Gravity Filtration Research Articles

A treatability index for reservoir water

A Treatability Index is developed to allow comparison of different reservoir waters according to their effects on a water treatments works. For the water treatment works which employs rapid gravity filters, the index is the product of the algal concentration, the clarification coefficients of algae and the filtration rate. The index is applied to reservoir waters within Thames Water area. Algae observed in reservoirs are grouped according to their shapes. Among these groups, twenty frequently observed species were selected and their clarification coefficients were measured. The treatability index was then evaluated for different waters and at different times of the year. The results were correlated to the filter run lengths and the development of headloss across the filters.

A mathematical filter backwash model

A mathematical model for deposit detachment and removal during the backwash of rapid gravity filters has been derived for a fluidising water wash. The model consists of two parts; part 1 determines the volume of deposit dislodged into suspension at any instant, and part 2 determines the change in concentration emerging from the filter bed at any stage during the backwash. Initial conditions for the backwash model are determined from a filtration model.

Determination of water-soluble vitamins in infant milk by high-performance liquid chromatography

A rapid, simple and reliable liquid chromatographic method has been developed for the simultaneous determination of nicotinamide, thiamin, riboflavin, pyridoxine, pyridoxal, pyridoxamine, cyanocobalamine and folic acid in liquid and powdered infant milk. Ion-pair chromatography with a reversed-phase C 18 column is used. Six vitamins were resolved in a single analysis; total analysis time never exceeded 55 min. A mobile phase of methanol-water (15:85), 5m M octanesulfonic acid, with 0.5% triethylamine at pH 3.6 and a flow-rate of 1.0 ml/min gave the most satisfactory separation of these vitamins using a UV detector set at different wavelengths. Sample preparation involves acidification to precipitate proteins, and centrifugation followed by gravity filtration. Linearity, precision, recovery and sensitivity were always satisfactory. Detection limits ranged from 0.02 to 0.10 μg/ml and determination limits ranged from 0.03 to 0.25 μg/ml.

The use of particle monitoring in the performance optimisation of conventional clarification and filtration processes

The application of laser diffraction particle monitoring to the performance optimisation of a pilot clarifier and full-scale rapid gravity filters (RGF), operating on water supply works in Hampshire, is described. Furthermore the dosing of powdered activated carbon (PAC) into the works' clarifiers has been evaluated in terms of RGF performance. A costly proposal to install a third filter medium was subsequently abandoned when it was found that particle numbers in the filtered water were consistently below 1×102/ml. Various combinations and doses of coagulants and flocculant aids, shown to give optimum particulates removal during intensive jar testing trials, were transferred to the pilot clarifier. Particle monitoring enabled a more accurate derivation of suitable blanket chemistry and optimum blanket heights than turbidity changes. Raw water turbidities were 10-15 NTU at start-up with corresponding counts beyond the upper limit of the particle monitor. An on-line dilution system was developed to overcome this problem. Latex bead (4.33 μm) and Lycopodium spore (4-5 μm) suspensions (about 1 × 109 particles) were injected into the pilot clarifier to assess the removal efficiency of Cryptosporidium-sized particles. Reductions of about 1.7 log and 2.6 log were achieved for the beads and spores, respectively. Particle distributions of various PAC's and a bentonite were obtained in order to assess their potential effects on the coagulation process during clarification. Bentonite was also beneficial as an on-line means of checking particle monitor response and calibration. The works' filters achieved 1.5 to 2.0 log removals of 2-5 μm particles without media addition or operational changes. Combined clarification and filtration gave better particulates removal than two-stage microfiltration.

Pressure signal analysis of combined water and air backwash of rapid gravity filters

Many problems encountered in water filtration are associated with the effectiveness of backwashing. Combined air and water backwashing especially using flow rate combinations resulting in “collapse-pulsing” provide the most effective cleaning. This study examines a technique of analysing pressure signal fluctuations in order to determine the onset of “collapse-pulsing”. The technique provides a non-subjective and practical method of “collapse-pulsing” determination. The resultant pressure fluctuation traces are examined in combination with headloss versus water velocity profiles at particular set air flow rates. The occurrence of significant fluctuation patterns at the position where headloss becomes constant implies that “collapse-pulsing” determined by this technique is the onset of three phase fluidisation.

