The performance of UASB reactors treating high-strength wastewaters.
Anaerobic wastewater treatment, a method that is widely used all over the world, is the biological treatment of wastewater without the use of air or elemental oxygen. During anaerobic treatment, the organic pollutants are converted to a gas containing methane (C[H.sub.4]), carbon dioxide (C[O.sub.2]), ammonia (N[H.sub.3]), and hydrogen sulfide ([H.sub.2]S) by anaerobic microorganisms. Anaerobic wastewater treatment has several advantages as compared with aerobic wastewater treatment. It uses less energy since aeration is not needed, the organic matter in the influent is converted into a valuable gas, and less surplus sludge is formed because the growth rate of anaerobic microorganisms is lower.
The increased utilization of anaerobic wastewater treatment systems has been associated with the development of high-rate reactors such as the anaerobic filter (AF), the downflow stationary fixed-film (DSFF) reactor, the upflow anaerobic sludge blanket (UASB) reactor, the fluidized-bed (FB) reactor, and the expanded-bed (EB) reactor, which are widely used in municipal and industrial wastewater treatment (Shivayogimath & Ramanujam, 1999; Tay & Zhang, 2000). The high-rate anaerobic reactors are able to separate hydraulic retention time (HRT) from solids retention time (SRT); therefore, higher volumetric loading rates can be applied, and enhanced removal efficiencies can be achieved. The most popular high-rate anaerobic reactor configuration in the world today is the upflow anaerobic sludge blanket (UASB) process that was developed in the 1970s by Lettinga and co-workers in the Netherlands (Lettinga, van Velsen, Hobma, de Zeeuw, & Klapwijk, 1980). It has been employed for the treatment of wastewaters from various industrial applications such as sugar, food, distillery, beverage, slaughterhouse, dairy, chemical, pulp and paper, petrochemical, and pharmaceuticals, as well as for treatment of domestic sewage (Lettinga & Hulshoff Pol, 1991; Rajeshwari, Balakrishnan, Kansal, Lata, & Kishore, 2000). Compared with other anaerobic processes, a UASB reactor is relatively simple, economical, and easy to operate. It does not require packing material to support biomass, clarifier, and mechanical mixing. Moreover, effluent recirculation is not necessary as in fluidized-bed reactors. A detailed scheme of a UASB reactor is shown in Figure 1.
A typical UASB reactor consists of three parts: 1) sludge bed, 2) sludge blanket, and 3) a three-phase separator known as a gas-liquid-solid (GLS) separator. The GLS separator is installed at the top of the reactor. In the UASB process, the influent is pumped into the bottom and passes upward through the granular sludge bed. The organic compounds in the influent are biologically converted to biogas. Gas and solids are then separated from the wastewater by the GLS separator. The mixing within the UASB reactor results from the influent flow and biogas production.
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The successful operation of a UASB reactor relies on the formation and stability of particles with high settleability and bioactivity (Akunna & Clark, 2000; Schmidt & Ahring, 1996). These particles are known as granules, and their formation is termed granulation (Yu, Fang, & Tay, 2001). Granulation is the result of microbial self-immobilization and, subsequently, aggregate formation and growth (Yan & Tay, 1997), and it involves different trophic bacterial groups and physico-chemical and microbiological interactions (Yu et al.). The granulation process is believed to be the most critical factor affecting the treatment efficiency of the UASB process (Show, Wang, Foong & Tay, 2004), and it is affected by temperature, the characteristics of the wastewater to be treated, the characteristics of the selected seed sludge, the availability of essential nutrients, divalent cations, pH and alkalinity, and process operating parameters (Tay & Yan). The granular sludge enhances settleability of the biomass, leads to an effective retention of bacteria in the reactor, and improves physiological conditions, making them favorable for bacteria and their interactions, especially syntrophs (Akunna & Clark, 2000; Show et al.).
Materials and Methods
Two identical pilot-scale UASB reactors were used in our study. The reactors were made from fiberglass and had 10 L in working volume (lower part: 105 cm high by 10 cm in diameter; upper part: 18 cm height by 18 cm in diameter). Three evenly distributed sampling ports were installed along the side of each of the reactors. The reactors were kept in controlled-temperature room at 37[degrees]C. A schematic of the lab-scale experimental setup is shown in Figure 2.
