"Dry" and "liquid" chitosan in stormwater treatment.
Updated: Aug 17, 2020
– This post offers my view on the topic based on my review of scientific literature and manufacturer’s documentation available online as well as my own hands-on experience with chitosan and other chemicals in water treatment. – Links to third party websites in this post are not to be considered as my endorsement of their products.
What is chitosan?
Chitosan, a unique positively charged polysaccharide, is one of the most popular polymers used in stormwater treatment in British Columbia and Washington State, due to its excellent biodegradability, abundant availability and low production cost.
Simply put, chitosan is a sugar. Both chitosan (left) and starch (right) are polymeric forms of a glucose derivative; and both of them degrade by enzymatic activity.
Images’ credit: Wikipedia (left: chitosan; right: starch)
What is the difference between dry and liquid forms of chitosan?
Liquid chitosan for water treatment is most commonly a 1-3% (by weight) solution of chitosan acetate. Dry chitosan is chitosan lactate flakes. Solubility of these two chitosan salts is comparable.
Why is solubility important? While “liquid” chitosan acetate is typically injected in the water stream using chemical metering pumps allowing for a relative control of the dose rates (assuming constant flow rate of the intake and no equipment malfunctions), "dry" chitosan is applied in "socks" or belts and is simply installed inside a pipe or a tube. As water flows through the pipe, chitosan lactate dissolves in the water and coagulates sediment particles. The dissolution rate will depend on the various conditions, resulting in inconsistent and often unpredictable dose rates.
Dissolution rate of the chitosan lactate depends on the following few conditions:
flow rate of the water going through the pipe
temperature of the water
composition of the water (mineral content, pH, etc)
design of the pipe/tube
Design of the pipe/tube is a big factor: in most cases, a belt of chitosan lactate will stay submerged in the water and the formed gel will quickly disintegrate with the next "slug" of water going through, potentially resulting in 1-3% concentration of the chitosan salt. Converting this % by weight concentration to mg/L, 1-3% conc'n = 10-30 g/L = 10,000-30,000 mg/L = 10,000-30,000 ppm of chitosan. As an MSDS of one of the major chitosan suppliers states on their safety sheet in Environmental Precautions section: "AVOID WATER; water will dissolve chitosan lactate forming a thick viscous solution or gelatinous mass."
"Dry" chitosan can be used in a controlled fashion if it is dosed with an automatic dispenser, the intake flow rate is constant, and sufficient mixing time is provided to ensure full solubility is achieved. Both conditions would need to be implemented to be certain in the "actual" dose rate applied: but doing so could make this process more expensive than injecting a liquid chitosan acetate solution and so this approach is less common than chitosan belts.
However, the “unpredictability” and risk of releasing high chitosan concentrations in the environment makes dry chitosan belts more suitable for the applications utilizing a series of large settling ponds where chitosan belts are installed in the ditches or conveyance channels draining into the settling pond.
Common “beliefs” about chitosan
Sand filtration will prevent release of chitosan residual into the environment.
Use Designation for Erosion and Sediment Control For Chitosan-Enhanced Sand Filtration using 1% HaloKlear® LiquiFloc (by WA Department of Ecology, June 2017) describes that "researchers passed clean water containing 2 mg/L of chitosan through a sand filter in a bench scale test and did not detect chitosan in the filtrate using a procedure with a detection limit of 0.03 mg/L".
At the time of the post publication, we could not find any further information on the retention capability of sand media toward chitosan salts and whether there were any other trials with higher concentrations of chitosan performed.
It is clear that in case of dry chitosan there is a higher potential to release much higher concentrations than 2 mg/L ( = 2 ppm). 2 ppm of chitosan acetate corresponds to only 200 ppm of 1% solution or 100 ppm of 2% solution (= the lower range of the dose rates typical for chitosan chemistry), so tests with higher chitosan concentrations are needed as well as a real evaluation of the retention capabilities of sand media through a proper column test and not a single pass of the water containing chitosan through a sand bed. While sand filtration will help remove sediment-bound chitosan, the retention capabilities of the sand filter to trap “free” chitosan are still yet to be determined at the time of the post publication.
Chitosan is environmentally safe.
It is a well known fact that chitosan completely degrades in the environment through enzymatic activity relatively quickly and does not bioaccumulate. However, the environmental fate alone is not sufficient to say a particular chemical is environmentally safe; it depends on its biological and chemical properties, as well as the application (use case).
Although chitosan has excellent biodegradation properties, it exhibits a relatively high aquatic toxicity, as compared to anionic polyacrylamides which are non-biodegradable but extremely safe in terms of aquatic toxicity. Based on toxicological studies performed/ordered by the manufacturer, 96-h LC50 (rainbow trout) of 1% solution (ChitoVan) is 173 pm, which means that if 173 ppm of this product blend is dosed into water free of sediment, 50% of rainbow trout would die within 96 hours. Converting the concentration back to chitosan acetate, the “dry ingredient” of the 1% solution, its LC50 would be around 1.73 ppm. Different brands are anticipated to have slightly different LC50 and other toxicity values due to varying manufacturing processes and source materials, but they are expected to be in the same order of magnitude.
How to use it safely?
With a 1% solution, dose rates generally range between 200 and 500 ppm. With a 1.5-2% solution, the typical range is 100-300 ppm. Since the typical application dose rates are higher than respective toxicity thresholds reported by the manufacturers, caution must be used when using chitosan for water treatment especially when effluent is discharged into a sensitive location such as fish-bearing stream or water body.
Optimal dose rates properly determined and set at the start of the treatment and verified with adequate frequency, pH control to maintain pH in the optimal range for coagulation by chitosan, well maintained sump, regular equipment maintenance (flow switch check, chemical injection lines, filter maintenance, etc.) are all important steps to ensure safe and effective use of chitosan.
Residual chitosan can be tested in the field using a colorimetric test kit manufactured by Dober or Dungeness Environmental. The method sensitivity (about 0.2 ppm) provides a safety factor of over 8 when comparing the method detection threshold with the LC50 of neat chitosan acetate. Flowlink provides supplementary on-site chitosan residual testing services as a part of its real-time, continuous water monitoring programs, and test kits are also available for stand-alone purchase.
NOTE!: with chitosan chemistry, it is safer to “underdose” than “overdose”. As far as we know, cationic charge of chitosan is what makes it harmful to fish . Additionally, addressing the issue when the treatment tanks are full of water “overdosed” with positively charged chitosan is much more difficult than dealing with “underdosed” sediment-laden water.
Chitosan can be a very good chemical of choice for sediment control / turbidity reduction process, that is assuming that the jar tests on site confirmed its effectiveness to be sufficient for achieving the treatment objectives:
Chitosan retained in the water treatment sludge is biodegradable and will leave no residue.
It does not introduce metals into the water, the only potential residual is chitosan itself.
Residual chitosan in the effluent can be tested in the field using a portable colorimetric test kit.
However, due to the acute toxicity characteristics of chitosan, to prevent harm to the environment, extreme care must be taken to ensure its proper and safe application.
Did you like this post? Read another post about chitosan "Is chitosan safe for use in stormwater treatment?"here. Have any questions? Talk to us.