Soil Based Wastewater Treatment
George W. Loomis
Dept. of Natural Resources Science,
Director of the Cooperative Extension On-Site Wastewater Training Center at the University of Rhode Island.
Regardless of whatever primary and/or secondary treatment steps that we subject wastewater to, most on-site wastewater treatment systems will have as a final step, the discharge of effluent to a soil absorption system The soil present on the selected building lot should be assessed by an experienced site evaluator for both its inherent suitability for on-site systems, and its expected ability to treat septic effluent in a manner which protects public and environmental health. Typically, we look for determinant factors such as depth to seasonal water table, presence of restrictive layers, permeability rates, and horizontal setback distances to drinking water wells, water bodies and wetlands, and other physical features. Based on this site evaluation, the site evaluator needs to judge the receiving soil’s ability to properly treat wastewater.
By utilizing component-based treatment steps to reduce septic tank effluent BOD, TSS, and microbial levels, we can often maximize hydraulic acceptance rates and treatment potential on marginal sites that barely meet regulatory code requirements. Understanding the basics of wastewater treatment in soils is fundamental to conducting a comprehensive site evaluation that produces a design which protects public and environmental health and promotes system longevity. The purpose of this document is to revisit those principles and the role of soils in on-site wastewater treatment.
Wastewater treatment should actually begin at the source; keeping gross solid generation within the household to an absolute minimum, conserving water, and avoiding excessive use of harsh chemicals, which may have a toxic effect on the septic microbes. The septic tank functions as both a quiescent zone where solids settle out of suspension, and as an anaerobic digestor. The digestion process is quite efficient, reaching maximum efficiency during the warmer times of the year. The solids separating ability in a septic tank is maximized in the colder time periods because less gas generation and particulate resuspension occurs then.
Water-tight septic tanks are critical for proper operation of any system, but they are especially important for advanced treatment systems incorporating secondary treatment steps and/or pressure dosing system. Periodic inspection and pumping septic tanks on an as needed basis helps assure that the digestion process is maximized and life cycle costs to the homeowner are kept to a minimum. Effluent filters places on the outlets of tanks are simple, yet effective, devices for retaining solids within the digestion step and safeguarding drainfields.
Pollutants common to septic tank effluent (STE) include: total suspended solids (TSS), biochemical oxygen demand (BOD), nutrients such as nitrogen and phosphorus, and pathogens (helminths, protozoa, bacteria, and viruses). The ability of soil to remove or inactivate these contaminants depends upon several soil factors: (a) physical – soil texture and structure, (b) chemical – the amount of soil particle surface area and their chemical properties, c biological – the presence of soil microbes that can utilize or degrade incoming pollutants, and (d) environmental conditions present in the soil. Along with these soil factors, chemical composition of the wastewater itself is the single most important non-soil factor governing the extent of wastewater treatment in soils.
Soil texture and structure can have a pronounced influence on the level of wastewater treatment that occurs in a soil. Mechanical filtering or straining processes can be effective in removing most of the suspended solids and some of the BOD load from effluent percolating through soils. Dissolved pollutants present in wastewater (not attached to organic matter) are not removed by mechanical filtering and typically must come in contact with soil surfaces for removal to happen.
Finely textures soils (those having more clay and silt sized particles) have a much greater surface per unit volume of soil than would a coarse textures (sandy/gravely) soil. As surface area increases, so does that soil’s chemical reactivity which is responsible for removing some of the incoming pollutants.
Soil surface area is important in the treatment process only when wastewater comes in contact with it. In a massive clayey soil with limited pore space, the relative amount of usable soil surfaces for wastewater treatment is small because wastewater does not come in contact with the entire soil volume. Ponded conditions often occur in poorly structured clayey soils which promote anaerobic conditions and poor treatment. Conversely, in coarse textured soils, rapid and localized wastewater movement occurs which does not allow sufficient time for biochemical processes to happen.
The key to optimal wastewater treatment is to promote unsaturated flow of wastewater throughout as much soil pore space as possible, thereby encouraging long retention times for biochemical reactions to occur. It is important t have soil micropores involved in the wastewater transport process so that pollutants make contact with soil surfaces. Well-structured soil, with even a limited number of macropores, can transmit large amounts of wastewater along preferential flow paths. In these cases where conduit-type flow happens, only a small amount of the soil volume is in contact with wastewater, and the beneficial effect of moving wastewater through smaller pores by capillary flow has been lost.
