Accelerated Gravity Removal of Snail Shells from Trickling Filter Plants. Adam Neumayer

Transcription

1 Accelerated Gravity Removal of Snail Shells from Trickling Filter Plants Adam Neumayer

2 ABSTRACT Accelerated gravity separation has been successfully used to remove and classify grit for over twenty years. Several installations and pilot study demonstrations in California, Oklahoma, and Nevada revealed that this process is extremely successful and highly efficient for removal of snails that plague Trickling Filter plants. There are several key benefits in using accelerated gravity for removal of snails. Accelerated gravity separation removes snails on a continuous basis preventing snail build-up and problems in downstream processes. Some of the pitfalls of alternate snail removal systems such as manual removal and the continued growth of new snails may be avoided. Important design criteria, including snail specific gravity, process location, snail collection, and flow rates must be considered for a successful process. CHARACTERISTICS OF THE SNAILS Sources of snails in wastewater treatment plants have been well documented. Usually they are limited to Trickling Filters and Rotating Biological Contact Media, although any quiescent tank creates the potential for snail growth. Several studies have investigated the use of chemicals and/or the increase of ph to kill the snails and prevent them from growing in the first place. Due to the snail s apparent high level of resistance, the studies have often had mixed results. Consequently, the purpose of this paper is not to offer another alternative to killing the snails but to offer a means to minimize the damage that they cause to the rest of the plant. If the plant has to live with them, continuous removal proves to minimize their impact. The genus of snail most commonly documented to be a nuisance in Trickling Filters is Physa, commonly known as the pouch snail. Of this genus, the P. gyrina and P. integra have been frequently cited as the most common species. The physical size range of the tested snails was observed to be from 3 mm to 15 mm. This has been documented in other studies as well (Lacan). Testing of various snail samples showed that, despite their physical size, their settling velocity is similar to that of much smaller sand particles. This can be attributed to differences in specific gravities due to the organic portion of the snail. Figure 1 details the distribution of snails tested, based on their settling velocity. Settling velocity ranged from 1 to 9 cm/s. The settling velocities were related to equivalent silica sand particles for sizing purposes and a common reference point. Observations have shown snail shells to be extremely abrasive. Thus it is paramount to continuously remove them from the process stream in order to minimize retention time and abrasive impact. Furthermore, continuous removal neutralizes large surges caused by intermittent removal. SES ANALYSIS SAMPLE # SES, micron FIGURE 1. Page 2 of 2

3 Process Problems Created by Snails Problems associated with snails in wastewater treatment plants were documented as early as (Lacan). Snails cause sedimentation problems in many of the processes downstream of the trickling filters. The aeration basin elements become covered, preventing proper oxygen transfer and hindering the process. The digesters fill at extraordinary rates. Pumps experience unusually high rates of abrasive wear. Plugged sludge lines, abraded sludge pump pistons, clogged fixed bed reactor nozzles, and shells in digesters have also been reported. Fixing these problems proves to be very costly. Also interesting to note are referenced facilities reporting pulverized shells settling in anaerobic digesters (Bishop). In one plant, this resulted in the plant hauling 660 tons of snails to a landfill at a cost of $20,000. Dayton, Ohio, a 40 mgd plant, has reported an average of 1,200 lb/day of snails. Medford, Oregon, a 17 mgd plant, has to dredge nearly 100 cubic yards per year (Smith). Methods of removal Several methods have been used to remove accumulations of snails causing problems in the plant. They have met with mixed results. Forced vortex grit removal units (1 gravity, or 1 G) have been used with poor results as the tank becomes a secondary breeding ground. This appears to be caused by the fact that the wall velocity of the unit is faster than the exit velocity of the unit. This is due to the tangential exit channel. Consequently, the forces generated may hold a portion of the snails in the unit, however, they are not strong enough to prevent the snail from adhering to the tank surfaces. Sedimentation tanks have also been used as a means of removing the snails from the process. In many cases, the tanks must be drained and the snails dredged by hand multiple times a year. Problems arise when snails bypass these tanks because of significant accumulation. Consequently, removing them on a continuous basis appears to be the key to success. To accomplish this, the snails are continuously drawn from the tank with the sludge and classified by their specific gravity. Accelerated gravity separation is an industry proven technique for accomplishing this. ACCELERATED GRAVITY SEPARATION Accelerated gravity separation is a process that has been used for many years for the removal of grit in wastewater treatment plants. Separation of particles is performed by tangential introduction of the slurry to the unit, with discharge from an opening in the top of the unit. This creates a condition where the velocity at the outside wall of the unit is much slower than the velocity at the center of the unit. The resulting condition is called a free vortex. The free vortex creates a radial centrifugal acceleration force field affecting particles with specific gravities greater than water. This generated force field, in combination with that due to gravity, causes these settleable particles to settle into and be hydraulically collected in a boundary layer at the bottom of the unit. Particles, such as organics, with specific gravities near that of water, are carried through the unit. This designed combination of force fields is referred to as accelerated gravity separation and is accomplished in a free vortex classifier. The Separation Process Flow is introduced tangentially to an enclosed cylindrical chamber to maintain a rotating body of fluid. The ideal free vortex is characterized by constant angular momentum. As a result, the tangential velocity becomes very large as the radius is decreased from the wall to the exit discharge cylinder. The kinetic energy associated with this tangential velocity is dissipated as the swirling effluent discharges into the outflow stream, resulting in headloss. The liquid-particle separation occurs within the unit as a result of centrifugal forces exceeding fluid drag forces that carry particles through the system. Classification and separation of particles according to specific gravity occurs within the boundary layer at the base of the unit. These units have been shown to effectively remove particles as small as 50 microns (270 mesh) at a specific gravi ty of 2.6. Page 3 of 3

