Investigation of E. coli and Virus Reductions Using Replicate, Bench-Scale Biosand Filter Columns and Two Filter Media.

The biosand filter (BSF) is an intermittently operated, household-scale slow sand filter for which little data are available on the effect of sand composition on treatment performance. Therefore, bench-scale columns were prepared according to the then-current (2006-2007) guidance on BSF design and run in parallel to conduct two microbial challenge experiments of eight-week duration. Triplicate columns were loaded with Accusand silica or crushed granite to compare virus and E. coli reduction performance. Bench-scale experiments provided confirmation that increased schmutzdecke growth, as indicated by decline in filtration rate, is the primary factor causing increased E. coli reductions of up to 5-log10. However, reductions of challenge viruses improved only modestly with increased schmutzdecke growth. Filter media type (Accusand silica vs. crushed granite) did not influence reduction of E. coli bacteria. The granite media without backwashing yielded superior virus reductions when compared to Accusand. However, for columns in which the granite media was first backwashed (to yield a more consistent distribution of grains and remove the finest size fraction), virus reductions were not significantly greater than in columns with Accusand media. It was postulated that a decline in surface area with backwashing decreased the sites and surface area available for virus sorption and/or biofilm growth and thus decreased the extent of virus reduction. Additionally, backwashing caused preferential flow paths and deviation from plug flow; backwashing is not part of standard BSF field preparation and is not recommended for BSF column studies. Overall, virus reductions were modest and did not meet the 5- or 3-log10 World Health Organization performance targets.

Autopsy of high-pressure membranes to compare effectiveness of MF and UF pretreatment in water reclamation.

A pilot-plant study was designed to compare the effectiveness of microfiltration (MF) and ultrafiltration (UF) as pretreatment for high-pressure membranes in reclamation of biologically treated wastewater effluent. Granular media, filtered secondary effluent from a full-scale wastewater treatment plant, was fed to MF and UF units that operated in parallel. Each of these filtrates served as the feedwater to two reverse osmosis (RO) units and one nanofiltration (NF) unit that operated in parallel. The decline in specific flux was substantially lower for high-pressure membranes receiving UF than MF pretreatment over the course of each of four pilot plant runs that lasted from 1 to 7 weeks. The removal of organic matter as measured by dissolved organic carbon (DOC) was somewhat higher by UF than MF pretreatment (about 15% by UF compared with 11% by MF). Addition of ferric chloride ahead of the UF unit, but not ahead of the MF unit, may account for this additional removal of organic matter. However, the additional DOC removal appeared insufficient to explain the differential in foulant accumulation between high-pressure membranes receiving UF and MF pretreatment. Extensive autopsy analyses of these high-pressure membranes showed from 35% to 56% less organic carbon on those receiving UF rather than MF pretreatment. A more specific indicator of a differential in organic fouling was the accumulation of polysaccharides and this showed from 27% to 38% less on UF- than on MF-pretreated membranes. Yet another possible source of foulants is inorganic material given that the inorganic and organic weight percentages were nearly equal (56% vs. 44%) on the membrane surface. One specific source was aluminum added for phosphorus removal. Less fouling of high-pressure membranes pretreated by UF than MF could be due to the following: (1) a small, but very important, colloidal fouling fraction may have passed through MF but was rejected by UF pretreatment; (2) organic fouling was not related to organics in either the MF or UF filtrates but rather to organics that are generated in situ by microbial activity on the membrane surface; and/or (3) less passage of colloidal Al-P that carried over from secondary wastewater treatment.

A net water production model for ultrafiltration including flow direction reversal and chemically assisted backwashing.

Flow direction reversal (FDR) was proposed as a novel method to increase net water production (NWP) during cross-flow ultrafiltration. The design of the pilot-plant study allowed measurement of specific flux recovery after each chemically assisted backwash (BW) combined with FDR and after each FDR at the midpoint of each BW/FDR cycle. The percent recovery of specific flux was higher following FDR (55%) than combined BW and FDR (53%) at lower chemical dosages; however, the percent increase in specific flux recovery by FDR was much lower (20%) when the chemical dose was doubled. A mathematical model was developed to predict the NWP achieved by any combination ofBW/ FDR and FDR frequency. For example, the advantage of introducing FDR was demonstrated at the lower chlorine dose, whereby the percent increase in NWP by alternating 15-minute intervals of BW/FDR with FDR over BW/ FDR alone was 10% for 30-minute BW/FDR intervals and 2% for 15-minute BW/FDR intervals.

Uncertainty analysis in a mechanistic model of bacterial regrowth in distribution systems.

The first generation of mechanistic models of bacterial regrowth in distribution systems (DS) provides insight into cause-and-effect relationships. However, the state of knowledge about the processes included in these models is insufficient to warrant deterministic predictions. Even if the process descriptions are reasonable, the uncertainty in values of key system constants limits predictions of bacterial growth. A new mechanistic model was developed to incorporate the accepted knowledge of physical, chemical, and biological processes with the hydraulic features in order to capture the unsteady state behavior of the DS. Sensitivitytesting showed that the extent of bacterial regrowth was affected mainly by the rate constants for chlorine decay reactions in bulk water and on the pipe wall and by the maximum growth rate constant of attached bacteria. A simple hypothetical network was used to evaluate the effects of uncertainty in these three system constants by running 100 Monte Carlo simulations. Cumulative probability plots showed a wide range of predictions for concentrations of bacteria and chlorine in bulk water at various nodes in the DS. The magnitude of these concentrations and the range of values were greatly affected by water residence time to each node. Once the chlorine residual is depleted, bacterial growth is mainly influenced by the amount of substrate available. However, high values of the coefficients for the maximum growth rate of attached bacteria, the chlorine decay in bulk water, and the chlorine decay by wall reaction did not necessarily lead to the maximum bacterial growth at a given location.

Comparison of bacterial regrowth in distribution systems using free chlorine and chloramine: a statistical study of causative factors.

Bacterial regrowth was investigated over a 15-month period in distribution systems (DSs) of Durham and Raleigh in North Carolina. These two water utilities were chosen because they are adjacent to one another, have similar service area characteristics, and treat surface waters of similar characteristics with conventional processes (coagulation-sedimentation and dual-media filtration). The finished waters have similar chemical quality and regrowth potential as measured by assimilable organic carbon (AOC). The major difference in treatment is the choice of final disinfectants (chlorine in Durham and chloramine in Raleigh). Ten sampling sites (monthly sampling) were chosen in each system to give wide geographic coverage and correspondingly, a wide range of water residence times. Significant losses were observed in both chlorine and chloramine residual in the DSs that produced bacterial regrowth as measured by heterotrophic plate count (HPC). The frequency distributions for log HPC (133 observations from Durham and 135 observations from Raleigh) were statistically the same in the chlorinated and chloraminated DSs. A correlation analysis indicated that disinfectant residual is the most important factor determining HPC level. However, the resulting R2 value for a non-linear regression model that also included AOC, temperature, and pH as independent variables was less than 0.7. Bacterial regrowth as measured by HPC, is dependent upon a complex interaction of chemical, physical, and operational parameters that may not be captured by such a simple statistical relationship.