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Loading Rates

Loading Rates. Sara Heger sheger@umn.edu http://septic.umn.edu. Loading Rates - The Thought Process. The system should last a long time The wastewater plugs the soil over the long term Designing the system for plugging is CRITICAL. How does soil treat wastewater?. Well. Horizontal

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Loading Rates

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  1. Loading Rates Sara Heger sheger@umn.edu http://septic.umn.edu

  2. Loading Rates - The Thought Process • The system should last a long time • The wastewater plugs the soil over the long term • Designing the system for plugging is CRITICAL

  3. How does soil treat wastewater? Well Horizontal Setback Aerobic soil Groundwater Aerobic soil is needed to treat – remove pathogens – and disperse the treated wastewater back into the environment

  4. What are aerobic soil conditions? • Pores filled primarily with air (oxygen) • Aerobic organisms present • Pores are open – not smeared • Air can move through pores – not compacted • Soil is NOT saturated or likely to become saturated

  5. Soil properties that influence wastewater treatment • Wetness conditions • Water movement • Texture • Structure • Restrictive zones or horizons • Landscape

  6. Types of Flow Unsaturated Saturated Pores air-filled Flow is adjacent to particles Controlled by moisture content and pore diameter Aerobic conditions Slower than saturated flow LTAR related to unsaturated flow Pores water-filled Flow is in large pores Controlled by soils and site conditions May result in anaerobic conditions Faster than unsaturated flow

  7. Flow from the trench Sidewall infiltration is limited to depth of ponding in trench Zone of saturated or nearly saturated flow Zone of unsaturated flow, majority of flow is vertical Zone of saturation or a restrictive layer, flow will be down hill

  8. Regulations vary: Vertical separation distance : The thickness of air filled soil required between the base of the drainfield and the water table. wt W WT

  9. Wastewater treatment and renovation in soils Controlling Factors : Environmental • Temperature, moisture, and oxygen levels Wastewater characteristics • Loading rates; wastewater strength • Types of pollutants

  10. Wastewater treatment and renovation in soils Controlling Factors: Soilproperties • Physical - filtration and sedimentation • Chemical - adsorption/precipitation (surface area) • Biological - uptake, incorporation, predation, • and transformations

  11. Time isneeded for treatment reactions • Biochemical processes depend on detention time • Detention time is closely related to • hydraulic loading rates • rate of wastewater movement through soils • soil texture, structure, and density have a huge influence on detention and reaction times

  12. Soil physical properties - Texture The relative proportion of soil separates (sand, silt & clay) in a soil. Texture influences: • Soil permeability and moisture content • Biomat formation • Treatment of effluent • System construction • - Soil smearing and compaction

  13. Soil structure & water movement Void spaces between soil peds transmit air and water. Type of structure determines: • Direction of voids (soil pores) • Direction of water movement • Relative rate of water movement • Retention time for treatment processes

  14. T 1 Biomat begins as incomplete layer at end of trench closest to D-Box where most of the loading is taking place.

  15. Trench with a fully-developed biomat T m (mature) Even distribution of wastewater has occurred due to biomat acting as a membrane-type filter.

  16. Biomat acting as a membrane filter T s (steady state) Soil under biomat is aerobic and air filed. Amount of organic material removed from underside of biomat membrane by soil aerobic bacteria roughly equals amounts added from septic tank.

  17. When organic inputs exceed removals and all soil pore spaces are clogged by organic material, then hydraulic failure occurs.

  18. All systems have Two Values Hydraulic Flow Organic Loading

  19. Hydraulic Flow

  20. Wastewater Loading • Wastewater quantity • Hydraulic loading • Residential 120-150 gallons per bedroom • Wastewater quality • Organic loading • Residential – 300 mg/l • Oxygen demand • Residential and commercial facilities

  21. Importance of hydraulic load • The daily flow must not exceed the system’s hydraulic capability • Hydraulic detention time (HDT) • Example: solids are not able to settle in a septic tank if the water moves through too quickly. • Hydraulic overload of the soil • Effluent surfacing

  22. Too much use • Clean water • Groundwater drainage • Footing drain • Cooling water • Water treatment • Too much use • Over use • Wash day • Cleaning service • Change in use • Master bath • Added bedroom

  23. Leaky Components Tanks Piping Treatment Components System measurement

  24. Measured Flows? How often is it Measured? • Annually plus--- • Average < 70% • Monthly • 70-75% • Weekly • 80% • Daily • Actual use • Surge flow is determined by measuring flow daily over an extended period of time

