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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

The Treatment Performance of Different Subsoils in Ireland Receiving On-Site Wastewater Effluent

Dep. of Civil, Structural and Environmental Engineering, Univ. of Dublin, Trinity College, Dublin 2, Ireland

^* Corresponding author (gilll{at}tcd.ie).

Received for publication February 5, 2007.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Current Irish guidelines require a comprehensive site assessment of a percolation area for wastewater disposal before planning permission is granted for dwellings in rural areas. For a site to be deemed suitable, the subsoil must have a percolation value equivalent to a field saturated hydraulic conductivity in the range 0.08 to 4.2 m d^–1 using a falling head percolation test. A minimum of 1.2 m of unsaturated subsoil must also exist below the invert of the percolation area receiving effluent from a septic tank (or 0.6 m for secondary treated effluent). During a 2-yr period, the three-dimensional performance of four percolation areas treating domestic wastewater was monitored. At each site samples were taken at 0, 10, and 20 m along each of the four percolation trenches at depths of 0.3, 0.6, and 1.0 m below each trench to ascertain the attenuation effects of the unsaturated subsoil. The two sites with septic tanks installed performed at least as well as the other two sites with secondary treatment systems installed and appeared to discharge a better quality effluent in terms of nutrient load. An average of 2.1 and 6.8 g total N d^–1 remained after passing through 1-m depth of subsoil beneath the trenches receiving septic tank effluent compared with 12.7 and 16.7 g total N d^–1 on the sites receiving secondary effluent. The research also indicates that the septic tank effluent was of an equivalent quality to the secondary treated effluent in terms of indicator bacteria (E. coli) after percolating through 0.6-m depth of unsaturated subsoil.

Abbreviations: BOD, biochemical oxygen demand • COD, chemical oxygen demand • cfu, colony forming units • EPA, Environmental Protection Agency • LTAR, long-term acceptance rate • MPN, most probable number • SE, secondary treated effluent • STE, septic tank effluent

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
The domestic wastewater of over one-third of the population in Ireland, or 400,000 dwellings, is treated by on-site systems (Department of the Environment, 2004) and with more than 25% of all water supplies provided by ground water (EPA, 2005), the protection of ground water resources from contamination by domestic wastewater effluent is imperative. Indeed, in many areas more than 30% of private domestic and farm wells are polluted by microbiological contamination (Daly, 2003). There is a popular misconception in Ireland that the effluent, after on-site "treatment" in septic tanks or packaged treatment plants, can be discharged into the soil without further processing. The reality is that the ground water is protected from pollution by the overlying subsoils of variable thicknesses and permeabilities. The percolation area (soil absorption system) is therefore an integral part of the overall on-site system, particularly since the main aquifers in Ireland occur in fissured or fractured bedrock formations.

A septic tank acts primarily as a settlement chamber providing quiescent, anaerobic conditions that facilitate the reduction of the organic and suspended solids content of wastewater, producing a consistent effluent that is suitable for subsequent treatment in the subsoil (Bouma, 1979; Payne and Butler, 1995). A well-constructed and maintained septic tank can remove 15 to 30% of the biochemical oxygen demand (BOD) and retain between 50 and 70% solids (Patterson et al., 1971; Goldstein and Wenk, 1972; EPA, 2000). However, the environment within the septic tank is largely ineffective in reducing the nutrient loading of the wastewater, acting only to convert the influent organic N to NH₄ and the organic P to inorganic P (Lawrence, 1973; Bauer et al., 1979; Zanini et al., 1998; Beal et al., 2005). Studies have shown that the removal of viruses, bacteria, and microorganisms within the tank is also small (Patterson et al., 1971; McCoy and Ziebell, 1975; Feachem et al., 1983; Canter and Knox, 1985; Van Cuyk et al., 2001).

