Mixing behaviour and treatment performance of a UASB reactor under different hydraulic loading rates

Martina Uldal

Water and Environmental Engineering Department of Chemical Engineering

Lund University

Email: martina.uldal@anoxkaldnes.com

Abstract Dispersion studies using red dye were done for three hydraulic loading rates on a pilot-scale UASB treating domestic wastewater in Vapi, India. The reactor was operated with hydraulic retention times varying between 10.6 and 26.5 h, and corresponding organic loading of ~0.20 kg COD/m3/d for 60 days. COD and TSS were used to evaluate the performance of organic matter removal. Methanogenic activity and Sulfidogenic activity tests were performed for evaluation of sludge quality. The UASB showed a poor mixing behaviour, with dead volumes and short-circuiting. Under ambient temperature conditions (20-35 ˚C) an average of 36 % COD removal and 48 % SS removal was achieved. However, the removal of organic matter showed a correlation with hydraulic loading rate. Also, a strong correlation between upflow velocity and retardation factor was found. The Methanogenic activity showed decreased activity at lower organic loading rates. In addition, at this state the sulfidogenic activity instead increased. Several parameters indicated a process inhibited by sulphate reducing bacteria, i.e. low methanogenic activity, high variability in COD reduction, high sulfidogenic activity and low gas production. Key words: anaerobic treatment; UASB; domestic wastewater; methanogenic activity; sulfidogenic activity; flow variations; hydrodynamic behaviour

1. Introduction The world today faces enormous challenges within the areas of sanitation, environmental protection and conservation of clean water supplies. As a consequence, the development of efficient wastewater treatment techniques has gained increased importance, with special attention been put on the use of anaerobic processes for treatment of liquid effluent. The result is the development of several high-rate anaerobic systems, characterized by their ability to retain large amounts of biomass even with the application of high hydraulic loads to the system (Chernicharo, 2007). Within the various anaerobic processes developed, the Upflow

Anaerobic Sludge Blanket (UASB) process has been applied far more than other modern anaerobic treatment systems. Though originally developed for industrial wastewater, recent research has proven its applicability also for domestic sewage, especially in tropical countries with warmer climate (van Haandel et al, 1994). The overall hydrodynamic behaviour of a UASB reactor is dependent on several factors, such as the gas production within the sludge bed, the influent distribution system, as well as advection and dispersion effects. These all together determine the resulting final performance of the given reactor. However, few data are available in the literature for UASB reactors that relate

process performance to mixing characteristics. This study was therefore an evaluation of the overall hydrodynamic behaviour of a pilot-scale UASB reactor treating domestic wastewater at three different hydraulic loading rates. The analysis is focused on the macro-mixing behaviour and process performance of the reactor, as well as the quality of the sludge.

2. Methods and material 2.1 Experimental unit The UASB pilot plant is located in Vapi, Gujarat, India at Vapi Common Effluent Treatment Plant (CETP). It has been operated on high-strength industrial wastewater for 2 years of time, before it was switched over to low-strength domestic wastewater. This study was conducted during the start-up phase with domestic wastewater. The pilot plant layout and its main features are given in Figure i and Table i, respectively.

2.2 Experimental design The influent flow rate was the parameter to be varied at different levels. The initial flow rate of 4000 l/h was lowered to 2000 l/h and thereafter increased to 5000 l/h. Corresponding HRT values are 26.5 h for 2000 l/h, 10.6 and 12.5 h for 5000 l/h and 4000 l/h, respectively. Each time step was maintained for approximately 15 days. Total time for the whole study was 2 months.