Seed sludge (3L) from an anaerobic plant treating brewery wastewater located in Adana was inoculated into each of the UASB reactors. The composition of the seed sludge is given in Table 1. The concentration of suspended solids and the concentration of volatile suspended solids in the seed sludge were 11,602 and 6,263 mg/L, respectively.
The UASB reactors were fed with synthetic wastewater prepared according to the procedure described by Tay and Yan (1997). The synthetic feed provided sufficient macro- and micronutrients for bacterial growth, but its composition was varied throughout study. We adjusted the pH and buffer capacity by adding sodium bicarbonate (NaHC[O.sub.3]). Each reactor was fed from a different storage tank. The characteristics of wastewater fed to each reactor were varied during the study. The synthetic wastewater was continuously fed into each UASB reactor by two pumps.
The hydraulic loading rates and the organic loading rates also were varied throughout the experiment. The hydraulic loading rates had ranges of 0.121-2.713 cubic meter per cubic meter per day ([m.sup.3]/[m.sup.3]-d) for Reactor 1 and 0.069-2.367 [m.sup.3]/[m.sup.3]-d for Reactor 2, and the organic loading rates had ranges of 1.27-21.72 kg of chemical oxygen demand per cubic meter per day (COD/[m.sup.3]-d) (HRT = 3 d) for Reactor 1 and 1.86-23.46 kg COD/[m.sup.3]-d (HRT = 5 d) for Reactor 2.
During the experiment, pH, COD, total solids (TS), total suspended solids (TSS), volatile solids (VS), total kjeldahl nitrogen (TKN), total phosphorus (TP), and alkalinity analyses were done for both influent and effluent. Moreover, MLSS and MLVSS were also sampled from three ports installed along the side of each of the reactors. The pH was measured with a pH meter (Orion Model SA 720). The other analyses were done according to Standard Methods for the Examination of Water and Wastewater (American Public Health Association [APHA], American Water Works Association [AWWA], and Water Pollution Control Federation [WPCF], 1989). Gas production was measured by a liquid displacement method (Agdag & Sponza, 2005).
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Results and Discussion
The reactors were filled with 3L of seed sludge, and synthetic wastewater was added. They were operated during an acclimation period lasting 30 days. Then the reactors were started up. During operation, the hydraulic loading rates, the organic loading rates, and the composition of the synthetic wastewater were varied. The COD removal efficiencies obtained at different hydraulic loading rates are given in Figure 3. The COD removal rates ranged between 74.27 and 84.75 percent for Reactor 1 and 73.03 and 82.83 percent for Reactor 2. As seen in Figure 3, when the hydraulic loading rate was increased, the COD removal rate decreased.
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The COD removal rate was lower for Reactor 2 than for Reactor 1 at about equal hydraulic loading rates. The difference can be attributed to differences in influent COD values. The COD values were 7,630-8,240 mg/L for Reactor 1 and 10,800-12,140 mg/L for Reactor 2. Paula and Foresti (1992) found that for an average initial COD concentration of 1,780-9,700 mg/L, corresponding to volumetric loading rates of 2.7-14.8 kg COD/[m.sup.3]-d, COD removal efficiencies varied from 98 to 80 percent. They suggested that the stepped increase of influent COD affected reactor performance in two ways. First, the reactor could assimilate the shock loads from the stepped increase of the influent COD and the volumetric loading rate beyond the transient periods following changes in the operation parameters. Second, the overall efficiency decreased gradually with the stepped increase of the initial COD and volumetric loading rate.
