Comparison of Coagulation/Flocculation
A wide variety of chemicals exist for use in clearing raw water of suspended solids in coagulation/flocculation processes. It is known that the effectiveness of these coagulants has a complex dependency on the nature of the raw water, being affected by such things as temperature, pH, and especially the specific proportions of organic, inorganic, and biological particles that constitute the suspended solids. Furthermore, it is typically found that combinations of coagulants can be used to achieve much higher performance and process efficiency, but this performance again depends on the complex nature of the water source.
Because of this complexity, no systematic criteria can be applied across all drinking water treatment facilities, so coagulant selection must be addressed by each facility according to its own circumstances. Jar testing has been the standard approach, enabling facilities to identify coagulants (or coagulant combinations) which achieve a water quality that is adequate for today's standards and doing so with acceptable process economy.
However, recent advances in analytical instrumentation has led to the ability to systematically determine optimal coagulant dosage, selection, and proportions, leading to significant advances in coagulation/flocculation performance and economy. The Innosol instrumentation can determine specific amounts of inorganic, organic, and biological suspended solids, to identify optimal coagulant dosage, selection, and proportions, while quantifiably determining cost-saving and performance-enhancing opportunities given a facilities current chemical and equipment scenario.
In addition, significant performance and economic improvements can be achieved by periodically altering coagulant usage in response to daily (and seasonal) fluctuations. However, such adjustments are only recently possible given the in-situ, real-time analysis provided by the Innosol instrumentation.
As a courtesy to our customers, we provide in the table below the advantages and disadvantages of various coagulants and coagulant aids, and brief explanations of their functioning.
|Hydrolyzing Metallic Salts||Alum (Aluminum Sulphate)||A standard in coagulation/flocculation. Attracts inorganic suspended solids very effectively.||Fast mixing is critical to proper functioning. Non-optimal pH leads to excessive dosage requirements, should be used between pH 5.5 and 7.5, typically requiring alkaline additives to achieve optimum pH. Performance substantially degrades at lower temperatures. Poor efficiency for attracting organic suspended solids. 2 Relatively large dosage required when used alone.|
|Ferric Chloride||Alternative to Alum. Ferric chloride is good at attracting inorganic SS. Gives more compact sludge. pH sensitivity is somewhat less than alum. Suitable for usage in the lime-softening process (pH 9).||Lower efficiency for removing organic suspended solids than alum. Fast mixing is critical to proper functioning. Should be used between pH 5.5 – 8.5, typically requiring alkaline additives to achieve optimum pH. Generally large dosage required.|
|Pre-Hydrolyzed Metal Salts||PACl / PAC (Polyaluminum Chloride)||Does not require addition of alkali to raw water for coagulation, and is much less sensitive to pH, operating withing pH 4.5 – 9.5. Mixing time not critical. Floc is tougher, and if substituted for hydrolyzing metallic salts, may be possible to reduce dose and avoid using coagulant aid. Suitable for high colour applications. Specific chemical composition depends on preparation.||Generally requires an on-site production process to prepare pre-hydrolyzed metallic salts from alum.|
|Polyiron Chloride||Generally requires an on-site production process to prepare pre-hydrolyzed metallic salts from iron chloride|
|Synthetic Cationic Polymers||Epichlorohydrin dimethylamine (epi-DMA)||Lower dosages required, producing denser sludge. When used in combination with metal salts, greatly reduces their dosage requirement, resulting in substantial economic benefits .||Determining correct proportion for mixing with inorganic coagulants and other additives has been challenging due to a historical lack of instrumentation for determining relative amounts of inorganic, organic, and biological suspended solids in raw water. 1|
2Organic material occurs in raw water as either organic suspended particles or as dissolved natural organic matter (NOM). Coagulation/flocculation is primarily concerned with elimination of suspended particles, but flocs also adsorb dissolved contaminants by a different mechanism. Interestingly, while hydrolyzing metal salts have poor capacity to eliminate organic particles, they have a good capacity for adsorbing NOM. On the other hand, synthetic cationic polymer coagulants eliminate organic particles effectively, but adsorb NOM poorly.
In addition, various additives can be used to aid in the coagulation and flocculation process. These may accelerate the flocculation process or strengthen the floc to make it easier to filter. Coagulant aids can be roughly broken into two classes based on their mechanism of action. Some coagulant aids, generally the synthetic polymers, bind to particles much like coagulants themselves. Others, generally the inorganic and also natural polymers, act as sites of nucleation to speed the formation of floc. Nucleating agents typically also increase the density of floc, and so speed settling. In the case of synthetic polymer coagulant aids there is a blurring between coagulant proper and coagulant aid. The distinction is that synthetic polymer coagulant aids are added mainly for how they improve sludge properties (density, strenth, floc-size), easing separation and speeding flocculation, and that they may not be efficient when used alone as primary coagulants.
The table below compares various coagulant aids.
|Synthetic Cationic Polymers||Polydiallyldimethyl ammonium chloride (poly-DADMAC)||Produce denser more shear-resistant sludge. For use with metallic coagulants.||Increases complexity of coagulation/flocculation stage but improves process performance and economy when used properly|
|Polydimethyl aminomethyl polyacrylamide|
|trimethyl ammonium chloride|
|Synthetic Neutral Polymers||polyacrylamides|
|Synthetic Anionic Polymer||Anionic Polymer A110 (Kemira)||Produce larger shear-resistant flocs by promoting bridging.|
|Natural Polymers||Sodium Alginate||Particularly suited for use with ferric salts, but if used properly can be effective when used with alum||These less expensive polymers are somewhat less efficient than synthetic polymers|
|Chitosan||Inexpensive additives for increasing settling velocity, and reducing coagulant dosage.|
|Inorganic Coagulant Aids||Aluminum Chloride||For use with organic polymer coagulants||Increases complexity of coagulation/flocculation, but improves process performance and economy when used properly|
|Activated Silica||Inexpensive additives for increasing settling velocity, reducing dosage.|
The North American Water Quality Association WQA selected the Innosol technology and instrumentation as one of the most important innovations. Search for Innosol here
"...Innovative Multi-Application Large Volume Contaminant Detector: Reduces Process Costs, Monitors Product Water Quality, and Enhances Consumer Safety...The new instrumentation platform, Innosol-PLF, allows in-situ (online) detection, sizing, counting, and advanced monitoring of a wide range of biological and inorganic suspended solids (and more) in thousands of gallons of water and other liquid media, allowing for a number of unique cost-saving applications in industrial, commercial, and municipal markets..."
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