Treatment of Septic Tank Effluent using the Puraflo Peat Biofiltration System
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Bord na Mona,
Newbridge, Co. Kildare, IRELAND,
PO Box 77457,
Greensboro, NC 27417
As a primary treatment system, septic tanks do not significantly reduce the polluting potential of the wastewater. The bulk of the treatment takes place in the soil through various physical, chemical and biological interactions between the effluent and soil colloids. In the United States approximately three billion m3 of septic tank effluent is discharged into the soils for treatment annually (Bitton & Gerba, 1984). However, less than 50% of these soils are thought to be capable of achieving an adequate reduction in the pollution potential of the waste (Patterson et al, 1971). Similarly, in Ireland there are an estimated 300,000 septic tanks serving a population in the region of 1.2 million people and discharging approximately 78 million m3 of wastewater to soil annually (Henry, 1988). Once again only half of these soils are considered capable of providing sufficient treatment to prevent groundwater or surface water pollution.
Septic tank systems are the most frequently reported source of groundwater contamination. Many public health professionals feel that the most critical effect of septic tank systems is the contamination of private water wells. The human health implications of such contamination are considerable. Outbreaks of typhoid fever, infectious hepatitis, gastrointestinal infections and infantile methaemoglobinaemia have all been linked to malfunctioning septic tanks. Almost half of the reported water disease outbreaks in the US every year are due to the consumption of contaminated groundwater (Keswick et al, 1982). Overflow from septic tanks are responsible for 42% of the reported outbreaks of disease (Craun, 1979).
Pollution of groundwater and surface waters by septic tank effluent can be chemical, or biological in nature, or both. The poor Microbiological quality of domestic well water supplies has been well documented. A study of rural groundwater sources in the U.S. showed 92% to be contaminated with coliform bacteria (Bitton & Gerba, 1984), while a similar study in western Ireland found that 68% of all rural groundwater supplies contained fecal coliforms, fecal Streptococci or both (Aldwell et al, 1986). Septic tank effluent was believed to be the main source of contamination in both cases.
In response to the problems identified, a large number of “alternative” on-site treatment technologies have been developed and tested in various parts of the U.S. and Europe over the past few decades. Many of these innovations have failed to provide the necessary safeguard against:
- Soil clogging with associated surface failure or,
- Contamination of groundwater sources.
Consequently their widespread application has not been universally accepted by local regulatory and public health personnel.
There is an urgent requirement for a reliable, cost effective and low maintenance on-site treatment and disposal system which can be used with equal succession a range of soil types and hydrogeological settings. This paper describes which Puraflo Peat Biofilter which has been developed and optimized in Ireland over the past 10 years. The technology has been introduced in the U.S. since 1993 where performance and acceptance by local health representatives and consumers alike has been very favorable.
The Puraflo Peat Biofilter is highly efficient treatment unit fro the assimilation and disposal of wastewater from domestic sources. Poor quality primary effluents are evenly distributed over a specialized fibrous peat media which is contained in a number of molded polyethylene modules. The effluent percolates through the unit and emerges as a clear innocuous liquid from the base of the system for disposal, usually by subsurface infiltration. A schematic representation of the Puraflo system is presented.
The required surface area/volume of treatment media and percolation area for a typical Puraflo Peat Biofilter system is based on anticipated water usage, soil classification and expected percolation rate; and the following assumptions with respect to the strength of septic tank effluent:
|Unit organic loading||0.0102||lbs.ft.-2.d-1||max.|
Any oil and grease present in the wastewater should be removed prior to dosing onto the treatment media.
The effluent flows under gravity from the septic tank to a pump chamber (sump). The sump is brought to ground level with a riser and covered by a manhole cover. A submersible pump is suspended six to twelve inches above the base of the sump. An audio and visual alarm in the dwelling alerts the homeowner in the event of a pump malfunction. The pressurized dosing system is activated by an automatic float switch or timer. The dosing mechanism pumps the effluent through a flow splitting manifold.
The manifold is located at the base of the polyethylene modules containing the biofibrous peat treatment media. The manifold is connected by flexible pipes to the rectangular distribution grid contained below the top of each module. The effluent dosing is accelerated by orifice plates placed inside the flexible piping and is evenly dosed onto the peat media. 15mm (1/2″) orifices are positioned at 9 o’clock and spaced at 30 cm (1″) intervals along the laterals of the closed loop rectangular grid. The distribution grid is placed about six inches below the surface of the biofibrous treatment media in each module such that any odors during dosing or pumping are suppressed. The surface area of the standard unit (excluding the 3 foot graded mound the encompasses the system) is less than 150 square feet.
The effluent moves laterally and vertically within the odules through a depth of two feet of biofibrous peat treatment media. The residence period or contact time in the media has been calculated to be somewhere between 36 and 48 hours. Treatment occurs in the media through a combination of physical, chemical and biological processes and emerges as an innocuous liquid which exits through holes just above the base of the module. The system is completely natural and requires no additives or chemicals of any kind to be used in the treatment process.
