Fate and transport of organics, and trace organics under SAT

Groundwater recharge with reclaimed wastewater and other sources of water is now being widely practiced in various parts of the world, especially in the arid and semi arid regions. Surface spreading basin (i.e. Soil Aquifer Treatment) is now a common practice for groundwater artificial recharge (Drewes, 2009). Soil Aquifer Treatment (SAT) is an economical and aesthetic wastewater reuse system. Since the soil and the aquifer can act as a natural filter, SAT system can remove suspended solids, biodegradable materials, bacteria, viruses, and other microorganisms (Bouwer, 1997). During SAT, secondary or tertiary treated wastewater infiltrates into the ground from an infiltration basin, percolates through the unsaturated zone, and finally mixes with native groundwater. Soil percolation includes several processes that occur during downward transport in the vadose zone. At the basin – soil interface, the combined effect of sedimentation, filtration, aeration and microbial growth leads to the formation of a biologically active zone that may be impermeable (Bouwer and Rice, 1984). The main problems concerning groundwater quality using reclaimed wastewater are pathogens, total minerals, heavy metals, and stable organic substances (Drewes and Jekel, 1998). Research has shown that a variety of organic compounds including veterinary and human antibiotics, other prescription and non-prescription drugs, widely used household and industrial chemicals including personal care products and products of oil use and combustion, steroids and reproductive hormones (Ternes. 1998, Kolpin et al., 2002), as well as bacterial, viral, and protozoan pathogens (Toze, 1999), can survive conventional wastewater treatment and persist in the aquatic environment. Very little information is available concerning the removal efficiency and removal mechanisms or processes of nitrogen, phosphorus, pharmaceuticals, heavy metals, trace metal, and BOD removal in soil aquifer treatment yet many experimental studies have been performed (laboratory and field) to attempt to ascertain the processes involved in removing organic matter, heavy metals, and other emerging pollutants (Drewes, 2009). The following sections briefly summarize the state of knowledge on the fate of organics and trace organics (e.g. pharmaceuticals) under soil aquifer treatment.

Fate of Organic Matter (OM) under SAT

SAT can effectively treat suspended and dissolved effluent organic matter (EfOM) present in source water. The removal efficiency of organic matter from the effluent was found to be very high under different field conditions with different initial organic contents. At higher concentrations of

TOC/DOC (>10 mg/L) in the infiltrated water, ca. 70-90% removal efficiency was achieved in field investigations (for details Table 2.3). After infiltration, organic matter of wastewater origin gradually transforms into organic matter that more closely resembles background Natural Organic Matter (NOM) (Drewes et al., 2006). Protein-like and soluble microbial products (SMP) may persist in the aquifer even after long travel times (Drewes et al., 2006). The depth of the unsaturated zone has no impact on organic matter removal in a short time of SAT operation (Cha et al., 2005).

About 50-60% of DOC removal during SAT of secondary and tertiary effluents takes place in the top 1.5 m of soil which is most predominantly of an oxic condition ( Sharma et al., 2008). Most of the removal of organic matter occurs within the top few centimeters or in the infiltration zone (Drewes, 2009). This is probably due to the abundance and distribution of microbes and their biological activity in the subsurface (Rauch and Drewes, 2005, Nema et al., 2004). A decrease in assimiable organic carbon (AOC) (Kortelainen and Kharu, 2006) and rapid removal of easily biodegradable organic matter during early stages of SAT below the infiltration pond (Fox et al., 2001) suggest a contribution of high microbial activity. Reports from the literature suggest that the microbial biomass (e.g., Holden and Fierer, 2005) is generally highest in surface soils and decreases rapidly with depth (most probably follows power functions) and overall composition of microbial communities changes significantly with soil depth. Mechanical filtration capacity of the top soil layer contributes to the removal of suspended organic matter, which results in a surface clogging layer over time.

Many field and controlled laboratory soil column studies suggest that biodegradation is predominantly the main process of organic matter removal (e.g., Fox et al., 2005; Quanrud et al., 2003b; Drewes and Jekel, 1998). Recent experimentation with 3-D fluorescence excitation/emission has also indicated the highly biologically active processes that occur during SAT (Drewes, 2009). The redox condition in a saturated aquifer may provide a suitable environment for degradation (Vanderzalm et al., 2006). The authors also reported that in addition to aerobic respiration, denitrification is responsible for minerilization of DOC in carbonate aquifers. DOC may be adsorbed on the soil particles (Kortelainen and Kharu. 2006). Humic substances (HS) are relatively resistant to degradation. The important mechanism of removal for HS is attachment to aquifer material followed by microbial degradation (Juhna et al., 2003). So, adsorption is likely less prominent in vadose zone organic carbon removal.

Adsorption processes are not sustainable for SAT operation as breakthrough of accumulated organics might occur over the long term. Field and column studies concluded that adsorbed organics are subsequently transformed or degraded and do not accumulate in the soil (Fox et al., 2005). Removal of OC might be a combination of degradation and adsorption processes accompanied by filtration through the upper soil (Idelovitch et al., 2003, Quanrud et al., 2003b). Better understanding is required to differentiate chemo-sorption from physical sorption under SAT. Recently Drewes, 2009 summarised that three mechanisms are responsible for the removal of EfOM in the sub surface. These

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are: 1) biotransformation and mineralization 2) physical adsorption 3) dilution with local groundwater. Very few studies tried to prove this statement by numerical modelling using either laboratory or field observed data. A first-order macroscopic kinetic model can describe the biodegradation of DOC during SAT.

