British Columbia Mine Reclamation Symposia

Practical sediment pond design with the use of flocculants Jones, Ron; Berdusco, Roger Joseph 1989

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th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  173  PRACTICAL SEDIMENT POND DESIGN WITH THE USE OF FLOCCULANTS Ron Jones, Environmental Control Officer Roger Berdusco, Administrator, Environmental & Geology February, 1989 FORDING COAL LIMITED FORDING RIVER OPERATIONS  INTRODUCTION Fording Coal Limited operates a large, open pit coal mine located  in  the  Rocky  Mountains  of  southeastern  British  Columbia (refer to site location map). The mine's capacity is 6.0 million tonnes of mostly metallurgical coal per annum. Mining operations commenced in 1972, employing both truck/shovel and dragline mining techniques in multiple seam pits. Total material moved annually is forty-eight million bank cubic meters of waste and over six million bank cubic meters of raw coal. There are two main mining areas: Greenhills on the west side of the Fording River and Eagle Mountain located on the east  side.  approximately  The 2,300  active  mining  hectares  of  operation  land.  Land  occupies  disturbances  include such activities as exploration, road construction, logging, clearing, grubbing, ditch construction, powerline installation, mining in large open pits, waste rock dumping or spoiling, coal preparation, coal stockpiling, and waste disposal of coarse washplant rejects and washplant tailings.  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  174  CANADA U.S.A.  175  This large land disturbance has the potential to cause erosion and  elevated  minimize  sediment  sediment  discharge  discharge  into  into the  the  Fording  Fording  River.  River,  To  Fording  Coal Limited operates six large settling ponds, several smaller catch basins, and twelve kilometres of diversion ditches. This paper discusses Fording Coal Limited's approach to manage suspended solids discharge from the mining area. The theoretical aspects of sediment settling rates, results of laboratory testing on minesite sediment samples, and the application of both theory and laboratory tests to develop practical sediment pond designs, are reviewed. SEDIMENT  PARTICLE  SIZE  &  THEORETICAL  SETTLING  RATE  CHARACTERISTICS Settling ponds and smaller catch basins are used to remove suspended solids in contaminated water. The ponds retain the water for a sufficient period of time called "detention time" so that the sediment particles can be removed by gravity settling. A particle that has a specific gravity greater than water will accelerate downward until it reaches a constant velocity. This settling rate velocity is dependent on factors particle's size, shape, and mass, and water turbulence, and viscosity.  such as the temperature,  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  176  Based  on  the  Canadian  classification, millimeter  a  (mm).  Soil  sand  Survey  particle  Particles  in  Committee  size  this  is  size  (C.S.S.C.)  0.05 range  to  2.0  should  require less than ten minutes to settle one meter. Usually silt particles in the size range of 0.01 to 0.05mm require approximately  1  to  1  1/2  hours  to  settle.  Fine  silt  particles in the size range of greater than 0.002 to 0.02mm require up to 24 hours. Coarse clay particles in the size range of 1 to 2 microns may require greater than 36 hours for settling. Fine clay or colloidal size particles of less than 1 micron may remain in suspension indefinitely. THE PROBLEM OF "FINE SEDIMENT TREATMENT"  Silt size sediment particles of greater than 0.01mm can be treated in a settling pond by plain sedimentation or gravity type settling. This can be accomplished by constructing a reasonably sized 6-10 hour detention time settling pond at moderate capital cost. However, sediment particles that are "fine",(less  than  0.01  to  0.001mm  in  size),  require  treatment in 36 hours or greater detention time settling ponds. These large structures have a usable water volume 3-6 times larger than a 6-10 hour pond, and can require a capital effective  outlay  6  treatment  times  greater.  of this  fine  The  problem  sediment  size  of  cost  fraction  prompted Fording Coal to investigate the use of flocculants and develop a "Practical Sediment Pond Design,"  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  177  MINESITE HYDROLOGY The 30 year normal precipitation average is 725mm per year at  the  Fording  River  minesite.  A  majority  of  this  precipitation, over 400mm, is in the form of snowfall. This is probably due to the high elevation of 1,700 meters in the Fording Valley and up to 2,300 meters in the Eagle  Mountain  mining  area.  