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Study on a novel reactor of sludge process reduction for domestic sewage treatment a

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En Xie , Xiao-yi Xu & Gu-yuan Luo

a

a

Key Laboratory of Three Gorges Reservoir Region's Eco-Environment, Ministry of Education , Chongqing University , Chongqing Municipality , China Accepted author version posted online: 13 Dec 2012.Published online: 08 Jan 2013.

To cite this article: En Xie , Xiao-yi Xu & Gu-yuan Luo (2013) Study on a novel reactor of sludge process reduction for domestic sewage treatment, Environmental Technology, 34:12, 1593-1599, DOI: 10.1080/09593330.2012.758670 To link to this article: http://dx.doi.org/10.1080/09593330.2012.758670

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Environmental Technology, 2013 Vol. 34, No. 12, 1593–1599, http://dx.doi.org/10.1080/09593330.2012.758670

Study on a novel reactor of sludge process reduction for domestic sewage treatment En Xie, Xiao-yi Xu∗ and Gu-yuan Luo Key Laboratory of Three Gorges Reservoir Region’s Eco-Environment, Ministry of Education, Chongqing University, Chongqing Municipality, China

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(Received 27 December 2011; final version received 10 December 2012 ) A laboratory-scale novel Sludge Reduction Reactor with Arc Guide Plate (SRR) for sludge process reduction was developed in this study. Pollutant removal efficiency and biomass yield for domestic sewage treatment in the Anaerobic/Anoxic/OxicSRR (A2/O-SRR) process were compared with performances in a control A2/O process. One of the competitive advantages in the SRR was that part of the inert suspended solids (ISS) could be separated and discharged out of system with flux at the bottom of the SRR. Mixed liquid volatile suspended solids (MLVSS) in the A2/O-SRR system also could be steadily kept at a good level under a relatively long solid retention time. The average MLVSS/mixed liquor suspended solids (MLSS) ratio of 77.5% in the A2/O-SRR was higher than that in the A2/O process. Average removal rates of chemical oxygen demand (COD), total nitrogen (TN) and NH+ 4 showed little difference, while total phosphorous (TP) removal efficiency in the A2/O-SRR decreased slightly (81.89% in the A2/O-SRR and 86.50% in the A2/O process) due to the reduction of sludge discharge. The A2/O-SRR system demonstrated a considerable sludge reduction effect, with the sludge reduction ratio of 43.8%, lower solid volume index and higher dehydrogenase activity (DHA) value in comparison to the control A2/O system. The mainly mechanisms of sludge reduction in the SRR have been proved to be the uncoupling effect under the condition of anaerobic/oxic circulation and the sludge lysis with the lack of substrate. Keywords: sludge process reduction; Sludge Reduction Reactor with Arc Guide Plate; anaerobic/oxygen-limited; sludge activity

Introduction Over the past several decades, the activated sludge process has been used world wide for municipal sewage biological treatment [1–3]. Moreover, the A2/O activated sludge process or its modified technologies used in wastewater treatment plants (WWTPs) are fairly common in China. They will inevitably bring the generation of excess sludge in a large amount [4]. Furthermore, costs of volume reduction, stabilization and disposal of excess sludge, etc., may take more than 40% of the total operational costs in WWTPs [5–7]. Reducing biomass production during the sewage treatment process, which is also called sludge process reduction, has been studied since the 1990s [8]. Different from volume reduction of excess sludge, sludge process reduction simultaneously occurs in the sewage treatment process, which means effective pollutant removal and low sludge yield in the whole system. The mechanism of sludge process reduction includes energy uncoupling of activated sludge under the alternating anaerobic-oxic conditions, cell lysis and metazoan predation [9–11]. The recent studies have demonstrated that long hydraulic residence time (HRT) and the accumulation of inert suspended solids (ISS) were the most important restricting factors in sludge process reduction technology [12]. The oxic-settling anaerobic (OSA) ∗ Corresponding

author. Email: [email protected]