A portion of the treated effluent is piped directly from the treatment media to a sampling chamber adjacent to the modules before being released to the stone base percolation area. This allow easy access to inspect system performance and quality of treatment. In sandy soils a 500 gallon per day typically covers a 320 square foot area (16 ft by 20 ft). However the module configuration and percolation area will vary to suit the site and soil conditions and is subject to regulatory approval. In poorer draining soil, radiating drains are connected to the footprint percolation area to increase the available dispersal area.
The treatment of the wastewater within the Puraflo system is achieved by a combination of unique physical, chemical, and biological interaction between the sewage and the fibrous media. Extensive scientific examination of the treatment media has revealed a complex chemical structure which permits a number of separate treatment/attenuation processes to occur simultaneously. The treatment mechanisms within the fixed film media can be summarized as follows:
Physical Filtration, Adsorption
Chemical Adsorption, Ion exchange
Biological Microbial assimilation
In nature Puraflo unit the biological transformation processes are known to be crucial in maintaining the treatment efficiency observed. The bulk of the treatment/assimilation processes are achieved by a divers microflora which adhere to the surface of the peat media. This microflora is largely composed of aerobic and faculatively aerobic heterotrophic bacteria from a large number of genera. The most important bacterial genera represent include:
The total bacterial population recorded per gram of peat has been measured at 1 x 10 9 cfus. Similarly, high numbers (up to 1 x 10 7 cfu/g) of fungal organisms have been isolated from the Puraflo sewage treatment units. A wide variety of “higher life” forms have also been recorded within the media matrix (ranging from protozoans, rotifers, and algae to nematode and annelid worms, insects and their larvae). These organisms play an important role in keeping the bacterial population “in check” thereby maintaining a balanced microflora and ultimately a stable ecosystem. The larger numbers of heterotrophic bacteria are found in the upper portions of filter media with nitrifiers becoming more prevalent are depths of 30cm or greater. Therefore, the degradation and assimilation of the carbonaceous element of the waste is effected within the upper portions of the filter bed with nitrification occurring at greater depths provided that sufficient oxygen is available.
The Puraflo system is primarily a pre-treatment system but is usually presented as a combined treatment and disposal unit. The treated effluent usually disperses under gravity through a level of stone base percolation area located beneath the modules.Alternative dispersal methods such as direct piping to remote trenches by gravity or under pressure can be utilized. Any disposal methods used with other pre-treatment systems such as LPP or drip can also be utilized with Puraflo.
The percolation area required are each site is a function of:
- The hydraulic conductivity of the soil;
- The long term acceptance rate or hydraulic loading on the disposal area;
- The average daily flows to the disposal area.
The site conditions under which we would recommend Puraflo Peat Biofilter installed are set out in Appendix A. Specific site criteria for use in North Carolina have yet to be finalized.
Because the PurafloR treatment technology is based on simple, passive biofiltration principles, there is no requirement for moving parts or expensive forced aeration mechanisms. The total running costs are thereby restricted to the power consumption of small submersible pumps which, for a standard single house installation, can be operated for less than a dollar per month.
The Puraflo system is a low-maintenance product and requires no desludging or backwashing. Provided that the primary septic tank is maintained by regular desludging. The system will simply require inspection of the sump/pump unit on an annual basis. An automatic alarm warning device if fitted to each system which will sound in the event of system malfunction.
The performance of the existing installed PurafloR systems has been extensively studied over the past eight year period. In the vast majority of installations the quality of final effluent produces has been very impressive and superior to other treatment options. Reduction of greater than 95% in the BOD and TSS content of the wastewater have been consistently recorded. Similarly, the NH3 -N levels in the primary wastewater has been reduced to <5mgl-1 (70-90% reduction).
The treatment efficiency in the unit has not been subject to significant seasonal variation with ambient air temperature fluctuations. This is again due to the combination of physical, chemical, and biological treatment processes within the system. The average quality of effluent from Puraflosystems (independently tested by reputable laboratories is presented in Table 2.3over).
The standard treatment system is currently being upgraded to provide an option for the removal of residual contaminants particularly nutrients (phosphate and nitrate). Initial field trials in the U.S. show some very positive results.
In addition to achieving considerable BOD, SS and NH3-N reductions in the wastewater, the PurafloR system is also very effective at eliminating enteric bacteria contained in the waste. The anti-microbial properties of the Puraflo system can be classified under two broad headings:
- The aggressive nature of the peaty media
- Microbial antagonism
The anti-microbial properties of the acidic peaty soils are well established in scientific literature. Bitton and Millen, (1984) and Gerba and Wolf, (1975) describe the accelerated decay of bacteria belonging the Enterobacteriacae group in peaty soils (pH 3-5) when compared to other soils. There are a number of reasons proposed for this. Firstly, the low pH directly affects the cells walls of the organisms in addition to limiting the amounts of nutrients available for uptake. Secondly, the trace amounts of phenols, bitumes and other complex hydrocarbons which are associated with peaty materials are directly toxic to certain bacteria in particular enteric organisms which find themselves in a hostile environment (low temperature high competition etc.) and are already in a stressed condition. Finally certain peaty soils have been demonstrated to contain a significant fungal species population (in addition to certain actinomycetes) which produce antibiotics and thus can adversely affect bacterial species in the zone of influence. It is important to note that the natural anti-microbial properties of the peat media are only effective on the “stressed” enteric organisms contained in the primary wastewater. The indigenous microflora associated with the treatment media are largely unaffected by the properties described.
