Source of Pollution or Groundwater Solution?

Transcription

1 Acknowledgement Source of Pollution or Groundwater Solution? Kitt Farrell-Poe, Ph.D. Water Quality Extension Specialist & Professor Agricultural & Biosystems Engineering University of Arizona This work was supported [in part] by the National Decentralized Water Resources Capacity Development Project with funding provided by the U.S. Environmental Protection Agency through a Cooperative Agreement (EPA No. CR ) with Washington University in St. Louis. The results have not been reviewed by EPA or Washington University in St. Louis. The views expressed in this presentation are solely those of NCSU, and University of Arkansas and EPA and Washington University in St. Louis do not endorse any products or commercial services mentioned in the presentation. Effluent Movement & Treatment Why is this important? How systems work Flow patterns Unsaturated Biomat Saturated Flow direction Lateral movement Vertical movement Treatment impacts Why is this important? Because an onsite wastewater treatment system should: Treat sewage Public health concerns Environmental quality concerns Disperse sewage Provide acceptable level of risk put the odds on our side Join hydrologic cycle Well Hydrologic Cycle Precipitation ET Saturated flow Restrictive layer Unsaturated flow Regional water table Hydrology Components Precipitation Evapotranspiration Runoff (surface flow) Infiltration Subsurface flow lateral flow, interflow, shallow groundwater flow Vertical seepage deep percolation, groundwater recharge Unsaturated zones Water table (saturated zones)

2 RAINFALL (51 /Year, Raleigh, NC) (12 /Year, Tucson, AZ) POTENTIAL ET (38 /Year, Raleigh, NC) (74 /Year, Tucson, AZ) CROPLAND FORESTLAND Recharge area Water table Discharge area INFILTRATION SURFACE RUNOFF WATER TABLE GROUNDWATER FLOW SURFACE WATER GROUNDWATER SYSTEM Decades Centuries Years Days AQUIFER RECHARGE (1/2 TO 2 Inch/Year) CONFINED LAYER AQUTARD CONFINED AQUIFER Flow lines Millennia Direction and rate of groundwater movement How Systems Work Sewage What is sewage? Onsite systems Flow patterns What is it? Water Bacteria food BOD Nutrients TSS Solids Pathogens Toxic chemicals Others?? How much is Produced? gpd per bedroom gpd per person Where is it Produced? Bathroom [60%] Kitchen [20%] Laundry [20%] System Overview Goals of Treatment Separate solids * Pretreatment Final treatment & dispersal (Soil Treatment Area (STA) Reduce organic materials (BOD)* Reduce nutrients Reduce pathogens Reduce toxic discharges * Septic tank Soil treatment area

3 Goals of Dispersal Disperse effluent evenly across soil treatment area to keep soil unsaturated and aerobic. Provide enough time in the soil for treatment to take place. Percolate to groundwater so original water can be recycled back into the water cycle. Pretreatment: Septic Tank Functions Separate solids, oils, and greases from liquid Anaerobic decomposition (some, not a lot) Keep sewage in tank at least hours Store solids Dissipate energy of wastewater As scum and sludge layers build and come closer together, the 2-day detention time becomes less and solids removal efficiency drops. Screen Soil Treatment Area Pathogen capture Receives effluent & transmits to soil Soil underneath provides treatment physical filtration biological activity chemical reactions Soil should remove disease-causing organisms and some chemicals of concern Returns the water back to the water cycle Courtesy of National Assn of Wastewater Transporters Pathogen removal How do pathogens get treated? They are physically filtered They are attached to the soil particles They die due to environmental conditions They are attacked by soil organisms Given time, the soil is a very effective treatment media Courtesy of National Assn of Wastewater Transporters

4 NUTRIENTS Phosphorus Surface water Nitrogen Groundwater Treatment Precipitation Dilution Denitrification Courtesy of National Assn of Wastewater Transporters Chemicals NH 4 + O2 NO 3 - VOC Pharmaceuticals Courtesy of National Assn of Wastewater Transporters Denitrification needs Low DO Food {BOD or organic matter} Bacteria Flow pattern in sub-surface trench Saturated Conditions Unsaturated flow Saturated Conditions Unsaturated flow Mounded Groundwater Saturated flow Courtesy of National Assn of Wastewater Transporters Pores are filled with water