© 2013 Taylor & Francis

process, one of the typical sludge reduction technologies, has been proved to have obvious low sludge yield due to the uncoupling mechanism with the extra HRT of 8–16 hours. However, it is generally accepted that sludge activity in the OSA system often tends to be low [13–15]. Activated sludge in suspended biomass bioreactors usually comprises four components: (1) the active organisms that contribute significantly to the volatile suspended solids (VSS); (2) endogenous residues of microorganisms; (3) the unbiodegradable or unmetabolized organic suspended solids from influent; and (4) inert material from influent [16–18]. Sludge process reduction is essentially concerned with the organic part of activated fractions. Primary settling tanks are usually cancelled in most WWTPs of China in order to save the organic carbon source for enhancing biological nitrogen and phosphorus removal, therefore, the mixed liquid volatile suspended solids/mixed liquor suspended solids (MLVSS/MLSS) ratio of activated sludge is typically only in the range of 50–60% and sometimes even as low as 30% [19], especially in mountainous cities with combined sewers. It is necessary and significant to decrease ISS content in the activated sludge mixture effectively for the stable operation of the sludge process reduction system.

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In this study, a novel system combining a Sludge Reduction Reactor with Arc Guide Plate (SRR) with the A2/O process was developed (China Patent number: 200910104248) [20]. Pilot tests were employed to compare the effectiveness of pollutants removal, sludge reduction effect and ISS separation in Anaerobic/Anoxic/OxicSRR (A2/O-SRR) and control A2/O systems. Meanwhile, sludge properties, operating parameters and the mechanism of sludge process reduction were also investigated in the paper.

Material and methods Reactor configuration and experimental design All the experimental devices were made of plexiglass. Figure 1 shows the control A2/O process and the A2/O-SRR process, respectively. The SRR is shown in Figure 2, which is a U-shaped reactor and can be divided into three parts: the anaerobic zone, oxygen-limited zone and circulating zone. Experiments were carried out to deduce the relationship between circulating velocity and separating efficiency for ISS in the SRR. Because ISS particles with diameter larger than 0.2 mm can be removed by the grit chamber [21], the study mainly focused on the separation of inorganic sand with particle size less than 0.2 mm in the SRR. Clear water experiments with appropriate sand particles were performed in the SRR, and different circulating velocities ranging from 0.20 to 0.55 m/s were controlled to obtain the optimum value. The initial amount of fine sand in this test was 100 g. In the two systems of the control A2/O and A2/O-SRR, the influent flow rates were both 10 l/h. The effective volumes of the A2/O reactor and SRR were 100 and 42 l, respectively. In the A2/O system, the HRT of the anaerobic tank, anoxic tank and aerobic tank were 1.5, 2.5 and 6 hours, respectively. Dissolved oxygen (DO) in an aerobic tank was in the range of 2.0–2.5 mg/l. According to the standard HJ 576 (2010), the sludge return ratio and internal recycle ratio of the mixing liquid were 70% and 200%, respectively [22]. MLSS in aeration tanks of the two systems were within

Figure 2. Sketches and flow pattern of the SRR: A. anaerobic zone; B. oxygen-limited zone; C. circulating zone; 1. stirrer; 2. vertical partition; 3. hole for gas pressure balance; 4. submerged propeller; 5. aerator; 6. sand outlet; 7. control position of circulating velocity; 8. control position of DO; 9. arc guide plate.

the range of 2900 ± 100mg/l. ˙ The HRT of the SRR was 8 hours with average MLSS of 6700 ± 100 mg/l. The DO value of the control point (shown in Figure 2) in the SRR was 1.0 mg/l. In the A2/O-SRR system, most of the excess sludge was discharged from the settling tank, while the excess sludge from the SRR accounting for 6.5% of the total volume. Influent and sludge cultivation The influent quality monitored during the study is summarized in Table 1. Raw wastewater from a campus sewer line was pumped into the A2/O-SRR and A2/O system after pre-sedimentation. The bioreactors were operated

Figure 1. The flow diagram of experimental device: (a) control A2/O system; (b) A2/O-SRR system; 1. influent tank; 2. pump; 3. DO and pH meter; 4. stirrer; 5. air pump; 6. computer; 7. settling tank; 8. airflow meter; 9. SRR.

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Table 1.

Indexes of influent quality.

