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Investigation on Recycling Dry Toilet Generated Blackwater by Anaerobic Digestion: from Energy Recovery to Sanitation

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Investigation on Recycling Dry Toilet Generated Blackwater by Anaerobic Digestion: from Energy Recovery to Sanitation sustainability Article Investigation on Recycling Dry Toilet Generated Blackwater by Anaerobic Digestion: From Energy Recovery to Sanitation Siqi Zuo 1,2,†, Xiaoqin Zhou 1,2,†, Zifu Li 1,2,*, Xuemei Wang 1,2 and Longbin Yu 1 1 School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (S.Z.); [email protected] (X.Z.); [email protected] (X.W.); [email protected] (L.Y.) 2 Beijing Key Laboratory of Resource—Oriented Treatment of Industrial Pollutants, University of Science and Technology Beijing, Beijing 100083, China * Correspondence: [email protected] † These two authors contributed equally to this work. Abstract: Anaerobic digestion (AD) has been widely adapted for blackwater treatment, however, the effect of water-conserving toilet generated blackwater on the AD process is still unknown. In this study, the anaerobic digestion process of dry toilet generated blackwater was investigated by means of a biomethane potential test. It was demonstrated that anaerobic digestion was inhibited and then adapted because of a high total ammonium nitrogen (TAN) level (3673.3 mg/L). The start-up period was 14.04 days and the biomethane potential of dry toilet blackwater was 402.36 mLCH4/gVS (55 days, 38 ◦C). Inhabitation and adaptation could be described as the increase of free ammonia nitrogen content and acetic acid concentration, followed by an enhancement of the relative abundance of acetic acid-type methanogens (from 33.53–61.52%). The main pathogen in dry toilet blackwater fermentation broth, Pseudomonas aeruginosa, kept multiplying in the first 8 days and then stabilized at a higher level than that of the beginning. This work showed the self-adjustment process and Citation: Zuo, S.; Zhou, X.; Li, Z.; Wang, X.; Yu, L. Investigation on pathogen dynamics of dry toilet blackwater anaerobic digestion and highlights the significance of Recycling Dry Toilet Generated dry toilet blackwater characteristics when designing and maintaining anaerobic digestion sanitary Blackwater by Anaerobic Digestion: treatment and reuse systems. From Energy Recovery to Sanitation. Sustainability 2021, 13, 4090. https:// Keywords: blackwater; dry toilet; sanitation; sustainable treatment doi.org/10.3390/su13084090 Academic Editor: Giovanni De Feo 1. Introduction Received: 17 January 2021 Ecological sanitation emphasizes closing the loop of water and nutrients during Accepted: 25 February 2021 wastewater management [1]. Source separation collection of blackwater (feces, urine and Published: 7 April 2021 flushing water) and greywater (other household wastewater) is necessary for resource recovery treatment [2]. In well-designed and developed systems, blackwater can be Publisher’s Note: MDPI stays neutral further separately collected and treated as urine and feces, which leads to higher recovery with regard to jurisdictional claims in efficiencies and better fertilizer products [3–7]. Blackwater contains high organic and published maps and institutional affil- nutrient levels (>50% of the organic content and 80–95% of the nutrients in domestic iations. wastewater) [8], and the separate treatment of blackwater can maximize the possibility of resource recovery. In places with poor sanitation arrangements and facilities, dry toilets have been widely used, which can lead to a pollution of the soil and drinking water without proper treatment. Copyright: © 2021 by the authors. Anaerobic digestion is an energy-efficient method for blackwater treatment to recover Licensee MDPI, Basel, Switzerland. energy and nutrients [9]. However, the stability of anaerobic digestion is susceptible This article is an open access article to fluctuations in substrate composition (such as total solids, chemical oxygen demand, distributed under the terms and C/N ratio, and total organic carbon), accumulation of toxic substances, sludge aging and conditions of the Creative Commons Attribution (CC BY) license (https:// other factors [10]. The organic loading rate (OLR) of organic waste anaerobic digestion creativecommons.org/licenses/by/ systems is normally controlled to be less than 4 g Volatile Solid/d·L[11]. Ammonia 4.0/). Sustainability 2021, 13, 4090. https://doi.org/10.3390/su13084090 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 4090 2 of 13 suppression is the most typical phenomenon in the anaerobic process because of nitrogen- rich materials. Ionized ammonium can directly inhibit the synthesis of methane-producing enzymes [12] and ammonia molecules can passively diffuse into cells, causing proton imbalance and potassium deficiency [13]. The biogas production process is normally