Dewatering Characteristics Of Sludge Research Articles

Optimizing sludge dewatering efficiency with ultrasonic Treatment: Insights into Parameters, Effects, and microstructural changes

Sludge dewatering plays a critical role in the efficient and cost-effective management of wastewater treatment plants. Ultrasonic treatment has emerged as a promising technique for improving dewatering processes. This study aims to evaluate the impact of ultrasonic treatment on sludge dewatering characteristics. A series of experiments were conducted to evaluate the dewatering characteristics of sludge under ultrasonic treatment. Experimental data was collected, and the effects of ultrasonic parameters on dewatering efficiency were analyzed. Ultrasound has the capacity to disintegrate sludge flocs, liberate tightly bound water, and enhance sludge dewatering capabilities. The application of ultrasound leads to the breakdown of sludge flocs, which facilitates a substantial amount of organic acids or carbonates. This, in turn, modifies the pH value of the sludge. Additionally, ultrasound induces instantaneous high temperature and pressure within the liquid phase, consequently elevating the temperature of the sludge slurry. Optimum ultrasound energy density and duration of ultrasound treatment exist. For the sludge samples analyzed in this investigation, it was determined that the optimal ultrasonic energy density is 9.8 W, while the optimal duration of ultrasound treatment is 30 s. Excessively escalating the sound energy density or prolonging the duration of ultrasound may yield unfavorable outcomes in terms of sludge dewatering effectiveness. To enhance sludge dewatering, it is crucial to select appropriate ultrasonic energy density and duration of ultrasonic treatment. This study demonstrates the positive impact of ultrasonic treatment on the dewatering characteristics of sludge. The findings provide valuable insights into the potential of ultrasonic technology for enhancing sludge dewatering.

Analysis on the properties of hydrolyzed amino acids in typical municipal sludge.

In this study, amino acids, proteins, and microbial communities in sludge from different wastewater treatment plants (WWTPs) were analyzed. The results showed that the bacterial communities of different sludge samples were similar at the phylum level, and the dominant bacterial species in sludge samples with the same treatment process were the consistent. The main amino acids in EPS of different layers were different, and the amino acid results of different sludge samples were quite different, but the content of hydrophilic amino acids in all samples was higher than that of hydrophobic amino acids. And the total content of glycine, serine, and threonine related to sludge dewatering was positively correlated with protein content in sludge. In addition, the content of nitrifying bacteria and denitrifying bacteria in sludge was also positively correlated with the content of hydrophilic amino acids. In this study, the correlations between proteins, amino acids, and microbial communities in sludge were analyzed respectively, and the internal relationship was found. And it provided ideas for further study of sludge dewatering characteristics in the future.

Analysis of the effect of using divalent and trivalent iron compounds on sludge dewatering performance in thickening units

Abstract Sludge is one of the by-products of wastewater treatment plants (WWTPs). To assist biological processes, part of the produced sludge is returned to the treatment process and the excess is removed from the treatment plant whereby it undergoes thickening and dewatering. Numerous studies have been conducted to investigate the effect of different coagulants on dewatering from sludge, but the effect of using divalent iron for this purpose has not been investigated so far. In this research, the effect of divalent and trivalent iron compounds (ferrous sulfate and ferric chloride) on the dewatering of excess sludge of the first module of the Bojnourd WWTP (Iran) has been investigated. In this regard, first, the effect of different doses of each coagulant to optimize the dewatering characteristics of sludge was investigated and then the effect of pH change by adding lime was investigated. The results showed that the addition of optimal doses of FeSO4 (0.6 and 0.4 g/l) and lime (0.664 and 1.5866 g/l) reduced the capillary suction time of sludge by 30.6 and 32.7%, respectively, while reducing the moisture content of sludge cake by 26 and 30.6%.

