Turbidity is reduced by removing particles from the water through filtration. Adequate filtration can be achieved by a variety of technologies: chemically assisted filtration, slow sand filtration, diatomaceous earth filtration, membrane filtration or an alternative proven filtration technology.
The chemically assisted filtration process generally includes chemical mixing, coagulation, flocculation, sedimentation (or dissolved air flotation) and rapid gravity filtration. Aluminum and ferric salts are used as primary coagulants. Cationic and anionic polymers are most commonly used as flocculation aids, and both, along with non-ionic polymers, have been used as filter aids. The coagulants and polymers are used to destabilize the generally negatively charged colloidal particles, which allows aggregation to occur via chemical and van der Waals interactions.88,89 The resulting (much larger) particles are filtered out when the water passes through sand beds or other single-, dual- or mixed-media granular filters. In systems where the combined water from all filters is monitored continuously, this treatment process is capable of producing water with a turbidity of less than 0.3 NTU; turbidities of less than 0.2 NTU have been demonstrated to be achievable on an ongoing basis. Filter loading rates generally range from 3.0 to 12.0 m/h.90,91
Changes in alkalinity, colour, turbidity and orthophosphate concentrations affect coagulation reactions and the properties and rate of settling of resulting floc particles. Temperature affects efficiency by influencing the rate of chemical reactions and the viscosity of water, thereby affecting the particle settling velocity and the filter backwash rate. The lower the temperature of the water, the more difficult it is to treat the water.
All filtration plants should provide for continuous monitoring of the effluent turbidity from each individual filter, as well as for continuous monitoring of the combined filtered water turbidity from all filters. Continuous monitoring is required because short-term turbidity spikes represent a process failure and potential health risk. Peak turbidity levels in the filtered water are a particular concern immediately after filter backwashing; therefore, all filters should be designed so that the filtered water immediately after filter backwashing is directed into a waste stream ("filter-to-waste" provision). When operating the filters, every effort should be made to minimize the magnitude and duration of turbidity spikes.92
Discharge of filter backwash water into a raw water reservoir should not be permitted unless the filter backwash water receives off-line treatment or is returned to a location upstream of the coagulant dosage point, so that all processes of a conventional or direct filtration plant are employed. The off-line treatment may be acceptable depending on the method used to treat the backwash water.
Following filtration, turbidity in a waterworks may increase if any of the following occur:
In 1989, the American Water Works Association Research Foundation sponsored a study that identified design provisions and operational practices at high-rate filtration plants. For the study, researchers chose 21 plants that were successful in producing finished water with turbidity of less than 0.2 NTU.91 In choosing the participating plants, consideration was given to geographic coverage as well as diversity of raw water types and treatment processes. In a different study, the Pennsylvania Department of Environmental Protection undertook performance evaluations of 150 surface water treatment plants that used filtration from 1988 to 1990 and found that a goal of 0.2 NTU was achievable for most plants.97 An internal report, prepared in 1995 for the same department, also found that filtration plants can readily achieve finished water with turbidities of less than 0.5 NTU and that most plants can achieve less than 0.2 NTU.90 In pilot tests involving treatment of Boston's low-turbidity surface water supply with dissolved air flotation, the turbidity goal of 0.1 NTU was met in more than 90% of the runs.98 Operational studies at specific plants have indicated that low turbidities in plant effluent are readily achievable when competent operations are in place.99,100 In another study, it was demonstrated that well-operated conventional treatment plants or direct filtration plants that produce water with low turbidity (less than 0.5 NTU) can achieve up to a 3-log reduction of Giardia cysts and up to a 2-log reduction of viruses.101 The same study demonstrated that source waters with low raw water turbidity require filter effluent turbidities to be substantially lower than 0.5 NTU in order to effectively remove Giardia cysts and viruses.
The U.S. EPA's 1997 Notice of Data Availability for Interim Enhanced Surface Water Treatment Rule (IESWTR) shows that systems serving more than 10 000 people are able to meet low turbidity limits. The same study indicated that chemically assisted filtration is able to achieve a 2-log reduction of Cryptosporidium through filtration.102,103 The U.S. EPA has now concluded that conventional treatment plants in compliance with the IESWTR or LT1ESWTR achieve an average of 3-log reduction of Cryptosporidium. Direct filtration plants achieve an average of 2.5-log reduction of Cryptosporidium.46
In slow sand filtration, filter effectiveness depends on the formation of schmutzdecke, which is a layer of bacteria, algae and other microorganisms on the surface of the sand, and the formation of a biopopulation within the sand bed. Raw water passes through the sand bed, where physical, chemical and biological mechanisms remove contaminants. The most important removal mechanism has been attributed to the biological process. No chemicals are added, nor is there a need to backwash.
