Upflow anaerobic sludge blanket in microalgae-based sewage treatment: Co-digestion for improving biogas production

Journal Pre-proofs Upflow anaerobic sludge blanket in microalgae-based sewage treatment: codigestion for improving biogas production Lucas Vassalle, Rubén Díez-Montero, Alcino Trindade Rosa Machado, Camila Moreira, Ivet Ferrer, Cesar Rossas Mota Filho, Fabiana Passos PII: DOI: Reference:

S0960-8524(19)31906-6 https://doi.org/10.1016/j.biortech.2019.122677 BITE 122677

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Bioresource Technology

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18 October 2019 20 December 2019 22 December 2019

Please cite this article as: Vassalle, L., Díez-Montero, R., Machado, A.T.R., Moreira, C., Ferrer, I., Filho, C.R.M., Passos, F., Upflow anaerobic sludge blanket in microalgae-based sewage treatment: co-digestion for improving biogas production, Bioresource Technology (2019), doi: https://doi.org/10.1016/j.biortech.2019.122677

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Upflow anaerobic sludge blanket in microalgae-based sewage treatment: co-digestion for improving biogas production

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Lucas Vassalleab*, Rubén Díez-Monterob, Alcino Trindade Rosa Machadoa, Camila Moreiraa, Ivet Ferrerb, Cesar Rossas Mota Filhoa and Fabiana Passosa

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a Department of Sanitary and Environmental Engineering, Universidade Federal de Minas Gerais (UFMG), Av. Antônio Carlos,

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b

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6627, Belo Horizonte, MG, Brazil. GEMMA – Group of Environmental Engineering and Microbiology, Department of Civil and Environmental Engineering, Universitat Politècnica de Catalunya-BarcelonaTech, c/Jordi Girona 1-3, Building D1, E-08034 Barcelona, Spain

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_____________________________________________ *Corresponding:

[email protected] / [email protected]

Telephone: +55 31 3409 1946 Fax number: +55 31 3409 1879

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Abstract Upflow anaerobic sludge blanket (UASB) reactors are widely used to treat domestic

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sewage and frequently require post-treatment. Little is known about the use of high rate algal

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ponds (HRAP) for post-treating UASB reactors’ effluent. This study aimed to evaluate a

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UASB reactor followed by a HRAP in terms of sewage treatment efficiency and biogas

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production, during one year at demonstration-scale. The UASB reactor co-treated raw sewage

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and the harvested microalgal biomass from the HRAP, which was recirculated to the reactor.

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An identical UASB reactor, treating only raw sewage, was used as control. The results

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showed an overall removal of 65% COD and 61% N-NH4 in the system. Furthermore,

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methane yield was increased by 25% after anaerobic co-digestion with microalgae, from 156

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to 211 NL CH4 kg-1 VS. An energy assessment was performed and showed a positive energy

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balance, with a net ratio of 2.11 to the annual average.

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Keywords: Bioenergy Recovery, Biogas, High Rate Algal Pond, UASB reactor, Sewage.

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1 – Introduction

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Over the years, the technology of upflow anaerobic sludge blanket (UASB) reactors has

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been expanding for treating domestic and industrial wastewater. UASB reactors have allowed

40

the expansion of the sewage treatment infrastructure to a vast population, especially in

41

locations with low availability of financial resources, land and/or skilled workers (Bressani-

42

Ribeiro et al., 2019). In this context, this technology have been successfully applied to treat

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domestic sewage in developing countries, especially with tropical climate like Brazil,

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Colombia, India and Africa (Chernicharo et al., 2018). Essentially, these reactors remove

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suspended and dissolved organic matter and, as a result, generate two co-products: a stabilised

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sludge and biogas. Biogas is mainly composed of methane, which allows its conversion into

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energy, transforming sewage treatment into a more sustainable platform and contributing to

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the circular economy.

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If compared with complete stirred tank reactors (CSTR) normally used for sludge

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digestion, the greatest advantage of the UASB technology for treating domestic sewage is its

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higher rate, allowing a low hydraulic retention time (6-10 hours HRT), but a high solid

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retention time (~ 30 days SRT) and relatively low cell growth, which implies low sludge

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generation (Daud et al., 2018). The applicability of UASB reactors treating sewage in tropical

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environmental conditions is undeniable in terms of economy, operation and area demand,

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particularly when compared to activated sludge or stabilisation ponds, for instance. However,

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the effluent from these reactors often need to be subjected to a post-treatment step in order to

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remove, above all, nutrients and pathogenic organisms in order to meet worldwide discharge

58

standards (Daud et al., 2018) . Numerous technologies have already been studied and proven

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efficient for post-treatment of UASB domestic effluent, among them are constructed

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wetlands, percolating trickling filters and polishing ponds (Mungray et al., 2012). However,

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there are very few studies evaluating the post-treatment of UASB effluent using high rate

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algal ponds (HRAP) (de Godos et al., 2016; Santiago et al., 2017; Villar-Navarro et al., 2018).