The second and perhaps the most important means by which the enteric organisms are extinguished in the PurafloR system is by a process referred to as microbial antagonism. This simply means that the stressed micro-organisms within the primary wastewater are out competed by the indigenous microflora. The low temperature, low pH, presence of UV light and production of certain microbial toxins within the peat media adversely effects the “foreign” organisms and as such they are largely ineffective in assimilating nutrients etc. which are necessary for their survival. The large retention time in the filter bed (36-48 hours) ensures that die off is maximized.
The performance of the Puraflo biofiltration system with respect to its antimicrobial properties, is clearly demonstrated in the Table below.
|Average Performance of the Puraflo System Single Family Treatment Unit|
|Total Coliforms (cfu/100ml)||3,900,000||6,000||99.9+|
|Fecal Coliforms (cfu/100mls)||910,000||760||99.9+|
*Staphlococcus aureus, Clostridium perfingers Pseudomonas aeruginosa. Shigella spp. and Slmonella spp.
In summary, it should be noted that the treatment processes which occur in the filter media include physical filtration/adsorption, chemical adsorption/ion exchange and biological transformations. Therefore, the system can be effectively used in situations of intermittent or seasonal loadings where the microbial biofilm may not be as stable as in standard loaded units. Furthermore, it is emphasized that the peat filter media will not, under normal circumstance, drain to less than 50% moisture content. Therefore, even in situations of seasonal loadings the media and consequently the microbial biofilm will not dry out. A healthy microbial population is thereby maintained.
The results of intensive scientific research on the operation of the Puraflo technology has conclusively proven that it can produce a high quality treated effluent which can subsequently be disposed to the subsurface without any adverse affects on groundwater/subsurface water quality or associated human health risks. The use of a fixed film filtration system, and particularly a peat based filter, has inherent advantages over other technologies. This has been clearly demonstrated in the success of the system in a wide range of soil types, geological/hydrogeological settings, and possibly of most importance, in a diverse range of climatic conditions. This allows a very efficient and natural method of recycling wastewater to be employed, by recharging the acquifer through the subsurface disposal of highly treated PurafloR effluent.
<30>The widespread use of on-site disposal systems will continue as towns and cities expand into unsewered areas. It is considered that Puraflo is a natural and cost effective solution to this problem.
The Puraflo Peat Biofilter can be used independently as a high quality pre-treatment system on any site and in any soil conditions, either on its own or with other disposal technologies. If at all possible, Bord na Mona recommend sub- surface dispersal of treated effluent and recommend its use where the following site conditions can be met:
- Topography and Landscape position: Where uniform slopes are less than 50%
- Soil Characteristics: Texture:
- In the following for soil textures:
- Soil Group I – Sandy Texture Soils,
- Soil Group II – Coarse Loamy Texture Soils,
- Soil Group III – Fine Loamy Texture Soils,
- Soil Group IV – Clayey Texture Soils.
Structure:Where soil structure qualifies as suitable under current State regulations and/or as determined by a professional soil scientist.
Mineralogy:Where clay Mineralogy qualifies as suitable under current State regulations and/or as determined by a professional soil scientist.
- Soil Wetness Conditions: Minimum of 6 inches of suitable unsaturated material below trench bottom
- Soil Depth: where a minimum of 12 inches of soil depth exists, dependent on the topography of the site, permeability and soil conditions. Fill material may be used to meet soil depth requirements when permitted by the individual State.
- Restrictive Horizons: In soils with depths of twenty four inches or greater above restrictive horizons that are three inches or more in thickness at any section. Suitable sand fill material may be used to meet soil depth requirements when permitted by the individual State.
The minimum space required in permeable sandy soils for a 500 gpd system is approx. 320 sq. ft. to accommodate the modules and the footprint percolation area beneath the modules. For other less permeable soils a larger area may be required depending on the expected infiltration rate and hydraulic load. See following Table:
|INFILTRATION RATES This chart correlates (approx.) perc rates, soil texture and infiltration rates for the purpose of sizing the Puraflo disposal area|
|Perc Rate||Soil Group||Infiltration Rate|
|20 or less||Group I – Sand, Loamy Sand||1.6639|
|Group II – Sandy Loam, Loam||1.1093
|Group III – Sandy Clay Loam, Silt Loam clay Loam, Silty Clay Loam, Silt||0.5546
|Group IV – Sandy Clay, Silty Clay, Clay||0.3328
- Design flow (gpd) divided by infiltrative rate = total effective disposal area (TEDA)
- Size pad and/or trenches such that: Pad area plus (trench length multiplied by (trench width plus 1′)) = TEDA