Filtration
Introduction
Slow 142
Enhanced sand filters have been used to remove particles from drinking water since the early 1800's. Although slow sand filtration is an old technology, the mechanisms responsible for particle removal are not well understood. Because slow sand filter performance gradually increases with time, it has often been assumed that the growth of biofilms is responsible for the gradual improvement in filter performance. Research conducted at Cornell suggests that biofilms are not responsible for significant particle removal and that most particles are removed by physical-chemical mechanisms. The particles that are captured on slow sand filters have been shown to significantly improve filter performance (Weber-Shirk and Dick, 1997). More recent research has shown that a filter aid can be extracted under acid conditions from particles harvested from Cayuga Lake or from Cayuga Lake sediment. The filter aid has been shown to greatly enhance bacteria removal. The filter aid is soluble at very low pH, and forms floc at neutral pH. This naturally occurring filter aid may be able to improve rapid sand filter performance.
Theory
In new slow sand filters with clean filter media, particles are initially removed by attaching to the filter media. However, as the filter media begins to be covered with removed particles, particles begin to attach to previously removed particles. If particle-particle interaction is more favorable than particle-media interaction then particle removal efficiency increases as the media becomes covered with particles. This improvement in filter performance with time is commonly observed in slow sand filters and is referred to as filter ripening. Filter ripening often takes several weeks to several months for new slow sand filters. Slow sand filters that operate with pristine water sources may never achieve efficient particle removal because the lack of particles in the source water results in a sparse coating of the filter media. Potential mechanisms of particle removal by slow sand filters are summarized in Figure 1. Physical-chemical removal mechanisms are responsible for most of the particle removal that occurs in slow sand filters. The one exception is that suspensionfeeding nanoflagellates attached to the filter media can capture a significant fraction of bacteria (Weber-Shirk and Dick, 1999). Thus, bacteria removal by suspension feeding predators is significant provided the influent bacteria concentration is sufficient to maintain a large predator population. Biofilms on the filter media have not been shown to significantly increase particle removal. Straining of bacteria-sized particles by the filter media and attachment of bacteria-sized particles to the filter media were shown to not be significant because the removal of bacteria by a clean filter column was negligible (Weber-Shirk and Dick, 1997). It is possible that straining becomes significant as filters clog and pores become smaller. Attachment of particles to previously removed particles is considered likely.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
143 Physicalchemical filter by medium Straining ripening may be the (fluid and gravitational by result of the previously forces) removed changes in pore particles Physical-Chemical geometry that enhance straining or to medium Attachment the modification of (electrochemical Particle to previously forces) filter media surfaces Removal removed particles Mechanisms that enhance the Attachment to ability of particles biofilms to attach. Biological Suspension feeders Decreasing the pore Capture by predators size to enhance Grazers straining is not a reasonable way to Figure 1. Particle removal mechanisms that potentially could improve particle be operative in slow sand filters. removal because the head loss through the filter increases rapidly as the pore size decreases. Thus, the best way to enhance physical-chemical ripening is to modify filter media surfaces. The filter aid may act by coating the surface and providing more favorable attachment sites. Filtration theory suggests that particle removal will be first order with respect to depth if the filter media is homogeneous (Iwasaki, 1937). In equation form the relationship between particle concentration, C, and depth is given by
dC = C 12.1 dz where is the filter coefficient with units of [1/L]. Setting appropriate integration limits
C0
C
0 dC = dz C L
12.2
where L is the depth of the filter bed and C is the influent particle concentration and o integrating gives:
ln C = L C0
12.3
Enhanced Filtration
144 Equation 12.3 can be used to evaluate the filter coefficient, . A list of previously measured filtration constants is given in Table 1. Filtration theory suggests Table 1. Typical values of filter coefficients adapted that filter performance would from (Tien and Payatakes, 1979). be optimal if the filter aid Filter Grain particle particle approach were applied uniformly medium size type size velocity (1/cm) throughout the filter. Uniform (mm) (m) (cm/hr) application is difficult, Calcium ?? Ferric 10 500 0.1 floc however, because the filter carbonate Calcium ?? Ferric 10 1000 0.044 aid will be captured first order carbonate floc with respect to depth if the Anthracite 0.77 Quartz 2-22 500 0.064 filter aid is applied using powder normal down flow operation. Sand 0.54 Chlorell 5 500 0.34 a It may be possible to apply Sand 0.647 Fullers 6 470 0.363 the filter aid during a gentle earth backwash thus enabling the Granular 0.594 Clay 4-40 500 0.102 filter aid to distribute more carbon uniformly. Application techniques that optimize the filter aid distribution require further study.