The use of particle monitoring in the performance optimisation of conventional clarification and filtration processes

The application of laser diffraction particle monitoring to the performance optimisation of a pilot clarifier and full-scale rapid gravity filters (RGF), operating on water supply works in Hampshire, is described. Furthermore the dosing of powdered activated carbon (PAC) into the works' clarifiers has been evaluated in terms of RGF performance. A costly proposal to install a third filter medium was subsequently abandoned when it was found that particle numbers in the filtered water were consistently below 1 × 102ml.Various combinations and doses of coagulants and flocculant aids, shown to give optimum particulates removal during intensive jar testing trials, were transferred to the pilot clarifier. Particle monitoring enabled a more accurate derivation of suitable blanket chemistry and optimum blanket heights than turbidity changes Raw water turbidities were 10–15 NTU at start-up with corresponding counts beyond the upper limit of the particle monitor. An on-line dilution system was developed to overcome this problem. Latex bead (4.33 µm) and Lycopodium spore (4–5 µm) suspensions (about 1 × 109 particles) were injected into the pilot clarifier to assess the removal efficiency of Cryptosporidium-sized particles. Reductions of about 1.7 log and 2.6 log were achieved for the beads and spores, respectively. Particle distributions of various PAC's and a bentonite were obtained in order to assess their potential effects on the coagulation process during clarification. Bentonite was also beneficial as an on-line means of checking particle monitor response and calibration.The works' filters achieved 1.5 to 2.0 log removals of 2–5 µm particles without media addition or operational changes. Combined clarification and filtration gave better particulates removal than two-stage microfiltration.

Filter backwash and start-up strategies for enhanced particulate removal

A significant consideration in forward planning for water treatment works design and operation concerns the effectiveness of a filtration plant in providing a barrier to particulates in the low micrometre size range, including Cryptosporidium oocysts. The performance of rapid gravity filtration plants is believed to be dependent on backwash and start-up regimes. It was the aim of this study to optimize direct sand filtration by identifying optimum filter backwash and start-up conditions which minimized the passage of particulates into the filtrate. The filter ripening period has long been identified as a cause for concern with respect to particulate passage into the filtrate; this work has shown that up to 40% of all particles that pass into supply during a 48 h run, do so in the first hour of operation. Optimum combined air water “collapse-pulsing” backwash durations were identified that reduced the number of 2–5 μm particles entering the filtrate, especially during the ripening period. Slow start-up was also found to reduce the number of 2–5 μm particles in the filtrate during the ripening period. The reductions in particulate passage resulting from a slow start was found to be media dependent, with smaller media requiring a longer slow start duration than coarser media.

A MODEL FOR FREENESS MEASUREMENT OF PAPERMAKING SUSPENSIONS

In the pulp and paper industry, it is often necessary to characterize the drainage capability or a pulp on the paper machines. The industry uses a standard measure called the freeness to represent this and other properties for papermaking pulps. The freeness is the total volume of water discharged from a side orifice of a specific configuration while the pulp suspension drains freely under gravity. In this paper, a model for the gravity filtration of pulp suspensions forming a compressible pulp mat is along with some empirical treatment of the flow is used to model the freeness test. The gravity drainage process is assumed to be described by a cake filtration process with the pressure at each instant being given by the gravity head. From the model, the critical parameters governing the freeness of a pulp suspension are shown to be the specific surface area and the specific volume of the pulp fibers in addition to the compressibility of the pulp mat. When these parameters are available from independent gravity drainage measurements, the freeness itself can be estimated. Estimated freeness values are in agreement with experimental measurements for pulps which are reasonably free of fines. When fines are present however, they get entrapped within the pulp mats decreasing their permeability. The model predictions are higher than experimental measurements in this case.

