Rapid Gravity Filters Research Articles

Possibility of useing the western Iraqi desert Silica sand at drinking water treatment fitters

This study was determined specified characteristics of Iraqi silica sand , touse it in the drinking water treatment rapid gravity filters. These properties includes grain size ,uniformity coefficient , grain shape , porosity , density , durability, chemical content and capability of solubility in the acid ..this study explained that the Iraqi silica sand has high degree at mechanical and chemical stabilities .The e filter column was operate for many cycles , the average results of raw water and treatedwater for variable values (turbidity , total suspended solids and total bacterial count) was taken . The study showed that possibility of use the Iraqi silica sand in the westernIraqi desert in the rapid gravity drinking water treatment plant filters . when the raw water has initial turbidity (5.24 NTU) ,the study and the experimental tests showedthat the average removal efficiency of turbidity , T.S.S ,and T.B.C of (82.9%,82.8%and 79.5%) respectively . when the raw water has initial turbidity (9.58 NTU) ,the study and the experimental tests showed that the average removal efficiency of turbidity , T.S.S ,and T.B.C of (79.4%,78.7% and 74.1%) respectively . when the rawwater has initial turbidity (28.35 NTU) ,the study and the experimental tests showedthat the average removal efficiency of turbidity , T.S.S ,and T.B.C of (72.6%,72.7%and 60.9%) respectively

Control of biofouling by Cordylophora caspia in freshwater using one-off, pulsed and intermittent dosing of chlorine: laboratory evaluation

Cordylophora caspia is a hydrozoan which causes biofouling in power plants and is an increasing problem in UK drinking water treatment works. Thermal control is not usually feasible without a ready source of hot water so laboratory experiments were conducted to assess whether using pulsed doses of chlorine is an alternative solution. C. caspia polyps disintegrated after a single 20 min dose (the length of one backwash cycle in water treatment work filter beds) of 2.5 ppm chlorine. Without further treatment colonies regenerated within 3 days, but repeated dosing with chlorine for 20 min each day inhibited this regeneration. The resistance of surviving colonies to chlorine increased over time, although colony size and polyp regeneration continued to fall. These results suggest pulsed treatment with chlorinated backwashes at 2 ppm could be used to control C. caspia biofouling in rapid gravity filters and this may have relevance to other settings where thermal control is not feasible.

Biofouling by bryozoans, Cordylophora and sponges in UK water treatment works

In recent years biofouling from native (bryozoans, sponges) and non-native (Cordylophora) animals has increased in UK water treatment works (WTW). A survey of six UK water companies and eight WTWs revealed that these taxa were more widespread and abundant than previously recognised. Primary problems related to the occlusion of underfloor nozzles and tailpipes in rapid gravity filter beds (RGFs). These cost the UK water industry pound 1.49 m between 2005 and 2009. Additional impacts came from skin irritation to operatives from sponge spicules and the potential for elevated bacterial pathogen levels. Sponges penetrated the furthest through the water treatment process, reaching the point of final chlorination at one WTW. A monitoring plate study showed pronounced seasonality in fouling, with most taxa peaking in mid to late summer before a winter die-off. Control options, including the use of chlorine, and the importance of resistant stages for each taxon are discussed.

Simulation on combined rapid gravity filtration and backwash models

Combined rapid gravity filtration and backwash models have been applied to simulate filtration and backwash cycles. The simulated results from the backwash model suggest that an optimum air flow rate exists to maximise particle removal efficiency in the backwash operation for a certain backwash system. The simulation of combined rapid gravity filtration and backwash models suggests that the filter should not be completely cleaned up in the backwash and a certain amount of particles retained on filter grains after backwash can be beneficial for subsequent filtration runs. This is consistent with the experimental results in the literature.