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The use of UASB reactors followed by HRAPs, may be conceived from the perspective

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of sustainability and co-products recovery, while promoting sanitation in terms of domestic

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sewage treatment in tropical developing countries. This suggests that generated co-products

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may minimise environmental and economic costs and impacts, or even be used for activating

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the local or regional economy. In the context of this work, microalgal biomass produced in

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HRAPs, may be converted into biogas and biosolids through anaerobic digestion in the first

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step of the sewage treatment plant (STP), i.e. the UASB reactors. In this scenario, the

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recirculation of microalgal biomass to be digested together with the raw sewage, is

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characterised as anaerobic co-digestion. In spite of the fact that co-digestion of microalgal

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biomass has been extensively investigated with many substrates, to the authors knowledge

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there is no studies using domestic sewage as co-substrate (Solé-Bundó et al., 2019a; Uggetti

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et al., 2017). Previous literature shows that co-digestion may increase the biodegradation rate

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and overall biogas production, due to a more adequate balance of solid to liquid ratio, macro

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and micronutrients (as carbon to nitrogen balance) and the dilution of toxic and inhibitory

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compounds, for instance (González-Fernández et al., 2012; Gutiérrez et al., 2016; Mata-

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Alvarez et al., 2014). In addition, it also enables simultaneous treatment infrastructure and

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final disposal of several residues.

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Most studies on microalgae-based STPs have been based on anaerobic co-digestion of

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microalgal biomass with primary or secondary sludge (Solé-Bundó et al., 2019b). Reported

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results have shown an increase in methane yield if compared to anaerobic mono-digestion

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(50-80 %) (Solé-Bundó et al., 2019a). According to the literature, the methane yield from

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anaerobic reactors mono-digesting microalgal biomass ranges from 0.10 to 0.25 L CH4 g VS-1

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(Solé-Bundó et al., 2018), while those values to sewage sludge substrate are around 0.35 L

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CH4 g VS-1 (Ramos-Suárez and Carreras, 2014). In terms of microalgal biomass co-digestion

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with waste activated sludge methane yield was about 0.47 L CH4 gVS-1 (Wang et al. 2013).

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Moreover, another study also confirmed that the co-digestion of microalgae with sewage

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sludge (a mixture of primary sludge, biosludge and chemical sludge) increased by 12% the

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biogas production compared to microalgae anaerobic digestion alone (Olsson et al., 2014). In

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addition, the anaerobic co-digestion of microalgae may have specific advantages, as

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increasing microalgae hydrolysis rate (Solé-Bundó et al., 2019b; Wang et al., 2013; Wang and

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Park, 2015).

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Based on the gaps presented in the literature and in order to propose a sustainable low-

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cost STP with the recovery of co-products, this study aimed to evaluate a UASB reactor

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followed by HRAP treating domestic sewage. The first part of this work focused on the

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sewage treatment efficiency. The second part, evaluated the anaerobic digestion of raw

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sewage and its anaerobic co-digestion with microalgal biomass harvested from the UASB

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reactor. Finally, an energy assessment was estimated to comprehend the process self-

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sufficiency. To the authors knowledge, this is the first time that the proposed STP flowsheet is

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investigated and evaluated in a demonstration-scale facility.

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2 - Material and Methods

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2.1. Experimental set-up and operation

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The demonstration-scale experimental set-up is shown in Figure 1. The set-up received

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raw sewage from a nearby STP located in Belo Horizonte, Brazil (coordinates 19º53'42'' S

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and 43º52'42''W, 800 m of altitude). Sewage reached the system by means of a pump

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(Netzsch® Germany) after a pre-treatment for the removal of coarse solids and grit. The study

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was conducted in two UASB reactors, i) UASBcont: fed only with domestic sewage, and 5

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ii) UASBco-dig: fed with domestic sewage and harvested microalgal biomass and used to

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evaluate the anaerobic co-digestion. Following, a settler was used to separate and concentrate

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the microalgal biomass from the HRAP effluent. Co-digestion operation was performed using

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a centrifuge pump (BCR 2000 – Schneider®) for recirculating the microalgal biomass from

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the settler to the co-digestion UASB reactor.

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The UASB reactors were made of fiberglass, with a working volume of 343 L each. The

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reactors were operated at a flow rate of 49 L h-1, a HRT of 7 hours, a sludge retention time

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(SRT) of 35 days and an organic loading rate (OLR) of 0.71 g VS L-1 day-1. The HRAPs were

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also made of fiberglass, with a working volume of 205 L each one and a surface area of 0.41

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m2 each. The ponds received the effluent from one of the UASB reactors, and were operated

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at a flow rate of 25.5 L day-1 each one, and a HRT of 8 days. Treated effluent was conducted

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by gravity to a 30 L a settler in which microalgal biomass was harvested. The settler was

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made of PVC and operated at 14 hours HRT. For anaerobic co-digestion, 10 L of microalgal

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biomass were conveyed to a plexiglass column located 4 m above the UASB and recirculated

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therefrom at a flow rate of 0.5 L h-1 into the bottom of the reactor. The recirculation flow rate

124

was selected in accordance with previous hydraulic tests that showed how higher flow rates

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(from 0.5 to 10 L h-1) led to the release of the reactor sludge blanket and effluent quality

126

deterioration. The biomass inlet flow was controlled by a needle-type flow-controlled valve.

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The system was operated continuously for 12 months. The UASBcont was fed only with raw

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sewage, while UASBco-dig was fed with raw sewage and harvested microalgal biomass (Figure

129

1).