A secondary treatment system can be installed as an alternative to a septic tank or to provide subsequent treatment of septic tank effluent (STE) before discharge to subsoil. Secondary treatment systems in the form of mechanical aeration systems, filter systems, or constructed wetlands provide a controlled aerobic environment for the accelerated microbial degradation of organic matter and often nitrification. This does not, however, result in a significant reduction in the total N loading across the process as the only reduction is normally due to the use of N in microbial growth. Most package wastewater treatment systems are not generally designed to remove P, although some P reduction (around 15%) is also achieved in bacterial assimilation, precipitation and adsorption (Metcalf and Eddy, 2003). An average 4 log bacteriological removal efficiency is reported for most treatment systems, although this still leaves substantial levels of bacteria in the secondary effluent (SE) considering typical total coliform influent concentrations into such units range from 10⁷ to10⁸ MPN (100 mL)^–1 (EPA, 2000). It is clear, therefore, that a typical STE is of poor quality, with a high pollution potential and, even with alternative or subsequent treatment by an engineered process, requires further treatment by natural processes in the subsoil before reaching the ground water.

The soil treatment system, comprised of a series of subsurface percolation trenches, is a crucial component of the conventional septic tank treatment system. The biogeochemical mechanisms for purification and hydraulic performance are complex and have been shown to be highly influenced by the biomat zone, which forms at the soil–gravel interface along the base and wetted sides of the percolation trenches (Beal et al., 2005). The chemical characteristics of the soil (cation exchange capacity and organic material content) have also been shown to be important (Dawes and Goonetilleke, 2003). Anaerobic activity has been attributed as the main clogging process in biomats (Siegrist and Boyle, 1987), which have generally been shown to have low hydraulic conductivities, in the region of 0.006 m d^–1 for clay subsoils up to 0.05 m d^–1 for sand (Bouma, 1975). This can lead to effluent backing-up above the biomat, leaving conditions below unsaturated for aerobic degradation processes to operate on percolating effluent. The development of a biomat takes several months but will eventually reach a steady state equilibrium (Hillel, 1980), which the long-term acceptance rate (LTAR)—the basis of several design codes in Europe and elsewhere (CEN, 2006)—attempts to define. There is a clear relationship between the organic loading rate and rate and extent of biomat development (Siegrist and Boyle, 1987) and so the provision of secondary treatment before the percolation trenches reduces the rate and extent of biomat growth (Siegrist and Boyle, 1987; Harrison et al., 2000; Potts et al., 2004). Research has shown that >90% removal efficiencies can be achieved under unsaturated subsoil conditions for BOD and SS by filtration, sorption, and biodegradation processes, although the removal of nutrients can be more limited (Viraraghavan and Warnock, 1976; USEPA, 1980; Jenssen and Siegrist, 1990; Van Cuyk et al., 2001; Rodgers et al., 2004; Beal et al., 2005). Nitrification of STE, however, is also commonly reported in the unsaturated zone beneath the biomat (Pell and Nyberg, 1989; Van Cuyk et al., 2001). Pathogens in particular can be dramatically reduced through 0.6 to 0.9 m of unsaturated subsoil (Kristiansen, 1981; Van Cuyk et al., 2004) yielding near complete removal of fecal coliform bacteria and >4 log reduction in viruses (Gerba et al., 1981; Emerick et al., 1997; Stevik et al., 1999; Van Cuyk et al., 2001). Most of the biological activity has been shown to occur within the initial zone of percolation: for example, a 4 log reduction in original coliform concentrations has been observed over a distance of less than the first 0.3-m depth of unsaturated subsoil (McCoy and Ziebell, 1975; Hagedorn et al., 1981).

Recommendations published by Ireland's Environmental Protection Agency (EPA, 2000) are aimed at defining subsoil conditions that will provide an acceptable level of treatment for on-site domestic wastewater effluent to protect ground water resources from contamination. The risk assessment–based approach is composed of an intensive site assessment procedure, involving a desk study and an on-site trial hole inspection and percolation test, which evaluates the suitability of the site and soil for treatment of on-site effluent against the vulnerability of local ground water resources. The percolation test is required to determine the assimilation capacity of the subsoil. It is recommended that the so-called T value (Mulqueen and Rodgers, 2001) (i.e., percolation rate) obtained from the standard falling head percolation test (CEN, 2006) must fall within the specified range of T = 1 to 50 (minutes per 25 mm water level fall) for subsoils receiving wastewater effluent. In addition, a minimum unsaturated subsoil depth of 1.2 m below the invert of the septic tank percolation trenches, or 0.6 m for trenches receiving SE, should exist before the site may be deemed suitable for on-site treatment of domestic wastewater effluent. Most on-site research to date has been performed in continental climates on sandy subsoils or under controlled laboratory conditions (Beal et al., 2005), with little field research previously performed in a temperate maritime climate or on the heterogeneous subsoils of northwestern Europe, which are largely a result of the recent glaciation. Hence, a 3-yr project funded by the EPA was undertaken to test out the efficacy of these recommendations and also examine the behavior of a subsoil with percolation characteristics outside the recommended range receiving domestic wastewater effluent. The study consisted of two sets of trials, each of 1 yr duration, designed to assess the following objectives:

1. The hydraulic and wastewater treatment performance of two subsoils of known permeability receiving STE
   
2. The hydraulic and wastewater treatment performance of two subsoils of known permeability receiving SE
   

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Site Selection and Construction
Four sites were required for the project (two sites receiving STE, two sites receiving SE) with different percolation T values across the range 5 to 60 (equivalent to field saturated hydraulic conductivities in the range 0.07–0.84 m d^–1) as determined by the onsite standardized Irish falling head percolation test, the t test (Mulqueen and Rodgers, 2001; CEN, 2006). Site location and trial hole inspections had to satisfy ground water protection schemes (Department of the Environment, 2004) and EPA guidelines (EPA, 2000) and there had to be at least four residents in each dwelling to ensure that the requisite hydraulic loading on four 20-m long percolation trenches was attained according to the design criteria in the guidelines.

A total of 74 sites were formally investigated in the Leinster region of Ireland, which involved digging a trial hole to determine at least 2-m depth of unsaturated subsoil to the bedrock/water table and carrying out the falling head percolation t tests. Only four sites (5.4%) were deemed suitable for the project with the most common reason for site rejection being the presence of a high water table, which is indicative of the pressure being exerted on water resources by on-site treatment of domestic wastewater in these regions. On such sites it is often possible to engineer a solution, through site improvement works such as raising the percolation area to make them suitable for on-site wastewater treatment, although the option of centralized sewer systems may also have to be considered for such areas in the future due to Ireland's burgeoning number of one-off housing developments.

Four suitable sites were identified as shown in Table 1 ; Sites 1 and 2 in County Kildare, Sites 3 and 4 in County Wicklow. Two-chamber septic tanks were installed on Sites 1 and 3 while naturally aerated peat secondary treatment systems (Puraflo, Bord na Mona) preceded by two-chamber septic tanks were installed at the other two sites. In the peat-based biofiltration system, septic tank effluent enters a sump from where it is pumped to a fibrous peat medium that is contained in molded polyethylene modules. No mechanical aeration of the treatment media is required as the peat is naturally aerated through a series of holes at the top of each module, thereby reducing the energy demand. The treatment of the septic tank effluent within the peat media is achieved by a combination of physical (filtration and adsorption), chemical (adsorption and ion exchange) and biological (microbial assimilation) processes resulting from the interactions between the effluent and the peat media and associated biofilm (Henry, 1996). The description of the subsoil characteristics are also given in Table 1. The indicative saturated permeability of the subsoil matrix (where moderate permeability is defined as 1 x 10^–4 to 1 x 10^–9 m s^–1 and low permeability as less than 1 x 10^–9 m s^–1) has been derived from grain size distributions (Swartz et al., 2003) and shows a clear difference compared with the measured field permeabilities. This indicates that the values obtained from the falling head percolation test must be highly influenced by lenses of higher permeability materials (sands, etc.) and macropore flows between cobbles and so forth. The "true" permeability across the whole percolation area probably lies somewhere in between these two values.

View larger version (12K): [in this window] [in a new window]   Fig. 1. Instrumentation layout: (a) cross-section of trench and (b) plan view of trenches.

On the morning preceding all sampling events, the lysimeters were put under a suction of 50.7 kPa using a vacuum-pressure hand pump. The specific suction pressure was adopted to prevent the extraction of bound moisture that would otherwise be unavailable to recharge, as recommended by Teagasc (the Irish agricultural research authority), and also advice from Soilmoisture Equipment Corporation that the practical limit applicable to the operational pathways for water flow in such heterogeneous soils, (i.e., the relevant field capacity) is about 65.9 kPa. Sampling was performed the following day using a vacuum-pressure pump and a 1000-mL conical flask and rubber bung with an extraction tube attached. The total volume of sample collected in each lysimeter was recorded and samples collected in 70-mL sterilized plastic sample tubes (Sarstedt Ltd.). The samples were then taken directly, on the same day, to the laboratory for analysis.