Researchers have observed variation in the ability of slow sand filters to reduce turbidity. Fox et al.104 found that when water was filtered at 0.12 m/h, after an initial ripening period had allowed the biopopulation to become established on new sand, the treated water turbidity was consistently less than 1.0 NTU. Raw water turbidity ranged from 0.2 to 10.0 NTU. Cleasby et al.105 reported that typical effluent turbidity was 0.1 NTU for raw water, with turbidity ranging from lower than 1.0 to 30.0 NTU, except during the first 2 days after scraping of the schmutzdecke. Pyper106 observed slow sand filtered water with turbidity of 0.1 NTU or lower 50% of the time and 1.0 NTU or lower 99% of the time; raw water turbidity in this study ranged from 0.4 to 4.6 NTU. Slezak and Sims107 reported that nearly half of the 27 slow sand filtration plants they surveyed produced filtered water turbidity of 0.4 NTU or lower; at the same time, 15% of the plants produced water with an average turbidity of 1.0 NTU or higher. Consistent 3-log reductions of particles sized from 2 to 4 μm upwards were also observed in this study. The size range of 7-12 µm is considered to be representative of the size of Giardia cysts. Bellamy et al.108 studied the water treatment efficiency of slow sand filtration to ascertain removal of Giardia cysts, total coliform bacteria, HPC bacteria, particles and turbidity. Results showed that slow sand filtration is an effective water treatment technology. Using a biologically mature filter, Giardia cyst removal was virtually 100%; total and faecal coliform removal was approximately 99%; particle removal averaged 98%; HPC bacteria removal ranged from negative to 99%, depending on the influent concentration; and turbidity removal ranged from 0 to 40%.103 The U.S. EPA has now concluded that slow sand filtration plants in compliance with the IESWTR or LT1ESWTR achieve an average of 3-log reduction of Cryptosporidium.46
Slow sand filtration is appropriate for use when raw water turbidities are relatively low (e.g., <10.0 NTU).
As is the case with chemically assisted filtration, a "filter-to-waste" feature should be provided so that the filtered water immediately after filter cleaning is directed into a waste stream.
Diatomaceous earth filters operate by passing water through a thin layer of diatomaceous earth about 3 mm thick supported on a septum or filter element. To prevent turbid water from clogging the filter, a small amount of diatomaceous earth is continually added as body feed to maintain a permeable filter cake. Once the head loss across the filter cake becomes too great or the filter cake begins to slough, the filter is removed from service and the filter cake is washed and reused. New precoat is applied, and the cycle starts again.
Diatomaceous earth filtration has been shown to attain excellent removal of Giardia cysts over a broad range of operating conditions. Cyst removals exceeding 99%, and often 99.9%, were reported by Lange et al.109 for filtration rates of 2.4-9.6 m/h and for temperatures from 3.5 to 15°C. Logsdon et al.110 reported that when sufficient diatomaceous earth and body feed were used, removal of 9-μm radioactive beads was nearly always 99.9% or higher. The same study reported that 11 filter runs were made with Giardia muris cysts at filtration rates of 2.2-3.5 m/h. Cyst removal exceeded 99% in all runs and exceeded 99.9% in five of the runs. The U.S. EPA has now concluded that diatomaceous earth filtration plants in compliance with the IESWTR or LT1ESWTR achieve an average of 3-log reduction of Cryptosporidium.46
Diatomaceous earth filtration is appropriate and effective in treating waters with low turbidity. Logsdon et al.110 reported that turbidity reductions of 56-78% were attained with diatomaceous earth when raw water turbidity ranged from 0.95 to 2.5 NTU. Pyper106 reported an average turbidity reduction of 75% with an effluent quality of 0.5 NTU.
As is the case with chemically assisted filtration, a "filter-to-waste" feature should be provided so that the filtered water immediately after filter backwashing is directed into a waste stream.
Four membrane treatment processes are currently used in the water industry, and all involve pressure-driven semi-permeable membranes. The most appropriate type of membrane depends on a number of factors, including targeted materials to be removed, source water quality characteristics, treated water quality requirements, membrane pore size, molecular weight cut-off, membrane materials and system/treatment configuration.111 The four processes are:
Reverse osmosis and nanofiltration are very effective for absolute removal of cysts, bacteria and viruses.112 Ultrafiltration (pore size 0.01 μm) and microfiltration (pore size 0.1 µm) are effective for absolute removal of Giardia cysts and partial removal of bacteria and viruses.113 Filtrate turbidity can be achieved consistently at or below 0.1 NTU.114,115
Prefiltration and/or the addition of a scale-inhibiting chemical may be required to protect membranes from plugging effects, fouling and/or scaling.
If membrane filtration is the sole treatment technology in use, then a form of virus inactivation should be incorporated into the treatment train after the filtering process.
A "filter-to-waste" feature should be provided for initial start-up and commissioning of the membrane system and for emergency diversion in the event of a membrane integrity breach.