Lab Scale Study of HRT and OLR Optimization in UASB Reactor for Pretreating Fortified Wastewater in Various Operational Temperatures Academic research paper on "Chemical engineering"

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APCBEE Procedia 1 (2012) 90 - 95

ICESD 2012: 5-7 January 2012, Hong Kong

Lab Scale Study of HRT and OLR Optimization in UASB Reactor for Pretreating Fortified Wastewater in Various Operational Temperatures

Soheil Farajzadehha a *, S.A.Mirbagheria, Soroush Farajzadehha, Jalal Shayegan

Civil Engineering Faculty, Khaje Nasir Toosi University of Technology, Tehran, 1996715433, Iran

Abstract

This study was carried out for examination of a lab-scale UASB reactor for optimization of organic loading rate and hydraulic retention time in various operational temperatures. The total volume of the reactor was 5 l with an effective height of 160 cm and diameter of 5 cm. This reactor was used to treat fortified municipal wastewater at volumetric organic loadings of 3.6, 7.2, 10.8, and 14.4 kg m-3 d-1 at temperatures 30 °C and 20 °C and PH 7.6 - 8.4 for the treatment of high-strength enriched municipal wastewater of Ekbatan local treatment plant that located in west of Tehran. The COD of this wastewater have enriched by molasses and milk powder. To prevent PH fluctuation of the influent, NaHCO3 and K2HPO4 were added to wastewater. The result of present work indicated an optimum range for organic loading (7.2 to 10.8 kg m-3 d-1) at all temperature condition with COD removal efficiency of about of 85% and 73 % in UASB reactor at 30 °C and low temperature about 20 °C. Moreover, optimum HRT for influent COD concentration of 1200mg/l is shown to be only 4 hours. Furthermore nitrate removal efficiency was about 80% at optimized organic loading range, with a sharp decrease beyond this range (e.g. up to 30% decrease for 14.4 kg/m3.d and up to 45% decrease for 3.6 kg/m3.d)

© 20121 The Authors.Pub lished by Elsevier B.V.

Selection and/or peer review under responsibility of Asia-Pacific Chemical, Biological & Environmental Engineering Society Keywords: fortified municipal wastewater, HRT, OLR, removal COD, UASB

1. Introduction

In some cases, domestic wastewater and industrial wastewater such dairy swage discharge into same

* Corresponding author. Tel.: +98-912-2868348; fax: +98-21-55422435. E-mail address: soheil.farajzadehha@gmail.com

2212-6708 © 2012 © 2012 The Authors. Published by Elsevier B.V.

Selection and/or peer review under responsibility of Asia-Pacific Chemical, Biological & Environmental Engineering Society doi:10.1016/j.apcbee.2012.03.016

collector system and then high value of nutrients such COD must be treated in wastewater treatment plants.

To solve these problems, it is thought to install up-flow anaerobic sludge blanket reactor (UASB) prior to ASP to reduce the BOD and COD feed and also admit excess sludge for volume reduction as well as anaerobic digestion. UASBR is widely used for sewage treatment in tropical countries, such as India and Brazil (Aiyuk et al, 2006). The current challenge in anaerobic technology is to develop the system to treat municipal sewage in extreme situations. For instance, in Palestine and Jordan sewage is characterized with high COD concentrations of more than 1000 mg/l with high fraction of suspended COD (CODs of up to 70%) and fluctuating temperature in the range of 15-25 °C (Halalsheh et al., 2005). Previous researches had demonstrated that the performance of one-stage UASB reactors at low temperatures (520 °C) is severely limited by the slow hydrolysis of entrapped solids that accumulate in the sludge bed when high loading rates are applied (Zeeman and Lettinga, 1999). The solids accumulation will impose a more frequent sludge discharge. Consequently, the excess sludge will increase, leading to a low solids retention time (SRT) and a concomitantly less stabilized sludge bed with a low specific methanogenic activity (SMA). Due to high rate of population growth in Iran and thus increasing the flow and loading rate of municipal wastewater, most of existing ASPs need to be retrofitted. As mentioned before, the aim of this work was to optimize the hydraulic retention time (HRT) and organic loading rate (OLR) for such retrofitted system.

