Abstract

There are hundreds of full-scale upflow anaerobic sludge blanket (UASB) reactors in operation in various parts of the tropical world, notably in India and Latin America, Brazil being the holder of the largest park of anaerobic reactors for sewage treatment in the world. Despite the recognized advantages of UASB reactors, there are problems that have prevented their maximum operational performance. Neglecting the existence and delaying the solution of these challenges can jeopardize the important advances made to date, impacting the future of anaerobic technology in Brazil and in other countries. This work aims to evaluate the operational performance of five full-scale UASB reactors in Brazil, taking into account a monitoring period ranging between two and six years. The main observed design, construction, and operational constraints are discussed. Some outlooks for important upcoming developments are also provided, considering that most of the observed drawbacks can be tackled without significant increases on reactor costs.

INTRODUCTION

The application of anaerobic technology for sewage treatment has significantly expanded in the last two decades (Noyola et al. 2012) and now Brazil has the largest park of upflow anaerobic sludge blanket (UASB) reactors (in terms of installed units) for sewage treatment in the world (Chernicharo et al. 2018). Amongst the main factors that led to such expansion, savings of implementation and operational costs can be highlighted, compared to those of conventional aerobic (e.g. activated sludge) or physicochemical processes. The reduction of operational costs is mainly ascribed to the low sludge production, which is in turn already thickened and digested, as well as the reduced (or zero) energy expenditure. On top of that, the average methane content in biogas of UASB reactors treating sewage opens possibilities for different uses (e.g. sludge sanitization or cogeneration of heat and power) (Possetti et al. 2019). Nevertheless, some constraints currently observed in full-scale systems still need to be solved. Especially the design, operational, and managerial aspects of UASB reactor need improvements, since the further expansion of the technology and its wider acceptance in the near future can be significantly hindered by sub-optimal functioning systems.

A great deal of problems associated with these anaerobic reactors has been ascribed to deficiencies in the initial design phases. However, inadequate construction practices and operational problems have also been reported in the literature and by sanitation service providers in different regions in Brazil (Miki 2010). In addition to the technical problems of design, construction, and operation, several constraints arise from situations that go beyond the limits of the sewage treatment plant itself, such as managerial issues (e.g. personnel administration, subcontracting) which often prevent a proper operational routine.

The aim of this work is to assess the operational performance of five full-scale UASB-based sewage treatment plants (STPs) in Brazil, highlighting the main observed design, construction, and operational constraints. Such problems are critically assessed focusing on possible improvements without significant increases on reactor costs.

MATERIAL AND METHODS

Five different anaerobic-based STPs located in the southeast region of Brazil were assessed in terms of chemical oxygen demand (COD) and biochemical oxygen demand (BOD) removal, as well as settleable solids in the effluent. This was carried out alongside a comprehensive evaluation of the main design, construction and operational constraints which may hamper the system performance. The monitoring period ranged between two and six years, comprising bimonthly samples. The evaluated parameters were COD, BOD and settleable solids. A complementary intensive sampling campaign was performed once or twice a week for three months, comprising the following parameters: alkalinity (as CaCO3), dissolved sulphate, sulphide and oxygen, total suspended solids (TSS), temperature, pH, total nitrogen, surfactants and soluble COD. The main goal was to further investigate possible reasons for problems pointed out by plant operators. Analytical procedures followed Standard Methods for the Examination of Water and Wastewater (APHA 2012).

The treatment flowsheet of four of the evaluated STPs comprises preliminary treatment units (screens and grit chambers) and UASB reactors followed by trickling filters (TFs) (associated with secondary settlers), as well as dewatering units (Figure 1). The flowsheet of the STP 5 does not comprise the post-treatment step of the anaerobic effluent. The main characteristics of the STPs are presented in Table 1.

