Removal of dissolved organic carbon and nitrogen during simulated soil aquifer treatment
Introduction
Unreliable rainfall patterns in recent years and declining groundwater levels are rendering opportunities for wastewater reuse an important issue for consideration in urban water management (Tchobanoglous et al., 2003). This makes Soil Aquifer Treatment (SAT) a very useful wastewater treatment option. SAT is an inexpensive, low-technology (Bouwer, 2000) wastewater treatment and reclamation option, able to generate high quality effluent from secondary treated wastewater for potable and non-potable uses (Cha et al., 2006; Fox et al., 2006). It is usually applied for final polishing of secondary treated effluents with the intention of replenishing groundwater reserves (Droste, 1997). During SAT, the treated effluent is intermittently spread in infiltration basins to allow percolation through the soil down to the groundwater aquifer. The saturated and unsaturated zones of the subsurface soils act as the medium in which physicochemical and biological reactions occur (Cha et al., 2006) to significantly reduce effluent parameters such as biochemical oxygen demand (BOD), total suspended solids (TSS) and pathogens to levels that even allow the use of the water for unrestricted irrigation (Bdour et al., 2009). SAT is also effective in the removal of nitrogen, phosphorus and trace metals (Kopchynski et al., 1996). Mixing of the infiltrated wastewater with the groundwater and the slow movement through the aquifer increases the contact time with the aquifer material leading to further purification of the water (Asano and Cotruvo, 2004; Dillon et al., 2006). Partially treated wastewater can be therefore highly upgraded by percolation through the soil to the groundwater under favourable soil and groundwater conditions (Pescod, 1992; Tchobanoglous et al., 2003; Asano and Cotruvo, 2004).
Biodegradation is an important process in SAT which brings about the breakdown of organic chemicals in soils by the action of microorganisms naturally present in the soil or introduced through engineered systems (Charbeneau, 2000). Bacteria are the most predominant microbial species involved in the stabilisation of organic matter (Gray, 2004) and also the most important microorganisms in the groundwater environment for the catalysis of oxidation reduction processes (Freeze and Cherry, 1979). There are about 107 to 108 bacteria in every 1 g of soil with the largest numbers being found in the surface layer (Ishizawa and Toyoda, 1964). Although biodegradation and adsorption are the most important removal mechanisms occurring during SAT, only a few studies have looked at the relative contribution of each mechanism to the removal process.
A number of methods have been developed to determine the microbial community structure and biomass in soils. Deoxyribonucleic acid (DNA) based fingerprint methods have been developed and are commonly used for describing the community structure. These methods are based on variations in ribosomal ribonucleic acid (rRNA) of microbial communities (Leckie, 2005). Microscopy can also be used for the enumeration of microorganisms in soils. It is however not suitable for use on coarse sands. Phospholipids fatty acid (PLFA) analysis is another widely used method for characterising microbial communities in soil and offers the advantage of deriving information on both community structure and total microbial biomass from the same sample (Findlay and Dobbs, 1993). It is based on the differences in the fatty acids contained in the cell membranes of diverse organisms (Leckie, 2005) and as no culturing of samples is required, the method is not selective but covers the entire range of the microbial community (Findlay and Dobbs, 1993). The cell membranes of viable organisms contain fatty acids as a component of its phospholipids, which are essential components of its cells. Fatty acids may be straight chained, branched or cyclic and could also be saturated or unsaturated (O'Leary, 1962). Unique to bacteria are the cyclopropane, β-OH and branched chain fatty acids (Lechevalier, 1989). Specific types of fatty acids are usually predominant in a given group of organisms and are used as biomarkers for that group of microorganisms (Zelles, 1999; Leckie, 2005). Fatty acids are named according to the total number of carbon atoms it contains, followed by the degree of unsaturation or the number of double bonds, and the position of the double bond from the aliphatic end, ω, of the molecule. cis or trans double bonds are denoted by the suffix c or t respectively and indicate the position of the adjacent hydrogen atoms to the double bond (Findlay and Dobbs, 1993; Zelles, 1999). In cis isomers, the two adjacent hydrogen atoms are bound on the same side of the double bond in the carbon chain whilst in trans isomers, they are bound on opposite sides of the double bond (Valenzuela and Morgado, 1999). Prefixes a, i, cy, br and d are used to signify anteiso, iso and cyclopropyl branched, branching type unknown and dicarboxylic acids respectively (Findlay and Dobbs, 1993; Zelles, 1999). The position of the methyl group in the molecule is indicated by a number followed by ME (methyl group). In an OH fatty acid, the prefixes α and β denote that OH groups of the molecule are located at positions 2 and 3 respectively (Zelles, 1999).