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2.2 Analytical methods

131

For evaluating the sewage treatment efficiency, four samples were taken from the liquid

132

phase: from the raw sewage, the two UASB reactors effluent and the HRAPs effluent, twice a

6

133

week. The parameters analysed were pH (at 10 AM), temperature and dissolved oxygen (DO)

134

using a Hach® (HQ30D) probe. COD was analysed by Hach® kit COD at high range. Total

135

and volatile suspended solids (TSS and VSS) were assessed according to Standard Methods

136

(APHA, 2012) and ammonium (N-NH4+) by ionic chromatography using Metrohm® - 940

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professional IC Vario - ionic chromatography.

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For microalgal biomass characterisation, total samples in solid phase were taken once a

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week from the settler. Total and volatile solids (TS and VS) and Total Kjeldahl Nitrogen

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(TKN) were analysed according to standard procedures (APHA, 2012). Total COD was

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analysed by Hach® kit COD at high range. For carbohydrates was measured using a phenol–

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sulphuric acid method after acid hydrolysis according to literature (Dubois et al., 1956). For

143

estimating the protein content, a conversion factor of 5.95 (López et al., 2010) was used based

144

on the results of TKN. The main microalgae species in the HRAPs were identified through

145

optic microscopy (Olympus BX-50) equipped with a camera (Olympus DP70).

146

Samples for biogas analysis were collected twice a week from both the control and co-

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digestion UASB reactors. Biogas production was measured twice a week using Ritter®

148

meters. In addition, biogas characterisation in terms of CH4, CO2, O2, CO and H2S were

149

analysed by means of a Geotech® brand portable meter. The results were expressed as

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methane yield, i.e. methane volume produced per mass of COD and VS fed to the reactor.

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Substrate biodegradability was calculated through the ratio between the methane yield in

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terms of CODin measured and the theoritical methane yield under standard conditions (350 ml

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CH4 g CODremoved-1) (Chernicharo, 2007).

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Climatic data (temperature, solar radiation and precipitation) were obtained from the

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meteorological station near the STP (Brasil National Meteorology Institute, INMET,

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http://www.inmet.gov.br).

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157 158

2.3 Energy Assessment To assess the STP energy self-sufficiency, an energy balance was estimated from the

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energy requirement of the proposed system and the energy produced from the average

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methane yield monitored during the experimental period (12 months). For the calculations,

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the UASB and HRAP systems were scaled-up and projected for receiving 10,000 population

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equivalent (PE). For the preliminary treatment of this STP, it was considered manual grid and

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gravity degritters, that is, without energy costs. The energy input (Ein) comprised: i) the

164

energy demand for sewage (and microalgal biomass) pumping (Ein, UASB) and, ii) the energy

165

demand for the HRAP paddle-wheel (Ein, HRAP). For the UASB co-digestion reactor, a total of

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300 m3 d-1 (20% from total flow) of microalgal biomass was recirculated during the whole

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year, since no great differences in microalgal biomass production was noticed due to similar

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solar irradiation conditions obtained in this study. It is important to note that the operation of

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the UASB reactor had no external energy requirement, since it was operated at environment

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temperature during the whole year. The energy produced (Eout) was calculated from the

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methane yield in the control (only domestic sewage) and co-digestion (domestic sewage and

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microalgal biomass) UASB reactors. This methodology was based and adapted from (Passos

173

et al., 2017) and main equations are described following.

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2.3.1 Energy input

175 176 177

The energy input for the system was calculated from Eq. (1). (1)

𝐸𝑖𝑛 = 𝐸𝑖𝑛 𝑈𝐴𝑆𝐵 + 𝐸𝑖𝑛 𝐻𝑅𝐴𝑃

To calculate de energy consumption of the UASB reactor, Eq (2) and (3) were used.

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The equations aim to estimate the energy used for pumping wastewater (and biomass, when

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co-digestion was applied). According to the equations, Epump UASB; Microalgae is the input energy

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for the UASB reactor (kWh d-1); Qp is the pump flow rate (m3 d-1), ϴ is the electricity

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consumption for pumping (kJ m-3) and 0.000278 is the conversion factor from kJ to kWh.

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𝐸𝑖𝑛 𝑈𝐴𝑆𝐵 = 𝐸𝑝𝑢𝑚𝑝 𝑈𝐴𝑆𝐵 + 𝐸𝑝𝑢𝑚𝑝 𝑀𝑖𝑐𝑟𝑜𝑎𝑙𝑔𝑎𝑒

183

𝐸𝑝𝑢𝑚𝑝 𝑈𝐴𝑆𝐵 𝑜𝑟 𝐸𝑝𝑢𝑚𝑝 𝑀𝑖𝑐𝑟𝑜𝑎𝑙𝑔𝑎𝑒 = 𝑄𝑝·θ· 0.000278

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(2) (3)

In addition, the energy consumed in HRAP was considered to be the energy required for

185

paddle-wheel operation and was calculated according to Equation 4, where Epaddle-wheel, is the

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input electricity for the HRAPs paddle-wheel (kWh d-1), QHRAP is the mixed liquor flow rate

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in motion (m3 s-1), γ is the specific weight of water at 20 °C (kN m-3), hf channels is the head loss

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in channels (m), hf reversals is the head loss in reversals (m) and ε is the paddle-wheel efficiency.