Perhaps the ideal treatment scenario is to incrementally apply small amounts of wastewater evenly to a soil interface over a twenty-four hour period, avoiding saturated conditions at any one point in time and encouraging capillary flow. This, of course, is not readily achievable with conventional gravity fed systems which receive wastewater flows on a demand type basis.
Soil surfaces are chemically reactive sites on soil particles where a host of treatment mechanisms can take place. Soil surfaces can be divided into four general categories:
- Silicate clay minerals are largely responsible for imparting a negative charge to soils particles which can attract and adsorb cations (positively charge ions) to these surfaces. Cations in septic tank effluent, such as ammonium (NH4+), can be adsorbed onto negatively charged surfaces; however, anions in solution will be repelled by the same surfaces and will not be removed by the adsorption process. Silicate clay minerals are effective in adsorbing bacteria, viruses, and many organic compounds.
- Hydrous oxides of iron, aluminum and manganese often occur as coatings on soil particles and are capable of attracting and holding certain anions (negatively charged ions) such as phosphate, and adsorbing and possible inactivating some viruses. Nitrate (NO3-) and chloride (Cl-) anions are not attracted to these hydrous oxides and move through soils unchecked.
- Calcium and magnesium carbonates can adsorb phosphate in a similar way to which hydrous oxides do. Calcium and magnesium carbonates are important phosphorus absorption materials in arid regions or in areas that have soils developed from parent material high in limestone. In some regions these carbonates can form dense layers called caliche which are impermeable to water movement and very problematic form a wastewater hydraulics perspective.
- Organic matter has a large reactive surface area and is extremely complex chemically. Naturally occurring soil organic matter can form chemical bonds with a variety of substances, and provide a food source for the growth of microbes necessary in the treatment process. Most septic drainfields are placed deeply in the soil solum; far below the organic-rich topsoil. The use of shallowly-placed drainfields makes use of these more biochemically active soil zones.
Soil Microbes and Wastewater Treatment
In general, aerobic soils are a hostile living environment for septic microorganisms. Soils contain naturally-occurring bacteria, fungi, actinomyces, and protozoa which make their homes on soil surfaces and are responsible for many biological wastewater treatment processes. Soil microbes play a role in organic matter degradation and removal of nitrogen, bacteria, and viruses. Providing aerobic conditions are maintained, soil microbe populations can actually benefit from the additions of nutrients, organic matter, and septic microbes present in septic tank effluent, because these materials serve as a food source. Some septic bacteria and viruses are retained in soils by surface adsorption processes which allows time for die-off, predation, and inactivation processes to occur.
Soil Environmental Conditions
Microbioligical processes in soil are sensitive to such soil environmental conditions as: temperature, oxygen levels, and moisture status. Cold temperatures reduce biological efficiency and treatment performance. As oxygen levels drop the efficiency and types of aerobic treatment processes are reduced markedly. Low oxygen levels, as may be found in failing drainfields or ones being inundated by high groundwater tables, favor the survival of anaerobic septic microbes and the inhibition of naturally- occurring (and beneficial) soil microbes.
Chemical Composition of Wastewater
Wastewater can be composed of a very diverse mixture of compounds, largely dependent upon what type of facility is creating the wastestream. Usually domestic single family flows are the easiest to deal with and high strength wastewater type flows such as commercial and restaurant wastes, are far more difficult to treat. In all cases, large systems compound and challenge soil treatment processes. Typical substances found in septic tank effluent are organic matter (as measured by BOD and TSS), nitrogen, phosphorus, and pathogens. Once introduced to a septic drainfield, these substances can become involved in a variety of physical, chemical, and biological process.
As mentioned earlier, BOD and TSS levels typical of domestic septic flows can be lowered significantly by passage through soils beneath a drainfield. This organic load adds to the eventual accumulation of a biological mat at the drainfield/native soil interface. Biomats begin to form closest to the point where wastewater is fed into a gravity trench and, with time, progress along the entire trench length creating a membrane type of filter. Organic material accumulates on the drain pipe side of the biomat where anaerobic conditions and microorganisms prevail. At the same time, reductions of biomat material occur from the native soil side of the mat primarily due to oxidation and mineralization processes. Unsaturated wastewater flow, and aerobic soil microorganisms and treatment processes, would predominate in the aerated soil beneath the biomat. At some point in time a dynamic equilibrium is formed where wastewater flow through the biomat reaches a steady state (often referred to as the long-term acceptance rate; LTAR).