2.3 Dispersion studies Tracer studies, using the red dye Procion Brilliant Red M 8B as inert tracer, were performed for each flow rate. The addition of tracer was done through a separate inlet via an external tank. Samples of the effluent were taken at the outlet of the reactor at different time steps throughout four retention times for each tracer study. The effluent samples where collected in larger buckets, from which a smaller sample of 100 mL was collected for analysis. For analysis of red dye, the samples were first centrifuged for 10 minutes at 2500 rpm. Thereafter the supernatant was analysed with

spectrophotometer (Merck SQ-118) at wavelength 550 nm at the CETP lab. The concentration red dye in the sample was estimated using a standard curve (absorbance as a function of concentration). The residence time distribution (RTD) in the reactor was described using dimensionless Eθ-curves from the dispersion model with small extents of dispersion were derived from the effluent tracer

Figure i: Layout of the UASB pilot plant at Vapi CETP.

All units are in meter.

Table i: Main features of the UASB reactor.

Design parameters Value

Design flow (l/h) 4000

Hydraulic retention time (h) 13.3

Total active volume (m3) 53

Total height (m) 7.8

Water depth (m) 6.7

Sludge depth (m) 2.5

Internal diameter (m) 3.17

Internal cross sectional area (m2) 7.9

Organic load (kg COD/m3/d) 0.25±0.08

TS (g/l) 74±7.6

VS (g/l) 32±3

Incoming wastewater

TCOD (mg/l) 106.9±25

SCOD (mg/l) 62±25

Total BOD5 (mg/l) 44.7±9.7

SS (mg/l) 37.8±18

Sulphide (mg/l) 1.46±0.8

Sulphate (mg/l) 20.6±5.2

data set (Levenspiel, 1972). Also, dispersion coefficients were calculated, and the mixing conditions in the reactor were evaluated for each flow rate.

2.4 Process performance Composite samples of the reactor influent and effluent were collected on a daily basis and analysed for CODt, CODf, NH4+, TKN, sulfate, sulfide, phenol, BOD and TSS. All laboratory analyses were carried out according to the Standard Methods (APHA, 1995). VFA was analysed using titration method (Buchauer, 1998), and is given as acetic acid equivalents (mg CH3COOH/l). Sludge sampling was carried out via side ports in the sludge zone of the reactor. Total gas production was monitored on-site by liquid displacement method, and pH was analysed in lab with a Control Dynamics pH-meter, as well as online with Online Electrode Method. The flowrate was controlled by a valve and continuously regulated by means of a pump. Composite sludge samples from different levels of the sludge bed were collected at regular interval throughout the study period for evaluation of sludge quality, i.e. determination of methanogenic and sulfidogenic activity. Total methanogenic activity (TMA) and acetoclastic methanogenic activity (AMA) were examined by the method developed by Valcke et al (1983). Sulfidogenic activity tests (SAT) were conducted using a method developed by IIT-Kanpur. Also, total solids (TS) and volatile solids (VS) was analysed for each sludge sample.

3. Results 3.1 Dispersion studies The Eθ-curve (Figure ii) obtained for the design flow condition (4000 l/h) show signs of a poorly mixed reactor with dead spaces. The peak arrives later than expected; mean residence time is 16.7 h. Compared with the theoretical hydraulic retention time of 13.25 h, this gives a

retardation coefficient of 1.26. In addition, the mass of red dye is divided in two peaks, with the second peak having a mean residence time of ~36 h, indicating that part of the red dye has been trapped inside the reactor in dead volume. The fractional weight of peak 1 was computed to 87 %, which can be thought of as effective reactor volume available for treatment. For the rest of the volume (13 %), the effective HRT is 36 hours. With an increased flow regime (Figure iii), again two peaks are visible, as in the case with design flow. The effective volume is now 83 % (Table ii), which implies no gain in effective volume for higher flow rates. At the decreased flow rate of 2000 l/h (Figure iv), a gross disturbance on the internal mixing in the reactor is apparent, with signs of internal circulation in the reactor. The effective flow increased with upflow velocity, but at higher flow rate than design flow, there was no gain in effective volume. Only a weak correlation between upflow velocity and dispersion coefficient was found. A strong dependency was however achieved between the dispersion coefficient and the retardation coefficient (data not shown).