inhibited when the total ammonia nitrogen (TAN) concentration exceeds 1500 mg/L and becomes toxic when TAN is ≥3000 mg/L. Pretreatment is usually employed to reduce ammonium concentration [14], or a support material is added to control ammonia inhibition in fermentation broths [15]. The anaerobic digestion of chicken manure was performed with high ammonia nitrogen content (TAN above 6 g/L). When average free ammonia nitrogen (FAN) concentration increased from 0.77 g/L to 0.86 g/L, the biogas yield dropped from 0.39 to 0.27 L/gVS [16]. BMP (biomethane potential) tests are generally considered to be an assessing method for an anaerobic digestion substrate. The BMP procedure involves adding small amounts of selected inoculum and substrate into serum bottles, creating anaerobic conditions and measuring biogas production over time. A cumulative biogas production curve is then obtained and the BMP value is typically expressed as a function of added volatile solids (VS) (mL·CH4/g·VS) [17]. However, the results of BMP tests can be influenced by inoculum characteristics (origin, concentration and activity), experimental conditions (gas measurement system, operational parameters, chemical parameters and inoculum to substrate ratio) [18], particle size and the nutrient media of the substrate [19]. To the best of our knowledge, a large amount of the domestic wastewater anaerobic digestion studies has focused on treatment performance and removal efficiency and mainly employed simulated blackwater or diluted blackwater from flush toilets. These types of blackwater normally have low chemical oxygen demand (COD) and TAN concentrations. Adhikari used a septic tank-upflow anaerobic sludge blanket (UASB) reactor to treat high concentration domestic sewage. The COD reached 1200 mg/L and the average removal efficiency of the COD was 88% [20]. Cunha added calcium for the recovery of phosphate granules (CaP granules) and methane from vacuum collected blackwater (BW) using an upflow anaerobic sludge blanket (UASB) reactor. The overall removal of COD was stable at above 80% [21]. In another study, a sludge blanket anaerobic baffled reactor was applied to treat blackwater. The temperature in the buffer tank ranged from 10–15 ◦C in wintertime and from 18–21 ◦C in summertime. On average, more than 78% of the influent load of the COD at a hydraulic retention time (HRT) of 3 days was removed [22]. When proposing and developing sustainable wastewater treatment systems, recovery of energy and nutrients has become the direction for future research [23]. However, mini- mal attention has been given to the effects of blackwater characteristics on methanogenic process stability and efficiency. The blackwater generated from dry toilets poses a challenge to decentralized treatment systems due to its high organic and ammonia load. Recently, re- searchers have begun to focus on the effect of blackwater generated from water-conserving toilets in anaerobic digestion. Studies of different blackwaters have suggested that differ- ing toilet types would affect energy recovery potential due to ammonia inhibition in the methanogenesis phase [24]. Granulated activated carbon was added into the anaerobic digestion of blackwater to decrease the inhibition of TAN (2800 ± 200 mg/L) and adjust the process; biochemical methane potential and COD removal increased from 34% to 53% and 37% to 56.7%, respectively [25]. The authors investigated blackwater produced by different types of toilets, including dry, vacuum and water saving flushing toilets, and re- vealed remarkable differences in their properties. TAN in various blackwaters ranged from 107–3673 mg/L, and the highest TAN in concentrated blackwater was at a toxic level. The inhibition behavior during high concentration blackwater anaerobic processes remains unknown. Considering the reuse of blackwater, pathogens can be another problem. Human feces contain a large number of pathogenic bacteria, such as Aeromonas spp., Escherichia coli, etc. (from WHO guidelines). Aerobic composting has been proved to be useful for eliminat- ing pathogens and parasites, and reducing phytotoxicity (cress seed germination index ≥80%) [26,27]. The biomethane potential (BMP) test of dry toilet generated blackwater Sustainability 2021, 13, 4090 3 of 13 was investigated in this study, and biogas production and methane content were measured and calculated. Volatile fatty acid (VFA), ammonia nitrogen, total nitrogen and microbial diversity during the start-up period were analyzed to obtain insights into the process. The results can serve as a theoretical basis for dry toilet blackwater anaerobic digestion process design and operation. 2. Material and Methods 2.1. Blackwater Blackwater was sampled from one household dry toilet in a rural village of Shunyi District, Beijing, used by 5 healthy adults for 1 weekday. A total of 5 L of blackwater
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