This  snowfall  usually  accumulates in the Fording Valley from November to early March. Snowfall accumulates in the mountains from October to April. Erosion and subsequent sediment problems at the minesite occur during the spring runoff from April to June. The runoff is dominated by snowmelt with the peak flows usually occurring near the end of May, as illustrated in the 1981 hydrograph of the upper Fording River in Figure 1. Although the spring runoff can peak well over one hundred times greater  than  base  flows  in  winter,  the  duration  is  a  relatively short period, usually 2 weeks to a maximum of 6 weeks. Maximum sediment loading in creeks usually occurs during this peak runoff period. PROVINCIAL GOVERNMENT REQUIREMENTS The  British  requires  Columbia  settling  Ministry  pond  of  discharge  Environment contain  less  milligrams per liter (mg/1) or 50 parts per million total suspended solids (TSS).  The design flow for  of TSS in settling ponds should be the 10 year, flood flow.  (M.O.E.) than  50  (ppm) of removal 24 hour  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  178  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  179  In the late 1970's and 1980-1981, Fording Coal Limited had difficulty meeting the B.C. Ministry of Environment 50 mg/1 TSS objective during peak spring runoff at several settling ponds. In addition, many of the large settling ponds were beginning to fill up with sediment and clean-outs costs were found to be prohibitive.  maintenance  LAB SEDIMENT SETTLING RATE STUDY  A study was initiated in 1980-1981 to find ways to improve existing settling pond trap efficiency (sediment removal) combined  with  clean-out  a  method  costs.  This  or  design  study  to  lower  involved  maintenance  sampling  twelve  different creeks and testing sediment settling rates using imoff cones in a laboratory situation. A "modified pipette" with  a  large  turbulence  and  size  tip  opening  flocculant  was  breakage  used  which  to  minimize  are  important  factors in achieving good lab test results. Ideal settling rate tests were also carried out on sediment samples with various  flocculants  at  different  dosage  rates  or  concentrations. The flocculants that worked best at the Fording River minesite were Cynamid magnifloc cation 587-C and anion 1849-A. The cation 587-C flocculant is added first  to  "coagulate"  or  destabilize  the  electrostatic  charges on the individual sediment particle. Then the heavy molecular  weight  anion  1849-A  flocculant  is  added  to  "flocculate" or agglomerate the small individual particles to form a large particle mass that settles quickly.  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  180  OPTIMUM FLOCCULANT DOSAGE  The optimum dosage or concentration for each flocculant was determined by carrying out sediment settling rate tests in the  Environmental  optimum  dosage  effectiveness  Service's rate  is  3.0  in  ppm,  laboratory.  terms as  of  shown  Magnifiée  quantity  in  Figure  587-C  used/cost  2.  Magnifloc  1849-A optimum dosage rate was also 3.0 ppm as shown in Figure 3. IDEAL SETTLING RATES WITH AND WITHOUT FLOCCULANTS The ideal settling  rate  tests  indicated  that  the  majority  (80  percent of the total) of incoming sediment was coarse and could be removed effectively by gravity settling in a 1 - 2 hour  detention  time,  without  the  addition  of  flocculants, as illustrated in Figure 4. Typical samples from settling pond inlets that contained high sediment concentrations were found to require very high flocculant Environment's  addition  rates  objective.  It  to was  meet  the  observed  Ministry  that  these  of  large  dosages did not change the coarse fraction settling rate time significantly, thereby wasting the use of flocculants (illustrated in Figure 5). However, for the fine sediment fraction that would not settle readily by gravity in 24 hours  time,  flocculants  were  proven  to  be  effective  in  meeting the B.C. Ministry of Environment 50 mg/1 objective, as  sh ow n  in  Fi gure s  6  and  7.  Idea l  sett lin g  tim e  wa s  reduced for the fine fraction from an excess of 24 hours to less than 2 hours with cost effective rates of flocculant addition.  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  181  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  182  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  183  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  184  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  185  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  186  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  187  NON-IDEAL SETTLING RATES IN SEDIMENT PONDS  The settling time for treatment of the "fine fraction" with flocculants is 2 hours, as determined in the laboratory under ideal conditions. But what is this 2 hour ideal settling time equivalent to in a non-ideal situation such as an actual settling pond? B.C.  