Parameter

Unit

Range

Average value

Number of samples

Relative standard deviation

COD NH+ 4 -N TN TP pH Water temperature

mg/l mg/l mg/l mg/l –

200–390 25.68–41.94 33.76–58.34 4.71–7.71 7–8 11–29

297.76 31.46 45.61 6.17 7.32 21.5

60 60 60 60 60 60

17.69% 16.50% 13.86% 11.02% 4.17% 19.98%



continuously for eight months from January to August in 2011 in the key laboratory of the Three Gorges Reservoir Region’s Eco-Environment (Ministry of Education), Chongqing University. The seed sludge was taken from the Tang Jiaqiao municipal sewage treatment plant located in Chongqing municipality. Samples were determined after a successful start-up. Analytical methods Dehydrogenase activity (DHA) was measured referring to the TTC (2, 3, 5-tripheyltetrazolium)-DHA method [23,24]; MLSS and MLVSS determination was carried out according to the weight principle. The DO and pH value of the mixture in the SRR were measured by online meter. The circulating velocity of the mixture in the SRR was measured by LS45A liquid flow meters. The other analyses of water quality indexes were performed according to the Standard Methods for the Examination of Water and Wastewater (fourth edition) [25]. Results and discussion Separation effects of non-volatile suspended solids Because the SRR had a specific configuration, a considerable portion of ISS in activated sludge could be separated by centrifugal force generating from the flow properties under the condition of an appropriate value of the circulation velocity [26]. According to the results from different circulating velocities in clear water experiments, the ISS separation efficiency maintained relatively high values greater than 95% with circulating velocity of 0.2 m/s, while it rapidly decreased with circulating velocity greater than 0.25 m/s. Research conducted by Deng [27] on the pilot suspended biomass bioreactors revealed that the average velocity of the mixture should be kept at least at about 0.25 m/s in order to avoid the settling of activated sludge. Therefore, the circulation velocity of the mixture in the SRR was set at around 0.25 m/s, which was helpful to achieve a satisfying separation efficiency of ISS without sludge sedimentation in the SRR. Separated ISS could be discharged out of the whole system via the sand outlet with a flux mixture. Non-volatile suspended solids (NVSS) were detected to characterize ISS approximately in the experiments [28]. The average amount

of NVSS separated from the SRR during the experiment period was 1.00 g/d, accounting for 70.61% of the total amount of excess sludge discharged from the SRR. The results also showed that NVSS of 1.54% and VSS of 0.26% in MLSS of the SRR could be discharged from the flux, which demonstrated a good separation effect of ISS in the SRR. The average values of NVSS/MLSS in excess sludge were 28.17% in the A2/O system and 34.01% in the A2/O-SRR system. Sludge activity in the A2/O-SRR process could be improved correspondingly, which helped to keep a stable pollutant removal efficiency of the A2/O-SRR system. Removal effects of pollutants As shown in Figure 3, pollutant removal efficiencies of the A2/O-SRR system and the control A2/O system showed a small difference. Average values of chemical oxygen demand (COD), total nitrogen (TN), NH+ 4 and total phosphorous (TP) removal ratio in the A2/O-SRR system were 94.91%, 81.12%, 87.64% and 81.89%, respectively, and 94.00%, 80.08%, 87.70% and 86.50% in the A2/O system. After the 24th day during the study, a certain kind of Tubificidae in large quantity was found in the upper part of the oxygen-limited area in the SRR. At the same time, pollutant concentration of outflow in the A2/O-SRR system increased slightly, which might be attributed to the predation of Tubificidae to zoogloea. However, the indexes of effluent quality expect TP could still meet the requirements listed in the B standard of first grade of the standard GB18918-2002 [29]. Sludge yield Sludge yield could be calculated by following Equation (1): Yobs =



MLSSES × VES (CODI − CODE ) × Q

(1)

where: Yobs is the apparent sludge yield of activated sludge (mgMLSS/mgCOD); VES is the daily discharge volume of excess sludge (l/d); MLSS ES is the discharged MLSS concentration of wasted sludge (mg/l);

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Figure 3.

Pollutant removal effects in the A2/O-SRR system and the A2/O system.