All STE, SE, and soil moisture samples were analyzed for ammonium (NH₄), nitrite (NO₂), nitrate (NO₃), chemical oxygen demand (COD), orthophosphate (ortho-P), and chloride (Cl) using a Merck Spectroquant Nova 60 spectrophotometer and associated reagent kits (USEPA approved). If parameter concentrations were above the detectable limit for a specified reagent the samples were diluted with a known volume of distilled water. During the sampling period, sets of samples from all sites were taken directly to an accredited laboratory for bacteriological analysis. All samples were analyzed for the indicator bacteria total coliforms and E. coli, with analysis also performed for enterococci, fecal streptococci, and fecal coliforms on some occasions. Samples were assayed for those bacteria associated with fecal contamination under the premise that their presence and attenuation kinetics in the subsoil were suggestive of the behavior of human pathogens.

	Results and Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Flow Measurement
The EPA guidelines (EPA, 2000) define the typical daily per capita hydraulic load to an on-site treatment system for single houses as 180 L d^–1. This figure is then used to determine the required size of a percolation area in conjunction with specified loading rates of 20 L m^–2 d^–1, for subsoils receiving STE, and 25 L m^–2 d^–1, for subsoils with T values between 21 and 50 receiving SE. The domestic wastewater generation, however, measured on all sites was considerably less than the EPA calculated figure with the per capita hydraulic load on each of the four sites as follows: Site 1 (104.7 L d^–1), Site 2 (60.2 L d^–1), Site 3 (82.3 L d^–1), and Site 4 (123.0 L d^–1), giving nominal hydraulic loading rates on the trenches of 11.6, 8.4, 9.1, and 13.7 L m^–2 d^–1 respectively, which can be compared with the recommended LTARs given in Table 1.

It was ensured that all roof water from rainfall on the research sites was diverted away from the wastewater system, as recommended in the guidelines. However, the higher design loading rate is deliberatively conservative to take into account a common scenario where builders often plumb the roof water into the wastewater system and the fact that even well-designed systems are often poorly installed, with little regulation during construction. This is obviously of concern given that the site investigation process showed that the majority of sites have high water tables and are often encountered in conjunction with fissured bedrock.

Method of Analysis and Effect of Dilution
As Cl does not take a significant part in any geochemical reaction (Marshall et al., 1999), it was used initially as a crude tracer to identify areas across the percolation areas that were receiving wastewater effluent. The results of the laboratory analysis for Cl at the three sample positions along each trench (i.e., 0, 10, and 20 m) were averaged at the same depth plane at which they were recorded. This enabled the identification of differences in loading rates between sampling distances on the same depth plane, thus highlighting any anomalies within each depth plane. From this, one of the two following methods, both of which assume isotropic and homogeneous soil properties, was chosen as the most representative conceptual model for the analysis of the attenuation of the percolating effluent:

1. Planar average. This method involved the averaging, across all four trenches, of the concentrations of each parameter across all sampling positions at the 0.3-, 0.6-, or 1.0-m depth planes. The difference between the average loading rates was then compared for the different depth planes.
   
2. Depth average. The average loading rate, across all four trenches, of each parameter within each plane was calculated at the three different sample distances along the trenches (0, 10, and 20 m) and the corresponding differences in loading rates between the planes compared at these three distances.
   

Loading rates at the different depths were calculated on a daily basis according to a mass balance of effluent flow plus any rainfall recharge. The rainfall available for dilution at the depth planes over the project duration, or effective rainfall, was calculated using rainfall figures and evapotranspiration figures obtained on site based on the Penman–Monteith method (FAO, 1998). The potential evapotranspiration calculation was used to calculate actual evapotranspiration. Where the soil moisture deficit was >40 mm the actual evapotranspiration was considered to occur at a slower rate than potential evapotranspiration and was calculated using the Aslyng scale (Keane, 2001). Daily effective rainfall, or recharge, was then calculated by subtracting the daily actual evapotranspiration and accumulated soil moisture deficit figures from the daily rainfall measurement. The mean annual rainfall at the sites was 988 mm yr^–1, of which 46.4% was effective rainfall on average across the year.