Study on the Effect of Chemical Conditioning on the Consolidation Characteristics of Municipal Sludge in Landfill Depot

Vacuum preloading is one of the effective methods for in-situ dewatering and reduction treatment of municipal sludge landfill, but because the consolidation coefficient of municipal sludge is 1 or 2 orders of magnitude smaller than that of sludge, the drainage consolidation technology will face the problem of vacuum for too long. In order to improve the consolidation characteristics of sludge, combined with the chemical conditioning technology in the field of water treatment, firstly, the chemical conditioner suitable for landfill sludge was selected. The effects of different conditioning combinations on sludge dewatering characteristics were studied by microscopic testing techniques (zeta potential analysis and environmental scanning electron microscope analysis) and vacuum filtration test and consolidation test. The results show that the combination of ferric chloride and calcium oxide can significantly improve the dewatering characteristics of sludge. After conditioning with 10% ferric chloride and 10% calcium oxide (relative to sludge dry base), the specific resistance of sludge is reduced from 2 × 1013m/kg to 3 × 1012m / kg, and the consolidation coefficient is 3.5-10 times higher than that of original sludge, which lays a foundation for improving the efficiency of vacuum preloading treatment. In addition, from the results of vacuum filtration test and consolidation test of sludge, it can be found that there is a good negative linear relationship between specific resistance and consolidation coefficient of sludge. Compared with the consolidation test, the vacuum filtration test is simple and easy, so it can be used as an effective means to select sludge conditioner or test the conditioning effect.

An investigation of agricultural use potential of dewatered sewage sludge

Background: One of the useful applications of Dewatered sludge (DWS) of municipal wastewater treatment plants (WWTPs) is its use as manure in agriculture; therefore, its quality characteristics should be specified. The aim of this research was to determine biological and physicochemical characteristics of DWS of Sari WWTP and compare them with standards, and also to investigate its potential use in agriculture. Methods: Sludge samples were taken from the sewage sludge of Sari WWTP. Sampling and analysis of samples parameters including fecal coliform, salmonella, helminth ova, carbon, nitrogen, C/N, phosphorus, organic matter, potassium, moisture, electrical conductivity, and PH, were performed during four seasons with three replications based on the standard method. Results: The fecal coliform, salmonella, and helminth ova of the DWS were 2.37×106 ± 1.06×106 MPN/1 g d.s weight, 47±12.92 MPN/4 g d.s weight, and 466±61.85 number/4 g d.s weight, respectively, therefore, the DWS of Sari WWTP was categorized in the class B of the EPA standard. The amounts of C/N, organic matter, carbon, nitrogen, phosphorus, potassium, moisture, electrical conductivity, and PH were obtained to be 12.7±1.15, 42.4±3.27%, 24.6±1.89%, 1.94±0.13%, 2.35±0.6%, 0.57±0.13%, 82±3.12%, 1.34±0.21 ds/m, and 7.41± 0.45, respectively. Conclusion: The DWS of Sari WWTP has a good fertility value but it cannot be safely used in agriculture and should be improved for class A by the Processes to Further Reduce Pathogens (PFRP), especially by composting.

Variations in sludge organic composition and dewatering behaviors through visible light driven photocatalysis: In-situ O2 generation

In this research, carbon and nitrogen doped TiO2 nano-particles were successfully synthesized and immobilized onto inner surface of a tubular thin film reactor (TTFR). CO3-NxOy-TiO2 nano-composite thin films (NCTF(s)) were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM-EDX), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Characterization results indicated that the CO3-NxOy-TiO2 NCTF has a broad adsorption edge in the visible light region. Decorated TTFR was used to observe the impacts of visible light irradiation on sludge dewatering characteristics and organic structure. In the experiments, H2O2, NaOCl and their various gravimetric combinations were added to the sludge as electron acceptor combination (EAC). Obtained experimental results revealed a photocatalytic synergy between hydrogen peroxide and chlorite molecules. The optimum H2O2 and OCl mixing dosage for photocatalytic and photolytic experiments was determined to be 0.02 g H2O2 g−1 DS and 4 × 10-3 g−1 NaOCl g−1 DS, respectively. The pseudo-first order kinetic constants (kobs) for volatile solids (VS) removal observed in EAC3 (with NCTF), EAC2 (with NCTF) and photolysis were 0.140 min−1, 0.045 min−1 and 0.025 min−1, respectively. The average VS removal rate depending on the applied unit photonic power to unit TTFR area was 218 ± 14 mgVSremoved W−1 h−1 cm−2. Experimental results obtained using different EAC(s) dosage schemes revealed that O2 was the main active specie and HO radicals also contributed to sludge treatment. Naphthalene, pyrene, anthracene and phenanthrene were detected in raw sludge and removed 87 ± 1% by the method.

