• Energy Research

Abstract Anaerobic digestion (AD) at low temperature (20 °C) for low-strength municipal sewage (COD of 500 mg/L) treatment was evaluated in two laboratory-scale up-flow anaerobic sludge blankets (UASBs), one with and one without granular activated carbon (GAC). During the 120-day operation, the addition of GAC significantly improved average COD removal (from 56% to 82%) and methane production (from 132 to 264 mL CH4/g feed-COD), allowing for a reduced hydraulic retention time (from 1 d to 0.25 d) and an increased organic loading rate (from 500 to 2000 mg COD/L/d). There was a significant (p

Proteins present in domestic and industrial wastewater, yet their role in nutrient and energy recovery during anaerobic digestion (AD) has not been well understood. This study aimed to examine the impact of feedwater protein content on the calcium phosphate (CaP) precipitation under a low supersaturation condition and the methane production in an AD process. Four 1.0 L upflow anaerobic sludge blanket (UASB) reactors were fed with the synthetic feeds with different protein contents. The results showed that the amino acid degradation caused the pH elevation, triggering the CaP precipitation. A 25% bovine serum albumin (BSA) content in the feedwater helped shape a superior microbial community that contributed to a methanisation rate of 82.7%. The high prevalence of the phylum Synergistetes and the methanogen Methanosaeta suggested that methane was mainly produced through the acetate utilization.

A pilot-scale UASB-Settler-Digester (USD) system was utilized to treat raw municipal wastewater collected from a sewer system at 10 °C. During the reactor operation, UASB sludge was continuously transferred from the UASB to a settler; concentrated sludge in the settler was then transferred to a digester operated at 35 °C. The results showed that the settler with a hydraulic retention time (HRT) of 3 h increased UASB sludge chemical oxygen demand (COD) concentration from 14.5 ± 2.5 g/L to 29.9 ± 4.1 g/L. With an HRT of 6 h, the USD system achieved a mean COD removal of 49.2%; and 23.9% influent COD was converted to methane. The specific methanogenic activities at 35 °C of the UASB and the digester sludge were 0.26 and 0.24 g CH4 COD/(g VSS d), respectively, and the stability values were 0.21 and 0.16 g CH4 COD/g COD, respectively. The stability of the settled sludge was similar to that of the recirculated UASB sludge. Compared to a UASB-digester system, the system with an added settler achieved similar COD removal and methane production, but reduced sludge recirculation rate (from 16% to 8% of the influent flow rate), which led to a 50% heating energy saving in the digester of the UASB-digester.

Municipal wastewater contains valuable organic contents that can be recovered as energy through anaerobic digestion. The operating temperature impacts the design and performance of anaerobic digestion systems and can be selected to accommodate different geographical zones, from tropical to frigid. Here the biological methane potential (BMP) and the effect of temperature on methane production are compared for municipal wastewater collected from different collection systems, including source-diverted toilet wastewater (i.e. blackwater) and conventionally collected sewage. The sewage and blackwater collections were performed in Canada. BMP tests were conducted at 20 and 35°C. The BMPs of municipal sewage and water-wasting toilet (6–9 l per toilet flush) blackwater were comparable at 20 and 35°C. Water-conserving toilet (1 l per toilet flush) blackwater had lower BMP than municipal sewage and water-wasting toilet blackwater at 35°C, due to FA inhibition. Under the lower-temperature condition (20°C), although no apparent inhibition in the methanogenic process was observed for water-conserving toilet blackwater, a much lower hydrolysis rate was observed.

