Pilot studies were conducted to investigate the effect of recycling spent filter backwash water (SFBW) on Cryptosporidium and particle removal during conventional sedimentation and dual‐media filtration. When SFBW recycle configurations (3,000–19,000 oocysts/100 L) were used, Cryptosporidium concentrations in settled water were as low as or lower than when no recycle was used (6,000–22,000 oocysts/100 L). Filtered water oocyst concentrations were typically below the detection limit of 1 oocyst/120 gal (~0.25/100 L). Overall process removal was about 5 log without recycle and about 5.7 log for each recycle scenario based on calculations using raw Cryptosporidium concentrations and filtered water detection levels. As with Cryptosporidium removal, removal of turbidity and particles > 2 μm for the entire process (sedimentation + filtration) was similar for operations with and without recycle. Typical median filtered water concentrations for particles > 2 μm were < 0.5 particles/mL for all recycle scenarios, including no recycle. Similarly, settled and filtered water turbidity levels for all studies were typically < 2 and < 0.03 ntu, respectively.
Dewatering of residuals containing arsenic (As) has attracted little research but will become increasingly important as As is removed from water by sorption to ferric or aluminum hydroxides. Lime conditioning proves to be an effective method for immobilizing As even at high pH. In contrast, increasing pH with addition of caustic soda or soda ash leaches As from residuals. Soluble calcium appears to neutralize the negative surface charge on the hydroxide solids to enhance As sorption. In many cases, As-containing residuals are stored in lagoons for months or even years. Although many parameters contribute to soluble As mobilization, the presence of calcium appears to assist in keeping As immobilized. AND MARC EDWARDSrsenic (As) has long been recognized as a human health issue, but concern about low-level As in drinking water is a recent development (Smith et al, 1992). The maximum contaminant level for As in public drinking water has been lowered from 50 to 10 µg/L because of increased likelihood of liver, lung, bladder, and kidney cancer. The revised As standard could result in stricter regulations on the processing and disposal of Ascontaining residuals.In order to be disposed of in a landfill, residuals must meet requirements set by individual states, usually the toxicity characteristic leaching procedure (TCLP), or in California, the waste extraction test (WET). In the past, TCLP limits have been based on drinking water standards, but these limits are not automatically changed as drinking water standards are revised. It is possible that the lowering of the drinking water standard might result in a future reduction of the 5-mg/L allowable As criteria for passing the TCLP or WET.There are two usual methods that water treatment plants use to prepare residuals for disposal to a landfill. One common practice is to place the residuals in a lagoon and allow the solids to dry naturally, after which the waste is shipped to a landfill (provided it passes the TCLP or WET). The water from these lagoons is frequently pumped back into the treatment plant, but in the case of unlined lagoons, any As mobilized might find its way into the groundwater. In other instances, mechanical dewatering of residuals may be required. In either case, in addition to the usually mandated TCLP or WET requirement, wastes must also pass the paint filter liquids test to ensure that no free liquids are placed in the landfill (LaGrega et al, 2001).Many parameters have the potential to affect the soluble As concentration in storage lagoons; they include pH, dissolved oxygen (DO) concentration, micro-A 2003
DEMONSTRATING CRYPTOSPORIDIUM REMOVAL using sporemonitoring at lime-softeningplants This study investigated the removal of Bacillus subtilis and other aerobic spore-forming bacteria at four lime-softening plants in the United States. Removal of aerobic spores has been shown through a compilation of literature to be a conservative indicator of Cryptosporidium removal during clarification and filtration. Results reported in this research demonstrated that lime softening involving two clarification stages and one filtration stage achieved 3.5-to 4.0-log removal of aerobic spores during routine monitoring spanning periods of a year or more. More than half of the filtered water samples at two facilities were below detection limits, meaning that the actual log spore removal at these facilities may have been even higher than reported. Two facilities with presedimentation basins prior to lime softening were able to demonstrate an additional 0.5-to 1.0-log spore removal. Given the conservatism of spore removal, it is anticipated that these plants can achieve at least the same amount of Cryptosporidium removal.
This article describes a comprehensive national investigation of trace contaminants in water treatment chemicals. In general, no pervasive problems with trace contaminants in US water treatment chemicals were found, but serious isolated contamination incidents continue to occur. The most commonly identified problems were transport‐related, typically the result of improperly cleaned or maintained delivery vehicles and transfer hoses. These and other related problems were routinely averted when utilities developed and implemented inspection and evaluation programs for incoming chemical deliveries. This research produced compositional data from more than 50 treatment‐chemical products. Trace contaminants contributed by coagulants partitioned primarily into residuals rather than finished water during full‐ and pilot‐scale studies conducted during this investigation. Mass balance calculations from these partitioning studies confirmed the compositional data reported for coagulants in this research, results that are much lower than limited data reported previously in the literature for samples collected before 1990.
A recent project focused on the recycling challenges posed by the potential presence of the pathogen Cryptosporidium in microfiltration (MF) residuals. After reviewing the various methods available for treating MF backwash streams, clarification was selected because it (1) is successful in treating spent filter backwash water from conventional plants, (2) is economical, and (3) is familiar to water plant operators and US regulators. Successful results from bench‐scale treatability tests guided the pilot‐ and full‐scale studies. Microbial performance indicators for the pilot test included inactivated Cryptosporidium oocysts and aerobic endospores. At a sedimentation overflow rate of 0.25 to 0.5 gpm/sq ft (0.6 to 1.2 m3/m2/h) and with the appropriate coagulant type and dosage, coagulation/clarification of MF backwash streams produced a recycle quality similar to the original raw water. Unlike spent filter backwash water from conventional water treatment plants, polymer alone was not sufficient to treat MF backwash because the feedwater had not been previously coagulated with a metal salt coagulant.
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