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𝐸𝑖𝑛𝑝𝑢𝑡 𝐻𝑅𝐴𝑃 = 𝐸𝑝𝑎𝑑𝑑𝑙𝑒 ― 𝑤ℎ𝑒𝑒𝑙 =

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HRAP flow (QHRAP) corresponded to the flow rate through the transversal area of the HRAPs

191

(Eq. (5), where υ is the water velocity (m s-1), d is the water depth (m) and W is the channel

192

width (m).

193

QHRAP = υ·d·W

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𝑄𝐻𝑅𝐴𝑃·𝛾(ℎ𝑓 𝑐ℎ𝑎𝑛𝑛𝑒𝑙𝑠 + ℎ𝑓 𝑟𝑒𝑣𝑒𝑟𝑠𝑎𝑙𝑠)·24 𝜀

(4) The

(5)

The head loss in channels and reversals was calculated according to Eqs. (6) and (7),

195

respectively, where hf channels is head loss in channels (m), L is the channel length (m), n is the

196

Manning factor and, hf reversals is the head loss in reversals (m) and g is the gravitational force

197

(m s-2).

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ℎ𝑓 𝑐ℎ𝑎𝑛𝑛𝑒𝑙𝑠 =

υ2·𝐿 1.486 2

( )( 𝑛

199

𝑑𝑊

)

(6)

1.26

𝑊 + 2𝑑

υ2

(7)

ℎ𝑓 reversals = 2·2·𝑔

9

200 201

2.3.2 Energy output The energy generated in the STP was calculated in terms of the methane yield produced.

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To calculate the energy output a lower calorific value of methane of 10 kWh m-3 CH4 (ξ) and

203

an energy conversion efficiency of 90% were considered (η1) (Eq. 8).

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Eout = (OLY ξ η1)

205

where E output, is the energy output from biogas (kWh d-1); OL is organic loading rate of the

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digester (kg VS d-1); Y is the average methane yield (m3 CH4 kg-1 VS); ξ is the lower calorific

207

value of methane (kWh·m-3 CH4); and η1 is the efficiency for energy generation.

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2.3.3 Energy balance and net energy ratio

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(8)

Finally, results were expressed in terms of energy balance and net energy ratio (Eq. 9

210

and 10). The final energy balance was calculated subtracting the energy output from the

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energy input, where ΔE is the final energy balance from methane yield (kWh d-1) (Eq. 9).

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E = 𝐸𝑜𝑢𝑡 ― 𝐸𝑖𝑛

213

(9)

While, the net energy ratio (NER) was calculated as the energy output (energy produced

214

by the system) over the energy input (energy consumed by the system) (Eq. (10)).

215

𝑁𝐸𝑅 =

216

Eout

(10)

E𝐼𝑛

This means that a positive energy balance is when the system has a surplus in terms of

217

energy production (i.e. E> 0 or NER>1). For the sake of comparison, the assessment was

218

carried out using the same scenarios for control and co-digested UASB reactors.

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2.4 Statistical analysis

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The Mann-Whitney U test for independent samples was used to evaluate COD and

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ammonia removal, in the complete pilot-scale system by comparing influent and effluent

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concentration values. For comparing results from anaerobic performance and biogas

223

production in control and co-digestion UASB reactors, the Wilcoxon statistical test was used

224

for paired or dependent samples. To perform the statistical analysis, Statistica 10.0® software

225

was used. The significance level of all tests was 95%.

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3 - RESULTS AND DISCUSSION

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3.1 Sewage treatment efficiency

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The physical-chemical parameters of raw sewage, UASBcont, UASBco-dig, HRAP

229

effluent and microalgal biomass are summarised in Table 1. It may be observed that for pH,

230

DO and temperature there was no significant variation for both reactors, indicating that the

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reactors operated similarly. It should be highlighted that both reactors were continuously fed

232

with the same domestic sewage and that the increase in organic loading rate for the co-

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digested reactor was due to the addition of microalgal biomass.

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COD removal in the UASBco-dig was 50%, considering the influent sewage COD and the

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recycled microalgal biomass COD. The overall removal of COD, after the HRAP, reached

236

65%. Similar results were found in a previous study carried out with a UASB reactor followed

237

by HRAP, in which a 65% COD removal was obtained (Villar-Navarro et al., 2018a). For the

238

UASBcont, the average COD removal was 57%. It may be noticed a lower COD removal in the

239

UASBco-dig when compared with the control. This may have occurred due to microalgal

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biomass recirculation in the co-digested reactor. Even with a slow recirculation (0.5 L h-1), the

241

upflow biomass movement may have caused a displacement of the UASB sludge blanket,

242

leading to a transport of stabilised organic matter from the solid to the liquid phase. In

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general, COD removals between 55 to 70% have been reported for UASB reactors and

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between 65 and 80% for UASB followed by polishing ponds system (Von Sperling, 2007). It

245

may be noticed that in this set-up, the UASBco-dig had slightly lower efficiency in comparison 11

246

with those reported in the literature for COD removal (50% vs. 55-70%), but when compared

247

the overall removal this data stay within range reported in literature (65% vs. 65-80%).