CODI , CODE is the COD concentration in influent and effluent, respectively (mg/l); Q is influent flow quantity (l/d). The sludge productions are listed in Table 2. The cumulative productions of excess sludge during the experimental period of the A2/O and A2/O-SRR

systems were 862.62 and 475.27 g, respectively. Calculated by Equation (1), the Yobs of the A2/O system and Yobs of the A2/O-SRR were 0.422 gMLSS/gCOD and 0.237 gMLSS/gCOD, respectively. Compared to the A2/O system, the corresponding reduction ratio of the excess sludge in the A2/O-SRR system was 43.8%.

Environmental Technology Table 2.

Cumulative production of activated sludge. Cumulative yield (g)

Cumulative removed COD (g/d)

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Experimental period (d) A/A/O A/A/O-SRR A/A/O A/A/O-SRR 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

0 29.02 59.37 84.59 110.99 140.59 165.20 185.55 212.13 237.20 265.93 291.42 315.57 343.65 373.32 402.17 433.38 458.41 484.35 510.50 535.98 566.19 602.15 641.68 680.83 712.28 740.75 776.18 807.80 836.82 862.62

0 16.02 32.43 48.66 64.87 80.94 96.76 113.79 130.72 147.72 164.70 181.10 197.64 214.24 230.77 246.59 262.12 277.36 293.28 309.81 327.03 342.85 359.28 376.25 389.93 404.43 419.10 432.97 446.97 461.21 475.27

0 61.92 122.64 191.04 268.08 339.84 422.64 510.72 587.04 670.56 741.36 820.32 905.52 981.60 1048.08 1115.52 1181.04 1260.72 1339.44 1421.28 1492.56 1559.28 1611.60 1656.48 1701.60 1764.72 1824.96 1871.04 1931.28 1989.84 2045.52

0 58.32 115.20 169.92 229.92 281.52 332.40 402.24 468.96 538.80 600.96 665.52 714.96 775.20 829.44 875.52 944.64 1020.00 1104.72 1187.04 1262.16 1317.12 1390.08 1458.24 1561.00 1646.83 1722.19 1807.63 1874.11 1940.83 2008.87

Sludge characteristics The average value of the MLVSS/MLSS ratio in the A2/O-SRR system during the experimental period was 77.5%, which was higher than that in the A2/O process (71.2%). Accordingly, the activity in activated sludge of the A2/O-SRR system was comparatively higher. As presented in Figure 4, the average DHA values of the activated sludge in the A2/O-SRR system and the A2/O system were 7.02 μgTF/mgVSS and 5.91 μgTF/mgVSS, respectively. The solid volume index (SVI) was 83.89 ml/gMLSS in the A2/O-SRR, which was lower than that in the A2/O system (97.54 ml/gMLSS). It seems that there would be a contradiction between the MLVSS/MLSS ratio and SVI, but these results demonstrate that the activated sludge properties in the A2/O-SRR might be quite different from that in the control A2/O system, such as microbial communities, extracellular polymeric substances, floc structure and so on. In this case, MLSS in the bioreactor could have a good compaction and sedimentation performance with relatively high sludge activity by combining the SRR with the A2/O system.

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The phosphorus content in the activated sludge in the aerobic zone was 4.19% in the A2/O system and 7.47% in the A2/O-SRR, respectively. The additional carbon source resulting from the lysis effect in the SRR was one of the most important reasons for this [30]. Further study is needed to obtain a deep understanding of the reasons why the phosphate accumulating ability of sludge in the A2/O-SRR system could be maintained at a comparatively high level. Equation (2) describes the mass balance of P in the whole system: TPin = Xp + TPef + TPex

(2)

where: TPin , TPef is the total phosphorus in influent and effluent, respectively (g/d); TPef comprises dissolved phosphorus and phosphorus in SSef . Xp is the phosphorus removal amount of microbial assimilation (g/d); TPex is the total phosphorus in excess sludge (g/d). According to the model of Lawrence–McCarty, Xp can be calculated via the following formula: XP =