4 In a free vortex classifier the tangential velocity multiplied by radius is a constant within the unit. Thus a small change in radius can equate to a substantial increase in centrifugal force. Furthermore, a small increase in inlet velocity results in a large increase in force holding the particles in the unit. Consequently, increased flow results in increased removal efficiency. This is the exact opposite of a gravity separator where increased flow results in decreased removal efficiency. Classification of particles according to specific gravity occurs within the boundary layer at the base of the free vortex unit. Efficient classification is due to the acceleration of liquid with a reduction of radius that occurs in a free vortex. It can be shown that two particles with equal settling velocity in a constant acceleration field will follow different paths within the free vortex boundary layer if they have different specific gravities. This is due to the greater influence of the drag force on larger particles in a field of increasing acceleration. The critical or cut point particle separation occurs at the classifier s discharge cylinder. At the wall of this cylinder, the centrifugal force tends to keep this particle from exiting with the outflow, is balanced by the drag force due to the outflow velocity flux through the discharge cylinder. The action of the axisymmetrical rotation of water above the base of the unit creates a boundary layer. A cylinder of liquid rotating over a flat base results in a radially inward velocity component within the viscous boundary layer. Dense particles held in the unit eventually settle by gravity into the boundary layer. The radial velocities within the layer drag the particle toward the center of the base. Design Criteria Free vortex classifier units are typically fed Secondary Clarifier sludge to desnail it. This eliminates the need to treat the full flow of the plant. In smaller plants, treating all of the plant flow is feasible. Single free vortex classifier units have a design range of 0.1 mgd to 10 mgd for a single unit. Flows larger than this can be accommodated by multiple units. Sludge concentrations should be kept below 1.5% to prevent large slurry viscosity changes. APPLICATIONS OF ACCELERATED GRAVITY SYSTEMS FOR SNAIL REMOVAL DEMONSTRATION UNIT APPLICATIONS Las Vegas, NV Clark County Sanitary District - Las Vegas, NV Plant # 2 is a 66 mgd wastewater treatment plant. In 1993, a free vortex classifier demonstration unit was set up to evaluate snail removal capabilities. The unit was tested on discharge from the trickling filters and influent to the secondary clarifiers. During the demonstration, the unit was operated around the clock for two days. Test showed an average removal rate of 95% of all snails sent to the unit. Monterey, CA Monterey Regional Water Pollution Control Agency operates a 17 mgd trickling filter plant. During the summer of 1990, the staff observed tiny black solids floating on the surface of the aeration basin, accumulations in their bioflocculation channels, and problems maintaining dissolved oxygen levels. They drained the tank to find that snails from the trickling filter had settled to a level above the fine bubble diffusers thus interfering with the oxygen transfer. The black solids proved to be fragments of snail shells. Consequently, they have to drain each of their three aeration basins down several times each year to remove truckloads of settled snails. Snails were also found in the solids handling system, the anaerobic digesters, and the heat exchangers. In May 1991, a pilot study was performed to test the free vortex classifier s snail removal capabilities. This system was rated at gpm and designed for removal of 50 micron (270 mesh) grit at a specific gravity of 2.6. The unit was used to remove large quantities of settled snails and performed very well. Page 4 of 4