248

In the UASBco-dig reactor no total nitrogen removal was observed, but an intense

249

mineralisation of organic nitrogen. Under anaerobic conditions, the decomposition of the

250

organic matter by anaerobic microorganisms, leads to the hydrolysis of proteins and urea and

251

the consequent increase in NH4+-N (Metcalf & Eddy et al., 2003). Average concentrations

252

observed were 34 mg N-NH4 L-1 for sewage and 41 mg N-NH4 L-1 for UASB effluent. For

253

HRAPs, there was an average removal of 61% of NH4+-N, with final effluent concentrations

254

of 16 mg N-NH4 L-1. According to the literature, the main removal pathway of NH4+ in HRAP

255

is due to microalgal biomass assimilation. Nitrification and volatilization could be another

256

removal route. Similar concentrations were found in studies that evaluated the NH4+-N

257

removal in HRAP with similar HRT (Doma et al., 2016).

258

TSS and VSS concentrations found in the effluent samples were comparable with the

259

typical values from domestic effluents (120-360 mg TSS L-1 and 90-280 mg VSS L-1)

260

(Metcalf & Eddy et al., 2003), from UASB reactor effluents (60-160 mg TSS L-1 and 30 mg

261

VSS L-1) (Chernicharo, 2007) and from HRAPs effluents used as UASB post-treatment units

262

(145 mg TSS L-1 and 124 VSS mg L-1) (Santiago et al. 2017) (Table 2). The results presented

263

an increase of TSS and VSS values from the UASB reactor to the HRAP by 122% and 137%

264

respectively. However, global removal for TSS and VSS were about 45% and 57%,

265

respectively. Results on HRAP fed with UASB effluent are still incipient in literature. For

266

instance, UASB reactors operating at a higher HRT (15 hours) and HRAP at lower HRT (4

267

and 6 days) compared to the HRT in both treatment units in this work, showed overall

268

removals of 60% for NH4+-N, and an average increase of 130% for TSS and 165% for VSS

269

due to microalgae growth (Santiago et al., 2017; Villar-Navarro et al., 2018). In respect to

12

270

studied set-up, both COD and NH4+-N were significantly decreased when influent and

271

effluent values were compared, indicating a removal of these parameters.

272

3.2. Biogas Production

273

On regard to the characteristics of harvested microalgal biomass recirculated to the

274

UASBco-dig reactor for co-digestion with a raw sewage, the sample had in average 2,393 mg

275

VS L-1, 172 mg TKN L-1, 395 mg L-1 of carbohydrates and 1,023 mg L-1 of proteins.

276

Regarding the total COD, these samples presented an average concentration of 3,763 mg L-1.

277

Carbohydrates and proteins corresponded to 17% and 43% of the biomass dry weight,

278

respectively. Regarding the microalgae species present in the HRAPs, Kirchneriella sp. was

279

predominant. Scenedesmus sp., Westella sp. and different species of diatoms were also present

280

in lower frequency.

281

Biogas prodution and composition of both UASB reactors are shown in Table 2. As can

282

be observed, biogas characteristics was typical of UASB reactors. As known, biogas

283

generated from anaerobic treatment in UASB reactors is commonly composed of higher-grade

284

methane and lower concentration by carbon dioxide, due to the high solubility of this gaseous

285

compound in the liquid (van Haandel and Lettinga, 1994). The biogas generated in UASB

286

reactors is normally a mixture of gases with volume concentrations of 60-85% methane

287

(CH4); 5-15% dioxide carbon (CO2); 2-25% nitrogen (N2); 0-0.3% carbon monoxide (CO); 0-

288

3% hydrogen (H2); 0-2% oxygen (O2); and 1,000-2,000 ppmv (parts per million by volume)

289

of hydrogen sulphide (H2S) (Silveira, 2015).

290

An increase of H2S was observed in the biogas of the UASBco-dig reactor. This increase

291

may be related to the oxygen introduced by microalgal biomass, which, being dissolved, may

292

have been rapidly consumed in the first centimetres of the sludge blanket for aerobic

293

conversion of organic compounds. Thus, the anaerobic environment of this reactor may have 13

294

allowed the reduction of sulphur compounds (as elemental sulphur), increasing concentration

295

in biogas. Similar dynamics are reported in the literature dealing with UASB reactors with

296

micro-aeration (Marinho, 2019)

297

Produced biogas was in average 304.42 NL kg-1 VS (149.81 NL kg-1 COD) for

298

UASBcont and 331.12 NL kg-1 VS (165.63 NL kg-1 COD) for UASBco-dig, which represents a

299

10% increase. Considering the methane yield, the increase after co-digesting sewage with

300

microalgal biomass was 25% (from 156 to 211 NL CH4 kg-1 VS). It is important to note that

301

the increase in the organic content input in terms of VS to the co-digested reactor was in

302

average 9% higher in relation to the UASBcont. Methane yield results in both reactors for the

303

different annual seasons are shown in Figure 2. As can be seen, average values were different

304

among the seasons.

305

Values obtained for anaerobic treatment of raw sewage may be compared with mono-

306

digestion of sewage sludge (110 to 160 NL kg-1 VS) (Gunaseelan, 1997). In terms of co-

307

digestion, previous results have shown that the methane yield increased after treatment with

308

other substrates (12-41% increase) (Jankowska et al., 2017; Solé-Bundó et al., 2019a). The

309

results found in this research are within the methane yield range reported in the literature for

310

sewage sludge and microalgae co-digestion (107 – 293 NL CH4 kg COD-1 and 168 – 291 CH4

311

kg VS-1) (Hlavínek et al., 2016; Jankowska et al., 2017; Mahdy et al., 2014; Solé-Bundó et al.,

312

2018). However, it is important to note that the observed range has a high variation, due to

313

differences in the reactor type, the operation mode, substrate characteristics and to the

314

microalgae species present in the system. Moreover, these data were obtained from CSTR or

315

BMP tests, while this study was conducted using a UASB reactor in demonstrate scale. It is

316

noteworthy that there is an incipient number of publications on microalgal biomass anaerobic

14

317

digestion using UASB reactors and no publications at all using co-digestion of microalgal

318

biomass and domestic sewage.