The infiltration rate, the level of pretreatment, and the soil type have no influence on the biological removal efficiency during SAT. The effluent water quality from SAT largely depends on the travel distance and residence/travel time. Longer travel time will allow breakdown of slowly biodegradable organics (Idelovitch and Michail, 1984). The guidelines proposed by Sharma et al., 2008 state that more than 90% removal of DOC can be achievable when the travel time is more than 50 days (Influent concentration range 2-24 mg/L). Nevertheless, for additional purification of influent treated wastewater, it would be helpful to let the water flow horizontally in the aquifer (saturated zone).

The impact of soil types on SAT operation has been examined by few studies. The removal of organics does not depend predominantly on soil type, although fine-grained soil has little advantage over other soil types (Quanrud et al., 1996). Sharma et al., 2008 suggested that soil type might have an impact on DOC removal and mentioned that sandy loam has better DOC removal efficiency than others.

The persistence or biodegradation of Total Organic Halogens (TOX) or Adsorbable Organic Halogen (AOX) have been poorly investigated. Cometabolism might be the main process of AOX removal (Drewes et al., 1998). On the other hand, sorption to clay mineral and organic matter is concluded to be the main predominant mechanism for AOX removal (Lin et al., 2008; Quanrud et al., 1996).

Table 2.2: Reductions in total organic carbon during SAT of wastewater effluent (extended from Quanrud et al., 2003b) (field and laboratory tests).

a dissolved organic carbon; ^b annual averages for two recharge seasons: ^C organic wastewater compounds

Fate of trace organics (EDC and Pharmaceuticals)

Endocryne Disrupting Compounds (EDCs) can be natural hormones or pharmaceuticals, estrogen replacement products, or steriods (Master et al., 2004). The presence of Endocrine Disrupting Compounds (EDCs) is of special concern because these compounds are associated with potential adverse health effects and toxicological effects on aquatic species (Snyder et al., 2004). Consequently, their presence in wastewater leads to the necessity of better understanding their fate and transport during aquifer storage and recovery operations. Mostly secondary treated (biological treatment followed by disinfection) wastewater is used for groundwater replenishment via the surface spreading system . Despite the treatment process, some pharmaceuticals and EDCs persist in treated effluent at very low concentrations (Benotti and Snyder, 2009). Only limited studies have been performed up to now to provide information on the mechanisms for the attenuation of EDCs during SAT. The fate and transport of pharmaceuticals and EDCs in the subsurface are controlled by many factors such as hydrogeological conditions, concentration, pH of recharge water, processes such as advection and dispersion, sorption and desorption, diffusion, microbiological and chemical transformation, pond operation (wetting and drying cycling scheme), etc. Drewes et al. (2002), showed that SAT could efficiently remove anti-inflammatory and lipid-regulating drugs. A period of less than six months of groundwater transport can efficiently remove some pharmaceuticals and Personal Care Products (PCPs), such as Diclofenac, Ibuprofen, Ketoprofen, Naproxen, Fenoprofen and Gemfibrozil, from secondary effluent under SAT. Antiepileptics such as Carbamazepine and Primidone persist in groundwater even after a long period of recharge (Drewes et al., 2003). A 23 day study within a 2.4 m long soil column showed about 70% removal of some organic compounds, but the study demonstrated that under recharge conditions similar to those in arid and semi arid climates, some pharmaceuticals (especially eight compounds: Carbamazepine, Sulfamethaxazole, Benzophenon, 5-methyl-1H-benzotiazole, N,N-diethyl-tolaumide, Tributylphosphate, (Tri- 2-choloroethyl) phosphate, and Cholesterol), pathogens, and other organic wastewater compounds (OWCs) can persist in treated effluent after soil aquifer treatment (Cordy et al., 2004). SAT can remove steriodal hormones. Estriol and testosterene were completely removed (< 0.6 ng/L) only after travel through 1.5m of porous media, where 17 - estradoil was attenuated by 90% at this same condition and was completely removed before reaching the water table. The mobility of these compunds is low in subsurface system. A field experiment was supported by a soil column study under saturated and anoxic flow condition. A 4 m long soil column reported complete removal of these three hormones. The primimary mechanisms for controlling the fate and transport of EDCs and pharmaceuticals are adsorption and biodegradation. ( Snyder et al., 2004, Mansell and Drewes, 2004a and Mansell et al., 2004b). Mansell et al., (2004b) reported 79%, 84%, and 98% removal of 17 - estradoil, estriol, and testosterene in a short passage of a soil column (0.30 m). 17 - estradoil is less mobile than others.

Column experiments and chemical nonequilibrium miscible-displacement models also imply rapid

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degradation/transformation of 17 - estradoil in the sorbed phase (Casey et al., 2003). In addtion to sorption processes, transformation might effect the transport of Sulfadiazine (SDZ) in soil. The transport of SDZ mainly depends on the input concentration and pulse duration. For better understanding of transport processes, using laboratory data and mathematical modeling, besides site-kinetic sorption with irreversility, possible transformation reaction, appropriate isotherms and rate laws are important information to have (Wehrhan et al., 2007). Because of the wide range of physiochemical properties (e.g. Log Kow) and microbial transformation mechanisms, it is not easy to understand comprehensively the behaviour of these contaminants in the subsurface environment (Benotti and Snyder, 2009).