The biomat’s long-time hydraulic capacity is maintained as long as the inputs and reductions of the mat are kept in equilibrium. If additions far exceed removal rates, then trench failure will occur and effluent breakout or surface ponding will likely happen. Conversely, should wastewater flow to this trench be interrupted, the biomat would dry out, decompose, and most of the original infiltration rate of the soil will be restored.
The biomat serves as a mechanical and biological filter and as a microbiological treatment zone. Though slower to establish and not as thickly developed in excessively permeable soils, biomats help to slow wastewater flow providing retention time needed for treatment to occur. Although not all the treatment dynamics are known, some removal of bacteria and viruses by the biomat membrane is generally accepted by the wastewater community.
Nitrogen present in septic tank effluent poses substantial public and environmental health risks in many area throughout North America. The US Environmental Protection Agency has established a nitrate-N concentration of 10 mg/liter (ppm) in groundwater used for potable use. This level has been enforced to prevent the occurrence of methemoglobinemia (blue baby syndrome). In salt and brackish waters, nitrogen is the limiting nutrient responsible for causing eutrophication (excessive aquatic plant growth and subsequent depletion of dissolved oxygen in the water column).
A variety of nitrogen retention, transformation, and removal processes can occur at the soil/drainfield interface and in the native soil or fill material beneath the drainfield (see the accompanying figure). Nitrogen in septic tank effluent is primarily in the organic and ammonium nitrogen (NH4+-N) forms, of which a small portion becomes trapped in the biomat interface by mechanical filtration and adsorption processes. The main transformation process here would be mineralization of organic-N to ammonium-N.
Once wastewater has moved through the biomat and into the soil beneath the drainfield, the predominant nitrogen retention reaction would be ammonium adsorption, while the main transformation reaction would be biological nitrification (the conversion of ammonium-N to nitrate-N). The nitrification process occurs only when aerobic conditions are present. Under wet soil conditions, as would be the case when drainfields are flooded by groundwater, ammonium-N would remain in that form and not undergo nitrification. Ammonium, as a positively charged ion (cation), can be weakly held by soil surface adsorption and cation exchange processes. Ammonium adsorption, although a retention process, is not considered a long-term removal mechanism.
Under aerobic conditions, as would occur in a properly designed and functioning drainfield, biological nitrification is the dominant transformation mechanism. In the nitrification process, NH4+-N is readily oxidized to NO3–N. Because NO3–N is a negatively charged ion (anion), it is not influenced by soil adsorption and cation exchange processes and can readily leach to the groundwater.
Plant uptake of NH4+-N and NO3–N may also take place by plant roots. Plant uptake, however, is considered to be minimal in the soil environment beneath a drainfield, because wastewater discharge in a typical conventional system is deeper than plant roots normally extend. In alternative septic system designs, which utilize shallow drainfields in upper soil zones, substantial uptake of NH4+-N and NO3–N can result. Under these conditions, plant uptake mechanisms remove nitrogen from solution and incorporate it into plant tissue. This sequestered nitrogen can, at a later time, be recycled back to the soil surface (form dead above-ground plant tissue) or recycled back into below surface soils when roots die. Although not a permanent removal process, plant uptake can continue to recycle nutrients over long periods of time.
Ammonia (NH3-N) volatilization, the conversion of NH4+-N to NH3-N gas is sometimes considered a possible nitrogen removal mechanism in soils. Because this process occurs at pH levels greater than 8.0-9.0, NH3-N volatilization is not a significant nitrogen removal pathway in regions with acidic soils.
Microorganisms can also incorporate inorganic forms of nitrogen (NO3–N) and NH4+-N) into cell tissue. The amount which becomes microbial biomass is relatively small and is not a permanent removal process, much being released back into the soil after microbial die-off.
Although microbes sequester only a small amount of nitrogen as biomass, they play a major role in biological denitrification – the reduction of NO3–N to nitrogen gases and perhaps the most significant nitrogen removal mechanism in soil environments. The end products of this process (N2 and N20) are harmless to public health and the environment. In order for the denitrification process to proceed, four specific conditions must be met: (a) oxidation of NH4+-N to NO3–N (nitrification); (b) subsequent anaerobic conditions; (c) presence of denitrifying bacteria; and (d) an adequate carbon (energy) source for the denitrifying bacteria present I the anaerobic zone.