Enhanced sludge solubilization and dewaterability by synergistic effects of nitrite and freezing

In this study, nitrite was added into sludge during freezing process to evaluate its role in waste activated sludge (WAS) solubilization and effect on sludge dewatering characteristics. The results showed that the introduction of 100 mg L−1 of nitrite could increase dissolved organic carbon (DOC) concentration from 29.5 to 48.8 mg DOC g−1 VSS under freezing conditions. More DOC was released with the increase of nitrite concentration. Freezing temperature, or freezing speed, also played a role in sludge solubilization. It was found that some readily-biodegradable low molecular weight (LMW) compounds, e.g. LMW protein, LMW polysaccharide, LMW neutrals, building blocks and LMW acids, were mainly released during the freezing process with the presence of nitrite. Interestingly, nitrite could also improve the sludge filterability at the lower nitrite concentration as a result of the increased sludge particle size. However, electrolytes (sodium nitrite) addition effects may mask such enhancement when nitrite concentration was high (800 mg L−1). The rheological characteristics of sludge could be well modeled by Herchel-Bulkley model and the introduction of nitrite into freezing process further increased sludge flowability and decreased sludge viscosity. These results indicated that freezing with the presence of suitable concentration of nitrite could promote sludge solubilization and dewaterability. As such, good liquid and solid separation can be achieved with the recovery of liquid stream as carbon source.

Influence analysis for the behavior of dewaterability of excess sludge in a two-stage vermifilter.

To improve excess sludge dewaterability, a two-stage vermifilter was developed to qualitatively and quantitatively analyze sludge physico-chemical properties (fractal dimension, zeta potential, extracellular polymeric substances (EPS), particle size distribution, etc.) and to correlate them with sludge dewatering characteristics (specific resistance to filtration (SRF) and capillary suction time (CST)). Results demonstrated that sludge dewatering performance was significantly improved after the primary vermifilter VF1 and the second-stage vermifilter VF2. In addition, the further VF2 treatment exhibited higher effects on sludge dewatering performance. The particle boundary of sludge after VF2 treatment was clearer and smoother than VF1 sludge (VF1S), apart from the fact that sludge morphological structure got denser and more compact. Comparing with VF1S, the fractal dimension D1 calculated within 1D topological space was closer to 1 after VF2 treatment, and the fractal dimension D2 within 2D topological space closer to 2, indicating a better dewatering performance after VF2 treatment. Additionally, the changes of sludge floc surface properties (such as zeta potential and EPS) resulted in small particles agglomerating into larger ones and then the increase of particle diameter. In summary, the two-stage vermifilter got a better sludge dewatering performance, and thus beneficial for subsequent processing of sludge.

Study of sludge moisture distribution and dewatering characteristic after cationic polyacrylamide (C-PAM) conditioning

Dewatering is a key step in sludge handling, and flocculant conditioning is the commonest way to improve the sludge dewatering. The sludge dewatering performance is largely affected by the sludge moisture distribution. But changes of sludge moisture distribution by flocculants were unclear and literature findings were controversial to each other. This paper investigated the changes of sludge moisture distribution and sludge dewatering characteristics after flocculant conditioning, and the flocculant used was cationic polyacrylamide (C-PAM). Results showed that C-PAM conditioning effectively changed the sludge moisture distribution, sludge viscosity, specific resistance of filtration (SRF), particle size, extracellular polymeric substances (EPS) concentration, and filtrate liquid viscosity. Especially, free water was increased by C-PAM conditioning and the highest value reached 95.04%. On the other hand, the interstitial water content was decreased by C-PAM conditioning. The optimal dosage was 3 mg g−1 TS considering both effects and cost. After C-PAM conditioning, the mean sludge particle size increased to 219.1 μm, the sludge SRF reduced by 87%, and the supernatant EPS decreased. Specifically, the filtrate liquid viscosity could also determine the optimal dosage of C-PAM and an equation was proposed for the correlation between filtrate viscosity and C-PAM dosage.