Table 1

Main characteristics of the assessed UASB/TF systems

General parameters STP 1 STP 2 STP 3 STP 4 STP 5 
Population equiv. (inhab) 391,959 44,395 1,000,000 103,921 78,815 
Design flow rate (L·s−1989 80 2,050 196 201 
Preliminary treatment CS (75 mm +25 mm), FS (6 mm), GC CS (50 mm +20 mm), GC CS (100 mm +25 mm), FS (6 mm), GC CS (50 mm +20 mm), GC CS (100 mm +15 mm), GC 
Design parameters UASB TFa DS UASB TFa DS UASB TFa DS UASB TFa DS UASB 
Design HRT (h) 6.4 – 2.30 6.8 – 3.05 7.2 – 4.40 8.3 – 3.77 10.0 
Design upflow velocity (m·h−10.72 – – 0.71 – – 0.63 – – 0.58 – – 0.51 
Number of reactors 12 24 
Unitary area (m2410 615 452 200 177 127 492 1319 1017 200 346 253 354 
Useful depth (m) 4.65 2.50 4.00 4.80 2.50 3.50 4.65 2.50 4.00 4.80 2.50 3.50 5.80 
Unitary useful volume (m31,905 1,539 1,809 960 442 443 2,212 3,299 4,069 960 865 886 1,940 
Operational parameters UASB TFa DS UASB TFa DS UASB TFa DS UASB TFa DS UASB 
Current flow rate (L·s−1320 64 1,700 95 89 
Current HRT (h) 13.2 – 7.85 8.3 – 3.83 8.7 – 5.32 8.4 – 7.74 18.1 
Current upflow velocity (m·h−10.35 – – 0.58 – – 0.52 – – 0.86 – – 0.30 
General parameters STP 1 STP 2 STP 3 STP 4 STP 5 
Population equiv. (inhab) 391,959 44,395 1,000,000 103,921 78,815 
Design flow rate (L·s−1989 80 2,050 196 201 
Preliminary treatment CS (75 mm +25 mm), FS (6 mm), GC CS (50 mm +20 mm), GC CS (100 mm +25 mm), FS (6 mm), GC CS (50 mm +20 mm), GC CS (100 mm +15 mm), GC 
Design parameters UASB TFa DS UASB TFa DS UASB TFa DS UASB TFa DS UASB 
Design HRT (h) 6.4 – 2.30 6.8 – 3.05 7.2 – 4.40 8.3 – 3.77 10.0 
Design upflow velocity (m·h−10.72 – – 0.71 – – 0.63 – – 0.58 – – 0.51 
Number of reactors 12 24 
Unitary area (m2410 615 452 200 177 127 492 1319 1017 200 346 253 354 
Useful depth (m) 4.65 2.50 4.00 4.80 2.50 3.50 4.65 2.50 4.00 4.80 2.50 3.50 5.80 
Unitary useful volume (m31,905 1,539 1,809 960 442 443 2,212 3,299 4,069 960 865 886 1,940 
Operational parameters UASB TFa DS UASB TFa DS UASB TFa DS UASB TFa DS UASB 
Current flow rate (L·s−1320 64 1,700 95 89 
Current HRT (h) 13.2 – 7.85 8.3 – 3.83 8.7 – 5.32 8.4 – 7.74 18.1 
Current upflow velocity (m·h−10.35 – – 0.58 – – 0.52 – – 0.86 – – 0.30 

HRT, hydraulic retention time; CS, coarse screen; FS, fine screen; GC, horizontal flow grit chamber.

aTrickling filters are packed with crushed stones.

Figure 1

Flowsheet of STPs 1 to 4* – UASB/TF systems. Source: adapted from von Sperling & Chernicharo (2005). *The flowsheet of STP 5 only comprises the preliminary treatment and the UASB reactor.

Figure 1

Flowsheet of STPs 1 to 4* – UASB/TF systems. Source: adapted from von Sperling & Chernicharo (2005). *The flowsheet of STP 5 only comprises the preliminary treatment and the UASB reactor.

It should be emphasized that the preliminary treatment step presents different characteristics amongst the evaluated STPs (e.g. space between bars; presence of sequential coarse screens and fine screening step). Additionally, four of the assessed STPs directly flare the produced biogas without energy recovery. For the excess sludge management, dewatering systems comprise natural units (drying beds) or mechanized equipment (e.g. centrifuge). Biogas recovery for sludge dewatering (thermal dryer) is implemented in STP 1.