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Figure iv: Residence time distribution for decreased

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3.2 Reactor performance The OLR for the three flow regimes was 0.08, 0.20 and 0.22 kgCOD/m3/d for 2000 l/h, 4000 l/h and 5000 l/h, respectively. The removal efficiency varied with flow rate, i.e. the highest removal efficiency was obtained for 5000 l/h, and the lowest for 2000 l/h (Figure v). On average the removal of suspended solids was 48 %, with a variation in removal efficiency of 18 % during the whole time period (Figure vi). Alkalinity was stable around 250 – 300 mg CaCO3/l throughout the whole time period.

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Figure vi: SS (mg/l) in influent and effluent as a function of

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Also, the concentration of VFA in influent and effluent followed each other very closely throughout the whole time period, at around 50 mg CH3COOH/l. The pH was stable without any large deviations between influent and effluent, indicating that the alkalinity available was enough to buffer the system. Gas production was not measurable during the first month, but started after approximately 30 days. The gas production was however much smaller than the theoretical gas production, and possibly there was gas escaping with the effluent or through other openings in the reactor.

Table ii: Summary of hydrodynamic parameters obtained during tracer studies.

Flow (l/h) HRTt (h) HRTe (h) σ2

σ D/uL u (m/h) D (m2/h) Retardation Veff

2000 26.5 23.2 8.0 2.8 0.0115 0.25 5.10E-06 0.87 40%

4000 13.25 16.7 2.7 1.7 0.0048 0.49 4.30E-06 1.26 87%

5000 10.6 25.5 20.2 4.5 0.037 0.61 4.10E-05 1.55 83%

3.3 Sludge quality analysis Figure vii shows the sulfidogenic activity together with the methanogenic activity, as a function of time. The methanogenic activity started at approximately 8-10 mL CH4/gVS/d and then increased until day 30, thereafter it started to decrease. This decrease coincides with the lower flow regime of 2000 l/h, which was operated during day 22 to 38. In opposite, the sulphate reducers seem to be favored by the longer hydraulic retention time and increase in activity during this time period. At the end of the period with 5000 l/h, the activity of the sulphate reducers is well over 35 mgS2-/gVSS/d. The methanogenic activity is at this time ~ 10 mL CH4/gVS/d.

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Figure vii: Results from sulfidogenic activity test (SAT)

and average methanogenic activity test (MAT)

4. Discussion 4.1 Hydrodynamic behaviour The RTD curves obtained for the three flow rates all show signs of poor mixing and dead volumes. Flow channels have formed inside the reactor and the mixing within the sludge bed is not enough. For the flow rate of 2000 l/h, the reactor also showed signs of short-circuiting, as the dye show up earlier than expected. For all three cases, it seems like a high fraction of the tracer mass is trapped inside the reactor and is released much later than expected. This indicates presence of dead volumes inside the

reactor, where the tracer is trapped. Inside these pockets the retention time is much longer than the overall hydraulic retention time for the system. Thus, the dead volumes could provide further explanation for the growth of SRB, who has a slightly lower growth rate than the methanogens. Only a weak correlation was found between dispersion coefficient and upflow velocity, wich is a contrary result compared with the results obtained in studies by Peña et al (2006) and Zeng et al (2005). These both found the dispersion coefficient to increase with an increased upflow velocity. As the dispersion coefficient depends on mean residence time and variance (see equation 6.13 and 6.14), it is possible that for the flow rate of 2000 l/h, the effect of short-circuiting levelled out the dispersion, which now was neglecteble. If only the 4000 l/h and 5000 l/h flow rates are considered, the correlation found by others of increased dispersion coefficient with increased upflow velocity is indeed apparent. A strong correlation was found between retardation coefficient and upflow velocity. With a higher upflow velocity, the retardation increased. The decreased retardation for lower upflow velocity can again be explained by short-circuiting, causing the tracer to travel through channels being build in the sludge bed, and thereby allowing for a faster passage through the reactor.