5 Unsaturated Conditions Unsaturated vs. Saturated flow Unsaturated Pores: Air -filled Saturated Pores: Water-filled Mov t next to particles Aerobic Controlled by Soil wetness Mov t in large pores Non aerobic Controlled by Slope Courtesy of National Assn of Wastewater Transporters Pores are filled with air & water along the soil particles Soil pore size Soil (Ksat) How wastewater is treated? Pore size & unsaturated flow Inside the trench the conditions are Anaerobic Under the trench Aerobic environment Bacteria Unsaturated Flow Large pores water will pulled predominantly by gravity Small pores water will move in all directions better & further Large Pores Small Pores How does unsaturated flow in the soil treatment area happen? Unsaturated flow is the key Biomat formation needed for gravity-fed soil treatment areas BOD Oxygen relationship TSS Pressure distribution Flow pattern in a gravity trench Biomat Growth (t = 0 = start )

6 Flow pattern in a gravity trench Biomat Growth (t = growth) Flow pattern in a gravity trench Biomat Growth (t=mature) Flow pattern with Pressure Distribution Pressure distribution System geometry HYDROLOGY OF A SEPTIC SYSTEM Infiltration from Trenches Vertical Movement through the Unsaturated Zone Lateral Movement in the Saturated Zone Least Permeable Ground Water Mounding and Formation of a Saturated Zone Slowly Permeable Layer

7 What happens when you put wastewater in the soil? Biomat forms Creating unsaturated flow This means: More contact with the soil particles Aerobic organisms treat the wastewater Biomat & Sidewalls Biomat develops along the bottom and then around the trench Ponding levels use sidewalls Excessive ponding depths create saturated flow Narrower allows more surface area Narrower allows better O2 transfer Long Term Acceptance Rate (LTAR) The biomat controls the ability of the soil to accept effluent: this is the LTAR Generally State codes dictate LTARs LTAR Texture/ Structure Other tests o Saturated conductivity o Percolation rates Soil Characteristics and Soil Sizing Factor (> 3' separation) Percolation Rate Soil Sizing Factor minutes per inch Soil Texture square feet/ gallon (mpi) per day(sqft/ gpd) faster 0.83 than 0.1* Coarse sand 0.1 to 5 Medium sand 0.83 Loamy sand 0.1 to 5** Fine sand to 15 Sandy loam to 30 Loam to 45 Silt loam 2.00 Silt 46 to 60 Clay loam 2.20 Sandy clay Silty clay over 61 to Clay *** Sandy clay Silty clay slow er than 120**** *Use systems for rapidly permeable soils: pressure distribution or serial distribution with no trench >25% of the total system. **Soil having 50% or more fine sand plus very fine sand. ***A mound must be used. ****An other or performance system must be used Influencing the Biomat Good Loading Hydraulic Organic Resting Depth of cover Oxygen availability Bad Peroxide Acid Terra lift??? Finding Saturation in soils Identify saturation by looking for free water in pits or auger holes. Water pushed air out of pit as it flowed in, so its pressure was greater than air pressure.

8 Finding Saturation in soils Identify saturation by looking for changes in soil color in pits or auger holes. Where the two flows meet Trenches Biomat: Flow control Unsaturated zone Separation Treatment Mounding Raising of saturated levels Groundwater Saturated flow Putting it together Backfilled Soil Soil Surface Air Space Gravel Distribution Wastewater Pipe Saturated Zone Water Flow Path Summary of Effluent Movement & Treatment in a Soil Treatment Area Saturated conditions Lack of treatment Preferential flow Excessive biomat growth Organic loading Construction soil damage Unsaturated Zone NOT TO SCALE Water Table or an Impermeable Layer Conclusions For treatment, flow from trench needs to be aerobic & unsaturated Biomat reduces/controls the flow from a system Flow is generally vertical Treated (and untreated) wastewater eventually joins the hydrologic (water) cycle Soil is a very effective treatment media & Treatment is the name of the GAME

9 Questions?