Effect of Free Cells and Additional Supporting Material on Performance of Polyethylene Glycol (PEG)-Pellet Reactor to Treat NH4-N Contaminated Groundwater
Wilawan Khanitchaidecha, Tatsuo Sumino, Futaba Kazama
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DOI: 10.4236/jwarp.2011.31002   PDF    HTML     3,754 Downloads   7,504 Views  

Abstract

To study the effect of free cells (suspended bacteria) on performance of entrapped bacteria system (i.e. polyethylene glycol (PEG)-pellet reactor) to treat NH4-N contaminated groundwater, two PEG-pellet reactors with a lot of free cells - Reactor A containing PEG-pellet and Reactor B containing PEG-pellet and supporting material - and the another control reactor without free cells (Reactor C) were set-up. Three reactors were operated under various NH4-N concentrations (40-60 mg/L) and various temperatures (5-25ºC). The results show that the free cells effected on the NH4-N removal efficiency significantly. The free cells developed to be a biofilm layer on the pellet surface for Reactor A, the biofilm layer caused the decreasing NH4-N diffusion and incomplete nitrification eventually. On the other hand, most free cells attached to the supporting material for Reactor B. Although the NH4-N could diffuse properly, the free cells consuming acetate caused the added acetate was insufficient for complete denitrification. However, the results suggest that the supporting material could reduce the effect of free cells on the reactor performance at low temperature as indicated by 1) higher efficiency and 2) lower activation energy (Ea) for nitrification and denitrification in Reactor B than Reactor A.

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Khanitchaidecha, W. , Sumino, T. and Kazama, F. (2011) Effect of Free Cells and Additional Supporting Material on Performance of Polyethylene Glycol (PEG)-Pellet Reactor to Treat NH4-N Contaminated Groundwater. Journal of Water Resource and Protection, 3, 12-21. doi: 10.4236/jwarp.2011.31002.

1. Introduction

Ammonium-nitrogen (NH4-N) is one of the most commonly found contaminations in groundwater in addition to Fluoride (F), Arsenic (As) and Iron (Fe) [1,2]. Although NH4-N does not directly affect heath, NH4-N is oxidised to nitrate-nitrogen (NO3-N) easily if exposed to oxygen (in air) for a long time. Furthermore, the consumption of high NO3-N can cause methemoglobinemia [3], and it has been known to be a risk factor for the development of gastric and intestinal cancer [4]. Therefore, NH4-N is a major concern in groundwater and the NH4-N contaminated groundwater needs to be purified.

There have been several methods published for removing NH4-N from the groundwater, such as swim-bed bioreactor [5], biofilter [6] and clinoptilolite zeolite [7]. Although the swim-bed bioreactor and biofilter could remove the NH4-N around 95-100%, the high NO3-N still remained in the treated water because both systems were designed for only nitrification (NH4-N converts to NO3-N under oxic condition). Therefore, another denitrification system (NO3-N converts to N2 under anoxic condition) is needed to complete NH4-N removal to N2 when using the swim-bed bioreactor and biofilter. In contrast, less NO3-N was found in the zeolite adsorption system [7], however the efficiency of 66.3-86.3% could not provide the safe drinking water (requiring < 1.5 mg/L of NH4-N in accordance with WHO standard [8]) when the groundwater contaminates high NH4-N (i.e. 62 mg/L as reported in Kathmandu’s groundwater [9]). Regarding the advantages of biological system involving low cost and simplicity, the biological NH4-N removal is really appropriated for groundwater purification in all areas including developing country that cannot afford high cost system.

One of biological NH4-N removal technologies is using an entrapped bacteria system. In the entrapped bacteria system, the bacteria (i.e. nitrifying bacteria, denitrifying bacteria) are entrapped within polymer (pellet or bead shape). The significant advantage of the entrapped bacteria system is that nitrification and denitrification can be achieved in a single reactor. As suggested by Pochana et al. [10], the oxygen gradient within the pellet/bead leads to both nitrification and denitrification occurred simultaneously on the pellet surface and core. Furthermore, other advantages are following: 1) dense bacteria in the system, 2) high removal capacity (gram NH4-N removed per day), 3) no sludge wash-out, 4) no settling system required and 5) less sensitivity to substrates shock-loading [11-14]. However, when the entrapped bacteria system is applied for treating high NH4-N water containing organic carbon, suspended bacteria (further referred as free cells) which possibly affect on the efficiency are found easily in the system [15].