319

Microalgal biomass production in HRAP and solar irradiation during different seasons

320

is shown in Figure 3. When compared to Figure 2, it may be noticed how the highest methane

321

yield was obtained in winter and autumn, when microalgal biomass production was the

322

highest (Figure 3). This difference was probably associated to seasonality changes and

323

pluviosity. Since Summer and Spring were months associated with highest incidence of rainy

324

days. In fact, in tropical regions it is common that warmer months is also the rainy season.

325

This probably affected the microalgal biomass production and concentration. As the

326

recirculation was constant and fixed at a volume of 10 L per day, there was an increase in the

327

recirculated organic load during those phases, leading to an increase in the methane yield

328

during that period. Higher volume of microalgal biomass recirculated to the reactor also may

329

have contributed to the insertion of a portion of oxygen in this reactor. It is known that a

330

micro aeration in anaerobic systems can improve the hydrolysis of organic matter and the

331

acidogenic phase in the reactor, leading to the oxidation of some available substrates by

332

aerobic metabolism (Botheju and Bakke, 2011). Furthermore, oxygen supplementation in

333

anaerobic digestion is efficient when done in batches and in small quantities, as it was

334

performed in this work. (Botheju et al., 2010). Moreover, in terms of biodegradability, the

335

mixed substrate fed to the UASBco-dig reactor was 26% higher compared to raw sewage (Table

336

2).

337

The annual average microalgal biomass production in the HRAP was 8.5 g TSS m-2 day.

338

This value is similar to that reported in a study developed in HRAP operated in tropical

339

climate and with the same HRT, where the authors obtained a production of 9.0 g TSS m-2

340

day (Park and Craggs, 2010). However, values between 13 and 35 g TSS m-2 day are

15

341

considered typical to this systems (Park and Craggs, 2011). The lower values may be related

342

to the low availability of carbon dioxide in domestic sewage. The inorganic carbon

343

concentration in the HRAP can be increased by introducing CO2 into the system. For instance,

344

microalgal biomass production was increased by 15% after addition of CO2 in HRAP

345

(Heubeck et al., 2007). Another important factor for microalgae growth in HRAPs is the

346

incidence of solar radiation (Solimeno et al., 2017). Therefore, the geographic region in which

347

the system is situated may interfere in the biomass production results. In this research, the

348

experiment was performed in the southeast region of Brazil with high solar irradiation

349

throughout the whole year (1000-1200 W m-2). Another important environmental factor is the

350

pluviometry, which causes dilution and biomass production decrease in small-scale systems

351

(~ 200 L). In this study, spring and summer had an accumulated rainfall of 178 mm and 277

352

mm, while for autumn and winter it was 37 mm and 18 mm, respectively. Probably, due to

353

this difference, biomass production during summer and spring were lower in comparison with

354

the other seasons. In addition, another factor that influences an optimal microalgae growth is

355

the temperature of the ponds, which must be close to 25ºC (Solimeno et al., 2017).

356

Temperatures in this study were ranged from 18ºC and 24ºC. Therefore, the temperature was

357

stable and did not seem to be a limiting factor in this case study.

358

Finally, literature reports that the amount of nutrients, especially ammonium, and pH

359

affects microalgae growth, with a possible inhibition of above 90% when ammonia is higher

360

than 50 mg N-NH4 L-1 and pH is higher than 8 (Azov et al., 1982). In this study, HRAPs had

361

average ammonium values of 16 NH4 L-1 and pH of 7.8. Therefore, it can be concluded that

362

low microalgal biomass production was more possibly associated with carbon deficiency and

363

pluviometry.

364

16

365 366

3.3 Energy assessment The energy assessment for UASBco-dig and UASBcont was performed to verify the energy

367

self-sustainability of the proposed STP when evaluating biogas production from co-digestion

368

of microalgal biomass with raw sewage. The energy production scenario was developed for a

369

system for 10,000 PE. The results obtained by season and the average annual period are

370

shown in Table 3.

371

As may be observed from the values, UASBco-dig. had a positive energy balance for all

372

weather seasons. On the contrary, UASBcont showed all negative values. Thus, it is possible to

373

conclude that the use of microalgal biomass in co-digestion with raw sewage improved the

374

potential in using biogas produced into energy. The energy balance and NER of the UASBco-

375

dig and

376

reported for an anaerobic system co-digesting microalgal biomass with primary sludge

377

(Passos et al., 2017). In the mentioned study, authors obtained energy ratio from 1.01 to 5.31

378

for different scenarios. It is important to note that the variation in energy production produced

379

in the scenarios was associated with the variation of the OLR input in the UASB reactors as

380

explained in section 3.2.