Survey of disinfection efficiency of small drinking water treatment plants: Challenges facing small water treatment plants in South Africa

A survey involving 181 water treatment plants across 7 provinces of South Africa: Mpumalanga, Limpopo, North West, Free State, KwaZulu-Natal, Eastern Cape and Western Cape was undertaken to identify the challenges facing small water treatment plants (SWTPs) in South Africa . Information gathered included ownership and design capacity of the plants, water sources, and various methods of disinfection, equipment currently employed and performance of the treatment plants. In general, the majority (over 80%) of the SWTPs surveyed in the designated provinces were owned by the district municipalities. The designed capacities of these plants varied between 1 and 60 M./d; the smallest capacity was 100 m3/d and the largest 120 M./d. The small water treatment plants abstracted their raw water from either surface or groundwater or a combination of both water sources with greater preponderance for surface water sources (over 86%). Water treatment practices were noted to be the conventional types mainly coagulation, flocculation, sedimentation, filtration and disinfection. Two types of coagulants namely polyelectrolyte (66%) and alum (18%) were commonly used by the water treatment plants across the provinces studied. Rapid gravity filtration, pressure filter and slow sand filtration systems accounted for 60%, 23% and 9% of the filtration systems across the provinces, respectively. The predominant types of disinfectants employed were chlorine gas (69%) followed by sodium (15%) and calcium (14%) hypochlorite. Over 50% of the various SWTPs did not complywith the SANS 241 Class I (< 1 NTU) and Class II (1 to 5 NTU)  recommended turbidity values. The recommended target range of 0.3 to 0.6 mg/. free chlorine residual concentrations at the point of use was not always met by 40% of the plants. Seventy percent of the SWTPs complied with the SANS 241 criteria of microbiological safety of drinking water vis-a-vis total and faecal coliforms. Operational problems affecting the efficiency of small water treatment plants included: inability to appropriately determine the flow rate, chemical dosage and turbidity, lack of chlorine residual at the point of use and lack of water quality monitoring. To produce safe drinking water, appropriate operational practices must be implemented in all small water treatment plants.

Simulation of combined rapid gravity filtration and backwash models

Combined rapid gravity filtration and backwash models are applied to simulate filtration and backwash cycles. The simulated results from the backwash model suggest that air flow rate can be optimised to maximise particle removal efficiency in the backwash for a particular system. The simulation of combined rapid gravity filtration and backwash models suggests that efficient backwash operation is essential for maintaining the life time of a filter. However, the filter is not advised to be completely cleaned up in the backwash and the particles retained on filter grains after the backwash can be beneficial for subsequent filtration runs.

Rapid gravity filters: Developments in desalination pre-treatment

Mark Hadley, senior process engineer at Severn Trent Services, looks at how rapid gravity filters have developed to meet the demands for efficient solids removal at the pre-treatment stage of desalination plants.

The impact of flow surges on rapid gravity filtration

In drinking water treatment flow fluctuations or surges can occur at the filtration stage. Pilot plant tests were carried out to investigate the impact of flow surges on filter performance in this study. Moreover a filtration model was applied to analyse the impact of flow surges on filter performance and predict the experimental data obtained from pilot plant tests. Experimental results showed that flow surges caused an increase in effluent particle concentration and head loss. Theoretical analysis in this study showed that the impact of a flow surge became more significant as the filtration run approaches the particle breakthrough stage. The model was also used to fit the data from one water treatment plant. The comparison demonstrated a good agreement.

Experiences of algae in UK waters: a treatment perspective

AbstractAlgae blooms are a seasonal problem in UK waters and during these periods interferences with treatment plants are reported. This paper presents an analysis of data from 2000 to 2005 demonstrating UK experiences of algae at water treatment works. Cell populations are lower than those reported in the 1970s and 1980s, but reach levels that adversely affect treatment processes. Diatoms and cyanobacteria dominate in spring and autumn respectively. A treatment works including pre‐oxidation, coagulation, flotation and filtration removes on average 96% of influent cells, while rapid gravity filters alone remove 63–75%. Cells present in the filtrate are typically either unicellular, micro‐algae, or flagellated algae. filter blockages in the spring and autumn are caused by large cells of complex morphology, including the diatoms Melosira and Asterionella. Overall, since the 1980s the key issue with respect to algae treatment has changed from one of treatability to that of process optimisation and economics.