Source diverted blackwater collected from toilets can be anaerobically digested to recover energy. The anaerobically digested blackwater (ADB) contains high levels of ammonium and low carbon to nitrogen (C/N) ratio. In the present study, ADB was treated by a two-stage nitritation-denitrification/anammox process in an integrated fixed film activated sludge-continuous flow reactor (IFAS-CFR). NH4+-N, NO2--N, total nitrogen (TN), and chemical oxygen demand (COD) removal efficiencies were 80%, 82%, 76%, and 78%, respectively. Anaerobic ammonium oxidation (anammox) and denitrification contributed to 44-48%, and 52-56% of total nitrogen removal, respectively. Both of the protein- and humic acid-like matters were removed during the process. An increase in feed load promoted the sustained growth of anammox bacteria-Candidatus Brocadia in the biofilm, as well as an increase of denitrifiers (Pseudomonas, Thermotonus, Phodanobacter, Caulobacter) in both biofilm and suspended biomass, which remained higher in the suspended biomass than in biofilm. Overall, biofilm had higher nitrogen removal efficiency than suspended biomass, while suspended biomass had a higher COD removal efficiency than biofilm.

The anaerobic treatment of low strength domestic sewage at low temperature is an attractive and important topic at present. The upflow anaerobic sludge bed (UASB)-digester system is one of the anaerobic systems to challenge low temperature and concentrations. The effect of sludge recirculation rate on a UASB-digester system treating domestic sewage at 15 °C was studied in this research. A sludge recirculation rate of 0.9, 2.6 and 12.5% of the influent flow rate was investigated. The results showed that the total chemical oxygen demand (COD) removal efficiency rose with increasing sludge recirculation rate. A sludge recirculation rate of 0.9% of the influent flow rate led to organic solids accumulation in the UASB reactor. After the sludge recirculation rate increased from 0.9 to 2.6%, the stability of the UASB sludge was substantially improved from 0.37 to 0.15 g CH4-COD/g COD, and the bio­gas production in the digester went up from 2.9 to 7.4 L/d. The stability of the UASB sludge and bio­gas production in the digester were not significantly further improved by increasing sludge recirculation rate to 12.5% of the influent flow rate, but the biogas production in the UASB increased from 0.37 to 1.2 L/d. It is recommended to apply a maximum sludge recirculation rate of 2–2.5% of the influent flow rate in a UASB-digester system, as this still allows energy self-sufficiency of the system.

This study investigated the effect of granular activated carbon (GAC) on the digestion of blackwater collected from different collection systems, by monitoring the biochemical methane production (BMP), adsorption of molecules to GAC and their impacts on the microbial community. Without GAC-amendment, BMP reached 35.6, 42.6 and 50.4% in 1L, 5L and 9L water-flushed blackwater, respectively. When 33.3 gL-1 GAC was added to the cultures, methane potential increased up to 53.1% in 1L water-flushed blackwater, while in 5L and 9L water-flushed conditions the BMP drastically decreased to 16.1 and 9.6%, respectively. The concentration of volatile fatty acids (VFA) in 5L and 9L water-flushed blackwater with GAC-amended cultures was not enhanced, in contrast with 1L water-flushed blackwater. Further tests showed 29.8% (±1.9%) of VFA and 86.0% of soluble chemical oxygen demand were removed by GAC adsorption in 9L water-flushed blackwater. A decrease in biomass density in 5L and 9L GAC-amended cultures was also observed, corroborated by a significant decrease in gene copy numbers of methanogenic archaeal communities. This study gives an insight on the effect of GAC on different strengths of blackwater, which is of relevance for further tests of long-time and full-scale application.

The present work evaluated the impacts of microbial communities, biomass activity and sludge morphology on anaerobic syntrophic reactions. Experiments were conducted using mature floc sludge and granular sludge under different food/microbes ratios, and with different sludge types (floc sludge, concentrated floc sludge and granular sludge) and sludge morphology (granules, vortexed granules, and granules with different particle sizes). The results show that the intact granules achieved the most effective syntrophic reaction among all sludge types. The granule structure facilitated the enrichment of syntrophic acetate oxidation bacteria (g_Syner-01 and g_Mesotoga) and methanogens, which corresponds to their superior specific methanogenic activity and high production of communication compounds. Despite the high diffusion and substrate uptake capacities, the disintegrated granules had low H2 consumption rates, which led to poor syntrophic activities. The results underline the importance of sludge spatial structures in promoting excellent syntrophic activities and the development of diverse microbial communities.