Denitrification is not a common transformation process in a conventionally designed septic system. In a properly designed and functioning drainfield, the absence of reduced conditions immediately following the nitrification process and the lack of a suitable carbon source generally limit denitrification mechanisms. In areas where reduced conditions are produced by high groundwater tables, denitrification may also be limited by the lack of nitrification (as a result of saturated conditions).
The overlying reason why the rate and extent of denitrification in conventional system is so unpredictable is that the conventional system is not specifically designed to remove nitrogen. Properly designed conventional system usually contain efficient nitrification steps in their drainfield components but lack the necessary mechanisms for denitrification to occur. Alternative nitrogen removal systems have additional steps incorporated in their designs to ensure that proper conditions exist for both nitrification and denitrification. These steps occur in discrete watertight components that are designed to provide a suitable environment for a particular treatment process to happen. All these steps (should) occur sequentially in a logical treatment train so that an initial process compliments and enhances the next subsequent treatment step. This type of scenario does not typically happen at a quantitative level in a conventional septic tank/soil absorption system.
Phosphate anions are negatively charged ions capable of being strongly adsorbed to hydrous oxides of iron, aluminum, and manganese and carbonate surfaces on soil particles. It is also taken up by plant roots and incorporated into microbial cell material and organic matter. Phosphate is not a toxic compound, but it is the limiting nutrient in freshwater lakes and ponds responsible for eutrophication.
Most soils have the ability to adsorb phosphate loads from septic systems fairly well, so the concern is minimal. However, coarse textured soils with limited surface areas (due to low hydrous oxide or carbonate contents) can eventually reach their phosphate adsorptive capacity and not provide sufficient treatment to protect adjacent water bodies. Phosphate removals are also limited in saturated soils, and in situations where localized channel-type wastewater flow occurs. This can be especially problematic in large system where hydraulic overloads are common. Similarly, sand filters and mound type filters also have limited surfaces at which phosphorus adsorption could occur. It is common to see good phosphorus removal levels rebound to nearly that of the incoming septic tank effluent, after the filters have matured and adsorption sites have become saturated with phosphorus.
The common types of pathogenic (disease causing) microorganisms is septic tank effluent are: helminths (septic worms), protozoa, bacteria, and viruses. In many cases the mobility of these organisms if often related to their size relative to soil particles and soil pore space. Helminths are roughly the size of sand particles, protozoa the size silt particles, bacteria the size of fine silt and coarse clay, and viruses the size of very fine clay. Due to their relative size, both helminths and protozoa are physically caught by soil mechanical filtering processes, limiting their movement through soil pore spaces. Mechanical filtering and entrapment are the main retention mechanisms which lead to eventual inactivation (die-off) of these larger septic microbes by processes such as predation by soil microbes or death due to unfavorable soil environmental factors.
As one would expect, bacteria and viruses would have a much greater potential to move, especially in coarse textured soils. Their interactions and fate in soils are not well understood. Viruses, while not physically trapped in soil pore spaces, are retained primarily by chemical and physical adsorption to clay or hydrous oxide surfaces. Bacteria are capable of being retained in soils by both entrapment and adsorption processes. It is important to note that retained bacteria and viruses, especially those retained by adsorptive processes, may not always die and may even be protected from inactivation (i.e. prolonged survival) by the adsorptive process. Retention, as the term implies, is not always a permanent process, and retained viruses can be resuspended during heavy rain events or periods of groundwater flooding.
Under aerobic soil conditions, survival of septic bacteria and viruses is poor because they do not compete well with natural soil microbes. Should the soil moisture conditions change to an aerobic state, then survival would shift in favor of the septic-borne anaerobes. Lower soil temperatures favor septic bacteria and virus survival, because native soil microbe activity (predation) is low. Acid soils promote more rapid die off of most species of septic bacteria, yet encourages viral persistence, perhaps due to increased adsorption under acid conditions.
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Gustafson, David. 1995. On-site Sewage Treatment Manual. University of Minnesota Cooperative Extension Publication. St.Paul, MN.
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