RESULTS AND DISCUSSION

Performance of the evaluated STPs

Figure 2(a)–2(c) show the influent and effluent COD, BOD and settleable solids concentrations for all of the assessed STPs. In all evaluated plants, except for STP 1, the anaerobic reactor fulfilled its main role of reducing organic loads. The median influent COD and BOD concentrations ranged between 550 and 840 mg·L−1 and 240 and 500 mg·L−1, respectively. The median COD and BOD removal efficiencies of the UASB reactors varied between 60 and 75% and 65 and 82%, respectively. For STP 2, 3 and 4 the operating hydraulic retention time (HRT) was slightly (up to 20%) higher than the design HRT (see Table 1). For STP 5, the operating HRT was 80% higher than the design criteria. This is an important aspect, since larger HRTs provide a better hydrolysis condition and consequently a better system performance. On the other hand, excessive HRTs tend to impair mixing conditions in the reactor due to the associated low upflow liquid velocities (<0.50 m·h−1). Nonetheless, the low average upflow velocity (0.30 m·h−1) observed for STP 5 does not seem to have hampered the performance of the anaerobic reactors.

Figure 2

Boxplot graphics for monitoring of BOD (a), COD (b) and settleable solids (c). IN: influent raw sewage; UASB EF: effluent from UASB reactors; Final EF: effluent from post-treatment system (trickling filters followed by secondary settlers) (for STP 5, Final EF represents the effluent from UASB reactors).

Figure 2

Boxplot graphics for monitoring of BOD (a), COD (b) and settleable solids (c). IN: influent raw sewage; UASB EF: effluent from UASB reactors; Final EF: effluent from post-treatment system (trickling filters followed by secondary settlers) (for STP 5, Final EF represents the effluent from UASB reactors).

One of the main characteristics of UASB reactors, when properly operated, is their high solids retention capacity, resulting in high sludge ages and a high degree of sludge stabilisation (Chernicharo et al. 2019). Specifically for STP 1, an apparent design flaw led to the implementation of a dewatering system (one centrifuge followed by a thermal dryer) whose capacity is below the sludge production in the treatment plant. The underestimation of the sludge handling system capacity can also be related to receiving different waste streams after the STP commissioning, as further addressed in this paper. Therefore, the UASB reactors of STP 1 have been operated with abnormal excess sludge above their maximum storage capacity, resulting in excessive solids washout with the anaerobic effluent (see Figure 2(c)), and consequently high effluent COD concentrations (see Figure 2(b)), even though the higher observed HRT.

Additionally, this excessive effluent solids load in STP 1 had a deleterious effect on the trickling filter units that comprise the post-treatment step. The average COD removal efficiency in such reactors was drastically decreased to zero (data not shown), which is masked in the final effluent by the polishing capacity of the secondary settlers. This situation can eventually evolve to clogging the trickling filters, as discussed below.

It should be noted that the practice of transferring secondary (aerobic) excess sludge from the post-treatment unit for thickening and digestion in UASB reactors was not implemented in the assessed STPs. Exception is made to STP 2, in which a daily recirculation flow of 35 L·s−1 for 5 min has been done from the secondary settlers. In this case, a proper sludge withdrawal routine in the UASB reactors assures the observed good effluent quality.

Main observed constraints and possible solutions

The main observed constraints in the evaluated STPs are depicted in Figure 3.

Figure 3

Main observed constraints in the assessed STPs: (a) accumulation of grit and debris in a channel prior to the grit chamber; (b) accumulation of scum inside a gas-liquid-solid (GLS) separator; (c) accumulation of scum in a settler compartment of a UASB reactor; (d) non-uniform particle size of a rock-bed trickling filter; (e) partial biogas flaring; and (f) corrosion in a concrete gas chamber of a UASB reactor.

Figure 3

Main observed constraints in the assessed STPs: (a) accumulation of grit and debris in a channel prior to the grit chamber; (b) accumulation of scum inside a gas-liquid-solid (GLS) separator; (c) accumulation of scum in a settler compartment of a UASB reactor; (d) non-uniform particle size of a rock-bed trickling filter; (e) partial biogas flaring; and (f) corrosion in a concrete gas chamber of a UASB reactor.