Self-cleaning gravity filter

Self-cleaning gravity filter

Ciliates in rapid gravity filters of waterworks exploiting deep groundwaters

Potable water is increasingly produced from deep (>100 m) tertiary groundwaters which often are completely reduced and contain high amounts of ammonium, methane, and hydrogen sulphide. They thus require special treatment which includes oxygenation and removal of the reduced contaminants by the biofilm developing in rapid gravity filters. The biofilm is heavily colonized with ciliates and microinvertebrates. A total of 38 species of ciliates was found in 42 samples taken from 4 waterworks in Germany during a period of 2 years. Only six species occurred in high numbers and in more than half of the samples: Acineria uncinata, Aspidisca lynceus, Cinetochilum margaritaceum, Colpidium colpoda, Glaucoma scintillans, and Holosticha pullaster. Five to thirteen species occurred per sample, and up to 6,000 individuals ml−1 biofilm were counted. There was a considerable fluctuation in the number of species and individuals, which could not be related to specific process parameters. The number of species and individuals decreased markedly from the filter surface to its centre. Colonization of the filters very likely occurs randomly via impure surface waters. The ciliate communities found consist mainly of alphamesosaprobic to polysaprobic species and thus closely resemble those known from activated-sludge processes. This is explained by the specific conditions near and in the biofilm, which is probably microaerobic and highly productive, providing microaerobic bacterial feeders with copious food. Obviously, it is the microenvironment which determines the occurrence of certain species. Thus, future research on the autecology of the indicator species used in the saprobic system should concentrate on their microenvironments. © 1996 Wiley-Liss, Inc.

Ciliates in rapid gravity filters of waterworks exploiting deep groundwaters.

Potable water is increasingly produced from deep (>100 m) tertiary groundwaters which often are completely reduced and contain high amounts of ammonium, methane, and hydrogen sulphide. They thus require special treatment which includes oxygenation and removal of the reduced contaminants by the biofilm developing in rapid gravity filters. The biofilm is heavily colonized with ciliates and microinvertebrates. A total of 38 species of ciliates was found in 42 samples taken from 4 waterworks in Germany during a period of 2 years. Only six species occurred in high numbers and in more than half of the samples: Acineria uncinata, Aspidisca lynceus, Cinetochilum margaritaceum, Colpidium colpoda, Glaucoma scintillans, and Holosticha pullaster. Five to thirteen species occurred per sample, and up to 6,000 individuals ml-1 biofilm were counted. There was a considerable fluctuation in the number of species and individuals, which could not be related to specific process parameters. The number of species and individuals decreased markedly from the filter surface to its centre. Colonization of the filters very likely occurs randomly via impure surface waters. The ciliate communities found consist mainly of alphamesosaprobic to polysaprobic species and thus closely resemble those known from activated-sludge processes. This is explained by the specific conditions near and in the biofilm, which is probably microaerobic and highly productive, providing microaerobic bacterial feeders with copious food. Obviously, it is the microenvironment which determines the occurrence of certain species. Thus, future research on the autecology of the indicator species used in the saprobic system should concentrate on their microenvironments.

Ammoniacal Synthesis of Lanthanum Aryloxide Complexes and Observation of a Unique Interconversion between Oxygen- and .eta.-Arene-Bridged Dimeric Species. X-ray Crystal Structures of La2(OAr)6(NH3)n (n = 0, 2), La(OAr)3(NH3)4, and La(OAr)3(THF)2 (Ar = 2,6-i-Pr2C6H3)

Reaction of LaCl{sub 3} with 3 equiv of sodium metal in liquid ammonia at -78 {degrees}C, followed by addition of 3 equiv of HOAr (Ar = 2,6-i-Pr{sub 2}C{sub 6}H{sub 3}) in toluene produces La(OAr){sub 3}(NH{sub 3}){sub x} (1) after warming to room temperature. Dissolution of 1 in toluene followed by 6 h reflux provides La{sub 2}(OAr){sub 6} (2) in good yield. An X-ray crystallographic study revealed that 2 contains a dimeric unit held together with two {eta}{sup 6}-arene bridges formed from aryloxide ligands with La-C(av) = 3.062(10) {Angstrom}. The La-O distances average 2.193(5) and 2.273(5) {Angstrom} for terminal and bridging aryloxide ligands, respectively. Dissolution of 1 in toluene followed by vacuum filtration and crystallization from toluene without reflux yields La{sub 2}(OAr){sub 6}(NH{sub 3}){sub 2}(3) in high yield. The X-ray crystal structure of 3 reveals an alternate dimeric structure bridged by oxygen atoms of the aryloxide ligands. X-ray crystallography has shown that a third compound may be isolated by gravity filtration and crystallization of 1 to produce La(OAr){sub 3}(NH{sub 3}){sub 4} (4). 4 displays a capped pseudo-octahedral geometry around the metal center with facial aryloxide ligands average 2.252(6) {Angstrom}. The La-N distances for the NH{sub 3} ligands average 2.750(9) {Angstrom} withmore » no significant lengthening of the capped La-N bond.« less