4.2 Process performance in relation to mixing characteristics The removal efficiency of organic matter varied with flow rate, and the highest removal efficiency was obtained for 5000 l/h. However, the variation in total COD removal efficiency was much higher at this flow rate, compared with the two other flow rates. The concentration of sulfide in the effluent increased with a lower flow rate, indicating increased activity from the SRB. Large dead spaces present at lower flow rate provided shelter to sulfate reducers to grow

and compete with the methanogens. At higher flow, the sulphate reducers where instead washed out from the system. Gas production started during the time period with lowest flow rate. This has however probably a stronger connection to the fact that the methanogens now had started to adapt to the new type of wastewater, than that the flow rate at this time was lower.

Other parameters, such as alkalinity, pH and volatile fatty acids, showed no signs of being affected of the different flow rates.

4.3 Effect of flowrate on methanogenic and sulfidogenic activity The sulphate reducing bacteria were favoured by the longer hydraulic retention time and increased in activity during the time period with flowrate of 2000 l/h. At the last activity test performed, the SAT was still around 40 mgS2-

/gVSS/d. Also the concentration of sulfide in the effluent rose during the time period with flow rate 2000 l/h, again indicating increased reduction of sulphate during this time period. In addition, the results obtained from the tracer studies at this flow rate indicated a large portion of dead space within the reactor. Large dead spaces where no fresh substrate is available will provide shelter to sulfate reducers to grow and compete with the methanogens. At higher flow, the sulphate reducers are instead washed out from the system. Also, it seemed like the time period with 5000 l/h, i.e. 15 days, was not long enough to give the methanogens a competitive advantage over the SRB. The acetoclastic and hydrogenotrophic methanogens showed similar patterns in variation, with the activity decreasing during the period with longer retention time. At the end of the period with 5000 l/h, the activity again starts to increase. For the last activity test performed, the methanogenic activity was ~ 10 mL CH4/gVS/d.

5. Conclusions

� Dispersion study showed signs of gross disturbances in mixing behaviour inside the reactor, compared to the expected dispersed plug flow, with dead volumes inside the reactor and signs of short-circuiting. A strong correlation was found between retardation coefficient and upflow velocity.

� The removal efficiency of organic matter varied with flow rate, and the highest removal efficiency was obtained for 5000 l/h. Other parameters where not affected by the changes in flow rate, indicating a certain process stability against external variations.

� Sulfidogenic activity increased with decreased flow rate. The opposite applied for methanogenic activity, indicating a competitive advantage for the SRB at lower flow rate. Several factor indicated that the methanogens where inhibited by sulphate reducing bacteria, i.e. low methanogenic activity, high variability in COD reduction, high sulfidogenic activity and low gas production.

References APHA. Standard Methods for the examination of water and wastewater. 19th ed. 1995. Buchauer, K. A comparison of two simple titration procedures to determine volatile fatty acids in influent of wastewater and sludge treatment processes. Water SA 24(1), 49-56. 1998. de Lemos Chernicharo, C. Anaerobic reactors. Biological wastewater treatment series, volume 4. IWA Publishing. 2007. Levenspiel, O. Chemical reaction engineering. John Wiley & Sons. 2nd ed. 1972.

Pena, M.R.; Mara, D.D.; Avella, G.P. Dispersion and treatment performance analysis of an UASB reactor under different hydraulic loading rates. Water Research 40, 445-452. 2006. Valcke, D.; Verstraete, W. A practical method to estimate the acetoclastic methanogenic biomass in anaerobic sludges. Journal Water Pollution Control Federation 55, 1191-1195. 1983.

Van Haandel, A. Lettinga, G. Anaerobic sewage treatment – a practical guide for regions with a hot climate. John Wiley & Sons. 1994. Zeng, Y.; Mu, S.J.; Lou, S.J.; Tartakovsky, B.; Guiot, S.R.; Wu, P. Hydraulic modeling and axial dispersion analysis of UASB reactor. Biochemical Engineering Journal 25 (2), 113–123. 2005.