2. Material and methods

2.1. UASB Reactor

A Plexiglas lab-scale UASB reactor was built for this study (Fig. 1). The total volume of the reactor was 5l and had three sections: 1- feed entrance section with conical shape of 5 cm height, 2- sludge bed and blanket zone with 5 cm diameter and 160 cm effective height and 3- settling zone with 10 cm diameter and 20 cm height. A gas-liquid-solid separator (GLS) device which was inverse conical was installed at the top portion of sludge bed and inside the settlement section. Diameter of its inset is designed 6 cm to produce gas total in portion of sludge bed conducted out by part of gas outline and gas did not enter the settlement section.

Table 1. Influence wastewater characteristics

composition / unit Quantity

parameter

PH mg/L 7.6-8.4

COD total mg/L 600-2400

BOD5 mg/L 400-1600

SS mg/L 190 -250

Alkalinity mg CaCO3/L 1200-3140

Temperature ° C 20,30

2.2. Operation and start-up of reactor.

The anaerobic seed sludge was collected from pegah Dairy Company. The one stage UASB reactor was startup during summer specially in conceding the middle of the hot period in Iran. The UASB culture was operated at an initial hydraulic retention time (HRT) of 6 h and reached steady state condition in terms of TOC removal and gas production and biomass granulation. the HRT was further shortened to 4 h.

Influent and effluent wastewater samples analysis daily for CODtotal and TSS over the hot and cold periods of investigation. After 210 days operating 36 influent and effluent samples were collected and analyzed during a period of 120 days for BOD, TKN, and NH4. The system was operated for 210 days of which the first 55 days were considered as a "start-up period". The first 120 days operated at 30 °C and latest 90 days operated at 20 °C.

Fig. 1. Shematic of lab-scale experimental setup

2.3. Characterization of wastewater

Influent wastewater: For this study, the main source of wastewater was from Ekbatan Wastewater Treatment Plant located in west of Tehran. The COD of this wastewater was enriched by molasses and milk powder. So the total COD could reach to 600, 1200, 1800 and 2400 mg/l. To prevent pH fluctuation of the influent, NaHCO3 and K2HPO4 were added to wastewater. Influent wastewater characteristic is presented in Tab. 2

3. Results and discussion

3.1. Results of HRT variation

The effect of various RTs of 3,4,5, and 6 hours on COD and TSS RE was investigated in 30°C to determine the best HRT for set to resume research(results are shown in fig2). As can be seen, the COD RE is the highest in the RTs of 4 and 5 hours.

The reason for this decrease in efficiency while reducing the HRT, in spite of increasing the turbulence in the reactor, is that the contact time of wastewater with sludge granules will be decreased. Therefore, less organic matter is utilized. The efficiency also was lowered by increasing the RT, because of lower amount of mixing because of a reduction in up flow liquid velocity. Rising gas bubbles and the up flow liquid velocity causes mixing in the reactor. The TSS RE in steady state condition for different RTs is also shown in fig.2. As can be seen, the TSS RE is has been risen by increasing RT up to 4 hours, but very smooth reduction( about 1%) was observed for higher HRTs. Therefore, optimum HRT for COD removal (about 80%) can be considered 4 h at 30°C. As shown in fig .2, increasing the retention time from 5 to 6 hours has almost no effect on COD removal efficiency reduction. This could be because of reduction in SRB bacteria activities, which is a good factor for methanogens bacteria. So the effect of a decrease in mixing is reduced.

Bïogas

Effluent UASB

Influent wastewate

Fig. 2. COD and TSS removal efficiency with HRT (OLR of 1200 mg/lit, at 30°c) Fig. 3. The removal efficiency of COD in different OLR 3.2. Results of OLR variation by temperature

The RE of COD and TSS for different organic loading rate of 3.6, 7.2, 10.8 and 14.4 kg COD/m3 .day (for COD concentration of 600, 1200, 1800 and 2400 mg/L, respectively and HRT=4 hours) in 3 various operational temperature is shown in fig.3 and fig.4.

As it can be seen, the COD RE is the maximum in the organic loading rate of 7.2 to 10.8 kg COD/m3d. The COD RE in this range is about 85%, 81% and 75% in 30, 25 and 20 degree of centigrade, but it reduce to 72%, 66% and 59% while decreasing the OLR to 3.6 kg COD/m3.d. These values reduce to 78%, 72% and 62% while increasing the organic loading rate to 14.4 kgCOD/m3.d. These results representing an optimum OLR of 7.2 to 10.8kg COD/m3.d in 30°c, 25 °c and 20 °c for UASB reactor. By increasing the COD RE from 1800 to 2400mg/L, the COD RE was decreased. The reason for this RE reduction is because of a higher sulphate concentration (from 120 to 200 mg/L) due to the addition of molasses to increase COD from 1800 to 2400). So, SRB and methanogenous bacteria compete to use the substrate.

