THE USE OF AMMONIA TO CONTROL SNAILS IN TRICKLING FILTERS

This paper describes a laboratory-scale study and the plant-scale application of using ammonia-containing solutions with elevated pH to control the growth of snails (Physa gyrina) in trickling filters (TFs). The laboratory study showed that NH3(aq) was the toxophore to P. gyrina and that the concentration that produced 100% mortality was a function of exposure time and NH3(aq) concentration. For a 1-hr exposure time, 150 mg NH3(aq)-N/L produced 100% mortality. This is equivalent to adding 392 mg ammonia N/L at a pH of 9.2. The plant-scale application of this method was at the Truckee Meadows Water Reclamation Facility (TMWRF) which is a 40 million gallons per day (mgd) plant treating the wastewater from the Cities of Reno and Sparks, NV. This plant employs nitrifying TFs following an activated sludge plant. Snail growth in the TFs had caused the plant's Total Nitrogen standard to be exceeded by devouring the biofilm needed to nitrify in the TFs. The TFs were modified to allow their isolation and operation in a recycle mode together with the addition of dosing chemicals for snail control. The source of the ammonia used for dosing was centrate from the centrifuge dewatering of anaerobically digested biosolids. The centrate contained an average ammonia N of 1,080 mg N/L and its pH was adjusted to an average value of 9.05 with 20% NaOH. The TFs were dosed for 2.1 hr, 0.75 times per month. After dosing the TFs were flushed with secondary effluent in recycle mode for 10 hr then returned to service. Spent dosing solutions were returned to the headworks. TMWRF has been in compliance for 20 months with its Total N standard, with one exception unrelated to TF performance.