Tips and Ideas
Controlling Snails in RBCâ€™s and Trickling Filters
Snail infestation seems to be due to Physa Gyrina or the Pouch Snail. Although snails are a nuisance, they become an operational problem when they eat slow growing nitrifiers on attached growth systems. With loss of nitrification, facilities may not meet their ammonia limits.
Many chemical suggestions on snail control do not have a dosage and some are just impractical, but they are interesting solutions to snail management. The snail management program you choose depends on your facilities set-up. If you feed alum or iron salts, and the piping is already in place, then this is a good place to begin. Others may be able to isolate a few sections of RBC's or trickling filters and try D.O. control, dose with chlorine, salt, or raise pH. The solution you choose may also affect the growth on the units. Another consideration would be approval from the DNR for chemical usage.
Most of the chemical methods are from a November 1995 Operations Forum article; "Slowing Down a Snail's Pace," pg. 20-22.
1. Chemical methods
- Salt - 10mg/l for 24 hrs, repeat every month or two.
- Aluminum Sulfate
- Cayenne Pepper - Humm...the Mexican snails may be immune to this.
- Sulfuric acid
- Hydrated lime (raise pH above 10.0 for 8 hours)
- Sodium Hydroxide (raise pH above 10.0 for 8 hours)
- Calgon Veligon - I have no idea what this is.
- Copper sulfate - 1lb for every 1,000 gal.
- Potassium permanganate
- Iron - one facility found that feeding 15mg/l of ferrous sulfate for a couple days a month.
- Hypochlorite - two dosages given.
2. Obtain a residual of 10mg/l after 24 hours, and dose at 60-70mg/l and leave the RBC's run for 3 days in the solution.
- Mulluskcides (Nalco Co.) Methaldehyde, nicolsamide, trifenmorph
3. Hydraulic or physical methods
- One plant rotates their filters out of service 4 hours out and then 8-12 hours on-line. When this mode of operation was stopped the snails returned.
- Reduce the speed of distributor arms. The slower speed appears to flush excess biomass and reduce snail populations. (This makes me wonder if the suggestion given above works because the filter is out of service or because all the flow is directed to other filters?)
- It was suggested that higher RBC rotations and possible higher flow though the units will shear snails off. Higher flows can be accomplished by taking units off-line thus allowing more flow through the on-line units, or by increasing the recycle rate.
4. Dry out the media
5. Temperature extremes. Freezing the media in the winter and heat in the summer.
6. Lower dissolved oxygen below 1.8-2.0. One facility fed primary effluent to the nitrifying section of the RBC's. This lowered D.O. and killed the snails. Trickling filters could be flooded for a few days.
7. Use a three phase control strategy. Because snail eggs are hard to kill, it is suggested that chemicals are only dosed to kill the adult snail. In two weeks, dose to kill the snails that were about to hatch. Then, in six weeks, dose to kill the eggs (now hatched) that were just laid at the time of the first dose. This schedule may need to be modified for a fourth treatment if temperatures are less then 15 degrees Celsius. The eggs in cold conditions will take longer to hatch, thus another dose may be needed in 4-6 weeks. See the November 1992 issue of Operations Forum; "RBC's, Escargot, and Caviar," pg. 17-21.
8. Dose high concentrations of ammonia preferably at a pH of 9.5. Undissociated aqueous ammonia is toxic to snails. One facility used their centrate from solids dewatering that was high in ammonia, and added sodium hydroxide to raise its pH. This was dosed through a bio-tower and recycled for 2 hours. After which, the spent solution is recycled back to the headworks. This is done once a month. See the Operations Forum section in the May 2000 issue of WE&T; "Escargot...Going...Gone," pg. 80-83.
9. Biological Treatment. Though none of these methods are known to be used, they are interesting.
- Predation, introduce a predator to the snail. Some suggestions: Spedonea, a snail- killing fly-Belostoma flumineum, a water bug-Procambarus simulans, freshwater crayfish.
- Competition. It was suggested to introduce a snail that would compete with the speices you have at your facility.
- Habitat destabilization. Reducing the calcium in the water, or lower the temperature below 10 degrees Celsius.