Although much effort has been invested in the NH4-N removal using the entrapped bacteria system [16-18], it appears no explanations for nitrification and denitrification under free cells occurrence. Moreover, the efficiency of entrapped bacteria system under free cells occurring has not been studied. Therefore, the aim of this research was to examine the effects of free cells on efficiency, nitrification and denitrification in the entrapped bacteria system. Furthermore, the addition of supporting material in the system to reduce the free cells effect was also studied.

2. Materials and Methods

2.1. Polyethylene Glycol (PEG)-Pellet Preparation and Bacteria Cultivation

The 1.8 L of PEG-pellet from the previous research [19] was cultivated in three 3 L open tanks operating 6 hours for aeration and 6 hours for non-aeration. The definitions of aeration and non-aeration are clarified by following: 1) aeration - air was supplied continuously to maintain dissolved oxygen (DO) in 5-6 mg/L, and 2) non-aeration - no air supply resulted in the decreasing DO value to zero by 2 hours. Other factors (i.e. temperature, pH, carbon addition) were same as the operating condition reported by Khanitchaidecha et al. [19]. For all tanks, approximately 2.2 L water (effluent) was replaced with fresh synthetic groundwater (influent) at daily interval until the total nitrogen (sum of NH4-N, NO2-N and NO3-N) removal efficiency reached to 90%. The preparation of synthetic NH4-N groundwater is explained subsequently. Since no chemicals for NO2-N and NO3-N were included in the synthetic groundwater (as listed in Table 1), the influent NO2-N and NO3-N were negligible. Therefore, the total nitrogen (N) removal efficiency can be calculated as (1).

(1)

2.2. Supporting Material Characteristic

Figure 1(a) shows a photograph of the supporting material used in this research. The supporting material was made from textile with polyester warp thread, which has high tensile strength. The weft was made of special acryl yarn, which has the most hydrophilic character among synthetic fibres. The supporting material was kindly supported by the Networking of Engineering and Textile Processing (NET) Company, Japan.

2.3. Synthetic Groundwater Preparation

According to the groundwater quality analysis by ENPHO (Environment and Public Health Organization) of Nepal in 2007 and 2008, the concentration levels (mg/L) of various water quality parameters are summarised in

(a)(b)(c)

Figure 1. Representative the entrapped bacteria system: (a) supporting material, (b) schematic diagram of Reactor B containing PEG-pellet and supporting material and (c) operating condition for all reactors.

Table 1. Raw groundwater quality in Chyasal area, Nepal and synthetic groundwater quality.

Table 1. The groundwater samples were collected from a dug well in Chyasal area in Lalitpur, Nepal; which is the case study site for this research. The synthetic groundwater was prepared based on the quality parameters reported by ENPHO by mixing the following chemicals (g/L): 0.48 of NaHCO3, 0.05 of KCl, 0.11 of CaCl2·2H2O, 0.10 of MgSO4·7H2O, 0.02 of Na2HPO4·12H2O and various (NH4)2SO4. The concentration of K+, Ca2+ and Mg2+ in the synthetic groundwater was same as that in the raw groundwater; however the NH4-N concentration was set as 40 and 60 mg/L because some areas have higher NH4-N than the value reported by ENPHO [1,9]. Since the phosphorus (PO4-P) and inorganic carbon (HCO3-) concentrations in the raw groundwater are quite low for NH4-N removal by biological process, the 2 mg/L of PO4-P and excess inorganic carbon were prepared in the synthetic groundwater as suggested by Sumino et al. [20].

Conflicts of Interest

The authors declare no conflicts of interest.

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