Evaluation on the dewatering process of cyanobacteria-containing AlCl3 and PACl drinking water sludge

Most researches focused on the high efficiency removal of cyanobacteria from water phase during drinking water treatment, but did not address the potential danger of cyanobacterial cells being transferred into the sludge, especially during the dewatering process. In this study, the characteristics of cell lysis and microcystin (MC) release in the filtration of cyanobacteria-containing sludge were investigated. By evaluating filtration rates, filtrate microcystins (MCs) and turbidity levels, and SEM micrographs of the sludge in different operating conditions, the influences of mechanical actions and physical/chemical effects on the integrity of cyanobacterial cells were explicated, and the corresponding operation conditions were also optimized. The results showed that the sludge accumulation on unit filter area had significant influence on sludge dewatering characteristics, and with increase in the volume of sludge filtrated, more and more sludge accumulated on the surface of the filter media, resulting in reduced filtration rates and potential cyanobacteria lysis. More hydrophilic filter media with lower porosity should be chosen to enhance filtration efficiency. Compared with positive pressure filtration, vacuum filtration with low-destructibility should be a better choice considering the MC release. Under increased vacuum pressure, cyanobacteria-containing sludge had increasing filtration rate without causing damage of cyanobacterial cells. While the storage time of cyanobacteria-containing sludge should be severely restricted in actual water supply factories (4d for AlCl3 sludge and 2d for PACl sludge) to avoid the adverse influences of cyanobacteria lysis on sludge dewaterability and filtrate quality. This study produced deeper knowledge on the fate of cyanobacterial cells and intracellular MCs in the sludge dewatering process, and the results provided valuable scientific references for volume reduction of cyanobacteria-containing sludge in the actual treatment operations.

Recovery of volatile fatty acids from fermentation of sewage sludge in municipal wastewater treatment plants

This work investigated the pilot scale production of short chain fatty acids (SCFAs) from sewage sludge through alkaline fermentation and the subsequent membrane filtration. Furthermore, the impact of the fermentation liquid on nutrient bioremoval was examined. The addition of wollastonite in the fermenter to buffer the pH affected the composition of the carbon source produced during fermentation, resulting in higher COD/NH4-N and COD/PO4-P ratios in the liquid phase and higher content of propionic acid. The addition of wollastonite decreased the capillary suction time (CST) and the time to filter (TTF), resulting in favorable dewatering characteristics. The sludge dewatering characteristics and the separation process were adversely affected from the use of caustic soda. When wollastonite was added, the permeate flux increased by 32%, compared to the use of caustic soda. When fermentation liquid was added as carbon source for nutrient removal, higher removal rates were obtained compared to the use of acetic acid.

Comparative study of dewatering characteristics of metal precipitates generated during treatment synthetic polymetallic and AMD solutions

The sludge dewatering characteristics expressed in terms of settling, filtration and centrifugation of metal precipitates generated during treatment of polymetallic solutions and synthetic acid mine drainage have been evaluated in this research. Results show that dewatering properties of metallic sludge vary depending on the type of matrix (Cl −; SO 4 2−), precipitating agent, and metals present in effluent. Metal hydroxides (at pH 10.0) and metal phosphates precipitates (at pH 7.0) are amorphous in nature, thus difficult to dewater. In these treatment methods, the substitution of chloride matrix by sulphate one improves considerably dewatering properties (specific resistance to filtration = 6.60 × 10 13 and 2.35 × 10 13 m/kg for the chloride and sulphate matrix, respectively). In the case of sulphide and carbonate treatments (pH 8.0), precipitates obtained are semi crystalline, and crystalline form, respectively, and no influence of the matrix was detected on dewatering characteristics.

Comparative study of dewatering characteristics of metal precipitates generated during treatment of monometallic solutions

The sludge dewatering properties (settling, filtration and centrifugation) of metal precipitates generated during treatment of monometallic solutions (0.020 mol/L) have been evaluated in this research. The precipitation tests carried out on 15 different metals gave metal removal yields generally similar to those predicted by MINEQL+ software, with the exception that kinetic aspects should be considered during precipitation of metal sulphides and phosphates. Hydroxides precipitation at pH 10.0 was the most efficient technique for the removal of eight metallic ions (Al 3+, Cd 2+, Co 2+, Fe 2+, Fe 3+, Mg 2+, Mn 2+, Ni 2+), whereas phosphates precipitation (at pH 6.0 with an addition of 0.0133 mol PO 4 3−/L) gave highest removal yields for Ba 2+, Ca 2+, Cr 3+. Sulphides precipitation (at pH 7.0 and using, 0.020 mol S 2−/L) has been found the most efficient technique only for Cu 2+ and Sn 2+ precipitation, whereas carbonates precipitation (at pH 8.0 and using 0.020 mol CO 3 2−/L) gave better removal yield only for Pb 2+. Results have also shown that metal phosphates have generally better dewatering characteristics (SVI, filtration capacity, SRF, sludge solids content) than metal carbonates, sulphides and hydroxides. In fact, considering only the sludge dewatering characteristics, phosphates precipitation appears the most appropriate technique for the precipitation of many metals (Al 3+, Ba 2+, Cd 2+, Co 2+, Cr 3+, Fe 2+, Fe 3+, Ni 2+ and Zn 2+). Metal hydroxides formation constitutes the best option for Ca 2+, Cu 2+, Mg 2+ and Sn 2+ removal, whereas precipitation of metal carbonates is particularly interesting for treatment of Mn 2+ and Pb 2+ containing solutions.

