF membrane. Furthermore, higher total organic carbon rejection rates were also observed in the TFNC-CNF membrane (>83.2%) than the PVDF membrane (<69.8%). Collectively, the superior flux recovery and high permeability makes the TFNC-CNF membrane a promising material for membrane bioreactors.

Submerged osmotic processes: Design and operation of hollow fiber forward osmosis modules

Submerged forward osmosis (FO) is of high interest to treat and concentrate complex streams where cross flow module suffers from limitations such as breakage of fragile compounds, viscosity issue or fouling and clogging limitations. In this study, submerged hollow fiber FO (Sub-HFFO) modules were designed using commercial HFFO membranes. Then, modules were validated, compared to commercial HFFO cross flow module, and tested in different operational conditions (draw concentration and type, feed concentration and feed tank scouring). Finally, fouling tests were performed using model foulants. Designed Sub-HFFO modules featured similar performances than commercial cross flow modules. It was demonstrated that draw circulation in Sub-HFFO can be done in pulsing mode without mass-transfer limitation due to the homogeneous flow distribution and self-supporting of the membranes, in opposition to Sub-FO plate and frame design. This study also confirmed that providing mixing near the membrane surface in the feed tank is a key factor to mitigate not only fouling but also external concentration polarization (ECP) limitation. Air scouring and recirculation tank proved to be efficient towards more sustainable operation; full recovery of initial flux after fouling was achieved thanks to osmotic backwashing cleaning.

Role of air scouring in anaerobic/anoxic tanks providing nitrogen removal by mainstream anammox conversion in a hybrid biofilm/suspended growth full-scale WWTP in China.

A full-scale wastewater treatment plant in China experienced unintentional anammox bacterial enrichment on biofilm carriers placed in the anaerobic and anoxic zones of an anaerobic/anoxic/oxic process under ambient temperatures and without bioaugmentation. Here, we show that microaerophilic conditions resulting from air scouring needed for biofilm carrier suspension in the anaerobic/anoxic zones can support a robust nitritation/anammox process. Results from an in situ on/off air scouring test showed that air scouring strongly induced both ammonia and total inorganic nitrogen removal in the anaerobic/anoxic zones. Ammonium concentration in the anaerobic and anoxic tanks remained constant or even slightly increased when air scouring was off, indicating that air scouring made a noticeable difference in nitrogen profiles in the anaerobic/anoxic zones. Various batch tests further indicated that partial denitrification is not likely to generate nitrite for anammox bacteria. Robust nitritation, and anammox on the carriers, can occur at low dissolved oxygen conditions, as measured in the full-scale facility. The observations show that mainstream deammonification without sidestream bioaugmentation at moderate temperature is feasible and further optimization by a more dedicated design can result in improved nitrogen removal in cases when chemical oxygen demand is limited in mainstream wastewater treatment. PRACTITIONER POINTS: Microaerophilic conditions in a full-scale IFAS reactor caused mainstream anammox in moderate temperate area. Robust nitritation, and anammox on the carriers, can occur at low dissolved oxygen conditions in anaerobic/anoxic tanks with air scouring. Anammox can function well with conventional nitrification and denitrification process at mainstream conditions for stable nitrogen removal.

Air flow and scour patterns around erosion control elements

Abstract A series of wind tunnel experiments was carried out using isolated, surface-mounted elements of varied idealized geometric form to evaluate the relative efficacy of these in wind erosion control. The topography of the bed surface was scanned in high resolution, before and after each wind erosion event, giving the net volume change. The perturbed airflow structure was sampled via laser Doppler anemometry. The volume of erosion initiated by the flow perturbation relative to that of the roughness element was greatest for a rectangular prism in either orientation (face-on 2.5; edge-on 4.0), minimal for a hemisphere (0.2), and below detection for a reference (streamlined) form. Roughness elements that trigger strong downwelling in the perturbed flow and vortex impingement on the bed surface appear to initiate particle motion. Consequently, they are less desirable for wind erosion control, even though they usually have large form drag and demonstrate substantial kinetic energy (KE) dissipation in the wake flow. Hemispheres, with generalized upwelling along both the windward and leeward faces and strong turbulent KE dissipation associated with flow separation along the sidewalls, may present an optimal solution with minimal bed scour.

Grain displacement during backwash of drinking water filters

Abstract Backwashing rapid sand filters causes inadvertent displacement of filter media grains from their previous depths. This displacement can affect the hydraulic function of filters by mixing or segregating media grains, and the function of biofilters through displacement of active biomass and coatings from proper depths. This study quantifies grain displacement in a pilot-scale filter using tracer grains of colored sand, glass beads, anthracite and garnet to determine the effect of grain size, density and shape on grain displacement. Statistical moments are used to describe the depth distributions resulting from displacement during backwashing. Results show that significant grain displacement occurs during backwash consisting of air scour, air-and-water wash and sub-fluidization water-only wash. Here, displacement is largely independent of grain size, density and shape. When fluidization backwash is used, greater displacement and more dependence on grain characteristics is seen. A variety of grain movement phenomena can be observed during the backwashing steps, indicating that grain movement and therefore the resulting displacement is highly inhomogeneous in four dimensions. These results have direct practical implications for the design of rapid sand filters and the optimization of backwashing procedures, while suggesting that the current widespread backwashing practice used in the case study country (Denmark) should be abandoned.