Nutrient loading on subsoils from on-site wastewater effluent, comparing septic tank and secondary treatment systems

Abstract

The performance of six separate percolation areas was intensively monitored to ascertain the attenuation effects of unsaturated subsoils with respect to on-site wastewater effluent: three sites receiving septic tank effluent, the other three sites receiving secondary treated effluent. The development of a biomat across the percolation areas receiving secondary treated effluent was restricted on these sites compared to those sites receiving septic tank effluent and this created significant differences in terms of the potential nitrogen loading to groundwater. The average nitrogen loading per capita at 1.0 m depth of unsaturated subsoil equated to 3.9 g total-N/d for the sites receiving secondary treated effluent, compared to 2.1 g total-N/d for the sites receiving septic tank effluent. Relatively high nitrogen loading was, however, found on the septic tank sites discharging effluent into highly permeable subsoil that counteracted any significant denitrification. Phosphorus removal was generally very good on all of the sites although a clear relationship to the soil mineralogy was determined.

Introduction

Groundwater is an important resource in Ireland which is at risk from the increasing numbers of decentralised houses and their respective on-site wastewater treatment systems. Domestic wastewater from over one third of the population is treated by on-site systems and, with more than 25% of all water supplies provided by groundwater (EPA, 2005), the protection of groundwater resources from contamination by domestic wastewater effluent is imperative. The unsaturated subsoil above the water table or bedrock into which on-site effluent is discharged (i.e. the percolation area) is therefore an integral part of the overall on-site treatment system, particularly since the main aquifers in Ireland occur in fissured or fractured bedrock formations overlain by subsoils of variable thickness and permeability (Misstear and Daly, 2000).

Groundwater needs to be protected from nitrogen pollution for both ecological reasons (nutrient enrichment of sensitive surface waters) and health based reasons, the latter mainly due to a perceived link between nitrate concentrations and methaemoglobinemia (“blue-baby syndrome”), increased incidence of cancer, adverse reproductive effects, and other possible effects. Indeed the limit for maximum nitrogen concentrations in groundwater is often founded on health based grounds at 50 mg-NO₃/L as defined by the EU Drinking Water directive (98/83/EC) and WHO guideline value, (2006). There is increasing scepticism, however, as to the simple link that has been made in the past between drinking nitrate in water and methemoglobinemia due to the generally poor correlation between drinking water nitrate and blue-baby syndrome (L'hirondel and L'hirondel, 2002, Fewtrell, 2004) and that numerous other factors such as hereditary enzyme deficiencies, protein intolerance and a variety of trace chemicals are now known to cause the disorder (Avery, 1999, Knobeloch et al., 2000, Ward et al., 2005). Elevated nitrate concentrations in groundwater however, are an indicator of wastewater or agricultural pollution and also indicate potential microbial risks if the groundwater is used as a drinking water supply.

In temperate fresh waters, dissolved phosphorus tends to be the limiting nutrient (Schindler, 1977) and excess loading onto lakes and rivers can initiate algal blooms and generate negative aesthetic and eutrophic conditions in such water bodies. In Ireland groundwater forms the baseflow of rivers, with annual average contributions ranging from less than 30% of total river flow where the catchment is underlain by poorly productive aquifers to over 80% where there are regionally important aquifers in good hydraulic connection to the river (Fitzsimons and Misstear, 2006). Therefore, groundwater must be protected from phosphorus pollution.

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 (Goldstein and Wenk, 1972, Viraraghavan, 1976, Canter and Knox, 1985). The environment within the septic tank is, however, largely ineffective in reducing the nutrient loading of the wastewater, acting only to convert the influent organic-N to ammonium achieving little total-N removal across the process (Lawrence, 1973, Canter and Knox, 1985, Beal et al., 2005, McCray et al., 2005). Equally, the anaerobic conditions in the tank convert most of the influent phosphorus, in the form of both organic and condensed phosphate (polyphosphate), to soluble ortho-P which passes out in the effluent (Bouma, 1979; Wilhelm et al., 1994; Zanini et al., 1998, Beal et al., 2005).

A secondary treatment system can be installed as an alternative to a septic tank or to provide subsequent treatment of septic tank effluent prior to discharge to subsoil. The controlled aerobic environment for the accelerated microbial degradation of organic matter and often, nitrification of ammonia does not, however, result in a significant reduction in the total-N loading across the process. Equally, most package wastewater treatment systems are not generally designed to remove phosphorus, although some phosphorus reduction (around 15%) is usually achieved in bacterial assimilation, precipitation and adsorption (Metcalf and Eddy Inc, 2003).

The soil treatment system, comprised of a series of subsurface percolation trenches, is a crucial component of the gravity flow treatment system with much research concentrating on the flow of effluent and the mechanisms of pollutant attenuation within the subsoil (Jenssen and Siegrist, 1990, Beal et al., 2005). 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. Reduced percolation rates through the biomat due to clogging as a result of anaerobic activity can cause the effluent to pond above the biomat but leaves unsaturated conditions below, for aerobic degradation processes to operate on percolating effluent (Bouma, 1975, Siegrist and Boyle, 1987, Beal et al., 2008). 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 (CEN, 2006) and elsewhere – attempts to define. There is a clear relationship between the organic loading rate and rate and extent of biomat development and so the provision of secondary treatment before the percolation trenches reduces the rate and extent of biomat growth (Siegrist and Boyle, 1987, Jantrania and Gross, 2006). The removal efficiencies of nutrients in unsaturated subsoil can be limited (Jenssen and Siegrist, 1990, Beal et al., 2005) although nitrification of septic tank effluent however, is also commonly reported in the unsaturated zone beneath the biomat (Pell and Nyberg, 1989b, Van Cuyk et al., 2001). Other studies have shown that denitrification of percolating nitrates in soils tends to occur more readily in fine-textured soils (i.e. clays and silt/clays) compared to coarse-textured soils (silts and sands) (Tucholke et al., 2007).

The attenuation of ortho-P within the subsoil treatment system is controlled by soil adsorption and mineral precipitation and is dependent not only on its clay content, but also on the presence of Al, Fe, or Mn in acidic soils and the presence of Ca in alkaline soils (Robertson, 2003). Phosphorus precipitation is dominated by iron and aluminium when pH <6 (Erickson et al., 2007) whilst at pH levels >6 the reactions are a combination of physical adsorption to Fe and Al oxides and precipitation as sparingly soluble calcium phosphates onto calcareous sands (Arias et al., 2001). Other studies have shown that conditions of low redox potential (i.e. saturated, partially anaerobic) reduce the mineralisation rate and increase the adsorption rate (Pell and Nyberg, 1989a).

Two sequential projects funded by the Irish EPA have been undertaken to test out the efficacy of on-site wastewater treatment systems on a range of sites with subsoils of different percolation characteristics receiving domestic wastewater effluent. The research objectives addressed by this paper were to investigate the nutrient loadings occurring at different depths beneath the different percolation trenches and assess the implications in terms of groundwater pollution with respect to housing development in unsewered, rural areas.