Analysis of the average Cl data for each depth plane at the three sampling locations along the trenches (Fig. 2 ) suggests a reasonably consistent loading rate across the percolation area. This was corroborated when the average chloride loading rates for all planes were compared (Fig. 3 ). Furthermore, it suggests that the effect of dilution by recharge events on contaminant concentrations between the three planes examined is minimal. As a result of the Cl analysis it was decided to assess the degree of contaminant attenuation on Site 1 using the planar average method discussed above.

View larger version (14K): [in this window] [in a new window]   Fig. 2. Site 1: chloride loading rates at the different sample position depths (0.3, 0.6, 1.0 m) along the percolation trench length (0, 10, 20 m).

View larger version (8K): [in this window] [in a new window]   Fig. 3. Site 1: average chloride loading rates across all depth planes across the percolation area.

View larger version (8K): [in this window] [in a new window]   Fig. 4. Site 2: average chloride concentrations at the 0.3-m depth plane at the three sample positions (0, 10, 20 m) along the trenches.

View larger version (8K): [in this window] [in a new window]   Fig. 5. Site 2: average chloride concentration measured on the three depth planes (0.3, 0.6, 1.0 m) at the 0-m sample position.

View larger version (23K): [in this window] [in a new window]   Fig. 6. Bromide tracer study results for showing time after injection of tracer that bromide levels over 0.2 mg L–1 were first picked up at the different sampling positions on (a) Site 1 and (b) Site 2. [D2 = 2 d after injection, n = not picked up by 8 d, – = no sample]

View larger version (17K): [in this window] [in a new window]   Fig. 7. Site 2: soil moisture tension plotted against effective rainfall for the 20-m sample position.

View larger version (18K): [in this window] [in a new window]   Fig. 8. Site 2: soil moisture tension plotted against effective rainfall for the 0-m sample position.

Chemical Analysis
Table 4 gives the quality of septic tank and secondary treated effluent discharged into the percolation trenches on each site over the research period whereas Table 5 lists the average loading rates measured of each parameter in the subsoil showing the attenuation with depth of the percolating effluent. The loads at each depth for all parameters were calculated as before taking into account the effect of dilution by effective rainfall over the zone of contribution. The concentrations for Site 1 were averaged across the whole percolation area for each depth plane. The concentrations for Sites 2, 3, and 4 are the averages of the four trenches at each site at each depth plane for the 0-m sample positions only.

The secondary treatment systems on Sites 2 and 4 clearly facilitated the nitrification of NH₄ in the STE, but while the SE underwent further slight nitrification within the subsoil, there did not appear to be a reduction in N load. The equivalent reduction in inorganic N load between the SE and point of discharge on Sites 2 and 4 was only 7 and 2%, respectively. Two sample t tests (Altman, 1991) show that statistically significantly higher inorganic N loads (p < 0.05) were recorded at the nominal point of discharge to ground water at these sites with an average of 16.7 and 12.7 g d^–1 total N loads passing the 1-m depth beneath the trenches receiving SE compared with and 6.8 and 2.1 g total N d^–1 on Sites 1 and 3 receiving STE. The reduction in inorganic N load between the STE and point of discharge on all sites shows that 70 and 75% reductions were achieved on Sites 1 and 3, respectively, whereas only 55 and 26% reduction were achieved on Sites 2 and 4. As NH₄ and NO₃ can be removed from the percolating effluent by immobilization and/or denitrification, the inhibition to biomat formation along the percolation trench base on the sites receiving SE would result in a reduction in microbial denitrification (promoted in the biomat by the reducing conditions), thereby reducing the mass of inorganic N released as gas. It is also possible that if localized saturated conditions existed within the subsoils on Sites 2 and 4, the organic load of the percolating effluent was insufficient to support the facultative heterotrophs required for denitrification.