Fig. 4. The removal efficiency of TSS in different OLR

Therefore the methanogens' activities are reduced with respect to the previous condition. Various research have demonstrated the competitive domination of sulfate reduction over methane production over in sulfate-rich environments [9,10]

3.3. Efficiency of COD removal in UASB reactor

The UASB reactor operated at a constant HRT of 4 throughout the study, while OLRs are 3.6, 7.2,

10.8 and 14.4 kg.m-3.d-1 Due to changes in influent Composition. UASB reactor operated at two operational temperatures. At First stage reactors efficiency investigated at 30 °C for various OLR mentioned above. Despite variation and 73 % COD removal for OLR of 3.b and 14.4 Moreover UASB reactor achieved 85 % COD removal efficiency for OLR 7.2 and 10.8 respectively. In the second stage, UASB reactor operated at 20 °C for above OLR and COD removal efficiency decreased approximately 13 % and 16 % for OLR of 14.4 and 3.6. COD removal efficiency had higher performance for OLR rate of 7.2 to 10.8 and only reduced by 10 %.

This relatively good performance could attribute to the long SRT (80 days) which could effectively increase efficiency of hydrolysis and subsequent digestion to organic matter. Results indicated that UASB performance in TSS removal is independent from temperature.

Fig. 5. COD Removal efficiency in both run time 3.4. Sludge physical characteristics

The mixed liquor suspended solid (MLSS) concentration in the sludge bed zone increased from 10g/l to 29g/l and mixed liquor volatile suspended solid (MLVSS)/MLSS ratio increased from 0.5 to about 0.85 at the end of the operation period. The sludge volume index (SVI) decreased from 55 to about 12 ml/g.

4. Conclusion

1. Existing ASPs in Iran could be easily retrofitted by adding a UASBR before aeration basin. This will increase the total influent capacity by a factor of 10 (BOD efficiency of about 90% was achieved for UASBR)

2. The UASBR was very stable when granules appeared after 40 days from start-up. Sludge washout was reduced to almost zero. Worth mentioning that SVI was less than 12.

3. As was noticed, the COD concentration of UASBR influent should be around 1200 to 2400 mg/l. for some cases the which the incoming COD is not as much, the COD may be increased by admitting excess sludge to UASBR resulting a significant reduction of excess sludge volume.

4. The optimum OLR was easily in the range of 7.2 to 10.8 kg/m3.d. However, this range could easily be expanded if the sulphate content of UASBR feed will be reduced.

5. HRT of 4 hour was found to be the optimum. Nevertheless, this could be reduced if the amount of granules is increased

Reference

[1]. Z.M Fu,F.L.YA ,Control of COD/N ratio for nutrient removal in modified membrane bioreactor(MBR) treating high strength wastewater, Bioresource technology 100(1) (2009) 136-141.

[2]. Anonymous, Biogas technology in the Netherlands, anaerobic digestion waste and wastwewater treatment with energy production, in: I.F .

[3]. Malina F.G Po, Design of anaerobic processes for the treatment of municipal and industrial wastes, Technomic, Lancaster, PA,1992, pp.119-120

[4]. Aiyuk S., Forrez I., De LI, K., van HA A., Verstraete, W., 2006, Anaerobic and Complementary Treatment of Domestic Sewage in Regions with Hot Climates - A review. Bio resource, Technol. 97 (17), 2225-2241.

[5]. A. TA, Treatment of combined dairy and domestic wastewater in an up-flow anaerobic sludge blanket(UASB) reactor followed by activated sludge( AS system) Desalination, 227(2008) 167-177.

[6]. YINgyu AN, The investigation of methanogenesis with shortcut nitrification and denitrification in a combined UASB with MBR, Bioresource technologiy 99, (2008) 3741-3720

[7]. APHA, Standard methods for examination of water and wastewater, 18th ed.American public health Association, Washington.

[8]. APHA, Standard methods for examination of water and wastewater, 20th ed.American public health Association, Washington.

[9]. W.AB, Hydrogen a substract for methanogenesis and sulphate reduction in anaerobic salt marsh sediment, Arch, Microbial 117(1)(1978)93-98

[10]. Jalal SH et al. , The effect of influent COSD and Upward flow velocity on the behaviour on the sulphate reducing bacteria, Process Biochemistry 40(2005)2305-2310