The satisfactory operation of the anaerobic reactor depends fundamentally on the adequate and accurate design and operation of the preliminary treatment (PT) units. Low liquid velocities (<0.3 m·s−1) or inadequate gate positioning/operation tend to build up grit and debris in PT channels (Figure 3(a)). Such accumulated material can be one of the main sources of odours in the STP, with reported H2S concentrations ranging from 2 to 37 ppmv (Al-Shammiri 2004). Additionally, the improper grit and coarse solids removal can lead to: (i) the obstruction of flow distribution structures and effluent collecting channels; (ii) the deposition/accumulation of grit in the bottom of UASB reactors; (iii) pipe obstructions, breakage, and loss of efficiency of the dewatering unit due to equipment wear and tear; and (iv) the scum formation in the upper part of the GLS separator.

The difficulties associated with the systematic scum removal from inside the GLS separator should be highlighted for the assessed STPs, especially regarding the operation of the scum removal devices as they were designed. Non- or insufficient scum withdrawal can lead to thickening and solidification of the scum layer (see Figure 3(b)), preventing the natural passage of the biogas generated in the reactor. This can cause the biogas to pass to the settler compartment, compromising the solids retention and leading to deterioration of the effluent quality (Lettinga & Hulshoff 1991; Souza et al. 2006). Additionally, biogas losses to the settler compartment have a direct effect on the increase in emission levels of odorous and greenhouse gases in the STP. Moreover, it reduces the recovery of biogas for purposes of burning and/or energy generation.

A possible solution refers to the implementation of GLS separators equipped with internal devices that allow the hydrostatic scum removal (as described in Chernicharo et al. 2015). Operational protocols tested in a full-scale UASB reactor indicated that an appropriate pressure arrangement in the gas line (5–10 cm.w.c) and the resulting level of scum within the gas chambers enabled the effective removal of the scum at approximately 75–90% of the initial volume recorded (Rosa et al. 2012).

Excess sludge accumulation in the assessed UASB reactors was observed, which tends to be worse in cases where the devices for monitoring the sludge concentration were not properly installed, not allowing the identification of the best moment for sludge discharge prior to deterioration of the effluent quality. The absence or removal of excess sludge at a frequency not compatible with the effective sludge production in the system can result in a more significant loss of solids to the settler compartment. Once in the settler, this sludge is entrapped in the scum layer and has the appearance of ‘floating sludge’ along with the scum (see Figure 3(c)). Apart from the aforementioned lack of capacity of the dewatering system, another factor that affected the production and management of sludge was related to inflow contributions not considered during the design stage of the plant. These contributions were mainly ascribed to chemical sludge from water treatment plants. In this case, the real sludge coefficient yield (Ysludge) can be significantly higher than that considered in the design, as also pointed out by Chernicharo et al. (2014).

A sort of complementary measures associated to the design and operational phases can be adopted to tackle this problem. Firstly, an accurate determination of the sludge production, and the provisioning of two sets of sludge withdrawal pipes can provide more operational flexibility (Chernicharo et al. 2019). Moreover, the use of balance tanks for sludge homogenization and thickening before the dewatering unit would assure flexibility for the sludge withdrawal from the UASB reactors.

Solids washout with the effluent of UASB reactors can also lead to clogging or flooding of the post-treatment units, especially when attached growth processes (e.g. trickling filters) are used. These phenomena can be even worse in cases of non-uniform particle size of rock-based support media in trickling filters (see Figure 3(d)). Therefore, the reduction in the void index can hamper oxygen transfer and consequently the performance for residual carbon and ammonia removal. Assuring a rigorous granulometry control of the crushed stones received during the STP construction phase is of utmost importance.