It should also be noted that the peat modules had to be adapted for the project by collecting all the percolated effluent to one outlet pipe to enable the effluent to be evenly distributed between the percolation trenches. This resulted in slightly flooded conditions in the base of the plastic modules, conditions that promoted some denitrification. A 51% reduction in inorganic N loading was recorded through the peat filter on Site 2 while a 25% reduction was recorded on Site 4. Such anoxic conditions present in this shallow flooded zone, however, would not be typical for other types of secondary treatment systems and therefore it must be considered that the inorganic N load at the point of discharge to ground water would normally be noticeably higher.

As discussed above, the NH₄ concentration measured at the known preferential flow sample points on Site 1 was much higher than that measured at the corresponding sample points in the other trenches, which were monitoring matrix soil moisture flow. It appears, therefore, that unlike COD, the reduction in inorganic N load on Sites 1 and 3 was a result of chemical and biological processes within the subsoil matrix rather than processes within the distribution gravel. The decrease in NH₄ concentration of the percolating effluent with depth, cannot be completely accounted for by nitrification (i.e., a corresponding increase in NO₃ concentration), which suggests that the reduction in total N occurred in the biomat and underlying subsoil either by denitrification, sorption or possibly the anammox pathway (Jetten et al., 1999).

The installation of secondary treatment systems on Sites 2 and 4 had little effect on the ortho-P load of the STE. These package treatment systems are not usually designed to remove P, apart from the reduction as a result of biological assimilation—generally in the order of 10 to 15%. They can, however, mineralize organic P in the influent wastewater. The attenuation of ortho-P within the subsoil treatment system is a function of subsoil texture and mineralogy, controlled by soil adsorption and mineral precipitation. The ability of a soil to fix P is dependent, not only on its clay content, but also on the presence of Al, Fe, or Mn in acidic soils, either as dissolved ions, as oxides or as hydrous oxides, and the presence of Ca in alkaline soils (Robertson, 2003). As the types of reaction that fix P are closely related to soil pH, it must be considered that the reduction in effluent pH that occurs across secondary treatment systems as a result of nitrification could reduce the potential for ortho-P fixation in the subsoil. The subsoils on Sites 1 and 3 had a higher clay content, which may account for the greater percentage removal of ortho-P above the 0.3-m depth plane compared to Sites 2 and 4. It was also found that the 0.0 to 0.3-m depth plane on Site 2 removed more ortho-P than the 0.3- to 0.6-m depth plane which was attributed to the higher clay content in the upper layer. During the sample period it was only possible to obtain four samples from the 1.0-m depth plane at the 0-m sample position on Site 2. When ortho-P loading rates are compared for these few occasions, there was a noticeable increase in ortho-P fixation between the 0.6- and 1.0-m depth planes (Table 6 ). X-ray diffraction analysis of a soil sample taken from this layer showed that while it contained calcite, it was devoid of Al, Fe, and Mn oxides, hydrous oxides, or dissolved ions and therefore, in such a medium, fixation would be confined to the high pH range alkaline conditions that did exist on Site 2.

Table 7 presents the E. coli concentrations measured on each site during the research period. The results of the bacteriological analysis demonstrate the ability of the subsoil and associated biomat to remove enteric bacteria from the percolating effluent. The presence of E. coli in samples obtained from the 1.0-m depth plane on Site 1 was confined to two lysimeters. A reduction in bacteriological concentration with time was evident at one of these sample points, suggesting that the initial high concentration measured may have been due to the presence of macropores that facilitated the initial movement of bacteria to the lysimeter porous cups but became blocked with time. If this data point is excluded, it appeared that, except for the one incidence where 100 cfu L^–1 was detected, complete removal of enteric bacteria was achieved by the 1.0-m depth plane. While the installation of the secondary treatment system greatly reduced the bacterial load, it can be seen that there is some evidence of E. coli contamination at the 1.0-m plane with depth on Site 2, even though there was no evidence of E. coli contamination on the 0.3- and 0.6-m depth planes. A particle-size analysis of the subsoil revealed it to have a high sand content and therefore it is possible that the associated grain size, and consequently pore size, facilitated the movement of bacteria through the subsoil, particularly as the specific sample was taken at the time of year (October) where the tensiometer on the 0.6-m plane indicated saturated conditions. The reduced biomat development, as discussed earlier, would also have had the effect of increasing the hydraulic load per unit area. The results from Sites 3 and 4 at the 0-m sampling positions indicate no evidence of E. coli contamination past the 0.3-m depth plane.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
The results from the field studies on four sites have shown that the septic tank and percolation area provided a comparable treatment performance with respect to ground water protection to the packaged secondary treatment system with percolation area, without the need for ongoing maintenance or energy consumption. The evidence from this research indicates that the septic tank effluent has achieved an equivalent quality to the secondary treated effluent after percolating through 0.6-m depth of unsaturated subsoil. The extra 0.6-m of unsaturated subsoil required for septic tank effluent can thus be considered to act as a safety buffer, particularly in terms of microbiological pollutants on sites; for example, with high permeability subsoil or during periods of high hydraulic loading from the household and/or during periods of intense rainfall events.