5 However, because of the massive quantities of snails removed, it was not possible to evaluate the system s removal efficiencies during the initial phase of the test. After most of the snails had been removed, the system was operated under less dramatic conditions to determine its snail removal efficiency. The test showed a removal rate of about 99% (volume) of the snails under normal loading conditions. FULL-SCALE INSTALLATIONS City of San Luis Obispo, CA Water Reclamation Facility The City of San Luis Obispo Water Reclamation Facility treats all of the wastewater generated by the city, the county airport, and California State Polytechnic University. The plant is designed for a peak flow of 14 mgd. Flow is sent through screening, aerated grit removal, and primary clarification to the Trickling Filters, which are followed by a secondary clarifier. Sludge from the secondary clarifiers is pumped to a free vortex classifier for snails to be removed and dewatered. Removing the snails prevents excessive build up in the effluent filters and digesters. At an average flow of 5 mgd they remove 0.11 yd 3 of snails per day. South San Luis Obispo County Sanitation District, CA - Oceana Regional Plant This 12 mgd Trickling Filter plant treats raw wastewater from the communities of Arroyo Grande, Oceano, and Grover Beach. Flow is pumped from a pump station to the primary clarifiers where it flows to the Trickling Filter. The Trickling Filter effluent flows to a secondary clarifier where the sludge is sent to a free vortex classifier to remove the snails thus minimizing deposition in the digesters and abrasive wear on the dewatering equipment. At an average flow of 2.5 mgd they remove about 0.29 yd 3 of snails per day. City of Lawton, OK Wastewater Treatment Plant The City of Lawton Wastewater Treatment Plant is a 24 mgd Trickling Filter facility. Wastewater from the city is pumped to flow measurement where if flows by gravity through aerated grit basins and primary clarification. Then it is pumped to the Trickling Filters where if flows by gravity to Trickling Filter clarifiers. The sludge from these is pumped to the free vortex classifier where the snails are removed and dewatered protecting the nitrification aeration basins and digesters. At an average flow of 13 mgd they remove an amazing 0.93 yd 3 per day. The free vortex classifier removes an impressive 2.77 yd 3 per day at a peak flow of 35 mgd. SUMMARY Accelerated gravity separation of snails from wastewater has proven to be a viable method to continually minimize additional process problems caused by snails. Once the snails have sloughed off from the Trickling Filters they can be effectively removed as fine grit. Using the properties of their specific gravity to collect them and prevent them from proliferating will bring downstream problems under control. Installations and pilot studies have shown promising results. With careful consideration of site-specific characteristics such as snail quantity and die-off periods, plant operation and maintenance snail control programs will be increasingly effective. Page 5 of 5

6 GLOSSARY mgd: Million gallons per day. Sand Equivalent Size: This is the behavior or settling velocity of a particle in water as compared to the settling velocity in water of a sand sphere (2.65 specific gravity) of specific diameter (microns). For example, a grit particle with an actual size of 150 microns may behave like a sand particle of 75 microns in size due to a coating with grease and organic material. This procedure can be performed on other particles such as snails to accurately size equipment. Table 1. Particle Size Equivalents Mesh Size Microns Inches Page 6 of 6

7 REFERENCES Bishop, Paul L.; Palsdottir, Gundy (1997) Nitrifying Biotower Upsets Due To Snails and Their Control, Water Science Technology Vol. 36 No. 1. p Lacan, Igor; Gray, Randall; Ritland, Greg; Jenkins, David; Resh, Vincent; Chan, Rick (2000) The Use of Ammonia To Control Snails In Trickling Filters, WEFTEC 2000 Metcalf & Eddy, inc. (1979a) Wastewater Engineering: Treatment, Disposal, Reuse, 3 rd Ed McGraw Hill, p , Smith, Jessica (2000) Wastewater workers scoop billions a year, Mail Tribune News. Water Environment Federation, WEF Manual Of Practices No. 8, 1992, Alexandria VA, WEF, ASCE Wilson, G.E. (1986) Coupling Free Vortex Energy Dissipation with Sediment Control, Water Forum 86: World Water Issues in Evolution, Volume 1 Wilson, G.E. (1996a) "Design Considerations in TEACUP Sludge Degritting", WEFTEC, Dallas, Texas Wilson, G.E., and Chang, D.P. (1984) "Light Grit", Proceedings, NCEE, ASCE, Los Angeles, California, pp Page 7 of 7