Achieving enhanced filter backwashing with combined air scour and sub-fluidising water at pilot and operational scale

The paper presents evidence to demonstrate that filter backwash sequences should include a combined air and water phase to maintain the long-term performance of rapid gravity filters when treating lowland river water without the use of coagulants or pre-chlorination. Techniques for assessing filter backwash effectiveness in maintaining clean media included laboratory analysis for retained organic and inorganic material, scanning electron microscopy and examination of trends in clean bed head-loss data. Backwashing with air scour followed by a water wash, without a combined air and water stage, failed to keep the filter media clean. It led to increased starting head-losses, fluctuating filtrate quality and ultimately the appearance of cracks in the bed. Backwashing with combined air and water at a sub-fluidising rate ensured consistent filtrate quality and maintained low clean bed head-losses. Data were obtained from pilot plants and full-scale works (200–790 Ml/day range) treating London's reservoir stored water prior to further biological filtration by slow sand filters. Air scour rates investigated were from 20 to 50 m/hr free air and were combined with water at 7–12 m/hr. These rates were successful for several different media types, including standard (0.7 mm) and coarse (1.2 mm) filter sand and dual-media (anthracite/sand) filters. Backwash launder position and configuration were evaluated at large scale and found to be critical to successful retention of media during combined air and water washes. The results confirm, for water containing living organisms, theoretical and laboratory findings of other workers using predominantly inert suspensions in clean water. The degree of filter maturation attributed to biofilm and inorganic coating of media was affected by backwashing conditions. The impact of filter maturation of filtrate quality is discussed.

Stainless‐steel piping

Stainless‐steel piping can be cost‐effective for potable water use.Stainless steel has been used extensively for potable water since the mid‐1960s in desalination plants for handling product water; in potable water treatment plants for gravity filtration and piping; in Tokyo, Japan, for small‐diameter household connection piping; and in New York City for large‐diameter risers and other piping. The most familiar use of stainless steel is for drinking fountains. Background information and general data are given on types 304 (UNS S30400) and 316 (S31600) stainless steel, and their current use in potable water applications is reported. The behavior of stainless steel used with raw, chlorinated, and finished water as well as of piping buried in soil is reviewed. Postfabrication cleanup and maintenance of exterior appearance and cleanliness are also reviewed. Guidelines for procurement and successful use are suggested.

On the inducibility of nitrate transport by tobacco cells

The question as to whether the nitrate transport system is induced by nitrate was addressed using a cell suspension of the XD line of Nicotiana tabacum L. cv. Xanthi as an experimental system. The cells were grown on area as the sole nitrogen source, and tungstate was used to render nitrate reductase non‐functional. To avoid shock due to vacuum filtration, the cells, were harvested by gravity filtration. Nitrate uptake by cells, which were harvested, transferred to fresh medium, and immediately exposed to nitrate (freshly harvested cells), displayed a lag period of about 3 h.In cells which were given incubation periods in fresh medium before exposure to nitrate (preincubated cells), the lag period was considerably shortened. After 3 h of preincubation in the absence of nitrate (recovered cells), the lag period was almost completely eliminated. Cycloheximide inhibited nitrate uptake by recovered cells within minutes, and prevented the development of nitrate uptake in freshly harvested cells. Cycloheximide did not affect uptake of α‐aminoisobutyric acid (AIB) within the first 2 h after its addition. Recovery of the membrane potential from a low value just after the harvest of the cells to a maximal value 3 h later, was observed using the lipophilic cation methyltriphenylphosphonium (MTPP+), supplied at low concentrations, as a probe. Depolarization of the membrane potential by MTPP+, at the millimolar range, caused a rapid inhibition of nitrate uptake by recovered cells. The results indicate that nitrate transport by the XD cells depends on the membrane potential and on protein components with short half life. In addition, it requires a continuous protein synthesis. The effects of physical manipulation on nitrate uptake are discussed.