Mathematical Modelling of Particle Removal and Head Loss in Rapid Gravity Filtration

A new filtration model has been developed to describe the entire three‐stage filtration run. The two‐stage assumption is applied for physical description of the rapid gravity filtration process. At the first stage of the filtration, previously deposited particles serve as collectors to improve particle removal efficiency and the detachment of deposited particles does not occur until the specific deposit reaches a certain transient specific deposit where the second stage starts. Particle detachment rate at the second stage is assumed to be a function of relative specific deposit and hydrodynamic conditions within filter media. The proposed filtration model has been applied to fit the data from one water treatment plant of Yorkshire Water (UK). The comparison demonstrates a good agreement between water treatment site data and the simulated results.

Cryptosporidiosis and Filtration of Water from Loch Lomond, Scotland

Previous evidence has suggested an association between consumption of unfiltered water from Loch Lomond, Scotland, and cryptosporidiosis. Before November 1999, this water had been only microstrained and disinfected with chlorine; however, since that time, physical treatment of the water (coagulation, rapid gravity filtration) has been added. To determine risk factors, including drinking water, for cryptosporidiosis, we analyzed data on laboratory-confirmed cases of cryptosporidiosis collected from 1997 through 2003. We identified an association between the incidence of cryptosporidiosis and unfiltered drinking water supplied to the home. The association supports the view that adding a filtration system to minimally treated water can substantially reduce the number of confirmed cryptosporidiosis cases.

Performance of Dual-Media Down-Flow Rapid Gravity Filters

The present study was conducted to evaluate the effect of variation of influent raw water turbidity, bed composition, and filtration rate on the performance of mono (sand) and dual media (sand and anthracite) rapid gravity filters in response to the effluent filtered water turbidity and headloss development. In order to evaluate each filter pe1formance, sieve analysis was made to characterize both media and to determine the effective size and uniformity coefficient. Effluent filtered water turbidity and the headloss development was recorded with time during each experiment.

Surface water clarification using M. oleifera seeds

Turbid surface water was treated using a pilot scale water treatment plant comprising coagulation, flocculation, sedimentation and rapid gravity filtration, using Moringa oleifera seeds/alum as coagulants. Turbidity removal of M. oleifera, alum, and the mixture of both M. oleifera/alum were compared, and results obtained were 7.2, 4.2 and 3.2 NTU, respectively. The turbidity achieved using M. oleifera/alum mixture and alum were less than the required standard of 5 NTU, while M. oleifera/alum mixture recorded the least turbidity value (3.2 NTU) with removal efficiency of 99%. The natural alkalinity of the water did not vary during the treatment processes. Therefore M. oleifera/alum mixture could be considered as a suitable alternative for partial replacement of alum as coagulant in surface water treatment, which is an added advantage since M. oleifera is a natural product with less or no side effects as compared to alum as a chemical agent.

Biomass and microbial activity in a biofilter during backwashing

Biomass and microbial activity in backwashing processes of a biofilter for tertiary treatment were investigated. The microbial groups revealed new distribution along the biofilter depth after low flow rate backwashing for a short time. Then the start-up process was accelerated by backwashing. The biomass profile and microbial activity profile both varying with depth before and after backwashing, can be mathematically described by quadratic equations. Using the profiles, the difference of oxygen demand can be calculated to determine the airflow rate during backwashing. Combined with the difference between biofilters and rapid gravity filters, analysis of biomass and microbial activity can determine more accurately the required airflow rate during backwashing.

Practical and theoretical analysis of relationships between particle count data and turbidity

Potential relationships between particle counts and turbidity have been examined by several researchers over recent years. This paper describes how a different approach to this issue was adopted. Particle count and turbidity data arising from the combined filtrate of a block of six rapid gravity filters are analysed for possible relationships. The results are compared with those of other researchers and it is demonstrated that no linear relationship exists between particle count and turbidity. This practical result is then corroborated by a theoretical analysis from first principles of the light scattering properties of suspended particles.