Previous Research Results
Previous research (Weber-Shirk and Dick, 1997) has shown that Cayuga Lake water particles can enhance filter performance and thus Cayuga Lake particles (CLP) from the Bolton Point Water Treatment Plant sedimentation basin were tested. Three filters were treated with 30 mL of concentrated CLP suspension from the Bolton Point Water Treatment Plant. One filter had the CLP mixed throughout the filter bed, one filter had the CLP mixed throughout the top 2 cm of the filter bed, and one filter had the CLP applied only to the top of the filter bed. The three application techniques were used because particles may improve filtration efficiency by providing surfaces to which bacteria attach more readily or because the pores within the sediment are smaller and thus more effective at straining particles. The filter with the particles distributed throughout the filter bed performed the best with approximately 99% removal of kaolin compared with 96% removal for the filter with the CLP on top of the filter bed. This result suggested that kaolin was being removed by attaching to CLP rather than by straining. CLP from the Bolton Point facility contain alum and possibly other polymers used in the water treatment process. Previous research also indicated that an acid treatment of Cayuga Lake sediment dissolves species that flocculate and attach to filter media at neutral pH. This Cayuga Lake Sediment Extract (CLSE) has been shown to rapidly ripen slow sand filters and achieve up to 6 log (99.9999%) removal of E. coli. The CLSE has also been shown to enhance E. coli removal at rapid sand filtration rates.
CEE Laboratory 453: Research in Environmental Engineering
Spring 2001
145
Filter Performance Evaluation
Several measurement techniques could be used to characterize filter performance. Particle concentrations could be measured using a particle counter, or measured indirectly using a turbidimeter. If the particle suspension absorbed a significant amount of light, a spectrophotometer could be used. A microscope could be used to count particles. If microorganisms are used as the source particles, they could be enumerated using standard microbiological techniques such as membrane filtration followed by growth on selective media. Turbidimeters measure the amount of light scatter caused by a suspension of particles. Because absorption and scattering of light are influenced by both size and surface characteristics of the suspended material, turbidity is not a direct quantitative measurement of 90 Detector Trans mitted the concentration of suspended light detector solids. In a turbidimeter the scattered light (measured at a right angle to the incident light) and the transmitted light intensities are Sample Lamp measured (Figure 2). The ratio of cell Lens scattered light to transmitted light is proportional to the turbidity of the Figure 2. Light path in a turbidimeter. sample. The constant of proportionality is determined by measuring a known standard.
Experimental Objectives
The purpose of this research is to evaluate the ability of the CLSE filter aid to enhance particle removal in a filter operating at rapid sand filtration rates. We will use tap water amended with kaolin, 2.5 cm diameter filter columns, and t rbidimeters. u Students will assembly the apparatus.
Experimental Methods
1) Setup 2.5 cm diameter filter column plumbing (Make all connections firmly and verify that the connections cant be pulled apart) including 1 L of clay suspension on a stirrer, peristaltic pump for metering in clay suspension and filter aid, flow meter, pressure reducing valve, and pressure sensor for head loss. 2) Add 8 cm of sand to the filter column (by mass). 3) Carefully observe the sand surface as you gradually increase the flow rate from zero in backwash mode. Measure the pressure required to begin to lift the bed. Continue backwashing the filter to clean the sand until the effluent turbidity is less than 0.5 NTU
Enhanced Filtration
146 4) Obtain head loss (in cm) as a function of flow rate (down flow mode) over a range of 1 to 25 m/hr (8.2 to 204 mL/min) using at least 5 data points. Use the rotometer to measure the flow rate. 5) Challenge the filter with a kaolin suspension (approximately 5 NTU) for 30 minutes to determine baseline filter performance. 6) Backwash the filter 7) Add the filter aid (the amount and method of application will be discussed during lab) 8) Set the down flow rate to 5 m/hr. 9) Measure the head loss to see if the filter aid increased the head loss 10) Pump a clay suspension into the filter influent so that the influent concentration is 10-mg/L kaolin. Measure effluent turbidity and head loss as a function of time for 30 minutes. Take turbidity measurements every 5 minutes and measure the head loss continuously using the Signal Monitor software. 11) Backwash the filter. 12) If you have time test the filter again to see if the filter aid improved filter performance even after backwashing. Figure 3. Picture of experiment setup.
CEE 453: Laboratory Research in Environmental Engineering
Spring 2001
147 Table 2. Filtration parameters.
parameter approach velocity column diameter column area Column length Media depth bulk density of media mass of media Backwash velocity flowrate (forward) flowrate (backwash) Influent clay concentration dilution factor clay stock concentration clay stock flowrate media residence time total residence time symbol value Va 5.0 d 2.5 A 4.9 Lcolumn 20.0 L 8.0 bulkdensity 1650 sandmass 64.8 Vb 50.0 Qd 40.9 Qb 409.1 C0 10.0 dilution 100 Cconcentrate 1000 Qc 0.41 thetam 0.96 thetac 2.4 units m/hr cm cm 2 cm cm kg/m 3 g m/hr mL/min mL/min mg/L mg/L mL/min min min
Prelab Questions
1) How much water is required to operate one of the laboratory filters for 2 hours? Dont include the water required to fill the filter initially. 2) Given the dimensions for the filter column, a glass density of 2.65 g/cm3 , and filter porosity of 0.4, estimate the mass of glass beads in one filter column. (Show your calculati...