Most of the treatment of the septic tank effluent took place in the distribution gravel and first 300 mm of subsoil where there was also a reduction in the total N load. Isolated incidences of low concentrations of E. coli were found in the subsoil on both septic tank sites.

The installation of a secondary treatment peat filter system greatly reduced the organic and bacterial load on the percolation area. Although the peat filters did have a limited effect on the N loads, this was felt to be mainly due to the particular modification required for the project and is not a normal process in most secondary package plants, which have little effect on nutrient loads. The secondary treated effluent itself did not receive substantial further treatment in the subsoil, (with the exception of P which was dependent on the mineral characteristics of the subsoil) and resulted in higher N loads moving down to the ground water when compared to the septic tank effluent percolation system. The total inorganic N load after 1.0-m depth of unsaturated subsoil, was significantly higher for the sites with secondary treatment systems installed [0.103 and 0.277 g total N (L d)^–1 when normalized per unit flow] than it was for the sites receiving just septic tank effluent [0.026 and 0.065 g total N (L d)^–1]. It is postulated that this reduction in the organic load in the secondary treatment systems reduces considerably the extent of the biomat and concentrates the effluent over a relatively small area. This affects the development of the clogged or saturated zones required for denitrification; it also deprives the bacteria of the carbon source essential for the process. However, the secondary treatment systems did substantially reduce the on-site wastewater bacterial loads to levels where only one incidence of enteric bacteria was found in the subsoils across both sites.

No discernable differences in treatment performance could be found between the sites according to their different percolation characteristics as measured by the on-site falling head percolation test. The actual distribution of effluent and percolation characteristics seemed to depend more on the development of the biomat, which is a function of the organic load in the effluent although the complexity of predicting effluent percolation rates through the typically heterogenic character of soils in Ireland has also been shown. The subsoil conditions beneath the biomats receiving higher hydraulic loadings but lower organic loadings in Sites 2 and 4 remained more or less unsaturated throughout the year, compared to the occasional saturated conditions found beneath the biomats receiving STE. This may be indicative that the macropores responsible for the relatively fast on-site percolation rates (obtained with clean water) had become blocked up under the higher organic loading, leaving the real permeability more reflective of the soil matrix material.

All sites monitored experienced less than the EPA design daily per capita hydraulic load of 180 L d^–1. As the percolation area design is presently based on hydraulic loading alone, it is possible that the footprint required for percolation areas could be reduced, although such a percolation area following a secondary treatment system does provide a safety buffer in the event of mechanical failure. Due to the confinement of effluent to <10 m of the percolation trenches on the sites receiving SE, the hydraulic loading rate experienced over the effective lengths of trench could have been greater than those specified by the EPA. A conclusion from this research, therefore, is that to achieve the desired hydraulic loading rate for sites receiving SE, a greater number of shorter percolation trenches would be recommended. In addition, secondary treatment systems should incorporate a denitrification process into the design, particularly for applications in nutrient sensitive areas. Finally, the optimization of the treatment potential of the percolation area also depends on attaining an even distribution and the importance of a properly designed distribution box cannot be overemphasized.

	ACKNOWLEDGMENTS

 
The results of this research form the basis of a report prepared as part of the Environmental Research Technological Development and Innovation Programme under the Productive Sector Operational Programme 2000–2006. The programme is financed by the Irish Government under the National Development Plan 2000–2006. It is administered on behalf of the Department of Environment, Heritage and Local Government by the Environmental Protection Agency, which has the statutory function of coordinating and promoting environmental research.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
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	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 

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