0.023YT Q(S0 − Se ) 1 + K d θc

(3)

where: 0.023 is the percentage of phosphorus in the microorganism cells based on the molecular formula of C60 H87 O23 N12 P (100%); YT is the yield coefficient (gMLSS/gCOD); Kd is the attenuation coefficient (d−1 ); Q is influent flow quantity (l/d); S0 , Se is the COD concentration in influent and effluent, respectively (g/l); θc is the solid retention time (SRT) (d). The calculation and measured values of phosphorus content in the excess sludge of the A2/O system were 3.78% and 3.97%, respectively. The calculation result of phosphorus content in the excess sludge of the A2/O-SRR system was 6.87%, while the measured values of phosphorus content of excess sludge from the SRR, settling tank, the mixing sample of the SRR and settling tank were 1.47%, 7.15% and 6.65%, respectively. Reasonable agreement of the measured and calculated results was obtained. Mechanism and characteristics of sludge reduction Average DO concentration at 10 locations in the SRR is shown in Figure 5. DO concentration ranged from 0.17 to 2.50 mg/l. There was an obvious concentration gradient of DO in the SRR due to the special inner configuration. Correspondingly, average DO concentration at the first four points was only 0.23 mg/l, and 1.45 mg/l at the other six

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Figure 4.

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SVI and DHA values in the aerobic area of the A2/O system and A2/O-SRR.

Figure 5. Concentration variety of DO in the SRR and DO sampling position.

level. In other words, the lysis mechanism of sludge process reduction might occur in the A2/O-SRR system. Consequently, influent concentration of substrate in the SRR was lower than that in the effluent of the SRR, except TP. Phosphorus uptake effects of bacteria in the oxygen-limited area led to the decrease of TP concentration in the SRR effluent. Alternating anaerobic/oxygen-limited conditions in the SRR could stimulate the growth metabolism of some special microorganisms using diversified metabolic pathways and could further lead to functional diversity of microbial community [34]. Moreover, it might be helpful to form a richer biodiversity of microorganisms, then the non-traditional removal of nitrogen might take place.

points. Good alternating anaerobic/oxygen-limited performance in the SRR could be obtained effectively, which led to an uncoupling phenomenon of the microorganisms’ growth [31,32]. Generally, microorganisms obtain energy in the aerobic and oxygen-limited zone and energy can be stored as adenosine triphosphate (ATP), then the distal γ phosphate groups can be rapidly hydrolysed to inorganic phosphate molecules in one minute after ATP is formed [33]. This process not only releases energy for bacteria cell synthesis (synthesis energy), but also maintains basic living activities for survival (maintenance energy). When mixing sludge is under the alternating anaerobic/oxygen-limited condition in the A2/O-SRR system, most of the energy generated in the above hydrolysis process may dissipate as heat instead of being used by microbes as synthesis energy. On the other hand, there also exists the phenomenon of microorganism failure to a certain degree when the sludge mixture in the SRR has a lower substrate concentration

Conclusion A novel activated sludge system combining a SRR with the A2/O process was developed in this paper. A considerably high sludge reduction effect of 43.8% was achieved compared with the control A2/O system for domestic sewage treatment. Meanwhile, it maintained good removal effects of pollutants. The amount of NVSS was comparatively high in the flux of the SRR; a substantial portion of ISS could be separated and discharged out of the A2/O-SRR system, which decreased the ISS accumulation effect to keep a good sludge activity in process. The SRR combined with the activated sludge system is a promising technology of sludge process reduction. Appropriate DO concentration and circulating velocity in the SRR were essential to keep the good effect of sludge process reduction. The obvious concentration gradient of DO in the range of 0.17–2.50 mg/l could be well formed in the SRR. Under the conditions of alternating

Environmental Technology anaerobic/oxygen-limited performance and a suitable HRT in the SRR, mechanisms of uncoupling and lysis together contributed to the occurrence of sludge process reduction.

Acknowledgement This research was supported by the International Scientific and Technological Cooperation Projects of the People’s Republic of China (No. 2007DFA90660).

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Study on a novel reactor of sludge process reduction for domestic sewage treatment.

A laboratory-scale novel Sludge Reduction Reactor with Arc Guide Plate (SRR) for sludge process reduction was developed in this study. Pollutant remov...
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