Is chitosan safe for use 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.
– This post was originally published on www.setwater.ca
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. Compare the molecular structures of chitosan (left) and starch (right) and pay attention to the monomers. Both chitosan and starch are polymeric forms of a glucose derivative; and both of them degrade by enzymatic activity.
Images’ credit: Wikipedia (left: chitosan; right: starch)
Chitosan "reputation" in the water treatment world.
Aside from making a point about its environmental safety, manufacturers of chitosan for water treatment will tell you that, in comparison to the synthetic polymers or inorganic coagulants, chitosan generates solids with a lower percentage of water creating a stronger and more stable floc. They will also state that with chitosan you will achieve faster settling times, be able to filter solids off easier and remove a variety of contaminants including fine sediment particles and heavy metals.
Some of these statements are a bit too positive or oversimplified and, as a result, not entirely accurate.
Is chitosan indeed environmentally-friendly? Does it, in fact, remove metals from the water? Is it actually very effective at coagulating fine sediment particles like clays? I would not answer with plain YES to either of these questions: there is a caveat or two to the answers…
This particular post will focus on the first question about environmental safety of chitosan. The other two questions about metal removal and its coagulation effectiveness on fine particles will be addressed in a separate blog post.
What questions should we ask to determine if a chemical 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, definitely a big plus. But is the environmental fate all we need to know about a chemical to make a conclusion about its safety for the environment?
I would argue that it is not enough. To really tell whether a chemical is environmentally-friendly, i.e. bringing no harm to the environment, not only do we need to know about what happens with the residual chemical after it’s been released into the environment but also understand its biological and chemical properties. Here are some of the key questions, I ask myself as a chemist when reviewing a particular chemical treatment process:
What are the transformations the chemical is undergoing through the treatment process? Are we dealing with exactly the same compound in the treated water as the compound we had introduced into the process or has a different compound been formed as a result of the chemical transformation(s)?
How are other chemicals used in the process reacting with the chemical we are trying to evaluate in regards to its safety?
What are the dose requirements to achieve the objective, e.g. what excess of the chemical is required in order to complete the desired (precipitation/oxidation/reduction/etc) reaction? When feeding an excess of the chemical, inevitably we will have a residual chemical after the treatment.
Is the residual chemical being removed by the complete process (e.g. by filtration or media adsorption) or does it remain in our discharge?
Is the residual safe for the aquatic organisms if it is released into the environment?
Can we test for the residual chemical with sufficient accuracy and reasonable frequency?
It's easy to see that the question about environmental safety is not always a quick and easy question to answer about any given chemical. But I would suggest that at a minimum, when reviewing safety of a chemical for use in water treatment and discharge into a storm sewer or a water stream, we need to have a good understanding of the following four points:
1. Environmental fate (which includes degradation, bio-concentration, sorption, etc)
2. Acute aquatic toxicity (often expressed as LC50 on commonly occurring fish species or other aquatic organisms)
3. Where in the complete treatment process and how (liquid/dry + dose rate) the chemical is to be used (upstream/downstream, how is optimal dose rate determined, what are the following process steps, contingency procedures, etc.)
4. Residual (what is it and how can we test for it)
Let’s give it a try and evaluate chitosan environmental safety on these four points?
1. Chitosan scores high on point #1, especially compared to synthetic polymers like anionic polyacrylamides which do not degrade easily. Just like most starches, chitosan is completely biodegradable. In my opinion, this is the strongest point for using chitosan chemistry.
2. This is the point where chitosan scores pretty low, compared to anionic polyacrylamides which are non-biodegradable but extremely safe in terms of aquatic toxicity. Here is a couple of links to Safety Data Sheets of chitosan acetate solution: Dober HaloKlear™(2%) and Dungeness Environmental ChitoVan™(1%). Different brands are anticipated to have slightly different LC50 and other toxicity values due to varying manufacturing processes and source materials, but I would expect them to be of the same order of magnitude.
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. Or at least this is what toxicological studies performed/ordered by the manufacturer must have produced as a result. Calculating back to chitosan acetate, the “dry ingredient” of the 1% solution, its LC50 would be around 1.73 ppm.
There are several reports by the manufacturers and users stating that chitosan toxicity is greatly reduced if the injection dose is controlled and optimized according to the influent profile (pH and TSS load). Chitosan toxicity is mainly caused by its cationic (positive) charge, so when it is bound to the negatively charged sediment particles its toxicity is effectively negated.
3. When using a 1% solution, dose rates generally range between 200 and 500 ppm. When using a 1.5-2% solution, dose rates typically range between 100 and 300 ppm. Since the typical application dosages 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.
One more note: with chitosan chemistry, it is safer to “underdose” than “overdose”. As we know, cationic charge of chitosan is what makes it dangerous for fish. Additionally, solving the issue when the treatment tanks are full of water “overdosed” with positively charged chitosan is much more difficult than the solving the opposite problem.
4. Properly manufactured chitosan will not introduce metals or other pollutants in the water. That is another very good point about chitosan chemistry.
There is no chemical transformation of chitosan in the process: the way it works is by coagulating negatively charged sediment particles through electrostatic interaction so the only residual we need to be concerned about is chitosan itself, and more specifically, the unreacted portion of it in case the dose rate and/or application conditions were not optimal.
Another positive comment about chitosan is that residual chitosan can be tested in the field using a colorimetric test kit supplied by both Dober and 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.
In summary, 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 does not work on all types of soils and source turbidity ranges the same way!):
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 fish acute toxicity characteristics of chitosan, to prevent harm to the environment, extreme care must be taken to ensure its proper and safe application (see point #3 above).