A study of dual polymer conditioning of aluminum-based drinking water treatment residual

Conditioning of an aluminum-based water treatment sludge by single and dual polymers was investigated in this study. Capillary suction time (CST), specific resistance to filtration (SRF) and settling rate were used to evaluate the sludge dewatering characteristics. Sludge dewaterability resulting from single and dual polymer conditioning were compared for the purpose of exploring the validity and effectiveness of dual polymer conditioning strategy. Unlike activated sludge derived from wastewater treatment, results from this study have demonstrated that conditioning of the alum sludge by the combined use of an cationic polymer (FO-4140) followed by an anionic polymer (LT-25) does not exhibit considerable advantage in further improvement of sludge dewaterability with comparison of single polymer conditioning. This study supports the view that for aluminum-based water treatment sludge, inter particle bridging seems to be the dominative mechanism and the charge neutralization plays a less important role in the conditioning process. In addition, an intrinsic relationship between CST and SRF was deducted and tested via the experimental data obtained from the study.

Improvement of Activated Sludge Dewaterability by Humus Soil Induced Bioflocculation

Effects of humus soil particles on the dewaterability of activated sludge were investigated. Cations leaching increased proportionally with the dosage of humus soil, and the leaching was not significant after 2 h. Divalent cations, Ca2+ and Mg2+, leaching from the humus soil played an important role in improving dewaterability of the biological sludge. On the contrary, dewaterability was not affected or slightly deteriorated by the monovalent cations, K+ and Na+ leached from the humus soil. Improvement in dewaterability of the sludge by addition of humus soil was higher than that of equivalent cations mixture. It seemed that the decrease of supracolloidal bio-particles (1 to 100 µm in diameter) resulted in diminishing of the blinding effect on cake and filter medium. SRF (specific resistance to filtration) of the humus soil added sludge varied in parallel with the M/D (monovalent to divalent cation) ratio, and the M/D ratio could be utilized as a useful tool for evaluation of the sludge dewatering characteristics. Long-term effects of humus soil on the improvement of activated sludge dewaterability were clearly identified by continuous operation results of a bench-scale MLE (Modified Ludzack Ettinger) system combined with a humus soil contactor. On the other hand, dewaterability of the control sludge was only slightly improved by a decrease in M/D ratio of the wastewater influent.

Effects of Surfactant Addition on Dewatering of Alum Sludges

In Taiwan, surfactants are frequently used in the flotation process to aid in solid/liquid separation. Their effect on the dewatering of alum sludge was investigated. Various amounts of cationic and anionic surfactants were added to sludge samples, and the dewatering characteristics of the sludge and the water content of sludge cakes were evaluated. Both surfactants improved the dewatering of the sludge by lowering the specific resistance to filtration, decreasing the bound water content, and increasing the dewatering rate of the sludge. Different combinations of anionic and cationic surfactants and polyelectrolytes were also experimented on to study the effect of surfactant addition on the dewatering characteristics of polyelectrolyte-conditioned sludge. Experimental results indicated that both cationic and anionic surfactants adversely affected the dewatering of the conditioned sludge. The addition of surfactant to the oppositely charged polyelectrolyte proved to be most detrimental to sludge dewatering due to the precipitation between surfactant and polyelectrolyte. The addition of cationic surfactant to the cationic polyelectrolyte-conditioned sludge had the least effect.

Correlation between dewatering index and dewatering performance of three mechanical dewatering devices

Efficient sludge conditioning can improve sludge dewatering characteristics and promote the separation of flocs from the liquid phase to achieve a high solid content. To optimize this process, a reliable dewatering index must be established. Although numerous researches investigated sludge conditioning, few are devoted to the correlation between the conditioning index and the dewatering efficiency. In this study, sludges were conditioned with both synthetic and natural polymers, and their dewatering characteristics after three different mechanical dewatering devices, the filter press, belt press, and the centrifuge, were compared. The result shows that the traditionally used dewatering index may not reflect sludge conditioning properly. Correlation between sludge conditioning indices and dewaterability depends upon both the type of sludge and the means of sludge conditioning and dewatering. It is therefore concluded that dewatering index must be properly chosen for optimum sludge conditioning.