Ministry  of  Environment  recommend  in  their  "Draft  Guidelines for the Design and Operation of Settling Ponds Used for Sediment Control in Mining Operations", 1980, that a multiplier of 1.2 be applied to allow for non-ideal settling in actual pond design. A further correction factor of 1.25 should be applied at the Fording River Operation's minesite sediment ponds to correct for the colder pond water temperature of 5-10 C versus lab temperature of 20-22 C. Thus a practical correction factor for the settling rate tests conducted in the lab and then applied to actual Fording Coal Limited settling ponds is: 1,2 x 1.25 -= 1.5 K 2 hours - 3 hour detention time with flocculants FORDING COAL LIMITED'S SEDIMENT POND DESIGN The basic  design of  Fording Coal  Limited's settling  consists of the following, as illustrated in Figure 8.  pond  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  188  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  189  1.  Primary Pond The first pond is small in size, with approximately 1-2 hours detention time and is used to trap the incoming coarse fraction (silt and sand size range > 0.01mm) of sediment.  An  average  of  80-90%  of  the  total  inlet  sediment load is removed by the primary pond without the  use  of  flocculants.  This  pond  is  specifically  designed to be dewatered and cleaned out at a low cost. This  may  be  accommodated  by  providing  access  for  equipment to remove sediment and providing alternate routes  for  discharge  incoming from  water  this  during  pond  maintenance.  contains  only  The  the  fine  suspended sediment fraction. 2.  Mixing Zone The length of channel that connects the primary pond with the secondary pond is considered the Mixing Zone for flocculants. First the cation and then the anion are added to enhance settling of the fine fraction carried over from the primary pond. Tests indicate a 2-5  percent  flocculants. flocculants,  grade  range  Excessive  is  grades  resulting in  lower  ideal may  for  mixing  overshear  settling  the  rates.  Lab  mixing tests also indicate that a time of 30 seconds to 3 minutes appears adequate for mixing the 587-C and 1849-A  flocculants  slight  improvement  at in  moderate settling  speed.  There  efficiency  is  with  a  the  longer mixing time period of 3 minutes versus the 30 second mixing time, especially for the heavy molecular weight anion 1849-A polymer.  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  190  3 .  Secondary Pond The second pond is large in size, with an approximate 3-6 hour detention time. It is used to trap the fine fraction consisting of fine silt down to clay (0.01 to 0.001mm) with the use of flocculants. Pond length to width ratios should be a minimum of 2:1 and ideally 5:1 to avoid short circuiting and dead space problems.  Fording Coal Limited's existing settling ponds have been upgraded  to  constructed  the  primary/secondary  ponds  are  also  pond  designed  system,  with  two  and  newly  ponds  and  flocculant addition. The settling ponds very seldom exceed the B.C. Ministry of Environment T.S.S. 50 mg/1 objective and maintenance costs for clean-out of the sediment are much reduced. Fording  Coal  Limited's  more  recent  settling  pond  design  includes provision for infiltration and/or exfiltration through in-situ alluvial sand-gravels beneath the pond dike. This design is more effective in removing the fine fraction of sediment and also reduces the capital costs of settling ponds by subsequent downsizing.  February, 1989 :pp  th  Proceedings of the 13 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  191  REFERENCES  1.  Cheremisinoff, P.N. and Young, R.A. Engineering Practice Handbook", Ann Publishers, 1976.  2.  Chow, V.T., "Handbook Hill Inc . , 1964.  3.  Ettinger, P.E., "Development of Methods to Improve Performance of Surface Mine Sediment Basins Phase I " , Skelly and Lay Consultants on behalf of United States Environmental Protection Agency, 1980.  4.  Gibbs, R.J. and Konevar, L.N., "Effect of Pipetting on Mineral Floes", Environ. Sci. Technology Vol. 16, No. 1, 1982.  5.  Goldman, S.J. et al, "Erosion & Handbook", McGraw Hill Inc., 1986.  6.  Howie, H.J., "Draft Guidelines Operation of Settling Ponds Used in Mining Operations", Province Ministry of Environment, 1980.  7.  Jones, R.G., "Erosion & Sedimentation Strategy", Fording Coal Limited, 1980.  8.  Kyca, K.P., "An Evaluation of Flocculant Dosages Applied Suspended Solids in Water", 1981. Fording Coal Limited.  9.  McKeague, J.A., "Manual on Soil Sampling & Method Analysis", Second Edition, 1978. Canadian Society Soil Science.  10.  Merritt, F.S,, "Standard Handbook for Civil Engineers", McGraw Hill Inc . , 1976.  11.  Syme, D.T., "The Optimum Mining Speeds & Mixing Times for Magniflocs 587-C & 1849-A", 1982. Fording Coal Limited.  of Applied  "Pollution Arbor Science  Hydrology",  Sediment  McGraw  Control  for the Design and for Sediment Control of British Columbia, Control  of of  

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