The Use and Application of Particle‐Count Data in Water Treatment

AbstractParticle counters are widely used in the water industry and generate a significant amount of data. To be useful to operators, such data need rapid interpretation. This paper describes how poor individual filter performance can be detected from particle‐size analysis of the combined filtrate from a block of six rapid‐gravity filters. Relationships between particle counts in the 2–5 μm size range and total count are explored, and the concept of an onsite real‐time alarm system, monitoring differential particle‐count data from combined filtrate, is developed.Consideration is given to the frequency of data collection. Whilst all major events were detected by collecting regimes with collection frequencies varying from 5 to 45 mins, it was found that the governing factor in determining collection frequency is the response time required by the operator to an incident.

A Practical Assessment of the Use of Particle Counters for Monitoring the Risk of Cryptosporidium and Giardia

AbstractThe paper describes a project which was carried out to establish whether particle counts can be used to predict the risk of Cryptosporidium and Giardia breakthrough at a large water‐treatment works. The project was based at a direct river‐abstraction works which treats up to 120 MI/d. On‐line particle counters were installed following three of the rapid‐gravity filters, and a fourth counter was used to measure particles in spot samples of raw and clarified water.There was a clear relationship between particle counts and the occasional low‐level (oo)cyst (crypto oocysts and giardia cysts) occurrence in the filter samples, and no (oo)cysts were found in the final water. There appear to be complex inter‐relationships between particle counts and other parameters such as raw‐water conditions, environmental factors and treatment‐plant variables.

Suspension concentration profiles during rapid gravity filter backwashing

Rapid gravity, granular media filters are widely used in the water and wastewater treatment industries. Regular backwashing to clean the filters is a vital part of their efficient operation. Experimental data on the development of suspension concentration profiles through laboratory scale filter beds during the backwash process were obtained. Previous attempts to obtain and record backwash profiles of this type have been unsuccessful due to the limited range of existing turbidimeters. The results have been used to validate a new model developed by the authors.

Performance testing and operational experience of a Fibrotex plant installed to treat a 40 Ml/d groundwater source

Clay Lane Water Treatment Works, owned and operated by Three Valleys Water (part of the Vivendi Group), is located in Watford in the South East of England. The raw water supplying the works is taken from eight chalk borehole sources in the surrounding area and treatment at Clay Lane consists of ozonation, rapid gravity filtration (GAC media) and disinfection with sodium hypochlorite. Ozone and GAC are used primarily for pesticide removal. In 1997 a decision was taken to install additional treatment at one of the sources, Eastbury, which can supply up to 40 Ml/d to Clay Lane and which has on occasions been subject to surface water influence and poor quality following heavy rainfall. Given the potential risk from contamination by Cryptosporidium oocysts, a Fibrotex plant capable of achieving 99% (2 log) removal of oocysts was selected as the appropriate treatment. This paper reviews the results obtained during the performance testing, both on site during commissioning and subsequently during the Cryptosporidium oocyst challenge test. In addition, the operation of the plant over the last 18 months is reviewed.

Spectral analysis of pressure variations during combined air and water backwash of rapid gravity filters

Simultaneous water and air backwashing has been established as the most effective cleaning regime for rapid gravity filters. For certain air and water wash rates the bed can reach a state termed “collapse-pulsing”. This has been identified as the optimum cleaning regime for simultaneous air and water backwashing. An empirical formula exists to predict collapse-pulsing, this is given in terms of the water, air and minimum fluidising velocities. However, no suitable method exists to confirm that the collapse-pulsing condition has been achieved, other than by visually subjective observations of the bed. Visual observation is not always physically possible or practical on industrial size filters. This study examines pressure fluctuations, taken at various heights in the bed, and their associated frequency spectra as a means of identifying the collapse-pulsing condition. Preliminary results show that, particularly in the upper sections of the bed, distinct peaks occurs in the frequency spectrum at collapse-pulsing.