  25. What is needed to Calculate Hydraulic Loading • Cycle counter reading • Dose Volume • Time between readings (actual operation) • Elapsed time meter • Pump Rate • Change in value = Total number of units • Minutes • Hours • Time between readings (actual operation) • Water Meter • Present and last reading • Time between readings Pump delivery rate

  26. Wastewater Quantity - Surges • Surge flows • Daily • Weekly • Seasonal • Flow equalization?

  27. Organic loading

  28. Commercial Wastewater • Strength • Usually greater than residential • Operation based • Food preparation • Restrooms • Laundry

  29. High Strength Wastewater • CIDWT glossary definition 1) Influent having • BOD5> 300 mg/L, • and/or TSS > 200 mg/L, • and/or fats, oils, and grease (FOG) > 50 mg/L entering a pretreatment component 2) Effluent from a septic tank or other pretreatment component that has: • BOD5> 170 mg/L, • and/or TSS > 60 mg/L, • and/or (FOG) > 25 mg/L and is applied to an infiltrative surface. • Local code definitions may vary

  30. Type of Restaurant Restaurants Sampled, # BOD5 mg/L TSS mg/L FOG mg/L Fast Food 8 1286 202 282 Service 5 1130 213 219 Golf Club 4 1010 142 200 Bar 3 874 184 132 Restaurant Results - Minnesota

  31. Restaurant Data - Lesikar 2004 Study • 28 restaurants located in Texas • Sampled during June, July, and August 2002 • 12 samples per restaurant and 336 total observations • Most conclusive study to date

  32. Percent of Data Captured (28 Restaurants)(Geometric Mean plus One Std. Dev.) Lesikar et. al (2006)

  33. Summary Statistics Lesikar et. al (2006)

  34. Wastewater Organic Rates

  35. Effluent Constituent Concentrations 1Siegrist, 2001

  36. Now that we know the hydraulic and organic values, we combine the two to determine the mass load What's Next? Mass loading

  37. Mass Loading • Calculate mass loading to a system • Concentration of constituent in the wastewater • Mass loading based on number of people • Mass (lb) = C (mg/l) x Q (gpd) x 0.00000834 • Mass (lb) = P (# of people) x OL (lbs per capita- day)

  38. Mass Loading Calculation Residential strength • Calculate mass loading to a system • Concentration in wastewater • Volume of wastewater • Mass (lb) = 140(mg/l) x 200(gpd) x 0.00000834 • Mass (lb) = 0.23 lbs per day Commercial strength • Mass (lb) = C (mg/l) x Q (gpd) x 0.00000834 • Mass (lb) = 500(mg/l) x 600(gpd) x 0.00000834 • Mass (lb) = 2.5 lbs per day

  39. Mass Loading • Calculate mass loading to a system • Number of people • Organic loading rate • Mass (lb) = P (# of people) x OL(lbs per capita- day) • Mass (lb) = 5 (# of people) x 0.17(lbs per capita- day) • Mass (lb) = 0.85 lbs per day

  40. Water Saving Devices • Decrease water quantity • Assuming no change in mass load • Wastewater strength increases

  41. Water Saving Device Example • Example 4.2 Increasing concentration of TSS • A 4 person household produces 0.56 lbs/day TSS without water saving devices (75 gpd/person). • Then that family switches to water savings devices, and so they only use 60 gpd/person. • What is the change in TSS concentration after water saving devices are installed?

  42. Example Cont. TSS Concentration (before) = ____ 0.56 lbs/day___ = 224 mg 300 gal x 0.00000834 L TSS Concentration (after) = ____ 0.56 lbs/day___ = 280 mg 240 gal x 0.00000834 L

  43. Now that we know the flow and the biological values, we combine the two to determine the mass load What's Next? Contour loading rate

  44. Much More Water

  45. Much More Water

  46. Much More Water • It appears to be OK to use conventional soil loading rates for sizing the infiltration areas for both septic systems and MSTS (i.e. getting the effluent from the media into the soil) • The question is what does the effluent do once it gets into the soil?

  47. Stop to Review • Actually 2 Loading Rates: • Infiltration/Absorption Loading • Amount of Effluent/Soil Texture • Organic Loading Rate • Contour/Mounding Loading Rate

  48. Two Loading Rates Infiltration Capacity

  49. Two Loading Rates Disperses

  50. What is GW Mounding? Simply, the rise in the groundwater when water is added by man.

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