In relation to biogas, there were problems associated with: (i) the absence of desirable gas line devices (e.g. water seal, automatic ignition); (ii) precarious pipeline installations (e.g. points of condensate accumulation) with materials not resistant to solar radiation, as polyvinyl chloride (PVC); and (iii) low efficiency of open flares (see Figure 3(e)). Open flares are usually used for biogas combustion in STPs in developing countries (Noyola et al. 2006). Nevertheless, because the flame is open (exposed to weather conditions), the intensity is low, and the ambient air quickly cools or even extinguishes the flame. Furthermore, the temperature and residence time in the combustion process are difficult to measure and, therefore, determining the conversion efficiency of the biogas is also difficult. The methane destruction efficiency, for example, is typically approximately 50% (Baukal 2013).

The installation of a simple hydraulic device (i.e. water seal) can be an effective measure to assure the minimum required pressure of 1,500 Pa (15 cm of water column) for biogas burning (Possetti et al. 2019). Moreover, the replacement of open flares by enclosed ones would provide the required CH4 and H2S abatement to prevent greenhouse gases emissions in the STP, as well as reduction in odorous emissions and corrosion related problems. The efficiency of compound destruction (CH4 and H2S) in enclosed burners can reach rates exceeding 99% (Wagner et al. 2017). Nevertheless, to achieve higher efficiencies, it is necessary to regulate the flow rate, as biogas production in UASB reactors shows variable behaviour (Possetti et al. 2019).

Odour nuisance was also reported as a major drawback of the assessed STPs. In the presence of sulphate, even at low concentrations, as for domestic sewage (50–200 mg·L−1; Barbosa & Sant'Anna 1989), sulphate-reducing bacteria can consume the fermentation products (as electron donors for sulphate reduction). In this way, hydrogen sulphide (H2S) is formed and will be present in the biogas and also dissolved in the bulk liquid. H2S is not only the primary reason for complaints about odour but it is also toxic when inhaled, and is highly corrosive, causing considerable damage to concrete and steel structures at ambient conditions, as shown in Figure 3(f). The dissolved H2S concentrations for the assessed STPs are shown in Figure 4. The values for STPs 1 to 4 are slightly below the typical range reported in the literature (5 and 22 mg·L−1) (Souza et al. 2012; Garcia et al. 2015; Azevedo et al. 2018). This can be probably ascribed to the release of dissolved gases to atmosphere due to turbulence points alongside the collection and transport of effluents from the assessed UASB reactors.

Figure 4

Boxplot graphic for monitoring of hydrogen sulphide (H2S).

Figure 4

Boxplot graphic for monitoring of hydrogen sulphide (H2S).

To control odour dispersing around the anaerobic-based STPs associated with diffusive emissions, preventive actions can be taken during the design phase to minimize turbulence during the conduction of anaerobic effluent. Moreover, downstream devices can be used to allow the controlled desorption and further treatment of gases dissolved in the effluent of anaerobic reactors (as proposed in Brandt et al. 2019).

CONCLUSIONS

Although many problems have been detected in the assessed full-scale anaerobic-based STPs, the overwhelming majority can be simply and effectively solved without significant increase on reactor costs. Nevertheless, to the best of our knowledge, the suggested improvements have not been implemented in a joint manner so far. In synthesis, the source of the main identified constraints is shared through design, construction and operational phases.

A crucial issue refers to the appropriateness of GLS separators. The design of a GLS separator equipped with internal hydrostatic scum removal, assuring its proper levelling during the construction stage, can prevent the scum accumulation both inside the GLS separator and in the settler compartment of the UASB reactor. Moreover, the major drawback of odour nuisance can be tackled by preventing or stimulating a hydraulic turbulent regime in the anaerobic effluent, depending on the further management strategy (e.g. extraction and treatment of dissolved gases).

From the STPs performance, it should be noted the robustness of the evaluated systems to assure an effluent that meets the discharge standards typically adopted in developing countries. In this instance, it is mandatory in establishing an adequate operational routine for excess sludge withdrawal.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the support obtained from the following Brazilian institutions: Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq; Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES; Fundação de Amparo à Pesquisa do Estado de Minas Gerais – FAPEMIG; Instituto Nacional de Ciência e Tecnologia em Estações Sustentáveis de Tratamento de Esgoto – INCT ETEs Sustentáveis (INCT on Sustainable Sewage Treatment Plants).

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