Evaluation of capital cost data. part 7: Liquid waste disposal with emphasis on physical treatment

AbstractCapital cost correlations are given for oil‐water separators, equalization basins, primary clarifiers, clarifiers and clarifier mechanisms, reverse osmosis unit, ultrafiltration units, gravity filters and miroscreens. Data are included for raw sewage, intermediate and recirculation pumping stations and for preliminary treatment (or bar screens, grit removal, overflow and bypass chamber and Parshall flume), for grit removal, comminution and gas stripping. The correlations are accurate to ± 30%, expressed in Canadian dollars and adjusted to a Marshall and Swift value of 1000. Details as to what components are included in the cost are given. Data are given for the labour and material costs to create a working process module.For inflation indices, a critique of the Engineering News Record, EPA small city, Marshall and Swift, Chemical Engineering and Southam inflation indices suggested that the Marshall and Swift index is most appropriate for the construction and process equipment studied here. Concerning the currency exchange, for the equipment and processes in this study, equipment costs the same whether expressed directly in US or Canadian dollars.To put the current data into perspective, several hundred sources of cost data were analyzed. The major correlations are compared, on a consistent basis, with current data.

Status of Declining Rate Filtration Design

Declining rate filtration is one of several flow control alternatives available for gravity filtration plants with four or more filters. Experimental and theoretical studies of declining rate filtration have provided the foundation for rational design procedures. This paper presents design procedures for estimating the filter system head needed for a declining rate filtration plant, and to predict the water level variations that will occur during operation. The paper also discusses the choice between inlet versus outlet rate limitation, the choice of type of rate limitation device, and the impact of operation of a declining rate filtration plant below full design load.

Declining Rate Filtration

Declining rate filtration is one of several flow control alternatives available for gravity filtration plants with four or more filters. Experimental and theoretical studies of declining rate filtration have provided the foundation for rational design procedures. This paper presents design procedures for estimating the filter system head needed for a declining rate filtration plant, and to predict the water level variations that will occur during operation. The paper also discusses the choice between inlet versus outlet rate limitation, the choice of type of rate limitation device, and the impact of operation of a declining rate filtration plant below full design load.

Slow sand/nanofiltration treatment for secondary treated wastewater

The subject of slow sand/nanofiltration of municipal wastewater is discussed. Results of testing in Phoenix, Tucson and Tempe, Arizona are given as evidence that this technique will provide a low salinity water suitable for many types of reuse. The slow sand filter is a gravity filter which utilizes biological processes to remove dissolved organics. Only the surface of the sand bed is disturbed when cleaned. The slow sand filter is an excellent prefilter to the nanofilter membrane reducing organic scaling. The nanofilter membrane reduces salinity and hardness nitrates, heavy metals and other pollutants as well as significant amounts of dissolved organics. The combination of slow sand/nanofiltration treats wastewater the way it needs to be treated in order to be either safely disposed of or reused without significant negative environmental impacts.

Uncoupling the microbial loop: growth and grazing loss rates of bacteria and heterotrophic nanoflagellates in the North Atlantic

Growth and grazing loss rates of bacteria and heterotrophic nanoflagellates were investigated in the northeast Atlantic in spring 1989. Experiments were conducted in 2 different oligotrophic areas at 18N. 30°W and at 33ON. 20°W. Numbers of bacteria and protozoa, namely heterotrophic nanoflagellates (HNF), heterotrophic dinoflagellates and ciliates, were low in both areas, and the average abundance of microorganisms in the euphotic zone varied little. The microbial loop was uncoupled experimentally by differential filtration. Microplankton samples were size-fractionated by gravity filtration and changes in numbers of bacteria and their potential predators were measured in microcosms over several days. Manmum potential growth rates were 0 087 h-' for bacteria and 0.123 h-' for HNF. Ingestion rates ranged from 9 to 36 bacteria HNF-' h-', clearance rates ranged from 0.3 to 3.0 X 10-' ml HNF-hBased on the experimental results, 3 modes of control of growth rates and abundances were ldent~fied (1) substrate control: both bacteria and HNF did not grow unless substrates were added; (2 ) substrate and grazing control both effective: substrate supply via recycled dissolved organic matter enhanced bacterial growth moderately, followed by increasing HNF numbers which reduced the bacterial peak; (3) grazing control: bacterial production was balanced immedately by grazing protozoans, namely HNF, and removal of predators led to a pronounced bacterial peak. Results confirmed the hypothesis of a linear food chain consisting of bacteria HNF larger protozoa which is tightly coupled in the natural environment.