381

UASBcont are shown in Figures 4 and 5. Positive energy balance was previously

The energy ratio obtained from UASBco-dig. varied from 1.7 to 2.8, which means that

382

from 70 to 180% more energy was produced that consumed. The energy produced in this case

383

was much higher compared to the energy produced when only sewage was digested in the

384

reactor (~ 5 times higher). In the latter scenario, more energy was consumed with pumping

385

the sewage and for HRAP paddle-wheels, than the energy produced from the methane yield.

386

The energy produced by this system has many applications, within the co-digestion STP

387

and also within the nearby community. Three possible energy applications will be discussed

388

below: i) conversion into electricity and heat in a combined heat and power (CHP) plants; ii) 17

389

conversion into heat using boilers and, iii) conversion into biomethane through biogas

390

upgrading.

391

Considering conversion of biogas into electricity and heat using a CHP motor, 35% of

392

energy is converted into electricity and 55% into heat. Therefore, in this case, in average 757

393

kWh d-1 of electricity and 1,188 kWh d-1 of heat would be produced annually. In this case, the

394

electricity produced could be injected into the grid to reduce the electricity consumption at the

395

plant, covering 82% of the STP demand.

396

Another option is to produce heat using a boiler. Considering the boiler efficiency 90%,

397

the heat balance would be in average 1.945 kWh d-1. In this case, the electricity required for

398

the STP could be provided through renewable sources, as photovoltaic systems or wind

399

power. Since UASB reactor was operated at ambient conditions, heat should be used for other

400

purposes in or outside STP. This also may be applied for the thermal energy produced using

401

the CHP plant. In the STP, an option is to sanitize the sludge produced, making it suitable for

402

agricultural use. Or by drying and reducing the sludge for its more cost-effective disposal.

403

Previous studies have shown that both applications are viable, with reductions in the sludge

404

volume by 46% and reduction in pathogens by up to 99.9% (Cartes et al., 2018; Kacprzak et

405

al., 2017; Rosa et al., 2018). Still in STP, the thermal energy could be used for pre-treating

406

microalgal biomass prior to the anaerobic reactor. This application could be used for

407

increasing biomass solubilization and biodegradability, therefore increasing methane yield in

408

the reactor. In fact, previous studies have shown that thermal pretreatment at 70-90 °C

409

increased methane yield of microalgal biomass up to 30% (Passos et al., 2014). Outside the

410

STP, thermal energy produced may also be used for cooking and heating water in the nearby

411

community.

18

412

Finally, a third option for converting biogas is through upgrading into biomethane. In

413

this case, purification and removal of CO2 is necessary to reach acceptable values for injection

414

into the distribution grid (> 95%) (Muñoz et al., 2015) and for the use as car engines (96%)

415

(Papacz, 2011) . The injection of biomethane into the grid may be used exploited by the

416

nearby population or for the STP car fleet. This scenario would enable a decrease in the use of

417

fossil fuels and, consequently, a decrease in the carbon footprint. In a previously study, biogas

418

purification was achieved in an adsorption column with microalgae and obtained 94-99% of

419

methane content (Marín et al., 2019) . Similar results were reported for biogas upgrading

420

using microalgae grown in HRAPs (Marín et al., 2018; Posadas et al., 2016).

421

Still in terms of the best energy conversion option, a previous study showed how a STP

422

using UASB reactors, would be more economically viable when converting the energy

423

produced from biogas into electricity and heat, if compared to the same system harnessing

424

only heat for a population equivalent above 200,000 (Valente, 2015). In any case, the

425

mentioned study did not consider any co-digestion step. Other approaches consider that

426

biomethane may be more economically viable compared to electricity or heat production

427

(Budzianowski and Budzianowska, 2015). However, the mentioned study criticizes the

428

insufficient political and economic incentive given to this technology, leaving it at the margin

429

of CHP or boiler technologies. On the whole, it is safe to say that an energetical assessment

430

should consider each local reality in order to evaluate the complete system viability.

431

4 – Conclusion

432

The propose STP consisting in a UASB + HRAP showed a great potential of organic

433

matter and nutrients removal (65% COD and 61% N-NH4). The evaluated system showed

434

that co-digestion with microalgae improved the methane yield (25% higher than control

435

increase). Moreover, the system showed a positive energy balance, with 70 to 180% more

436

energy produced than consumed throughout the year. Further studies should be performed to 19

437

verify the real economic impact of the proposed co-digestion on energy production from

438

biogas generated by UASB reactors and consider the implementation of low-cost pre-

439

treatment and energy demand of HRAP microalgal biomass.

440

Acknowledgements

441

The authors would like to acknowledge the following organisms for their help and

442

funding: National Council for Scientific and Technological Development from Brazilian

443

Ministry of Education – CNPq; to the Institute of Sustainable Sewage Treatment Plants -

444

INCT ETEs; Sustentáveis and their funders: Coordination for the Improvement of Higher

445

Education Personnel – CAPES; Foundation for Research of the State of Minas Gerais –

446

FAPEMIG; CNPQ and to National Health Foundation – FUNASA from Brasil. Lucas

447

Vassalle would like to acknowledge the CNPQ to his scholarship 204026/2018-0. Rubén

448

Díez-Montero would like to thank the Spanish Ministry of Industry and Economy for his

449

research grant (FJCI-2016-30997).