We also investigated TKN, TP, TS, TSS, and VS removal efficiencies at different hydraulic loading rates. The TKN and TP removal rates are given in Figure 4 and Figure 5. The TKN and TP removal rates were quite low. Because anaerobic treatment does not remove phosphate, ammonia, and sulfide, it is efficient only for removal of biodegradable organic matter. Therefore, certain post-treatment methods must be applied to remove these compounds and to meet standards for discharge into surface water or for irrigation purposes (Buisman, Post, Geraats, & Lettinga, 1989; Seghezzo, Zeeman, Lier, Hamelers & Lettinga, 1998; Tawfik, Klapwijk, Gohary & Lettinga, 2002; Ghangrekar & Kahalekar, 2003). For example, Rim & Han (2000) used a combined process of UASB and biofilm process to remove nitrogen. They achieved 80-95 percent TCOD and 90 percent ammonia nitrogen (N[H.sub.3]-N) removal rates.
The TS, TSS, and VS removal rates had ranges of 69.81-84.75 percent, 73.39-85.06 percent, and 72.23-87.42 percent, respectively for Reactor 1, and 67.71-84.02 percent, 64.86-85.45 percent, and 63.20-84.93 percent, respectively, for Reactor 2. Results are shown in Figure 6.
COD, TS, TSS and VS removal rates and biogas production volume were investigated at different organic loading rates. The hydraulic retention times were 3 d and 5 d for Reactor 1 and Reactor 2. The organic loading rates had ranges of 1.49-18.46 kg COD/[m.sup.3]-d for Reactor 1 and 2.12-25.85 kg COD/[m.sup.3]-d for Reactor 2. The relationship between organic loading rate, COD removal rate, and produced biogas volume is shown in Figure 7. At these organic loading rates, the COD removal rates had ranges of 64.37-82.45 percent for Reactor 1 and 55.24-84.47 percent for Reactor 2, and biogas volume was 25.10-55.44 L for Reactor 1 and 23.45-60.28 L for Reactor 2. When the organic loading rate was increased, the COD removal rate was decreased. Moreover, at higher hydraulic retention times, higher COD removal rates were obtained. From these results, it can be concluded that hydraulic retention time and organic loading rate had an effect on reactor performance.
The following conclusions may be drawn from our study:
1. It is feasible to use a UASB process for treatment of high-strength wastewaters. Both UASB reactors removed COD up to 84 percent.
2. Varying influent COD concentration affects COD removal rates. At about equal hydraulic loading rates, a higher removal efficiency was achieved in Reactor 1, which had a lower influent COD concentration than Reactor 2. Moreover, COD removal rates decreased when the hydraulic and organic loading rates were increased. Similarly, TP, TKN, TS, TSS, and VS removal rates also decreased. The performance of the reactors was highly dependent on hydraulic loading rate, organic loading rate, and hydraulic retention time.
3. TKN and TP removal rates of UASB reactors are low; therefore, certain post-treatment methods should be applied if removal of these constituents is desired.
4. UASB reactors can be used to treat high-strength wastewaters with high efficiency, but effluent characteristics may not always meet discharge standards for receiving waters. Therefore, the UASB process may be best suited for use as a pre-treatment process.
Acknowledgements: The research reported in this paper was supported with funds provided by the Research Fund of the Firat University (FUNAF, Project 486).
Corresponding author: Sibel Aslan, Environmental Engineer and Research Assistant, Firat University, Faculty of Engineering, Department of Environmental Engineering, Elazig, Turkey 23119. E-mail: email@example.com.
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Although most of the information presented in the Journal refers to situations within the United States, environmental health and protection know no boundaries. The Journal periodically runs International Perspectives to ensure that issues relevant to our international constituency, representing over 60 countries worldwide, are addressed. Our goal is to raise diverse issues of interest to all our readers, irrespective of origin.
Sibel Aslan, M.D.
Nusret Sekerdag, Ph.D.
TABLE 1 Composition of Seed Sludge Parameter Concentration pH 6.87 COD (mg/L) 8,800 TKN (mg/L) 97.44 TP (mg/L) 132.52 Alkalinity (mg/L CaC[O.sub.3]) 1650 TS (mg/L) 11,602 VS (mg/L) 6,263 TSS (mg/L) 5,072
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|Title Annotation:||INTERNATIONAL PERSPECTIVES; upflow anaerobic sludge blanket|
|Author:||Aslan, Sibel; Sekerdag, Nusret|
|Publication:||Journal of Environmental Health|
|Article Type:||Technical report|
|Date:||Jan 1, 2008|
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