Most SAT systems employ well treated secondary effluents from conventional wastewater treatment or tertiary effluents. Therefore little work has been done to demonstrate the applicability of SAT in treating poorly treated effluents. In addition, SAT has always involved wastewater infiltrating through an unsaturated soil zone before reaching the groundwater, and a minimum depth of 3 m to groundwater is usually recommended (Tchobanoglous et al., 1999) to safeguard the quality of the underlying aquifer. The unsaturated zone promotes biological activities for the removal of wastewater contaminants such as organic matter and also promotes nitrification due to its conduciveness for re-aeration mechanisms to occur, which ensure the continuous availability of dissolved oxygen in this zone. Therefore little work has been done to demonstrate the applicability of SAT in treating poorly treated effluents and its use in shallow aquifers has not been well explored. However, during SAT groundwater levels are expected to fluctuate and in very shallow aquifers coupled with high organic matter content, treatment efficiency may be highly compromised. This is because in contrast to the unsaturated zone, groundwater environments are inclined to being devoid of dissolved oxygen due to non-replenishment of consumed oxygen during hydro chemical and biochemical reactions, as a result of no contact existing between the circulating groundwater and the atmosphere (Freeze and Cherry, 1979). Due to low oxygen water solubility, which ranges from 9 mg L−1 at 25 °C to 11 mg L−1 at 5 °C, even low concentrations of organic matter in groundwater can cause depletion of all the dissolved oxygen (Freeze and Cherry, 1979), which could impact negatively on the treatment process.
Previous laboratory studies by Cha et al. (2005) revealed that the depth of the unsaturated zone (between 0 and 1 m) has little or no effect on the removal of dissolved organic carbon (DOC). DOC concentration of the applied wastewater was however low (4.5 mg L−1). For effluent DOC ranging from 4 to 20 mg L−1 applied under field conditions, Fox et al. (2006) also found no correlation between the depth of the unsaturated zone and the treatment efficiency attained. In their study, a minimum of 3 m unsaturated zone was available at the field sites. This study therefore i) explores the impact of the length of travel through unsaturated zone on the removal of DOC and nitrogen, using wastewater simulating poor quality effluent, ii) investigates the biological community involved in the removal of DOC under saturated and unsaturated conditions and iii) examines the effect of soils type on the removal efficiencies.
Section snippets
Materials and methods
The experimental setup consisted of one unsaturated soil column, SC1, and two saturated soil columns, SC2 and SC3, all of inner diameter 150 mm and length 1 m. The setup is shown in Fig. 1. The soil columns were set up in a controlled temperature room set to 20 ± 0.5 °C. Influent wastewater kept at 4 °C in the fridge was continuously aerated and warmed up to the column temperature before application to the soil columns by placing a portion of the influent tubing in a warm water bath. SC1 and
Wastewater treatment under unsaturated conditions
Fig. 2 a shows the distribution of water in the unsaturated soil column for each level of water table (WT75, WT500 and WT800) over periods of 5weeks for WT75 and 2weeks each for WT500 and WT800. The removal of DOC and sulphate in the soil column under these experimental conditions are shown in Fig. 2b and c. The removal of nitrogen is also shown in Fig. 3.
Measurements of unsaturated zone thickness in the soil column were found to be 850 mm, 425 mm and 125 mm corresponding to WT75, WT500 and
Conclusions
As evidenced by Phospholipid Fatty Acids (PLFAs) analysis, this study revealed that for both saturated and unsaturated conditions, a large concentration of microorganisms exist near the infiltration surface resulting in a greater proportion of the removal to occur within the first few cm of the soil. PLFAs analysis of the soil has also revealed the presence of aerobic and anaerobic bacteria in addition to fungi. This suggested that aerobic and anaerobic biodegradation mechanisms supported by
Acknowledgements
This study was carried out with support from the Netherlands Organisation for International Cooperation in Higher Education (Nuffic).
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