Effects of humus soil on the settling and dewatering characteristics of activated sludge

Effects of humus soil on the settling and dewatering characteristics of the activated sludge were investigated. Addition of the humus soil improved the sedimentation characteristics of the activated sludge remarkably. The initial lag phase was diminished and the zone settling velocity increased as the humus soil dosage increased. For estimation of the chemical effect of the humus soil on sludge settling, cation mixtures equivalent to the cations leaching from the humus soil were added and only the chemical effect was examined. The physical effect was then isolated by subtracting the chemical effect. The chemical effect on sludge settling was about30∼50% of the equivalent humus soil powder dosage and the physical effect was more important in improvement of the sludge settling properties. Dewatering characteristics of the activated sludge were dependent on both the concentration and distribution of the cations leaching from the humus soil. Divalent cations, calcium and magnesium from the humus soil decreased the bound water content of the sludge, which in turn increased floc density and cake solids content. However, excess sodium ion increased the SRF (specific resistance to filtration) slightly. Addition of humus soil to thickened activated sludge improved the dewatering properties, which could be well explained by the effect of the cations leaching. The SRF showed a good trend with the monovalent to divalent cation (M/D) ratio, which could be utilized as a useful tool for evaluation of the sludge dewatering characteristics.

Process optimisation of a combined system of floating medium and sand filter in prawn farm effluent treatment

A treatment technology known as “a combined system of floating medium and sand filter (FMSF)” was developed and tested successfully with prawn farm effluent. This system has a remarkable techno-economical advantages. Laboratory and semi pilot-scale studies were carried out to optimise the filter bed depth, backwash method and other operating conditions. The dewatering characteristics of sludge from filter backwash was also analysed. The filter was operated at a high rate. The experimental results indicated that: (i) in case of no in-line chemical addition, the smaller ratio between floating medium and sand filter depth gave rise better filter performance. At the filtration rate of 7.5–20 m3/m2.h and with an in-line chemical addition, the suitable depth of floating medium varied from 400–1000 mm for a sand filter depth of 400 mm; (ii) frequent (once in every 90–120 minutes) but short duration of backwash (not more than 60 seconds) was found to be suitable. During the backwash, the water and air were sent for 30 seconds in upward direction and then followed with upflow of water for another 30 seconds. Backwash water amount comprised only 1.2–1.8% of the filtered water production. A mechanical backwash system using rotating paddles is a promising alternative for floating medium filter; and (iii) the filterability of the sludge from filter backwash was low in case of no in-line chemical addition (specific resistance, r = 9.34 × 1010 m/kg) but improved with in-line flocculant addition (r = 3.07 × 109 − 1.29 × 1010 m/kg).

Process optimisation of a combined system of floating medium and sand filter in prawn farm effluent treatment

A treatment technology known as “a combined system of floating medium and sand filter (FMSF)” was developed and tested successfully with prawn farm effluent. This system has a remarkable techno-economical advantages. Laboratory and semi pilot-scale studies were carried out to optimise the filter bed depth, backwash method and other operating conditions. The dewatering characteristics of sludge from filter backwash was also analysed. The filter was operated at a high rate. The experimental results indicated that: (i) in case of no in-line chemical addition, the smaller ratio between floating medium and sand filter depth gave rise better filter performance. At the filtration rate of 7.5 – 20 m3/m2.h and with an in-line chemical addition, the suitable depth of floating medium varied from 400-1000 mm for a sand filter depth of 400 mm; (ii) frequent (once in every 90–120 minutes) but short duration of backwash (not more than 60 seconds) was found to be suitable. During the backwash, the water and air were sent for 30 seconds in upward direction and then followed with upflow of water for another 30 seconds. Backwash water amount comprised only 1.2–1.8% of the filtered water production. A mechanical backwash system using rotating paddles is a promising alternative for floating medium filter; and (iii) the filterability of the sludge from filter backwash was low in case of no in-line chemical addition (specific resistance, r = 9.34 × 1010 m/kg) but improved with in-line flocculant addition (r = 3.07 × 109 – 1.29 × 1010 m/kg).