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Turbidity reduction is often necessary to provide water suitable for discharge to the environment or a municipal sewer system. Conventional clarification is the most commonly used process for turbidity reduction. Conventional clarification consists of coagulation, flocculation (Step 2 on the schematic below) and sedimentation (Step 3).
Sometimes, depending on the discharge quality targets and project specifics (space limitations, source water quality, etc.) incorporating filtration (Step 4) into the treatment process is necessary to meet the water quality objectives for turbidity.
Coagulation & flocculation - what is the difference?
Coagulation is the process of colloid destabilization achieved through charge neutralization. Electrostatic interaction of the charged sediment particles repelling each other prevents them from forming agglomerates and settling.
NOTE! Coagulation is often necessary when the turbidity of the source water is low and/or caused by clay particles and colloids.
Once the surface charge on the sediment particles is neutralized, the particles can get closer to each other and be eventually bridged together, typically through the flocculation step. Flocculation is the process of bringing the destabilized particles together to form larger agglomerates often called “flocs”. Flocculants are sometimes called coagulant aids and can be anionic, non-ionic, or cationic polymers. Frequently, anionic polyacrylamides with large molecular weights are used as a low cost coagulant aids.
NOTE! Some compounds can play a role of both coagulant and flocculent, e.g. chitosan.
Types of coagulants.
Aluminum salts, such as ACH (aluminum chlorohydrate), PAC (polyaluminum chloride)
Iron salts such as ferric chloride or ferric sulphate
Organic coagulants (cationic polyelectrolytes):
Polyamines, including chitosan
Blends (inorganic coagulant + cationic polyelectrolytes)
NOTE! Organic coagulants are often ineffective on waters with low turbidities, with blends giving the best clarification results, however there may be a downside (read about the residuals below).
Organic coagulants, if used at the appropriate concentrations, do not contribute to the TDS in the discharge (find out why here), while inorganic coagulants and blends leave soluble iron or aluminum carryover in the effluent. Residual dissolved Al or Fe concentration will be dictated by the corresponding metal solubility at the given conditions (pH, temperature, matrix effect), while total metal concentration will depend on the clarification process effectiveness.
Site discharges directed to a water body have to meet regulatory water quality guidelines for Freshwater Aquatic Life protection in Canada and British Columbia (CCME and BCWQGs).
While in case of iron salts, meeting the BCWQGs and CCME guidelines can be achieved following a well designed and executed treatment process, meeting CCME guidelines for total aluminum (100 ppb at pH > 6.5 and 5 ppb at pH < 6.5) is nearly impossible using a conventional clarification process even when supplemented by micro-filtration. One of the reasons residual aluminum in treated water is undesirable and should be avoided is a possible link between aluminum and adverse neurological effects such as Alzheimer’s disease.
Field test methods for chemical residuals.
Iron (Fe) colorimetric test.
This simple test takes 3 min and requires basic tools - a colorimetric test kit (can be purchased here), a beaker or a glass jar, a syringe, and a 0.45 µM filter if testing for dissolved iron.
Detection range: 0 - 5 mg/L.
Detection limit: 0.1 mg/L.
The test is sensitive enough to confirm with sufficient level of confidence that the residual Fe concentration is below the BCWQ and/or CCME guideline.
Aluminum (Al) colorimetric test.
This simple test takes about 5 min and requires basic tools - a colorimetric test kit (can be purchased here), a beaker or a glass jar, a syringe, and a 0.45 µM filter if testing for dissolved aluminum.
Detection range: 0 - 0.5 mg/L.
Detection limit: 0.1 - 0.2 mg/L.
The test is not sensitive enough to confirm with sufficient level of confidence that the residual Al concentration is below the BCWQG and/or CCME guideline, but it will indicate if a serious guideline exceedance is observed.
Photometer, a more sensitive method for on-site analysis of Al, Fe & other parameters.
A portable photometer works by measuring color intensity. "Light is passed through a test tube containing the sample solution, and then through a colored filter onto a photodetector. Filters have been chosen so that light of a specific wavelength is selected. When the solution is completely colorless, all of the light passes through the sample. With colored samples, light is absorbed and that which passes through the sample is proportionately reduced." (Quote is taken from YSI 9300 Manual, read more here)
Detection range: 0 - 0.5 mg/L
Detection limit: 0.02 mg/L
Detection range: 0 - 5 mg/L
Detection limit: 0.02 mg/L
Chitosan colorimetric test.
The test manufactured by both Dober and Dungeness Environmental compares a field water sample with a blank and a standard solution of 0.2 mg/L chitosan. If there is greater than 0.2 ppm of free residual chitosan then a brown color will develop on the test filter. If the color is as dark as the standard, it means that the treated water has more than 0.2 ppm of chitosan.
Detection limit: 0.2 mg/L (ppm)
Flowlink can perform the residual chemical tests as a part of its online water monitoring program, or can simply supply (rental or sale) the residual test kits.
Here you can find an up-to-date summary of metal water quality guidelines for Freshwater and Saltwater Aquatic Life protection in Canada and British Columbia (CCME and BCWQGs). These regulatory guidelines typically apply to site discharges directed to a water body.