Soil Physical Properties and Wastewater Treatment

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1 Soil Physical Properties and Wastewater Treatment John R. Buchanan, Ph.D., P. E. Associate Professor Biosystems Engineering and Soil Science Department

2 Soil Physical Properties and Wastewater Treatment Ultimate goal To convert wastewater back to water Intermediate goal To protect environmental and public health We separate humans from their wastes by putting wastewater in the soil And we separate the wastes out of the water by putting wastewater in the soil

3 Important Note We are very dependent on the soil to provide treatment This is not subsurface wastewater disposal This is wastewater renovation that disperses water back into the hydrologic cycle recharges groundwater baseflow to streams evapotranspiration

4 With this Dependence on the Soil We have to understand that all soils are not created equal We have to understand the renovation potential provided by different soils and quantify these differences by understanding the physical properties of soil chemical properties of soil and the biological properties of soil

5 When using the Soil for Wastewater Treatment We have to: Understand the quantity of waste constituents in the wastewater let s call it wastewater strength Understand how much wastewater we are working with wastewater volume Understand how wastes are removed from water wastewater treatment

6 This Session Will focus on the soil physical properties The next session will focus on soil chemical properties soil biological properties All with the intention of converting wastewater back to water

7 What is Soil? Minerals Organic matter Water Air It s not dirt unless it s under your fingernails

8 What Makes a Good Soil? Not too shallow Not too much clay Not too much sand Minimum shrink-swell Can support vegetation Oxidized conditions

9 Soil should be a uniform brown, yellow, or red

10 When we do Soil Evaluations What are we looking for? Information to put in the soil profile log With this information we can go to the Soil Loading Rate Table

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12 Soil Loading Rate Table

13 Soil Physical Properties All of these properties interact to determine water movement through the soil Horizons Texture Structure Morphology

14 With this Information We get a loading rate With the loading rate we calculate the size of our system based in the infiltrative surface area

15 Soil Physical Properties Other properties include temperature restrictive horizons aeration

16 Soil Physical Properties Allow for the movement of effluent into the soil (infiltration) and through the soil (percolation) If we cannot get the water into the soil then we cannot provide treatment

17 Department of Soil Science, University of Saskatchewan Soil Processes

18 Texture Distribution of sand, silt and clay particles relative percentages of sand, silt, and clay 100 = %sand + %silt + %clay

19 Alberta Guidelines for Texture

20 Natural Soil Structure Description of how soil particles are aggregated or clustered Granular Angular Blocky Subangular Blocky Platy Wedge Prismatic Columnar Grades degree of distinctness of structure

21 Relationship of Structure to Porosity

22 Structure and Water Movement

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29 What does Texture and Structure have to do with Wastewater Renovation? Texture is related to micropore flow Small pores Exposes wastewater to more soil-particle surface area Attachments site for chemical and biological reactions Structure is related to macropore flow Larger pores Key aspect to moving water through soil

30 Texture and Treatment Surface area for attachment sites 1 lb of sandy soil has 3 acres of surface area 1 lb of loamy soil has 15-acres of surface area

31 Reactive Surface Area Clay is the reactive fraction of the soil 1,000,000,000 clay particles could fit in a sand particle sand surface area 24 mm 2 clay surface area 24,000 mm 2

32 Infiltrative Surface Interface where water enters the soil primarily, the trench bottom the sidewalls are a bonus Surface area is determined by wastewater volume wastewater strength ability of soil to infiltrate water

33 Infiltration and Percolation Before a volume of water can go through the infiltrative surface there has to be an equal volume open within the soil If the soil is saturated no new infiltration until some water percolates During saturated conditions the limiting factor is percolation

34 Permeability Limiting Factors if water is applied at a rate greater than it can move through profile, then effluent will pond in the trenches Water holding capacity if too much water is added than the soil can hold, then saturated conditions and deep drainage occurs or in sandy soils, the water will move through too quickly

35 Moisture Holding Capacity Water is held by the soil particles Water and soil particles are attracted to each other This layer of water is where the microorganisms that renovate wastewater live and work.

36 Saturated Soil All the pore space is filled with water Very little oxygen is available for microorganisms Water will drain by gravity

37 Non-Saturated Soils Dry but not bone dry water is tightly held by surface forces on soil particle water movement is based on balancing those forces

38 Dissolved Ions Pull Water toward Surfaces

39 Roots will Pull Water and Ions Away from Surfaces

40 Non-Saturated Flow Is based on balancing these forces Fine textured soils have more tension but the porosity is too small to percolate much water

41 Field Condition The amount of moisture when drainage no longer occurs Non-saturated flow Aerobic conditions for microorganisms

42 Wilting Point Particles hold moisture with greater tension Infiltration is rapid Wasted opportunity to renovate wastewater

43 Soil-Water Volume Difference between wilting point and field condition is a volume of water that is aerobic and can provide excellent renovation of wastewater

44 Aeration Gas transfer We need aerobic conditions to minimize the biomat build up the accumulation of biosolids in the soil pores Oxygen demand is the mass of O 2 needed to maintain aerobic conditions

45 Aerobic Biodegradation of Soil microbes will consume O 2 release CO 2 These gases need be able to move through the soil profile or soil will go anaerobic similar to saturated conditions Organic Matter

46 Assume that. Septic tank effluent has a biochemical oxygen demand (BOD) of 150 mg/l For each liter of effluent the soil needs to allow 150 mg of O 2 to pass through the soil this doesn t sound like much, but

47 When it comes to Gas Transfer The Soil is Limited Air is only 21% O 2 have to move a lot of air to get the oxygen CO 2 has to leave the soil just like our lungs The deeper the infiltrative surface the more limited oxygen transfer becomes

48 Aeration Processes Convection atmospheric pressure helps to push air into the soil we have about 14 psi of air pressure on the soil surface Diffusion movement to less concentration as oxygen is consumed, a gradient is formed

49 Back to the Example Three bedroom home average flow 225 imp gpd (1026 L) 150 mg/l BOD 153 grams (0.34 lb) of O 2 per day 1.62 lb of air per day Air is typically lb/ft 3 21 ft 3 of air movement per day divided over the whole drainfield assuming uniform distribution

50 Part of Breathing. Is losing water vapor the soil must allow for evaporation Evapotranspiration water moving from the soil to the atmosphere liquid phase to gas phase cooling process roots pull water out of micropores so new water can be added

51 Trench Width and Spacing Narrower trenches allows more surface area Narrower trenches allows better O 2 transfer

52 Using the Soil as a Wastewater Treatment Plant Must think past simple infiltration Will the water be sufficiently renovated before it re-enters the hydrologic cycle? What is sufficient treatment? Onsite systems must be managed Cannot be installed and forgotten These systems can have a tremendous influence on our environmental quality

53 Questions