450 451

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Wang, M., Park, C., 2015. Investigation of anaerobic digestion of Chlorella sp. and

606

Micractinium sp. grown in high-nitrogen wastewater and their co-digestion with waste

607

activated sludge. Biomass and Bioenergy 80, 30–37.

608

https://doi.org/10.1016/j.biombioe.2015.04.028

609

50.

Wang, M., Sahu, A.K., Rusten, B., Park, C., 2013. Bioresource Technology Anaerobic

610

co-digestion of microalgae Chlorella sp . and waste activated sludge. Bioresour.

611

Technol. 142, 585–590. https://doi.org/10.1016/j.biortech.2013.05.096

612

27

613 614 615

28

616 617

29

618 619

30

620 621

31

622 623 624

32

625

Figure Caption

626

Figure 1. Diagram of the demo-scale experimental set-up

627

Figure 2. Methane yield in UASB reactors with and without co-digestion

628

Figure 3. Biomass production in the HRAP and solar radiation over the year.

629

Figure 4. Energy balance

630

Figure 5. NER energy for the system

631 632 633

33

634

Table 1. Physical-chemical characterisation of the different sampling points evaluated: Raw sewage, UASB

635

co-digestion (UASBco-dig), control UASB (UASBcont) and HRAPeff (n = 84) Samples UASBco-dig

UASBcont

HRAPeff

Mean ± SD

Mean ± SD

Mean ± SD

Mean ± SD

pH

7.7 ± 0.2

7.4 ± 0.3

7.3 ± 0.3

7.8 ± 0.5

DO (mg L-1)

1.39 ± 1.45

0.39 ± 0.37

0.29 ± 0.13

8.60 ± 2.75

Temperature (ºC)

24.3 ± 1.8

23.2 ± 2.0

23.6 ± 1.9

21.3 ± 3.5

COD (mg O2 L-1)

422.9 ±136.7

223.8 ±89.8

180.6 ±66.6

147.9 ±71.6

TSS (mg L-1)

251.6 ±95.3

63.4 ± 54.3

-

140.7 ± 100.9

VSS (mg L-1)

207.39 ±70.3

37.78 ± 25.7

-

88.43 ± 49.1

N-NH4+ (mg L-1)

34.21 ± 7.8

41.85 ± 5.9

-

16.67 ± 3.9

Parameters

Raw Sewage

636 637

34

638

Table 2. Biogas production and composition (n = 84)

Parameter

UASBco-dig

UASBcont

Mean ± SD

Min/Máx

Mean ± SD

Min/Máx

30.23 ± 12.97

13.23 / 58.08-

22.16 ±11.59

6.97 / 54.74-

165.63 ± 71.44

78.98 / 488.75

149.82 ±64.54

58.29 / 483.57

105.81 ± 45.40

45.73 / 283.96

77.56 ± 40.56

24.38 / 292.08

331.12 ± 133.97

124.00 / 739.76

304.42 ± 116.11

82.34 / 612.36

210.79 ± 78.12

87.37 / 487.06

156.33 ± 45.40

69.69 / 378.54

CH4 (%)

63.63 ± 7.01

40.80 / 80.40

51.22 ± 9.77

18.40 /70.80

CO2 (%)

6.74 ±5.67

2.49 / 51.82

6.21 ±3.55

0.20 / 20.62

O2 (%)

1.39 ±2.07

0.20 /10.80

0.30 ±0.12

0.20 /0.80

CO (ppm)

6.77 ±3.55

0.00 / 35.00

7.82 ±6.03

0.00 / 36.00

H2S (ppm)

1843.58 ±359.70

362 / 3463

1558.32 ±582.70

104 / 2629

Balance (%) (N2 + H2)

26.43 ± 9.05

19.36 / 31.52

40.70 ± 16.35

13.44 / 53.33

Biodegradability (%) Biogas production (L kg-1 COD) Methane yield (LCH4 kg-1COD) Biogas production (L kg-1 VS) Methane yield (LCH4 kg-1VS)

639 640

35

641

Table 3. Results of the average seasonal energy assessment UASB+HRAP Winter Parameters

Spring

UASB

Summer

UASB

Autumn

UASB

Annual

UASB

UASB

Co-dig

Cont

Co-dig

Cont

Co-dig

Cont

Co-dig

Cont

Co-dig

Cont

EIn UASB (kWh d-1)

901

751

901

751

901

751

901

751

901

751

EIn HRAP (kWh d-1)

22

0

22

0

22

0

22

0

22

0

EIn (kWh d-1)

923

751

923

751

923

751

923

751

923

751

EOut (kWh d-1)

1875

411

1559

413

1761

359

2589

589

1945

448

∆ E (kWh d-1)

952

-340

636

-338

838

-392

1666

-162

1022

-303

NER

2,03

0,55

1,69

0,55

1,91

0,48

2,81

0,78

2,11

0,60

642 643

36

644 645 646 647 648 649 650 651

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

652 653 654 655 656

37

657 658

38

Highlights

659 660



661 662

domestic sewage 

663 664

UASB+HRAP treatment system removed 65% COD and 61% N-NH4 from

Co-digestion with microalgae improved 25% methane yield in UASB compared to control



665

After UASB co-digestion 70 to 180% more energy was produced than consumed in the STP.

666



STP showed a positive net energy ratio of 2.11 in average during the year.

667



Microalgae recirculation in UASB reactor improved STP energy self-sustainability.

668

39